U.S. patent application number 15/635550 was filed with the patent office on 2017-10-19 for semiconductor device.
The applicant listed for this patent is Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Kiyoshi KATO, Jun KOYAMA, Shunpei YAMAZAKI.
Application Number | 20170301380 15/635550 |
Document ID | / |
Family ID | 44059540 |
Filed Date | 2017-10-19 |
United States Patent
Application |
20170301380 |
Kind Code |
A1 |
YAMAZAKI; Shunpei ; et
al. |
October 19, 2017 |
SEMICONDUCTOR DEVICE
Abstract
An object of the present invention is to provide a semiconductor
device combining transistors integrating on a same substrate
transistors including an oxide semiconductor in their channel
formation region and transistors including non-oxide semiconductor
in their channel formation region. An application of the present
invention is to realize substantially non-volatile semiconductor
memories which do not require specific erasing operation and do not
suffer from damages due to repeated writing operation. Furthermore,
the semiconductor device is well adapted to store multivalued data.
Manufacturing methods, application circuits and driving/reading
methods are explained in details in the description.
Inventors: |
YAMAZAKI; Shunpei;
(Setagaya, JP) ; KOYAMA; Jun; (Sagamihara, JP)
; KATO; Kiyoshi; (Atsugi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Semiconductor Energy Laboratory Co., Ltd. |
Atsugi-shi |
|
JP |
|
|
Family ID: |
44059540 |
Appl. No.: |
15/635550 |
Filed: |
June 28, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14828669 |
Aug 18, 2015 |
9705005 |
|
|
15635550 |
|
|
|
|
14451671 |
Aug 5, 2014 |
9135958 |
|
|
14828669 |
|
|
|
|
14059475 |
Oct 22, 2013 |
8804396 |
|
|
14451671 |
|
|
|
|
13680378 |
Nov 19, 2012 |
8619454 |
|
|
14059475 |
|
|
|
|
12947846 |
Nov 17, 2010 |
8339828 |
|
|
13680378 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G11C 16/28 20130101;
H01L 27/105 20130101; G11C 7/10 20130101; H01L 27/1156 20130101;
G11C 7/12 20130101; H01L 21/02631 20130101; G11C 11/4097 20130101;
H01L 29/26 20130101; H01L 27/115 20130101; G11C 5/06 20130101; H01L
21/02565 20130101; G11C 7/18 20130101; G11C 5/147 20130101; H01L
27/11551 20130101; H01L 29/22 20130101; H01L 29/24 20130101; G11C
7/22 20130101; H01L 2924/0002 20130101; H01L 27/1207 20130101; H01L
23/528 20130101; G11C 8/08 20130101; H01L 29/7869 20130101; H01L
27/0688 20130101; H01L 27/108 20130101; H01L 2924/00 20130101; G11C
16/0433 20130101; G11C 11/24 20130101; H01L 29/78603 20130101; H01L
27/1225 20130101; H01L 2924/0002 20130101; H01L 29/78 20130101;
H01L 21/02554 20130101 |
International
Class: |
G11C 7/10 20060101
G11C007/10; H01L 27/12 20060101 H01L027/12; H01L 27/06 20060101
H01L027/06; H01L 23/528 20060101 H01L023/528; G11C 8/08 20060101
G11C008/08; G11C 7/22 20060101 G11C007/22; G11C 7/18 20060101
G11C007/18; H01L 29/786 20060101 H01L029/786; G11C 7/12 20060101
G11C007/12 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 20, 2009 |
JP |
2009-264623 |
Claims
1. (canceled)
2. A circuit comprising: a measurement unit configured to perform a
plurality of measurements of a voltage between a supply voltage
node and a reference node by taking a single measurement in each of
a plurality of consecutive measurement cycles, wherein the
measurement unit comprises a plurality of measurement circuits; a
comparison unit configured to compare each of the plurality of
measurements to a reference voltage; and a register unit configured
to store results of each comparison performed by the comparison
unit, and further configured to provide data based on the
corresponding unique one of the plurality of measurements.
3. The circuit according to claim 2, wherein each of the
measurement circuits is configured to perform a measurement
responsive to assertion of an enable signal.
4. The circuit according to claim 2, wherein the comparison unit
comprises a plurality of comparators, wherein each of the
comparators includes a first input coupled to receive the reference
voltage, and a second input coupled to an output of a corresponding
unique one of the plurality of measurement circuits of the
measurement unit.
5. The circuit according to claim 2, wherein the register unit
includes a plurality of storage locations, wherein each of the
storage locations corresponds to one of a plurality of comparators
of the comparison unit.
6. A circuit comprising: a measurement unit configured to perform a
plurality of measurements of a voltage by taking a single
measurement in each of a plurality of consecutive measurement
cycles, wherein the measurement unit comprises: a plurality of
measurement circuits comprising: a comparison unit configured to
compare each of the plurality of measurements to a reference
voltage; and a register unit configured to store results of each
comparison performed by the comparison unit, and further configured
to provide data based on the corresponding unique one of the
plurality of measurements.
7. The circuit according to claim 6, wherein each of the
measurement circuits is configured to perform a measurement
responsive to assertion of an enable signal.
8. The circuit according to claim 6, wherein the comparison unit
comprises a plurality of comparators, wherein each of the
comparators includes a first input coupled to receive the reference
voltage, and a second input coupled to an output of a corresponding
unique one of the plurality of measurement circuits of the
measurement unit.
9. The circuit according to claim 6, wherein the register unit
includes a plurality of storage locations, wherein each of the
storage locations corresponds to one of a plurality of comparators
of the comparison unit.
10. A method comprising: sequentially performing a plurality of
measurements of a voltage between a supply voltage node and a
reference node, wherein each of the plurality of measurements is
performed over a plurality of consecutive measurement cycles, and
wherein each of the plurality of measurements is performed by a
corresponding unique one of a plurality of measurement circuits;
comparing, using a comparison unit, each of the plurality of
measurements to a reference voltage; storing results of each
comparison in a register unit; and generating an output data in
each of plurality of measurement cycles.
11. The method according to claim 10, wherein the comparison unit
comprises a plurality of comparators, wherein each of the
comparators includes a first input coupled to receive the reference
voltage, and a second input coupled to an output of a corresponding
unique one of the plurality of measurement circuits.
Description
TECHNICAL FIELD
[0001] The present invention relates to a semiconductor device
using semiconductor elements and a manufacturing method
thereof.
BACKGROUND ART
[0002] Memory devices using semiconductor elements are roughly
classified into volatile memory devices that lose their memory
content when power supply is stopped and nonvolatile memory devices
that can hold their memory content even when power supply is
stopped.
[0003] As a typical example of a volatile memory device, a dynamic
random access memory (DRAM) is given. In a DRAM, a transistor
included in a memory element is selected and an electric charge is
accumulated in a capacitor, so that information is memorized.
[0004] Owing to the above-described principle, a charge in a
capacitor is lost when information data is read in a DRAM; thus, it
is necessary to perform rewriting of the information data, so that
information is memorized again after the information data reading.
In addition, there is leakage of current in a transistor included
in a memory element so that a charge accumulated at an electrode of
the capacitor is lost or a charge flows into the capacitor, even if
the transistor is not selected to perform any operation, whereby
data holding period is short. Therefore, it is necessary to perform
rewriting in a predetermined cycle (refresh operation) and it is
difficult to reduce power consumption satisfactorily. Further,
since memory content is lost when the power is not supplied to the
DRAM, another memory device using a magnetic material or an optical
material is needed to store information for a long period.
[0005] As another example of a volatile memory device, a static
random access memory (SRAM) is given. In an SRAM, memory content is
stored using a circuit such as a flip flop, so that refresh
operation is not needed. In view of this point, SRAM is
advantageous over a DRAM. However, an inconvenient in that a cost
per storage capacity becomes high because a circuit such as a flip
flop is used. Further, in view of the point that memory content is
lost when the power is not supplied, an SRAM is not superior to a
DRAM.
[0006] As a typical example of a nonvolatile memory device, flash
memory is given. A flash memory includes a floating gate between a
gate electrode and a channel formation region in a transistor. A
flash memory stores memory content by storing a charge in the
floating gate, so that a data holding period is extremely long
(semi-permanent), and thus has an advantage that refresh operation
which is necessary in a volatile memory device is not needed (for
example, see Patent Document 1).
[0007] However, in flash memory, there is a problem in that a
memory element does not function after performing writing
operations a predetermined number of times because a gate
insulating layer included in the memory element is deteriorated by
a tunnel current occurring each time a writing operation is
performed. In order to relieve an adverse effect of this
phenomenon, a method consisting in equalizing the number of writing
operations between the memory elements is employed, for example.
However, a complicated peripheral circuit is needed to apply this
method. Further, even if such a method is employed, the basic
problem of lifetime is not resolved. That is, a flash memory is
unsuitable for applications in which information is rewritten with
high frequency.
[0008] Further, high voltage is required to store a charge in the
floating gate or to remove a charge in the floating gate.
Furthermore, a relatively long time is required for storing or
removing a charge and the speed of writing and erasing cannot
easily be increased.
REFERENCE
Patent Document
[0009] [Patent Document 1] Japanese Published Patent Application
No. S57-105889
DISCLOSURE OF INVENTION
[0010] In view of the above problem, one object of one embodiment
of the present invention is to provide a semiconductor device
having a new structure, the semiconductor device being capable of
holding memory content in the state where power is not supplied and
having no limitation on the number of writing operations. Another
object of one embodiment of the present invention is to provide a
semiconductor having a structure allowing using easily multivalued
information.
[0011] One embodiment of the present invention is a semiconductor
device having a stack of a transistor using an oxide semiconductor
and a transistor using a material other than an oxide
semiconductor. For example, the semiconductor device can employ the
following structures.
[0012] One embodiment of the present invention is a semiconductor
device including: a source line; a bit line; a first signal line; a
plurality of second signal lines; a plurality of word lines; a
plurality of memory cells connected in parallel between the source
line and the bit line; a driver circuit for the second signal lines
and the word lines to which an address signal is input and which
drives the plurality of second signal lines and the plurality of
word lines so that a memory cell specified by the address signal is
selected from the plurality of memory cells; a driver circuit of
the first signal line selecting and outputting any of a plurality
of writing potentials to the first signal line; a reading circuit
to which a potential of the bit line and a plurality of reference
potentials are input and which read data by comparing the potential
of the bit line and the plurality of reference potentials; a
potential generating circuit generating and supplying the plurality
of writing potentials and the plurality of reference potentials to
the driver circuit of the first signal line and the reading
circuit; and a boosting circuit supplying a potential to the
potential generating circuit. One of the memory cells includes: a
first transistor having a first gate electrode, a first source
electrode, and a first drain electrode; a second transistor having
a second gate electrode, a second source electrode, and a second
drain electrode; and a third transistor having a third gate
electrode, a third source electrode, and a third drain electrode.
The first transistor is provided over a substrate containing a
semiconductor material. The second transistor includes an oxide
semiconductor layer. The first gate electrode and one of the second
source electrode and the second drain electrode are electrically
connected to each other. The source line and the first source
electrode are electrically connected to each other. The first drain
electrode and the third source electrode are electrically connected
to each other. The bit line and the third drain electrode are
electrically connected to each other. The first signal line and the
other of the second source electrode and the second drain electrode
are electrically connected to each other. One of the second signal
lines and the second gate electrode are electrically connected to
each other. One of the word lines and the third gate electrode are
electrically connected to each other.
[0013] Additionally, in the above structure, the semiconductor
device includes a capacitor electrically connected to the first
gate electrode and one of the second source electrode and the
second drain electrode.
[0014] Another embodiment of the present invention is a
semiconductor device including: a source line; a bit line; a first
signal line; a plurality of second signal lines; a plurality of
word lines; a plurality of memory cells connected in parallel
between the source line and the bit line; a driver circuit for the
second signal lines and the word lines to which an address signal
is input and which drives the plurality of second signal lines and
the plurality of word lines so that a memory cell specified by the
address signal is selected from the plurality of memory cells; a
driver circuit of the first signal line selecting and outputting
any of a plurality of writing potentials to the first signal line;
a reading circuit to which a potential of the bit line and a
plurality of reference potentials are input and which has a
reference memory cell, and reads data by comparing conductance of
the specified memory cell and conductance of the reference memory
cell; a potential generating circuit generating and supplying the
plurality of writing potentials and the plurality of reference
potentials to the driver circuit of the first signal line and the
reading circuit; and a boosting circuit supplying a potential to
the potential generating circuit. One of the plurality of memory
cells includes: a first transistor having a first gate electrode, a
first source electrode, and a first drain electrode; a second
transistor having a second gate electrode, a second source
electrode, and a second drain electrode; and a third transistor
having a third gate electrode, a third source electrode, and a
third drain electrode. The first transistor is provided over a
substrate containing a semiconductor material. The second
transistor includes an oxide semiconductor layer. The first gate
electrode and one of the second source electrode and the second
drain electrode are electrically connected to each other. The
source line and the first source electrode are electrically
connected to each other. The first drain electrode and the third
source electrode are electrically connected to each other. The bit
line and the third drain electrode are electrically connected to
each other. The first signal line and the other of the second
source electrode and the second drain electrode are electrically
connected to each other. One of the second signal lines and the
second gate electrode are electrically connected to each other. One
of the word lines and the third gate electrode are electrically
connected to each other.
[0015] Another embodiment of the present invention is a
semiconductor device including: a source line; a bit line; a first
signal line; a plurality of second signal lines; a plurality of
word lines; a plurality of memory cells connected in parallel
between the source line and the bit line; a driver circuit of the
second line and the word line to which an address signal and a
plurality of reference potentials are input and which drives the
plurality of second signal lines and the plurality of word lines so
that a memory cell specified by the address signal is selected from
the plurality of memory cells and which selects and outputs any of
the plurality of reference potentials to one selected word line; a
driver circuit of the first signal line selecting and outputting
any of a plurality of writing potentials to the first signal line;
a reading circuit to which the bit line is connected and which
reads data by reading conductance of the specified memory cell to
read data; a potential generating circuit generating and supplying
the plurality of writing potentials and the plurality of reference
potentials to the driver circuit of the first signal line and the
reading circuit; and a boosting circuit supplying a potential to
the potential generating circuit. One of the memory cells includes:
a first transistor having a first gate electrode, a first source
electrode, and a first drain electrode; a second transistor having
a second gate electrode, a second source electrode, and a second
drain electrode; and a capacitor. The first transistor is provided
over a substrate containing a semiconductor material. The second
transistor is includes an oxide semiconductor layer. The first gate
electrode, one of the second source electrode and the second drain
electrode, and one of electrodes of the capacitor are electrically
connected to one another. The source line and the first source
electrode are electrically connected to each other. The bit line
and the first drain electrode are electrically connected to each
other. The first signal line and the other of the second source
electrode and the second drain electrode are electrically connected
to each other. One of the second signal lines and the second gate
electrode are electrically connected to each other. One of the word
lines and the other of the electrodes of the capacitor are
electrically connected to each other.
[0016] In the above structure, the first transistor includes the
following elements: a channel formation region provided over the
substrate containing the semiconductor material, impurity regions
formed with the channel formation region sandwiched therebetween,
the first gate insulating layer over the channel formation region,
a first gate electrode over the first gate insulating layer, and
the first source electrode and the first drain electrode both of
which are electrically connected to the impurity regions.
[0017] Further, in the above structure, the second transistor
includes the following elements: the second gate electrode over the
substrate containing the semiconductor material, a second gate
insulating layer over the second gate electrode, the oxide
semiconductor layer over the second gate insulating layer, and the
second source electrode and the second drain electrode both of
which are electrically connected to the oxide semiconductor
layer.
[0018] In the above structure, the third transistor includes the
following elements: a channel formation region provided over the
substrate containing the semiconductor material, impurity regions
formed with the channel formation region sandwiched therebetween,
the third gate insulating layer over the channel formation region,
a third gate electrode over the third gate insulating layer, and
the third source electrode and the third drain electrode both of
which are electrically connected to the impurity regions
[0019] In the above structure, the substrate containing the
semiconductor material is preferably a single crystal semiconductor
substrate. In particular, the semiconductor material is preferably
silicon. Further, as a substrate containing the semiconductor
material, an SOI substrate may also be used.
[0020] Preferably, in the above structure, the oxide semiconductor
layer contains an In--Ga--Zn--O based oxide semiconductor material.
In particular, the oxide semiconductor layer preferably contains an
In.sub.2Ga.sub.2ZnO.sub.7 crystal. In addition, preferably, the
hydrogen concentration of the oxide semiconductor layer is less
than or equal to 5.times.10.sup.19 atoms/cm.sup.3. The off-state
current of the second transistor is preferably less than or equal
to 1.times.10.sup.-13 A.
[0021] In addition, in the above structure, the second transistor
can be provided in a region overlapping with the first
transistor.
[0022] Note that in this specification, "over" and "below" do not
necessarily mean "directly on" and "directly under", respectively,
in the description of a physical relationship between components.
For example, the expression of "a first gate electrode over a gate
insulating layer" may refer to the case where another component is
interposed between the gate insulating layer and the first gate
electrode. In addition, the terms "above" and "below" are just used
for convenience of explanations and they can be interchanged unless
otherwise specified.
[0023] In this specification, the term "electrode" or "wiring" does
not limit the function of components. For example, an "electrode"
can be used as a part of a "wiring", and the "wiring" can be used
as a part of the "electrode". In addition, the term "electrode" or
"wiring" can also mean a combination of a plurality of "electrodes"
and "wirings", for example.
[0024] Further, functions of a "source" and a "drain" might be
switched when transistors having different polarities are employed
or a direction of current flow is changed in circuit operation, for
example. Therefore, the terms "source" and "drain" can be switched
in this specification.
[0025] Note that in this specification, the expression of
"electrically connected" includes the case of electrical connection
through "an object having any electrical function". Here, there is
no particular limitation on "an object having any electrical
function" as long as the object enables transmission and reception
of an electrical signal between components connected by the
object.
[0026] For example, the category "an object having any electrical
function" can include a switching element such as a transistor, a
resistor, an inductor, a capacitor, and other elements having
several functions, as well as electrodes and wirings.
[0027] In general, the term "SOT substrate" means a substrate
having a silicon semiconductor layer over an insulating surface. In
this specification, the term "SOT substrate" also means a substrate
having a semiconductor layer using a material other than silicon
over an insulating surface. That is, a semiconductor layer included
in the "SOT substrate" is not limited to a silicon semiconductor
layer. Additionally, a substrate in an "SOT substrate" is not
limited to a semiconductor substrate such as a silicon wafer, and
may be a non-semiconductor substrate such as a glass substrate, a
quartz substrate, a sapphire substrate, and a metal substrate. That
is, an "SOT substrate" also includes a conductive substrate and an
insulating substrate over which a layer is formed using a
semiconductor material. In addition, in this specification, a
"semiconductor substrate" means a substrate of only a semiconductor
material and also a general substrate of a material including a
semiconductor material. In other words, in this specification, an
"SOT substrate" is also included in the broad category of a
"semiconductor substrate".
[0028] Moreover, in this specification, a semiconductor material
other than an oxide semiconductor may be any semiconductor material
as long as it is a semiconductor material other than an oxide
semiconductor material. For example, silicon, germanium, silicon
germanium, silicon carbide, gallium arsenide, or the like can be
given. Besides, an organic semiconductor material and the like can
be used. Note that in the case where a material included in a
semiconductor device and the like is not particularly specified, an
oxide semiconductor material or a semiconductor material other than
an oxide semiconductor material may be used.
[0029] One embodiment of the present invention provides, in its
lower portion, a semiconductor device including a transistor using
a material other than an oxide semiconductor in its channel
formation region, and, in its upper portion, a transistor using an
oxide semiconductor in its channel formation region.
[0030] A transistor using an oxide semiconductor has extremely low
off-state current; therefore, by using the transistor, memory
content can be stored for a relatively long time. That is, refresh
operation can become unnecessary, or frequency of refresh operation
can be reduced considerably, so that power consumption can be
reduced substantially. Further, even in the case where power is not
supplied, memory content can be stored for a long time.
[0031] In addition, high voltage is not needed for writing
information in the semiconductor device and there is no problem of
deterioration of elements. For example, since there is no need to
perform injection of electrons to a floating gate and extraction of
electrons from a floating gate which are needed in a conventional
nonvolatile memory, deterioration such as deterioration of a gate
insulating layer does not occur. That is, the semiconductor device
according to one embodiment of the present invention does not have
a limit on the number of times of writing which is a problem in a
conventional nonvolatile memory, and reliability thereof is
drastically improved. Further, information is written according to
an on state and an off state of the transistor, whereby high-speed
operation can be easily realized. Additionally, when information is
written, there is an advantage that operation for erasing the
previously stored information is not needed.
[0032] Further, the transistor formed using a material other than
an oxide semiconductor can be operated at sufficiently high speed;
therefore, by using the transistor, stored contents can be read out
at high speed.
[0033] Moreover, the semiconductor device according to one
embodiment of the present invention can easily multivalue data by
being provided with a boosting circuit, so that storage capacity
can be increased.
[0034] Accordingly, a semiconductor device having unprecedented
characteristics can be realized by being provided with a
combination of a transistor using a material other than an oxide
semiconductor material and a transistor using an oxide
semiconductor material.
BRIEF DESCRIPTION OF DRAWINGS
[0035] In the accompanying drawings:
[0036] FIG. 1 is a circuit diagram for describing a semiconductor
device;
[0037] FIG. 2A is a cross-sectional view for describing a
semiconductor device and FIG. 2B is a plan view for describing the
semiconductor device;
[0038] FIGS. 3A to 3H are cross-sectional views each for explaining
a manufacturing step of a semiconductor device;
[0039] FIGS. 4A to 4G are cross-sectional views each for explaining
a manufacturing step of a semiconductor device;
[0040] FIGS. 5A to 5D are cross-sectional views each for explaining
a manufacturing step of a semiconductor device;
[0041] FIG. 6 is a cross-sectional view of a transistor formed
using an oxide semiconductor;
[0042] FIG. 7 is an energy band diagram (schematic diagram) along
an A-A' section in FIG. 6;
[0043] FIG. 8A is a diagram illustrating a state where a positive
voltage (V.sub.G>0) is applied to a gate electrode (GE), and
FIG. 8B is a diagram illustrating a state where a negative
potential (V.sub.G<0) is applied to the gate electrode (GE);
[0044] FIG. 9 is a diagram illustrating the relationship between
the vacuum level and the electron affinity (.chi.) of an oxide
semiconductor;
[0045] FIG. 10 is a graph illustrating a C-V characteristic;
[0046] FIG. 11 is a graph illustrating a relation between V.sub.g
and (1/C).sup.2;
[0047] FIG. 12 is a cross-sectional view for describing a
semiconductor device;
[0048] FIGS. 13A and 13B are cross-sectional views for describing a
semiconductor device;
[0049] FIGS. 14A and 14B are cross-sectional views for describing a
semiconductor device;
[0050] FIGS. 15A and 15B are cross-sectional views for describing a
semiconductor device;
[0051] FIG. 16 is a circuit diagram for describing a memory
element;
[0052] FIG. 17 is a circuit diagram for describing a semiconductor
device;
[0053] FIG. 18 is a circuit diagram for describing a driver
circuit;
[0054] FIG. 19 is a circuit diagram for describing a driver
circuit;
[0055] FIG. 20 is a circuit diagram for describing a reading
circuit;
[0056] FIG. 21 is a circuit diagram for describing a potential
generating circuit;
[0057] FIGS. 22A and 22B are circuit diagrams for describing a
boosting circuit;
[0058] FIG. 23 is a circuit diagram for describing a differential
sense amplifier circuit;
[0059] FIG. 24 is a circuit diagram for describing a latch sense
amplifier;
[0060] FIGS. 25A and 25B are timing charts for explaining an
operation;
[0061] FIG. 26 is a circuit diagram for describing a semiconductor
device;
[0062] FIG. 27 is a circuit diagram for describing a reading
circuit;
[0063] FIG. 28 is a timing chart for explaining an operation;
[0064] FIG. 29 is a circuit diagram for describing a reading
circuit;
[0065] FIG. 30 is a timing chart for explaining an operation;
[0066] FIG. 31 is a circuit diagram for describing a memory
element;
[0067] FIG. 32 is a circuit diagram for describing a semiconductor
device;
[0068] FIG. 33 is a circuit diagram for describing a reading
circuit;
[0069] FIG. 34 is a circuit diagram for describing a driver
circuit;
[0070] FIG. 35 is a timing chart for explaining an operation;
[0071] FIG. 36 is a graph illustrating a relation between a
potential of a node A and a potential of a word line; and
[0072] FIGS. 37A to 37F are diagrams each describing an electronic
appliance.
BEST MODE FOR CARRYING OUT THE INVENTION
[0073] Hereinafter, examples of embodiments of the present
invention will be described with reference to the drawings. Note
that the present invention is not limited to the following
descriptions and it will be readily appreciated by those skilled in
the art that modes and details can be modified in various ways
without departing from the spirit and the scope of the present
invention. Therefore, the invention should not be interpreted as
being limited to the description of the following embodiment
modes.
[0074] Note that for the easy understanding, the position, size,
range and the like of each component illustrated in the drawings
and the like are not actual ones in some cases. Therefore, the
present invention is not limited to the position, size, and range
and the like disclosed in the drawings and the like.
[0075] Note that in this specification, ordinal numbers such as
"first", "second", and "third" are used in order to avoid confusion
among components, and the terms do not limit the components
numerically.
Embodiment 1
[0076] In this embodiment, structures and manufacturing methods of
semiconductor devices according to one embodiment of the disclosed
invention are described with reference to FIG. 1, FIGS. 2A and 2B,
FIGS. 3A to 3H, FIGS. 4A to 4G, FIGS. 5A to 5D, FIG. 6, FIGS. 7A
and 7B, FIGS. 8A and 8B, FIG. 9, FIG. 10, FIG. 11, FIG. 12, FIGS.
13A and 13B, FIGS. 14A and 14B, and FIGS. 15A and 15B.
<Circuit Configuration of Semiconductor Device>
[0077] FIG. 1 illustrates an example of a circuit configuration of
a semiconductor device. The semiconductor device includes a
transistor 160 which uses a material other than an oxide
semiconductor and a transistor 162 which uses an oxide
semiconductor. Note that a mark "OS" is added to the transistor 162
in FIG. 1 to show that the transistor 162 uses an oxide
semiconductor.
[0078] Here, a gate electrode of the transistor 160 is electrically
connected to one of a source electrode and a drain electrode of the
transistor 162. A first wiring (which is denoted as "1st Line" and
also called a source line) and a second wiring (which is denoted as
"2nd Line" and also called a bit line) are electrically connected
to a source electrode of the transistor 160 and a drain electrode
of the transistor 160, respectively. Further, a third wiring (which
is denoted as "3rd Line" and also called a first signal line) and a
fourth wiring (which is denoted as "4th Line" and also called a
second signal line) are electrically connected to the other of the
source electrode and the drain electrode of the transistor 162 and
to a gate electrode of the transistor 162, respectively.
[0079] The transistor 160 which uses a material other than an oxide
semiconductor can operate at sufficiently high speed; thus, can
read stored contents and the like at high speed. In addition,
off-state current is extremely small in the transistor 162 which
uses an oxide semiconductor. Therefore, when the transistor 162 is
turned off, a potential of the gate electrode of the transistor 160
can be held for an extremely long time.
[0080] The advantage that the potential of the gate electrode can
be held for an extremely long time enables writing, holding, and
reading of information to be performed as described below.
[0081] Description is made on writing and holding of information
first. First, a potential of the fourth wiring is set to a
potential putting the transistor 162 in an on state. Accordingly, a
potential of the third wiring is applied to the gate electrode of
the transistor 160 (writing data operation). After that, the
potential of the fourth wiring is set to a potential putting the
transistor 162 in an off state; accordingly, the potential of the
gate electrode of the transistor 160 is held (holding data
operation).
[0082] Since the off-state current of the transistor 162 is
extremely small, the potential of the gate electrode of the
transistor 160 is held for a long time. For example, when the
potential of the gate electrode of the transistor 160 is a
potential putting the transistor 160 in an on state, the on state
of the transistor 160 is kept for a long time. When the potential
of the gate electrode of the transistor 160 is a potential to make
the transistor 160 be in an off state, the off state of the
transistor 160 is kept for a long time.
[0083] Next, description is made on the reading of information
operation. When an on state or an off state of the transistor 160
is kept as described above and a predetermined potential (a low
potential) is applied to the first wiring, the value of a potential
of the second wiring varies depending on a state of the transistor
160 which is an on state or an off state. For example, when the
transistor 160 is in an on state, the potential of the second
wiring is lowered by being affected by the potential of the first
wiring. On the other hand, when the transistor 160 is in an off
state, the potential of the second wiring is not changed.
[0084] In this manner, by comparing the potential of the first
wiring with the potential of the second wiring in a state where
information is held, the information can be read.
[0085] Then, description is made on rewriting of information.
Rewriting of information is performed in a manner similar to that
of the writing and holding of information which are described
above. That is, the potential of the fourth wiring is set to a
potential putting the transistor 162 in an on state, whereby the
transistor 162 is put in an on state. Accordingly, the potential of
the third wiring (a potential relating to new information) is
applied to the gate electrode of the transistor 160. After that,
the potential of the fourth wiring is set to a potential putting
the transistor 162 in an off state, whereby the transistor 162 is
put in an off state; thus, the new information is held.
[0086] As described above, in the semiconductor device according to
one embodiment of the disclosed invention, information can be
directly rewritten by performing rewriting of information. Erasing
operation, necessary in a flash memory and the like, is thus not
needed; therefore, reduction in operation speed due to erasing
operation can be suppressed. In other words, high-speed operation
of a semiconductor device is realized.
[0087] Note that, in the above description, an n-type transistor
(an n-channel transistor) using electrons as carriers is used;
however, a p-channel transistor using holes as carriers, needless
to say, can be used instead of an n-channel transistor.
[0088] It is also needless to say that a capacitor may be added to
the gate electrode of the transistor 160 so that the potential of
the gate electrode of the transistor 160 is easily held.
<Plan Structure and Cross-Sectional Structure of a Semiconductor
Device>
[0089] An example of a structure of the above semiconductor device
is illustrated in FIGS. 2A and 2B. FIGS. 2A and 2B are a
cross-sectional view of the semiconductor device and a plan view
thereof, respectively. Here, FIG. 2A corresponds to a cross-section
taken along line A1-A2 and line B1-B2 of FIG. 2B. The semiconductor
device illustrated in FIGS. 2A and 2B includes the transistor 160
using a material other than an oxide semiconductor in a lower
portion and the transistor 162 using an oxide semiconductor in an
upper portion. Note that although n-channel transistors are
described as the transistors 160 and 162, p-channel transistors may
be employed. A p-channel transistor can be preferably used as the
transistor 160, in particular.
[0090] The transistor 160 includes: a channel formation region 116
which is provided in a substrate 100 containing a semiconductor
material; impurity regions 114 between which the channel formation
region 116 is sandwiched and high-concentration impurity regions
120 between which the channel formation region 116 is sandwiched
(impurity regions 114 and high-concentration impurity regions 120
are also collectively called impurity regions); a gate insulating
layer 108a provided over the channel formation region 116: a gate
electrode 110a provided over the gate insulating layer 108a; a
source or drain electrode 130a electrically connected to a first
impurity region 114 on one side of the channel formation region
116; and a source or drain electrode 130b electrically connected to
a second impurity regions 114 on another side of the channel
formation region 116.
[0091] Here, sidewall insulating layers 118 are provided for side
surfaces of the gate electrode 110a. Moreover, at least parts of
the side wall insulating layers 118 are comprised between the high
concentration impurity regions 120 formed in regions of the
substrate 100, when seen from above, and metal compound regions 124
are present over the high concentration impurity regions 120.
Further, an element insulation insulating layer 106 is formed over
the substrate 100 so as to surround the p-type transistor 160, and
an interlayer insulating layer 126 and an interlayer insulating
layer 128 are formed so as to cover the p-type transistor 160. The
source or drain electrode 130a is electrically connected to a first
metal compound region 124 on the one side of the channel formation
region 116, and the source or drain electrode 130b is electrically
connected to a second metal compound region 124 on the other side
of the channel formation region 116 through openings in the
interlayer insulating layer 126 and the interlayer insulating layer
128. In other words, the source or drain electrode 130a is
electrically connected to a first high concentration region 120 and
to the first impurity region 114 which are on the one side of the
channel formation region 116 through the first metal compound
region 124 on the one side of the channel formation region 116, and
the source or drain electrode 130b is electrically connected to a
second high concentration region 120 and to the second impurity
region 114 which are on the other side of the channel formation
region 116 through the second metal compound region 124 on the
other side the channel formation region 116. Further, the gate
electrode 110a is electrically connected to an electrode 130c
provided in a similar manner to the source or drain electrode 130a
and the source or drain electrode 130b.
[0092] The transistor 162 includes: a gate electrode 136d provided
over the interlayer insulating layer 128; a gate insulating layer
138 provided over the gate electrode 136d; an oxide semiconductor
layer 140 provided over the gate insulating layer 138; and a source
or drain electrode 142a and a source or drain electrode 142b which
are provided over and electrically connected to the oxide
semiconductor layer 140.
[0093] Here, the gate electrode 136d is provided so as to be
embedded in an insulating layer 132 which is formed over the
interlayer insulating layer 128. Furthermore, similarly to the gate
electrode 136d, an electrode 136a, an electrode 136b, and an
electrode 136c are formed in contact with the source or drain
electrode 130a, the source or drain electrode 130b, and the
electrode 130c, respectively.
[0094] Over the transistor 162, a protective insulating layer 144
is provided in contact with part of the oxide semiconductor layer
140. An interlayer insulating layer 146 is provided over the
protective insulating layer 144. Here, in the protective insulating
layer 144 and the interlayer insulating layer 146, openings
reaching the source or drain electrode 142a and the source or drain
electrode 142b are formed. In the openings, an electrode 150d and
an electrode 150e are formed to be in contact with the source or
drain electrode 142a and the source or drain electrode 142b,
respectively. Similarly to the electrodes 150d and 150e, an
electrode 150a, an electrode 150b, and an electrode 150c are formed
to be in contact with the electrode 136a, the electrode 136b, and
the electrode 136c, respectively, through openings provided in the
gate insulating layer 138, the protective insulating layer 144, and
the interlayer insulating layer 146.
[0095] Here, the oxide semiconductor layer 140 is preferably an
oxide semiconductor layer highly purified by removing an impurity
such as hydrogen. Specifically, hydrogen concentration in the oxide
semiconductor layer 140 is less than or equal to 5.times.10.sup.19
atoms/cm.sup.3, preferably, less than or equal to 5.times.10.sup.18
atoms/cm.sup.3, or more preferably, less than or equal to
5.times.10.sup.17 atoms/cm.sup.3. In addition, it is desirable that
the oxide semiconductor layer 140 contain sufficient oxygen so that
defects due to oxygen vacancies are reduced. In the oxide
semiconductor layer 140 which is highly purified by sufficiently
reducing the hydrogen concentration, carrier concentration is less
than or equal to 1.times.10.sup.12/cm.sup.3, preferably less than
or equal to 1.times.10.sup.11/cm.sup.3. In this manner, by using an
oxide semiconductor which is made to be an i-type (intrinsic) oxide
semiconductor or a substantially i-type oxide semiconductor, the
transistor 162 which has extremely favorable off-state current
characteristics can be obtained. For example, when a drain voltage
V.sub.d is +1 V or +10 V and a gate voltage V.sub.g ranges from -5
V to -20 V, off-state current is less than or equal to
1.times.10.sup.-13 A. When the oxide semiconductor layer 140 which
is highly purified by sufficiently reducing the hydrogen
concentration and in which the defects due to oxygen vacancies are
reduced is used and off-state current of the transistor 162 is
reduced, a semiconductor device having a new structure can be
realized. Note that the hydrogen concentration in the oxide
semiconductor layer 140 was measured by secondary ion mass
spectroscopy (SIMS).
[0096] Furthermore, an insulating layer 152 is provided over the
interlayer insulating layer 146. An electrode 154a, an electrode
154b, an electrode 154c, and an electrode 154d are provided so as
to be embedded in the insulating layer 152. Here, the electrode
154a is in contact with the electrode 150a; the electrode 154b, the
electrode 150b; the electrode 154c, the electrodes 150c and 150d;
and the electrode 154d, the electrode 150e.
[0097] That is, in the semiconductor device illustrated in FIGS. 2A
and 2B, the gate electrode 110a of the transistor 160 is
electrically connected to the source or drain electrode 142a of the
transistor 162 through the electrodes 130c, 136c, 150c, 154c, and
150d.
<Method for Manufacturing Semiconductor Device>
[0098] Next, an example of a method for manufacturing the
above-described semiconductor device will be described. First, a
method for manufacturing the transistor 160 in the lower portion
will be described with reference to FIGS. 3A to 3H and next, a
method for manufacturing the transistor 162 in the upper portion
will be described with reference to FIGS. 4A to 4G and FIGS. 5A to
5D.
<Method for Manufacturing a Transistor in a Lower
Portion>
[0099] First, the substrate 100 which contains a semiconductor
material is prepared (see FIG. 3A). As the substrate 100 which
contains a semiconductor material, a single crystal semiconductor
substrate or a polycrystalline semiconductor substrate containing
silicon, silicon carbide, or the like, a compound semiconductor
substrate containing silicon germanium or the like, an SOT
substrate, or the like can be used. Here, an example in which a
single crystal silicon substrate is used as the substrate 100
containing a semiconductor material is described. Note that in
general, the term "SOT substrate" means a substrate having a
silicon semiconductor layer over an insulating surface. In this
specification, the term "SOT substrate" also means a substrate
having a semiconductor layer using a material other than silicon
over an insulating surface. That is, a semiconductor layer included
in the "SOT substrate" is not limited to a silicon semiconductor
layer. In addition, the SOT substrate includes a substrate which
has a semiconductor layer over an insulating substrate such as a
glass substrate, with an insulating layer between the semiconductor
layer and the insulating substrate.
[0100] Over the substrate 100, a protective layer 102 which
functions as a mask for forming an element insulation insulating
layer is formed (see FIG. 3A). As the protective layer 102, for
example, an insulating layer formed using silicon oxide, silicon
nitride, silicon nitride oxide, or the like can be used. Note that
an impurity element giving n-type conductivity or an impurity
element giving p-type conductivity may be added to the substrate
100 before or after the above step to control the threshold voltage
of the transistor. As the impurity giving n-type conductivity,
phosphorus, arsenic, or the like can be used when the semiconductor
material contained in the substrate 100 is silicon. As the impurity
giving p-type conductivity, boron, aluminum, gallium, or the like
can be used, for example.
[0101] Next, part of the substrate 100 in a region which is not
covered with the protective layer 102 (an exposed region) is
removed by etching with the use of the above protective layer 102
as a mask. Thus, an insulated semiconductor region 104 is formed
(see FIG. 3B). For the etching, dry etching is preferably
performed, but wet etching can be performed. An etching gas and an
etchant can be selected as appropriate depending on a material of
the object to be etched.
[0102] Next, an insulating layer is formed so as to cover the
semiconductor region 104 and is selectively removed in a region
which overlaps with the semiconductor region 104, whereby the
element insulation insulating layer 106 is formed (see FIG. 3B).
The insulating layer is formed using silicon oxide, silicon
nitride, silicon nitride oxide, or the like. As a method for
removing the insulating layer, there are etching and polishing
treatment such as CMP, and any of them can be employed. Note that
the protective layer 102 is removed either after the semiconductor
region 104 is formed or after the element insulation insulating
layer 106 is formed.
[0103] Then, an insulating layer is formed over the semiconductor
region 104 and a layer containing a conductive material is formed
over the insulating layer.
[0104] The insulating layer serves as a gate insulating layer later
and preferably has a single-layer structure or a stacked-layer
structure film containing silicon oxide, silicon nitride oxide,
silicon nitride, hafnium oxide, aluminum oxide, tantalum oxide, or
the like obtained by using a CVD method, a sputtering method, or
the like. Alternatively, the above insulating layer may be obtained
by oxidizing or nitriding a surface of the semiconductor region 104
by high-density plasma treatment or thermal oxidation treatment.
The high-density plasma treatment can be performed using, for
example, a mixed gas of a combination of a rare gas such as He, Ar,
Kr, or Xe and oxygen, nitrogen oxide, ammonia, nitrogen, or
hydrogen. There is no particular limitation on the thickness of the
insulating layer, but the thickness can be greater than or equal to
1 nm and less than or equal to 100 nm, for example.
[0105] The layer containing a conductive material can be formed
using a metal material such as aluminum, copper, titanium,
tantalum, or tungsten. Alternatively, the layer containing a
conductive material may be formed using a semiconductor material
such as polycrystalline silicon containing a conductive material.
There is also no particular limitation on a method for forming the
layer containing a conductive material, and any of a variety of
deposition methods such as an evaporation method, a CVD method, a
sputtering method, and a spin coating method is applicable. Note
that in this embodiment, an example of the case where the layer
containing a conductive material is formed using a metal material
is described.
[0106] After that, by selectively etching the insulating layer and
the layer containing a conductive material, the gate insulating
layer 108a and the gate electrode 110a are formed (see FIG.
3C).
[0107] Next, an insulating layer 112 which covers the gate
electrode 110a is formed (see FIG. 3C). Phosphorus (P), arsenic
(As), or the like is then added to the semiconductor region 104,
whereby the impurity regions 114 with a shallow junction depth are
formed (see FIG. 3C). Note that although phosphorus or arsenic is
added here so that an n-channel transistor is formed, an impurity
element such as boron (B) or aluminum (Al) may be added in the case
of forming a p-channel transistor. Note also that the channel
formation region 116 is formed in the semiconductor region 104
under the gate insulating layer 108a by formation of the impurity
regions 114 (see FIG. 3C). Here, the concentration of the added
impurity can be set as appropriate; in the case where a
semiconductor element is highly miniaturized, the concentration is
preferably set to be high. Further, a process in which the
insulating layer 112 is formed after formation of the impurity
regions 114 may be employed instead of the process employed here in
which the impurity regions 114 are formed after formation of the
insulating layer 112.
[0108] Then, the sidewall insulating layers 118 are formed (see
FIG. 3D). An insulating layer is formed so as to cover the
insulating layer 112 and then is subjected to highly anisotropic
etching, whereby the sidewall insulating layers 118 can be formed
in a self-aligned manner. It is preferable that the insulating
layer 112 be partly etched at this time so that a top surface of
the gate electrode 110a and top surfaces of the impurity regions
114 are exposed.
[0109] After that, an insulating layer is formed so as to cover the
gate electrode 110a, the impurity regions 114, the side wall
insulating layers 118, and the like. Phosphorus (P), arsenic (As),
or the like is then added to regions which are in contact with the
impurity regions 114, whereby the high-concentration impurity
regions 120 are formed (see FIG. 3E). Next, the above insulating
layer is removed and a metal layer 122 is formed so as to cover the
gate electrode 110a, the sidewall insulating layers 118, the
high-concentration impurity regions 120, and the like (see FIG.
3E). Any of a variety of film formation methods such as a vacuum
evaporation method, a sputtering method, and a spin coating method
is applicable to formation of the metal layer 122. It is preferable
that the metal layer 122 be formed using a metal material that
reacts with a semiconductor material included in the semiconductor
region 104 so as to form a metal compound having low resistance.
Examples of such a metal material include titanium, tantalum,
tungsten, nickel, cobalt, and platinum.
[0110] Next, heat treatment is performed, whereby the metal layer
122 reacts with the semiconductor material. Accordingly, the metal
compound regions 124 which are in contact with the
high-concentration impurity regions 120 are formed (see FIG. 3F).
Note that, in the case of using polycrystalline silicon for the
gate electrode 110a, a portion of the gate electrode 110a which is
in contact with the metal layer 122 also has the metal compound
region.
[0111] As the heat treatment, irradiation with a flash lamp can be
employed. Although it is needless to say that another heat
treatment method may be used, a method by which heat treatment for
an extremely short time can be achieved is preferably used in order
to improve the controllability of chemical reaction in formation of
the metal compound. Note that the metal compound regions 124 are
formed through reaction of the metal material with the
semiconductor material and have sufficiently high conductivity. By
formation of the metal compound regions 124, electric resistance
can be sufficiently reduced and element characteristics can be
improved. The metal layer 122 is removed after formation of the
metal compound regions 124.
[0112] The interlayer insulating layers 126 and 128 are formed so
as to cover the components formed in the above steps (see FIG. 3G).
The interlayer insulating layers 126 and 128 can be formed using a
material containing an inorganic insulating material such as
silicon oxide, silicon nitride oxide, silicon nitride, hafnium
oxide, aluminum oxide, or tantalum oxide. Alternatively, an organic
insulating material such as polyimide or acrylic can be used. Note
that although the interlayer insulating layer 126 and the
interlayer insulating layer 128 have a two-layer structure here,
the structure of the interlayer insulating layers is not limited to
this. A surface of the interlayer insulating layer 128 is
preferably subjected to CMP, etching, or the like so as to be
flattened after the interlayer insulating layer 128 is formed.
[0113] After that, openings reaching the metal compound regions 124
are formed in the interlayer insulating layers, and then the source
or drain electrode 130a and the source or drain electrode 130b are
formed in the openings (see FIG. 3H). For example, the source or
drain electrode 130a and the source or drain electrode 130b can be
formed as follows: a conductive layer is formed in a region
including the openings by a PVD method, a CVD method, or the like;
and then, part of the conductive layer is removed by etching, CMP,
or the like.
[0114] Note that in the case of forming the source or drain
electrode 130a and the source or drain electrode 130b by removing
part of the conductive layer, surfaces thereof are preferably
processed to be flat. For example, in the case where a titanium
film, a titanium nitride film, or the like is formed to have a
small thickness in the region including the openings and a tungsten
film is then formed so as to be embedded in the openings, CMP which
is performed after that can remove an unnecessary portion of the
tungsten film, titanium film, titanium nitride film, or the like,
and improve the flatness of the surfaces. By flattening surfaces
including the surfaces of the source or drain electrode 130a and
the source or drain electrode 130b as described above, favorable
electrodes, wirings, insulating layers, semiconductor layers, or
the like can be formed in a subsequent step.
[0115] Note that although only the source or drain electrode 130a
and the source or drain electrode 130b which are in contact with
the metal compound regions 124 are described, an electrode which is
in contact with the gate electrode 110a (e.g., the electrode 130c
of FIG. 2A) and the like can be formed in the same step. There is
no particular limitation on a material used for the source or drain
electrode 130a and the source or drain electrode 130b and any of a
variety of conductive materials can be used. For example, a
conductive material such as molybdenum, titanium, chromium,
tantalum, tungsten, aluminum, copper, neodymium, or scandium can be
used.
[0116] Through the above process, the transistor 160 which uses the
substrate 100 containing a semiconductor material is formed. Note
that electrodes, wirings, insulating layers, or the like may be
formed as well after the above process is performed. When a
multilayer wiring structure in which an interlayer insulating layer
and a conductive layer are stacked is employed as a wiring
structure, a highly-integrated semiconductor device can be
provided.
<Method for Manufacturing a Transistor in an Upper
Portion>
[0117] Next, a process through which the transistor 162 is
manufactured over the interlayer insulating layer 128 is described
with reference to FIGS. 4A to 4G and FIGS. 5A to 5D. Note that the
transistor 160 and the like below the transistor 162 are omitted in
FIGS. 4A to 4G and FIGS. 5A to 5D, which illustrate a manufacturing
process of a variety of electrodes over the interlayer insulating
layer 128, the transistor 162, and the like.
[0118] First, the insulating layer 132 is formed over the
interlayer insulating layer 128, the source or drain electrode
130a, the source or drain electrode 130b, and the electrode 130c
(see FIG. 4A). The insulating layer 132 can be formed by a PVD
method, a CVD method, or the like. A material containing an
inorganic insulating material such as silicon oxide, silicon
nitride oxide, silicon nitride, hafnium oxide, aluminum oxide, or
tantalum oxide can be used for the insulating layer 132.
[0119] Next, openings reaching the source or drain electrode 130a,
the source or drain electrode 130b, and the electrode 130c are
formed in the insulating layer 132. At this time, another opening
is formed in a region where the gate electrode 136d is to be
formed. A conductive layer 134 is formed so as to be embedded in
the openings (see FIG. 4B). The above openings can be formed by
etching with the use of a mask, for example. The mask can be formed
by exposure using a photomask, for example. For the etching, either
wet etching or dry etching may be performed but dry etching is
preferable in view of the fine patterning. The conductive layer 134
can be formed by a film formation method such as a PVD method or a
CVD method. Examples of a material for the conductive layer 134
include a conductive material such as molybdenum, titanium,
chromium, tantalum, tungsten, aluminum, copper, neodymium, and
scandium, an alloy of any of these, and a compound containing any
of these (e.g., nitride of any of these).
[0120] Specifically, for example, the conductive layer 134 can be
formed as follows: a titanium film is formed to have a small
thickness by a PVD method in a region including the openings and a
titanium nitride film is then formed to have a small thickness by a
CVD method; and then, a tungsten film is formed so as to fill the
openings. Here, the titanium film formed by a PVD method has a
function of reducing formation of an oxide film at an interface
with an electrode in a lower portion (here, the source or drain
electrode 130a, the source or drain electrode 130b, the electrode
130c, or the like) so that contact resistance with the lower
electrode is reduced. In addition, the subsequently formed titanium
nitride film has a bather property such that diffusion of a
conductive material is prevented. Further, after a barrier film is
formed using titanium, titanium nitride, or the like, a copper film
may be formed by a plating method.
[0121] After the conductive layer 134 is formed, part of the
conductive layer 134 is removed by etching, CMP, or the like so
that the insulating layer 132 is exposed and the electrodes 136a,
136b, and 136c, and the gate electrode 136d are formed (see FIG.
4C). Note that when the electrodes 136a, 136b, and 136c, and the
gate electrode 136d are formed by removing part of the above
conductive layer 134, the process is preferably performed so that
flattened surfaces are obtained. By flattening surfaces of the
insulating layer 132, the electrodes 136a, 136b, and 136c, and the
gate electrode 136d, favorable electrodes, wirings, insulating
layers, semiconductor layers, and the like can be formed in a
subsequent step.
[0122] After that, the gate insulating layer 138 is formed so as to
cover the insulating layer 132, the electrodes 136a, 136b, and
136c, and the gate electrode 136d (see FIG. 4D). The gate
insulating layer 138 can be formed by a sputtering method, a CVD
method, or the like. The gate insulating layer 138 preferably
contains silicon oxide, silicon nitride, silicon oxynitride,
silicon nitride oxide, aluminum oxide, hafnium oxide, tantalum
oxide, or the like. Note that the gate insulating layer 138 may
have a single-layer structure or a stacked-layer structure. For
example, the gate insulating layer 138 of silicon oxynitride can be
formed by a plasma CVD method using silane (SiH.sub.4), oxygen, and
nitrogen as source gases. There is no particular limitation on the
thickness of the gate insulating layer 138, but the thickness can
be greater than or equal to 10 nm and less than or equal to 500 nm,
for example. When a stacked-layer structure is employed, the gate
insulating layer 138 is preferably formed by stacking a first gate
insulating layer with a thickness greater than or equal to 50 nm
and less than or equal to 200 nm and a second gate insulating layer
with a thickness greater than or equal to 5 nm and less than or
equal to 300 nm over the first gate insulating layer.
[0123] Note that an oxide semiconductor which is made to be an
i-type oxide semiconductor or a substantially i-type oxide
semiconductor by removing an impurity (an oxide semiconductor which
is highly purified) is extremely sensitive to an interface energy
levels or to the electric charges trapping at the interface;
therefore, when such an oxide semiconductor is used for an oxide
semiconductor layer, an interface between the oxide semiconductor
layer and a gate insulating layer is important. In other words, the
gate insulating layer 138 which is to be in contact with the highly
purified oxide semiconductor layer needs to be of high quality.
[0124] For example, a high-density plasma CVD method using
microwave (2.45 GHz) is favorable because a dense and high-quality
gate insulating layer 138 having high withstand voltage can be
formed thereby. In this manner, the density of energy levels at the
interface can be reduced and interface characteristics can be
favorable when the highly purified oxide semiconductor layer and
the high quality gate insulating layer are in contact with each
other.
[0125] Needless to say, even when such a highly purified oxide
semiconductor layer is used, another method such as a sputtering
method or a plasma CVD method can be employed as long as an
insulating layer having good quality can be formed as the gate
insulating layer. Alternatively, an insulating layer whose film
quality and interface characteristics are modified by heat
treatment after being formed may be applied. In any case, a layer
is acceptable which is of good quality as the gate insulating layer
138, and which reduces interface state density between the gate
insulating layer and the oxide semiconductor layer so that a good
interface is formed.
[0126] Moreover, when an impurity is contained in an oxide
semiconductor, in the bias temperature test (the BT test) at a
temperature of 85.degree. C. for 12 hours with electric field
strength of 2.times.10.sup.6 V/cm, bond between impurities and the
main components of the oxide semiconductor are cut by a strong
electric field (B: bias) and a high temperature (T: temperature),
and generated dangling bonds lead to a drift in the threshold
voltage (V.sub.th).
[0127] On the other hand, a transistor which is stable even in the
BT test can be provided by removing impurities in the oxide
semiconductor, especially hydrogen or water, and realizing good
interface characteristics between the gate insulating layer and the
oxide semiconductor layer as described above.
[0128] Then, an oxide semiconductor layer is formed over the gate
insulating layer 138 and processed by a method such as etching
using a mask so that the oxide semiconductor layer 140 having an
island-shape is formed (see FIG. 4E).
[0129] As the oxide semiconductor layer, an oxide semiconductor
layer formed using any of the following materials can be applied:
four-component metal oxides such as In--Sn--Ga--Zn--O;
three-component metal oxides such as In--Ga--Zn--O, In--Sn--Zn--O,
In--Al--Zn--O, Sn--Ga--Zn--O, Al--Ga--Zn--O, and Sn--Al--Zn--O;
two-component metal oxides such as In--Zn--O, Sn--Zn--O, Al--Zn--O,
Zn--Mg--O, Sn--Mg--O, and In--Mg--O; single-component metal oxides
such as In--O, Sn--O, and Zn--O. In addition, the above oxide
semiconductor materials may contain SiO.sub.2.
[0130] As the oxide semiconductor layer, a thin film represented by
InMO.sub.3 (ZnO).sub.m (m>0) can be used. Here, M represents one
or more metal elements selected from Ga, Al, Mn, and Co. For
example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.
An oxide semiconductor film which represented by InMO.sub.3
(ZnO).sub.m (m>0), which includes Ga as M, is referred to as an
In--Ga--Zn--O-based oxide semiconductor, and a thin film of the
In--Ga--Zn--O-based oxide semiconductor is referred to as an
In--Ga--Zn--O-based oxide semiconductor film (an
In--Ga--Zn--O-based amorphous film).
[0131] In this embodiment, as the oxide semiconductor layer, an
amorphous oxide semiconductor layer is formed by a sputtering
method with the use of an In--Ga--Zn--O-based oxide semiconductor
target for film formation. Note that by adding silicon to the
amorphous oxide semiconductor layer, crystallization can be
suppressed; therefore, the oxide semiconductor layer may be formed
using a target which contains SiO.sub.2 at greater than or equal to
2 wt. % and less than or equal to 10 wt. %.
[0132] As a target for forming the oxide semiconductor layer by a
sputtering method, a metal oxide containing zinc oxide as a main
component can be used, for example. As the oxide semiconductor
target for film formation containing In, Ga, and Zn, a target
having a composition ratio of
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:1 (molar ratio) or the like
can also be used. Alternatively, as the oxide semiconductor target
for film formation containing In, Ga, and Zn, a target having a
composition ratio of In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:2
(molar ratio) or a target having a composition ratio of
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:4 (molar ratio) can be
used. The filling rate of the oxide semiconductor target for film
formation is greater than or equal to 90% and less than or equal to
100%, preferably greater than or equal to 95% (e.g., 99.9%). By
using an oxide semiconductor target for film formation whose
filling rate is high, a dense oxide semiconductor layer is
formed.
[0133] An atmosphere for formation of the oxide semiconductor layer
is preferably a rare gas (typically argon) atmosphere, an oxygen
atmosphere, or a mixed atmosphere of a rare gas (typically argon)
and oxygen. Specifically, a high-purity gas, in which the
concentration of impurities such as hydrogen, water, hydroxyl, and
hydride is reduced to a concentration of approximately several
parts per million (and even better, to several parts per billion),
is preferable.
[0134] At the time of forming the oxide semiconductor layer, the
substrate is placed in a treatment chamber which is kept in a
reduced-pressure state and the substrate temperature is higher than
or equal to 100.degree. C. and lower than or equal to 600.degree.
C., preferably, higher than or equal to 200.degree. C. and lower
than or equal to 400.degree. C. By forming the oxide semiconductor
layer while the substrate is heated, the concentration of
impurities contained in the oxide semiconductor layer can be
reduced. In addition, damage due to sputtering is reduced. While
moisture remaining in the treatment chamber is removed, a
sputtering gas from which hydrogen and moisture are removed is
introduced, and the oxide semiconductor layer is formed with use of
metal oxide as a target. In order to remove remaining moisture in
the treatment chamber, a sorption vacuum pump is preferably used. A
cryopump, an ion pump, or a titanium sublimation pump can be used.
The evacuation unit may be a turbo pump provided with a cold trap.
A hydrogen atom, a compound containing a hydrogen atom, such as
water (H.sub.2O) (and also preferably a compound containing a
carbon atom), or the like is removed from the deposition chamber
while reduced pressure is maintained with the cryopump, thereby
reducing the concentration of impurities contained in the oxide
semiconductor layer formed in the deposition chamber.
[0135] For example, the film formation conditions can be set as
follows: the distance between a substrate and a target is 100 mm;
the pressure is 0.6 Pa; the direct-current (DC) power is 0.5 kW;
and the atmosphere is an oxygen atmosphere (the proportion of the
oxygen flow rate is 100%). It is preferable that a pulsed
direct-current (DC) power supply be used because powder substances
(also referred to as particles or dust) can be reduced and the film
thickness can be uniform. The thickness of the oxide semiconductor
layer is greater than or equal to 2 nm and less than or equal to
200 nm, preferably greater than or equal to 5 nm and less than or
equal to 30 nm. Note that an appropriate thickness depends on an
applied oxide semiconductor material, and the thickness of the
oxide semiconductor layer may be set as appropriate depending on
the material.
[0136] Note that before the oxide semiconductor layer is formed by
a sputtering method, dust attached to a surface of the gate
insulating layer 138 is preferably removed by reverse sputtering in
which an argon gas is introduced and plasma is generated. Here, the
reverse sputtering means a method for improving the quality of a
surface of the object to be processed by ions striking on the
surface, while general sputtering is achieved by ions striking on a
sputtering target. Methods for making ions strike the surface of
the object to be processed include a method in which a high
frequency voltage is applied on the surface in an argon atmosphere
and plasma is generated in the vicinity of the substrate. Note that
a nitrogen atmosphere, a helium atmosphere, an oxygen atmosphere,
or the like may be used instead of the argon atmosphere.
[0137] For the etching of the oxide semiconductor layer, either dry
etching or wet etching may be used. Needless to say, a combination
of dry etching and wet etching may be employed. The etching
conditions (an etching gas, etching solution, etching time,
temperature, or the like) are set as appropriate, depending on the
material so that the oxide semiconductor layer can be etched into a
desired shape.
[0138] Examples of the etching gas for dry etching are a gas
containing chlorine (a chlorine-based gas such as chlorine
(Cl.sub.2), boron trichloride (BCl.sub.3), silicon tetrachloride
(SiCl.sub.4), or carbon tetrachloride (CCl.sub.4)) and the like.
Alternatively, a gas containing fluorine (a fluorine-based gas such
as carbon tetrafluoride (CF.sub.4), sulfur hexafluoride (SF.sub.6),
nitrogen trifluoride (NF.sub.3), or trifluoromethane (CHF.sub.3));
hydrogen bromide (HBr); oxygen (O.sub.2); any of these gases to
which a rare gas such as helium (He) or argon (Ar) is added; or the
like may be used.
[0139] As a dry etching method, a parallel plate reactive ion
etching (RIE) method or an inductively coupled plasma (ICP) etching
method can be used. In order to etch the layer into a desired
shape, the etching conditions (the amount of electric power applied
to a coil-shaped electrode, the amount of electric power applied to
an electrode on a substrate side, the temperature of the electrode
on the substrate side, or the like) are set as appropriate.
[0140] As an etchant used for wet etching, a mixed solution of
phosphoric acid, acetic acid, and nitric acid, an ammonia hydrogen
peroxide solution (31 wt % hydrogen peroxide in water: 28 wt %
ammonia water: water=5:2:2), or the like can be used.
Alternatively, ITO07N (manufactured by Kanto Chemical Co., Inc.) or
the like may be used.
[0141] Then, the oxide semiconductor layer is preferably subjected
to first heat treatment. By this first heat treatment, the oxide
semiconductor layer can be dehydrated or dehydrogenated. The first
heat treatment is performed at a temperature higher than or equal
to 300.degree. C. and lower than or equal to 750.degree. C.,
preferably, higher than or equal to 400.degree. C. and lower than
the strain point of the substrate. For example, the substrate is
introduced into an electric furnace using a resistance heating
element or the like and the oxide semiconductor layer 140 is
subjected to heat treatment in a nitrogen atmosphere at a
temperature of 450.degree. C. for one hour. During this time, the
oxide semiconductor layer 140 is prevented from being exposed to
the air so that entry of water or hydrogen is prevented.
[0142] Note that a heat treatment apparatus is not limited to an
electrical furnace, and may include a device for heating an object
to be processed by heat conduction or heat radiation given by a
medium such as a heated gas or the like. For example, a rapid
thermal anneal (RTA) apparatus such as a lamp rapid thermal anneal
(LRTA) apparatus or a gas rapid thermal anneal (GRTA) apparatus can
be used. An LRTA apparatus is an apparatus for heating an object to
be processed by radiation of light (an electromagnetic wave)
emitted from a lamp such as a halogen lamp, a metal halide lamp, a
xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or
a high pressure mercury lamp. A GRTA apparatus is an apparatus for
heat treatment using a high-temperature gas. As the gas, an inert
gas which does not react with an object to be processed by heat
treatment, such as nitrogen or a rare gas such as argon is
used.
[0143] For example, as the first heat treatment, GRTA may be
performed as follows. The substrate is placed in an inert gas which
has been heated to a high temperature of 650.degree. C. to
700.degree. C., heated for several minutes, and taken out from the
inert gas. GRTA enables high-temperature heat treatment for a short
time. In addition, such heat treatment is applicable even when a
temperature exceeds the strain point of the substrate because it
takes only a short time.
[0144] Note that the first heat treatment is preferably performed
in an atmosphere which contains nitrogen or a rare gas (e.g.,
helium, neon, or argon) as its main component and does not contain
water, hydrogen, or the like. For example, the purity of nitrogen
or a rare gas (e.g., helium, neon, or argon) introduced into the
heat treatment apparatus is greater than or equal to 6 N
(99.9999%), preferably greater than or equal to 7 N (99.99999%)
(that is, the concentration of impurities is less than or equal to
1 ppm, preferably less than or equal to 0.1 ppm).
[0145] In some cases, the oxide semiconductor layer might be
crystallized to be a microcrystalline layer or a polycrystalline
layer depending on the condition of the first heat treatment or the
material of the oxide semiconductor layer. For example, the oxide
semiconductor layer may be crystallized to become a
microcrystalline oxide semiconductor layer having a degree of
crystallization of greater than or equal to 90%, or greater than or
equal to 80%. Further, depending on the condition of the first heat
treatment or the material of the oxide semiconductor layer, the
oxide semiconductor layer may become an amorphous oxide
semiconductor layer containing no crystalline component.
[0146] The oxide semiconductor layer might become an oxide
semiconductor layer in which a crystal (with a grain diameter
greater than or equal to 1 nm and less than or equal to 20 nm,
typically greater than or equal to 2 nm and less than or equal to 4
nm) is mixed in an amorphous oxide semiconductor (e.g. at a surface
of the oxide semiconductor layer).
[0147] In addition, electric characteristics of the oxide
semiconductor layer can be changed by providing a crystal layer
over the amorphous surface of the oxide semiconductor layer. For
example, in the case of forming the oxide semiconductor layer with
the use of an In--Ga--Zn--O-based oxide semiconductor target for
the film formation, the electric characteristics of the oxide
semiconductor layer can be changed by forming a crystal portion
represented by In.sub.2Ga.sub.2ZnO.sub.7 in which a crystal grains
are aligned and which exhibits electrical anisotropy.
[0148] More specifically, for example, by aligning the crystal
grain in such a manner that the c-axis of In.sub.2Ga.sub.2ZnO.sub.7
is oriented in a direction perpendicular to a surface of the oxide
semiconductor layer, conductivity in a direction parallel to the
surface of the oxide semiconductor is improved, and an insulating
property in the direction perpendicular to the surface of the oxide
semiconductor layer can be increased. Further, such a crystal
portion has a function of suppressing entry of an impurity such as
water or hydrogen into the oxide semiconductor layer.
[0149] Note that the above oxide semiconductor layer which includes
the crystal portion can be formed by heating a surface of the oxide
semiconductor layer by GRTA. When a sputtering target in which the
amount of Zn is smaller than that of In or Ga is used, more
favorable formation can be achieved.
[0150] The first heat treatment performed on the oxide
semiconductor layer 140 can be performed on the oxide semiconductor
layer which has not yet been processed into the island-shaped
layer. In that case, after the first heat treatment, the substrate
is taken out of the heating apparatus and a photolithography step
is performed.
[0151] Note that the above heat treatment can dehydrate or
dehydrogenate the oxide semiconductor layer 140 and thus can be
called dehydration treatment or dehydrogenation treatment. It is
possible to perform such dehydration treatment or dehydrogenation
treatment at any timing, for example, after the oxide semiconductor
layer is formed, after the source or drain electrodes are stacked
over the oxide semiconductor layer 140, or after a protective
insulating layer is formed over the source or drain electrodes.
Such dehydration treatment or dehydrogenation treatment may be
performed more than once.
[0152] Next, the source or drain electrode 142a and the source or
drain electrode 142b are formed in contact with the oxide
semiconductor layer 140 (see FIG. 4F). The source or drain
electrode 142a and the source or drain electrode 142b can be formed
in such a manner that a conductive layer is formed so as to cover
the oxide semiconductor layer 140 and then selectively etched.
[0153] The conductive layer can be formed by a PVD method such as a
sputtering method, a CVD method such as a plasma CVD method. As a
material of the conductive layer, an element selected from
aluminum, chromium, copper, tantalum, titanium, molybdenum, and
tungsten, an alloy containing any of the above elements as its
component, or the like can be used. Further, a material containing
one or more elements selected from manganese, magnesium, zirconium,
beryllium, and thorium as a component may be used. A material in
which aluminum and one or more elements selected from titanium,
tantalum, tungsten, molybdenum, chromium, neodymium, and scandium
are combined is also applicable for the material of the conductive
layer.
[0154] Alternatively, the conductive layer may be formed using
conductive metal oxide. As conductive metal oxide, indium oxide
(In.sub.2O.sub.3), tin oxide (SnO.sub.2), zinc oxide (ZnO), indium
oxide-tin oxide alloy (In.sub.2O.sub.3--SnO.sub.2, which is
abbreviated to ITO in some cases), indium oxide-zinc oxide alloy
(In.sub.2O.sub.3--ZnO), or any of these metal oxide materials in
which silicon or silicon oxide is contained can be used.
[0155] The conductive layer may have either a single-layer
structure or a stacked-layer structure of two or more layers. For
example, a single-layer structure of an aluminum film containing
silicon, a two-layer structure of an aluminum film and a titanium
film stacked thereover, a three-layer structure in which a titanium
film, an aluminum film, and a titanium film are stacked in this
order, and the like can be given.
[0156] Here, ultraviolet rays, a KrF laser beam, or an ArF laser
beam are preferably used for exposure for forming an etching
mask.
[0157] The channel length (L) of the transistor is determined by a
distance between a lower edge portion of the source or drain
electrode 142a over the oxide semiconductor layer 140 and a lower
edge portion of the source or drain electrode 142b over the oxide
semiconductor layer 140, as shown in FIG. 4E. In the case where
exposure for a pattern in which the channel length (L) is less than
25 nm, exposure for making a mask is performed in the extreme
ultraviolet range of several nanometers to several tens of
nanometers which is extremely short wavelength. In the exposure
using extreme ultraviolet light, the resolution is high and the
focus depth is large. Therefore, the channel length (L) of the
transistor to be formed later can be greater than or equal to 10 nm
and less than or equal to 1000 nm, whereby operation speed of a
circuit can be increased. Further, off-state current of the
transistor is extremely small, which prevents increase in power
consumption.
[0158] Materials of the layers and etching conditions are adjusted
as appropriate so that the oxide semiconductor layer 140 is not
removed when the conductive layer is etched. Note that in some
cases, the oxide semiconductor layer 140 is partly etched in this
step so as to be an oxide semiconductor layer having a groove (a
depressed portion) depending on the materials and etching
conditions.
[0159] An oxide conductive layer may be formed between the oxide
semiconductor layer 140 and the source or drain electrode 142a, and
between the oxide semiconductor layer 140 and the source or drain
electrode 142b. An oxide conductive layer and the conductive layer
for forming the source or drain electrode 142a and the source or
drain electrode 142b can be formed in immediate succession
(successive deposition). The oxide conductive layer can function as
a source region or a drain region. By providing such an oxide
conductive layer, resistance of the source and drain regions can be
reduced and high-speed operation of the transistor can be
realized.
[0160] In order to reduce the number of the masks and steps,
etching may be performed with the use of a resist mask formed using
a multi-tone mask which is a light-exposure mask through which
light is transmitted so as to have a plurality of intensities. A
resist mask formed with the use of a multi-tone mask has a shape
with a plurality of thicknesses (a step-like shape) and further can
be changed in shape by ashing; therefore, the resist mask can be
used in a plurality of etching steps for processing into different
patterns. That is, a resist mask corresponding to at least two or
more kinds of different patterns can be formed by one multi-tone
mask. Thus, the number of light-exposure masks can be reduced and
the number of corresponding photolithography steps can be also
reduced, whereby simplification of a process can be realized.
[0161] Note that plasma treatment using a gas such as N.sub.2O,
N.sub.2, or Ar is preferably performed after the above step. By
this plasma treatment, water attached to a surface of the oxide
semiconductor layer which is exposed is removed. Alternatively,
plasma treatment may be performed using a gas containing oxygen
such as a mixed gas of oxygen and argon. In this manner, the oxide
semiconductor layer is supplied with oxygen and defects resulted
from oxygen deficiency can be reduced.
[0162] After that, the protective insulating layer 144 which is in
contact with part of the oxide semiconductor layer 140 is formed
without exposure to the air (see FIG. 4G).
[0163] The protective insulating layer 144 can be formed by
appropriately employing a method such as a sputtering method, by
which an impurity such as water or hydrogen is prevented from
entering the protective insulating layer 144. The protective
insulating layer 144 is formed to a thickness greater than or equal
to 1 nm. As a material which can be used for the protective
insulating layer 144, there are silicon oxide, silicon nitride,
silicon oxynitride, silicon nitride oxide, and the like. The
protective insulating layer 144 may have a single-layer structure
or a stacked structure. The substrate temperature for formation of
the protective insulating layer 144 is preferably higher than or
equal to room temperature and lower than or equal to 300.degree. C.
The atmosphere for formation of the protective insulating layer 144
is preferably a rare gas (typically argon) atmosphere, an oxygen
atmosphere, or a mixed atmosphere of a rare gas (typically argon)
and oxygen.
[0164] Presence of hydrogen in the protective insulating layer 144
causes entry of the hydrogen to the oxide semiconductor layer,
extraction of oxygen in the oxide semiconductor layer by the
hydrogen, or the like, and the resistance of the backchannel side
of the oxide semiconductor layer is made low, which may form a
parasitic channel. Therefore, it is important that a formation
method in which hydrogen is not used is employed so that the
protective insulating layer 144 contains as little hydrogen as
possible.
[0165] In addition, it is preferable that the protective insulating
layer 144 be formed while remaining moisture in the treatment
chamber is removed. This is for preventing hydrogen, hydroxyl, or
water from being contained in the oxide semiconductor layer 140 and
the protective insulating layer 144.
[0166] In order to remove remaining moisture in the treatment
chamber, a sorption vacuum pump is preferably used. A cryopump, an
ion pump, or a titanium sublimation pump is preferably used. The
evacuation unit may be a turbo pump provided with a cold trap. A
hydrogen atom, a compound containing a hydrogen atom, such as water
(H.sub.2O), or the like is removed from the deposition chamber
which is evacuated with the cryopump, thereby reducing the
concentration of impurities contained in the protective insulating
layer 144 formed in the deposition chamber.
[0167] As a sputtering gas used in formation of the protective
insulating layer 144, a high-purity gas from which an impurity such
as hydrogen, water, hydroxyl, or hydride is reduced to a
concentration of approximately several parts per million
(preferably several parts per billion) is preferably used.
[0168] Next, second heat treatment (preferably at a temperature
higher than or equal to 200.degree. C. and lower than or equal to
400.degree. C., for example, higher than or equal to 250.degree. C.
and lower than or equal to 350.degree. C.) in an inert gas
atmosphere or an oxygen atmosphere is preferably performed. For
example, the second heat treatment is performed in a nitrogen
atmosphere at 250.degree. C. for one hour. The second heat
treatment can reduce variation in the electric characteristics of
the thin film transistor. Further, the oxide semiconductor layer
can be supplied with oxygen by the second heat treatment.
[0169] Further, an additional heat treatment may be performed at a
temperature higher than or equal to 100.degree. C. and lower than
or equal to 200.degree. C. for one hour or more and 30 hours or
less in the air. This heat treatment may be performed at a fixed
temperature. Alternatively, the following temperature cycle may be
applied plural times repeatedly: the temperature is increased from
room temperature to a temperature higher than or equal to
100.degree. C. and lower than or equal to 200.degree. C. and then
decreased to room temperature. Further, this heat treatment may be
performed under a reduced pressure before formation of the
protective insulating layer. The reduced pressure allows the heat
treatment time to be short. Note that this heat treatment may be
performed instead of the second heat treatment; alternatively, this
heat treatment may be performed before and/or after the second heat
treatment is performed.
[0170] Then, the interlayer insulating layer 146 is formed over the
protective insulating layer 144 (see FIG. 5A). The interlayer
insulating layer 146 can be formed by a PVD method, a CVD method,
or the like. A material containing an inorganic insulating material
such as silicon oxide, silicon nitride oxide, silicon nitride,
hafnium oxide, aluminum oxide, or tantalum oxide can be used for
the interlayer insulating layer 146. Further, a surface of the
interlayer insulating layer 146 is preferably subjected to CMP,
etching, or the like so as to be flattened after the interlayer
insulating layer 146 is formed.
[0171] Next, openings reaching the electrodes 136a, 136b, and 136c,
the source or drain electrode 142a, and the source or drain
electrode 142b are formed in the interlayer insulating layer 146,
the protective insulating layer 144, and the gate insulating layer
138; then, a conductive layer 148 is formed so as to fill the
openings (see FIG. 5B). The above openings can be formed by etching
with the use of a mask, for example. The mask can be formed by
exposure using a photomask, for example. For the etching, either
wet etching or dry etching may be performed but dry etching is
preferable in view of the fine patterning. The conductive layer 148
can be formed by a film formation method such as a PVD method or a
CVD method. Examples of a material for the conductive layer 148
include a conductive material such as molybdenum, titanium,
chromium, tantalum, tungsten, aluminum, copper, neodymium, and
scandium, an alloy of any of these, and a compound containing any
of these (e.g., nitride of any of these).
[0172] Specifically, for example, the conductive layer 148 can be
formed as follows: a titanium film is formed to have a small
thickness by a PVD method in a region including the openings and a
titanium nitride film is then formed to have a small thickness by a
CVD method; and then, a tungsten film is formed so as to fill the
openings. Here, the titanium film formed by a PVD method has a
function of reducing an oxide film at an interface with an
electrode in a lower portion (here, the electrodes 136a, 136b, and
136c, the source or drain electrode 142a, the source or drain
electrode 142b, or the like), so that contact resistance with the
electrode in a lower portion is reduced. In addition, the
subsequently formed titanium nitride film has a barrier property
such that diffusion of a conductive material is prevented. Further,
after a barrier film is formed using titanium, titanium nitride, or
the like, a copper film may be formed by a plating method.
[0173] After the conductive layer 148 is formed, part of the
conductive layer 148 is removed by etching, CMP, or the like so
that the interlayer insulating layer 146 is exposed and the
electrodes 150a, 150b, 150c, 150d and 150e are formed (see FIG.
5C). Note that when the electrodes 150a, 150b, 150c, 150d and 150e
are formed by removing part of the above conductive layer 148,
processing is preferably performed to obtain flattened surfaces. By
flattening surfaces of the interlayer insulating layer 146 and the
electrodes 150a, 150b, 150c, 150d and 150e, electrodes, wirings,
insulating layers, semiconductor layers, and the like can be
favorably formed in a later step.
[0174] Further, the insulating layer 152 is formed and openings
reaching the electrodes 150a, 150b, 150c, 150d and 150e are formed
in the insulating layer 152; then, a conductive layer is formed so
as to fill the openings. After that, part of the conductive layer
is removed by etching, CMP, or the like so that the insulating
layer 152 is exposed and the electrodes 154a, 154b, 154c, and 154d
are formed (see FIG. 5D). This step is similar to that of forming
the electrode 150a and the like; therefore, detailed description is
omitted here.
[0175] When the transistor 162 is manufactured in the
above-described manner, the hydrogen concentration of the oxide
semiconductor layer 140 is less than or equal to
5.times.10.sup.19/cm.sup.3 and off-state current of the transistor
162 is less than or equal to 1.times.10.sup.-13 A. Thus, the
transistor 162 having excellent characteristics can be obtained by
employing the highly purified oxide semiconductor layer 140 in
which the hydrogen concentration is sufficiently reduced and
defects resulted from oxygen deficiency are reduced. In addition, a
semiconductor device having excellent characteristics which
includes the transistor 160 using a material other than an oxide
semiconductor in a lower portion and the transistor 162 using an
oxide semiconductor in an upper portion can be manufactured.
[0176] Note that although many researches on a physical property of
density of state (DOS) or the like in an oxide semiconductor have
been conducted, they do not suggest an idea of substantially
reducing density of localized energy levels in an energy gap. In
one embodiment of the disclosed invention, water or hydrogen which
induces localized energy levels is removed from an oxide
semiconductor, whereby a highly purified oxide semiconductor is
manufactured. This is based on an idea of substantially reducing
localized energy levels and realizing manufacture of excellent
industrial products.
[0177] Note that when hydrogen, water, or the like is removed,
oxygen is also removed in some cases. Therefore, it is favorable
that an oxide semiconductor be further purified (made to be an
i-type oxide semiconductor) by supplying oxygen to dangling bonds
of metal which are generated by oxygen deficiency so that localized
energy levels resulting from oxygen deficiency are reduced. For
example, localized states resulted from oxygen deficiency can be
reduced in the following manner: an oxide film having oxygen in
excess is formed in close contact with a channel formation region;
and heat treatment at 200.degree. C. to 400.degree. C., typically,
approximately 250.degree. C., is performed so that oxygen is
supplied to the oxide semiconductor from the oxide film. Further,
an inert gas may be changed to a gas containing oxygen during the
second heat treatment. An oxide semiconductor can be supplied with
oxygen by being subjected to a temperature decreasing step in an
oxygen atmosphere or an atmosphere from which hydrogen, water, or
the like is sufficiently reduced, after the second heat
treatment.
[0178] It can be considered that a factor of deterioration of
characteristics of an oxide semiconductor is a shallow level
resulted from excessive hydrogen at 0.1 eV to 0.2 eV below the
conduction band, a deep level resulted from oxygen deficiency, or
the like. A technical idea of thoroughly removing hydrogen and
sufficiently supplying oxygen in order to correct these defects
ought to be valid.
[0179] In the disclosed invention, since an oxide semiconductor is
highly purified, carrier concentration of the oxide semiconductor
is sufficiently low.
[0180] With the use of Fermi-Dirac distribution function at normal
temperature, intrinsic carrier density of an oxide semiconductor
having an energy gap at 3.05 eV to 3.15 eV is
1.times.10.sup.-7/cm.sup.3, which is much lower than intrinsic
carrier density of 1.45.times.10.sup.10/cm.sup.3 of silicon.
[0181] Accordingly, the number of holes that are minority carriers
is extremely small. Leakage current of an insulated gate field
effect transistor (IGFET) in an off-state with less than or equal
to 100 aA/.mu.m at normal temperature, preferably, less than or
equal to 10 aA/.mu.m, or more preferably less than or equal to 1
aA/.mu.m can be realized. Note that "1 aA/.mu.m" means that flowing
current per micrometer of channel width of a transistor is 1 aA
(1.times.10.sup.-18 A).
[0182] In fact, SiC (3.26 eV), GaN (3.42 eV), and the like are
known as semiconductors having a wide energy gap greater than or
equal to 3 eV. Transistor characteristics similar to the above
described characteristics are expected to be obtained with the use
of these semiconductors. However, it is practically impossible to
form a thin film of these semiconductor materials because they need
a process temperature higher than or equal to 1500.degree. C. In
addition, the process temperature is so high that these materials
cannot be stacked in three dimensions over a silicon integrated
circuit. On the other hand, an oxide semiconductor can be deposited
as a thin film by sputtering at room temperature to 400.degree. C.
and can be dehydrated or dehydrogenated (removal of hydrogen or
water from an oxide semiconductor layer) and supplied with oxygen
(supply of oxygen to an oxide semiconductor layer) at 450.degree.
C. to 700.degree. C.; thus, an oxide semiconductor can be stacked
in three dimensions over a silicon integrated circuit.
[0183] Note that although an oxide semiconductor generally has
n-type conductivity, in one embodiment of the disclosed invention,
an oxide semiconductor is made to be an i-type oxide semiconductor
by removing an impurity such as water or hydrogen and supplying
oxygen that is a component of the oxide semiconductor. From this
aspect, different from the case of silicon which is made to be an
i-type silicon by adding an impurity, one embodiment of the
disclosed invention includes a novel technical idea.
[0184] Note that in this embodiment, the case where the transistor
162 using the oxide semiconductor is a bottom gate type is
described; however, the present invention is not limited thereto.
The structure of the transistor 162 may be a top gate type or a
dual gate type. A dual gate type transistor refers to a transistor
including two gate electrodes formed above and below a channel
region with a gate insulating layer interposed therebetween.
<Electrical Conduction Mechanism of Transistor Using Oxide
Semiconductor>
[0185] Here, the electrical conduction mechanism of the transistor
using an oxide semiconductor is described with reference to FIG. 6,
FIG. 7, FIGS. 8A and 8B, and FIG. 9. Note that the description
below is only consideration and the effect of one embodiment of the
disclosed invention is not denied thereby.
[0186] FIG. 6 is a cross-sectional view illustrating a dual gate
type transistor (a thin film transistor) using an oxide
semiconductor. An oxide semiconductor layer (OS) is provided over a
gate electrode (GE) with a gate insulating layer (GI) therebetween
and a source electrode (S) and a drain electrode (D) are provided
thereover. An insulating layer is provided so as to cover the
source electrode (S) and the drain electrode (D).
[0187] FIG. 7 represents an energy band diagram (schematic view)
corresponding to a cross-section taken along line A-A' in FIG. 6.
In FIG. 7, a black circle ( ) and a white circle (.smallcircle.)
represent an electron and a hole and have electric charges (-q,
+q), respectively.
[0188] With a positive voltage (V.sub.D>0) applied to the drain
electrode, the dashed line shows the case where no voltage is
applied to the gate electrode (V.sub.G=0) and the solid line shows
the case where a positive voltage is applied to the gate electrode
(V.sub.G>0). In the case where no voltage is applied to the gate
electrode, carriers (electrons) are not injected to the oxide
semiconductor side from an electrode because of a high potential
barrier, so that a current does not flow, which means an off state.
On the other hand, when a positive voltage is applied to the gate
electrode, potential barrier is lowered, and thus a current flows,
which means an on state.
[0189] FIGS. 8A and 8B are energy band diagrams (schematic views)
each corresponding to a cross-section taken along line B-B' in FIG.
6. FIG. 8A illustrates an on state in which a positive voltage
(V.sub.G>0) is applied to the gate electrode (GE) and carriers
(electrons) flow between the source electrode and the drain
electrode. FIG. 8B illustrates an off state in which a negative
voltage (V.sub.G<0) is applied to the gate electrode (GE) and
minority carriers do not flow.
[0190] FIG. 9 is a diagram illustrating the relationship between
the vacuum level and the work function of a metal (.phi.m) and the
relationship between the vacuum level and the electron affinity
(.chi.) of an oxide semiconductor.
[0191] At normal temperature, electrons in the metal are in
degenerate states and the Fermi level is located in the conduction
band. On the other hand, a conventional oxide semiconductor is an
n-type semiconductor, in which the Fermi level (E.sub.F) is away
from the intrinsic Fermi level (E.sub.i) located in the middle of a
band gap and is located closer to the conduction band. Note that it
is known that hydrogen is a donor in an oxide semiconductor and is
one factor causing an oxide semiconductor to be an n-type
semiconductor.
[0192] On the other hand, an oxide semiconductor according to one
embodiment of the disclosed invention is made to be an intrinsic
(i-type) or extremely close to an intrinsic oxide semiconductor by
removal of hydrogen that is a factor which makes the oxide
semiconductor have n-type conductivity so as to be highly purified
in such a manner that elements (impurity elements) that are not
main components thereof are contained as little as possible. In
other words, the oxide semiconductor according to one embodiment of
the disclosed invention is not an oxide semiconductor which is made
to be an i-type oxide semiconductor by adding an impurity element
but an i-type (intrinsic) or almost i-type oxide semiconductor
which is highly purified by removing an impurity such as hydrogen
or water as much as possible. In this manner, the Fermi level
(E.sub.f) can be extremely close to the intrinsic Fermi level
(E.sub.i).
[0193] It is said that when a band gap (E.sub.g) of an oxide
semiconductor is 3.15 eV, electron affinity (.chi.) thereof is 4.3
eV. The work function of titanium (Ti) contained in the source and
drain electrodes is almost equivalent to the electron affinity
(.chi.) of the oxide semiconductor. In that case, at an interface
between the metal and the oxide semiconductor, a Schottky barrier
against an electron passage is not formed.
[0194] At that time, the electron moves in the vicinity of the
interface between the gate insulating layer and the highly purified
oxide semiconductor (the lowest portion of the oxide semiconductor
which is stable in terms of energy) as illustrated in FIG. 8A.
[0195] In addition, as illustrated in FIG. 8B, when a negative
potential is applied to the gate electrode (GE), the current value
is extremely close to zero because holes that are minority carriers
are substantially nonexistent.
[0196] In such a manner, an intrinsic (i-type) or substantially
intrinsic oxide semiconductor is obtained by being highly purified
such that an element other than its main element (i.e., an impurity
element) is contained as little as possible. Thus, characteristics
of the interface between the oxide semiconductor and the gate
insulating layer become obvious. For that reason, the gate
insulating layer needs to be able to form a favorable interface
with the oxide semiconductor. Specifically, it is preferable to
use, for example, an insulating layer formed by a CVD method using
high-density plasma generated with a power supply frequency in the
range of the VHF band to the microwave band, an insulating layer
formed by a sputtering method, or the like.
[0197] When the oxide semiconductor is highly purified and the
interface between the oxide semiconductor and the gate insulating
layer is made favorable, in the case where the transistor has a
channel width (W) of 1.times.10.sup.4 .mu.m and a channel length
(L) of 3 .mu.m, for example, it is possible to realize an off-state
current of 10.sup.-13 A or less and a subthreshold swing (S value)
of 0.1 V/dec. (with a 100-nm-thick gate insulating layer).
[0198] The oxide semiconductor is highly purified as described
above so as to contain an element other than its main element
(i.e., an impurity element) as little as possible, so that the
transistor can operate in a favorable manner.
<Carrier Concentration>
[0199] In a technical idea according to the disclosed invention, an
oxide semiconductor layer is made as close as possible to an
intrinsic (i-type) oxide semiconductor layer by sufficiently
reducing carrier concentration thereof. Hereinafter, a method for
calculating the carrier concentration and carrier concentration
actually measured are described with reference to FIG. 10 and FIG.
11.
[0200] First, a method for calculating the carrier concentration is
easily explained. The carrier concentration can be calculated in
such a manner that an MOS capacitor is manufactured and results of
CV measurement (CV characteristics) of the MOS capacitor are
evaluated.
[0201] Specifically, carrier concentration N.sub.d is calculated in
the following manner: C-V characteristics are obtained by plotting
relations between a gate voltage (V.sub.g) and a capacitance (C) of
an MOS capacitor; a graph of a relation between the gate voltage
V.sub.g and (1/C).sup.2 is obtained with the use of the C-V
characteristics; a differential value of (1/C).sup.2 in a weak
inversion region of the graph is found; and the differential value
is substituted into Formula 1. Note that e, .epsilon..sub.0, and
.epsilon. in Formula 1 represent elementary electric charge, vacuum
permittivity, and relative permittivity of an oxide semiconductor,
respectively.
[ Formula 1 ] N d = - ( 2 e 0 ) / d ( 1 / C ) 2 dV ( 1 )
##EQU00001##
[0202] Next, measurement of actual carrier concentration
corresponding to that calculated by the above method is described.
For the measurement, a sample (a MOS capacitor) which was formed as
follows was used: a titanium film was formed to a thickness of 300
nm over a glass substrate; a titanium nitride film was formed to a
thickness of 100 nm over the titanium film; an oxide semiconductor
layer using an In--Ga--Zn--O-based oxide semiconductor was formed
to a thickness of 2 .mu.m over the titanium nitride film; and a
silver film was formed to a thickness of 300 nm over the oxide
semiconductor layer. Note that the oxide semiconductor layer was
formed using an oxide semiconductor target containing In, Ga, and
Zn (In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:1 (molar ratio)) by a
sputtering method. Further, a formation atmosphere of the oxide
semiconductor layer was a mixed atmosphere of argon and oxygen (a
flow ratio was Ar:O.sub.2=30 (sccm):15 (sccm)).
[0203] The C-V characteristics and the relation between the gate
voltage (V.sub.g) and (1/C).sup.2 are illustrated in FIG. 10 and
FIG. 11, respectively. The carrier concentration calculated using
Formula 1 from the differential value of (1/C).sup.2 in a weak
inversion region of the graph of FIG. 11 was
6.0.times.10.sup.10/cm.sup.3.
[0204] As described above, by using an oxide semiconductor which is
made to be an i-type or substantially i-type oxide semiconductor
(for example, carrier concentration is less than or equal to
1.times.10.sup.12/cm.sup.3, preferably, less than or equal to
1.times.10.sup.11/cm.sup.3), a transistor which has extremely
favorable off-state current characteristics can be obtained.
<Examples of Variations>
[0205] Examples of variations of a structure of a semiconductor
device are described with reference to FIG. 12, FIGS. 13A and 13B,
FIGS. 14A and 14B, and FIGS. 15A and 15B. Note that in the
following examples, the structure of the transistor 162 is
different from that already described. However, the structure of
the transistor 160 is similar to that already described.
[0206] In an example illustrated in FIG. 12, a semiconductor device
including the transistor 162 having the gate electrode 136d under
the oxide semiconductor layer 140 and the source or drain electrode
142a and the source or drain electrode 142b which are in contact
with the oxide semiconductor layer 140 at a bottom surface of the
oxide semiconductor layer 140. Since a plan structure may be
appropriately changed corresponding to a cross-sectional structure,
only the cross-sectional structure is described here.
[0207] As an important difference between the structure illustrated
in FIG. 12 and that illustrated in FIGS. 2A and 2B, there are
connection positions where the source or drain electrode 142a and
the source or drain electrode 142b are connected to the oxide
semiconductor layer 140. That is, in the structure illustrated in
FIGS. 2A and 2B, the source or drain electrode 142a and the source
or drain electrode 142b are in contact with a top surface of the
oxide semiconductor layer 140; on the other hand, in the structure
illustrated in FIG. 12, the source or drain electrode 142a and the
source or drain electrode 142b are in contact with the bottom
surface of the oxide semiconductor layer 140. In addition,
resulting from this difference in contact positions, a position of
another electrode, another insulating layer, or the like is
changed. As for details of each component, FIGS. 2A and 2B can be
referred to.
[0208] Specifically, the semiconductor device includes: the gate
electrode 136d provided over the interlayer insulating layer 128;
the gate insulating layer 138 provided over the gate electrode
136d; the source or drain electrode 142a and the source or drain
electrode 142b which are provided over the gate insulating layer
138; and the oxide semiconductor layer 140 in contact with top
surfaces of the source or drain electrode 142a and the source or
drain electrode 142b.
[0209] Here, the gate electrode 136d is provided so as to be
embedded in the insulating layer 132 which is formed over the
interlayer insulating layer 128. Furthermore, similarly to the gate
electrode 136d, an electrode 136a, an electrode 136b, and an
electrode 136c are formed in contact with the source or drain
electrode 130a, the source or drain electrode 130b, and the
electrode 130c, respectively.
[0210] In addition, over the transistor 162, a protective
insulating layer 144 is provided in contact with part of the oxide
semiconductor layer 140. An interlayer insulating layer 146 is
provided over the protective insulating layer 144. Here, in the
protective insulating layer 144 and the interlayer insulating layer
146, openings reaching the source or drain electrode 142a and the
source or drain electrode 142b are formed. In the openings, the
electrode 150d and the electrode 150e are formed to be in contact
with the source or drain electrode 142a and the source or drain
electrode 142b, respectively. Similarly to the electrodes 150d and
150e, the electrode 150a, the electrode 150b, and the electrode
150c are formed to be in contact with the electrode 136a, the
electrode 136b, and the electrode 136c, respectively, in openings
provided in the gate insulating layer 138, the protective
insulating layer 144, and the interlayer insulating layer 146.
[0211] Furthermore, the insulating layer 152 is provided over the
interlayer insulating layer 146. The electrode 154a, the electrode
154b, the electrode 154c, and the electrode 154d are provided so as
to be embedded in the insulating layer 152. Here, the electrode
154a is in contact with the electrode 150a; the electrode 154b, the
electrode 150b; the electrode 154c, the electrodes 150c and 150d;
and the electrode 154d, the electrode 150e.
[0212] FIGS. 13A and 13B each illustrate an example in which the
gate electrode 136d is provided over the oxide semiconductor layer
140. Here, FIG. 13A illustrates an example in which the source or
drain electrode 142a and the source or drain electrode 142b are in
contact with the oxide semiconductor layer 140 at the bottom
surface of the oxide semiconductor layer 140; and FIG. 13B
illustrates an example in which the source or drain electrode 142a
and the source or drain electrode 142b are in contact with the
oxide semiconductor layer 140 at the top surface of the oxide
semiconductor layer 140.
[0213] The structures of FIGS. 13A and 13B are largely different
from those of FIGS. 2A and 2B and FIG. 12 in that the gate
electrode 136d is provided over the oxide semiconductor layer 140.
Further, an important difference between the structure in FIG. 13A
and the structure in FIG. 13B is that which of the bottom surface
or the top surface of the oxide semiconductor layer 140 the source
and drain electrodes 142a and 142b are in contact with.
Furthermore, resulting from these differences, a position of
another electrode, another insulating layer, or the like is
changed. As for details of each component, FIGS. 2A and 2B or other
drawings can be referred to.
[0214] Specifically, in FIG. 13A, the semiconductor device
includes: the source or drain electrode 142a and the source or
drain electrode 142b which are provided over the interlayer
insulating layer 128; the oxide semiconductor layer 140 which is in
contact with the top surfaces of the source or drain electrode 142a
and the source or drain electrode 142b; the gate insulating layer
138 provided over the oxide semiconductor layer 140; and the gate
electrode 136d over a region of the gate insulating layer 138 which
is overlapped with the oxide semiconductor layer 140.
[0215] In FIG. 13B, the semiconductor device includes: the oxide
semiconductor layer 140 provided over the interlayer insulating
layer 128; the source or drain electrode 142a and the source or
drain electrode 142b which are provided in contact with the top
surface of the oxide semiconductor layer 140; the gate insulating
layer 138 provided over the oxide semiconductor layer 140, the
source or drain electrode 142a, and the source or drain electrode
142b; and the gate electrode 136d over a region of the gate
insulating layer 138 which is overlapped with the oxide
semiconductor layer 140.
[0216] Note that in the structures illustrated in FIGS. 13A and
13B, a component (e.g., the electrode 150a, the electrode 154a, or
the like) which the structure illustrated in FIGS. 2A and 2B or the
like has can be omitted in some cases. In such a case,
simplification of the manufacturing process can be achieved
secondarily. Needless to say, a component which is not essential
can be omitted also in the structure illustrated in FIGS. 2A and 2B
or the like.
[0217] FIGS. 14A and 14B each illustrate an example of a structure
in which the semiconductor device has a relatively large size and
the gate electrode 136d is provided under the oxide semiconductor
layer 140. In this case, a wiring, an electrode, or the like does
not need to be formed so as to be embedded in the insulating layer
because flatness or coverage of a surface is not needed to be
extremely high. For example, the gate electrode 136d and the like
can be formed in such a manner that a conductive layer is formed
and then patterned. Note that although not illustrated, the
transistor 160 can be manufactured similarly.
[0218] An important difference between the structure in FIG. 14A
and the structure in FIG. 14B is that which of the bottom surface
or the top surface of the oxide semiconductor layer 140 the source
and drain electrodes 142a and 142b are in contact with. In
addition, resulting from this difference, a position of another
electrode, another insulating layer, or the like is changed. As for
details of each component, FIGS. 2A and 2B or other drawings can be
referred to.
[0219] Specifically, in FIG. 14A, the semiconductor device
includes: the gate electrode 136d provided over the interlayer
insulating layer 128; the gate insulating layer 138 provided over
the gate electrode 136d; the source or drain electrode 142a and the
source or drain electrode 142b which are provided over the gate
insulating layer 138; and the oxide semiconductor layer 140 in
contact with the top surfaces of the source or drain electrode 142a
and the source or drain electrode 142b.
[0220] In FIG. 14B, the semiconductor device includes: the gate
electrode 136d provided over the interlayer insulating layer 128;
the gate insulating layer 138 provided over the gate electrode
136d; the oxide semiconductor layer 140 provided over the gate
insulating layer 138 so as to overlap with the gate electrode 136d;
and the source or drain electrode 142a and the source or drain
electrode 142b which are provided in contact with the top surface
of the oxide semiconductor layer 140.
[0221] Note that a component which the structure illustrated in
FIGS. 2A and 2B or the like are omitted in some cases also in the
structures illustrated in FIGS. 14A and 14B. Also in this case,
simplification of the manufacturing process can be achieved.
[0222] FIGS. 15A and 15B each illustrate an example of a structure
in which the semiconductor device has a relatively large size and
the gate electrode 136d is provided over the oxide semiconductor
layer 140. Also in this case, a wiring, an electrode, or the like
does not need to be formed so as to be embedded in the insulating
layer because flatness or coverage of a surface is not needed to be
extremely high. For example, the gate electrode 136d and the like
can be formed in such a manner that a conductive layer is formed
and then patterned. Note that although not illustrated, the
transistor 160 can be manufactured similarly.
[0223] An important difference between the structure in FIG. 15A
and the structure in FIG. 15B is that which of the bottom surface
or the top surface of the oxide semiconductor layer 140 the source
and drain electrodes 142a and 142b are in contact with. In
addition, resulting from this difference, a position of another
electrode, another insulating layer, or the like is changed. As for
details of each component, FIGS. 2A and 2B or other drawings can be
referred to.
[0224] Specifically, in FIG. 15A, the semiconductor device
includes: the source or drain electrode 142a and the source or
drain electrode 142b which are provided over the interlayer
insulating layer 128; the oxide semiconductor layer 140 which is in
contact with the top surfaces of the source or drain electrode 142a
and the source or drain electrode 142b; the gate insulating layer
138 provided over the source or drain electrode 142a, the source or
drain electrode 142b, and the oxide semiconductor layer 140; and
the gate electrode 136d over a region of the gate insulating layer
138 which is overlapped with the oxide semiconductor layer 140.
[0225] In FIG. 15B, the semiconductor device includes: the oxide
semiconductor layer 140 provided over the interlayer insulating
layer 128; the source or drain electrode 142a and the source or
drain electrode 142b which are provided in contact with the top
surface of the oxide semiconductor layer 140; the gate insulating
layer 138 provided over the source or drain electrode 142a, the
source or drain electrode 142b, and the oxide semiconductor layer
140; and the gate electrode 136d over a region of the gate
insulating layer 138 which is overlapped with the oxide
semiconductor layer 140.
[0226] Note that a component which the structure illustrated in
FIGS. 2A and 2B or the like has can be omitted in some cases also
in the structures illustrated in FIGS. 15A and 15B. Also in this
case, simplification of the manufacturing process can be
achieved.
[0227] As described above, according to one embodiment of the
disclosed invention, a semiconductor device having a new structure
is realized. Although the transistor 160 and the transistor 162 are
stacked in this embodiment, the structure of the semiconductor
device is not limited thereto. Further, although an example in
which the channel length direction of the transistor 160 and that
of the transistor 162 are perpendicular to each other is described,
the positions of the transistors 160 and 162 are not limited to
this. In addition, the transistors 160 and 162 may be provided to
overlap with each other.
[0228] Note that although in this embodiment a semiconductor device
per minimum storage unit (one bit) is described for easy
understanding, the structure of the semiconductor device is not
limited to this. A more developed semiconductor device can be
formed by appropriately connecting a plurality of semiconductor
devices. For example, it is possible to make a NAND-type or
NOR-type semiconductor device by using a plurality of the above
described semiconductor devices. The structure of the wiring is not
limited to that illustrated in FIG. 1 and can be changed as
appropriate.
[0229] In the semiconductor device according to this embodiment,
the low off-state current characteristics of the transistor 162
enable information to be held for an extremely long time. In other
words, refreshing operation, which is needed in DRAM memories and
the like, is not necessary; thus, power consumption can be
suppressed. In addition, the semiconductor device can be
substantially used as a nonvolatile memory.
[0230] Since information is written by switching operation of the
transistor 162, high voltage is not needed and an element is not
deteriorated in the semiconductor device. Further, information is
written or erased according to an on state and an off state of the
transistor, whereby high-speed operation can be easily realized.
Moreover, there is an advantage in that operation for erasing
information which is necessary in flash memory and the like is not
needed.
[0231] Furthermore, a transistor using a material other than an
oxide semiconductor can operate at sufficiently high speed; thus,
high-speed reading of stored content can be realized by using the
semiconductor device.
[0232] The methods and structures described in this embodiment can
be combined as appropriate with any of the methods and structures
described in the other embodiments.
Embodiment 2
[0233] In this embodiment, a circuit configuration and operation of
a semiconductor device according to one embodiment of the present
invention will be described.
[0234] An example of a circuit diagram of a memory element
(hereinafter also referred to as a memory cell) including the
semiconductor device is illustrated in FIG. 16. A memory cell 200
illustrated in FIG. 16 is a multi-valued memory cell and includes a
source line SL, a bit line BL, a first signal line S1, a second
signal line S2, a word line WL, a transistor 201, a transistor 202,
a transistor 203, and a capacitor 205. The transistors 201 and 203
are formed using a material other than an oxide semiconductor, and
the transistor 202 is formed using an oxide semiconductor.
[0235] Here, a gate electrode of the transistor 201 is electrically
connected to one of a source electrode and a drain electrode of the
transistor 202. In addition, the source line SL is electrically
connected to a source electrode of the transistor 201, and a source
electrode of the transistor 203 is electrically connected to a
drain electrode of the transistor 201. The bit line BL is
electrically connected to a drain electrode of the transistor 203,
and the first signal line S1 is electrically connected to the other
of the source electrode and the drain electrode of the transistor
202. The second signal line S2 is electrically connected to a gate
electrode of the transistor 202, and the word line WL is
electrically connected to a gate electrode of the transistor 203.
Additionally, one of electrodes of the capacitor 205 is
electrically connected to the gate electrode of the transistor 201
and one of the source electrode and the drain electrode of the
transistor 202. The other of the electrodes of the capacitor 205 is
supplied with a predetermined potential, for example, GND.
[0236] Next, operation of the memory cell 200 illustrated in FIG.
16 is described. In the case where the memory cell 200 is a
four-valued memory cell is described. Four states of the memory
cell 200 are set to data "00b", "01b", "10b", and "11b", and
respective potentials of a node A at that time are set to V.sub.00,
V.sub.01, V.sub.10, and V.sub.11
(V.sub.00<V.sub.01<V.sub.10<V.sub.11), respectively.
[0237] When writing is performed to the memory cell 200, the source
line SL is set to 0 [V], the word line WL is set to 0 [V], the bit
line BL is set to 0 [V], and the second signal line S2 is set to 2
[V]. When the data "00b" is to be written, the first signal line S1
is set to V.sub.00 [V]. When the data "01b" is to be written, the
first signal line S1 is set to V.sub.01 [V]. When the data "10b" is
to be written, the first signal line S1 is set to V.sub.10 [V].
When the data "11b" is to be written, the first signal line S1 is
set to V.sub.11 [V]. At this time, the transistor 203 is put in an
off state and the transistor 202 is put in an on state. Note that
when writing data is completed, the second signal line S2 is set to
0 [V] so as to turn off the transistor 202, before a potential of
the first signal line S1 is changed.
[0238] As a result, after writing one of the data "00b", "01b",
"10b", and "11b", a potential of a node connected to the gate
electrode of the transistor 201 (hereinafter referred to as the
node A) becomes approximately one of V.sub.00 [V], V.sub.01 [V],
V.sub.10 [V], and V.sub.11 [V]. A charge corresponding to the
potential of the first signal line S1 is accumulated in the node A;
however, since off-state current of the transistor 202 is extremely
small or is substantially 0, a potential of the gate electrode of
the transistor 201 is held for a long time.
[0239] When reading is performed from the memory cell 200, first,
the bit line BL is pre-charged to a voltage V.sub.pc[V]. The source
line SL is set to V.sub.s.sub._.sub.read [V], the word line WL is
set to 2 [V], the second signal line S2 is set to 0 [V], and the
first signal line S1 is set to 0 [V]. At this time, the transistor
203 takes an on state and the transistor 202 takes an off state.
Note that the potential V.sub.pc is set to lower than
V.sub.00-V.sub.th. Additionally, V.sub.s.sub._.sub.read is set to
higher than V.sub.11-V.sub.th.
[0240] As a result, current flows from the source line SL to the
bit line BL, and the bit line is charged to a potential represented
by (the potential of the node A)--(a threshold voltage Vth of the
transistor 201). Consequently, the potential of the bit line BL
becomes one of V.sub.00-V.sub.th, V.sub.01-V.sub.th,
V.sub.10-V.sub.th, and V.sub.11-V.sub.th corresponding to the data
"00b", "01b", "10b", and "11b", respectively. Since the potentials
applied to the bit line and corresponding to the data are different
from each other, a reading circuit connected to the bit line BL can
read the data "00b", "01b", "10b", and "11b".
[0241] A block circuit diagram of a semiconductor device according
to one embodiment of the present invention including m.times.n bits
of storage capacity is illustrated in FIG. 17.
[0242] The semiconductor device according to one embodiment of the
present invention includes the following components: an m number of
word lines WL and an m number of second signal lines S2; an n
number of bit lines BL, an n number of first signal lines S1, and
an n number of source lines SL; a memory cell array 210 including a
plurality of memory cells 200 (1, 1) to 200 (m, n) arranged in a
matrix with m cells in vertical (rows) and n cells in horizontal
(columns) (both m and n are natural numbers); and peripheral
circuits such as reading circuits 211, first signal line driver
circuits 212, a driver circuit 213 for the second signal lines and
word lines, and a potential generating circuit 214. As another
peripheral circuit, a refresh circuit and the like may be
provided.
[0243] Each of the memory cells, for example, a memory cell 200 (i,
j) is considered (here, i is an integer greater than or equal to 1
and less than or equal to m and j is an integer greater than or
equal to 1 and less than or equal to n). The memory cell 200 (i, j)
is connected to the bit line BL (j), the first signal line S1 (j),
the source line SL (j), the word line WL (i), and the second signal
line S2 (i). In addition, the bit lines BL (1) to BL (n) and the
source lines SL (1) to SL (n) are connected to the reading circuits
211. The first signal lines S1 (1) to S1 (n) are connected to the
first signal line driver circuits 212. The word lines WL (1) to WL
(m) and the second signal lines S2 (1) to S2 (m) are connected to
the driver circuit 213 for the second signal lines and the word
lines.
[0244] An example of the driver circuit 213 for the second signal
lines and the word lines is illustrated in FIG. 18. The driver
circuit 213 for the second signal lines and the word lines includes
a decoder 215. The decoder 215 is connected to the second signal
lines S2 and the word lines WL through switches. Further, the
second signal lines S2 and the word lines WL are connected to GND
(a ground potential) through the switches. The switches are
controlled by a read enable signal (an RE signal) or a write enable
signal (a WE signal). An address signal ADR is input to the decoder
215 from the exterior.
[0245] When the address signal ADR is input to the driver circuit
213 for the second signal lines and the word lines, rows specified
by the address (hereinafter also referred to as selected rows) are
asserted (activation) and rows other than the rows specified by the
address (hereinafter also referred to as non-selected rows) are
de-asserted (inactivation). Further, when the RE signal is
asserted, the word line WL is connected to an output of the decoder
215, and when the RE signal is de-asserted, the word line WL is
connected to the GND. When the WE signal is asserted, the second
signal line S2 is connected to the output of the decoder 215, and
when the WE signal is de-asserted, the second signal line S2 is
connected to the GND.
[0246] An example of the first signal line driver circuit 212 is
illustrated in FIG. 19. The first signal line driver circuit 212
includes a multiplexer (MUX1). Input data DI and the writing
potentials V.sub.00, V.sub.01, V.sub.10, and V.sub.11 are input to
the multiplexer (MUX1). An output terminal of the multiplexer
(MUX1) is connected to the first signal line S1 through a switch.
Additionally, the first signal line S1 is connected to GND through
a switch. The switches are controlled by a write enable signal (a
WE signal).
[0247] When DI is input to the first signal line driver circuit
212, the multiplexer (MUX1) selects writing potential V.sub.w in
accordance with the value of DI from the writing potentials
V.sub.00, V.sub.01, V.sub.10, and V.sub.11. The behavior of the
multiplexer (MUX1) is shown in Table 1. When the WE signal is
asserted, the selected writing potential V.sub.w is applied to the
first signal line S1. When the WE signal is de-asserted, 0 [V] is
applied to the first signal line S1 (the first signal line S1 is
connected to the GND).
TABLE-US-00001 TABLE 1 DI[1] DI[0] MUX1output 0 0 Corresponds to
V00 0 1 Corresponds to V01 1 0 Corresponds to V10 1 1 Corresponds
to V11
[0248] An example of the reading circuit 211 is illustrated in FIG.
20. The reading circuit 211 includes a plurality of sense amplifier
circuits, a logic circuit 229, and the like. One input terminal of
each of the sense amplifier circuits is connected to the bit line
BL and a wiring to which a potential V.sub.pc is applied through
switches. Any of reference potentials V.sub.ref0, V.sub.ref1, and
V.sub.ref2 is input to the other input terminal of each of the
sense amplifier circuits. An output terminal of each of the sense
amplifier circuits is connected to an input terminal of the logic
circuit 229. Note that the switches are controlled by a read enable
signal (an RE signal).
[0249] A state of a memory cell can be read as a digital signal
with three bits by setting values of each of the reference
potentials V.sub.ref0, V.sub.ref1, and V.sub.ref2 so as to satisfy
V.sub.00-V.sub.th<V.sub.ref0<V.sub.01-V.sub.th<V.sub.ref1<V.s-
ub.10-V.sub.th<V.sub.ref2<V.sub.11-V.sub.th. For example, in
the case of the data "00b", a potential of the bit line BL is
V.sub.00-V.sub.th. Here, the potential of the bit line is smaller
than any of the reference potentials: V.sub.ref0, V.sub.ref1, and
V.sub.ref2, whereby, all of outputs SA_OUT0, SA_OUT1, and SA_OUT2
of the sense amplifier circuits become "0". Similarly, in the case
of the data "01b", the potential of the bit line BL becomes
V.sub.01-V.sub.t, so that the outputs SA_OUT0, SA_OUT1, and SA_OUT2
of the sense amplifier circuits become "1", "0", and "0",
respectively. In the case of the data "10b", the potential of the
bit line BL is V.sub.10-V.sub.th, whereby the outputs SA_OUT0,
SA_OUT1, and SA_OUT2 of the sense amplifier circuits become "1",
"1", and "0", respectively. In the case of the data "11b", the
potential of the bit line BL is V.sub.11-V.sub.th, so that the
outputs SA_OUT0, SA_OUT1, and SA_OUT2 of the sense amplifier
circuits become "1", "1", and "1", respectively. Then, using the
logic circuit 229 shown in a logic table Table 2, data DO with two
bits is generated and output from the reading circuit 211.
TABLE-US-00002 TABLE 2 SA_OUT0 SA_OUT1 SA_OUT2 D01 D00 0 0 0 0 0 1
0 0 0 1 1 1 0 1 0 1 1 1 1 1
[0250] Note that in the reading circuit 211 illustrated here, when
the RE signal is de-asserted, the source line SL is connected to
GND and 0 [V] is applied to the source line SL. At the same time,
the potential V.sub.pc [V] is also applied to the bit line BL and a
terminal of the sense amplifier circuits connected to the bit line
BL. When the RE signal is asserted, V.sub.s.sub._.sub.read [V] is
applied to the source line SL, whereby a potential reflecting data
is charged to the bit line BL. Then, the reading is performed. Note
that the potential V.sub.pc is set to lower than V.sub.00-V.sub.th.
Additionally, V.sub.s.sub._.sub.read is set to higher than
V.sub.11-V.sub.th.
[0251] Note that "potentials of the bit line BL" compared in
reading include a potential of node of input terminals of the sense
amplifier circuits connected to the bit line BL through switches.
That is, potentials compared in the reading circuit 211 do not need
to be exactly the same as the potentials of the bit line BL.
[0252] An example of the potential generating circuit 214 is
illustrated in FIG. 21. In the potential generating circuit 214, a
potential is divided between V.sub.dd and GND by resistance,
whereby desired potentials can be obtained. Then the generated
potentials are output through an analogue buffer 220. In such a
manner, the writing potentials V.sub.00, V.sub.01, V.sub.10, and
V.sub.11 and the reference potentials V.sub.ref0, V.sub.ref1, and
V.sub.ref2 are generated. Note that a configuration in which
V.sub.00<V.sub.ref0<V.sub.01<V.sub.ref1<V.sub.10<V.sub.ref-
2<V.sub.11 is illustrated in FIG. 21; however, a potential
relation is not limited thereto. Potentials required can be
generated as appropriate by adjusting a resistor and nodes to which
the reference potentials are connected. Further, V.sub.00,
V.sub.01, V.sub.10, and V.sub.11 may be generated using another
potential generating circuit than that of V.sub.ref0, V.sub.ref1,
and V.sub.ref2.
[0253] A potential boosted in a boosting circuit may be supplied to
the potential generating circuit 214 instead of the power supply
potential V.sub.dd. This is because the absolute value of the
potential difference can be increased by supplying an output of the
boosting circuit to the potential generating circuit, so that a
higher potential can be supplied.
[0254] Note that even in the case where the power supply potential
V.sub.dd is directly supplied to the potential generating circuit,
the power supply potential V.sub.dd can be divided into a plurality
of potentials. However, since in this case, it is difficult to
distinguish adjacent potentials from each other, writing mistakes
and reading mistakes would increase. In the case where the output
of the boosting circuit is supplied to the potential generating
circuit, the absolute value of the potential difference can be
increased, so that a sufficient potential difference between the
adjacent potentials can be secured even if the number of partitions
is increased.
[0255] Thus, storage capacity of a memory cell can be increased
without increasing writing mistakes and reading mistakes.
[0256] As an example of a boosting circuit in which boosting of
four stages is performed, a boosting circuit 219 is illustrated in
FIG. 22A. In FIG. 22A, the power supply potential V.sub.dd is
supplied to an input terminal of a first diode 402. An input
terminal of a second diode 404 and one terminal of a first
capacitor 412 are connected to an output terminal of the first
diode 402. Similarly, an input terminal of a third diode 406 and
one terminal of a second capacitor 414 are connected to an output
terminal of the second diode 404. Connections of other parts are
similar to the above; therefore, detailed explanation is omitted.
However, the connection can be represented as follows: one terminal
of an n-th capacitor is connected to an output terminal of an n-th
diode (where n represents an integer). Note that an output of a
fifth diode 410 becomes an output V.sub.out of the boosting circuit
219.
[0257] In addition, a clock signal CLK is input to the other
terminal of the first capacitor 412 and the other terminal of a
third capacitor 416. An inverted clock signal CLKB is input to the
other terminal of the second capacitor 414 and the other terminal
of a fourth capacitor 418. That is, the clock signal CLK is input
to the other terminal of the (2k-1)-th capacitor and the inverted
clock signal CLKB is input to the other terminal of the 2k-th
capacitor (where k represents an integer). Note that a ground
potential GND is input to the other terminal of a capacitor of the
last stage.
[0258] When the clock signal CLK is high, that is, when the
inverted clock signal CLKB is low, the first capacitor 412 and the
third capacitor 416 are charged, and potentials of node N1 and node
N3 capacitively coupled with the clock signal CLK are increased by
predetermined voltage. On the other hand, potentials of node N2 and
node N4 capacitively coupled with the inverted clock signal CLKB
are decreased by predetermined voltage.
[0259] Therefore, a charge moves through the first diode 402, the
third diode 406, and the fifth diode 410, and the potentials of
node N2 and node N4 are increased to a predetermined value.
[0260] Next, when the clock signal CLK becomes low and the inverted
clock signal CLKB becomes high, potentials of node N2 and node N4
further increase. On the other hand, the potentials of node N1,
node N3, and node N5 are decreased by a predetermined voltage.
[0261] Accordingly, a charge moves through the second diode 404 and
the fourth diode. As a result, potentials of node N3 and node N5
are increased to a predetermined potential. Thus, each of
potentials of nodes becomes
V.sub.N5>V.sub.N4(CLKB=High)>V.sub.N3 (CLK=High)>V.sub.N2
(CLKB=High)>V.sub.N1 (CLK=High)>V.sub.N2
(CLKB=High)>V.sub.N1 (CLK=High)>V.sub.dd, whereby boosting is
performed. Note that the boosting circuit 219 is not limited to a
circuit performing four stages of boosting. The number of stages of
the boosting can be changed as appropriate.
[0262] Note that the output V.sub.out of the boosting circuit 219
is significantly affected by variations between the characteristics
of diodes. For example, a diode is provided by connecting a source
electrode and a gate electrode of a transistor to each other, but
in this case, the diode is affected by variation in the threshold
value of the transistor.
[0263] In order to control the output V.sub.out with high accuracy,
a structure in which the output V.sub.out is fed back may be
employed. FIG. 22B illustrates an example of a circuit
configuration in the case where the output V.sub.out is fed back.
The boosting circuit 219 in FIG. 22B corresponds to the boosting
circuit 219 in FIG. 22A.
[0264] An output terminal of the boosting circuit 219 is connected
to one input terminal of a sense amplifier circuit through a
resistance R.sub.1. In addition, the one input terminal of the
sense amplifier circuit is grounded through a resistance R.sub.2.
That is, a potential V.sub.1 corresponding to the output V.sub.out
is input to the one input terminal of the sense amplifier circuit.
Here, V.sub.1=V.sub.outR.sub.2/(R.sub.1+R.sub.2).
[0265] Further, the reference potential V.sub.ref is input to the
other input terminal of the sense amplifier circuit. That is,
V.sub.1 and V.sub.ref are compared in the sense amplifier circuit.
An output terminal of the sense amplifier circuit is connected to a
control circuit. A clock signal CLK0 is input to the control
circuit. The control circuit outputs the clock signal CLK and the
inverted clock signal CLKB to the boosting circuit 219 in response
to the output of the sense amplifier circuit.
[0266] When V.sub.1>V.sub.ref, an output sig_1 of the sense
amplifier circuit is asserted, and the control circuit stops
supplying the clock signal CLK and the inverted clock signal CLKB
to the boosting circuit 219. Thus, the boosting operation is
stopped, so that the potential V.sub.out stops increasing. Then, a
circuit connected to the output of the boosting circuit 219
consumes electricity, whereby the potential V.sub.out gradually
decreases.
[0267] When V.sub.1<V.sub.ref, the output sig_1 of the sense
amplifier circuit is de-asserted, and the control circuit starts to
supply the clock signal CLK and the inverted clock signal CLKB to
the boosting circuit 219. Thus, the boosting operation is
performed, so that the potential V.sub.out gradually increases.
[0268] In such a manner, the output potential V.sub.out of the
boosting circuit 219 can be kept equal to a predetermined value by
feeding back the output potential V.sub.out of the boosting circuit
219. The above configuration is especially effective in the case
where the characteristics of the diodes vary. Moreover, it is also
effective in the case where a predetermined potential is to be
generated in accordance with the reference potential V.sub.ref.
Note that a plurality of potentials can be generated in the
boosting circuit 219 by using a plurality of different reference
potentials.
[0269] The absolute value of a potential difference can be
increased by supplying an output of a boosting circuit to a
potential generating circuit. Therefore, a higher potential can be
generated without a change of the minimum unit of the potential
difference. That is, storage capacity of a memory cell can be
increased.
[0270] FIG. 23 illustrates a differential sense amplifier as an
example of a sense amplifier circuit. The differential sense
amplifier includes input terminals V.sub.in (+) and V.sub.in (-)
and an output terminal V.sub.out, and amplifies a difference
between V.sub.in (+) and V.sub.in (-). The output V.sub.out is
relatively high when V.sub.in (+)>V.sub.in (-), and is
relatively low when V.sub.in (+)<V.sub.in (-).
[0271] FIG. 24 illustrates a latch sense amplifier as an example of
a sense amplifier circuit. The latch sense amplifier includes
input-output terminals V1 and V2 and input terminals of control
signals Sp and Sn. First, power supply is stopped by setting the
signal Sp at high and the signal Sn at low. Next, potentials to be
compared are applied to V1 and V2. After that, when power is
supplied by setting the signal Sp at low and the signal Sn at high,
output V1 becomes high and output V2 becomes low when the
potentials before supplying the power verify V1>V2. Output V1
becomes low and output V2 becomes high when potentials before
supplying the power verify V1<V2. In such a manner, a potential
difference between V1 and V2 is amplified.
[0272] FIG. 25A illustrates an example of a timing chart of a
writing operation. The case where data "10b" is a written to a
memory cell is illustrated in the timing chart in FIG. 25A. The
selected second signal line S2 becomes 0 [V] earlier than the first
signal line S1. During the writing period, the potential of the
first signal line S1 becomes V.sub.10. Note that the word line WL,
the bit line BL, and the source line SL are all at 0 [V]. In
addition, FIG. 25B illustrates an example of a timing chart of
reading operation. The case where data "10b" is read out from a
memory cell is illustrated in the timing chart in FIG. 25B. The
selected word line WL is asserted and the source line SL takes the
potential V.sub.s.sub._.sub.read [V], whereby the bit line BL is
charged to V.sub.10-V.sub.th [V] corresponding to the data "10b" of
the memory cell. As a result, SA_OUT0, SA_OUT1, and SA_OUT2 become
"1", "1", and "0", respectively. Note that the first signal line S1
and the second signal line S2 are at 0 [V].
[0273] Here, examples of specific operation potentials (voltages)
are described. For example, the following can be obtained: the
threshold voltage of the transistor 201 is approximately 0.3 V, the
power supply voltage V.sub.DD is 2 V, V.sub.11 is 1.6 V, V.sub.10
is 1.2 V, V.sub.01 is 0.8 V, V.sub.00 is 0V, V.sub.ref0 is 0.6 V,
V.sub.ref1 is 1.0 V, and V.sub.ref2 is 1.4 V. The potential
V.sub.pc is preferably 0 V, for example.
[0274] Although, the first signal line S1 is arranged in the bit
line BL direction (the column direction) and the second signal line
S2 is arranged in the word line WL direction (the row direction) in
this embodiment, one embodiment of the present invention is not
limited thereto. For example, the first signal line S1 may be
arranged in the word line WL direction (the row direction) and the
second signal line S2 may be arranged in the bit line BL direction
(the column direction). In such a case, the driver circuit to which
the first signal line S1 is connected and the driver circuit to
which the second signal line S2 is connected may be arranged as
appropriate.
[0275] In this embodiment, operation of four-valued memory cells,
i.e., writing any of four different states to one memory cell or
reading any of four different states from one memory cell is
described. By appropriately changing the circuit configuration,
operation of n valued memory cells, i.e., writing any of n
different states (n is integer greater than or equal to 2) to one
memory cell or reading any of n different states from one memory
cell, can be performed.
[0276] For example, in eight-valued memory cells, storage capacity
becomes three times as large as two-valued memory cells. When
writing data, eight values of potentials are prepared to decide a
potential of a node A and eight states are prepared. When reading
data, seven reference potentials capable of distinguishing the
eight states are prepared. One sense amplifier is provided and
comparison is performed seven times, so that data can be read.
Further, the number of comparison times may be reduced to three
times by feeding back the result of comparison. In a reading method
for driving the source line SL, data can be read in one comparison
by providing seven sense amplifiers. Furthermore, a plurality of
sense amplifiers can be provided and comparison is performed plural
times.
[0277] Generally, in 2.sup.k-valued memory cells (k is integer
greater than or equal to 1), memory capacity is k times as large as
that of two-valued memory cells. When writing data, 2.sup.k values
of writing potentials to decide a potential of a node A are
prepared, each value corresponding to one of 2.sup.k states. When
reading data, 2.sup.k-1 values of reference potentials enabling to
distinguish 2.sup.k states may be set up. One sense amplifier is
provided and comparison is performed 2.sup.k-1 times, so that data
can be read. Further, the number of comparison times may be reduced
to k times by feeding back the result of comparison. In a reading
method for driving the source line SL, data can be read in one
comparison by providing a 2.sup.k-1 number of sense amplifiers.
Furthermore, a plurality of sense amplifiers can be provided and
comparison is performed plural times.
[0278] The semiconductor device according to this embodiment can
store information for quite a long time because of low off-state
current characteristics of the transistor 202. That is, refresh
operation which is necessary in DRAM and the like is not needed, so
that power consumption can be reduced. In addition, the
semiconductor device of this embodiment can be used as a
substantially nonvolatile memory device.
[0279] Since writing data and the like are performed by switching
operation of the transistor 202, high voltage is not needed and
there is no problem of deterioration of elements. Further,
high-speed operation can be easily realized because writing
information and erasing information are performed by turning
transistors on or off. Furthermore, information can be directly
rewritten by controlling potentials input to transistors.
Accordingly, erasing operation which is necessary in flash memory
and the like is not needed, and decrease in operation speed due to
erasing operation can be prevented.
[0280] Moreover, a transistor using a material other than an oxide
semiconductor material can operate at sufficiently high speed;
therefore, by using the transistor, memory content can be read at
high speed.
[0281] The semiconductor device according to this embodiment is a
multivalued semiconductor device, so that storage capacity per area
can be increased. Therefore, the size of the semiconductor device
can be reduced and the semiconductor device can be highly
integrated. Additionally, potentials of nodes which take a floating
state when writing operation is performed can be directly
controlled; thus, threshold voltages of the semiconductor device
can be easily controlled with high accuracy which is required for a
multivalued memory. Therefore, verification of states after writing
data which is required to a multivalued-type memory can be omitted,
and in such a case, time required for writing data can be
shortened.
Embodiment 3
[0282] In this embodiment, a circuit configuration and operation of
a semiconductor device according to one embodiment of the present
invention are described.
[0283] In this embodiment, a reading operation different from that
of Embodiment 2 is described with the use of a circuit
configuration of a memory element illustrated in FIG. 16. Note that
the capacitor 205 shown in FIG. 16 is not always necessary and can
be omitted in some cases. The memory element considered is a
multivalued memory element, and a case where the memory element is
a four-valued memory element is described in this embodiment. Four
states of a memory cell 200 are data "00b", "01b", "10b", and
"11b", and corresponding potentials of a node A are V.sub.00,
V.sub.01, V.sub.10, and V.sub.11
(V.sub.00<V.sub.01<V.sub.10<V.sub.11), respectively.
[0284] When writing is performed to the memory cell 200, the source
line SL is set to 0 [V], the word line WL is set to 0 [V], the bit
line BL is set to 0 [V], and the second signal line S2 is set to 2
[V]. When the data "00b" is to be written, the first signal line S1
is set to V.sub.00 [V]. When the data "01b" is to be written, the
first signal line S1 is set to V.sub.01 [V]. When the data "10b" is
to be written, the first signal line S1 is set to V.sub.10 [V].
When the data "11b" is to be written, the first signal line S1 is
set to V.sub.11 [V]. At this time, the transistor 203 is put in an
off state and a transistor 202 to an on state. Note that when
writing data is completed, the second signal line S2 is set to 0
[V] so as to turn off the transistor 202, before a potential of the
first signal line S1 is changed.
[0285] As a result, after writing the data "00b", "01b", "10b", or
"11b", the potential of a node connected to a gate electrode of the
transistor 201 (hereinafter, referred to as a node A) is
approximately V.sub.00 [V], V.sub.01 [V], V.sub.10 [V], or V.sub.11
[V], respectively. A charge is accumulated in the node A in
accordance with the potential of the first signal line S1, and
since off-state current of the transistor 202 is extremely small or
substantially 0, the potential of the gate electrode of the
transistor 201 is held for a long time.
[0286] Next, in the case where reading of the memory cell 200 is
performed, the source line SL is set to 0 [V], the word line WL is
set to V.sub.DD, the second signal line S2 is set to 0 [V], the
first signal line S1 is set to 0 [V], and a reading circuit 211
connected to the bit line BL is in an operation state. At this
time, the transistor 203 is in an on state and the transistor 202
is in an off state.
[0287] As a result, an effective resistance value of the memory
cell 200 between a source line SL and a corresponding bit line BL
is function of the state of the memory cell 200. As the potential
of the node A is increased, the effective resistance value is
reduced. The reading circuit can read out the data "00b", "01b",
"10b", and "11b" from potential differences caused by differences
between resistance values. Note that in the case of data other than
data "00b" in which the potential of the node A is the lowest
value, it is preferable that the transistor 201 be in an on
state.
[0288] FIG. 26 illustrates a block circuit diagram of another
example of a semiconductor device according to one embodiment of
the present invention, which includes memory capacity of m.times.n
bits.
[0289] The semiconductor device illustrated in FIG. 26 includes an
m number of word lines WL, an m number of second signal lines S2,
an n number of bit lines BL, an n number of first signal lines S1,
a memory cell array 210 in which the plurality of memory cells 200
(1, 1) to 200 (m, n) are arranged in a matrix of m cells in
vertical (rows) by n cells in horizontal (columns) (both m and n
are natural numbers), and peripheral circuits such as reading
circuits 221, first signal line driver circuits 212, a driver
circuit 213 for the second signal lines and the word lines, and a
potential generating circuit 214. A refresh circuit or the like may
be provided as another peripheral circuit.
[0290] Each of the memory cells, for example, a memory cell 200 (i,
j) is considered (here, i is an integer greater than or equal to 1
and less than or equal to m and j is an integer greater than or
equal to 1 and less than or equal to n). The memory cell 200 (i, j)
is connected to a bit line BL (j), a first signal line S1 (j), a
word line WL (i), a second signal line S2 (i), and a source wiring.
A potential Vs (0V for example) is applied to the source wiring. In
addition, the bit lines BL (1) to BL (n) are connected to the
reading circuits 221, the first signal lines S1 (1) to S1 (n) are
connected to the first signal line driver circuits 212, the word
lines WL (1) to WL (m) and the second signal lines S2 (1) to S2 (m)
are connected to the driver circuit 213 for the second signal lines
and the word lines.
[0291] Note that, the configurations of the potential generating
circuit 214, the driver circuit 213 for the second signal lines and
the word signal lines, and the first signal line driver circuit 212
may be the same as the configurations of FIG. 21, FIG. 18, and FIG.
19, for example.
[0292] FIG. 27 illustrates an example of the reading circuit 221.
The reading circuit 221 includes a sense amplifier circuit, a
reference cell 225, a logic circuit 229, a multiplexer (MUX2),
flip-flop circuits FF0, FF1, and FF2, a bias circuit 223, and the
like. The reference cell 225 includes a transistor 216, a
transistor 217, and a transistor 218. The transistor 216, the
transistor 217, and the transistor 218 included in the reference
cell 225 correspond to the transistor 201, the transistor 202, and
the transistor 203 included in the memory cell, respectively, and
form the same circuit configuration as the memory cell. It is
preferable that the transistor 216 and the transistor 218 be formed
using materials other than an oxide semiconductor, and the
transistor 217 be formed using an oxide semiconductor. In addition,
in the case where the memory cell includes the capacitor 205, it is
preferable that the reference cell 225 also include a capacitor.
Two output terminals of the bias circuit 223 are connected to the
bit line BL and a drain electrode of the transistor 218 included in
the reference cell 225, respectively, through a switch. In
addition, the output terminals of the bias circuit 223 are
connected to input terminals of the sense amplifier circuit. An
output terminal of the sense amplifier circuit is connected to the
flip-flop circuits FF0, FF1, and FF2. Output terminals of the
flip-flop circuits FF0, FF1, and FF2 are connected to input
terminals of the logic circuit 229. Signals RE0, RE1, and RE2,
reference potentials V.sub.ref0, V.sub.ref1, and V.sub.ref2, and
GND are input to the multiplexer (MUX2). An output terminal of the
multiplexer (MUX2) is connected to one of a source electrode and a
drain electrode of the transistor 217 included in the reference
cell 225. The bit line BL and the drain electrode of the transistor
218 included in the reference cell 225 are connected to a wiring
V.sub.pc through switches. Note that the switches are controlled by
a signal .phi.A.
[0293] The reading circuit 221 has a structure in which a
comparison of the conductance of the memory cell with the
conductance of the reference cell 225 is performed by comparing
potential values output from the memory cell and the reference
cell. This structure includes one sense amplifier circuit. In this
structure, the comparison is performed three times in order to read
out the four states. In other words, the comparison of the
conductance of the memory cell with the conductance of the
reference cell 225 is performed in the case of each of three values
of reference potentials. The three comparisons are controlled by
the signals RE0, RE1, RE2, and .phi.A. The multiplexer (MUX2)
selects any of the three values of reference potentials V.sub.ref0,
V.sub.ref1, and V.sub.ref2, and GND in accordance with the values
of the signals RE0, RE1, and RE2. The behavior of the multiplexer
(MUX2) is illustrated in Table 3. The flip-flop circuits FF0, FF1,
and FF2 are controlled by the signals RE0, RE1, and RE2,
respectively, and store the value of an output signal SA_OUT of the
sense amplifier.
TABLE-US-00003 TABLE 3 RE0 RE1 RE2 VwL 0 0 0 Corresponds to GND 1 0
0 Corresponds to Vref0 0 1 0 Corresponds to Vref1 0 0 1 Corresponds
to Vref2
[0294] The values of the reference potentials are determined so as
to be
V.sub.00<V.sub.ref0<V.sub.01<V.sub.ref1<V.sub.10<V.sub.ref-
2<V.sub.11. Thus, the four states can be read out from the
results of the three comparisons. The values of the flip-flop
circuits FF0, FF1, and FF2 are "0", "0", and "0", respectively in
the case of the data "00b". The values of the flip-flop circuits
FF0, FF1, and FF2 are "1", "0", and "0", respectively in the case
of the data "01b". The values of the flip-flop circuits FF0, FF1,
and FF2 are "1", "1", and "0", respectively in the case of the data
"10b". The values of the flip-flop circuits FF0, FF1, and FF2 are
"1", "1", and "1", respectively in the case of the data "11b". In
this manner, the state of the memory cell can be read out as a
3-bit digital signal. After that, with the use of the logic circuit
229 whose a logic value table is shown in Table 2, 2-bit data DO is
generated and output from the reading circuit.
[0295] Note that in the reading circuit illustrated in FIG. 27,
when a signal RE is de-asserted, the bit line BL and the reference
cell 225 are connected to the wiring V.sub.pc so that pre-charge is
performed. When the signal RE is asserted, electrical continuity
between the bit line BL and the bias circuit 223 and between the
reference cell 225 and the bias circuit 223 is established.
[0296] Note that the pre-charge is not necessarily performed. In
this circuit, it is preferable that the circuits which generate two
signals input to the sense amplifier circuit have almost the same
structure. For example, it is preferable that the structure of the
transistors in the reference cell 225 be the same as the structure
of the corresponding transistors in the memory cell. It is
preferable that the corresponding transistors in the bias circuit
223 and the switch have the same structure.
[0297] A timing chart of the writing operation is the same as FIG.
25A. An example of a timing chart of the reading operation is shown
in FIG. 28. FIG. 28 shows a timing chart in the case where the data
"10b" is read out from the memory cell. V.sub.ref0, V.sub.ref1, and
V.sub.ref2 are input to an output MUX2_OUT of the multiplexer
(MUX2) in respective periods in which the signals RE0, RE1, and RE2
are asserted. In a first half of each of the periods, the signal
.phi.A is asserted and a predetermined potential is applied to a
node B of the transistor included in the reference cell 225. In a
latter half of each of the periods, the signal .phi.A is
de-asserted, the predetermined potential is held in the node B of
the transistor included in the reference cell 225 and the drain
electrode of the transistor 218 included in the reference cell 225
is connected to the bias circuit 223. Then, a result of the
comparison in the sense amplifier circuit is stored in each of the
flip-flop circuits FF0, FF1, and FF2. In the case of the data of
the memory cell is "10b", the values of the flip-flop circuits FF0,
FF1, and FF2 are "1", "1", and "0", respectively. Note that the
first signal line S1 and the second signal line S2 are at 0
[V].
[0298] Next, a reading circuit which is different from that
illustrated in FIG. 20 and a reading method are described.
[0299] FIG. 29 illustrates an example of a reading circuit 231. The
reading circuit 231 includes a sense amplifier circuit, a plurality
of reference cells (a reference cell 225a, a reference cell 225b,
and a reference cell 225c), the logic circuit 229, the flip-flop
circuits FF0, FF1, and FF2, the bias circuit 223, and the like.
[0300] The reference cell each includes a transistor 216, a
transistor 217, and a transistor 218. Transistors 216, 217, and 218
correspond to transistor 201, 202, and 203, respectively, and form
the same circuit configuration as that of the memory cell 200. It
is preferable that the transistors 216 and the transistors 218 be
formed using materials other than an oxide semiconductor, and the
transistors 217 be formed using an oxide semiconductor. In
addition, in the case where the memory cells include capacitors
205, it is preferable that the reference cell also include a
capacitor. Two output terminals of the bias circuit 223 are
connected to the bit line BL and the drain electrodes of the
transistors 218 included in the plurality of reference cells,
respectively, through switches. In addition, the output terminals
of the bias circuit 223 are connected to input terminals of the
sense amplifier circuit. An output terminal of the sense amplifier
circuit is connected to the flip-flop circuits FF0, FF1, and FF2.
Output terminals of the flip-flop circuits FF0, FF1, and FF2 are
connected to input terminals of the logic circuit 229. The bit line
BL and the drain electrodes of the transistors 218 included in the
reference cells are connected to a wiring V.sub.pc through
switches. Note that the switches are controlled by a read enable
signal (an RE signal).
[0301] The reading circuit 231 has a configuration in which a
comparison of the conductance of the memory cell with the
conductance of the plurality of the reference cells is performed by
comparing potential values output from the memory cells and the
reference cell. This configuration includes one sense amplifier
circuit. In this structure, the comparison is performed three times
in order to read out the four states. That is, the reading circuit
231 has a structure in which the comparison of the conductance of
the memory cell with the conductance of each of the three reference
cells is performed. The three comparisons are controlled by the
signals RE0, RE1, and RE2. V.sub.ref0, V.sub.ref1 and V.sub.ref2
are input to the gate electrode of the transistor 216 of the three
respective reference cells through the transistors 217. Before
reading, the signal .phi.A is asserted, all the transistors 217 are
turned on, and writing to the reference cells is performed. The
writing to the reference cells may be performed once before the
reading operation. Needless to say, writing may be performed once
when reading is performed several times, or may be performed every
time when reading is performed. In addition, the flip-flop circuits
FF0, FF1, and FF2 are controlled by the signals RE0, RE1, and RE2,
and store the value of the output signal SA_OUT of the sense
amplifier.
[0302] The values of the reference potentials are determined so as
to verify
V.sub.00<V.sub.ref0<V.sub.01<V.sub.ref1<V.sub.10<V.-
sub.ref2<V.sub.11. Thus, the four states can be read out from
the results of the three comparisons. The values of the flip-flop
circuits FF0, FF1, and FF2 are "0", "0", and "0", respectively in
the case of the data "00b". The values of the flip-flop circuits
FF0, FF1, and FF2 are "1", "0", and "0", respectively in the case
of the data "01b". The values of the flip-flop circuits FF0, FF1,
and FF2 are "1", "1", and "0", respectively in the case of the data
"10b". The values of the flip-flop circuits FF0, FF1, and FF2 are
"1", "1", and "1", respectively in the case of the data "11b". In
this manner, the state of the memory cell can be read out as a
3-bits digital signal. After that, with the use of the logic
circuit 229 which is represented in a logic value table shown in
Table 2, 2-bit data DO is generated and output from the reading
circuit.
[0303] Note that in the reading circuit illustrated in FIG. 29,
when the RE signal is de-asserted, the bit line BL and the
reference cells are connected to the wiring V.sub.pc so that
pre-charge is performed. When the RE signal is asserted, electrical
continuity between the bit line BL and the bias circuit 223 and
between the reference cells and the bias circuit 223 is
established.
[0304] Note that the pre-charge is not necessarily performed. In
this circuit, it is preferable that the circuits which generate two
signals input to the sense amplifier circuit have almost the same
structure. For example, it is preferable that the structure of the
transistors in the reference cells be the same as the structure of
the corresponding transistors in the memory cell. It is preferable
that the corresponding transistors in the bias circuit 223 and the
switch have the same structure.
[0305] A timing chart of the writing operation is the same as FIG.
25A. An example of a timing chart of the reading operation is shown
in FIG. 30. FIG. 30 shows a timing chart in the case where the data
"10b" is read out from the memory cell. The reference cell 225a,
the reference cell 225b, and the reference cell 225c are selected
and connected to the bias circuit 223 in respective terms in which
the signals RE0, RE1, and RE2 are asserted. Then, a result of
comparison in the sense amplifier circuit is stored in each of the
flip-flop circuits FF0, FF1, and FF2. In the case of the data of
the memory cell is "10b", the values of the flip-flop circuits FF0,
FF1, and FF2 are "1", "1", and "0", respectively. Note that the
first signal line S1 and the second signal line S2 have 0 [V].
[0306] Examples of specific operation potentials (voltages) are
described. For example, the following can be obtained: the
threshold voltage of the transistor 201 is approximately 0.3 V, the
power supply potential V.sub.DD is 2 V, V.sub.11 is 1.6 V, V.sub.10
is 1.2 V, V.sub.01 is 0.8 V, V.sub.00 is 0V, V.sub.ref0 is 0.6 V,
V.sub.ref1 is 1.0 V, and V.sub.ref2 is 1.4 V. The potential
V.sub.pc is preferably 0 V, for example.
[0307] Although, the first signal line S1 is arranged in the bit
line BL direction (the column direction) and the second signal line
S2 is arranged in the word line WL direction (the row direction) in
this embodiment, one embodiment of the present invention is not
limited thereto. For example, the first signal line S1 may be
arranged in the word line WL direction (the row direction) and the
second signal line S2 may be arranged in the bit line BL direction
(the column direction). In such a case, the driver circuit to which
the first signal line S1 is connected and the driver circuit to
which the second signal line S2 is connected may be arranged as
appropriate.
[0308] In this embodiment, operation of four-valued memory cells,
i.e., writing any of four different states to one memory cell or
reading any of four different states from one memory cell is
described. By appropriately changing the circuit configuration,
operation of n-valued memory cells, i.e., writing any of n
different states (n is an integer greater than or equal to 2) to
one memory cell or reading any of n different states from one
memory cell, can be performed.
[0309] For example, in eight-valued memory cells, storage capacity
becomes three times as large as two-valued memory cells. When
writing data, eight values of potentials corresponding to eight
states are prepared to decide a potential of a node A. When reading
data, seven reference potentials capable of distinguishing the
eight states are prepared. One sense amplifier is provided and
comparison is performed seven times, so that data can be read.
Further, the number of comparison times may be reduced to three
times by feeding back the result of comparison. In a reading method
for driving the source line SL, data can be read in one comparison
by providing seven sense amplifiers. Furthermore, a plurality of
sense amplifiers can be provided and comparison is performed plural
times.
[0310] Generally, in 2.sup.k-valued memory cells (k is an integer
greater than or equal to 1), memory capacity is k times as large as
that of two-valued memory cells. When writing data, 2.sup.k values
of writing potentials to decide a potential of a node A are
prepared, each value corresponding to one of 2.sup.k states. When
reading data, 2.sup.k-1 values of reference potentials enabling to
distinguish 2.sup.k states may be set up. One sense amplifier is
provided and comparison is performed 2.sup.k-1 times, so that data
can be read. Further, the number of comparison times may be reduced
to k times by feeding back the result of comparison. In a reading
method for driving the source line SL, data can be read in one
comparison by providing a 2.sup.k-1 number of sense amplifiers.
Furthermore, a plurality of sense amplifiers can be provided and
comparison is performed plural times.
[0311] The semiconductor device according to this embodiment can
store information for quite a long time because of low off-state
current characteristics of the transistor 202. That is, refresh
operation which is necessary in DRAM and the like is not needed, so
that power consumption can be reduced. In addition, the
semiconductor device of this embodiment can be used as a
substantially nonvolatile memory device.
[0312] Since writing information and the like are performed by
switching operation of the transistor 202, high voltage is not
needed and there is no problem of deterioration of elements.
Further, high-speed operation can be easily realized because
writing information and erasing information are performed by
turning transistors on or off. Furthermore, information can be
directly rewritten by controlling potentials input to transistors.
Accordingly, erasing operation which is necessary in flash memory
and the like is not needed, and decrease in operation speed due to
erasing operation can be prevented.
[0313] In addition, the transistor formed using a material other
than an oxide semiconductor can be operated at sufficient high
speed; therefore, by using the transistor, stored contents can be
read out at high speed.
[0314] The semiconductor device according to this embodiment is a
multivalued semiconductor device, so that storage capacity per area
can be increased. Therefore, the size of the semiconductor device
can be reduced and the semiconductor device can be highly
integrated. Additionally, potentials of a node which takes a
floating state when writing operation is performed can be directly
controlled; thus, threshold voltages of the semiconductor device
can be easily controlled with high accuracy which is required for a
multivalued memory. Therefore, verification of states after writing
data which is necessary to a multivalued-type memory can be
omitted, and in such a case, time required for writing data can be
shortened.
Embodiment 4
[0315] In this embodiment, a circuit configuration and operation of
a semiconductor device which is different from Embodiment 2 and
Embodiment 3 are described as an example.
[0316] FIG. 31 illustrates an example of a circuit diagram of a
memory cell included in the semiconductor device. A memory cell 240
illustrated in FIG. 31 includes a source line SL, a bit line BL, a
first signal line S1, a second signal line S2, a word line WL, a
transistor 201, a transistor 202, and a capacitor 204. The
transistor 201 is formed using a material other than an oxide
semiconductor, and the transistor 202 is formed using an oxide
semiconductor.
[0317] Here, a gate electrode of the transistor 201, one of a
source electrode and a drain electrode of the transistor 202, and
one of electrodes of the capacitor 204 are electrically connected
to one another. In addition, the source line SL and a source
electrode of the transistor 201 are electrically connected to each
other. The bit line BL and a drain electrode of the transistor 201
are electrically connected to each other. The first signal line S1
and the other of the source electrode and the drain electrode of
the transistor 202 are electrically connected to each other. The
second signal line S2 and a gate electrode of the transistor 202
are connected to each other. The word line WL and the other of the
electrodes of the capacitor 204 are electrically connected to each
other.
[0318] Next, operation of the memory cell 240 illustrated in FIG.
31 is described. Here, a four-valued memory cell is employed. Four
states of the memory cell 240 are data "00b", "01b", "10b", and
"11b", and potentials of a node A in the four states are V.sub.00,
V.sub.01, V.sub.10, and V.sub.11
(V.sub.00<V.sub.01<V.sub.10<V.sub.11), respectively.
[0319] When writing is performed to the memory cell 240, the source
line SL is set to 0 [V], the word line WL is set to 0 [V], the bit
line BL is set to 0 [V], and the second signal line S2 is set to
V.sub.DD. When the data "00b" is written, the first signal line S1
is set to V.sub.00 [V]. When the data "01b" is written, the first
signal line S1 is set to V.sub.01 [V]. When the data "10b" is
written, the first signal line S1 is set to V.sub.10 [V]. When the
data "11b" is written, the first signal line S1 is set to V.sub.11
[V]. At this time, the transistor 203 becomes in an off state and
the transistor 202 becomes in an on state. Note that when writing
data is finished, the second signal line S2 is set to 0 [V] so as
to turn off the transistor 202, before a potential of the first
signal line S1 is changed.
[0320] As a result, after the writing of the data "00b", "01b",
"10b", or "11b" (the potential of the word line WL is set to 0
[V]), the potential of a node connected to a gate electrode of the
transistor 201 (hereinafter, referred to as a node A) is
approximately V.sub.00 [V], V.sub.01 [V], V.sub.10 [V], or V.sub.11
[V], respectively. A charge is accumulated in the node A in
accordance with the potential of the first signal line S1, and
since off-state current of the transistor 202 is extremely small or
substantially 0, the potential of the gate electrode of the
transistor 201 is held for a long time.
[0321] Next, in the case where reading of the memory cell 240 is
performed, the source line SL is set to 0 [V], the second signal
line S2 is set to 0 [V], the first signal line S1 is set to 0 [V],
and a reading circuit connected to the bit line BL is in an
operation state. At this time, the transistor 202 is in an off
state.
[0322] The word line WL is set to V.sub.--WL [V]. The potential of
the node A of the memory cell 240 depends on the potential of the
word line WL. As the potential of the word line WL increases, the
potential of the node A of the memory cell 240 increases. For
example, the potential of the word line WL applied to the memory
cells in the four different states is changed from a low potential
to a high potential, the transistor 201 of the memory cell of the
data "11b" is turned on first, and then, the memory cell of the
data "10b", the memory cell of the data "01b", and the memory cell
of the data "00b" are turned on in this order. In other words, by
appropriately selecting the potential of the word line WL, the
states of the memory cells (that is, the data included in the
memory cells) can be distinguished. By appropriately selecting the
potential of the word line WL, the memory cell in which the
transistor 201 is in an on state is in a low resistance state, and
the memory cell in which the transistor 201 is in an off state is
in a high resistance state; therefore, when the resistance state is
distinguished by the reading circuit, the data "00b", "01b", "10b",
and "11b" can be read out.
[0323] FIG. 32 illustrates a block circuit diagram of another
example of a semiconductor device according to one embodiment of
the present invention, which includes memory capacity of m.times.n
bit.
[0324] The semiconductor device illustrated in FIG. 32 includes an
m number of the word lines WL, an m number of the second signal
lines S2, an n number of the bit lines BL, an n number of the first
signal lines S1, a memory cell array 210 in which the plurality of
memory cells 240 (1, 1) to 240 (m, n) are arranged in a matrix of m
cells in vertical (rows) by n cells in horizontal (columns) (both m
and n are natural numbers), and peripheral circuits such as reading
circuits 231, first signal line driver circuits 212, a driver
circuit 223 for the second signal lines and the word lines, and a
potential generating circuit 214. A refresh circuit or the like may
be provided as another peripheral circuit.
[0325] Each of the memory cells, for example, a memory cell 240 (i,
j) is considered (here, i is an integer greater than or equal to 1
and less than or equal to m and j is an integer greater than or
equal to 1 and less than or equal to n). The memory cell 240 (i, j)
is connected to a bit line BL (j), a first signal line S1 (j), a
word line WL (i), a second signal line S2 (i), and a source wiring
SL. A potential Vs (0V for example) is applied to the source wiring
SL. In addition, the bit lines BL (1) to BL (n) are connected to
the reading circuit 231, the first signal lines S1 (1) to S1 (n)
are connected to the first signal line driver circuits 212, the
word lines WL (1) to WL (m) and the second signal lines S2 (1) to
S2 (m) are connected to the driver circuit 223 for the second
signal lines S2 and the word lines WL.
[0326] Note that the configurations illustrated in FIG. 19 and FIG.
21 can be used for the configurations of the first signal line
driver circuit 212 and the potential generating circuit 214,
respectively.
[0327] FIG. 33 illustrates an example of the reading circuit. The
reading circuit includes a sense amplifier circuit, flip-flop
circuits, a bias circuit 224, and the like. The bias circuit 224 is
connected to the bit line BL through a switch. Further, the bias
circuit 224 is connected to an input terminal of the sense
amplifier circuit. A reference potential V.sub.r is input to the
other input terminal of the sense amplifier circuit. An output
terminal of the sense amplifier circuit is connected to input
terminals of flip-flop circuits FF0 and FF1. Note that the switch
is controlled by a read enable signal (an RE signal). The reading
circuit can read data out by reading out the voltage output by a
specified memory cell to the bit line BL to which it is connected.
The potential of the bit line BL is function of the conductance of
the memory cell. Note that reading of the conductance of the memory
cell indicates reading of an on or off state of the transistor 201
included in the memory cell.
[0328] The reading circuit illustrated in FIG. 33 includes the
sense amplifier circuit and performs comparison twice in order to
distinguish the four different states. The two comparisons are
controlled by signals RE0 and RE1. The flip-flop circuits FF0 and
FF1 are controlled by the signals RE0 and RE1, respectively, and
store the value of an output signal of the sense amplifier circuit.
An output DO[1] of the flip-flop circuit FF0 and an output DO[0] of
the flip-flop circuit FF1 are output from the reading circuit.
[0329] Note that in the reading circuit illustrated, when the RE
signal is de-asserted, the bit line BL is connected to the wiring
V.sub.pc and pre-charge is performed. When the RE signal is
asserted, electrical continuity between the bit line BL and the
bias circuit 224 is established. Note that pre-charge is not
necessarily performed.
[0330] FIG. 34 illustrates the driver circuit 223 for the second
signal lines S2 and the word lines WL, as another example.
[0331] In the driver circuit 223 for the second signal lines and
the word lines illustrated in FIG. 34, when an address signal ADR
is input, rows specified by the address (a selected row) are
asserted, and the other rows (non-selected rows) are de-asserted.
The second signal line S2 is connected to a decoder output when a
WE signal is asserted, and connected to GND when the WE signal is
de-asserted. The word line WL in the selected row is connected to
the output V.sub.--WL of a multiplexer (MUX3) and the word line WL
in the non-selected row is connected to GND. The multiplexer (MUX3)
selects any of the three values of reference potentials V.sub.ref0,
V.sub.ref1, and V.sub.ref2, and GND in response to the values of
the signals RE0, RE1, and DO0. The behavior of the multiplexer
(MUX3) is shown in Table 4.
TABLE-US-00004 TABLE 4 RE0 RE1 D0[1] VwL 0 0 * Corresponds to GND 1
0 * Corresponds to Vref1 0 1 0 Corresponds to Vref0 0 1 1
Corresponds to Vref2
[0332] The three values of reference potentials V.sub.ref0,
V.sub.ref1, and V.sub.ref2 (V.sub.ref0<V.sub.ref1<V.sub.ref2)
are described. In the case where V.sub.ref0 is selected as the
potential of the word line WL, a potential with which the
transistor 201 of the memory cell of the data "00b" is turned off
and the transistor 201 of the memory cell of the data "01b" is
turned on is selected as V.sub.ref0. In addition, in the case where
V.sub.ref1 is selected as the potential of the word line WL, a
potential with which the transistor 201 of the memory cell of the
data "01b" is turned off and the transistor 201 of the memory cell
of the data "10b" is turned on is selected as V.sub.ref1. In
addition, in the case where V.sub.ref2 is selected as the potential
of the word line WL, a potential with which the transistor 201 of
the memory cell of the data "10b" is turned off and the transistor
201 of the memory cell of the data "11b" is turned on is selected
as V.sub.ref2.
[0333] In the reading circuit, reading is performed by the two
comparisons. A first comparison is performed using V.sub.ref1. A
second comparison is performed using V.sub.ref2 when the value of
the flip-flop FF0 is "0" which results from comparison with the use
of V.sub.ref1, or using V.sub.era when the value of the flip-flop
FF0 is "1" which results from comparison with the use of
V.sub.ref1. In the above manner, the four states can be read out by
the two comparisons.
[0334] A timing chart of writing operation is the same as FIG. 25A.
An example of a timing chart of reading operation is shown in FIG.
35. FIG. 35 shows a timing chart in the case where the data "10b"
is read out from the memory cell. V.sub.ref1 and V.sub.ref2 are
input to the selected respective word lines WL, and the comparison
result in the sense amplifier circuit is stored in the flip-flop
circuits FF0 and FF1 in respective terms in which the signals RE0
and RE1 are asserted. In the case of the data of the memory cell is
"10b", the values of the flip-flop circuits FF0 and FF1 are "1" and
"0". Note that the first signal line S1 and the second signal line
S2 have 0 [V].
[0335] Examples of specific operation potentials (voltages) are
described. For example, the threshold voltage V.sub.th of the
transistor 201 is 2.2 V. The potential of the node A depends on
capacitance C1 between the word line WL and the node A and gate
capacitance C2 of the transistor 202, and here, for example,
C1/C2>>1 when the transistor 202 is in an off state, and
C1/C2=1 when the transistor 202 is in an on state. FIG. 36 shows a
relationship between the potential of the node A and the potential
of the word line WL in the case where the source line SL has 0 [V].
From FIG. 36, it is found that the reference potentials V.sub.ref0,
V.sub.ref1, and V.sub.ref2 are preferably 0.6 V, 1.0 V, and 1.4 V,
respectively in the case where when writing is performed, the
potential of the node A of the data "00b" is 0V, that of the data
"01b" is 0.8 V, that of the data "10b" is 1.2 V, and that of the
data "11b" is 1.6 V.
[0336] Note that the potential of the node A of the transistor 201
after the writing (the potential of the word line WL is 0 [V]) is
preferably lower than or equal to the threshold voltage of the
transistor 201.
[0337] Although, the first signal line S1 is arranged in the bit
line BL direction (the column direction) and the second signal line
S2 is arranged in the word line WL direction (the row direction) in
this embodiment, one embodiment of the present invention is not
limited thereto. For example, the first signal line S1 may be
arranged in the word line WL direction (the row direction) and the
second signal line S2 may be arranged in the bit line BL direction
(the column direction). In such a case, the driver circuit to which
the first signal line S1 is connected and the driver circuit to
which the second signal line S2 is connected may be arranged as
appropriate.
[0338] In this embodiment, operation of four-valued memory cells,
i.e., writing any of four different states to one memory cell or
reading any of four different states from one memory cell is
described. By appropriately changing the circuit configuration,
operation of n-valued memory cells, i.e., writing any of n
different states (n is an integer greater than or equal to 2) to
one memory cell or reading any of n different states from one
memory cell, can be performed.
[0339] For example, in eight-valued memory cells, storage capacity
becomes three times as large as that of two valued memory cells.
When writing data, eight values of potentials are set up to decide
a potential of a node A, each value corresponding to one of eight
states. When reading data, seven reference potentials enabling to
distinguish the eight states are set up. One sense amplifier is
provided and comparison is performed seven times, so that data can
be read. Further, the number of comparison times may be reduced to
three times by feeding back the result of comparison. In a reading
method for driving the source line SL, data can be read in one
comparison by providing seven sense amplifiers. Furthermore, a
plurality of sense amplifiers can be provided and comparison can be
performed plural times.
[0340] Generally, in 2.sup.k-valued memory cells (k is an integer
greater than or equal to 1), memory capacity is k times as large as
that of two-valued memory cells. When writing data, 2.sup.k values
of writing potentials to decide a potential of a node A are set up,
each value corresponding to one of 2.sup.k states. When reading
data, 2.sup.k-1 values of reference potentials enabling to
distinguish 2.sup.k states may be set up. One sense amplifier is
provided and comparison is performed 2.sup.k-1 times, so that data
can be read. Further, the number of comparison times may be reduced
to k times by feeding back the result of comparison. In a reading
method for driving the source line SL, data can be read in one
comparison by providing a 2.sup.k-1 number of sense amplifiers.
Furthermore, a plurality of sense amplifiers can be provided and
comparison can be performed plural times.
[0341] The semiconductor device according to this embodiment can
store information for a relatively long time because of low
off-state current characteristics of the transistor 202. That is,
refresh operation which is necessary in DRAM and the like is not
needed, so that power consumption can be reduced. In addition, the
semiconductor device of this embodiment can be used as a
substantial nonvolatile memory device.
[0342] Since writing information and the like are performed by
switching operation of the transistor 202, high voltage is not
needed and there is no problem of deterioration of elements.
Further, high-speed operation can be easily realized because
writing information and erasing information are performed by
turning transistors on or off. Furthermore, information can be
directly rewritten by controlling potentials input to transistors.
Accordingly, erasing operation which is necessary in flash memory
and the like is not needed, and decrease in operation speed due to
erasing operation can be prevented.
[0343] Further, the transistor formed using a material other than
an oxide semiconductor can be operated at sufficient high speed;
therefore, by using the transistor, stored contents can be read out
at high speed.
[0344] The semiconductor device according to this embodiment is a
multivalued semiconductor device, so that storage capacity per area
can be increased. Therefore, the size of the semiconductor device
can be reduced and the semiconductor device can be highly
integrated. Additionally, potentials of node which takes a floating
state when writing operation is performed can be directly
controlled; thus, threshold voltages of the semiconductor device
can be easily controlled with high accuracy which is required for a
multivalued memory. Therefore, verification of states after writing
data which is necessary to a multivalued-type memory can be
omitted, and in such a case, time required for writing data can be
shortened.
Embodiment 5
[0345] In this embodiment, examples of an electronic appliance in
which the semiconductor device obtained according to any of the
above embodiments is mounted are described with reference to FIGS.
37A to 37F. The semiconductor device obtained according to any of
the above embodiments can store information even without supply of
power. Degradation due to writing and erasing is not caused.
Moreover, operation speed thereof is high. Thus, with the use of
the semiconductor device, an electronic appliance having a new
structure can be provided. Note that the semiconductor device
according to any of the above embodiments is integrated and mounted
on a circuit board or the like to be mounted on an electronic
appliance.
[0346] FIG. 37A illustrates a laptop personal computer which
includes the semiconductor device according to any of the above
embodiments and includes a main body 301, a housing 302, a display
portion 303, a keyboard 304, and the like. When the semiconductor
device according to one embodiment of the present invention is
applied to the laptop personal computer, information can be stored
even without supply of power. In addition, degradation due to
writing and erasing is not caused. In addition, operation speed
thereof is high. Thus, it is preferable that the semiconductor
device according to one embodiment of the present invention be
applied to the laptop personal computer.
[0347] FIG. 37B illustrates a portable information terminal (PDA)
which includes the semiconductor device according to any of the
above embodiments and is provided with a main body 311 including a
display portion 313, an external interface 315, an operation button
314, and the like. In addition, a stylus 312 is included as an
accessory for operation. When the semiconductor device according to
one embodiment of the present invention is applied to the PDA,
information can be stored even without supply of power. In
addition, degradation due to writing and erasing is not caused. In
addition, operation speed thereof is high. Thus, it is preferable
that the semiconductor device according to one embodiment of the
present invention be applied to the PDA.
[0348] FIG. 37C illustrates an e-book reader 320 as an example of
electronic paper including the semiconductor device according to
any of the above embodiments. The e-book reader 320 includes two
housings, a housing 321 and a housing 323. The housing 321 and the
housing 323 are combined with a hinge 337 so that the e-book reader
320 can be opened and closed with the hinge 337 as an axis. With
such a structure, the e-book reader 320 can be used like a paper
book. When the semiconductor device according to one embodiment of
the present invention is applied to the electronic paper,
information can be stored even without supply of power. In
addition, degradation due to writing and erasing is not caused. In
addition, operation speed thereof is high. Thus, it is preferable
that the semiconductor device according to one embodiment of the
present invention be applied to the electronic paper.
[0349] A display portion 325 is incorporated in the housing 321 and
a display portion 327 is incorporated in the housing 323. The
display portion 325 and the display portion 327 may display one
image, or may display different images. When the display portions
325 and 327 display different images, for example, a display
portion on the right side (the display portion 325 in FIG. 37C) can
display text and a display portion on the left side (the display
portion 327 in FIG. 37C) can display graphics.
[0350] FIG. 37C illustrates an example in which the housing 321 is
provided with an operation portion and the like. For example, the
housing 321 is provided with a power button 331, operation keys
333, a speaker 335, and the like. Pages can be turned with the
operation keys 333. Note that a keyboard, a pointing device, or the
like may also be provided on the surface of the housing, on which
the display portion is provided. Furthermore, an external
connection terminal (an earphone terminal, a USB terminal, a
terminal that can be connected to various cables such as an AC
adapter and a USB cable, or the like), a recording medium insertion
portion, and the like may be provided on the back surface or the
side surface of the housing. Further, the e-book reader 320 may
have a function of an electronic dictionary.
[0351] The e-book reader 320 may be configured to transmit and
receive information wirelessly. Through wireless communication,
desired book data or the like can be purchased and downloaded from
an e-book server.
[0352] Note that the electronic paper can be applied to an
electronic appliance in any field which can display information.
For example, the electronic paper can be used for posters,
advertisements in vehicles such as trains, display in a variety of
cards such as credit cards, and the like in addition to e-book
readers.
[0353] FIG. 37D illustrates a mobile phone including the
semiconductor device according to any of the above embodiments. The
mobile phone includes two housings, the housing 340 and the housing
341. The housing 341 includes a display panel 342, a speaker 343, a
microphone 344, a pointing device 346, a camera lens 347, an
external connection terminal 348, and the like. The housing 341
includes a solar cell 349 for charging the mobile phone, an
external memory slot 350, and the like. In addition, an antenna is
incorporated in the housing 341. When the semiconductor device
according to one embodiment of the present invention is applied to
the mobile phone, information can be stored even without supply of
power. In addition, degradation due to writing and erasing is not
caused. In addition, operation speed thereof is high. Thus, it is
preferable that the semiconductor device according to one
embodiment of the present invention be applied to the mobile
phone.
[0354] The display panel 342 is provided with a touch panel
function. A plurality of operation keys 345 which is displayed as
images is illustrated by dashed lines in FIG. 37D. Note that the
mobile phone includes a boosting circuit for raising a voltage
output from the solar cell 349 to a voltage which is necessary for
each circuit. Further, in addition to the above structure, a
structure in which a noncontact IC chip, a small recording device,
or the like is incorporated may be employed.
[0355] A display direction of the display panel 342 is
appropriately changed in accordance with the usage mode. Further,
the camera lens 347 is provided on the same surface as the display
panel 342; thus, it can be used as a video phone. The speaker 343
and the microphone 344 can be used for videophone, recording,
playback, and the like without being limited to verbal
communication. Moreover, the housings 340 and 341 in a state where
they are developed as illustrated in FIG. 37D can be slid so that
one is lapped over the other; therefore, the size of the mobile
phone can be reduced, which makes the mobile phone suitable for
being carried.
[0356] The external connection terminal 348 can be connected to
various kinds of cables such as an AC adapter or a USB cable, which
enables charging and data communication. Moreover, by inserting a
recording medium into the external memory slot 350, the mobile
phone can deal with storing and moving a large capacity of data.
Further, in addition to the above functions, an infrared
communication function, a television reception function, and the
like may be provided.
[0357] FIG. 37E illustrates a digital camera including the
semiconductor device according to any of the above embodiments. The
digital camera includes a main body 361, a display portion (A) 367,
an eyepiece portion 363, an operation switch 364, a display portion
(B) 365, a battery 366, and the like. When the semiconductor device
according to one embodiment of the present invention is applied to
the digital camera, information can be stored even without supply
of power. In addition, degradation due to writing and erasing is
not caused. In addition, operation speed thereof is high. Thus, it
is preferable that the semiconductor device according to one
embodiment of the present invention be applied to the digital
camera.
[0358] FIG. 37F illustrates a television set including the
semiconductor device according to any of the above embodiments. In
the television set 370, a display portion 373 is incorporated in a
housing 371. The display portion 373 can display an image. Here,
the housing 371 is supported by a stand 375.
[0359] The television set 370 can be operated by an operation
switch of the housing 371 or a separate remote controller 380.
Channels and volume can be controlled by an operation key 379 of
the remote controller 380 so that an image displayed on the display
portion 373 can be controlled. Furthermore, the remote controller
380 may be provided with a display portion 377 for displaying
information output from the remote controller 380. When the
semiconductor device according to one embodiment of the present
invention is applied to the television set, information can be
stored even without supply of power. In addition, degradation due
to writing and erasing is not caused. In addition, operation speed
thereof is high. Thus, it is preferable that the semiconductor
device according to one embodiment of the present invention be
applied to the television set.
[0360] Note that the television set 370 is preferably provided with
a receiver, a modem, and the like. With the receiver, a general
television broadcast can be received. Furthermore, when the
television set 370 is connected to a communication network by wired
or wireless connection through the modem, one-way (from a
transmitter to a receiver) or two-way (between a transmitter and a
receiver, between receivers, or the like) information communication
can be performed.
[0361] The methods and structures described in this embodiment can
be combined as appropriate with any of the methods and structures
described in the other embodiments.
[0362] This application is based on Japanese Patent Application
serial no. 2009-264623 filed with Japan Patent Office on Nov. 20,
2009, the entire contents of which are hereby incorporated by
reference.
* * * * *