U.S. patent application number 17/172228 was filed with the patent office on 2021-08-05 for semiconductor device and manufacturing method thereof.
The applicant listed for this patent is Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Kengo AKIMOTO, Hidekazu MIYAIRI, Akiharu MIYANAGA, Kojiro SHIRAISHI, Shunpei YAMAZAKI.
Application Number | 20210242346 17/172228 |
Document ID | / |
Family ID | 1000005524869 |
Filed Date | 2021-08-05 |
United States Patent
Application |
20210242346 |
Kind Code |
A1 |
YAMAZAKI; Shunpei ; et
al. |
August 5, 2021 |
SEMICONDUCTOR DEVICE AND MANUFACTURING METHOD THEREOF
Abstract
To offer a semiconductor device including a thin film transistor
having excellent characteristics and high reliability and a method
for manufacturing the semiconductor device without variation. The
summary is to include an inverted-staggered (bottom-gate structure)
thin film transistor in which an oxide semiconductor film
containing In, Ga, and Zn is used for a semiconductor layer and a
buffer layer is provided between the semiconductor layer and source
and drain electrode layers. An ohmic contact is formed by
intentionally providing a buffer layer containing In, Ga, and Zn
and having a higher carrier concentration than the semiconductor
layer between the semiconductor layer and the source and drain
electrode layers.
Inventors: |
YAMAZAKI; Shunpei;
(Setagaya, JP) ; MIYAIRI; Hidekazu; (Isehara,
JP) ; MIYANAGA; Akiharu; (Hadano, JP) ;
AKIMOTO; Kengo; (Atsugi, JP) ; SHIRAISHI; Kojiro;
(Isehara, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Semiconductor Energy Laboratory Co., Ltd. |
Atsugi-shi |
|
JP |
|
|
Family ID: |
1000005524869 |
Appl. No.: |
17/172228 |
Filed: |
February 10, 2021 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
16783577 |
Feb 6, 2020 |
10930792 |
|
|
17172228 |
|
|
|
|
15584264 |
May 2, 2017 |
10559695 |
|
|
16783577 |
|
|
|
|
12511252 |
Jul 29, 2009 |
9666719 |
|
|
15584264 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 29/66969 20130101;
H01L 27/1225 20130101; H01L 29/7869 20130101; H01L 29/78618
20130101; H01L 29/42384 20130101 |
International
Class: |
H01L 29/786 20060101
H01L029/786; H01L 29/423 20060101 H01L029/423; H01L 29/66 20060101
H01L029/66 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 2008 |
JP |
2008-197137 |
Claims
1. (canceled)
2. A semiconductor device comprising: a gate electrode; an oxide
semiconductor film over the gate electrode; an insulating film over
and in contact with a channel formation region of the oxide
semiconductor film, the insulating film comprising silicon oxide; a
first layer over and in contact with the oxide semiconductor film;
a second layer over and in contact with the oxide semiconductor
film; a source electrode over and in contact with the first layer,
the source electrode being electrically connected to the oxide
semiconductor film; and a drain electrode over and in contact with
the second layer, the drain electrode being electrically connected
to the oxide semiconductor film, wherein the oxide semiconductor
film comprises indium, gallium, and zinc, wherein the insulating
film does not contact the source electrode or the drain electrode,
wherein the source electrode or the drain electrode comprises a
first conductive layer and a second conductive layer over the first
conductive layer, and wherein, in a cross sectional view in a
channel length direction, the first layer or the second layer
comprises a first region which is overlapped by the second
conductive layer and a second region which extends beyond an end
portion of the second conductive layer in the channel length
direction.
3. The semiconductor device according to claim 2, wherein the first
layer or the second layer is in contact with a side surface of the
oxide semiconductor film.
4. The semiconductor device according to claim 2, wherein a width
of the insulating film in the channel length direction is smaller
than a width of the gate electrode in the channel length
direction.
5. The semiconductor device according to claim 2, wherein an end
portion of the first conductive layer extends beyond the end
portion of the second conductive layer in the channel length
direction.
6. The semiconductor device according to claim 2, wherein the first
conductive layer comprises titanium.
7. The semiconductor device according to claim 2, wherein the
source electrode or the drain electrode further comprises a third
conductive layer over the second conductive layer.
8. The semiconductor device according to claim 2, wherein the first
layer or the second layer is a buffer layer including an impurity
imparting n-type conductivity.
9. The semiconductor device according to claim 2, wherein the first
layer or the second layer comprises an oxide semiconductor
comprising indium, gallium, and zinc.
10. A semiconductor device comprising: a gate electrode; an oxide
semiconductor film over the gate electrode; an insulating film over
and in contact with a channel formation region of the oxide
semiconductor film, the insulating film comprising silicon oxide; a
first layer over and in contact with the oxide semiconductor film;
a second layer over and in contact with the oxide semiconductor
film; a source electrode over and in contact with the first layer,
the source electrode being electrically connected to the oxide
semiconductor film; and a drain electrode over and in contact with
the second layer, the drain electrode being electrically connected
to the oxide semiconductor film, wherein the oxide semiconductor
film comprises indium, gallium, and zinc, wherein the oxide
semiconductor film comprises a first region overlapped by the
source electrode or the drain electrode and a second region in
contact with the insulating film, wherein a thickness of the first
region of the oxide semiconductor film is smaller than a thickness
of the second region of the oxide semiconductor film, wherein the
insulating film does not contact the source electrode or the drain
electrode, wherein the source electrode or the drain electrode
comprises a first conductive layer and a second conductive layer
over the first conductive layer, and wherein, in a cross sectional
view in a channel length direction, the first layer or the second
layer comprises a first region which is overlapped by the second
conductive layer and a second region which extends beyond an end
portion of the second conductive layer in the channel length
direction.
11. The semiconductor device according to claim 10, wherein the
first layer or the second layer is in contact with a side surface
of the oxide semiconductor film.
12. The semiconductor device according to claim 10, wherein a width
of the insulating film in the channel length direction is smaller
than a width of the gate electrode in the channel length
direction.
13. The semiconductor device according to claim 10, wherein an end
portion of the first conductive layer extends beyond the end
portion of the second conductive layer in the channel length
direction.
14. The semiconductor device according to claim 10, wherein the
first conductive layer comprises titanium.
15. The semiconductor device according to claim 10, wherein the
source electrode or the drain electrode further comprises a third
conductive layer over the second conductive layer.
16. The semiconductor device according to claim 10, wherein the
first layer or the second layer is a buffer layer including an
impurity imparting n-type conductivity.
17. The semiconductor device according to claim 10, wherein the
first layer or the second layer comprises an oxide semiconductor
comprising indium, gallium, and zinc.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to a semiconductor device
having a circuit including a thin film transistor (hereinafter
referred to as a TFT) using an oxide semiconductor film for a
channel formation region and a method for manufacturing the
semiconductor device. For example, the present invention relates to
an electronic appliance mounted with an electro-optic device
typified by a liquid crystal display panel or a light-emitting
display device having an organic light-emitting element as a
component.
[0002] Note that the semiconductor devices in this specification
refer to all the devices which can operate by using semiconductor
characteristics, and an electro-optic device, a semiconductor
circuit, and an electronic appliance are all included in the
semiconductor devices.
2. Description of the Related Art
[0003] In recent years, active matrix display devices (such as
liquid crystal display devices, light-emitting display devices, and
electrophoretic display devices) in each of which a switching
element including a thin film transistor (TFT) is provided in each
of display pixels arranged in matrix have been actively developed.
An active matrix display device is advantageous because a switching
element is provided in each pixel (or each dot) and thus lower
voltage driving is possible as compared to a passive matrix display
device in the case where pixel density is increased.
[0004] In addition, a technique applied to an electronic device or
an optical device, in which a thin film transistor (TFT) or the
like is formed using an oxide semiconductor film for a channel
formation region, has attracted attention. As an oxide
semiconductor film, a TFT using ZnO and a TFT using
InGaO.sub.3(ZnO).sub.m are given for example. Patent Documents 1
and 2 and the like disclose a technique applied to a switching
element or the like of an image display device, in which a TFT
formed using such an oxide semiconductor film is formed over a
substrate having a light-transmitting property.
REFERENCES
[Patent Document 1] Japanese Published Patent Application No.
2007-123861
[Patent Document 2] Japanese Published Patent Application No.
2007-96055
SUMMARY OF THE INVENTION
[0005] A thin film transistor using an oxide semiconductor film for
a channel formation region is required to operate rapidly, be
manufactured through a relatively simple process, and be
sufficiently reliable.
[0006] In forming a thin film transistor, a metal material having
low resistance is used for source and drain electrode layers. In
particular, in the case of manufacturing display devices performing
large-area display, the problem of signal delay due to wiring
resistance becomes significant. Therefore, a metal material having
low electric resistance is desirably used as a material of a wiring
or an electrode. On the other hand, in the case of the thin film
transistor structure in which an oxide semiconductor film is
directly in contact with source and drain electrode layers formed
using a metal material having low electric resistance, contact
resistance might be high. As one of factors causing high contact
resistance, the following condition is given: Schottky junction is
formed at the interface between the oxide semiconductor film and
the source and drain electrode layers.
[0007] In addition, capacitance is caused in a portion where the
oxide semiconductor film and the source and drain electrode layers
are directly in contact with each other, and frequency
characteristics (referred to as f characteristics) are low, which
might hinder rapid operation of the thin film transistor.
[0008] An object of an embodiment of the present invention is to
provide a thin film transistor using an oxide semiconductor film
containing indium (In), gallium (Ga), and zinc (Zn), in which
contact resistance between an oxide semiconductor layer and source
and drain electrode layers is reduced, and a method for
manufacturing the thin film transistor.
[0009] An object is to improve operation characteristics and
reliability of a thin film transistor using an oxide semiconductor
film containing In, Ga, and Zn.
[0010] An object is to reduce variations in electric
characteristics of thin film transistors each using an oxide
semiconductor film containing In, Ga, and Zn. In particular, in a
liquid crystal display device, in the case where there are large
variations between elements, display unevenness due to variations
in TFT characteristics might be caused.
[0011] Also in a display device having a light-emitting element, in
the case where variations in on current (Ion) of TFTs (TFTs each
provided in a driver circuit or TFTs each provided in a pixel and
supplying a current to a light-emitting element) arranged so that a
certain current flows to pixel electrodes are large, luminance of a
display screen might be varied. As described above, an object of an
embodiment of the present invention is to achieve at least one of
the above objects.
[0012] The summary of an embodiment of the present invention is
that an inverted-staggered (bottom-gate) thin film transistor is
included in which an oxide semiconductor layer containing In, Ga,
and Zn is used as a semiconductor layer and a buffer layer is
provided between the semiconductor layer and source and drain
electrode layers.
[0013] In this specification, a semiconductor layer formed using an
oxide semiconductor film containing In, Ga, and Zn is also referred
to as an "IGZO semiconductor layer".
[0014] An ohmic contact of an IGZO semiconductor layer and source
electrode layers is necessary and the contact resistance
therebetween is desirably as low as possible. Similarly, an ohmic
contact of the IGZO semiconductor layer and a drain electrode layer
is necessary and the contact resistance therebetween is desirably
as low as possible.
[0015] Therefore, an ohmic contact is formed by intentionally
providing a buffer layer having a higher carrier concentration than
the IGZO semiconductor layer between the IGZO semiconductor layer
and the source and drain electrode layers.
[0016] For the buffer layer, an oxide semiconductor film having
n-type conductivity and containing In, Ga, and Zn is used. An
impurity element imparting n-type conductivity may be included in
the buffer layer. As the impurity element, for example, magnesium,
aluminum, titanium, scandium, yttrium, zirconium, hafnium, boron,
thallium, germanium, tin, lead, or the like can be used. In the
case where magnesium, aluminum, titanium, or the like is contained
in the buffer layer, there is an effect of blocking oxygen, and the
like, so that the oxygen concentration of a semiconductor layer can
be maintained within an optimal range by heat treatment or the like
after film formation.
[0017] The buffer layer functions as an n.sup.+ layer and can also
be referred to as a drain region or a source region.
[0018] A semiconductor device according to an embodiment of the
present invention includes a thin film transistor including a gate
electrode; a gate insulating film covering the gate electrode; an
IGZO semiconductor layer over the gate electrode with the gate
insulating film interposed therebetween; a channel protective layer
in a region overlapping with a channel formation region of the IGZO
semiconductor layer; source and drain electrode layers over the
IGZO semiconductor layer; and a buffer layer between the
semiconductor layer and the source and drain electrode layers. The
carrier concentration of the buffer layer is higher than that of
the IGZO semiconductor layer. The IGZO semiconductor layer is
electrically connected to the source and drain electrode layers
through the buffer layer.
[0019] In the above structure, a second buffer layer having a
carrier concentration higher than that of the semiconductor layer
and lower than that of the buffer layer may be provided between the
semiconductor layer and the buffer layer. The second buffer layer
functions as an n.sup.- layer.
[0020] The oxide semiconductor film (IGZO film) containing In, Ga,
and Zn has such a characteristic that the increase in carrier
concentration leads to increase in hole mobility. Thus, the
relation between a carrier concentration and hole mobility of the
oxide semiconductor film containing In, Ga, and Zn is as shown in
FIG. 29. In an embodiment of the present invention, the carrier
concentration of the IGZO film suitable for a channel of the
semiconductor layer is preferably less than 1.times.10.sup.17
atoms/cm.sup.3 (more preferably 1.times.10.sup.11 atoms/cm.sup.3 or
more) (channel concentration range 1), and the carrier
concentration of the IGZO film suitable for the buffer layer is
preferably 1.times.10.sup.18 atoms/cm.sup.3 or more (more
preferably 1.times.10.sup.22 atoms/cm.sup.3 or less) (buffer layer
concentration range 2). The above carrier concentration of the IGZO
film is a value in the state where the temperature is room
temperature and source, drain, and gate voltages are not applied,
in the case where the IGZO film is used for the semiconductor
layer.
[0021] When the carrier concentration of the IGZO film for the
channel exceeds the above range (the channel concentration range
1), the thin film transistor might be normally on.
[0022] Note that the carrier concentration and the hole mobility of
the IGZO film can be measured by hole effect measurement. As an
example of an equipment used for hole effect measurement, the
specific resistance/hole measuring system ResiTest 8310
(manufactured by TOYO Corporation) can be given. With the specific
resistance/hole measuring system ResiTest 8310, the direction and
strength of a magnetic field are changed in a certain cycle and in
synchronization therewith, only a hole electromotive voltage caused
in a sample is detected, so that AC (alternate current) hole
measurement can be performed. Even in the case of a material with
low mobility and high resistivity, a hole electromotive voltage can
be detected.
[0023] In the above structure, the source and drain electrode
layers preferably contain titanium. For example, a multilayer film
of a titanium film, an aluminum film, and a titanium film has low
resistance and hillocks are unlikely to be generated in the
aluminum film.
[0024] The thin film transistor according to an embodiment of the
present invention is provided with a channel protective layer;
therefore, a region on a side reverse of the side of a surface of
an IGZO semiconductor layer in contact with a gate insulating film,
a so-called back channel, can be protected from damages (such as
reduction in film thickness due to plasma or an etchant at the time
of etching and oxidation) in the process. Thus, reliability of a
semiconductor device can be improved.
[0025] A method for manufacturing a semiconductor device, according
to an embodiment of the present invention, includes a step of
forming a gate electrode layer over a substrate; a step of forming
a gate insulating film over the gate electrode layer; a step of
forming an IGZO semiconductor layer over the gate insulating film;
a step of forming a channel protective layer over the IGZO
semiconductor layer and in a region overlapping with a channel
formation region; a step of forming a pair of buffer layers having
n-type conductivity over the IGZO semiconductor layer; and a step
of forming source and drain electrode layers over the buffer
layers. The pair of buffer layers having n-type conductivity are
formed using an oxide semiconductor layer containing In, Ga, and
Zn. The carrier concentration of the buffer layers is higher than
that of the IGZO semiconductor layer. The IGZO semiconductor layer
and the source and drain electrode layers are electrically
connected to each other through the buffer layers.
[0026] Further, by successively forming the gate insulating film,
the semiconductor film, and the channel protective layer without
exposing them to the air, productivity can be increased and in
addition, interfaces of the stacked films can be formed without
being contaminated by an atmospheric component such as water vapor
and impurity elements and dusts floating in the air. Thus,
variations in thin film transistor characteristics can be
reduced.
[0027] In other words, by successively forming the gate insulating
film, the oxide semiconductor film containing In, Ga, and Zn to be
the semiconductor film, and the insulating film to be the channel
protective layer without exposing them to the air, productivity can
be increased and in addition, interfaces of the stacked films can
be formed without being contaminated by an atmospheric component
such as water vapor and impurity elements and dusts floating in the
air. Thus, variations in thin film transistor characteristics can
be reduced.
[0028] Successive film formation in this specification means that a
series of steps from a first film formation step by a sputtering
method to a second film formation step by a sputtering method are
performed by controlling an atmosphere in which a process substrate
is provided so that it is constantly in vacuum or an inert gas
atmosphere (a nitrogen atmosphere or a rare gas atmosphere) without
being exposed to a contaminated atmosphere such as the air. By the
successive film formation, film formation can be performed without
reattachment of moisture or the like to the process substrate,
which has been cleaned.
[0029] Performing the series of steps from the first film formation
step to the second film formation step in the same chamber is in
the range of the successive film formation in this
specification.
[0030] Further, performing the series of steps from the first film
formation step to the second film formation step in different
chambers, in which after the first film formation step is
completed, a substrate is transferred between the chambers without
being exposed to the air and the second film formation is
performed, is also in the range of the successive film formation in
this specification.
[0031] Note that the case where there is a substrate transfer step,
an alignment step, a slow cooling step, a step of heating or
cooling a substrate so that the temperature of the substrate is
suitable to the second film formation step, or the like between the
first film formation step and the second film formation step is
also in the range of the successive film formation in this
specification.
[0032] However, the case where there is a step in which liquid is
used, such as a cleaning step, wet etching, or resist formation,
between the first film formation step and the second film formation
step is not in the range of the successive film formation in this
specification.
[0033] Further, by forming the gate insulating film, the
semiconductor layer, and the channel protective layer in an oxygen
atmosphere (or an atmosphere containing oxygen at 90% or more and a
rare gas (such as argon) at 10% or less), reduction in reliability
due to deterioration, a shift of thin film transistor
characteristics to the normally-on side, and the like can be
suppressed. Further, the buffer layer having n-type conductivity is
preferably formed in a rare gas (such as argon) atmosphere.
[0034] A method for manufacturing a semiconductor device, according
to an embodiment of the present invention, includes a step of
forming a gate electrode layer over a substrate; a step of forming
a gate insulating film over the gate electrode layer; a step of
forming an IGZO semiconductor layer over the gate insulating film;
a step of forming a channel protective layer over the IGZO
semiconductor layer and in a region overlapping with a channel
formation region; a step of forming a pair of buffer layers having
n-type conductivity over the IGZO semiconductor layer; and a step
of forming source and drain electrode layers over the buffer
layers. The pair of buffer layers having n-type conductivity are
formed using an oxide semiconductor layer containing In, Ga, and
Zn. The carrier concentration of the buffer layers is higher than
that of the IGZO semiconductor layer. The IGZO semiconductor layer
and the source and drain electrode layers are electrically
connected to each other through the buffer layers. The gate
insulating film, the semiconductor layer, and the channel
protective layer are successively formed without being exposed to
the air.
[0035] A semiconductor device according to an embodiment of the
present invention is a thin film transistor including a gate
electrode; a gate insulating film covering the gate electrode; a
semiconductor layer over the gate electrode with the gate
insulating film interposed therebetween; a channel protective layer
in a region overlapping with a channel formation region of the
semiconductor layer; source and drain electrode layers over the
semiconductor layer; and a buffer layer between the semiconductor
layer and the source and drain electrode layers. The semiconductor
layer and the buffer layer are formed using an oxide semiconductor
containing indium, gallium, and zinc. The carrier concentration of
the buffer layer is higher than that of the semiconductor layer.
The semiconductor layer is electrically connected to the source and
drain electrode layers through the buffer layer.
[0036] In the semiconductor device, the buffer layer may include
n-type impurity.
[0037] In the semiconductor device, the carrier concentration of
the semiconductor layer may be less than 1.times.10.sup.17
atoms/cm.sup.3, and the carrier concentration of the buffer layer
may be 1.times.10.sup.18 atoms/cm.sup.3 or more.
[0038] In the semiconductor device, a second buffer layer having a
carrier concentration higher than that of the semiconductor layer
and lower than that of the buffer layer may be provided between the
semiconductor layer and the buffer layer.
[0039] In the semiconductor device, the source and drain electrode
layers may contain titanium.
[0040] A method for manufacturing a semiconductor device, according
to an embodiment of the present invention, includes a step of
forming a gate electrode layer over a substrate; a step of forming
a gate insulating film over the gate electrode layer; a step of
forming a semiconductor layer over the gate insulating film; a step
of forming a channel protective layer over the semiconductor layer
and in a region overlapping with a channel formation region; a step
of forming a pair of buffer layers having n-type conductivity over
the semiconductor layer; and a step of forming source and drain
electrode layers over the buffer layers. The semiconductor layer
and the buffer layers having n-type conductivity are formed using
an oxide semiconductor layer containing In, Ga, and Zn. The carrier
concentration of the buffer layers is higher than that of the
semiconductor layer. The semiconductor layer and the source and
drain electrode layers are electrically connected to each other
through the buffer layers.
[0041] A method for manufacturing a semiconductor device, according
to an embodiment of the present invention, includes a step of
forming a gate electrode layer over a substrate; a step of forming
a gate insulating film over the gate electrode layer; a step of
forming a semiconductor layer over the gate insulating film; a step
of forming a channel protective layer over the semiconductor layer
and in a region overlapped with a channel formation region; a step
of forming a pair of buffer layers having n-type conductivity over
the semiconductor layer; and a step of forming source and drain
electrode layers over the buffer layers. The semiconductor layer
and the buffer layers are formed using an oxide semiconductor layer
containing indium, gallium, and zinc. The carrier concentration of
the buffer layers is higher than that of the semiconductor layer.
The semiconductor layer and the source and drain electrode layers
are electrically connected to each other through the buffer layers.
The gate insulating film, the semiconductor layer, and the channel
protective layer are successively formed without being exposed to
the air.
[0042] In the method for manufacturing a semiconductor device, the
gate insulating film, the semiconductor layer, and the channel
protective layer may be formed by a sputtering method.
[0043] In the method for manufacturing a semiconductor device, the
gate insulating film, the semiconductor layer, and the channel
protective layer may be formed in an oxygen atmosphere.
[0044] In the method for manufacturing a semiconductor device, the
buffer layer may be formed in a rare gas atmosphere.
[0045] In the method for manufacturing a semiconductor device, the
carrier concentration of the semiconductor layer may be less than
1.times.10.sup.17 atoms/cm.sup.3, and the carrier concentration of
the buffer layer may be 1.times.10.sup.18 atoms/cm.sup.3 or
more.
[0046] In the method for manufacturing a semiconductor device, the
buffer layer may be formed containing magnesium, aluminum, or
titanium.
[0047] According to an embodiment of the present invention, a thin
film transistor having a small amount of photocurrent, low
parasitic capacitance, a high on-off ratio, and favorable dynamic
characteristics (f characteristics) can be manufactured.
Accordingly, a semiconductor device including a thin film
transistor having excellent electric characteristics and high
reliability can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] In the accompanying drawings:
[0049] FIGS. 1A and 1B are views illustrating a semiconductor
device according to an embodiment of the present invention;
[0050] FIGS. 2A to 2D are views illustrating a method for
manufacturing a semiconductor device, according to an embodiment of
the present invention;
[0051] FIGS. 3A1-3A2, 3B to 3D are views illustrating a method for
manufacturing a semiconductor device, according to an embodiment of
the present invention;
[0052] FIGS. 4A to 4D are views illustrating a method for
manufacturing a semiconductor device, according to an embodiment of
the present invention;
[0053] FIGS. 5A and 5B are views illustrating a semiconductor
device according to an embodiment of the present invention;
[0054] FIGS. 6A and 6B are views illustrating a semiconductor
device according to an embodiment of the present invention;
[0055] FIGS. 7A and 7B are views illustrating a semiconductor
device according to an embodiment of the present invention;
[0056] FIG. 8 is a view illustrating a semiconductor device
according to an embodiment of the present invention;
[0057] FIG. 9 is a top schematic diagram illustrating a
multi-chamber manufacturing apparatus;
[0058] FIGS. 10A and 10B are block diagrams each illustrating a
display device;
[0059] FIG. 11 is a diagram illustrating a structure of a signal
line driver circuit;
[0060] FIG. 12 is a timing chart showing operation of a signal line
driver circuit;
[0061] FIG. 13 is a timing chart showing operation of a signal line
driver circuit;
[0062] FIG. 14 is a diagram illustrating a structure of a shift
register;
[0063] FIG. 15 is a diagram illustrating a connecting structure of
flip-flops illustrated in FIG. 14;
[0064] FIGS. 16A and 16B are views illustrating a liquid crystal
display device to which an embodiment of the present invention is
applied;
[0065] FIG. 17 is a view illustrating electronic paper to which an
embodiment of the present invention is applied;
[0066] FIGS. 18A and 18B are views illustrating a light-emitting
display device to which an embodiment of the present invention is
applied;
[0067] FIG. 19 is a diagram illustrating a light-emitting display
device to which an embodiment of the present invention is
applied;
[0068] FIGS. 20A to 20C are views each illustrating a
light-emitting display device to which an embodiment of the present
invention is applied;
[0069] FIGS. 21A and 21B are views illustrating a light-emitting
display device to which an embodiment of the present invention is
applied;
[0070] FIGS. 22A1, 22A2, and 22B are views each illustrating a
liquid crystal display device to which an embodiment of the present
invention is applied;
[0071] FIG. 23 is a view illustrating a liquid crystal display
device to which an embodiment of the present invention is
applied;
[0072] FIGS. 24A and 24B are diagrams each illustrating an
electronic appliance to which an embodiment of the present
invention is applied;
[0073] FIG. 25 is a diagram illustrating an electronic appliance to
which an embodiment of the present invention is applied;
[0074] FIGS. 26A and 26B are diagrams each illustrating an
electronic appliance to which an embodiment of the present
invention is applied;
[0075] FIG. 27 is a diagram illustrating an electronic appliance to
which an embodiment of the present invention is applied;
[0076] FIG. 28 is a diagram illustrating an electronic appliance to
which an embodiment of the present invention is applied; and
[0077] FIG. 29 is a graph showing a relation between a carrier
concentration and hole mobility.
DETAILED DESCRIPTION OF THE INVENTION
[0078] The embodiments of the present invention will be described
with reference to the accompanying drawings. However, the present
invention is not limited to the following description, and it will
be easily understood by those skilled in the art that various
changes and modifications can be made to the modes and their
details without departing from the spirit and scope of the present
invention. Therefore, the present invention should not be construed
as being limited to the description in the following embodiments.
Note that a common reference numeral refers to the same part or a
part having a similar function throughout the drawings in the
structure of the present invention described below, and the
description thereof is omitted.
Embodiment 1
[0079] In this embodiment, a thin film transistor and a
manufacturing process thereof will be described with reference to
FIGS. 1A and 1B and FIGS. 2A to 2D.
[0080] FIGS. 1A and 1B illustrate a thin film transistor having a
bottom gate structure according to this embodiment. FIG. 1A is a
plan view and FIG. 1B is a cross-sectional view taken along line
A1-A2 in FIG. 1A. In the thin film transistor illustrated in FIGS.
1A and 1B, a gate electrode 101 is formed over a substrate 100; a
gate insulating film 102 is formed over the gate electrode 101; an
amorphous oxide semiconductor layer 103 functioning as a channel
formation region is formed over the gate electrode 101 with the
gate insulating film 102 interposed therebetween; a channel
protective layer 106 is formed in a region overlapping with the
channel formation region of the amorphous oxide semiconductor layer
103; buffer layers 104a and 104b are formed over the amorphous
oxide semiconductor layer 103; and source and drain electrode
layers 105a and 105b are formed in contact with the buffer layers
104a and 104b.
[0081] An oxide semiconductor containing In, Ga, and Zn is used for
the semiconductor layer 103, and the buffer layers 104a and 104b
having a higher carrier concentration than the semiconductor layer
103 are intentionally provided between the semiconductor layer 103
and the source and drain electrode layers 105a and 105b, whereby an
ohmic contact is formed.
[0082] The buffer layers 104a and 104b are formed by using an oxide
semiconductor having n-type conductivity and containing In, Ga, and
Zn. An impurity element imparting n-type conductivity may be
included in the buffer layer. As the impurity element, for example,
magnesium, aluminum, titanium, scandium, yttrium, zirconium,
hafnium, boron, thallium, germanium, tin, lead, or the like can be
used. In the case where magnesium, aluminum, titanium, or the like
is contained in the buffer layer, there is an effect of blocking
oxygen and the like, so that the oxygen concentration of the
semiconductor layer 103 can be maintained within an optimal range
by heat treatment or the like after film formation.
[0083] The buffer layers 104a and 104b function as n.sup.+ layers
and can also be referred to as source and drain regions.
[0084] A method for manufacturing the thin film transistor
illustrated in FIGS. 1A and 1B is described with reference to FIGS.
2A to 2D. First, the gate electrode 101, the gate insulating film
102, a semiconductor film 133, the channel protective layer 106 are
formed over the substrate 100 (see FIG. 2A).
[0085] As the substrate 100, any of the following substrates can be
used: non-alkaline glass substrates manufactured by a fusion method
or a float method, such as a barium borosilicate glass substrate,
an aluminoborosilicate glass substrate, an aluminosilicate glass
substrate, and the like; ceramic substrates; plastic substrates
having heat resistance high enough to withstand a process
temperature of this manufacturing process; and the like.
Alternatively, a metal substrate of a stainless alloy, which is
provided with an insulating film over its surface, may be used. In
the case where the substrate 100 is mother glass, the substrate may
have any of the following sizes: the first generation (320
mm.times.400 mm), the second generation (400 mm.times.500 mm), the
third generation (550 mm.times.650 mm), the fourth generation (680
mm.times.880 mm or 730 mm.times.920 mm), the fifth generation (1000
mm.times.1200 mm or 1100 mm.times.1250 mm), the sixth generation
(1500 mm.times.1800 mm), the seventh generation (1900 mm.times.2200
mm), the eighth generation (2160 mm.times.2460 mm), the ninth
generation (2400 mm.times.2800 mm or 2450 mm.times.3050 mm), the
tenth generation (2950 mm.times.3400 mm), and the like.
[0086] Further, an insulating film may be provided as a base film
over the substrate 100. The base film can be formed to have a
single-layer structure or a layered structure of a silicon oxide
film, a silicon nitride film, a silicon oxynitride film, and/or a
silicon nitride oxide film by a CVD method, a sputtering method, or
the like.
[0087] The gate electrode 101 is formed using a metal material. As
the metal material, aluminum, chromium, titanium, tantalum,
molybdenum, copper, or the like is applied. A preferred example of
the gate electrode is formed using aluminum or a stack of aluminum
and barrier metal. As the barrier metal, refractory metal such as
titanium, molybdenum, or chromium is applied. The barrier metal is
preferably provided for preventing hillocks and oxidation of
aluminum.
[0088] The gate electrode 101 is formed to a thickness of from 50
to 300 nm. By forming the gate electrode to a thickness of 300 nm
or less, disconnection of a semiconductor film and wirings which
are formed later can be prevented. Further, by forming the gate
electrode to a thickness of 150 nm or more, resistance of the gate
electrode can be reduced and thus the size of the substrate can be
increased.
[0089] Note that since a semiconductor film and a wiring are to be
formed over the gate electrode 101, it is desired that the gate
electrode 101 be processed to have tapered end portions in order to
prevent disconnection. In addition, although not illustrated, in
this step, a wiring or a capacitor wiring connected to the gate
electrode can also be formed at the same time.
[0090] The gate electrode 101 can be formed by using a sputtering
method, a CVD method, a plating method, a printing method, or a
conductive nanopaste of silver, gold, copper, or the like.
Alternatively, the gate electrode 101 can be formed by discharging
droplets containing conductive particles or the like by an ink-jet
method and baking it.
[0091] Note that here, an aluminum film and a molybdenum film are
formed as a conductive film over the substrate by a sputtering
method and is etched with the use of a resist mask formed using a
first photomask of this embodiment, so that the gate electrode 101
is formed, as illustrated in FIG. 2A.
[0092] This embodiment describes an example in which a multilayer
film including stacked two insulating films is used as the gate
insulating film 102. The first and second gate insulating films
102a and 102b can each be formed using a silicon oxide film, a
silicon nitride film, a silicon oxynitride film, or a silicon
nitride oxide film with a thickness of from 50 to 150 nm. Here, a
mode is described in which a silicon nitride film or a silicon
nitride oxide film is formed as the first gate insulating film 102a
and a silicon oxide film or a silicon oxynitride film is formed as
the second gate insulating film 102b. Note that the gate insulating
film can be formed to have a single layer structure of a silicon
oxide film, a silicon nitride film, a silicon oxynitride film, or a
silicon nitride oxide film, instead of such a two-layer structure.
Alternatively, the gate insulating film may be formed to have a
three-layer structure.
[0093] By forming the first gate insulating film 102a using a
silicon nitride film or a silicon nitride oxide film, adhesiveness
between the substrate and the first gate insulating film 102a is
increased, and in the case where a glass substrate is used as the
substrate, an impurity can be prevented from diffusing into an
oxide semiconductor film from the substrate and further, the gate
electrode 101 can be prevented from being oxidized. That is to say,
film peeling can be prevented, and thus electric characteristics of
a thin film transistor to be completed later can be improved.
Further, the first gate insulating film 102a and the second gate
insulating film 102b each preferably have a thickness of 50 nm or
more so that they can cover unevenness of the gate electrode
101.
[0094] Here, a silicon oxynitride film refers to a film that
contains oxygen and nitrogen such that the amount of oxygen is
larger than that of nitrogen and contains oxygen, nitrogen,
silicon, and hydrogen at concentrations of 55 to 65 at. %, 1 to 20
at. %, 25 to 35 at. %, and 0.1 to 10 at. %, respectively. Further,
a silicon nitride oxide film refers to a film that contains oxygen
and nitrogen such that the amount of nitrogen is larger than that
of oxygen and includes oxygen, nitrogen, silicon, and hydrogen at
concentrations of 15 to 30 at. %, 20 to 35 at. %, 25 to 35 at. %,
and 15 to 25 at. %, respectively.
[0095] Further, for the second gate insulating film 102b in contact
with the semiconductor layer 103, silicon oxide, aluminum oxide,
magnesium oxide, aluminum nitride, yttrium oxide, hafnium oxide, or
the like can be used.
[0096] The first gate insulating film 102a and the second gate
insulating film 102b can each be formed by a CVD method, a
sputtering method, or the like. Here, a silicon nitride film is
formed by a plasma CVD method as the first gate insulating film
102a.
[0097] In particular, it is desirable to successively form the
second gate insulating film 102b in contact with the semiconductor
film 133, and the semiconductor film 133. By the successive film
formation, an interface of the stacked films can be formed without
being contaminated by an atmospheric component such as water vapor
and impurity elements and dusts floating in the air. Thus,
variations in thin film transistor characteristics can be
reduced.
[0098] As for an active matrix display device, electric
characteristics of a thin film transistor included in a circuit are
significant and performance of the display device depends on the
electric characteristics. The threshold voltage (V.sub.th) is
particularly significant among the electric characteristics of the
thin film transistor. Even when field-effect mobility is high, if
the value of the threshold voltage is high or below zero, it is
difficult to control a circuit. A thin film transistor in which the
value of the threshold voltage is high and the absolute value of
the threshold voltage is large might not be able to achieve a
switching function and might be a load in the state where the
driving voltage is low. Further, in the case where the value of the
threshold voltage is below zero, a thin film transistor is likely
to be in a so-called normally-on state in which a current flows
between a source electrode and a drain electrode even when a gate
voltage is 0 V.
[0099] In the case of an n-channel thin film transistor, it is
desirable that only after a positive voltage is applied as a gate
voltage, a channel be formed and a drain current flows. A
transistor in which a channel is not formed unless the driving
voltage is high and a transistor in which even when a negative
voltage is applied, a channel is formed and then a drain current
flows are not suitable as a thin film transistor used for a
circuit. Therefore, also in the case of a thin film transistor
using an oxide semiconductor film containing In, Ga, and Zn, a
channel is desirably formed at a threshold voltage of the gate
voltage which is a positive and close to 0 V.
[0100] A threshold voltage of a thin film transistor is considered
to significantly depend on the interface of a semiconductor layer,
that is, the interface between a semiconductor layer and a gate
insulating film. By forming the interface thereof so that it is
clean, electric characteristics of a thin film transistor can be
improved and a manufacturing process can be prevented from being
complicated. Thus, both mass productivity and high performance of
the thin film transistor can be achieved.
[0101] In particular, when moisture is at the interface between an
oxide semiconductor layer and a gate insulating film, there occurs
a problem such as degradation of electric characteristics of a thin
film transistor, variations in threshold voltages, or normally-on.
By successively forming the oxide semiconductor layer and the gate
insulating film, such a hydrogen compound can be removed.
[0102] Thus, the gate insulating film and the oxide semiconductor
film are successively formed under reduced pressure by a sputtering
method without being exposed to the air, so that a thin film
transistor having a favorable interface, a small leakage current,
and high current driving capability can be achieved.
[0103] Further, it is preferable to form the gate insulating film
and the oxide semiconductor film containing In, Ga, and Zn in an
oxygen atmosphere (or an atmosphere containing oxygen at 90% or
more and a rare gas (such as argon) at 10% or less).
[0104] By thus successively forming the films by a sputtering
method, productivity and reliability of an interface of the thin
films can be increased. Further, by forming the gate insulating
film and the semiconductor layer in an oxygen atmosphere so that
they contain a large mount of oxygen, reduction in reliability due
to deterioration and normally-on of a thin film transistor can be
suppressed.
[0105] Further, it is also desirable to form an insulating film to
be a channel protective layer 106, following the formation of the
semiconductor film. By the successive film formation, an interface
of the stacked films can be formed in a region on a side reverse of
the side of a surface of the semiconductor film in contact with a
gate insulating film, a so-called back channel, without being
contaminated by an atmospheric component such as water vapor and
impurity elements and dusts floating in the air. Thus, variations
in thin film transistor characteristics can be reduced.
[0106] For a method for successive film formation, a multi-chamber
sputtering apparatus equipped with a plurality of film formation
chambers, a sputtering apparatus provided with a plurality of
targets, or a pulsed laser deposition (PLD) apparatus may be
used.
[0107] In the case of depositing silicon oxide for the insulating
film, the insulating film can be formed by a high frequency
sputtering method or a reactive sputtering method using silicon
oxide (artificial quartz) or single crystal silicon as a
target.
[0108] Note that here, a semiconductor film and a silicon oxide
film to be a channel protective layer are successively formed
without being exposed to the air after formation of a silicon oxide
film as the second gate insulating film 102b in contact with the
semiconductor film by using a multi-chamber sputtering apparatus
provided with a single crystal silicon target and a target for the
semiconductor film.
[0109] The semiconductor layer 103 is formed using an amorphous
oxide semiconductor film. For the amorphous oxide semiconductor
film, a composite oxide of an element such as indium, gallium,
aluminum, zinc, or tin can be used. For example, indium oxide
containing zinc oxide (IZO), oxide containing In, Ga, and Zn
(IGZO), and oxide containing zinc oxide and tin oxide (ZTO) are
given as examples of the composite oxide.
[0110] In the case of oxide containing indium oxide, gallium oxide,
and zinc oxide, the composition ratio of the metal elements is
highly flexibly set and the oxide functions as a semiconductor
layer at a wide range of mixture ratio. For example, indium oxide
containing zinc oxide at 10 wt %, a material in which indium oxide,
gallium oxide, and zinc oxide are mixed in an equimolar ratio, and
oxide containing In, Ga, and Zn at a ratio of 2.2:2.2:1.0 can be
given.
[0111] The oxide semiconductor film 133 used for the semiconductor
layer 103 is preferably formed to a thickness of from 2 nm to 200
nm, more preferably from 20 to 150 nm. Further, when the oxygen
vacancy rate in the film is increased, a carrier concentration is
increased and thus thin film transistor characteristics are
degraded. Therefore, the composition is determined so that oxygen
vacancy is suppressed.
[0112] The amorphous oxide semiconductor film 133 can be formed by
a reactive sputtering method, a pulsed laser deposition method (PLD
method), or a sol-gel method. Among gas phase methods, a PLD method
is suitable in terms of easiness in controlling the composition of
materials and a sputtering method is suitable in terms of mass
productivity as described above. Here, a method using oxide
containing In, Ga and Zn (IGZO) is described as an example of a
method for forming the semiconductor film 133.
[0113] A target with a diameter of 8 inch obtained by mixing indium
oxide (In.sub.2O.sub.3), gallium oxide (Ga.sub.2O.sub.3), and zinc
oxide (ZnO) in an equimolar ratio and performing sintering is used,
a substrate is provided 170 mm apart from the target, and direct
current (DC) sputtering is performed with an output of 500 W, so
that the semiconductor film 133 is formed. The semiconductor film
133 is formed to a thickness of 50 nm under the conditions that the
chamber pressure is 0.4 Pa and the gas composition ratio of Ar to
O.sub.2 is 10/5 sccm. It is desirable that an oxygen partial
pressure in film formation be set higher than that in forming a
transparent conductive film of indium tin oxide (ITO) or the like
to control the oxygen concentration in a film formation atmosphere
so that oxygen vacancy is suppressed. Further, it is preferable to
use a pulsed direct current (DC) power supply because dusts can be
reduced and the thickness distribution of the semiconductor layer
can be uniform.
[0114] Note that the semiconductor layer 103 may be subjected to a
plasma treatment. By the plasma treatment, damages of the
semiconductor layer 103 due to etching can be repaired. The plasma
treatment is preferably performed in an atmosphere of O.sub.2 or
N.sub.2O, or preferably an atmosphere of N.sub.2, He, or Ar
containing oxygen. Alternatively, the plasma treatment may be
performed in the above atmosphere to which Cl.sub.2 or CF.sub.4 is
added. Note that the plasma treatment is preferably performed with
non-bias.
[0115] Note that in this embodiment, a semiconductor film is formed
over the second gate insulating film 102b formed in the former step
without exposing the second gate insulating film 102b to the air by
using a multi-chamber sputtering apparatus provided with a target
for an oxide semiconductor film and a single crystal silicon
target. In the following step, an insulating film to be the channel
protective layer 106 is formed over the formed semiconductor film
without exposing the semiconductor film to the air.
[0116] The channel protective layer 106 is formed using an
insulating film in a region overlapping with a channel formation
region of the semiconductor layer 103 as in FIG. 2A. The insulating
film functioning as the channel protective layer 106 can be formed
using an inorganic material (such as silicon oxide, silicon
nitride, silicon oxynitride, or silicon nitride oxide). Further, a
film formed using one or a plurality of kinds of photosensitive or
nonphotosensitive organic materials (organic resin materials) (such
as polyimide, acrylic, polyamide, polyimide amide, resist, and
benzocyclobutene) or a stack of such films can be used. Further,
siloxane may be used.
[0117] The insulating film to be the channel protective layer 106
can be formed by a vapor phase growth method such as a plasma CVD
method or a thermal CVD method, or a sputtering method.
Alternatively, an application method such as a spin coating method
which is a wet process may be used. Alternatively, the insulating
film may be selectively formed by a droplet discharge method, a
printing method (a method for forming a pattern, such as screen
printing or offset printing), or the like.
[0118] Note that here, a silicon oxide film to be the channel
protective layer 106 is formed without exposing the oxide
semiconductor film 133 formed in the former step to the air by
using a multi-chamber sputtering apparatus provided with a single
crystal silicon target and a target for the oxide semiconductor
film.
[0119] Next, the silicon oxide film formed over the semiconductor
film 133 is selectively etched by using a resist mask formed with
the use of a second photomask of this embodiment, so that the
channel protective layer 106 is formed as in FIG. 2A.
[0120] Next, the semiconductor film 133 formed over the gate
insulating films is etched by using a resist mask formed with the
use of a third photomask of this embodiment, so that the
semiconductor layer 103 is formed.
[0121] Note that as a method for etching a film of oxide containing
In, Ga, and Zn (IGZO), a wet etching method can be used. An organic
acid such as a citric acid or an oxalic acid can be used for an
etchant. For example, a film of oxide containing In, Ga, and Zn
(IGZO) with a thickness of 50 nm can be etched in 150 seconds by
using the ITO07N (manufactured by Kanto Chemical Co., Inc.).
[0122] The pair of buffer layers 104a and 104b over the amorphous
oxide semiconductor film are formed by using an oxide semiconductor
film having n-type conductivity and containing In, Ga, and Zn.
[0123] Alternatively, an oxide semiconductor film having n-type
conductivity and containing In, Ga, and Zn may be doped with
another metal. As the dopant, magnesium, aluminum, titanium,
scandium, yttrium, zirconium, hafnium, boron, thallium, germanium,
tin, lead, and the like can be given. In the case where magnesium,
aluminum, titanium, or the like is contained in the buffer layers,
there is an effect of blocking oxygen and the like, so that the
oxygen concentration of the semiconductor layer can be maintained
within an optimal range by heat treatment or the like after film
formation.
[0124] In an embodiment of the present invention, the carrier
concentration of the semiconductor layer is preferably less than
1.times.10.sup.17 atoms/cm.sup.3 (more preferably 1.times.10.sup.11
atoms/cm.sup.3 or more) (the channel concentration range 1), and
the carrier concentration of the IGZO film suitable for the buffer
layers is preferably 1.times.10.sup.18 atoms/cm.sup.3 or more (more
preferably 1.times.10.sup.22 atoms/cm.sup.3 or less) (the buffer
layer concentration range 2). Further, a second buffer layer
functioning as an n.sup.- layer having a higher carrier
concentration than the semiconductor layer and a lower carrier
concentration than the buffer layers may be provided between the
semiconductor layer and the buffer layers.
[0125] The buffer layers 104a and 104b have a higher carrier
concentration than the semiconductor layer formed using oxide
containing In, Ga, and Zn (IGZO) and thus has excellent
conductivity. Therefore, in the case of providing the buffer layers
104a and 104b, contact resistance can be reduced as compared to the
case where the semiconductor layer 103 and the source and drain
electrode layers 105a and 105b are directly bonded to each other.
Further, by providing the buffer layers 104a and 104b at the
bonding interface between the semiconductor layer 103 and the
source and drain electrode layers 105a and 105b, an electric field
concentrated at the bonding interface can be reduced.
[0126] Note that in order that the buffer layers 104a and 104b may
cover the semiconductor layer 103 reliably, patterning may be
performed so that the buffer layers are also provided over part of
the channel protective layer 106 as in FIG. 2B.
[0127] The oxide semiconductor film having n-type conductivity and
containing In, Ga, and Zn, which is to be the buffer layers 104a
and 104b, is preferably formed to a thickness of from 2 nm to 100
nm.
[0128] The oxide semiconductor film having n-type conductivity and
containing In, Ga, and Zn, which is to be the buffer layers 104a
and 104b, can be formed by a sputtering method or a pulsed laser
deposition method (PLD method).
[0129] Note that here, the oxide semiconductor film having n-type
conductivity and containing In, Ga, and Zn formed over the
semiconductor layer 103 and the channel protective layer 106 is
subjected to dry etching or wet etching using a resist mask formed
with the use of a fourth photomask of this embodiment, so that the
buffer layers 104a and 104b are formed.
[0130] The source and drain electrode layers 105a and 105b are
formed using a conductive film, the same material as the gate
electrode 101. In particular, a layer in contact with the buffer
layers 104a and 104b is preferably a titanium film. As specific
examples of the conductive film, a single titanium film, a layered
film including a titanium film and an aluminum film, and a
three-layer film in which a titanium film, an aluminum film, and a
titanium film are sequentially stacked are given.
[0131] Here, a three-layer film including a titanium film, an
aluminum film, and a titanium film is formed over the buffer layers
104a and 104b and the channel protective layer by a sputtering
method as in FIG. 2C. Next, the conductive film formed over the
channel protective layer 106 is etched to be divided by using a
resist mask formed with the use of a fifth photomask of this
embodiment, so that the source and drain electrode layers 105a and
105b are formed as in FIG. 2D. Note that the three-layer film in
which a titanium film, an aluminum film, and a titanium film are
sequentially stacked can be etched by using a hydrogen peroxide
solution or a heated hydrochloric acid as an etchant.
[0132] Note that in this embodiment, formation of the buffer layers
104a and 104b and formation of the source and drain electrode
layers 105a and 105b are performed separately, so that the lengths
of a part where end portions of the buffer layer 104a and the
source or drain electrode layer 105a overlap with each other and a
part where end portions of the buffer layer 104b and the source or
drain electrode layer 105b overlap with each other can be easily
controlled.
[0133] The characteristics of the thin film transistor using oxide
containing In, Ga, and Zn (IGZO) described in this embodiment for
the semiconductor layer 103 can be improved by performing heat
treatment on the semiconductor layer 103 formed. In specific, an on
current is increased and variations in transistor characteristics
are reduced.
[0134] The semiconductor layer 103 is heated at preferably
300.degree. C. to 400.degree. C. Here, the semiconductor layer 103
is heated at 350.degree. C. for one hour. The heat treatment may be
performed at any timing after formation of the semiconductor layer
103. For example, the heat treatment may be performed after the
semiconductor layer 103 and the insulating film to be the channel
protective layer 106 are successively formed, after the channel
protective layer 106 is patterned, or after the oxide semiconductor
film having n-type conductivity and containing In, Ga, and Zn,
which is to be the buffer layers 104a and 104b, is formed.
Alternatively, the heat treatment may be performed after the
conductive film to be the source and drain electrode layers 105a
and 105b is formed or after a sealing film of the thin film
transistor is formed. Alternatively, the heat treatment of the
semiconductor layer 103 may be performed also as a heat curing
treatment of a planarization film formed over the thin film
transistor.
[0135] According to the above description, the amorphous oxide
semiconductor layer 103, the channel protective layer 106, the
buffer layers 104a and 104b, and the source and drain electrode
layers 105a and 105b which are illustrated in FIGS. 1A and 1B are
formed.
[0136] The thin film transistor according to an embodiment of the
present invention has a layered structure including the gate
electrode, the gate insulating films, the semiconductor layer (the
oxide semiconductor layer containing In, Ga, and Zn), the buffer
layers, the channel protective layer, and the source and drain
electrode layers. By using the buffer layers having a higher
carrier concentration than the semiconductor layer, parasitic
capacitance can be reduced with the thickness of the semiconductor
layer kept small.
[0137] The thin film transistor according to an embodiment of the
present invention is provided with the channel protective layer
106; therefore, a region on a side reverse of the side of a surface
of an oxide semiconductor film in contact with the gate insulating
film 102b, a so-called back channel, can be protected from damages
(such as reduction in film thickness due to plasma or an etchant at
the time of etching and oxidation) in the process. Thus,
reliability of the thin film transistor can be improved.
[0138] Note that since the channel protective layer 106 functions
as an etching stopper in the etching step for forming the
semiconductor layer 103, it can also be referred to as a channel
stopper layer.
[0139] Further, in this embodiment, the edges of the source and
drain electrode layers 105a and 105b are on outer sides than the
edges of the buffer layers 104a and 104b over the channel
protective layer 106 so that they are apart from each other;
therefore, a leakage current and a short circuit between the source
and drain electrode layers 105a and 105b can be prevented.
[0140] Thus, by applying an embodiment of the present invention, a
thin film transistor having a small amount of photocurrent, low
parasitic capacitance, and a high on-off ratio can be obtained and
thus a thin film transistor having favorable dynamic
characteristics can be manufactured. Accordingly, a semiconductor
device including a thin film transistor having excellent electric
characteristics and high reliability can be provided.
Embodiment 2
[0141] In this embodiment, the structure of a thin film transistor
in which a buffer layer contains formed using an oxide
semiconductor having n-type conductivity and containing In, Ga, and
Zn, which is different from the structure of Embodiment 1, will be
described with reference to FIGS. 3A1 to 3D. In addition, in this
embodiment, the same reference numerals are used for the same
portions as those in Embodiment 1 and detailed description of the
portions is omitted.
[0142] Through a similar process to that of Embodiment 1, a channel
protective layer 106 is formed using an insulating film in a region
that overlaps a channel formation region of a semiconductor layer
103 as illustrated in FIG. 3A1. Note that by the etching processing
of the channel protective layer 106, the surface of the
semiconductor layer 103 to which buffer layer 104 are to be bonded
may be etched as illustrated in FIG. 3A2. By etching the surface of
the oxide semiconductor layer to which the buffer layer 104 are to
be bonded, the buffer layer 104 can be more favorably bonded to the
oxide semiconductor layer.
[0143] In other words, through a process similar to that of
Embodiment 1, the channel protective layer 106 is formed over a
semiconductor film 133 and in a region overlapping with a gate
electrode 101. Note that in a step of forming the channel
protective layer 106, the surface of the semiconductor film 133 may
be etched as illustrated in FIG. 3A2. The surface of the
semiconductor film 133 in an opening portion of the channel
protective layer 106 is etched; as a result, the surface can be
favorably bonded to an oxide semiconductor film 134 having n-type
conductivity and containing In, Ga, and Zn that is formed next and
is to be buffer layers. Note that in this embodiment, description
will be made based on the embodiment of FIG. 3A2.
[0144] In this embodiment, titanium oxide is deposited as the oxide
semiconductor film 134 having n-type conductivity and containing
In, Ga, and Zn to be buffer layers as illustrated in FIG. 3B. After
the oxide semiconductor film 134 having n-type conductivity and
containing In, Ga, and Zn to be buffer layers is formed by a method
similar to that described in Embodiment 1, without patterning of
the oxide semiconductor film 134 having n-type conductivity and
containing In, Ga, and Zn, a conductive film 105 to be source and
drain electrode layers 105a and 105b is stacked over the oxide
semiconductor film 134 having n-type conductivity and containing
In, Ga, and Zn as illustrated in FIG. 3C.
[0145] The conductive film 105 is formed by a method similar to
that of Embodiment 1. Here, a three-layer film is formed by a
sputtering method as the conductive film 105. For example, a
titanium film, an aluminum film, and a titanium film can be used
for source and drain electrode layers 105a1 and 105b1, source and
drain electrode layers 105a2 and 105b2, and source and drain
electrode layers 105a3 and 105b3, respectively.
[0146] In other words, the source and drain electrode layers (105a
and 105b) in which the first conductive layers (105a1, 105b1)
formed of titanium, the second conductive layers (105a2, 105b2)
formed of aluminum, and the third conductive layers (105a3, 105b3)
formed of titanium are stacked are formed using the conductive film
105 formed by stacking titanium for the first conductive film,
aluminum for the second conductive film, and titanium for the third
conductive film.
[0147] Next, the conductive film 105 is etched using a resist mask
that is formed with the use of a fourth photomask of this
embodiment.
[0148] The source and drain electrode layers 105a2, 105a3, 105b2,
and 105b3 are etched by wet etching using the source and drain
electrode layers 105a1 and 105b1 as etching stoppers. By using the
same mask that is used in the above wet etching, the source and
drain electrode layers 105a1 and 105b1, buffer layers 104a and
104b, and the semiconductor layer 103 are etched by dry etching.
Accordingly, as illustrated in FIG. 3D, an edge portion of the
source or drain electrode layer 105a1 is aligned with an edge
portion of the buffer layer 104a, and an edge portion of the source
or drain electrode layer 105b1 is aligned with an edge portion of
the buffer layer 104b. In addition, edge portions of the source or
drain electrode layers 105a2 and 105a3 and edge portions of the
source or drain electrode layers 105b2 and 105b3 are on inner sides
than the edge portion of the source or drain electrode layer 105a1
and the edge portion of the source or drain electrode layer 105b1,
respectively.
[0149] In other words, first, the titanium film, which is the third
conductive film, is etched to form the third conductive layers
(105a3, 105b3), and next the aluminum film, which is the second
conductive film, is etched using the titanium film, which is the
first conductive film, as an etching stopper to form the second
conductive layers (105a2, 105b2). Further, the titanium film, which
is the first conductive film, and the oxide semiconductor film 134
having n-type conductivity and containing In, Ga, and Zn are
dry-etched using the same resist mask that is used in the above wet
etching, so that the third conductive layers (105a1, 105b1) and the
buffer layers (104a, 104b) are formed. By formation of the source
and drain electrode layers (105a, 105b) through this process, edge
portions of the first conductive layers (105a1, 105b1) are aligned
with edge portions of the buffer layers (104a, 104b), and edge
portions of the second conductive layers (105a2, 105b2) and the
third conductive layers (105a3, 105b3) are on inner sides than the
edge portions of the first conductive layers (105a1, 105b1). Note
that the cross-sectional view at this stage is illustrated in FIG.
3D.
[0150] Thus, when the etching selectivity of the conductive film
used for the source and drain electrodes to the buffer layers and
the semiconductor layer is low, it is preferable to perform an
etching step plural times under different etching conditions by
stacking a conductive film functioning as an etching stopper.
[0151] Further, heat treatment of the formed semiconductor layer
103 is performed in a manner similar to that of Embodiment 1.
[0152] According to this embodiment, since the buffer layers 104a
and 104b and the source and drain electrode layers 105a and 105b
are patterned using the resist mask formed with the same photomask,
the number of photomasks used can be reduced compared to that of
Embodiment 1. Thus, the number of steps can be reduced by combining
plural steps, whereby yield can be improved and manufacturing time
can be shortened.
Embodiment 3
[0153] In this embodiment, the structure of a thin film transistor
including a buffer layer, which is different from the structures of
Embodiments 1 and 2, will be described with reference to FIGS. 4A
to 4D. In addition, in this embodiment, the same reference numerals
are used for portions the same as those in Embodiment 1 and a
detailed description of the portions is omitted.
[0154] Through a process similar to that of Embodiment 2, a channel
protective layer 106 is formed over a semiconductor film 133 of
oxide containing In, Ga, and Zn (IGZO) to be a semiconductor layer
103 as illustrated in FIG. 4A.
[0155] In this embodiment, without formation of the semiconductor
layer 103 by selective etching of the semiconductor film 133, an
oxide semiconductor film having n-type conductivity and containing
In, Ga, and Zn to be buffer layers 104a and 104b is formed over the
semiconductor film 133 by a method similar to that of Embodiment 2.
Next, the buffer layers 104a and 104b and the semiconductor layer
103 are formed as illustrated in FIG. 4B, by using a resist mask
that is formed with the use of a third photomask of this
embodiment.
[0156] Source and drain electrode layers 105a and 105b are formed
using a conductive film in a manner similar to that of Embodiment
1. Here, a three-layer film of a titanium film, an aluminum film,
and a titanium film is formed by a sputtering method as the
conductive film over the buffer layers 104a and 104b and the
channel protective layer 106. Next, the conductive film is
partially removed by etching with the use of a resist mask formed
with a fourth photomask of this embodiment, so that the source and
drain electrode layers 105a and 105b are formed as illustrated in
FIG. 4C. FIG. 4D is a plan view and FIG. 4C is a cross-sectional
view taken along line A1-A2 of FIG. 4D.
[0157] Further, heat treatment of the formed semiconductor layer
103 is performed in a manner similar to that of Embodiment 1.
[0158] According to this embodiment, since the buffer layers 104a
and 104b and the semiconductor layer 103 are patterned at the same
time, the number of photomasks used can be reduced compared to that
of Embodiment 1. Thus, the number of steps can be reduced by
combining plural steps, whereby yield can be improved and
manufacturing time can be shortened.
Embodiment 4
[0159] In this embodiment, a thin film transistor including a
plurality of electrically-connected gate electrodes and a buffer
layer will be described with reference to FIGS. 5A and 5B, FIGS. 6A
and 6B, and FIGS. 7A and 7B. FIG. 5A is a plan view, and FIG. 5B is
a cross-sectional view taken along line A1-A2 of FIG. 5A. FIG. 6A
is a plan view, and FIG. 6B is a cross-sectional view taken along
line A1-A2 of FIG. 6A. FIG. 7A is a plan view, and FIG. 7B is a
cross-sectional view taken along line A1-A2 of FIG. 7A. In
addition, in this embodiment, the same reference numerals are used
for portions the same as those in Embodiment 1 and detailed
description of the portions is omitted.
[0160] Note that the structure in which two channel formation
regions are connected is employed in this embodiment; however, the
structure is not limited thereto and the structure may be a
so-called multi-gate structure (the structure including two or more
channel formation regions which are connected in series) such as a
triple-gate structure in which three channel formation regions are
connected.
[0161] As the mode of the thin film transistor of this embodiment
in which two channel regions are connected, there are three modes:
a mode in which two channel regions are connected by only the
buffer layer 104c (FIGS. 5A and 5B), a mode in which two channel
regions are connected by the buffer layer 104c and the conductive
layer 105c (FIGS. 6A and 6B), and a mode in which two channel
regions are connected by the semiconductor layer 103, the buffer
layer 104c, and the conductive layer 105c (FIGS. 7A and 7B). By
changing the portion of the photomask for an appropriate layer,
which is located between a first gate electrode 101a and a second
gate electrode 101b, these thin film transistors can be formed by a
method similar to that of Embodiment 1.
[0162] Such a multi-gate structure is very effective for reducing
the off current value.
Embodiment 5
[0163] This embodiment will describe the structure of a thin film
transistor including buffer layers, which is different from the
structures of Embodiments 1 to 4 with reference to FIG. 8. Note
that since the thin film transistor of this embodiment except the
buffer layers can be formed by a method similar to the method
described in Embodiment 1, only the buffer layers will be
specifically described.
[0164] The buffer layers of this embodiment each include a first
buffer layer and a second buffer layer. The first buffer layers
104a and 104b are in contact with source and drain electrodes and
second buffer layers 114a and 114b are sandwiched between the first
buffer layers 104a and 104b and the semiconductor layer 103.
[0165] That is to say, the buffer layers of this embodiment are the
first buffer layer 104a in contact with one of the source electrode
or the drain electrode, the first buffer layer 104b in contact with
the other, the second buffer layer 114a sandwiched between the
first buffer layer 104a and the semiconductor layer 103, and the
second buffer layer 114b sandwiched between the first buffer layer
104b and the semiconductor layer 103.
[0166] The first buffer layers 104a and 104b and the second buffer
layers 114a and 114b are each formed using an oxide semiconductor
having n-type conductivity and containing In, Ga, and Zn.
[0167] Alternatively, an oxide semiconductor having n-type
conductivity and containing In, Ga, and Zn, which is doped with
another metal, may be used. As the dopant, for example, magnesium,
aluminum, titanium, scandium, yttrium, zirconium, hafnium, boron,
thallium, germanium, tin, lead, and the like can be given. By the
doping, the carrier concentration in the buffer layer can be
increased.
[0168] As a method for forming the buffer layers, for example, a
co-sputtering method for sputtering a target obtained by sintering
oxide containing In, Ga, and Zn (IGZO) and a target of a compound
containing a dopant imparting n-type conductivity at the same time
may be employed. By the co-sputtering method, a mixed layer of
oxide containing In, Ga, and Zn (IGZO) and a compound containing a
dopant can be formed, and the first buffer layers 104a and 104b can
be formed separately from the second buffer layers 114a and
114b.
[0169] The first buffer layers 104a and 104b and the second buffer
layers 114a and 114b have a higher carrier concentration than the
semiconductor layer 103 of oxide containing In, Ga, and Zn (IGZO)
and thus has excellent conductivity. The composition of the first
buffer layers 104a and 104b is selected so that the buffer layers
104a and 104b have a higher carrier concentration than the second
buffer layers 114a and 114b. That is to say, the first buffer
layers 104a and 104b function as n.sup.+ layers whereas the second
buffer layers 114a and 114b function as n.sup.- layers.
[0170] The carrier concentration of the semiconductor layer 103 is
preferably less than 1.times.10.sup.17 atoms/cm.sup.3 (more
preferably 1.times.10.sup.11 atoms/cm.sup.3 or more) (the channel
concentration range 1), and the carrier concentration of the IGZO
film suitable for the buffer layers 104a and 104b functioning as
n.sup.+ layers is preferably 1.times.10.sup.18 atoms/cm.sup.3 or
more (more preferably 1.times.10.sup.22 atoms/cm.sup.3 or less)
(the buffer layer concentration range 2).
[0171] By making the carrier concentration gradient so that the
carrier concentration is increased from the semiconductor layer 103
toward the source and drain electrode layers 105a and 105b, contact
resistance between the semiconductor layer 103 and the source and
drain electrode layers 105a and 105b can be reduced.
[0172] Further, by providing the buffer layers having the gradient
in which a carrier concentration is increased from the
semiconductor layer 103 side toward the side of the source and
drain electrode layers 105a and 105b at the bonding interface
between the semiconductor layer 103 and the source and drain
electrode layers 105a and 105b, an electric field concentrated at
the bonding interface can be alleviated.
[0173] The thin film transistor including the stacked buffer layers
according to an embodiment of the present invention has a small off
current, and thus a semiconductor device including such a thin film
transistor can have excellent electric characteristics and high
reliability.
[0174] This embodiment can be implemented by being combined with
any of the other embodiments as appropriate.
Embodiment 6
[0175] In this embodiment, a manufacturing example of an
inverted-staggered thin film transistor is described, in which at
least a gate insulating film and an oxide semiconductor film are
formed to be stacked successively without being exposed to the air.
In this embodiment, steps up to a step of the successive film
formation are described and steps after the successive film
formation may be performed in accordance with any one of
Embodiments 1 to 5 to manufacture a thin film transistor.
[0176] When films are successively formed without being exposed to
the air, such a multi-chamber manufacturing apparatus illustrated
in FIG. 9 is preferably used.
[0177] At the center of the manufacturing apparatus, a transfer
chamber 80 equipped with a transfer mechanism for transferring a
substrate (typically, a transfer robot 81) is provided. A cassette
chamber 82 in which a cassette case holding a plurality of
substrates carried into and out of the transfer chamber 80 is set
is connected to the transfer chamber 80.
[0178] In addition, a plurality of treatment chambers are connected
to the transfer chamber 80 through gate valves 84 to 88. In this
embodiment, an example in which five treatment chambers are
connected to the transfer chamber 80 having a hexagonal top shape
is illustrated. Note that by changing the top shape of the transfer
chamber 80, the number of treatment chambers which can be connected
to the transfer chamber can be changed. For example, three
treatment chambers can be connected to a transfer chamber having a
tetragonal shape, or seven treatment chambers can be connected to a
transfer chamber having an octagonal shape.
[0179] At least one treatment chamber among the five treatment
chambers is a sputtering chamber in which sputtering is performed.
The sputtering chamber is provided with, at least inside the
chamber, a sputtering target, a mechanism for applying electric
power or a gas introduction means for sputtering the target, a
substrate holder for holding a substrate at a predetermined
position, and the like. Further, the sputtering chamber is provided
with a pressure control means for controlling the pressure in the
chamber, so that the pressure is reduced in the sputtering
chamber.
[0180] Examples of a sputtering method include an RF sputtering
method in which a high-frequency power source is used for a
sputtering power source, a DC sputtering method, and a pulsed DC
sputtering method in which a bias is applied in a pulsed manner. An
RF sputtering method is mainly used in the case of forming an
insulating film, and a DC sputtering method is mainly used in the
case of forming a metal film.
[0181] In addition, there is also a multi-source sputtering
apparatus in which a plurality of targets of different materials
can be set. With the multi-source sputtering apparatus, films of
different materials can be deposited to be stacked in the same
chamber, or a plurality of kinds of materials can be deposited at
the same time by electric discharge in the same chamber.
[0182] In addition, there are also a sputtering apparatus provided
with a magnet system inside the chamber and used for a magnetron
sputtering method, and a sputtering apparatus used for an ECR
sputtering method in which plasma generated with the use of
microwaves is used without using glow discharge.
[0183] In the sputtering chamber of this embodiment, any of various
sputtering methods described above is used as appropriate. In
addition, as a deposition method, there are also a reactive
sputtering method in which a target substance and a sputtering gas
component are chemically reacted with each other during deposition
to form a thin compound film thereof, and a bias sputtering method
in which a voltage is also applied to a substrate during
deposition.
[0184] In addition, among the five treatment chambers, one of the
other treatment chambers than the sputtering chamber is a heating
chamber in which a substrate is preheated or the like before
sputtering, a cooling chamber in which a substrate is cooled after
sputtering, or a chamber in which plasma treatment is
performed.
[0185] Next, an example of an operation of the manufacturing
apparatus is described.
[0186] A substrate cassette storing a substrate 94 whose surface on
which deposition is performed faces downward is set in the cassette
chamber 82, and the cassette chamber 82 is placed in a reduced
pressure state by a vacuum evacuation means provided in the
cassette chamber 82. Note that in each of the treatment chambers
and the transfer chamber 80, the pressure is reduced in advance by
a vacuum evacuation means provided in each chamber. Accordingly,
during being transferred between the treatment chambers, the
substrate is not exposed to the air and can be kept clean.
[0187] Note that at least a gate electrode is provided in advance
over the substrate 94 surface on which deposition is performed
faces downward. For example, a base insulating film such as a
silicon nitride film or a silicon nitride oxide film may also be
provided by a plasma CVD method between the substrate and the gate
electrode. When a substrate made of glass containing alkali metal
is used as the substrate 94, the base insulating film has an effect
of preventing movable ions such as sodium ions from being
discharged from the substrate and entering a semiconductor region
above the substrate to change electric characteristics of a
TFT.
[0188] Here, a substrate over which a silicon nitride film covering
the gate electrode is formed by a plasma CVD method to form a first
layer of a gate insulating film is used. The silicon nitride film
formed by a plasma CVD method is dense, so that generation of pin
holes or the like can be suppressed by using this silicon nitride
film as the first layer of the gate insulating film. Although this
embodiment describes an example in which the gate insulating film
has a layered structure, the present invention is not limited
thereto, and the gate insulating film may be a single layer or have
a layered structure including three or more layers.
[0189] Then, the gate valve 83 is opened and the substrate 94 which
is the first substrate is picked up from the cassette by the
transfer robot 81. After that, the gate valve 84 is opened and the
substrate 94 is transferred to a first treatment chamber 89, and
then, the gate valve 84 is closed. In the first treatment chamber
89, by heating the substrate 94 by a heater or a lamp, moisture or
the like attached to the substrate 94 is removed. In particular,
when the gate insulating film contains moisture, electric
characteristics of a TFT could possibly be changed; therefore,
heating before sputtering deposition is effective. Note that in the
case where moisture has been sufficiently removed at the time when
the substrate is set in the cassette chamber 82, this heating
treatment is not necessary.
[0190] In addition, plasma treatment may be performed on the
surface of the first layer of the gate insulating film by providing
a plasma treatment means in the first treatment chamber 89.
Furthermore, heating for removing moisture may be performed in the
cassette chamber 82 by providing a heating means in the cassette
chamber 82.
[0191] Then, the gate valve 84 is opened and the substrate is
transferred to the transfer chamber 80 by the transfer robot 81.
After that, the gate valve 85 is opened and the substrate is
transferred to a second treatment chamber 90, and the gate valve 85
is closed.
[0192] Here, the second treatment chamber 90 is a sputtering
chamber in which sputtering is performed using an RF magnetron
sputtering method. In the second treatment chamber 90, a silicon
oxide film (SiO.sub.x film (x>0)) is formed as a second layer of
the gate insulating film. Alternatively, for the second layer of
the gate insulating film, an aluminum oxide film (Al.sub.2O.sub.3
film), a magnesium oxide film (MgO.sub.x film (x>0)), an
aluminum nitride film (AlN.sub.x film (x>0), an yttrium oxide
film (YO.sub.x film (x>0), or the like may be used instead of a
silicon oxide film.
[0193] A small amount of a halogen element such as fluorine or
chlorine may be added to the second layer of the gate insulating
film so that movable ions such as sodium ions can be immobilized.
As a method for adding a small amount of a halogen element,
sputtering is performed by introducing a gas containing a halogen
element into the chamber. It is to be noted that in the case where
a gas containing a halogen element is introduced, the evacuation
means of the chamber is needed to be provided with an abatement
system. The peak of the concentration of a halogen element to be
contained in the gate insulating film is measured by a secondary
ion mass spectrometer (SIMS) and is preferably in the range of from
1.times.10.sup.15 cm.sup.-3 to 1.times.10.sup.20 cm.sup.-3.
[0194] When the SiO.sub.x film (x>0) is formed, a sputtering
method in which artificial quartz is used as a target and a rare
gas, typically, argon is used, or a reactive sputtering method in
which single crystal silicon is used as a target and chemically
reacted with an oxygen gas to obtain a SiO.sub.x film (x>0) can
be used. Here, artificial quartz is used as a target, and
sputtering is performed in an atmosphere containing only oxygen, an
atmosphere containing oxygen at 90% or more and Ar at 10% or less
so that a SiO.sub.x film contains oxygen as much as possible. Thus,
a SiO.sub.x film (x>0) containing an excessive amount of oxygen
is formed.
[0195] After the SiO.sub.x film (x>0) is formed, the gate valve
85 is opened, and the substrate is transferred to the transfer
chamber 80 by the transfer robot 81 without being exposed to the
air. Then, the gate valve 86 is opened, the substrate is
transferred to a third treatment chamber 91, and the gate valve 86
is closed.
[0196] In this embodiment, the third treatment chamber 91 is a
sputtering chamber in which sputtering is performed using a DC
magnetron sputtering method. In the third treatment chamber 91, an
oxide semiconductor film containing In, Ga, and Zn (IGZO film) is
formed as a semiconductor layer. The IGZO film can be formed using
an oxide semiconductor target containing In, Ga, and Zn, under a
rare gas atmosphere or an oxygen atmosphere. Here, an oxide
semiconductor containing In, Ga, and Zn is used as a target and
sputtering is performed by a pulsed DC sputtering method under an
atmosphere containing only oxygen, an atmosphere containing oxygen
at 90% or more and Ar at 10% or less so that the IGZO film contains
oxygen as much as possible. Thus, an IGZO film containing an
excessive amount of oxygen is formed.
[0197] By thus successively forming the SiOx film containing an
excessive amount of oxygen and the IGZO film containing an
excessive amount of oxygen without exposing them to the air, an
interface state between the films can be stabilized because both
the films contain an excessive amount of oxygen, and the
reliability of a TFT can be improved. If the substrate is exposed
to the air before formation of the IGZO film, moisture or the like
is attached and the interface state is adversely affected, which
may cause phenomena such as variation in threshold voltage,
degradation in electric characteristics, and a normally-on TFT.
Moisture is a hydrogen compound. When the films are successively
formed without being exposed to the air, the hydrogen compound can
be prevented from existing at the interface. Therefore, by
successive film formation, variation in threshold voltage can be
reduced, degradation in electric characteristics can be prevented,
or shift of the TFT characteristics to the normally-on side can be
reduced, desirably, the shift of the TFT characteristics can be
prevented.
[0198] In addition, in the second treatment chamber 90 which is a
sputtering chamber, both an artificial quartz target and an oxide
semiconductor target containing In, Ga, and Zn are placed, and the
films are successively formed by using a shutter; therefore, the
films can be stacked in the same chamber. A shutter is provided
between the target and the substrate; a shutter is opened for a
target which is deposited, and a target which is not deposited is
shielded by a shutter. Advantages of the process in which the films
are stacked in the same chamber are as follows: the number of
chambers which are used can be reduced, and particles or the like
can be prevented from being attached to the substrate during
transfer of the substrate between different chambers.
[0199] Next, the substrate is transferred to the transfer chamber
80 with a transferring robot 81 by opening the gate valve 86 and
transferred to a fourth treatment chamber 92 by opening a gate
valve 87 without being exposed to the air, and then the gate valve
87 is closed.
[0200] Here, the fourth treatment chamber 92 is a sputtering
chamber in which sputtering is performed using an RF magnetron
sputtering method. In the fourth treatment chamber 92, a silicon
oxide film (S.sub.iO.sub.x film (x>0)) is formed as an
insulating film to be a channel protective layer. Alternatively,
for the channel protective layer, an aluminum oxide film
(Al.sub.2O.sub.3 film), a magnesium oxide film (MgO.sub.x film
(x>0)), an aluminum nitride film (AlN.sub.x film (x>0), an
yttrium oxide film (YO.sub.x film (x>0), or the like may be used
instead of a silicon oxide film.
[0201] A small amount of a halogen element such as fluorine or
chlorine may be added to the channel protective layer so that
movable ions such as sodium ions can be immobilized. As a method
for adding a small amount of a halogen element, sputtering is
performed by introducing a gas containing a halogen element into
the chamber. It is to be noted that in the case where a gas
containing a halogen element is introduced, the evacuation means of
the chamber is needed to be provided with an abatement system. The
peak of the concentration of a halogen element to be contained in
the gate insulating film is measured by a secondary ion mass
spectrometer (SIMS) and is preferably in the range of from
1.times.10.sup.15 cm.sup.-3 to 1.times.10.sup.20 cm.sup.-3.
[0202] When the SiO.sub.x film (x>0) is formed for the channel
protective layer, a sputtering method in which artificial quartz is
used as a target and a rare gas, typically, argon is used, or a
reactive sputtering method in which single crystal silicon is used
as a target and chemically reacted with an oxygen gas to obtain a
SiO.sub.x film (x>0) can be used. Here, artificial quartz is
used as a target, and sputtering is performed in an atmosphere
containing only oxygen, an atmosphere containing oxygen at 90% or
more and Ar at 10% or less so that a SiOx film contains oxygen as
much as possible. Thus, a SiO.sub.x film (x>0) containing an
excessive amount of oxygen is formed.
[0203] By thus successively forming the SiO.sub.x film (x>0)
containing an excessive amount of oxygen and the IGZO film
containing an excessive amount of oxygen without exposing them to
the air, an interface state between the films can be stabilized
because all the three layers contain an excessive amount of oxygen,
and the reliability of a TFT can be improved. If the substrate is
exposed to the air before formation of the IGZO film, moisture or
the like is attached and the interface state is adversely affected,
which may cause phenomena such as variation in threshold voltage,
degradation in electric characteristics, and a normally-on TFT.
Moisture is a hydrogen compound. When the films are successively
formed without being exposed to the air, the hydrogen compound can
be prevented from existing at the interface. Therefore, by
successive film formation, variation in threshold voltage can be
reduced, degradation in electric characteristics can be prevented,
or shift of the TFT characteristics of a TFT to the normally-on
side can be reduced, desirably, the shift of the TFT
characteristics can be prevented.
[0204] In addition, in the second treatment chamber 90 which is a
sputtering chamber, both a artificial quartz target and an oxide
semiconductor target containing In, Ga, and Zn are placed, and the
three films are successively formed by using a shutter; therefore,
the films can be stacked in the same chamber. Advantages of the
process in which the films are stacked in the same chamber are as
follows: the number of chambers which are used can be reduced, and
particles or the like can be prevented from being attached to the
substrate during transfer of the substrate between different
chambers.
[0205] After the above steps for deposition on a substrate in a
cassette case are repeated so that a plurality of substrates are
processed, the vacuum in the cassette chamber is released and the
substrates and the cassettes are taken out.
[0206] Next, to pattern the IGZO film, the channel protective layer
is selectively etched and further the IGZO film is selectively
etched. To form the pattern, dry etching or wet etching may be used
or selective etching may be performed twice. In this stage, the
surface of the gate insulating film is exposed in a region where
the IGZO film is removed.
[0207] Next, the channel protective layer is etched so that only a
portion thereof overlapping with the gate electrode, that is, only
a portion thereof overlapping with a portion to be a channel
formation region of the IGZO film is left. Here, the channel
protective layer is etched under the condition that the etching
rate is sufficiently different from that of the IGZO film. In the
case where there is not a sufficient difference in etching rate
between the channel protective layer and the IGZO film in etching
the channel protective layer, the surface portion of the IGZO film
is partially etched and thus a region thinner than a region
overlapped with the channel protective layer is formed in the IGZO
film. Note that in the case where the channel protective layer is
formed using the same material as the gate insulating film, the
gate insulating film is also etched by the etching. Therefore, it
is preferable to use a material different from that of the gate
insulating film for the channel protective layer to prevent the
gate insulating film from being etched. In this embodiment, the
gate insulating film has a two-layer structure in which the upper
layer is a SiO.sub.x film (x>0) and thus might be removed and
the lower layer is a silicon nitride film and functions as an
etching stopper.
[0208] Next, the substrate is set in the cassette chamber of the
multi-chamber manufacturing apparatus illustrated in FIG. 9
again.
[0209] Next, after pressure in the cassette chamber is reduced, the
substrate is transferred to the transfer chamber 80 and then to the
third treatment chamber 91. Here, sputtering is performed by a
pulsed DC sputtering method in an atmosphere containing only a rare
gas, so that the oxide semiconductor film having n-type
conductivity and containing In, Ga, and Zn to be buffer layers is
formed. The oxide semiconductor film having n-type conductivity and
containing In, Ga, and Zn has a lower oxygen concentration than the
IGZO film containing an excessive amount of oxygen. Further, the
oxide semiconductor film having n-type conductivity and containing
In, Ga, and Zn preferably has a higher carrier concentration than
the IGZO film containing an excessive amount of oxygen. As a
target, an oxide semiconductor containing In, Ga and Zn which
further contains Mg, Al, or Ti may be used. Mg, Al, and Ti are
materials which is likely to perform oxidative reaction; thus when
any of the materials is contained in the oxide semiconductor film
having n-type conductivity and containing In, Ga, and Zn, an effect
of blocking oxygen and the like can be obtained, so that the oxygen
concentration of the semiconductor layer can be maintained within
an optimal range even if heat treatment or the like is performed
after film formation. The oxide semiconductor film having n-type
conductivity and containing In, Ga, and Zn functions as source and
drain regions.
[0210] Next, the substrate is transferred to the transfer chamber
80 with a transferring robot 81 by opening the gate valve 87 and
transferred to a fifth treatment chamber 93 by opening a gate valve
88 without being exposed to the air, and then the gate valve 88 is
closed.
[0211] Here, the fifth treatment chamber 93 is a sputtering chamber
using a DC magnetron sputtering method. In the fifth treatment
chamber 93, a metal multilayer film to be source and drain
electrodes is formed. The sputtering chamber of the fifth treatment
chamber 93 is provided with both a titanium target and an aluminum
target and successive film formation is performed by using a
shutter; thus, films are formed in the same chamber. Here, an
aluminum film is stacked over a titanium film and further, a
titanium film is stacked over the aluminum film.
[0212] By thus successively forming an oxide semiconductor film
having n-type conductivity and containing In, Ga, and Zn and a
metal multilayer film without exposing them to the air, a favorable
interface state can be achieved and contact resistance can be
reduced between the oxide semiconductor film having n-type
conductivity and containing In, Ga, and Zn and the metal multilayer
film.
[0213] After the above steps for deposition on a substrate in a
cassette case are repeated so that a plurality of substrates are
processed, the vacuum in the cassette chamber is released and the
substrates and the cassettes are taken out.
[0214] Next, the metal multilayer film is selectively etched to
form the source and drain electrodes. Further, the oxide
semiconductor film having n-type conductivity and containing In,
Ga, and Zn is selectively etched using the source and drain
electrodes as masks, so that source and drain regions are formed.
In the etching of the oxide semiconductor film having n-type
conductivity and containing In, Ga, and Zn, the channel protective
layer functions as an etching stopper.
[0215] Through the above steps, the inverted-staggered thin film
transistor including the channel protective layer can be
formed.
[0216] Although an example is described in which the IGZO film
containing an excessive amount of oxygen and the oxide
semiconductor film having n-type conductivity and containing In,
Ga, and Zn are formed in the same chamber through the above steps,
the present invention is not particularly limited to this.
Alternatively, the IGZO film containing an excessive amount of
oxygen and the oxide semiconductor film having n-type conductivity
and containing In, Ga, and Zn may be formed in different
chambers.
[0217] In this embodiment, a multi-chamber manufacturing apparatus
is shown as an example, but an in-line manufacturing apparatus in
which sputtering chambers are connected in series may be used and
successive film formation may be performed without being exposed to
the air.
[0218] The apparatus illustrated in FIG. 9 has a so-called
face-down treatment chamber in which the deposition target surface
of the substrate faces downward, but may also have a vertical
placement treatment chamber in which a substrate is placed
vertically. The vertical placement treatment chamber has an
advantage that a footprint is smaller than that of a face-down
treatment chamber and can be effectively used in the case where a
large-area substrate which could be bent due to its weight is
used.
Embodiment 7
[0219] In this embodiment, an example will be described below in
which at least a part of a driver circuit and a thin film
transistor to be disposed in a pixel portion are formed over one
substrate.
[0220] The thin film transistor to be disposed in the pixel portion
is formed according to any of Embodiments 1 to 5. Further, the thin
film transistor described in any of Embodiments 1 to 5 is an
n-channel TFT, and thus a part of a driver circuit that can include
an n-channel TFT among driver circuits is formed over the same
substrate as the thin film transistor of the pixel portion.
[0221] FIG. 10A illustrates an example of a block diagram of an
active matrix liquid crystal display device. The display device
illustrated in FIG. 10A includes, over a substrate 5300, a pixel
portion 5301 including a plurality of pixels each provided with a
display element; a scan line driver circuit 5302 that selects each
pixel; and a signal line driver circuit 5303 that controls a video
signal input to a selected pixel. The pixel portion 5301 is
connected to the signal line driver circuit 5303 by a plurality of
signal lines S1 to Sm (not illustrated) that extend in a column
direction from the signal line driver circuit 5303, and to the scan
line driver circuit 5302 by a plurality of scan lines G1 to Gn (not
illustrated) that extend in a row direction from the scan line
driver circuit 5302. The pixel portion 5301 includes a plurality of
pixels (not illustrated) arranged in matrix so as to correspond to
the signal lines S1 to Sm and the scan lines G1 to Gm. Each pixel
is connected to a signal line Sj (any one of the signal lines S1 to
Sm) and a scan line Gi (any one of the scan lines G1 to Gn).
[0222] In addition, the thin film transistor described in any of
Embodiments 1 to 5 is an n-channel TFT, and a signal line driver
circuit including the n-channel TFT is described with reference to
FIG. 11.
[0223] The signal line driver circuit illustrated in FIG. 11
includes a driver IC 5601, switch groups 5602_1 to 5602_M, a first
wiring 5611, a second wiring 5612, a third wiring 5613, and wirings
5621_1 to 5621_M. Each of the switch groups 5602_1 to 5602_M
includes a first thin film transistor 5603a, a second thin film
transistor 5603b, and a third thin film transistor 5603c.
[0224] The driver IC 5601 is connected to the first wiring 5611,
the second wiring 5612, the third wiring 5613, and the wirings
5621_1 to 5621_M. The switch groups 5602_1 to 5602_M are connected
to the wirings 5621_1 to 5621_M corresponding to the switch groups
5602_1 to 5602_M, respectively, and are each connected to the first
wiring 5611, the second wiring 5612, the third wiring 5613. Each of
the wirings 5621_1 to 5621_M is connected to three signal lines via
the first thin film transistor 5603a, the second thin film
transistor 5603b, and the third thin film transistor 5603c. For
example, the wiring 5621_J of the J-th column (one of the wirings
5621_1 to 5621_M) is connected to a signal line Sj-1, a signal line
Sj, and a signal line Sj+1 via the first thin film transistor
5603a, the second thin film transistor 5603b, and the third thin
film transistor 5603c included in the switch group 5602_J.
[0225] Note that a signal is input to each of the first wiring
5611, the second wiring 5612, and the third wiring 5613.
[0226] Note that the driver IC 5601 is desirably formed over a
single crystalline substrate. The switch groups 5602_1 to 5602_M
are desirably formed over the same substrate as the pixel portion
illustrated in any of Embodiments 1 to 5. Therefore, the driver IC
5601 and the switch groups 5602_1 to 5602_M are preferably
connected via an FPC or the like.
[0227] Next, operation of the signal line driver circuit
illustrated in FIG. 11 is described with reference to a timing
chart in FIG. 12. Note that the timing chart in FIG. 12 shows the
case where the scan line Gi of the i-th row is selected. A
selection period of the scan line Gi of the i-th row is divided
into a first sub-selection period T1, a second sub-selection period
T2, and a third sub-selection period T3. In addition, the signal
line driver circuit in FIG. 11 operates as in FIG. 12 even when a
scan line of another row is selected.
[0228] Note that the timing chart in FIG. 12 shows the case where
the wiring 5621_J of the J-th column is connected to the signal
line Sj-1, the signal line Sj, and the signal line Sj+1 via the
first thin film transistor 5603a, the second thin film transistor
5603b, and the third thin film transistor 5603c.
[0229] The timing chart in FIG. 12 shows timing at which the scan
line Gi of the i-th row is selected, timing 5703a of on/off of the
first thin film transistor 5603a, timing 5703b of on/off of the
second thin film transistor 5603b, timing 5703c of on/off of the
third thin film transistor 5603c, and a signal 5721_J input to the
wiring 5621_J of the J-th column.
[0230] Note that in the first sub-selection period T1, the second
sub-selection period T2, and the third sub-selection period T3,
different video signals are input to the wirings 5621_1 to 5621_M.
For example, a video signal input to the wiring 5621_J in the first
sub-selection period T1 is input to the signal line Sj-1, a video
signal input to the wiring 5621_J in the second sub-selection
period T2 is input to the signal line Sj, and a video signal input
to the wiring 5621_J in the third sub-selection period T3 is input
to the signal line Sj+1. In addition, in the first sub-selection
period T1, the second sub-selection period T2, and the third
sub-selection period T3, the video signals input to the wiring
5621_J are denoted by Data_j-1, Data_j, and Data_j+1.
[0231] As illustrated in FIG. 12, in the first sub-selection period
T1, the first thin film transistor 5603a is turned on, and the
second thin film transistor 5603b and the third thin film
transistor 5603c are turned off. At this time, Data_j-1 input to
the wiring 5621_J is input to the signal line Sj-1 via the first
thin film transistor 5603a. In the second sub-selection period T2,
the second thin film transistor 5603b is turned on, and the first
thin film transistor 5603a and the third thin film transistor 5603c
are turned off. At this time, Data_j input to the wiring 5621_J is
input to the signal line Sj via the second thin film transistor
5603b. In the third sub-selection period T3, the third thin film
transistor 5603c is turned on, and the first thin film transistor
5603a and the second thin film transistor 5603b are turned off. At
this time, Data_j+1 input to the wiring 5621_J is input to the
signal line Sj+1 via the third thin film transistor 5603c.
[0232] As described above, in the signal line driver circuit in
FIG. 11, by dividing one gate selection period into three, video
signals can be input to three signal lines from one wiring 5621 in
one gate selection period. Therefore, in the signal line driver
circuit in FIG. 11, the number of connections of the substrate
provided with the driver IC 5601 and the substrate provided with
the pixel portion can be approximately 1/3 of the number of signal
lines. The number of connections is reduced to approximately 1/3 of
the number of the signal lines, so that reliability, yield, and the
like of the signal line driver circuit in FIG. 11 can be
improved.
[0233] Note that there are no particular limitations on the
arrangement, the number, a driving method, and the like of the thin
film transistors, as long as one gate selection period can be
divided into a plurality of sub-selection periods and video signals
can be input to a plurality of signal lines from one wiring in the
respective sub-selection periods as illustrated in FIG. 11.
[0234] For example, when video signals are input to three or more
signal lines from one wiring in each of three or more sub-selection
periods, it is only necessary to add a thin film transistor and a
wiring for controlling the thin film transistor. It is to be noted
that when one selection period is divided into four or more
sub-selection periods, one sub-selection period is shorter.
Therefore, one selection period is desirably divided into two or
three sub-selection periods.
[0235] As another example, one selection period may be divided into
a precharge period Tp, the first sub-selection period T1, the
second sub-selection period T2, and the third sub-selection period
T3 as illustrated in a timing chart in FIG. 13. The timing chart in
FIG. 13 shows timing at which the scan line Gi of the i-th row is
selected, timing 5803a of on/off of the first thin film transistor
5603a, timing 5803b of on/off of the second thin film transistor
5603b, timing 5803c of on/off of the third thin film transistor
5603c, and a signal 5821_J input to the wiring 5621_J of the J-th
column. As illustrated in FIG. 13, the first thin film transistor
5603a, the second thin film transistor 5603b, and the third thin
film transistor 5603c are tuned on in the precharge period Tp. At
this time, a precharge voltage Vp input to the wiring 5621_J is
input to each of the signal line Sj-1, the signal line Sj, and the
signal line Sj+1 via the first thin film transistor 5603a, the
second thin film transistor 5603b, and the third thin film
transistor 5603c. In the first sub-selection period T1, the first
thin film transistor 5603a is turned on, and the second thin film
transistor 5603b and the third thin film transistor 5603c are
turned off. At this time, Data_j-1 input to the wiring 5621_J is
input to the signal line Sj-1 via the first thin film transistor
5603a. In the second sub-selection period T2, the second thin film
transistor 5603b is turned on, and the first thin film transistor
5603a and the third thin film transistor 5603c are turned off. At
this time, Data_j input to the wiring 5621_J is input to the signal
line Sj via the second thin film transistor 5603b. In the third
sub-selection period T3, the third thin film transistor 5603c is
turned on, and the first thin film transistor 5603a and the second
thin film transistor 5603b are turned off. At this time, Data_j+1
input to the wiring 5621_J is input to the signal line Sj+1 via the
third thin film transistor 5603c.
[0236] As described above, in the signal line driver circuit in
FIG. 11 to which the timing chart in FIG. 13 is applied, the video
signal can be written to the pixel at high speed because the signal
line can be precharged by providing a precharge selection period
before a sub-selection period. Note that portions in FIG. 13 which
are similar to those of FIG. 12 are denoted by common reference
numerals and detailed description of the portions which are the
same and portions which have similar functions is omitted.
[0237] Further, a structure of a scan line driver circuit is
described. The scan line driver circuit includes a shift register
and a buffer. Additionally, the scan line driver circuit may
include a level shifter in some cases. In the scan line driver
circuit, when the clock signal (CLK) and the start pulse signal
(SP) are input to the shift register, a selection signal is
generated. The generated selection signal is buffered and amplified
by the buffer, and the resulting signal is supplied to a
corresponding scan line. Gate electrodes of transistors in pixels
of one line are connected to the scan line. Further, since the
transistors in the pixels of one line have to be turned on at the
same time, a buffer through which a large current can flow can be
used.
[0238] One mode of a shift register which is used for a part of a
scan line driver circuit is described with reference to FIG. 14 and
FIG. 15.
[0239] FIG. 14 illustrates a circuit configuration of a shift
register. The shift register illustrated in FIG. 14 includes a
plurality of flip-flops 5701_i (any one of flip-flops 5701_1 to
5701_n). The shift register is operated with input of a first clock
signal, a second clock signal, a start pulse signal, and a reset
signal.
[0240] Connection relations of the shift register in FIG. 14 are
described. In the i-th stage flip-flop 5701_i (any one of the
flip-flops 5701-1 to 5701_n) in the shift register of FIG. 14, a
first wiring 5501 illustrated in FIG. 15 is connected to a seventh
wiring 5717_i-1; a second wiring 5502 illustrated in FIG. 15 is
connected to a seventh wiring 5717_i+1; a third wiring 5503
illustrated in FIG. 15 is connected to a seventh wiring 5717_i; and
a sixth wiring 5506 illustrated in FIG. 15 is connected to a fifth
wiring 5715.
[0241] Further, a fourth wiring 5504 illustrated in FIG. 15 is
connected to a second wiring 5712 in flip-flops of odd-numbered
stages, and is connected to a third wiring 5713 in flip-flops of
even-numbered stages. A fifth wiring 5505 illustrated in FIG. 15 is
connected to a fourth wiring 5714.
[0242] It is to be noted that the first wiring 5501 in FIG. 15 of
the first stage flip-flop 5701_1 is connected to a first wiring
5711. Moreover, the second wiring 5502 in FIG. 15 of the n-th stage
flip-flop 5701_n is connected to a sixth wiring 5716.
[0243] Note that the first wiring 5711, the second wiring 5712, the
third wiring 5713, and the sixth wiring 5716 may be referred to as
a first signal line, a second signal line, a third signal line, and
a fourth signal line, respectively. The fourth wiring 5714 and the
fifth wiring 5715 may be referred to as a first power supply line
and a second power supply line, respectively.
[0244] Next, FIG. 15 illustrates details of the flip-flop
illustrated in FIG. 14. The flip-flop illustrated in FIG. 15
includes a first thin film transistor 5571, a second thin film
transistor 5572, a third thin film transistor 5573, a fourth thin
film transistor 5574, a fifth thin film transistor 5575, a sixth
thin film transistor 5576, a seventh thin film transistor 5577, and
an eighth thin film transistor 5578. Note that each of the first
thin film transistor 5571, the second thin film transistor 5572,
the third thin film transistor 5573, the fourth thin film
transistor 5574, the fifth thin film transistor 5575, the sixth
thin film transistor 5576, the seventh thin film transistor 5577,
and the eighth thin film transistor 5578 is an n-channel transistor
and is turned on when the gate-source voltage (V.sub.gs) exceeds
the threshold voltage (V.sub.th).
[0245] Next, connection structures of the flip-flop illustrated in
FIG. 14 are described below.
[0246] A first electrode (one of a source electrode and a drain
electrode) of the first thin film transistor 5571 is connected to
the fourth wiring 5504. A second electrode (the other of the source
electrode and the drain electrode) of the first thin film
transistor 5571 is connected to the third wiring 5503.
[0247] A first electrode of the second thin film transistor 5572 is
connected to the sixth wiring 5506. A second electrode of the
second thin film transistor 5572 is connected to the third wiring
5503.
[0248] A first electrode of the third thin film transistor 5573 is
connected to the fifth wiring 5505. A second electrode of the third
thin film transistor 5573 is connected to a gate electrode of the
second thin film transistor 5572. A gate electrode of the third
thin film transistor 5573 is connected to the fifth wiring
5505.
[0249] A first electrode of the fourth thin film transistor 5574 is
connected to the sixth wiring 5506. A second electrode of the
fourth thin film transistor 5574 is connected to the gate electrode
of the second thin film transistor 5572. A gate electrode of the
fourth thin film transistor 5574 is connected to a gate electrode
of the first thin film transistor 5571.
[0250] A first electrode of the fifth thin film transistor 5575 is
connected to the fifth wiring 5505. A second electrode of the fifth
thin film transistor 5575 is connected to the gate electrode of the
first thin film transistor 5571. A gate electrode of the fifth thin
film transistor 5575 is connected to the first wiring 5501.
[0251] A first electrode of the sixth thin film transistor 5576 is
connected to the sixth wiring 5506. A second electrode of the sixth
thin film transistor 5576 is connected to the gate electrode of the
first thin film transistor 5571. A gate electrode of the sixth thin
film transistor 5576 is connected to the gate electrode of the
second thin film transistor 5572.
[0252] A first electrode of the seventh thin film transistor 5577
is connected to the sixth wiring 5506. A second electrode of the
seventh thin film transistor 5577 is connected to the gate
electrode of the first thin film transistor 5571. A gate electrode
of the seventh thin film transistor 5577 is connected to the second
wiring 5502. A first electrode of the eighth thin film transistor
5578 is connected to the sixth wiring 5506. A second electrode of
the eighth thin film transistor 5578 is connected to the gate
electrode of the second thin film transistor 5572. A gate electrode
of the eighth thin film transistor 5578 is connected to the first
wiring 5501.
[0253] Note that the points at which the gate electrode of the
first thin film transistor 5571, the gate electrode of the fourth
thin film transistor 5574, the second electrode of the fifth thin
film transistor 5575, the second electrode of the sixth thin film
transistor 5576, and the second electrode of the seventh thin film
transistor 5577 are connected are each referred to as a node 5543.
The points at which the gate electrode of the second thin film
transistor 5572, the second electrode of the third thin film
transistor 5573, the second electrode of the fourth thin film
transistor 5574, the gate electrode of the sixth thin film
transistor 5576, and the second electrode of the eighth thin film
transistor 5578 are connected are each referred to as a node
5544.
[0254] Note that the first wiring 5501, the second wiring 5502, the
third wiring 5503, and the fourth wiring 5504 may be referred to as
a first signal line, a second signal line, a third signal line, and
a fourth signal line, respectively. The fifth wiring 5505 and the
sixth wiring 5506 may be referred to as a first power supply line
and a second power supply line, respectively.
[0255] In addition, the signal line driver circuit and the scan
line driver circuit can be formed using only the n-channel TFTs
described in any of Embodiments 1 to 5. The n-channel TFT described
in any of Embodiments 1 to 5 has a high mobility, and thus a
driving frequency of a driver circuit can be increased. Further, in
the case of the n-channel TFT described in any of Embodiments 1 to
5, since parasitic capacitance is reduced by a buffer layer,
frequency characteristics (also referred to as f characteristics)
is excellent. For example, a scan line driver circuit using the
n-channel TFT described in any of Embodiments 1 to 5 can be
operated at high speed, and thus a frame frequency can be increased
and insertion of black images can be realized.
[0256] In addition, when the channel width of the transistor in the
scan line driver circuit is increased or a plurality of scan line
driver circuits are provided, for example, higher frame frequency
can be realized. When a plurality of scan line driver circuits are
provided, a scan line driver circuit for driving even-numbered scan
lines is provided on one side and a scan line driver circuit for
driving odd-numbered scan lines is provided on the opposite side;
thus, increase in frame frequency can be realized.
[0257] Further, when an active matrix light-emitting display device
is manufactured, a plurality of thin film transistors are arranged
in at least one pixel, and thus a plurality of scan line driver
circuits are preferably arranged. FIG. 10B is a block diagram
illustrating an example of an active matrix light-emitting display
device.
[0258] The display device illustrated in FIG. 10B includes, over a
substrate 5400, a pixel portion 5401 having a plurality of pixels
each provided with a display element, a first scan line driver
circuit 5402 and a second scan line driver circuit 5404 that
selects a pixel, and a signal line driver circuit 5403 that
controls input of a video signal to the selected pixel.
[0259] When the video signal input to a pixel of the display device
illustrated in FIG. 10B is a digital signal, the pixel emits or
does not emit light by switching of on/off of a transistor. Thus,
grayscale can be displayed using an area ratio grayscale method or
a time ratio grayscale method. An area ratio grayscale method
refers to a driving method by which one pixel is divided into a
plurality of subpixels and the subpixels are driven independently
based on video signals so that grayscale is displayed. Further, a
time ratio grayscale method refers to a driving method by which a
period during which a pixel is in a light-emitting state is
controlled so that grayscale is displayed.
[0260] Since the response speed of light-emitting elements is
higher than that of liquid crystal elements or the like, the
light-emitting elements are more suitable for a time ratio
grayscale method than liquid-crystal display elements.
Specifically, in the case of performing display with a time gray
scale method, one frame period is divided into a plurality of
subframe periods. Then, in accordance with video signals, the
light-emitting element in the pixel is set in a light-emitting
state or a non-light-emitting state in each subframe period. By
dividing one frame into a plurality of subframes, the total length
of time, in which pixels actually emit light in one frame period,
can be controlled with video signals so that gray scales can be
displayed.
[0261] In the example of the display device illustrated in FIG.
10B, in the case where two TFTs of a switching TFT and a current
control TFT are arranged in one pixel, the first scan line driver
circuit 5402 generates a signal which is input to a first scan line
serving as a gate wiring of the switching TFT, and the second scan
line driver circuit 5404 generates a signal which is input to a
second scan line serving as a gate wiring of the current control
TFT; however, one scan line driver circuit may generate both the
signal which is input to the first scan line and the signal which
is input to the second scan line. In addition, for example, there
is a possibility that a plurality of the first scan lines used for
controlling the operation of the switching element are provided in
each pixel, depending on the number of transistors included in the
switching element. In that case, one scan line driver circuit may
generate all signals that are input to the plurality of first scan
lines, or a plurality of scan line driver circuits may generate
signals that are input to the plurality of first scan lines.
[0262] In addition, also in the light-emitting device, a part of
the driver circuits that can include n-channel TFTs among driver
circuits can be formed over the same substrate as the thin film
transistors of the pixel portion. Alternatively, the signal line
driver circuit and the scan line driver circuit can be formed using
only the n-channel TFTs described in any of Embodiments 1 to 5.
[0263] Moreover, the above-described driver circuit can be used for
electronic paper that drives electronic ink using an element
electrically connected to a switching element, without being
limited to applications to a liquid crystal display device or a
light-emitting device. Electronic paper is also referred to as an
electrophoretic display device (electrophoretic display) and has
advantages in that it has the same level of readability as plain
paper, it has lower power consumption than other display devices,
and it can be made thin and lightweight.
[0264] Electrophoretic displays can have various modes. An
electrophoretic display contains a plurality of microcapsules
dispersed in a solvent or a solute, each of which contains first
particles which are positive-charged and second particles which are
negative-charged. By applying an electric field to the
microcapsules, the particles in the microcapsules are moved in
opposite directions to each other and only the color of the
particles concentrated on one side is exhibited. Note that the
first particles and the second particles each contain a pigment and
do not move without an electric field. Moreover, the colors of the
first particles and the second particles are different from each
other (the colors include achromatic color).
[0265] In this way, an electrophoretic display is a display that
utilizes a so-called dielectrophoretic effect by which a substance
that has a high dielectric constant moves to a high-electric field
region. An electrophoretic display does not need to use a
polarizing plate and a counter substrate, which are required in a
liquid crystal display device, and both the thickness and weight of
the electrophoretic display device can be half of those of a liquid
crystal display device.
[0266] A solution in which the aforementioned microcapsules are
dispersed throughout a solvent is referred to as electronic ink.
This electronic ink can be printed on a surface of glass, plastic,
cloth, paper, or the like. Furthermore, with the use of a color
filter or particles each containing a pigment, color display is
possible, as well.
[0267] In addition, if a plurality of the aforementioned
microcapsules are arranged as appropriate over an active matrix
substrate so as to be interposed between two electrodes, an active
matrix display device can be completed, and display can be
performed by application of an electric field to the microcapsules.
For example, the active matrix substrate obtained in Embodiment 2
can be used.
[0268] Note that the first particles and the second particles in
the microcapsules may each be formed using a single material
selected from a conductive material, an insulating material, a
semiconductor material, a magnetic material, a liquid crystal
material, a ferroelectric material, an electroluminescent material,
an electrochromic material, or a magnetophoretic material or formed
of a composite material of any of these.
Embodiment 8
[0269] A thin film transistor according to an embodiment of the
present invention is manufactured, and a semiconductor device
having a display function (also referred to as a display device)
can be manufactured using the thin film transistor for a pixel
portion and further for a driver circuit. Further, part or whole of
a driver circuit can be formed over the same substrate as a pixel
portion, using a thin film transistor according to an embodiment of
the present invention, whereby a system-on-panel can be
obtained.
[0270] The display device includes a display element. As the
display element, a liquid crystal element (also referred to as a
liquid crystal display element) or a light-emitting element (also
referred to as a light-emitting display element) can be used.
Light-emitting elements include, in its category, an element whose
luminance is controlled by a current or a voltage, and specifically
include an inorganic electroluminescent (EL) element, an organic EL
element, and the like. Further, a display medium whose contrast is
changed by an electric effect, such as an electronic ink, can be
used.
[0271] In addition, the display device includes a panel in which
the display element is sealed, and a module in which an IC
including a controller or the like is mounted on the panel. An
embodiment of the present invention relates to one mode of an
element substrate before the display element is completed in a
manufacturing process of the display device, and the element
substrate is provided with means for supplying a current to the
display element in each of a plurality of pixels. Specifically, the
element substrate may be in a state provided with only a pixel
electrode of the display element, a state after a conductive film
to be a pixel electrode is formed and before the conductive film is
etched to form the pixel electrode, or any of other states.
[0272] Note that a display device in this specification means an
image display device, a display device, or a light source
(including a lighting device). Further, the display device includes
any of the following modules in its category: a module to which a
connector such as a flexible printed circuit (FPC), tape automated
bonding (TAB) tape, or a tape carrier package (TCP) is attached; a
module having TAB tape or a TCP which is provided with a printed
wiring board at the end thereof; and a module having an integrated
circuit (IC) which is directly mounted on a display element by a
chip on glass (COG) method.
[0273] In this embodiment, a liquid crystal display device will be
described as an example of a semiconductor device according to an
embodiment of the present invention.
[0274] FIGS. 16A and 16B illustrate an active-matrix liquid crystal
display device to which an embodiment of the present invention is
applied. FIG. 16A is a plan view of the liquid crystal display
device. FIG. 16B is a cross-sectional view taken along line V-X of
FIG. 16A. A thin film transistor 201 used in the semiconductor
device can be manufactured similarly to the thin film transistor
described in Embodiment 4 and is a highly reliable thin film
transistor including an IGZO semiconductor layer and a buffer layer
formed using an oxide semiconductor layer having n-type
conductivity and containing In, Ga, and Zn. Any of the thin film
transistors described in Embodiments 1, 3, and 5 can also be used
as the thin film transistor 201 of this embodiment.
[0275] The liquid crystal display device of this embodiment in FIG.
16A includes a source wiring layer 202, an inverted-staggered thin
film transistor 201 with a multi-gate structure, a gate wiring
layer 203, and a capacitor wiring layer 204.
[0276] Further, in FIG. 16B, in the liquid crystal display device
of this embodiment, a substrate 200 provided with the thin film
transistor 201 with a multi-gate structure, an insulating layer
211, an insulating layer 212, an insulating layer 213, an electrode
layer 255 used for a display element, an insulating layer 261
serving as an alignment film, and a polarizing plate 268 and a
substrate 266 provided with an insulating layer 263 serving as an
alignment film, an electrode layer 265 used for a display element,
a coloring layer 264 serving as a color filter, and a polarizing
plate 267 face to each other with a liquid crystal layer 262
interposed therebetween; thus, a liquid crystal display element 260
is formed.
[0277] Although FIGS. 16A and 16B illustrate an example of a
transmissive liquid crystal display device, an embodiment of the
present invention can also be applied to a reflective liquid
crystal display device and a transflective liquid crystal display
device.
[0278] While FIGS. 16A and 16B illustrate an example of the liquid
crystal display device in which the polarizing plate 267 is
provided in a position outer than the substrate 266 (on the viewer
side) and the coloring layer 264 and the electrode layer 265 used
for a display element are provided in a position inner than the
substrate 266 in that order, the polarizing plate 267 may be
provided in a position inner than the substrate 266. The layered
structure of the polarizing plate and the coloring layer is not
limited to that shown in FIG. 16B and may be set as appropriate
depending on materials of the polarizing plate and the coloring
layer or conditions of manufacturing steps. Further, a
light-blocking film serving as a black matrix may be provided.
[0279] The electrode layers 255 and 265 each serving as a pixel
electrode layer can be formed using a light-transmitting conductive
material such as indium oxide containing tungsten oxide, indium
zinc oxide containing tungsten oxide, indium oxide containing
titanium oxide, indium tin oxide containing titanium oxide, indium
tin oxide (hereinafter referred to as ITO), indium zinc oxide, or
indium tin oxide to which silicon oxide is added.
[0280] A conductive composition containing a conductive high
molecule (also referred to as a conductive polymer) can be used for
the electrode layers 255 and 265. The pixel electrode formed using
the conductive composition preferably has a sheet resistance of
10000 ohms per square or less and a transmittance of 70% or more at
a wavelength of 550 nm. Further, the resistance of the conductive
high molecule contained in the conductive composition is preferably
0.1 .OMEGA.cm or less.
[0281] As the conductive high molecule, a so-called 7c-electron
conjugated conductive polymer can be used. For example, polyaniline
or a derivative thereof, polypyrrole or a derivative thereof,
polythiophene or a derivative thereof, a copolymer of two or more
kinds of them, and the like can be given.
[0282] Through the above process, a highly reliable liquid crystal
display device as a semiconductor device can be manufactured.
[0283] This embodiment can be implemented by being combined with
any of the structures described in other embodiments, as
appropriate.
Embodiment 9
[0284] In this embodiment, an example of electronic paper will be
described as a semiconductor device according to an embodiment of
the present invention.
[0285] FIG. 17 illustrates active matrix electronic paper as an
example of a semiconductor device to which an embodiment of the
present invention is applied. A thin film transistor 581 used for
the semiconductor device can be manufactured similarly to the thin
film transistor described in Embodiment 4 and is a highly reliable
thin film transistor including an IGZO semiconductor layer and a
buffer layer formed using an oxide semiconductor layer having
n-type conductivity and containing In, Ga, and Zn. Alternatively,
any of the thin film transistors described in Embodiments 1, 3, and
4 may be used as the thin film transistor 581 of this
embodiment.
[0286] The electronic paper in FIG. 17 is an example of a display
device using a twisting ball display system. The twisting ball
display system refers to a method in which spherical particles each
colored in black and white are arranged between a first electrode
layer and a second electrode layer which are electrode layers used
for a display element, and a potential difference is generated
between the first electrode layer and the second electrode layer to
control orientation of the spherical particles, so that display is
performed.
[0287] The thin film transistor 581 is an inverted-staggered thin
film transistor with a multi-gate structure, and a source electrode
and a drain electrode thereof is in contact with a first electrode
layer 587 at an opening formed in an insulating layer 585, whereby
the thin film transistor 581 is electrically connected to the first
electrode layer 587. Between the first electrode layer 587 and a
second electrode layer 588, spherical particles 589 each having a
black region 590a, a white region 590b, and a cavity 594 around the
regions, which is filled with liquid, are provided. A space around
the spherical particles 589 is filled with a filler 595 such as a
resin (see FIG. 17).
[0288] In FIG. 17, an electrode layer containing a
light-transmitting conductive high molecule is used as the first
electrode layer. An inorganic insulating film is provided over the
first electrode layer 587. The inorganic insulating film serves as
a barrier film which prevents ionic impurities from diffusing from
the first electrode layer 587.
[0289] Further, instead of the twisting ball, an electrophoretic
element may be used. A microcapsule having a diameter of about 10
.mu.m to 200 .mu.m in which transparent liquid, positively charged
white microparticles, and negatively charged black microparticles
are encapsulated, is used. In the microcapsule which is provided
between the first electrode layer and the second electrode layer,
when an electric field is applied by the first electrode layer and
the second electrode layer, the white microparticles and black
microparticles move to opposite sides from each other, so that
white or black can be displayed. A display element using this
principle is an electrophoretic display element and is called
electronic paper in general. The electrophoretic display element
has higher reflectance than a liquid crystal display element and
thus, an auxiliary light is unnecessary, power consumption is low,
and a display portion can be recognized in a dim place. In
addition, even when power is not supplied to the display portion,
an image which has been displayed once can be maintained.
Accordingly, a displayed image can be stored even if a
semiconductor device having a display function (which may be
referred to as a display device simply or a semiconductor device
provided with a display device) is distanced from an electric wave
source.
[0290] Through the above process, highly reliable electronic paper
as a semiconductor device can be manufactured.
[0291] This embodiment can be implemented by being combined with
any of the structures described in other embodiments, as
appropriate.
Embodiment 10
[0292] In this embodiment, an example of a light-emitting display
device will be described as a semiconductor device according an
embodiment of the present invention. As a display element included
in a display device, a light-emitting element utilizing
electroluminescence is described here. Light-emitting elements
utilizing electroluminescence are classified according to whether a
light-emitting material is an organic compound or an inorganic
compound. In general, the former is referred to as an organic EL
element, and the latter is referred to as an inorganic EL
element.
[0293] In an organic EL element, by application of a voltage to a
light-emitting element, electrons and holes are separately injected
from a pair of electrodes into a layer containing a light-emitting
organic compound, and a current flows. The carriers (electrons and
holes) are recombined and thus, the light-emitting organic compound
is excited. The light-emitting organic compound returns to a ground
state from the excited state, thereby emitting light. Owing to such
a mechanism, this light-emitting element is referred to as a
current-excitation light-emitting element.
[0294] The inorganic EL elements are classified according to their
element structures into a dispersion-type inorganic EL element and
a thin-film inorganic EL element. A dispersion-type inorganic EL
element has a light-emitting layer where particles of a
light-emitting material are dispersed in a binder, and its light
emission mechanism is donor-acceptor recombination type light
emission that utilizes a donor level and an acceptor level. A
thin-film inorganic EL element has a structure where a
light-emitting layer is sandwiched between dielectric layers, which
are further sandwiched between electrodes, and its light emission
mechanism is localized type light emission that utilizes
inner-shell electron transition of metal ions. Note that
description is made here using an organic EL element as a
light-emitting element.
[0295] FIGS. 18A and 18B illustrate an active-matrix light-emitting
display device as an example of a semiconductor device to which an
embodiment of the present invention is applied. FIG. 18A is a plan
view of the light-emitting display device, and FIG. 18B is a
cross-sectional view taken along line Y-Z of FIG. 18A. Note that
FIG. 19 illustrates an equivalent circuit of the light-emitting
display device illustrated in FIGS. 18A and 18B.
[0296] Thin film transistors 301 and 302 used for a semiconductor
device can be manufactured similarly to any of the thin film
transistors described in Embodiments 1 and 2 and are highly
reliable thin film transistors each including an IGZO semiconductor
layer and a buffer layer formed using an oxide semiconductor layer
having n-type conductivity and containing In, Ga, and Zn.
Alternatively, any of the thin film transistors described in
Embodiments 3 to 5 may be used as the thin film transistors 301 and
302 of this embodiment.
[0297] The light-emitting display device of this embodiment
illustrated in FIG. 18A and FIG. 19 includes the thin film
transistors 301 and 302 with a multi-gate structure, a
light-emitting element 303, a capacitor element 304, a source
wiring layer 305, a gate wiring layer 306, and a power supply line
307. The thin film transistors 301 and 302 are n-channel thin film
transistors.
[0298] In FIG. 18B, the light-emitting display device of this
embodiment includes the thin film transistor 302; an insulating
layer 311; an insulating layer 312; an insulating layer 313; a
partition wall 321; and a first electrode layer 320, an
electroluminescent layer 322, and a second electrode layer 323
which are used for the light-emitting element 303.
[0299] The insulating layer 313 is preferably formed using an
organic resin such as acrylic, polyimide, or polyamide or using
siloxane.
[0300] Since the thin film transistor 302 in the pixel is n-type in
this embodiment, a cathode is desirably used as the first electrode
layer 320 which is a pixel electrode layer. Specifically, for the
cathode, a material with a low work function such as Ca, Al, CaF,
MgAg, or AlLi can be used.
[0301] The partition wall 321 is formed using an organic resin
film, an inorganic insulating film, or organic polysiloxane. It is
particularly preferable that the partition wall 321 be formed using
a photosensitive material and an opening be formed over the first
electrode layer 320 so that a sidewall of the opening is formed as
an inclined surface with continuous curvature.
[0302] The electroluminescent layer 322 may be formed using a
single layer or a plurality of layers stacked.
[0303] The second electrode layer 323 is formed as an anode to
cover the electroluminescent layer 322. The second electrode layer
323 can be formed using a light-transmitting conductive film using
any of the light-transmitting conductive materials listed in
Embodiment 7 for the pixel electrode layer. The second electrode
layer 323 may be formed using a titanium nitride film or a titanium
film instead of the above-described light-transmitting conductive
film. The light-emitting element 303 is formed by overlapping of
the first electrode layer 320, the electroluminescent layer 322,
and the second electrode layer 323. After that, a protective film
may be formed over the second electrode layer 323 and the partition
wall 321 in order to prevent entry of the air (oxygen, hydrogen,
moisture, carbon dioxide, or the like) into the light-emitting
element 303. As the protective film, a silicon nitride film, a
silicon nitride oxide film, a DLC film, or the like can be
formed.
[0304] Further, in a practical case, it is preferable that a
display device completed up to the state illustrated in FIG. 18B be
packaged (sealed) with a protective film (such as a laminate film
or an ultraviolet curable resin film) or a cover material with high
air-tightness and little degasification so that the display device
is not exposed to the outside air.
[0305] Next, structures of the light-emitting element will be
described with reference to FIGS. 20A to 20C. A cross-sectional
structure of a pixel will be described by taking an n-channel
driving TFT as an example. Driving TFTs 7001, 7011, and 7021 used
for the semiconductor devices illustrated in FIGS. 20A to 20C can
be manufactured similarly to the thin film transistor described in
Embodiment 1 and are highly reliable thin film transistors each
having an IGZO semiconductor layer and a buffer layer formed using
an oxide semiconductor layer having n-type conductivity and
containing In, Ga, and Zn. Alternatively, any of the thin film
transistors described in Embodiments 2 to 4 can be used as the
driving TFTs 7001, 7011, and 7021.
[0306] In order to extract light emitted from the light-emitting
element, at least one of the anode and the cathode is required to
transmit light. A thin film transistor and a light-emitting element
are formed over a substrate. A light-emitting element can have a
top emission structure, in which light emission is extracted
through the surface opposite to the substrate; a bottom emission
structure, in which light emission is extracted through the surface
on the substrate side; or a dual emission structure, in which light
emission is extracted through the surface opposite to the substrate
and the surface on the substrate side. The pixel structure
according to an embodiment of the present invention can be applied
to a light-emitting element having any of these emission
structures.
[0307] A light-emitting element having a top emission structure
will be described with reference to FIG. 20A.
[0308] FIG. 20A is a cross-sectional view of a pixel in the case
where the driving TFT 7001 is of an n-type and light is emitted
from a light-emitting element 7002 to an anode 7005 side. In FIG.
20A, a cathode 7003 of the light-emitting element 7002 is
electrically connected to the driving TFT 7001, and a
light-emitting layer 7004 and the anode 7005 are stacked in this
order over the cathode 7003. The cathode 7003 can be formed using
any of a variety of conductive materials as long as it has a low
work function and reflect light. For example, Ca, Al, CaF, MgAg,
AlLi, or the like is desirably used. The light-emitting layer 7004
may be formed using a single layer or a plurality of layers
stacked. When the light-emitting layer 7004 is formed using a
plurality of layers, the light-emitting layer 7004 is formed by
stacking an electron-injecting layer, an electron-transporting
layer, a light-emitting layer, a hole-transporting layer, and a
hole-injecting layer in this order over the cathode 7003. Note that
it is not necessary to form all of these layers. The anode 7005 is
formed using a light-transmitting conductive film such as a film of
indium oxide containing tungsten oxide, indium zinc oxide
containing tungsten oxide, indium oxide containing titanium oxide,
indium tin oxide containing titanium oxide, indium tin oxide
(hereinafter referred to as ITO), indium zinc oxide, or indium tin
oxide to which silicon oxide is added.
[0309] The light-emitting element 7002 corresponds to a region
where the cathode 7003 and the anode 7005 sandwich the
light-emitting layer 7004. In the case of the pixel illustrated in
FIG. 20A, light is emitted from the light-emitting element 7002 to
the anode 7005 side as indicated by an arrow.
[0310] Next, a light-emitting element having a bottom emission
structure will be described with reference to FIG. 20B. FIG. 20B is
a cross-sectional view of a pixel in the case where the driving TFT
7011 is of an n-type and light is emitted from a light-emitting
element 7012 to a cathode 7013 side. In FIG. 20B, the cathode 7013
of the light-emitting element 7012 is formed over a
light-transmitting conductive film 7017 that is electrically
connected to the driving TFT 7011, and a light-emitting layer 7014
and an anode 7015 are stacked in this order over the cathode 7013.
Note that a light-blocking film 7016 for reflecting or blocking
light may be formed to cover the anode 7015 when the anode 7015 has
a light-transmitting property. For the cathode 7013, various
materials can be used as in the case of FIG. 20A as long as they
are conductive materials having a low work function. It is to be
noted that the cathode 7013 is formed to have a thickness that can
transmit light (preferably, approximately 5 nm to 30 nm). For
example, an aluminum film with a thickness of 20 nm can be used as
the cathode 7013. In a similar manner to the case of FIG. 20A, the
light-emitting layer 7014 may be formed using either a single layer
or a plurality of layers stacked. The anode 7015 is not required to
transmit light, but can be formed using a light-transmitting
conductive material as in the case of FIG. 20A. As the
light-blocking film 7016, a metal or the like that reflects light
can be used for example; however, it is not limited to a metal
film. For example, a resin or the like to which black pigments are
added may alternatively be used.
[0311] The light-emitting element 7012 corresponds to a region
where the cathode 7013 and the anode 7015 sandwich the
light-emitting layer 7014. In the case of the pixel illustrated in
FIG. 20B, light is emitted from the light-emitting element 7012 to
the cathode 7013 side as indicated by an arrow.
[0312] Next, a light-emitting element having a dual emission
structure will be described with reference to FIG. 20C. In FIG.
20C, a cathode 7023 of a light-emitting element 7022 is formed over
a light-transmitting conductive film 7027 which is electrically
connected to the driving TFT 7021, and a light-emitting layer 7024
and an anode 7025 are stacked in this order over the cathode 7023.
As in the case of FIG. 20A, the cathode 7023 can be formed using
any of a variety of conductive materials as long as it has a low
work function. It is to be noted that the cathode 7023 is formed to
have a thickness that can transmit light. For example, a film of Al
having a thickness of 20 nm can be used as the cathode 7023. As in
FIG. 20A, the light-emitting layer 7024 may be formed using either
a single layer or a plurality of layers stacked. The anode 7025 can
be formed using a light-transmitting conductive material as in the
case of FIG. 20A.
[0313] The light-emitting element 7022 corresponds to a region
where the cathode 7023, the light-emitting layer 7024, and the
anode 7025 overlap with one another. In the case of the pixel
illustrated in FIG. 20C, light is emitted from the light-emitting
element 7022 to both the anode 7025 side and the cathode 7023 side
as indicated by arrows.
[0314] Note that, although an organic EL element is described here
as a light-emitting element, an inorganic EL element can
alternatively be provided as a light-emitting element.
[0315] In this embodiment, the example is described in which a thin
film transistor (a driving TFT) which controls the driving of a
light-emitting element is electrically connected to the
light-emitting element; however, a structure may be employed in
which a TFT for current control is connected between the driving
TFT and the light-emitting element.
[0316] A semiconductor device described in this embodiment is not
limited to the structures illustrated in FIGS. 20A to 20C and can
be modified in various ways based on the spirit of techniques
according to the present invention.
[0317] Through the above process, a highly reliable light-emitting
display device as a semiconductor device can be manufactured.
[0318] This embodiment can be implemented by being combined with
any of the structures described in other embodiments, as
appropriate.
Embodiment 11
[0319] Next, a structure of a display panel, which is an embodiment
of the semiconductor device of the present invention, will be
described below. In this embodiment, a liquid crystal display panel
(also referred to as a liquid crystal panel), which is an
embodiment of a liquid crystal display device having a liquid
crystal element as a display element, and a light-emitting display
panel (also referred to as a light-emitting panel), which is an
embodiment of a semiconductor device having a light-emitting
element as a display element, will be described.
[0320] Next, the appearance and a cross section of a light-emitting
display panel, which is an embodiment of the semiconductor device
of the present invention, will be described with reference to FIGS.
21A and 21B. FIG. 21A is a top view of a panel in which a highly
reliable thin film transistor including an IGZO semiconductor layer
and a buffer layer formed using an oxide semiconductor layer having
n-type conductivity and containing In, Ga, and Zn and a
light-emitting element are sealed between a first substrate and a
second substrate with a sealant. FIG. 21B is a cross-sectional view
taken along line H-I of FIG. 21A.
[0321] A sealant 4505 is provided so as to surround a pixel portion
4502, signal line driver circuits 4503a and 4503b, and scan line
driver circuits 4504a and 4504b which are provided over a first
substrate 4501. In addition, a second substrate 4506 is provided
over the pixel portion 4502, the signal line driver circuits 4503a
and 4503b, and the scan line driver circuits 4504a and 4504b.
Accordingly, the pixel portion 4502, the signal line driver
circuits 4503a and 4503b, and the scan line driver circuits 4504a
and 4504b are sealed together with a filler 4507, by the first
substrate 4501, the sealant 4505, and the second substrate
4506.
[0322] The pixel portion 4502, the signal line driver circuits
4503a and 4503b, and the scan line driver circuits 4504a and 4504b
formed over the first substrate 4501 each include a plurality of
thin film transistors, and a thin film transistor 4510 included in
the pixel portion 4502 and a thin film transistor 4509 included in
the signal line driver circuit 4503a are illustrated as an example
in FIG. 21B.
[0323] Each of the thin film transistors 4509 and 4510 corresponds
to a thin film transistor including an IGZO semiconductor layer and
a buffer layer formed using an oxide semiconductor layer having
n-type conductivity and containing In, Ga, and Zn, and any of the
thin film transistors described in Embodiments 1 to 5 can be used
as the thin film transistors 4509 and 4510. In this embodiment, the
thin film transistors 4509 and 4510 are n-channel thin film
transistors.
[0324] Moreover, reference numeral 4511 denotes a light-emitting
element. A first electrode layer 4517 which is a pixel electrode
included in the light-emitting element 4511 is electrically
connected to a source electrode layer or a drain electrode layer of
the thin film transistor 4510. Note that the structure of the
light-emitting element 4511 is not limited to that described in
this embodiment. The structure of the light-emitting element 4511
can be changed as appropriate depending on the direction in which
light is extracted from the light-emitting element 4511, or the
like.
[0325] In addition, a variety of signals and a potential are
supplied to the signal line driver circuits 4503a and 4503b, the
scan line driver circuits 4504a and 4504b, or the pixel portion
4502 from FPCs 4518a and 4518b.
[0326] In this embodiment, a wiring 4516 is formed using the same
material as that of the source electrode layer or the drain
electrode layer. The wiring 4516 is connected to the pixel portion
4502, the signal line driver circuits 4503a and 4503b, or the scan
line driver circuits 4504a and 4504b through a contact hole (not
illustrated) provided in an insulating film that covers the thin
film transistors 4509 and 4510. In addition, a connection terminal
4515 is formed over the wiring 4516 that is provided over an edge
portion of the first substrate 4501, by using the same material as
that of the first electrode layer 4517.
[0327] The connection terminal 4515 is electrically connected to a
terminal included in the FPC 4518a through an anisotropic
conductive film 4519.
[0328] The second substrate 4506 located in the direction in which
light is extracted from the light-emitting element 4511 needs to
have a light-transmitting property. In that case, a
light-transmitting material such as a glass plate, a plastic plate,
a polyester film, or an acrylic film is used.
[0329] As the filler 4507, an ultraviolet curable resin or a
thermosetting resin can be used, instead of an inert gas such as
nitrogen or argon. For example, PVC (polyvinyl chloride), acrylic,
polyimide, an epoxy resin, a silicone resin, PVB (polyvinyl
butyral), or EVA (ethylene vinyl acetate) can be used. In this
embodiment, nitrogen is used for the filler 4507.
[0330] In addition, if needed, an optical film, such as a
polarizing plate, a circularly polarizing plate (including an
elliptically polarizing plate), a retardation plate (a quarter-wave
plate or a half-wave plate), or a color filter, may be provided as
appropriate on a light-emitting surface of the light-emitting
element. Further, the polarizing plate or the circularly polarizing
plate may be provided with an anti-reflection film. For example,
anti-glare treatment by which reflected light can be diffused by
projections and depressions on the surface so as to reduce the
glare can be performed.
[0331] Driver circuits formed using a single crystal semiconductor
film or polycrystalline semiconductor film over a substrate
separately prepared may be mounted as the signal line driver
circuits 4503a and 4503b and the scan line driver circuits 4504a
and 4504b. Alternatively, only the signal line driver circuits or
part thereof, or the scan line driver circuits or part thereof may
be separately formed and mounted. This embodiment is not limited to
the structure illustrated in FIGS. 21A and 21B.
[0332] Next, the appearance and a cross section of a liquid crystal
display panel, which is an embodiment of the semiconductor device
of the present invention, will be described with reference to FIGS.
22A1, 22A2, and 22B. FIGS. 22A1 and 22A2 are each a top view of a
panel in which highly reliable thin film transistors 4010 and 4011
each having an IGZO semiconductor layer and a buffer layer formed
using an oxide semiconductor layer having n-type conductivity and
containing In, Ga, and Zn, and a liquid crystal element 4013 are
sealed between a first substrate 4001 and a second substrate 4006
with a sealant 4005. FIG. 22B is a cross-sectional view taken along
line M-N of FIGS. 22A1 and 22A2.
[0333] The sealant 4005 is provided so as to surround a pixel
portion 4002 and a scan line driver circuit 4004 which are provided
over the first substrate 4001. The second substrate 4006 is
provided over the pixel portion 4002 and the scan line driver
circuit 4004. Therefore, the pixel portion 4002 and the scan line
driver circuit 4004 are sealed together with liquid crystal 4008,
by the first substrate 4001, the sealant 4005, and the second
substrate 4006. A signal line driver circuit 4003 that is formed
using a single crystal semiconductor film or a polycrystalline
semiconductor film over a substrate separately prepared is mounted
in a region that is different from the region surrounded by the
sealant 4005 over the first substrate 4001.
[0334] Note that the connection method of a driver circuit which is
separately formed is not particularly limited, and a COG method, a
wire bonding method, a TAB method, or the like can be used. FIG.
22A1 illustrates an example of mounting the signal line driver
circuit 4003 by a COG method, and FIG. 22A2 illustrates an example
of mounting the signal line driver circuit 4003 by a TAB
method.
[0335] The pixel portion 4002 and the scan line driver circuit 4004
provided over the first substrate 4001 include a plurality of thin
film transistors. FIG. 22B illustrates the thin film transistor
4010 included in the pixel portion 4002 and the thin film
transistor 4011 included in the scan line driver circuit 4004.
[0336] Each of the thin film transistors 4010 and 4011 corresponds
to a thin film transistor including an IGZO semiconductor layer and
a buffer layer formed using an oxide semiconductor layer having
n-type conductivity and containing In, Ga, and Zn, and any of the
thin film transistors described in Embodiments 1 to 5 can be used
as the thin film transistors 4010 and 4011. In this embodiment, the
thin film transistors 4010 and 4011 are n-channel thin film
transistors.
[0337] A pixel electrode layer 4030 included in the liquid crystal
element 4013 is electrically connected to the thin film transistor
4010. A counter electrode layer 4031 of the liquid crystal element
4013 is formed on the second substrate 4006. A portion where the
pixel electrode layer 4030, the counter electrode layer 4031, and
the liquid crystal layer 4008 overlap with one another corresponds
to the liquid crystal element 4013. Note that the pixel electrode
layer 4030 and the counter electrode layer 4031 are provided with
an insulating layer 4032 and an insulating layer 4033 respectively
which each function as an alignment film, and sandwich the liquid
crystal layer 4008 with the insulating layers 4032 and 4033
interposed between the pixel electrode layer 4030 and the counter
electrode layer 4031.
[0338] Note that the first substrate 4001 and the second substrate
4006 can be formed by using glass, metal (typically, stainless
steel), ceramic, or plastic. As plastic, a fiberglass-reinforced
plastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester
film, or an acrylic resin film can be used. Alternatively, a sheet
with a structure in which an aluminum foil is sandwiched between
PVF films or polyester films may be used.
[0339] Reference numeral 4035 denotes a columnar spacer obtained by
selectively etching an insulating film and is provided to control
the distance between the pixel electrode layer 4030 and the counter
electrode layer 4031 (a cell gap). Further, a spherical spacer may
alternatively be used.
[0340] Further, a variety of signals and a potential are supplied
to the signal line driver circuit 4003 which is formed separately,
the scan line driver circuit 4004, or the pixel portion 4002 from
an FPC 4018.
[0341] In this embodiment, a connection terminal 4015 is formed
using the same conductive film as that of the pixel electrode layer
4030 included in the liquid crystal element 4013, and a wiring 4016
is formed using the same conductive film as that of gate electrode
layers of the thin film transistors 4010 and 4011.
[0342] The connection terminal 4015 is electrically connected to a
terminal included in the FPC 4018 through an anisotropic conductive
film 4019.
[0343] Further, FIGS. 22A and 22B illustrate an example in which
the signal line driver circuit 4003 is formed separately and
mounted on the first substrate 4001; however, this embodiment is
not limited to this structure. The scan line driver circuit may be
separately formed and then mounted, or only part of the signal line
driver circuit or part of the scan line driver circuit may be
separately formed and then mounted.
[0344] FIG. 23 illustrates an example in which a liquid crystal
display module is formed as a semiconductor device by using a TFT
substrate 2600 manufactured according to an embodiment of the
present invention.
[0345] FIG. 23 illustrates an example of a liquid crystal display
module, in which the TFT substrate 2600 and a counter substrate
2601 are fixed to each other with a sealant 2602, and a pixel
portion 2603 including a TFT or the like, a display element 2604
including a liquid crystal layer, and a coloring layer 2605 are
provided between the substrates to form a display region. The
coloring layer 2605 is necessary to perform color display. In the
case of the RGB system, respective coloring layers corresponding to
colors of red, green, and blue are provided for respective pixels.
Polarizing plates 2606 and 2607 and a diffusion plate 2613 are
provided outside the TFT substrate 2600 and the counter substrate
2601. A light source includes a cold cathode tube 2610 and a
reflective plate 2611, and a circuit substrate 2612 is connected to
a wiring circuit portion 2608 of the TFT substrate 2600 through a
flexible wiring board 2609 and includes an external circuit such as
a control circuit or a power source circuit. The polarizing plate
and the liquid crystal layer may be stacked with a retardation
plate interposed therebetween.
[0346] For the liquid crystal display module, a TN (twisted
nematic) mode, an IPS (in-plane-switching) mode, an FFS (fringe
field switching) mode, an MVA (multi-domain vertical alignment)
mode, a PVA (patterned vertical alignment) mode, an ASM (axially
symmetric aligned micro-cell) mode, an OCB (optical compensated
birefringence) mode, an FLC (ferroelectric liquid crystal) mode, an
AFLC (antiferroelectric liquid crystal) mode, or the like can be
used.
[0347] Through the above process, a highly reliable display panel
as a semiconductor device can be manufactured.
[0348] This embodiment can be implemented by being combined with
any of the structures described in other embodiments, as
appropriate.
Embodiment 12
[0349] A semiconductor device according to an embodiment of the
present invention can be applied to a variety of electronic
appliances (including an amusement machine). Examples of electronic
appliances are a television set (also referred to as a television
or a television receiver), a monitor of a computer or the like,
electronic paper, a camera such as a digital camera or a digital
video camera, a digital photo frame, a mobile phone handset (also
referred to as a mobile phone or a mobile phone device), a portable
game console, a portable information terminal, an audio reproducing
device, a large-sized game machine such as a pachinko machine, and
the like. In particular, a liquid crystal display device, a
light-emitting device, an electrophoretic display device, or the
like to which a thin film transistor according to an embodiment of
the present invention is applied as described in Embodiments 8 to
11 can be used for a display portion of an electronic appliance.
Specific examples will be described below.
[0350] A semiconductor device according to an embodiment of the
present invention can be applied to electronic paper as described
in Embodiment 9. Electronic paper can be used for electronic
appliances of a variety of fields as long as they can display data.
For example, electronic paper can be applied to an electronic book
(e-book), a poster, a transportation advertisement in a vehicle
such as a train, displays of various cards such as a credit card,
and the like. Examples of the electronic appliances are illustrated
in FIGS. 24A, 24B, and FIG. 25.
[0351] FIG. 24A illustrates a poster 1601 formed using electronic
paper. In the case where an advertising medium is printed paper,
the advertisement is replaced by manpower; however, by using
electronic paper to which a semiconductor device according to an
embodiment of the present invention is applied, the advertising
display can be changed in a short time. Further, since a thin film
transistor having excellent electric characteristics is used, an
image can be stably displayed without being distorted. Note that
the poster may transmit and receive data wirelessly.
[0352] FIG. 24B illustrates an advertisement 1602 in a vehicle such
as a train. In the case where an advertising medium is printed
paper, the advertisement is replaced by manpower; however, by using
electronic paper to which a semiconductor device according to an
embodiment of the present invention is applied, the advertising
display can be changed in a short time without a lot of manpower.
Further, since a thin film transistor having excellent electric
characteristics is used, an image can be stably displayed without
being distorted. Note that the advertisement in a vehicle may
transmit and receive data wirelessly.
[0353] FIG. 25 illustrates an example of an electronic book 2700.
For example, the electronic book 2700 includes two housings, a
housing 2701 and a housing 2703. The housing 2701 and the housing
2703 are combined with a hinge 2711 so that the electronic book
2700 can be opened and closed with the hinge 2711 as an axis. With
such a structure, the electronic book 2700 can be operated like a
paper book.
[0354] A display portion 2705 and a display portion 2707 are
incorporated in the housing 2701 and the housing 2703,
respectively. The display portion 2705 and the display portion 2707
may display one image or different images. In the case where the
display portion 2705 and the display portion 2707 display different
images, for example, a display portion on the right side (the
display portion 2705 in FIG. 25) can display text and a display
portion on the left side (the display portion 2707 in FIG. 25) can
display graphics.
[0355] FIG. 25 illustrates an example in which the housing 2701 is
provided with an operation portion and the like. For example, the
housing 2701 is provided with a power switch 2721, an operation key
2723, a speaker 2725, and the like. With the operation key 2723,
pages can be turned. Note that a keyboard, a pointing device, and
the like may be provided on the surface of the housing, on which
the display portion is provided. Further, 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 electronic book 2700 may have a function of
an electronic dictionary.
[0356] The electronic book 2700 may transmit and receive data
wirelessly. The structure may be employed in which a desired book
data or the like is purchased and downloaded from an electronic
book server wirelessly.
[0357] FIG. 26A illustrates an example of a television set 9600. In
the television set 9600, a display portion 9603 is incorporated in
a housing 9601. The display portion 9603 can display an image.
Further, the housing 9601 is supported by a stand 9605 here. Any of
the display devices described in Embodiments 8 to 11 can be applied
to the display portion 9603.
[0358] The television set 9600 can be operated by an operation
switch of the housing 9601 or a separate remote controller 9610.
Channels and volume can be controlled by an operation key 9609 of
the remote controller 9610 so that an image displayed on the
display portion 9603 can be controlled. Further, the remote
controller 9610 may be provided with a display portion 9607 for
displaying data output from the remote controller 9610.
[0359] Note that the television set 9600 is provided with a
receiver, a modem, and the like. With the receiver, a general
television broadcast can be received. Further, when the television
set 9600 is connected to a communication network by wired or
wireless connection via the modem, one-way (from a transmitter to a
receiver) or two-way (between a transmitter and a receiver or
between receivers) data communication can be performed.
[0360] FIG. 26B illustrates an example of a digital photo frame
9700. For example, in the digital photo frame 9700, a display
portion 9703 is incorporated in a housing 9701. The display portion
9703 can display various images. For example, the display portion
9703 can display data of an image shot by a digital camera or the
like to function as a normal photo frame.
[0361] Note that the digital photo frame 9700 is provided with an
operation portion, an external connection portion (a USB terminal,
a terminal that can be connected to various cables such as a USB
cable, or the like), a recording medium insertion portion, and the
like. Although they may be provided on the surface on which the
display portion is provided, it is preferable to provide them on
the side surface or the back surface for the design of the digital
photo frame 9700. For example, a memory storing data of an image
shot by a digital camera is inserted in the recording medium
insertion portion of the digital photo frame, whereby the image
data can be transferred and displayed on the display portion
9703.
[0362] The digital photo frame 9700 may transmit and receive data
wirelessly. The structure may be employed in which desired image
data is transferred wirelessly to be displayed.
[0363] FIG. 27 illustrates an example of a digital player 2100 that
is a portable audio device. The digital player 2100 includes a
housing 2130, a display portion 2131, a memory portion 2132, an
operation portion 2133, earphones 2134, a control portion 2137, and
the like. Note that headphones or wireless earphones may be used
instead of the earphones 2134. Any of the display devices described
in Embodiments 8 to 11 can be applied to the display portion
2131.
[0364] Further, images and sounds (music) can be recorded and
reproduced with the use of the memory portion 2132 by operating the
operation portion 2133. Note that power consumption of the display
portion 2131 can be suppressed through display of white text on
black background. Note that a memory provided in the memory portion
2132 may be removable.
[0365] FIG. 28 illustrates an example of a mobile phone handset
1000. The mobile phone handset 1000 is provided with a display
portion 1002 incorporated in a housing 1001, an operation button
1003, an external connection port 1004, a speaker 1005, a
microphone 1006, and the like. Any of the display devices described
in Embodiments 8 to 11 can be applied to the display portion
1002.
[0366] When the display portion 1002 of the mobile phone 1000
illustrated in FIG. 28 is touched with a finger or the like, data
can be input into the mobile phone 1000. Further, operations such
as making calls and composing mails can be performed by touching
the display portion 1002 with a finger or the like.
[0367] There are mainly three screen modes of the display portion
1002. The first mode is a display mode mainly for displaying an
image. The second mode is an input mode mainly for inputting data
such as text. The third mode is a display-and-input mode in which
two modes of the display mode and the input mode are combined.
[0368] For example, in the case of making a call or composing a
mail, a text input mode mainly for inputting text is selected for
the display portion 1002 so that text displayed on a screen can be
inputted. In that case, it is preferable to display a keyboard or
number buttons on almost all area of the screen of the display
portion 1002.
[0369] When a detection device including a sensor for detecting
inclination, such as a gyroscope or an acceleration sensor, is
provided inside the mobile phone 1000, display in the screen of the
display portion 1002 can be automatically switched by determining
the direction of the mobile phone 1000 (whether the mobile phone
1000 is set horizontally or vertically for a landscape mode or a
portrait mode).
[0370] The screen modes are switched by touching the display
portion 1002 or operating the operation button 1003 of the housing
1001. Alternatively, the screen modes may be switched depending on
the kind of the image displayed on the display portion 1002. For
example, when a signal of an image displayed on the display portion
is the one of moving image data, the screen mode is switched to the
display mode. When the signal is the one of text data, the screen
mode is switched to the input mode.
[0371] Further, in the input mode, when input by touching the
display portion 1002 is not performed for a certain period while a
signal detected by the optical sensor in the display portion 1002
is detected, the screen mode may be controlled so as to be switched
from the input mode to the display mode.
[0372] The display portion 1002 may function as an image sensor.
For example, an image of the palm print, the fingerprint, or the
like is taken by touching the display portion 1002 with the palm or
the finger, whereby personal authentication can be performed.
Further, by providing a backlight or sensing light source emitting
a near-infrared light for the display portion, an image of a finger
vein, a palm vein, or the like can be taken.
[0373] This embodiment can be combined with any of the structures
described in the other embodiments as appropriate.
[0374] This application is based on Japanese Patent Application
serial no. 2008-197137 filed with Japan Patent Office on Jul. 31,
2008, the entire contents of which are hereby incorporated by
reference.
* * * * *