U.S. patent application number 14/335401 was filed with the patent office on 2014-11-06 for semiconductor storage device and manufacturing method thereof.
The applicant listed for this patent is Renesas Electronics Corporation. Invention is credited to Hirotaka HAMAMURA, Digh HISAMOTO, Toshiyuki MINE, Yasuhiro SHIMAMOTO, Itaru YANAGI.
Application Number | 20140327066 14/335401 |
Document ID | / |
Family ID | 39675406 |
Filed Date | 2014-11-06 |
United States Patent
Application |
20140327066 |
Kind Code |
A1 |
YANAGI; Itaru ; et
al. |
November 6, 2014 |
SEMICONDUCTOR STORAGE DEVICE AND MANUFACTURING METHOD THEREOF
Abstract
In a non-volatile memory in which writing/erasing is performed
by changing a total charge amount by injecting electrons and holes
into a silicon nitride film serving as a charge accumulation layer,
in order to realize a high efficiency of a hole injection from a
gate electrode, the gate electrode of a memory cell comprises a
laminated structure made of a plurality of polysilicon films with
different impurity concentrations, for example, a two-layered
structure comprising a p-type polysilicon film with a low impurity
concentration and a p.sup.|-type polysilicon film with a high
impurity concentration deposited thereon.
Inventors: |
YANAGI; Itaru; (Kunitachi,
JP) ; MINE; Toshiyuki; (Fussa, JP) ; HAMAMURA;
Hirotaka; (Kodaira, JP) ; HISAMOTO; Digh;
(Kokubunji, JP) ; SHIMAMOTO; Yasuhiro;
(Tokorozawa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Renesas Electronics Corporation |
Kawasaki-shi |
|
JP |
|
|
Family ID: |
39675406 |
Appl. No.: |
14/335401 |
Filed: |
July 18, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13845005 |
Mar 17, 2013 |
8816426 |
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14335401 |
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11965391 |
Dec 27, 2007 |
8410543 |
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13845005 |
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Current U.S.
Class: |
257/324 |
Current CPC
Class: |
H01L 27/115 20130101;
H01L 27/11573 20130101; H01L 27/105 20130101; H01L 29/513 20130101;
H01L 27/11521 20130101; H01L 27/11568 20130101; H01L 29/40114
20190801; G11C 16/0466 20130101; H01L 27/11526 20130101; H01L
29/42344 20130101; H01L 29/40117 20190801; H01L 29/792
20130101 |
Class at
Publication: |
257/324 |
International
Class: |
H01L 29/792 20060101
H01L029/792 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 1, 2007 |
JP |
2007-23301 |
Jun 12, 2007 |
JP |
2007-155472 |
Claims
1. A semiconductor storage device, comprising: a non-volatile
memory having an insulating gate type field-effect transistor
causing electric field effect to a surface of a semiconductor
substrate from a gate electrode via a gate insulating film formed
on a main surface of the semiconductor substrate; the gate
insulating film including a first insulating film configuring a
charge retention layer, a second insulating film formed between the
first insulating film and the semiconductor substrate, and a third
insulating film formed between the first insulating film and the
gate electrode; and the non-volatile memory using, as retention
charges for information storage, holes injected into the charge
retention layer from the gate electrode via the third insulating
film and electrons injected into the charge retention layer from
the semiconductor substrate via the second insulating film at a
time of applying a positive voltage to the gate electrode, wherein
the gate electrode includes a first silicon layer formed to be in
contact with the gate insulating film and a second silicon layer
formed to be separated from the gate insulating film via the first
silicon layer and doped with an impurity with a concentration
higher than that of the first silicon layer.
2-33. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims priority from Japanese Patent
Applications No. JP 2007-023301 filed on Feb. 1, 2007, and No. JP
2007-155472 filed on Jun. 12, 2007, the contents of which are
hereby incorporated by reference into this application.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to a semiconductor storage
device and a manufacturing method thereof and, in particular, to an
effective technique applied to a semiconductor storage device
including a non-volatile memory using hole injection from a gate
electrode and hot electron injection from a semiconductor substrate
as a carrier injection system.
BACKGROUND OF THE INVENTION
[0003] Currently, LSIs obtained by integrating semiconductor
devices are used for controlling various systems to constitute
infrastructures supporting society. Since an operation of a current
LSI is based upon conduction of an arithmetic processing according
to a program, in most cases, it is inevitable that a program is
stored, so that a non-volatile memory which is one of semiconductor
memories incorporated into the LSI becomes considerably important
as a semiconductor device. When the LSI is used in various
applications, programs are required for re-programming to handle
the applications. So a rewritable non-volatile memory in which
storage information is maintained even when a power source of the
LSI is turned off is essential.
[0004] As a representative non-volatile memory, a so-called
floating gate type memory and a memory having an insulating layer
for a charge accumulation layer have been known. Especially, the
latter memory where insulating films are stacked and charges are
accumulated in interfaces therebetween or traps in the films is not
required for formation of another electrically conductive layer
that the floating gate type memory requires. Therefore, it is known
that the memory can be formed in a CMOS-LSI process with a good
consistency. Since both characteristics of retaining charges and
rewriting can be achieved, an insulating film obtained by
laminating a silicon nitride film and a silicon oxide film is
widely used as the charge accumulation layer. A non-volatile memory
including such a laminated insulating film is commonly known as
MONOS (Metal-Oxide-Nitride-Oxide-Semiconductor) type memory.
[0005] As a representative example of the MONOS type memory, there
is a two-transistor cell obtained by connecting a memory transistor
and a selection transistor in series. The memory transistor
performs injection/discharge of charges over a whole channel region
by using a direct tunnel current and an F-N (Folwer-Nordheim)
tunnel current generated by a bias applied between the channel and
a gate electrode.
[0006] However, the above-mentioned MONOS type memory is required
to have a sufficient charge retention characteristic while
conducting injection/discharge of charges, and that causes various
problems. For example, when a laminated insulating film is made
thicker for securing the charge retention characteristic
sufficiently for a practical use of the MONOS type memory, it
becomes difficult to perform writing/erasing of data, so that a
time period for writing/erasing exceeds a practical range.
[0007] On the other hand, a system in which rewriting of storage
information is performed by injecting two electrically different
charges (electrons and holes) as hot carriers instead of
discharging charges has been proposed in U.S. Pat. No. 6,215,148
(Patent Document 1). The system is for performing a charge
injection effectively even in a thick insulating film by injecting
hot carriers. According to the system, electrons and holes can be
alternately injected at a local place. Therefore, different charge
injection states are produced at end portions of a planar type MOS
transistor in a channel direction, namely, end portions of a source
and a drain, so that they can be read as charge information.
[0008] A MONOS type memory using the above-mentioned hot carrier
injection system adopts a device structure of a MOS transistor
basically, and a gate insulating film is obtained by replacing an
ordinary silicon oxide film with a three-layered insulating film
made of a silicon oxide film, a silicon nitride film, and a silicon
oxide film. As a method for configuring a memory array, it has been
proposed that the formation of a source and a drain under a thick
element isolation oxide film, and a source and a drain are formed
in a line shape in an extending direction of a gate electrode to
use them as wires. When focusing on one memory cell in respective
memory arrays, in many cases, basic operations of the memory cell
are similar, and are explained below.
[0009] A plan arrangement and a sectional structure of the
above-mentioned MONOS type memory are shown in FIG. 1 and FIG. 2.
In figures, a reference numeral 100 denotes a silicon substrate,
200 and 300 denote diffusion layers of a source and a drain, 910
and 940 denote silicon oxide films, 920 denotes a silicon nitride
film, and 500 denotes a gate electrode made of a doped polysilicon
film.
[0010] Writing operation, erasing operation, and reading operation
of the above-mentioned MONOS type memory are explained with
reference to FIG. 3, FIG. 4, FIG. 5, and FIG. 6.
[0011] As shown in FIG. 3, the gate electrode 500 (a word line WL)
is applied with 15 V, the diffusion layer 200 (BL1) is applied with
0 V, and the diffusion layer 300 (BL2) is applied with 5 V at a
writing time. Electrons accelerated by an electric field of a
channel are put in a hot carrier state and are injected into a
charge accumulation portion at an end portion of the diffusion
layer 300 (BL2). This is known as an avalanche phenomenon and a
substrate bias acceleration are used as a method for producing the
hot carrier.
[0012] As shown in FIG. 4, the gate electrode 500 (word line WL) is
applied with -6 V, the diffusion layer 200 (BL1) is applied with 0
V, and the diffusion layer 300 (BL2) is applied with 6 V at an
erasing time. Holes are generated at the end portion of the
diffusion layer 300 (BL2) by tunneling phenomenon between the bands
and they are injected into the charge accumulation portion by
acceleration caused by bias between the diffusion layer (BL2) and
the substrate.
[0013] As shown in FIG. 5, by applying 3 V to the gate electrode
500 (word line WL), applying 1 V to the diffusion layer 200 (BL1),
and applying 0 V to the diffusion layer 300 (BL2), an amount of
channel current flowing in a direction of the arrow shown in FIG. 5
is read as accumulated charge information at a reading time. That
is, a threshold is high when electrons have been injected at an end
portion of the diffusion layer 300 (BL2) so that no channel current
flows, but the threshold is low when holes have been injected at
the end portion so that much channel current flows.
[0014] In the case of the above-mentioned MONOS type memory, the
threshold is largely influenced by charges injected at an end
portion of a diffusion layer serving as a source side at the
reading time, but does not depend on a charge state of an end
portion of the diffusion layer serving as a drain side so much.
Therefore, one memory cell can be used as 2 bits by using the
above-mentioned diffusion layer 200 and the diffusion layer 300 in
an interchanging manner. FIG. 6 shows that that reading operation
is performed by injecting electrons and holes into the diffusion
layer 200 (BL1) and the diffusion layer 300 (BL2), respectively.
Here, it is shown that the holes (shown by a white circle) and the
electrons (shown by a black circle) are accumulated in the
diffusion layer 200 (BL1) and in the diffusion layer 300 (BL2),
respectively.
[0015] U.S. Pat. No. 5,969,383 (Patent Document 2) and U.S. Pat.
No. 6,477,084 (Patent Document 3) disclose a memory cell called
"split gate structure" as another example of the MONOS type memory.
In the memory cell, two MOS transistors (a selection transistor and
a memory transistor), basically each based upon an n-channel type
MOS transistor, are coupled to each other in a state that the
memory transistor is stacked in a vertical manner beside the
selection transistor. An equivalent circuit of the memory cell is
shown in FIG. 9. FIG. 7 and FIG. 8 are a plan view and a sectional
view of the memory cell corresponding to the circuit shown in FIG.
9. In figures, reference numerals 210 and 310 denote n.sup.-
diffusion layers, 900 denotes a gate insulating film made of a
silicon oxide film, 500 denotes a selector gate, 550 denotes a
memory gate, and 960 denotes a side wall spacer made of a silicon
oxide film.
[0016] Here, an operating method for the memory cell will be first
explained, but a manufacturing method for the memory cell and the
like will be later explained in detail by using embodiments
thereof. A circuit configuration of a memory array using the memory
cells is shown in FIG. 10. Respective gate electrodes (selector
gate 500 and a memory gate 550) of the selection transistor and the
memory transistor configure word lines indicated by SGL and MGL.
The diffusion layer 300 of the selection transistor configures a
bit line BL, and the diffusion layer 200 of the memory transistor
configures a source line SL.
[0017] Representative writing/erasing operations of the
above-mentioned memory cell are shown in FIG. 11 and FIG. 12. A
gate insulating film 950 of the memory gate 550 has a MONOS
structure that a silicon nitride film is interposed between two
layers of silicon oxide films. A gate insulating film 900 of the
selector gate 500 comprises a silicon oxide film. The diffusion
layers 200 and 300 are formed by ion implantation of impurities
using the selector gate 500 and the memory gate 550 as masks,
respectively. As basic operations of the memory cell, there are
four states of (1) writing, (2) erasing, (3) retaining, and (4)
reading. However, the designations of the four states are used as
typical ones, and the "writing" and the "erasing" can be called in
an inversed manner. Although the operations are explained based on
typical ones, it is considered that there are various different
operations. Here, a memory cell configured by two n-channel type
MOS transistors (a selection transistor and a memory transistor)
will be described, but a memory cell configured by two p-channel
type MOS transistors can also be similarly explained in
principle.
[0018] (1) The writing operation is shown in FIG. 11. A positive
potential is applied to the diffusion layer 200 on the memory gate
550 side and the same ground potential as that of a silicon
substrate 100 is applied to the diffusion layer 300 on the selector
gate 500 side. By applying a high gate overdrive voltage to the
memory gate 550 with respect to the silicon substrate 100, a
channel under the memory gate 550 is put in ON state. In this case,
by setting the potential of the selector gate 500 to be higher than
the threshold by about 0.1 V to 0.2 V, switching to ON state is
conducted. At this time, since the most intense electric field
occurs near a boundary between two gate electrodes (500, 550), many
hot electrons are generated to be injected into a gate insulating
film 950 on the memory gate 550 side. A reference numeral 800 shows
generation of carries by impact ionization. The electrons and holes
are shown by a white circle and a hatching black circle,
respectively. This phenomenon is also known as source side
injection (SSI).
[0019] A feature of a hot electron injection according to this
system is that the injection is intensively performed on the end
portion of the memory gate 550 on the selector gate 500 side
because the electric field concentrates near the boundary between
the selector gate 500 and the memory gate 550. In a floating gate
type memory, a charge retention layer is made of an electrically
conductive film. By contrast, in the insulating film type memory,
electrons are accumulated in an insulating film, therefore,
electrons are retained at an extremely small region.
[0020] (2) The erasing operation is shown in FIG. 12. By applying a
negative potential to the memory gate 550 and applying a positive
potential to the diffusion layer 200 on the memory gate 550 side,
intense inversion occurs in an overlapping region of the memory
gate 550 at the end portion of the diffusion layer 200 and the
diffusion layer 200, so that tunnel phenomenon between the bands is
caused, and hot holes 810 are produced. In the memory cell, the
holes generated are accelerated toward the channel direction, and
are attracted by bias of the memory gate 550 to be injected into
the gate insulating film 950, so that the erasing operation is
performed. It is shown that the holes generated produce secondary
electron-hole pairs 820 in FIG. 12. These carriers are also
injected into the gate insulating film 950. That is, the threshold
of the memory gate 550 which has been increased due to charge of
electrons is pulled down by charges of the holes injected.
[0021] (3) at the charge retaining time, the charge is retained as
charge of carriers injected into the gate insulating film 950.
Since movement of the carriers in the gate insulating film 950 is
extremely small and slow, the charge is stably retained even if a
voltage is not applied to the electrode.
[0022] (4) at the reading time, by applying a positive potential to
the diffusion layer 300 on the selector gate 500 side and applying
a positive potential to the selector gate 500, a channel under the
selector gate 500 is turned ON. Here, by application of a proper
memory gate potential (that is, an intermediate potential between
the threshold in the writing state and the threshold in the erasing
state) in which a difference between the thresholds of the memory
gate 550 given by the writing/the erasing states can be
discriminated, charge information retained is read as a
current.
[0023] As described above, the feature of the carrier injection
using hot carriers lies in that the injection can be performed
efficiently even if an insulating film between the silicon
substrate and the silicon nitride film which is the charge
retention layer is thick. This is because energy of hot carriers is
approximately equal to a barrier potential of the insulating film
measured from a silicon band. However, injection of carriers in
high energy state from the substrate side via the insulating film
produces many defects on an interface between the channel of the
substrate and the insulating film. So carriers are trapped in the
defects at a reading time, so that failures at the reading time
such as degradation or fluctuation of the threshold are caused.
[0024] Especially, since band offset of the silicon oxide film
serving as the gate insulating film 900 directly existing on the
channel with respect to silicon is higher to holes than to
electrons, a hot hole injection requires higher energy holes than
an electron energy in the hole electron injection. Therefore, when
the hot hole injection is adopted, damage imparted to an interface
between the channel and the insulating film becomes more severe
than that in a case that the hot electron injection is adopted.
[0025] In such a hot carrier injection system such as a source side
injection (SSI) and a band-to-band tunnel hot hole injection
(BTBTHH injection), a carrier injection is performed at a local
place, so that carriers injected into the charge accumulation layer
(silicon nitride film) gradually diffuse over time, which causes
fluctuation in the threshold and degradation of the charge
retention characteristic.
[0026] These problems are caused by degradation of a channel
interface due to an injection of holes from the substrate by using
hot carriers. It is also thought that the problems are caused by a
local injection of holes from a high electrical field position.
Therefore, when the injection of holes is performed from the
substrate, holes are required to be injected non-locally without
using hot carriers while a film thickness assuring sufficient
charge retention characteristic is ensured.
[0027] As a method of solving the problem, an injection of holes
without using hot carriers is disclosed in 2004 International
Reliability Symposium Proc. pp. 527-530 by E. Lusky et al
(Non-Patent Document 1). In a conventional laminated gate
insulating film structure using a silicon oxide film as a barrier
layer, since a silicon oxide film formed between a silicon nitride
film and a silicon substrate is thick, it is impossible to perform
a tunnel injection of holes from a channel. Therefore, in
Non-Patented Document 1, a laminated film made of a extremely thin
silicon oxide film, an extremely thin silicon nitride film, and an
extremely thin silicon oxide film is formed instead of the silicon
oxide film, so that such holes can tunnel easily from the substrate
side at an application of electric field.
[0028] However, since it is necessary to use the substrate side as
a channel at a reading time, it is necessary to form the silicon
oxide film thickly in order to maintain channel characteristic and
suppress change in the threshold due to disturbance. In addition,
since charges are moved through the barrier film at an injection
time of holes, the holes pass through a channel interface, whereby
degradation of an interface characteristic cannot be avoided.
Further, it is thought that carries are trapped in the silicon
nitride film in the laminated film made of the extremely thin
silicon oxide film, the extremely thin silicon nitride film, and
the extremely thin silicon oxide film during writing/erasing
operations, so that fluctuation in the threshold due to release of
trapped carriers can not be avoided.
[0029] As a non-local hole injection system which does not cause
degradation of an interface, a non-local hole injection from a gate
electrode is thought. However, since a valence band offset between
silicon oxide and silicon is as high as 4.7 eV, it is difficult to
conduct a hole injection from the gate electrode when an insulating
film positioned under the gate electrode is a thick silicon oxide
film.
[0030] As mentioned in Japanese Patent Application Laid-open
Publication No. 2004-303918 (Patent Document 4), a system that
holes are injected from a gate electrode side by applying a
positive voltage to the gate electrode, in which a gate insulating
film has a laminated film structure in which a silicon oxide film,
a silicon nitride film, and a silicon oxynitride (SiON) film are
formed successively from a substrate side, has been proposed.
[0031] A sectional view of the memory cell is shown in FIG. 16. A
reference numeral 200 denotes a source, 300 denotes a drain, 901
denotes a silicon oxide film, 920 denotes a silicon nitride film,
930 denotes a silicon oxynitride film, and 500 denotes a gate
electrode. A charge accumulation layer is the silicon nitride film
920. Electrons are hot-electron-injected from a substrate at a
writing time, and holes are injected from the gate electrode at an
erasing time. According to this system, by adopting the insulating
film on the gate electrode 500 side made of silicon oxynitride
(SiON) having a band gap smaller than that of silicon oxide and
having a low barrier wall to holes, it is made possible to cause
holes to F-N (Folwer-Nordheim)-tunnel by a small electric field at
an erasing time.
[0032] Incidentally, as shown in FIG. 15, it is generally thought
that an interface state between a polysilicon film configuring the
gate electrode 500 and the silicon oxynitride film 930 is made
excellent by interposing a thin silicon oxide film 940 with a
thickness of about 1 nm between an interface of the gate electrode
500 and the silicon oxide nitride film 930. In this case, since the
holes can directly tunnel from the gate electrode 500 through the
thin silicon oxide film 940 with a thickness of about 1 nm, the
thin silicon oxide film 940 is not a main factor for determining a
transmittance at a hole injecting time. Main factors for
determining an injection efficiency of the holes are a film
thickness of the silicon oxynitride film 930 and a band offset to
the holes thereof.
SUMMARY OF THE INVENTION
[0033] As seen from Patent Document 4, it is found that a hole
injection from a gate electrode is effective when degradation of a
channel interface should be suppressed in a non-local hole
injection system.
[0034] In the above Patent Document 4, however, since the hole
injection from the gate electrode is performed by injecting holes
which are not hot carriers in a state of applying a positive
voltage to the gate electrode, electrons are also injected from a
substrate at an injection time of the holes from the gate electrode
side. Especially, when a film thickness of an insulating film
between the gate electrode and the charge accumulation layer is
thick or when a barrier of the insulating film to the holes is
high, an amount of electrons injected from the substrate is more
than an injection amount of holes, which results in difficulty of
injection of holes. Naturally, a hole injection which does not use
hot carriers is inferior to that using hot carriers in injection
efficiency, so that a rewriting speed of the former becomes slower
than that of the latter.
[0035] In order to solve these problems and realize a hole
injection from the gate electrode at a high efficiency, it is
necessary to thin the insulating film between the gate electrode
and the charge accumulation layer or to lower the valence band
offset corresponding to the barrier to the holes in the gate
electrode. However, lowering of the band offset of the insulating
film and thinning the insulating film between the gate electrode
and the charge accumulation layer deteriorate retention
characteristics of electrons and holes trapped in the silicon
nitride film.
[0036] An object of the present invention is to provide a
non-volatile semiconductor storage device which can perform a
non-local hole injection from a gate electrode at a high efficiency
while maintaining a thickness and a band offset of an insulating
film under the gate electrode to secure a sufficient charge
retention characteristic.
[0037] The above and the other objects and a novel feature of the
present invention will become apparent from the description of the
present specification and the accompanying drawings.
[0038] Representative ones of the inventions disclosed in the
present application will be briefly explained below.
[0039] (1) A semiconductor storage device according to an
embodiment of the present application comprises a non-volatile
memory having an insulating gate type field-effect transistor
causing electric field effect to a surface of a semiconductor
substrate from a gate electrode via a gate insulating film formed
on a main surface of the semiconductor substrate, the gate
insulating film including a first insulating film configuring a
charge retention layer, a second insulating film formed between the
first insulating film and the semiconductor substrate, and a third
insulating film formed between the first insulating film and the
gate electrode, and the non-volatile memory using, as retention
charges for information storage, holes injected into the charge
retention layer from the gate electrode via the third insulating
film and electrons injected into the charge retention layer from
the semiconductor substrate via the second insulating film at a
time of applying a positive voltage to the gate electrode, wherein
the gate electrode includes a first silicon layer formed to be in
contact with the gate insulating film and a second silicon layer
formed to be separated from the gate insulating film via the first
silicon layer and doped with an impurity with a concentration
higher than that of the first silicon layer.
[0040] (2) A semiconductor storage device according to another
embodiment of the present application comprises a non-volatile
memory having an insulating gate type field-effect transistor
causing electric field effect to a surface of a semiconductor
substrate from a gate electrode via a gate insulating film formed
on a main surface of the semiconductor substrate, the gate
insulating film including a first insulating film configuring a
charge retention layer, a second insulating film formed between the
first insulating film and the semiconductor substrate, and a third
insulating film formed between the first insulating film and the
gate electrode, and the non-volatile memory using, as retention
charges for information storage, holes injected into the charge
retention layer from the gate electrode via the third insulating
film and electrons injected into the charge retention layer from
the semiconductor substrate via the second insulating film at a
time of applying a positive voltage to the gate electrode, wherein
the gate electrode is configured of a Group III-V compound
semiconductor film.
[0041] (3) In a manufacturing method for a semiconductor storage
device, according to still another embodiment of the present
application, the semiconductor device comprises a non-volatile
memory having an insulating gate type field-effect transistor
causing electric field effect to a surface of a semiconductor
substrate from a gate electrode via a gate insulating film formed
on a main surface of the semiconductor substrate, the gate
insulating film including a first insulating film configuring a
charge retention layer, a second insulating film formed between the
first insulating film and the semiconductor substrate, and a third
insulating film formed between the first insulating film and the
gate electrode, and the non-volatile memory using, as retention
charges for information storage, holes injected into the charge
retention layer from the gate electrode via the third insulating
film and electrons injected into the charge retention layer from
the semiconductor substrate via the second insulating film at a
time of applying a positive voltage to the gate electrode, wherein,
when a level is formed on an interface between the gate electrode
and the third insulating film, whereby a positive voltage is
applied to the gate electrode, a barrier potential wall of the
third insulating film to the holes in the gate electrode is smaller
than a valence band offset occurring when the third insulating film
and a silicon layer are joined to each other.
[0042] Effects obtained by the representative ones of the
inventions disclosed in the present application will be briefly
explained below.
[0043] A non-volatile semiconductor storage device which can
perform a non-local hole injection from a gate electrode at a high
efficiency while maintaining a thickness and a band offset of an
insulating film under the gate electrode to secure a sufficient
charge retention characteristic can be realized.
BRIEF DESCRIPTIONS OF THE DRAWINGS
[0044] FIG. 1 is a plan view of a MONOS type memory adopting a hot
carrier injection system;
[0045] FIG. 2 is a sectional view of the MONOS type memory adopting
a hot carrier injection system;
[0046] FIG. 3 is an explanatory sectional view of a writing
operation of the MONOS type memory adopting a hot carrier injection
system;
[0047] FIG. 4 is an explanatory sectional view of an erasing
operation of the MONOS type memory adopting a hot carrier injection
system;
[0048] FIG. 5 is an explanatory sectional view of a reading
operation of the MONOS type memory adopting a hot carrier injection
system;
[0049] FIG. 6 is an explanatory sectional view of a reading
operation of a MONOS type memory using one memory cell as two
bits;
[0050] FIG. 7 is a plan view of a MONOS type memory having a split
gate;
[0051] FIG. 8 is a sectional view of the MONOS type memory having a
split gate;
[0052] FIG. 9 is an equivalent circuit diagram of the MONOS type
memory having a split gate;
[0053] FIG. 10 is a circuit diagram of a memory array using the
MONOS type memories having a split gate;
[0054] FIG. 11 is an explanatory view of a writing operation of the
MONOS type memory having a split gate;
[0055] FIG. 12 is an explanatory view of an erasing operation of
the MONOS type memory having a split gate;
[0056] FIG. 13 is a sectional view showing a manufacturing step for
a memory cell according to a first embodiment;
[0057] FIG. 14 is a sectional view showing a manufacturing step for
a memory cell continued from FIG. 13;
[0058] FIG. 15 is a sectional view of a MONOS type memory in which
holes are injected from a gate electrode side;
[0059] FIG. 16 is a sectional view of the MONOS type memory in
which holes are injected from a gate electrode side;
[0060] FIG. 17 is a sectional view showing a manufacturing step for
the memory cell according to the first embodiment;
[0061] FIG. 18A is a band diagram of a gate insulating film
comprising a three-layered insulating film;
[0062] FIG. 18B is a band diagram of a gate insulating film
comprising a four-layered insulating film according to the first
embodiment;
[0063] FIG. 19 is a band diagram of a gate insulating film when a
positive voltage is applied to a gate electrode of the memory cell
of the first embodiment;
[0064] FIG. 20 is a band diagram showing an enlarged part of the
gate insulating film shown in FIG. 19;
[0065] FIG. 21 is a graph showing results of simulation of hole
tunnel currents flowing from the gate electrode when a positive
voltage is applied to the gate electrode of the memory cell of the
first embodiment;
[0066] FIG. 22 is a band diagram set for calculating Curve A in
FIG. 21;
[0067] FIG. 23 is a band diagram set for calculating Curve B in
FIG. 21;
[0068] FIG. 24 is a band diagram of the gate insulating film of the
memory cell of the first embodiment at a charge retention time;
[0069] FIG. 25 is a sectional view showing a forming step for the
gate electrode of the memory cell of the first embodiment;
[0070] FIG. 26 is a sectional view showing a forming step of the
gate electrode continued from FIG. 25;
[0071] FIG. 27 is a sectional view showing a forming step of the
gate electrode continued from FIG. 26;
[0072] FIG. 28 is a sectional view showing a forming step of the
gate electrode continued from FIG. 27;
[0073] FIG. 29 is a band diagram of the vicinity of a gate
electrode when a positive voltage is applied to a gate electrode of
a memory cell of a second embodiment;
[0074] FIG. 30 is a band diagram of the vicinity of a gate
electrode when a positive voltage is applied to a gate electrode of
a memory cell of a third embodiment;
[0075] FIG. 31 is a band diagram of the vicinity of a gate
electrode when a positive voltage is applied to a gate electrode of
a memory cell of a fourth embodiment;
[0076] FIG. 32 is an explanatory schematic sectional view of a
charge retention state of a charge accumulation layer made of a
polysilicon film;
[0077] FIG. 33 is a plan view showing a memory cell of an eighth
embodiment;
[0078] FIG. 34 is a sectional view showing the memory cell of the
eighth embodiment;
[0079] FIG. 35 is a sectional view showing a manufacturing method
for the memory cell of the eighth embodiment;
[0080] FIG. 36 is a sectional view showing a manufacturing method
for the memory cell continued from FIG. 35;
[0081] FIG. 37 is a sectional view showing a manufacturing method
for the memory cell continued from FIG. 36;
[0082] FIG. 38 is a sectional view showing a manufacturing method
for the memory cell continued from FIG. 37;
[0083] FIG. 39 is a sectional view showing a manufacturing method
for the memory cell continued from FIG. 38;
[0084] FIG. 40 is a sectional view showing a manufacturing method
for the memory cell continued from FIG. 39;
[0085] FIG. 41 is a sectional view showing a manufacturing method
for the memory cell continued from FIG. 40;
[0086] FIG. 42 is a sectional view showing a manufacturing method
for the memory cell continued from FIG. 41;
[0087] FIG. 43 is a sectional view showing a manufacturing method
for the memory cell continued from FIG. 42;
[0088] FIG. 44 is a sectional view showing a manufacturing method
for the memory cell continued from FIG. 43;
[0089] FIG. 45 is a sectional view showing another manufacturing
method for a memory cell of the eighth embodiment;
[0090] FIG. 46 is a graph showing boron concentration distributions
when a laminated film made of a non-doped polysilicon film and a
p-type polysilicon film is activated by laser anneal;
[0091] FIG. 47A is a graph showing Vg-Id characteristics of a
transistor of a laminated gate MONOS in an initial stage, and in
completion of writing and erasing;
[0092] FIG. 47B is a graph showing Vg-Id characteristics of a
transistor of a single layer gate MONOS in an initial stage, and in
completion of writing and erasing;
[0093] FIG. 48 is a graph showing time dependence of threshold
fluctuation from a write state to an erase state when a positive
voltage is applied to a gate electrode in MONOS;
[0094] FIG. 49 is a sectional view of MONOS having a laminated
layer gate electrode;
[0095] FIG. 50 is a sectional view showing a manufacturing method
for a memory cell and a peripheral circuit of a ninth
embodiment;
[0096] FIG. 51 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
50;
[0097] FIG. 52 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
51;
[0098] FIG. 53 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
52;
[0099] FIG. 54 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
53;
[0100] FIG. 55 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
54;
[0101] FIG. 56 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
55;
[0102] FIG. 57 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
56;
[0103] FIG. 58 is a sectional view showing a manufacturing method
for memory cell and a peripheral circuit of a tenth embodiment;
[0104] FIG. 59 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
58;
[0105] FIG. 60 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
59;
[0106] FIG. 61 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
60;
[0107] FIG. 62 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
61;
[0108] FIG. 63 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
62;
[0109] FIG. 64 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
63;
[0110] FIG. 65 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
64;
[0111] FIG. 66 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
65;
[0112] FIG. 67 is a sectional view showing a manufacturing method
for a memory cell and a peripheral circuit of an eleventh
embodiment;
[0113] FIG. 68 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
67;
[0114] FIG. 69 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
68;
[0115] FIG. 70 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
69;
[0116] FIG. 71 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
70;
[0117] FIG. 72 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
71;
[0118] FIG. 73 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
72;
[0119] FIG. 74 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
73;
[0120] FIG. 75 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
74;
[0121] FIG. 76 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
75;
[0122] FIG. 77 is a sectional view showing a manufacturing method
for a memory cell and a peripheral circuit of a twelfth
embodiment;
[0123] FIG. 78 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
77;
[0124] FIG. 79 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
78;
[0125] FIG. 80 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
79;
[0126] FIG. 81 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
80;
[0127] FIG. 82 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
81;
[0128] FIG. 83 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
82;
[0129] FIG. 84 is a sectional view showing a manufacturing method
for a memory cell and a peripheral circuit of a thirteenth
embodiment;
[0130] FIG. 85 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
84;
[0131] FIG. 86 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
85;
[0132] FIG. 87 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
86;
[0133] FIG. 88 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
87;
[0134] FIG. 89 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
88;
[0135] FIG. 90 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
89;
[0136] FIG. 91 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
90;
[0137] FIG. 92 is a sectional view showing a manufacturing method
of a memory cell and a peripheral circuit of a fourteenth
embodiment;
[0138] FIG. 93 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
92;
[0139] FIG. 94 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
93;
[0140] FIG. 95 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
94;
[0141] FIG. 96 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
95; and
[0142] FIG. 97 is a sectional view showing a manufacturing method
for the memory cell and the peripheral circuit continued from FIG.
96.
DESCRIPTIONS OF THE PREFERRED EMBODIMENTS
[0143] Hereinafter, Embodiments of the present invention will be
explained in detail with reference to the drawings. Incidentally,
same members are attached with same reference numerals in principle
throughout all figures for explaining embodiments and repetitive
explanation is omitted.
[0144] The drawings are illustrative, and a relationship between
thicknesses and sizes in plan views, ratio of thicknesses of
respective layers, and the like should be determined based upon the
following explanation.
[0145] The following embodiments are for describing a device and a
method for embodying the technical idea of the present invention,
and the technical idea of the present invention is not for
specifying materials, formations, structures, arrangements,
operation voltages or the like of component parts as mentioned in
the embodiments.
First Embodiment
[0146] A high efficiency hole injection from a gate electrode side
which is a feature of the present invention can be understood from
an one-dimensional band structure in a direction perpendicular to a
substrate. Then, a memory cell having the same device structure as
that of an ordinary MOS transistor will be explained as a
representative example. Thereafter, an electron injection method is
examined and a case that the electron injection method is applied
to a memory cell structure suitable for high-speed operation is
explained. For the memory cell structures, an optimal structure
according to applications of a memory can be selected.
[0147] Manufacturing steps of a memory cell of the present
embodiment will be explained with reference to FIG. 13 to FIG. 15,
and FIG. 17. The figures correspond to a section A-A of a plan
layout shown in FIG. 1, where explanations are made using a process
technique corresponding to the so-called 0.13 .mu.m generation.
[0148] As shown in FIG. 13, a device isolation region 990 is formed
on a main surface of a p-type silicon substrate 100 by using a
known Shallow Trench Isolation (STI) process. A p-type well region
(not shown) having an impurity concentration of 10.sup.17
atom/cm.sup.3 on a substrate surface is formed by performing ion
implantation of boron and an activation anneal process.
[0149] Next, as shown in FIG. 14, after a silicon oxide film 910
with a film thickness of 4 nm is formed by thermally oxidizing a
surface of the substrate, a silicon nitride film 920 with a film
thickness of 6 nm and a silicon oxynitride film 930 with a film
thickness of 5 nm are sequentially deposited on an upper portion of
the silicon oxide film 910 by using CVD (chemical vapor deposition)
process. SiH.sub.2Cl.sub.2+NH.sub.3 is used as material gas for the
deposition of the silicon nitride film 920 performed by the CVD
process. Subsequently, the silicon oxynitride film 930 is formed by
adding oxidant (N.sub.2O) into the material gas and restricting a
flow rate of NH.sub.3. The silicon oxynitride film 930 has such a
feature that its band gap is larger than that of the silicon
nitride film 920. Here, a composition ratio of oxygen and nitrogen
in the silicon oxynitride film 930 is set to 1:1.
[0150] Next, a thin silicon oxide film 940 with a film thickness of
about 1 nm is formed on the silicon oxynitride film 930. Such an
extremely thin silicon oxide film 940 can be formed by ISSG
(In-Situ Steam Generation)-oxidizing a surface of the silicon
oxynitride film 930. A film thickness of the silicon oxynitride
film 930 is determined by considering reduction of the film
thickness of the silicon oxide film 940. An insulating film with
four layers configuring the gate insulating film can be obtained
through the steps described above.
[0151] Next, as shown in FIG. 15, after a doped polysilicon film is
deposited on an upper portion of the silicon oxide film 940 by CVD
process, a gate electrode 500 and a gate insulating film are formed
by dry-etching the doped polysilicon film and the insulating film
with four layers. In the memory cell according to the present
embodiment, the doped polysilicon film configuring the gate
electrode 500 has a laminated structure comprising a plurality of
polysilicon films with different impurity concentrations in order
to perform a hole injection from the gate electrode 500 at a high
efficiency. Specifically, a two-layered film comprising an n-type
polysilicon film with a low impurity concentration and a
p.sup.+-type polysilicon film with a high impurity concentration
deposited on the upper portion thereof, a two-layered film
comprising a p-type polysilicon film with a low impurity
concentration and a p.sup.+-type polysilicon film with a high
impurity concentration deposited on the upper portion thereof, a
two-layered film comprising a non-doped polysilicon film and a
p.sup.+-type polysilicon film with a high impurity concentration
deposited on the upper portion thereof, or the like can be adopted.
A method of forming a gate electrode 500 with such a laminated
structure will be explained in detail later.
[0152] Next, after arsenic is ion-implanted into a surface of the
substrate, diffusion layers 200 and 300 serving as a source and a
drain of the memory cell are formed by thermally treating the
substrate to activate the arsenic. The implantation energy for the
arsenic here is set to 40 keV and a dose amount of the arsenic is
set to 2.times.10.sup.15 atom/cm.sup.2. Thermal treatment
conditions are set to 950.degree. C. and 60 seconds.
[0153] Next, as shown in FIG. 17, after connection holes 520 and
530 are formed in a thick interlayer insulating film 510 deposited
on the substrate, a wire 620 and a wire 630 are electrically
connected to the diffusion layer 200 and the diffusion layer 300
through the connection holes 520 and 530 by forming the wires 620
and 630 on the interlayer insulating film 510, respectively.
Thereafter, upper portion wires are formed on upper layers of the
wires 620 and 630, but the explanation of which is omitted.
Incidentally, the wires 620 and 630 have been here formed on the
interlayer insulating film 510, but the diffusion layers 200 and
300 may be formed in a line shape in an extending direction of the
gate electrode 500 to be utilized as wires. One of the diffusion
layers 200 and 300 may be shared by memory cells adjacent to each
other in a channel direction.
[0154] FIG. 18B shows a band structure of a gate insulating film
comprising the above-mentioned insulating film with four layers
(the silicon oxide film 910, the silicon nitride film 920, the
extremely thin silicon oxynitride film 930, and the silicon oxide
film 940). FIG. 18A shows a band structure of a gate insulating
film comprising an insulating film with three layers (the silicon
oxide film 910, the silicon nitride film 920, and the silicon oxide
film 940) used in the conventional MONOS type memory. In both the
gate insulating films, the silicon nitride film 920 trapping
charges mainly becomes a charge accumulation layer.
[0155] In the above-mentioned gate insulating film comprising the
insulating film with three layers, a band offset 10 to electrons
and a band offset 20 to holes between the silicon oxide film 910 or
940 and the silicon nitride film 920 are respectively mentioned as
1.1 eV and 2.9 eV (Applied Physics, Volume 70, No. 9, 2001, p.
1050). On the other hand, in the gate insulating film of the
present embodiment, offset barriers between the silicon oxynitride
film 930 and the charge accumulation layer (the silicon nitride
film 920) are 0.5 eV to electron 11 and 0.8 eV to hole 21.
[0156] In the memory cell of the present embodiment, the reason why
the gate insulating film is made to have a structure with four
layers of the silicon oxide film 910, the silicon nitride film 920,
the silicon oxynitride film 930, and the silicon oxide film 940 is
because the silicon oxynitride film 930 has an offset barrier to
holes in the gate electrode 500 lower than that of the silicon
oxide film and a hole injection from the gate electrode 500 becomes
easy as compared with the conventional MONOS type memory.
Naturally, similar effect can be obtained when the above-mentioned
gate insulating film with a four-layered structure is applied to
the conventional MONOS type memory.
[0157] Writing/erasing of the memory cell is performed by injecting
electrons and holes into the silicon nitride film 920 configuring
the charge accumulation layer to change a total charge amount.
Incidentally, naming of writing and erasing depends on a system and
it does not depend on a basic operation of a memory cell.
Therefore, the writing/erasing is herein explained as an injection
method of electrons and holes.
[0158] Injection of electrons is performed by bringing hot
electrons generated in a substrate into the silicon nitride film
920. When the conventional hot electron injection is used, a steep
electric field slope is provided to a current path between the
source and the drain, for example, by applying 0 V to the source,
applying 5 V to the drain, and applying 10 V to the gate electrode
500, and hot carriers generated in the current path are brought
into the gate electrode 500 side and are injected into the silicon
nitride film 920. In the following embodiments, the electron
injection means the hot electron injection from the substrate,
unless otherwise instructed.
[0159] On the other hand, when holes are injected from the gate
electrode 500 into the silicon nitride film 920, a positive
potential (for example, 10 V) is applied to the gate electrode 500,
so that holes generated at the gate electrode 500 are caused to
pass through the silicon oxynitride film 930 by F-N
(Folwer-Nordheim) tunnel effect (FIG. 19). For example, it is
necessary to set the voltages of both the source and the drain to 0
V so as not to generate hot electrons from the substrate at the
time of the hole injection. Incidentally, in FIG. 19, a solid line
shows the band diagram of the memory cell according to the present
embodiment and a dotted line shows the band diagram of a
conventional memory cell having a conventional single layer
polysilicon gate. Explanation about FIG. 19 will be made in detail
later. The silicon oxide film 940 on the silicon oxynitride film
930 is mainly for making a stable interface with the polysilicon
film configuring the gate electrode 500 but a film thickness
thereof is as extremely thin as about 1 nm, so that holes directly
pass through the silicon oxide film 940 by the tunnel phenomenon.
Therefore, the silicon oxide film 940 is not a main factor for
determining the hole transmission at the injection time.
[0160] The main factor for determining the hole transmission at the
injection time is the silicon oxynitride film 930. Since the
silicon oxynitride film 930 has a smaller band gap than that of the
silicon oxide film, the silicon oxynitride film 930 has a higher
injection efficiency of holes due to the smaller gap. The injection
efficiency of holes becomes higher as the film thickness of the
silicon oxynitride film 930 is reduced. However, as described
above, thinning of the silicon oxynitride film 930 and lowering of
the band offset of the silicon oxynitride film 930 with respect to
the charge accumulation layer deteriorate the charge retention
characteristic. On the other hand, thickening of the silicon
oxynitride film 930 and improving of the band offset deteriorate
the efficiency of the hole injection from the gate electrode 500,
and cause the electron injection amount from the substrate to be
main injected carriers, whereby holes cannot be accumulated.
[0161] As described above, in the present embodiment, the gate
electrode 500 is made of a two-layered film of an n-type
polysilicon film with a low impurity concentration and a
p.sup.+-type polysilicon film with a high impurity concentration
deposited on the upper portion thereof, or a two-layered film of a
p-type polysilicon film with a low impurity concentration and a
p.sup.+-type polysilicon film with a high impurity concentration
deposited on the upper portion thereof, or a two-layered film of a
non-doped polysilicon film and a p.sup.|-type polysilicon film with
a high impurity concentration deposited on the upper portion
thereof, or the like. Therefore, the hole injection from the gate
electrode 500 is performed at a high efficiency.
[0162] A solid line in FIG. 19 shows a band diagram of a gate
insulating film when a positive voltage is applied to the gate
electrode 500 to inject holes from the gate electrode 500. A
reference numeral 1010 denotes an n-type polysilicon film with a
low impurity concentration, or a p-type polysilicon film with a low
impurity concentration, or a non-doped polysilicon film, and a
reference numeral 1020 denotes a p.sup.+-type polysilicon film with
a high impurity concentration. On the other hand, a broken line in
FIG. 19 shows a band diagram when the gate electrode is made of a
p-type polysilicon single layer film with an even impurity
concentration.
[0163] As shown in FIG. 19, by configuring the gate electrode 500
with two polysilicon layers (1010, 1020), the polysilicon film 1010
(the n-type polysilicon film with a low impurity concentration, or
the p-type polysilicon film with a low impurity concentration, or
the non-doped polysilicon film) can be depleted excellently when a
positive voltage is applied to the gate electrode 500 at a time of
hole injection. This is the same principle that, in p-n junction, a
depletion layer in a lower impurity concentration layer becomes
long while one with a higher impurity concentration becomes short.
Thus, by depleting the polysilicon film near the interface with the
gate insulating film, quantum levels can be formed near the
interface.
[0164] FIG. 20 shows an enlarged band diagram of the vicinity of
the above-mentioned interface. A quantum level (QL) in FIG. 20
shows a ground state of a triangle-well produced at the interface.
That is, the holes are raised energetically by the energy of the
ground state, so the energetic heights of the silicon oxynitride
film 930 and the silicon oxide film 940 appear lower by the raised
energy. Therefore, the holes can be injected at a higher efficiency
as compared with the gate electrode made up of a polysilicon single
layer film. Though the energy of the holes in the gate electrode
500 varies depending on the film thickness of the polysilicon film
1010 and the gate voltage, it is raised by about 0.5 to 0.8 eV by
formation of the quantum level (QL) due to the depletion under such
a situation that the thickness of the polysilicon film 1010 is 5 nm
or less and the gate electrode voltage is in a range of about 8 to
13 V.
[0165] Regarding the n-type or the p-type polysilicon film 1010
with a low impurity concentration, it is ideally desirable that the
impurity concentration is lower. On the contrary, regarding the
p.sup.+-type polysilicon film 1020 with a high impurity
concentration, since a voltage can be applied to the polysilicon
film 1010 more efficiently as the impurity concentration becomes
higher, the injection efficiency of holes is increased. It is
desirable that the impurity concentration of the polysilicon film
1010 is 10.sup.17 atom/cm.sup.3 or less, and it is desirable that
the impurity concentration of the polysilicon film 1020 is
10.sup.19 atom/cm.sup.3 or more.
[0166] FIG. 21 is a graph showing the result of a simulation of a
hole tunnel current flowing from the gate electrode 500 when a
positive voltage is applied to the gate electrode 500 like the band
diagram shown in FIG. 22, and in addition, the formation of the
quantum level (QL) is taken into consideration. The polysilicon
film 1120 in FIG. 22 is a non-doped silicon film with a film
thickness of 2 nm, and the polysilicon film 1110 is a p.sup.+-type
polysilicon film having an impurity concentration of
5.times.10.sup.20 atom/cm.sup.3 or more. It is assumed that the
silicon oxynitride film 1130 has a valence band offset to silicon
of 2.8 eV, a relative dielectric constant of 6.1, and a film
thickness of 6 nm. A reference numeral 1140 denotes a p-type
silicon substrate, FL(S) denotes a Fermi level of the silicon
substrate, and FL (G) denotes Fermi level of the gate electrode,
respectively. A curve in FIG. 21 shows a calculation result
obtained from this case.
[0167] On the other hand, a curve "B" in FIG. 21 shows a similar
calculation result when a p.sup.+-type polysilicon single layer
film (1110) with a high impurity concentration as shown in FIG. 23
is used in the gate electrode. In the simulation of the tunnel
current, Transfer-Matrix method (for example, HIROSHI MIZUTA, et
al., IEEE TRANSACTION OF ELECTRON DEVICES, VOL 35, NO. 11, NOVEMBER
1988) well-known as a numerical computing technique was used.
[0168] From the above-mentioned simulation result, when the curve
"A" and the curve "B" are compared with each other, it is
understood that when a gate electrode with a two-layered
polysilicon structure is adopted (curve "A"), a hole tunnel current
is improved about ten times even during the application of a low
voltage of about 2 V. It is also understood improvement of a tunnel
current effect up to about 1000 times can be obtained according to
increasing the gate voltage. This means that the hole injection
time can be reduced up to about 1/10 to 1/1000. From the estimation
based upon the simulation, it is found that the film thickness is 6
nm or less so that an effect of the high efficiency hole injection
due to the quantum level can be obtained clearly.
[0169] The depletion in the gate electrode 500 can achieve such an
effect that the carrier retention characteristic is improved. FIG.
24 shows a band diagram of a gate insulating film with a
four-layered structure at a charge retention time. In FIG. 24, a
solid line shows use of the above-mentioned gate electrode with the
two-layered polysilicon structure, and a broken line shows use of a
gate electrode with a p-type polysilicon single layer structure. As
compared with the p-type polysilicon single layer structure in
which the gate electrode is not depleted, in the two-layered
polysilicon structure, an electric field produced by trap charges
(TC) is applied to the depletion layer owing to the effect of the
depletion. Therefore, electric fields applied to the silicon
oxynitride film 930 and the silicon oxide film 940 are reduced, so
that electrons are not easy to disappear. That is, the charge
retention characteristic is improved. Such a fact can be similarly
happen at the hole retention time.
[0170] Next, a manufacturing method of the gate electrode 500
having a two-layered polysilicon structure will be explained with
reference to FIG. 25 to FIG. 28. This manufacturing method is one
example of manufacturing methods in view of easy method of making
the gate electrode 500, and it does not limit the present
invention.
[0171] First, after a silicon oxide film 910, a silicon nitride
film 920, a silicon oxynitride film 930, and a silicon oxide film
940 are formed on a substrate by the above-mentioned methods shown
in FIG. 13 and FIG. 14, a non-doped amorphous silicon film 1210 or
an amorphous silicon film 1210 with a low impurity concentration is
deposited on the silicon oxide film 940 to have a film thickness of
6 nm by CVD process, as shown in FIG. 25. The amorphous silicon
film 1210 is made to be the polysilicon film 1010 shown in FIG. 19
at the subsequent step.
[0172] Next, as shown in FIG. 26, a thin silicon oxide film 1220
with a film thickness of about 1 nm is formed on a surface of the
amorphous silicon film 1210 by performing thermal oxidizing process
for about 5 minutes in a gas atmosphere including 20% oxygen at a
temperature of 400.degree. C. to 450.degree. C. The thin silicon
oxide film 1220 has a role of a barrier layer preventing diffusion
of boron and a role of producing grain aggregates of polysilicon in
an interface between the gate electrode 550 and the silicon oxide
film 940 to cause electric field concentration at a hole injection
time. These roles will be described later.
[0173] Next, as shown in FIG. 27, after a non-doped amorphous
silicon film or an amorphous silicon film having low impurity
concentration with a film thickness of 6 nm is deposited on the
silicon oxide film 1220 by CVD process, the amorphous silicon film
is poly-crystallized to obtain a polysilicon film 1230 by
performing anneal for about 30 minutes in nitrogen atmosphere at a
temperature of about 900.degree. C. When anneal is performed, a
portion of the silicon oxide film 1220 enters into the interface
between the amorphous silicon film 1210 and the silicon oxide film
940 so that grain aggregates (not shown) of polysilicon are formed
in the interface.
[0174] Next, after washing with hydrofluoric acid, as shown in FIG.
28, a p-type polysilicon film 1240 with a low impurity
concentration is deposited on the polysilicon film 1230 by CVD
process, and anneal is then performed in order to activate
impurities (boron) in the p-type polysilicon film 1240. The film
thickness of the polysilicon film 1240 is set to about 200 nm.
[0175] When the above-mentioned anneal for activation is performed,
boron in the p-type polysilicon film is diffused. However, the thin
silicon oxide film 1220 is interposed between the p-type
polysilicon film 1240 and the underlying amorphous silicon film
1210, therefore, boron in the p-type polysilicon film 1240 is
hardly diffused in the amorphous silicon film 1210. Incidentally,
when the above-mentioned anneal for activation is performed by
using a laser anneal process, a diffusion distance of boron can be
controlled more accurately. In this case, a step of forming the
silicon oxide film 1220 on a surface of the amorphous silicon film
1210 can be omitted.
[0176] Though a diagrammatic representation is omitted, the gate
electrode 500 comprising multiple layers made of polysilicon films
with different impurity concentrations can be obtained by
dry-etching the p-type polysilicon film 1240, the polysilicon film
1230, the silicon oxide film 1220, the amorphous silicon film 1210,
and a lower layer as the laminated insulation film (the silicon
oxide film 940, the oxynitride silicon film 930, the silicon
nitride film 920, and the silicon oxide film 910). Since the film
thickness of the silicon oxide film 1220 interposed between
polysilicon films is extremely thin (about 1 nm) and holes can
tunnel through the silicon film 1220 directly, the silicon oxide
film 1220 hardly blocks an injection of the holes from the gate
electrode 500.
[0177] Incidentally, instead of the above-mentioned step, a step
described below can be adopted. That is, after an n-type amorphous
silicon film with a high impurity concentration is deposited on the
silicon oxide film 940, a p-type polysilicon film 1240 is deposited
on the n-type amorphous silicon film. Thereafter, anneal for
activating boron in the p-type polysilicon film 1240 is performed.
In this case, since a portion of boron in the p-type polysilicon
film 1240 diffuses in the underlying n-type amorphous silicon film
to neutralize impurities (phosphorus) in the n-type amorphous
silicon film, a gate electrode 500 with a two-layered structure
comprising a non-doped polysilicon film or a polysilicon film with
a low impurity concentration and a p-type polysilicon film 1240 can
be obtained.
[0178] Also, instead of the above-mentioned silicon oxide film
1220, a silicon nitride film with a film thickness of about 1 nm
may be used. The silicon nitride film can be formed by thermally
treating the amorphous film 1210 in an ammonia atmosphere at a
temperature of about 700.degree. C. In this case, since no grain
aggregate of polysilicon is formed in an interface between the
amorphous silicon film 1210 and the silicon oxide film 940, the
effect in which the grain aggregates cause electric field
concentration can not be obtained. However, since the silicon
nitride film has a potential barrier to holes in gate electrode
lower than the silicon oxide film, the holes directly tunnel
easily, so that the holes can be injected at a higher
efficiency.
[0179] Also, instead of the silicon oxide film 1220, a metal thin
film having such a property that diffusion of boron is made
difficult may be used. As a representative metal thin film
material, TiN can be adopted. The present invention is not limited
to this material, and an optimal material may be selected
considering consistency with other processes.
[0180] Since the above-mentioned manufacturing method of the gate
electrode 500 is not required for introduction of novel materials,
such a merit can be obtained that the manufacturing method has a
high matching property with conventional semiconductor
manufacturing processes. In addition, since the memory cell
according to the present embodiment does not utilize hot holes at a
time of a hole injection, there is such a merit that the insulating
film under the gate electrode 500 is not degraded. Further, since
the gate electrode material is polysilicon like the conventional
art, such a merit can be obtained that no defect is produced at a
joint surface with the insulating film positioned under the gate
electrode 500 so that a stable operation is obtained.
[0181] In the memory cell according to the present embodiment, an
effect of lowering the barrier to holes due to the quantum level
formation is in a range of about 0.5 to 0.8 eV, which is slightly
smaller than that in a memory cell according to an embodiment
described later. However, the example explained above considers
only the ground state and, for example, when a voltage applied to
the gate electrode 500 is raised to make further upper levels, the
effect of lowering the barrier can get the raised level energy, so
that a hole injection at a further high efficiency is made
possible.
Second Embodiment
[0182] It is apparent from the above-mentioned first embodiment
that a high efficiency hole injection from the gate electrode can
be realized by lowering the potential barrier of the insulating
film under the gate viewed from holes in polysilicon gate. In the
present embodiment, lowering of the potential barrier with respect
to holes in the gate is realized by using semiconductor materials
with a wide gap for the gate electrode instead of by forming the
quantum levels.
[0183] FIG. 29 shows a band diagram near a gate electrode when the
gate electrode of a memory cell comprises a silicon carbide (SiC)
film and a predetermined gate electrode voltage is applied thereto.
A solid line CB (SiC) shown in a gate electrode part in figure
denotes a conduction band of SiC, and a solid line VB (SiC) denotes
a valence band of SiC, respectively. A dotted line CB (Si) denotes
a conduction band of silicon and a dotted line VB (Si) denotes a
valence band of silicon, respectively.
[0184] As described in IEEE Electron Device Letters, Vol. 18, No.
12, December 1997 (A. Agarwal et al.), in band offsets between
silicon oxide and SiC, a valence band offset is 3.05 eV in SiC of
4H-SiC, and a valence band offset is 3.2 eV in SiC of 6H-SiC. This
means that a barrier of the silicon oxide film to holes in the gate
electrode is lowered by about 1.5 to 1.6 eV as compared with a case
that silicon is used for the gate electrode, when assuming that the
valence band offset between silicon and the silicon oxide is about
4.7 eV. In this embodiment, this effect of lowering the barrier is
larger than that of lowering the barrier by the formation of the
quantum level (QL) according to the first embodiment. Therefore, a
probability that the holes in the gate electrode tunnel through the
silicon oxide film 940 and the silicon oxynitride film 930 is
higher than that in the gate electrode with the two-layered
structure shown in the first embodiment, so that a hole injection
is made possible with a further high efficiency.
[0185] As the semiconductor material with a wide gap used for the
gate electrode, in addition to SiC, Group-III-V compound
semiconductor such as AlN, InN, GaN, InGaN, GaAs, AlGaAs or ZnO can
be used.
Third Embodiment
[0186] The high efficiency hole injection from the gate electrode
can also be realized by producing hot holes on the gate electrode.
It can be said that this method lowers a barrier of the insulating
film to holes in the gate electrode like the first and second
embodiments.
[0187] In the present embodiment, as shown in a band diagram in
FIG. 30, the gate electrode comprises a two-layered film made of an
n-type polysilicon film and n.sup.| type (or p.sup.|-type)
polysilicon film with a high impurity concentration to generate
avalanche hot holes. FIG. 30 is a band diagram near the gate
electrode just after applying a gate voltage. Also, FIG. 30 shows
production of the avalanche hot holes (Ha).
[0188] In order to produce the avalanche hot holes, it is necessary
to accelerate carriers by deep depletion such as shown in FIG. 30,
so that it is necessary to make an impurity concentration in the
n-type polysilicon film near the gate electrode lower than
10.sup.18 atom/cm.sup.3. When the impurity concentration in the
n-type polysilicon film is higher than 10.sup.18 atom/cm.sup.3,
Zener Tunnel (inter-band tunnel) phenomenon in which electrons move
from a valance band to a conduction band occurs just after applying
a gate voltage. As a result, holes produced on the valence band
form an inversion layer, so a deep depletion required for the
avalanche hot holes cannot occur. It is desirable that the impurity
concentration in the n.sup.+ type (or p.sup.+-type) polysilicon
film on the upper portion of the n-type polysilicon film is
10.sup.19 atom/cm.sup.3 or more because a depletion can be made
deeper by a higher impurity concentration.
[0189] Incidentally, it is necessary to set the film thickness of
the above-mentioned n-type polysilicon film considering an
acceleration distance of carriers required for generation of
avalanche hot holes. Since the acceleration distance of carriers is
increased as the film thickness is increased, a hole injection at a
high efficiency can be expected. However, a voltage applied to the
gate electrode is also increased correspondingly, and a voltage to
be applied at a reading time is also increased similarly.
Accordingly, the film thickness of the n-type polysilicon film must
be set considering the gate voltage capable of being applied to a
memory to be used. The gate electrode of the present embodiment can
be manufactured by the method shown in the first embodiment.
[0190] Since the memory cell according to the present embodiment
adopts the carrier injection system utilizing the hot holes, the
holes can be injected from the gate electrode at a higher
efficiency than that in the memory cells of the other embodiments.
The silicon oxide film 940 and the silicon oxynitride film 930 tend
to be degraded by using the hot holes, but since a hot hole
junction from the substrate does not occur, a channel interface is
not damaged and transistor characteristics such as a reading
characteristic are not degraded.
[0191] In order to suppress degradation of the insulating film due
to utilization of the hot holes, it is effective to perform NO
anneal to an interface between the gate electrode and the gate
insulating film. It is also effective that, after the interface
between the gate electrode and the gate insulating film is nitrided
with plasma, anneal is performed in nitrogen atmosphere.
Incidentally, an electric field produced by carriers accumulated in
the silicon nitride film 920 is well applied to a part (the n-type
polysilicon film) of the gate electrode because the gate insulating
film existing under the gate electrode is made to have the
four-layered structure explained in the first embodiment.
Therefore, the carrier retention characteristic is improved due to
the same reason as described in the first embodiment.
Fourth Embodiment
[0192] In a memory cell according to the present embodiment, a hole
injection at high efficiency is realized by producing levels in an
interface between a gate electrode and an underlying silicon oxide
film 940, and by making holes to tunnel through the silicon oxide
film 940 via the levels.
[0193] FIG. 31 is a band diagram at a time of an application of a
gate voltage in the memory cell according to the present
embodiment. By producing the levels in the interface between the
gate electrode and the silicon oxide film 940, holes (H) are
trapped in the levels. Therefore, the holes (H) tunnel through the
silicon oxide film 940 without dropping in a bottom of the valence
band of a polysilicon film (gate electrode) at a time of depletion
of the gate electrode. It is said again that the barrier of the
insulating film is lowered with respect to the holes in the gate
electrode.
[0194] In order to produce levels in the interface between the gate
electrode and the silicon oxide film 940, a surface of the silicon
oxide film 940 is thinly nitrided by heat or nitrided with plasma
to form trap levels of silicon nitride. Alternatively, there is
such a method that the interface between the gate electrode and the
silicon oxide film 940 is damaged by plasma irradiation to form the
levels. Further, it is also thought that the levels are formed by
ion-implanting fluorine into the interface or inserting a metal
thin film for traps in the silicon oxide film 940. A feature of the
memory cell according to the present embodiment lies in that a
manufacturing method thereof is simple and the memory cell is easy
to manufacture.
Fifth Embodiment
[0195] The memory cells according to the first to fourth
embodiments have the gate insulating film between the substrate and
the gate electrode, and the gate insulating film comprises four
layer films of a silicon oxide film 910/a silicon nitride film
920/a silicon oxynitride film 930/a thin silicon oxide film 940
disposed in this order from the substrate side. On the other hand,
even when the gate insulating film comprises a three-layered
structure (1) of a silicon oxide film/a silicon nitride film/a
silicon oxynitride film disposed in this order from the substrate
side or it comprises a three-layered film (2) of a silicon oxide
film/a silicon nitride film/a silicon oxide film disposed in this
order from the substrate side, a high efficiency hole injection
from the gate electrode can be realized.
[0196] When the above-mentioned three-layered film (1) is adopted,
since the silicon oxide film 940 is not present, a hole injection
is made possible with higher efficiency, as compared with the case
that the four-layered film according to the first to fourth
embodiments is adopted. Further, when the three-layered film (2) is
adopted, since the silicon oxide has a larger band gap than that of
the silicon oxynitride, a retention characteristic of carriers
accumulated in the silicon nitride film is improved as compared
with the case that the four-layered film according to the first to
fourth embodiments is adopted.
Sixth Embodiment
[0197] In the first to fifth embodiments, the MONOS type memories
have been explained. In addition, in a so-called floating gate type
memory where the charge accumulation layer is made of a polysilicon
film, a high efficiency hole injection from a gate electrode to a
floating gate can be realized by applying any one of the gate
electrode structures of the first to fourth embodiments.
[0198] A memory cell including a charge accumulation layer made of
a polysilicon film becomes sensitive to defects in insulating films
on a substrate side and a gate electrode side in which the
polysilicon film is interposed therebetween. This is because the
polysilicon film is an electrically conductive film and when a
defect (D) occurs at a particular site in a gate insulating film
901, for example, as shown in FIG. 32, charges (C) accumulated in
the polysilicon film (the charge accumulation layer) reach the
defect easily, so that charges (C) leak therefrom. On the other
hand, when a film which is non-conductive like the silicon nitride
film and has a discrete trap property is used, the charges (C) need
the energy to move through sites in the discrete traps in order to
reach the defect, so that the charges do not leak easily.
[0199] Accordingly, when the charge accumulation layer is formed of
a polysilicon film, the insulating films on the substrate side and
the gate electrode side in which the polysilicon film is interposed
therebetween must be made thick, and a gate voltage required for a
hole injection and an electron injection is increased
correspondingly. However, such a problem that electrons are locally
present in a part of the charge accumulation layer, which occurs
when the charge accumulation layer is made of a silicon nitride
film and a local injection with hot carriers is performed, can be
avoided.
Seventh Embodiment
[0200] In the first to sixth embodiments, the memory cells that
holes are injected from the gate electrode have been explained.
However, a high efficiency injection can be realized similarly even
in a memory cell in which a negative voltage is applied to a gate
electrode and electrons are injected from the gate electrode.
[0201] When this system is adopted, since not only electrons but
also holes can be injected from the gate electrode, writing/erasing
operations can be performed without deteriorating a channel
interface at all. Therefore, degradation of transistor
characteristics can be prevented at a reading time.
Eighth Embodiment
[0202] The present invention can be applied to a MONOS type memory
having a split gate structure such as explained in FIG. 7 to FIG.
9. That is, by applying the gate electrode structure of any one of
the first to fourth embodiments to the memory gate 550 of the MONOS
type memory having the split gate structure shown in FIG. 7 to FIG.
9, it is made possible to inject holes from the memory gate 550
into the charge accumulation layer at a high efficiency. Also,
since the gate insulating film 950 of the memory gate 550 is made
to have the same four-layered structure as that of the gate
insulating film of the first embodiment, injection efficiency of
the holes can be further increased.
[0203] FIG. 33 is a plan view showing a memory cell according to
the present embodiment and FIG. 34 is a sectional view taken along
a line A-A in FIG. 33. Since a carrier injection system in the
memory cell of the present invention is to inject holes from the
gate electrode and to inject hot electrons from the substrate, the
erasing operation is different from the above-mentioned operation
in an operation of the memory cell. That is, for example, when the
erasing operation is executed, a positive potential of 10 V is
applied to the memory gate 550, and 0 V is applied to a diffusion
layer (source) 200, a diffusion layer (drain) 300, a selector gate
500, and a silicon substrate 100, respectively. The holes are
injected into a charge accumulation layer (a silicon nitride film
920) from the memory gate 550. Here, by applying the gate electrode
structure of the present invention shown in the first to fourth
embodiments to the memory gate 550, a high efficiency injection of
holes is made possible. The same operation as the above-mentioned
operations can be performed in writing and reading. Since a hole
injection at a high efficiency is made possible at the erasing
time, a threshold can be made low. Therefore, in the memory cell
according to the present embodiment where memory cells are selected
by the selection transistor, since the memory gate 550 can be read
at 0 V, operations at low voltages and high speeds can be realized.
In addition, since a bias is not applied to the memory gate 550 at
any time other than the writing and erasing time, the embodiment is
effective for charge retention.
[0204] Next, a method of manufacturing a memory cell of the present
embodiment will be explained with reference to FIG. 35 to FIG. 44.
Left side figures on FIG. 35 to FIG. 44 are sectional views taken
along a line A-A in FIG. 33, and right side figures thereon are
sectional views taken along line a B-B in FIG. 33. Here,
explanations are made using a process technique corresponding to so
called 0.13 .mu.m generation.
[0205] As shown in FIG. 35, a device isolation region 150 is first
formed on a main surface of a p-type silicon substrate using a
conventional Shallow Trench Isolation (STI) process. Also, a p-type
well region (not shown) is formed on a substrate surface by
performing ion implantation of boron and an anneal process for
activation. Next, after a gate insulating film 900 comprising a
silicon oxide film with a film thickness of 2.5 nm is formed by
thermally oxidizing the substrate surface, a polysilicon film with
a film thickness of about 200 nm is deposited on the gate
insulating film 900 using CVD process. A selection gate 500 is then
formed by dry-etching the polysilicon film. Note that, since the
gate insulating film 900 under the selection gate 500 is extremely
thin, it is not shown in the following figures (FIG. 36 to FIG.
44).
[0206] Next, as shown in FIG. 36, a gate insulating film 950 is
formed on the substrate. The gate insulating film 950 comprises a
four-layered film of the silicon oxide film 910, the silicon
nitride film 920, the silicon oxynitride film 930, and the silicon
oxide film 940 described in the above-mentioned first embodiment. A
manufacturing method thereof is the same as described in the
above-mentioned first embodiment.
[0207] Next, as shown in FIG. 37, after a p-type polysilicon film
550p with a film thickness of 70 nm is deposited on the gate
insulating film 950 by CVD process, as shown in FIG. 38, a
spacer-like memory gate 550 is formed on a side wall of the
selection gate 500 by performing anisotropic etching on the p-type
polysilicon film 550p. Here, the p-type polysilicon film 550p
comprises a two-layered film of a p-type polysilicon film with a
low impurity concentration and a p.sup.+-type polysilicon film with
a high impurity concentration deposited on the upper portion
thereof, as explained in the first embodiment. The memory gate 550
can be formed by using gate electrode materials as explained in the
second to fourth embodiments, instead of the p-type polysilicon
film 550p.
[0208] Next, as shown in FIG. 39, the memory gate 550 on one side
of the selection gate 500 is removed by performing dry etching
using a photoresist film 700 as a mask so that the memory gate 550
is left only on another side wall.
[0209] Next, as shown in FIG. 40, a gate insulating film 950 is
left on the one side wall of the selection gate 500 and under the
memory gate 550 by dry etching the gate insulating film 950.
Subsequently, after arsenic is ion-implanted into the surface of
the substrate, n.sup.- diffusion layers 210 and 310 are formed by
thermally treating the substrate to activate the arsenic. Here, the
implant energy of the arsenic is set to 10 keV and a dose amount
thereof is set to 1.times.10.sup.15 atom/cm.sup.2. The n.sup.-
diffusion layers 210 and 310 are formed to make the memory cell
have an LDD (lightly doped drain) structure.
[0210] Next, as shown in FIG. 41, after a silicon oxide film 961
with a film thickness of 100 nm is deposited on the substrate by
CVD process, as shown in FIG. 42, side wall spacers 960 are formed
on a side wall of the selection gate 500 and a side wall of the
memory gate by performing anisotropic etching to the silicon oxide
film 961. Subsequently, after arsenic is ion-implanted into the
surface of the substrate, diffusion layers 200 and 300 serving as a
source and a drain for the memory cell are formed by thermally
treating the substrate to activate the arsenic. Here, the implant
energy of the arsenic is set to 40 keV and the dose amount thereof
is set to 2.times.10.sup.15 atom/cm.sup.2. The thermal treatment
condition is set to 950.degree. C. and 60 seconds.
[0211] Next, as shown in FIG. 43, silicide layers 501 are formed on
respective surfaces of the selection gate 500, the memory gate 550,
and the diffusion layers 200 and 300 by a conventional salicide
process. The silicide layer 501 is made of Co silicide, for
example.
[0212] Next, as shown in FIG. 44, after a thick interlayer
insulating film 510 is deposited on the substrate, a wire 600 is
formed on the interlayer insulating film 510 by a conventional
wiring process. A MONOS type memory with a split gate structure in
which holes can be injected from the gate electrode side at a high
efficiency can be completed through the above-mentioned steps.
[0213] Incidentally, in the above explanation, the memory gate 550
is formed by self-alignment to the selection gate 500. However,
after an electrically conductive film is deposited so as to cover
an upper portion of the selection gate 500, a memory gate 550 can
be formed by dry etching the electrically conductive film using the
photoresist film as a mask, as shown in FIG. 45. The electrically
conductive film comprising the memory gate 550 is made of gate
electrode materials as explained in the first to fourth
embodiments.
Ninth Embodiment
[0214] In order to form an ideal laminated gate electrode
comprising a non-doped polysilicon film and a p-type polysilicon
film, the laser anneal which can well suppress diffusion of boron
at a time of activating the boron is preferably used, as described
in the first embodiment. This is because rapid heating/cooling can
be made possible, as compared with a conventional rapid thermal
anneal (RTA).
[0215] A boron concentration distribution when a laminated film
made of a non-doped polysilicon film and a p-type polysilicon film
is practically activated by laser anneal is shown in FIG. 46. Curve
"A" in FIG. 46 is a boron concentration profile by SIMS analysis
when a laser anneal for activating the boron is performed to a gate
electrode. Here, the gate electrode is formed by laminating a
non-doped polysilicon film (i-Si) with a film thickness of 6 nm and
a boron-doped polycrystalline silicon film (p.sup.+-Si) with a film
thickness of 200 nm on an insulating film in which a silicon oxide
film (film thickness=4 nm), a silicon nitride film (film
thickness=5 nm), and a silicon oxynitride film (film thickness=5
nm) are stacked in this order from a silicon substrate. The
activation of the boron is performed by the laser anneal at a
temperature of 1200.degree. C. for 800 .mu.sec, after performing
thermal treatment at a temperature of 900.degree. C. for 30 minutes
in N.sub.2 atmosphere. A result (Curve B), by SIMS analysis, of a
gate electrode comprising a p-type polysilicon single layer film is
also shown as a comparative profile in FIG. 46.
[0216] It is understood from FIG. 46 that the boron concentration
rapidly decreases between the non-doped polysilicon film and the
boron-doped polycrystalline silicon film in the laminated gate
electrode. Especially, the non-doped polysilicon film (i-Si) has a
region of about 4 nm width whose boron concentration is one or more
orders of magnitude lower compared to an average boron
concentration of the boron-doped polycrystalline silicon film.
Since this region can effectively bend a band at a time of applying
a gate voltage, the region may be substantially regarded as a
non-doped polysilicon film.
[0217] Meanwhile, an electrical thickness of the non-doped
polysilicon film is 3.9 nm calculated by C-V measurement using the
relative dielectric constant (=11.9) of silicon. As these results,
diffusion of the boron is suppressed to about 2 nm, and it is
understood that the effective thickness regarded as non-doped
polysilicon is a thickness of a part of the non-doped polysilicon
film which has a boron concentration of one or more orders of
magnitude lower than that of the boron-doped polycrystalline
silicon film. Therefore, a layer called as "non-doped polysilicon
film" in the present invention indicates a region whose boron
concentration is one digit lower than an average of the boron
concentrations of the boron-doped polycrystalline silicon film,
unless otherwise instructed. Also, the film thickness of the
non-doped polysilicon film is considered as a film thickness of an
Si layer electrically measured by C-V measurement.
[0218] It is desirable that, even after the laser anneal is
performed, the boron diffusion is suppressed as small as possible
at later steps, so it is preferable to use laser anneal for
activation of impurities when the diffusion layers (source and
drain) are formed. Ideally, it is desirable to activate the gate
electrode and the diffusion layers at once by laser anneal.
[0219] FIGS. 47A and 47B are graphs showing writing/erasing
characteristics of MONOS having a gate electrode comprising a
p-type polysilicon single layer film and MONOS having a laminated
gate electrode which is formed under the above conditions. Curves
"C", "D", and "E" in FIG. 47A are Vg-Id characteristics of a
transistor in an initial stage of the laminated gate MONOS, and
after writing and erasing operations, respectively. Curves "F",
"G", and "H" in FIG. 47B are Vg-Id characteristics of a transistor
in an initial stage of the single layer gate MONOS, after writing
thereof, and after erasing thereof, respectively.
[0220] FIG. 48 is a graph showing a time dependency of a change in
threshold during a time period from a write state to an erase state
when applying a positive voltage to the gate electrode in the
MONOS. A solid line shows a change in threshold of MONOS having a
laminated gate electrode, and a broken line shows a change in
threshold of MONOS having a single layer gate electrode.
[0221] FIG. 49 is a sectional view of MONOS having a laminated
layer gate electrode. In FIG. 49, a reference numeral 100 denotes a
silicon substrate, 401 denotes an n-type diffusion layer (a source
or a drain), 402 denotes a silicon oxide film with a film thickness
of 4 nm, 403 denotes a silicon nitride film with a film thickness
of 5 nm, and 404 denotes a silicon oxynitride film with a film
thickness of 5 nm. A reference numeral 405 denotes a laminated gate
electrode manufactured by the above-mentioned manufacturing method
and condition for a sample of the SIMS analysis. This laminated
gate electrode is treated for activation by laser anneal together
with diffusion layers at the same time.
[0222] From FIG. 48 and FIG. 49, it is understood that an amount of
injected holes of the laminated gate MONOS at an erasing time is
more than that of the single layer gate MONOS and a rate of erasing
of the former is faster than that of the latter by about 100 times.
Thus, formation of the laminated layer gate electrode conducted by
laser anneal is very useful in the implementation of the present
invention. It is also effective to exclude processes requiring high
temperatures as much as possible after activation of a gate
electrode by laser anneal.
[0223] Thus, in the present embodiment and following embodiments, a
process for activating a laminated gate electrode of a memory cell
by using laser anneal and examples of a manufacturing method in
consideration of how to match with the process are shown.
[0224] A manufacturing method explained in the present embodiment
is to form a one-transistor (NROM) type memory cell having a
laminated layer gate electrode comprising a non-doped polysilicon
film and a p-type polysilicon film and a CMOS peripheral circuit on
the same silicon substrate. In the following respective figures,
left side thereof is sectional view of memory cell region, and
right side thereof is sectional view of peripheral circuit
region.
[0225] As shown in FIG. 50, a gate oxide film 406 of a CMOS
transistor is first formed on a surface of a silicon substrate 100
in which a device isolation region 150, an n-type well 151, and a
p-type well 152 are formed by a well-known method. That is, the
gate oxide film 406 is formed by thermally oxidizing the surface of
the silicon substrate 100 to have a film thickness of about 3 nm.
Subsequently, after a non-doped polysilicon film 407a with a film
thickness of about 150 nm is deposited on the gate oxide film 406
by CVD process, the non-doped polysilicon film 407a and the gate
oxide film 406 on a memory cell region are removed by dry etching,
wet etching, or wet cleaning.
[0226] Next, as shown in FIG. 51, a gate insulating film 408 of a
memory cell having a charge accumulation portion and a barrier film
is deposited. For example, the gate insulating film 408 may be a
silicon oxide film, a silicon nitride film, a silicon oxynitride
film, or a laminated film made thereof. Also, the charge
accumulation layer may be a high-k trap film, a polysilicon
floating gate, or other configurations having a charge accumulation
layer. Here, for example, a laminated film comprising a silicon
oxide film with a film thickness of 4 nm, a silicon oxide film with
a film thickness of 5 nm, and a silicon oxynitride film with a film
thickness of 5 nm is deposited by CVD process.
[0227] Next, as shown in FIG. 52, after a non-doped amorphous
silicon film 409a or an amorphous silicon film 409a with a low
impurity concentration is deposited on the gate insulating film 408
by CVD process, thermal treatment is performed at a temperature of
900.degree. C. for about 30 minutes in N.sub.2 atmosphere. The
thermal treatment is performed for changing amorphous silicon into
polysilicon, and therefore, boron is more effectively prevented
from diffusing into the amorphous silicon film 409a at a later
laser anneal. It is desirable that the impurity concentration in
the amorphous silicon film 409a is one digit lower than that in a
p-type polysilicon film deposited on the amorphous silicon film
409a. It is also desirable that the film thickness of the amorphous
silicon film 409a is 6 nm or less. Even if the thickness is 6 nm or
more, an effect can be obtained to some extent, but when the
thickness is excessively thick, depletion of the gate electrode is
grown at a reading time, so that a read characteristic is
degraded.
[0228] Next, after a p-type polysilicon film 410p with a film
thickness of about 150 nm is deposited on the amorphous silicon
film 409a, the p-type polysilicon film 410p and the amorphous
silicon film 409a on the peripheral circuit region are removed by
dry etching, wet etching, or wet cleaning.
[0229] Next, as shown in FIG. 53, a gate electrode 411 of the
memory cell comprising a laminated layer made of the amorphous
silicon film 409a and the p-type polysilicon film 410p is formed by
patterning the p-type polysilicon film 410p, the amorphous silicon
film 409a, and the gate insulating film 408 on the memory cell
region by dry etching.
[0230] Next, the gate insulating film 408 deposited on the
non-doped polysilicon film 407a on the peripheral circuit region is
removed. Thereafter, as shown in FIG. 54, the non-doped polysilicon
film 407a on an n-MOS transistor formation region is changed, by
ion-implanting phosphorus therein, to an n-type polysilicon 407n,
and also the non-doped polysilicon film 407a on a p-MOS transistor
formation region is changed, by ion-implanting boron therein, to a
p-type polysilicon film 407p. The implantation energy of phosphorus
is set to about 10 Kev and the dose amount is set to about
6.times.10.sup.15/cm.sup.2. Also, the implantation energy of boron
is set to about 5 Kev and the dose amount is set to about
4.times.10.sup.15/cm.sup.2.
[0231] Next, as shown in FIG. 55, a gate electrode 412 of the n-MOS
transistor and a gate electrode 413 of the p-MOS transistor are
formed by patterning the n-type polysilicon film 407n, the p-type
polysilicon film 407p, and the gate oxide film 406 by using dry
etching.
[0232] Next, in order to form each of diffusion layers (source and
drain) of the memory cell, the n-MOS transistor, and the p-MOS
transistor, arsenic is ion-implanted into a p-type well 152 on the
memory cell region and also a p-type well 152 on the peripheral
circuit region, and boron is ion-implanted into the n-type well 151
on the peripheral circuit region, and after that, these impurities
are activated, for example, by laser anneal at a temperature of
1200.degree. C. for 800 .mu.sec. At the same time, activation of
impurities implanted into the gate electrodes 411, 412, and 413 is
also performed.
[0233] Since the CMOS peripheral circuit is required to operate at
a high speed, it is necessary to form silicide layers on surfaces
of diffusion layers (source and drain) later. However, since the
activation by laser anneal causes only a small diffusion of
impurities, only shallow diffusion layers are formed. If a silicide
layer is formed on a surface of the shallow diffusion layer, it
causes a junction leakage in a MOS transistor. Therefore, a
diffusion layer in a peripheral circuit region is made deeper by
implanting impurities therein in multi-stage manner, and then
activation is performed by laser anneal so as to form a deeper
diffusion layer even by using laser anneal for activation.
[0234] For example, when arsenic is ion-implanted, a diffusion
layer with a depth of about 40 to 50 nm can be formed after
activation by performing three stage implantations of (1)
implantation energy=5 KeV and dose
amount=1.times.10.sup.15/cm.sup.2, (2) implantation energy=15 KeV
and dose amount=2.times.10.sup.15/cm.sup.2, and (3) implantation
energy=30 KeV and dose amount=1.times.10.sup.15/cm.sup.2.
Accordingly, when the thickness of the silicide layer is set to 20
nm or less, the silicide does not penetrate the diffusion layer, so
that a normal transistor operation is made possible. Thus, it is
desirable that the CMOS peripheral circuit has a diffusion layer
deeper than that of the memory cell and a high impurity
concentration of the diffusion layer. However, in order to simplify
the process, the diffusion layer of the p-type well 152 of the
memory cell region may be formed deeply by implanting arsenic in a
multi-stage manner, or a silicide layer may also be formed on a
surface of the diffusion layer of the memory cell.
[0235] Incidentally, when arsenic is ion-implanted into the p-type
well 152 of the memory cell, arsenic is also ion-implanted in a
surface of the gate electrode 411 (a p-type polysilicon film 410p),
so that a p-n junction is formed in the gate electrode 411 of the
memory cell when the concentration of arsenic becomes high.
Accordingly, it is desirable that the concentration of arsenic to
be ion-implanted into the p-type well 152 of the memory cell region
is made lower than that of arsenic to be ion-implanted into the
p-type well 152 on the peripheral circuit region. Alternatively,
such a process can be adopted that, after the p-type polysilicon
film 410p is deposited at the step shown in FIG. 52, a silicon
oxide film is deposited on a surface of the p-type polysilicon film
410p, and the silicon oxide film is removed after ion-implantation
for formation of diffusion layer is performed.
[0236] As shown in FIG. 56, through the preceding steps, the n-type
diffusion layers 414 (source and drain) of the memory cell are
formed in the p-type well 152 of the memory cell region. In
addition, the n-type diffusion layers 415 (source and drain) of the
n-MOS transistor are formed in the p-type well 152 on the
peripheral circuit region and the p-type diffusion layers 416
(source and drain) of the p-type transistor are formed in the
n-type well 151.
[0237] Next, as shown in FIG. 57, a silicon oxide film 417 with a
film thickness of about 50 to 100 nm is deposited and side wall
spacers 417s are then formed on side walls of the gate electrodes
412 and 413 by dry-etching the silicon oxide film 417 on the
peripheral circuit region. Thereafter, silicide layers 418 made of
Co silicide are formed on respective surfaces of the gate
electrodes 412 and 413, the n-type diffusion layer 415, and the
p-type diffusion layer 416 of the CMOS peripheral circuit.
Incidentally, as described above, silicide layers 418 may be formed
on respective surfaces of the gate electrode 411 and the n-type
diffusion layer 414 of the memory cell.
[0238] Thus, the n-type diffusion layer 415 and the p-type
diffusion layer 416 of the CMOS peripheral circuit are formed
deeper than the n-type diffusion layer 414 of the memory cell,
whereby the silicide layer 418 can be prevented from penetrating
them. Therefore, a memory cell realizing a hole injection from a
gate at a high efficiency and a CMOS peripheral circuit realizing a
high-speed operation can be manufactured on the same silicon
substrate 100.
Tenth Embodiment
[0239] In the ninth embodiment, the deep diffusion layer is formed
by multi-stage implantation of impurities so that the silicide
layer does not penetrate the diffusion layer. In the present
embodiment, a CMOS peripheral circuit having a deep diffusion layer
is formed by a process in which an ion-implantation for a diffusion
layer is performed before a laminated gate electrode of a memory
cell is formed, and a gate electrode of the memory cell is formed
after activation of the diffusion layer is performed by RTA.
[0240] As shown in FIG. 58, a gate oxide film 406 for a CMOS
transistor is first formed on a surface of a silicon substrate 100
in which a device isolation region 150, an n-type well 151, and a
p-type well 152 are formed. The film thickness of the gate oxide
film 406 is set to about 3 nm. Subsequently, after a non-doped
polysilicon film (not shown) with a film thickness of about 150 nm
is deposited on the gate oxide film 406 by CVD process, phosphorus
is ion-implanted into the non-doped polysilicon film on a memory
cell region and the non-doped polysilicon film on an n-MOS
transistor formation region to form n-type polysilicon films 407n,
and boron is ion-implanted into the non-doped polysilicon film on a
p-MOS formation region to form a p-type polysilicon film 407p. The
implantation energy of phosphorus is set to about 10 KeV and the
dose amount thereof is set to about 6.times.10.sup.12/cm.sup.2. The
implantation energy of boron is set to about 5 KeV and the dose
amount thereof is set to about 4.times.10.sup.15/cm.sup.2.
[0241] Next, after the non-doped polysilicon film and the gate
oxide film 406 on the memory cell region are removed, a gate
electrode 412 of an n-MOS transistor and a gate electrode 413 of a
p-MOS transistor are formed by patterning the n-type polysilicon
films 407n, the p-type polysilicon film 407p, and the gate oxide
film 406, as shown in FIG. 59.
[0242] Next, as shown in FIG. 60, after a photoresist film 420
having the same shape as the gate electrode is formed in a region
where a gate electrode of the memory cell is formed at a later
step, as shown in FIG. 61, ion-implantation of impurities is
performed in order to form respective diffusion layers (source and
drain) of the memory cell, the n-MOS transistor, and the p-MOS
transistor. At this time, arsenic is ion-implanted into the p-type
well 152 on the memory cell region under the conditions of the
implantation energy=25 KeV and the dose
amount=1.times.10.sup.15/cm.sup.2. Arsenic is ion-implanted into
the p-type well 152 on the peripheral circuit region under the
conditions of the implantation energy=40 KeV and the dose
amount=4.times.10.sup.15/cm.sup.2, and boron is ion-implanted into
the n-type well 151 under the conditions of the implantation
energy=10 KeV and the dose amount=2.times.10.sup.15/cm.sup.2. At
this time, a wider impurity profile in a depth direction of the
silicon substrate 100 may be formed in order to form a deeper
diffusion layer by performing the ion-implantation in the
multi-stage manner. Especially, it is desirable that a CMOS
transistor requiring a silicide layer for realizing a high-speed
operation has a deeper diffusion layer and a higher impurity
concentration than those of the diffusion layer of the memory
cell.
[0243] Next, activation treatment of the above-mentioned impurities
is performed by RTA, whereby n-type diffusion layers 414 (source
and drain) of the memory cell are formed in the p-type well 152 on
the memory cell region. Also, the n-type diffusion layers 415
(source and drain) of the n-MOS transistor are formed in the p-type
well 152 on the peripheral circuit region, and p-type diffusion
layers 416 (source and drain) of the p-MOS transistor are formed in
the n-type well 151.
[0244] Next, as shown in FIG. 62, after the photoresist film 420 on
the memory region is removed, a gate insulating film 408 of the
memory cell having a charge accumulation portion and a barrier film
is deposited as shown in FIG. 63. The gate insulating film 408 may
be, for example, a silicon oxide film, a silicon nitride film, a
silicon oxynitride film, or a laminated film made thereof. The
charge accumulation film may be a high-k trap film or a polysilicon
floating gate, or other configurations having a charge accumulation
layer. Here, for example, a laminated film comprising a silicon
oxide film with a film thickness of 4 nm, a silicon oxide film with
a film thickness of 5 nm, and a silicon oxynitride film with a film
thickness of 5 nm is deposited by CVD process.
[0245] Next, a non-doped amorphous silicon film 409a or an
amorphous silicon film 409a with a low impurity concentration is
deposited on the gate insulating film 408. Thermal treatment is
then performed at a temperature of 900.degree. C. for about 30
minutes in N.sub.2 atmosphere. Thereafter, a p-type polysilicon
film 410p with a film thickness of about 150 nm is deposited on the
amorphous silicon film 409a. It is desirable that the impurity
concentration of the amorphous silicon film 409a is one or more
orders of magnitude lower than that of the p-type polysilicon film
410p like the ninth embodiment. It is also desirable that the film
thickness of the amorphous silicon film 409a is 6 nm or less.
[0246] Next, as shown in FIG. 64, after the p-type polysilicon film
410p and the amorphous silicon film 409a on the peripheral circuit
region are removed, a gate electrode 411 of the memory cell
comprising a laminated film made of the amorphous silicon film 409a
and the p-type polysilicon film 410p is formed by patterning the
p-type polysilicon film 410p, the amorphous silicon film 409a, and
the gate insulating film 408 on the memory cell region by dry
etching.
[0247] Next, as shown in FIG. 65, after the gate insulating film
408 on the peripheral circuit region is removed, as shown in FIG.
66, a silicon oxide film 417 is deposited by the same method as
used in the ninth embodiment. After side wall spacers 417s are
formed on side walls of the gate electrodes 412 and 413 by
dry-etching the silicon oxide film 417 on the peripheral circuit
region, silicide layers 418 are formed on respective surfaces of
the gate electrodes 412 and 413, the n-type diffusion layer 415,
and the p-type diffusion layer 416 on the CMOS peripheral
circuit.
[0248] Thus, by forming the n-type diffusion layer 415 and the
p-type diffusion layer 416 on the CMOS peripheral circuit to be
deeper than the n-type diffusion layer 414 of the memory cell, the
silicide layer 418 can be prevented from penetrating so that a
memory cell realizing a hole injection from a gate at a high
efficiency and a CMOS peripheral circuit realizing a high-speed
operation can be manufactured on the same silicon substrate
100.
[0249] According to the present embodiment, since the diffusion
layer 414 is formed before the gate electrode 411 of the memory
cell is formed, even if the impurity concentration in the diffusion
layer 414 is made higher, no p-n junction is formed in the gate
electrode 411.
Eleventh Embodiment
[0250] In the present embodiment, a manufacturing method of forming
a split gate type memory cell and a CMOS peripheral circuit as
shown in the eighth embodiment on the same silicon substrate will
be explained.
[0251] As shown in FIG. 67, first, after a gate oxide film 406 with
a film thickness of about 3 nm is formed on a surface of a silicon
substrate 100 in which a device isolation region 150, an n-type
well 151, and a p-type well 152 are formed, a non-doped polysilicon
film 407a with a film thickness of about 150 nm is then deposited
on the gate oxide film 406. Thereafter, phosphorus is ion-implanted
into the non-doped polysilicon film 407a on the memory cell region
to form an n-type polysilicon film 407n. The implantation energy of
phosphorus is set to about 10 KeV and the dose amount thereof is
set to about 6.times.10.sup.15/cm.sup.2.
[0252] Next, as shown in FIG. 68, after a selection gate 421 is
formed by patterning the n-type polysilicon film 407n and the gate
oxide film 406 on the memory cell region, a gate insulating film
408 of the memory cell having a charge accumulation portion and a
barrier film is deposited. The gate insulating film 408 is, for
example, a laminated film comprising a silicon oxide film with a
film thickness of 4 nm, a silicon oxide film with a film thickness
of 5 nm, and a silicon oxynitride film with a film thickness of 5
nm. However, it may be made of other materials such as shown in the
ninth and tenth embodiments.
[0253] Next, as shown in FIG. 69, after a non-doped amorphous
silicon film 409a or an amorphous silicon film 409a with a low
impurity concentration is deposited on the gate insulating film 408
and thermal treatment is then performed at a temperature of
900.degree. C. for about 30 minutes in N.sub.2 atmosphere, a p-type
polysilicon film 410p with a film thickness of about 150 nm is
deposited on the amorphous silicon film 409a. It is desirable that
the impurity concentration of the amorphous silicon film 409a is
one digit lower than that of the p-type polysilicon film 410p like
the ninth embodiment. It is also desirable that the film thickness
of the amorphous silicon film 409a is 6 nm or less.
[0254] Next, as shown in FIG. 70, spacer-like memory gates 422
comprising a laminated layer film made of the amorphous silicon
film 409a and the p-type polysilicon film 410p are formed by
patterning the p-type polysilicon film 410p and the amorphous
silicon film 409a on the memory cell region by dry etching.
[0255] Next, as shown in FIG. 71, one of the memory gates 422
formed on both sides of the selection gate 421 is removed by
dry-etching using a photoresist film 423 as a mask.
[0256] Next, as shown in FIG. 72, while the gate insulating film
408 is left on one of side walls of the selection gate 421 and
under the memory gate 422, the gate insulating films 408 on the
other regions are removed by etching.
[0257] Next, as shown in FIG. 73, phosphorus is ion-implanted into
the non-doped polysilicon film 407a on the n-MOS transistor
formation region to form an n-type polysilicon film 407n. Also,
born is ion-implanted into the non-doped polysilicon film 407a on
the p-MOS formation region to form a p-type polysilicon film 407p.
The implantation energy of phosphorus is set to about 10 KeV and
the dose amount thereof is set to about 6.times.10.sup.15/cm.sup.2.
The implantation energy of boron is set to about 5 KeV and the dose
amount thereof is set to about 4.times.10.sup.15/cm.sup.2.
[0258] Next, as shown in FIG. 74, a gate electrode 412 of the n-MOS
transistor and a gate electrode 413 of the p-MOS transistor are
formed by patterning the n-type polysilicon film 407n, the p-type
polysilicon film 407p, and the gate oxide film 406 by
dry-etching.
[0259] Next, as shown in FIG. 75, arsenic is ion-implanted into a
p-type well 152 on the memory cell region and a p-type well 152 on
the peripheral circuit region and boron is ion-implanted into an
n-type well 151 on the peripheral circuit region in order to form
respective diffusion layers (source and drain) of the memory cell,
the n-MOS transistor, and the p-MOS transistor. Thereafter, these
impurities are activated, for example, by laser anneal at a
temperature of 1200.degree. C. for 800 .mu.sec like the ninth
embodiment. At the same time, activation of impurities implanted
into the gate electrodes 421, 412, and 413 is also performed.
[0260] Since the CMOS peripheral circuit is required to operate at
a high speed, it is necessary to form silicide layers on surfaces
of diffusion layers (source and drain) later. However, the
activation by laser anneal causes only a small diffusion of
impurities, so that only shallow diffusion layers are formed. If a
silicide layer is formed on a surface of the shallow diffusion
layer, it causes junction leakage in a MOS transistor. Therefore,
the diffusion layer in a peripheral circuit region is made deeper
by implanting impurities therein in multi-stage manner, and then
activation is performed by laser anneal so as to form a deeper
diffusion layer even by using laser anneal for activation.
[0261] For example, when arsenic is ion-implanted, the diffusion
layer with a depth of about 40 to 50 nm can be formed after
activation by performing three stage implantation of (1) the
implantation energy=5 KeV and dose
amount=1.times.10.sup.15/cm.sup.2, (2) the implantation energy=15
KeV and the dose amount=2.times.10.sup.15/cm.sup.2, and (3) the
implantation energy=30 KeV and the dose
amount=1.times.10.sup.15/cm.sup.2. Accordingly, when the thickness
of the silicide layer is set to 20 nm or less, the silicide does
not penetrate the diffusion layer, so that a normal transistor
operation is made possible. Thus, it is desirable that the CMOS
peripheral circuit has a diffusion layer deeper than that of the
memory cell and has a higher impurity concentration of the
diffusion layer. However, in order to simplify the process, the
diffusion layer of the p-type well 152 of the memory cell region
may be formed deeply by implanting arsenic in a multi-stage manner,
or a silicide layer may also be formed on a surface of the
diffusion layer of the memory cell.
[0262] Incidentally, when arsenic is ion-implanted in the p-type
well 152 of the memory cell, arsenic is also ion-implanted in a
surface of the gate electrode 421 (p-type polysilicon film 410p),
so that a pn junction is formed in the gate electrode 421 of the
memory cell when the concentration of arsenic becomes high.
Accordingly, it is desirable that the concentration of arsenic to
be ion-implanted into the p-type well 152 of the memory cell region
is made lower than that of arsenic to be ion-implanted into the
p-type well 152 on the peripheral circuit region.
[0263] Through the preceding steps, the n-type diffusion layers 424
(source and drain) of the memory cell are formed in the p-type well
152 of the memory cell region. In addition, the n-type diffusion
layers 415 (source and drain) of the n-MOS transistor are formed in
the p-type well 152 on the peripheral circuit region and the p-type
diffusion layers 416 (source and drain) of the p-type transistor
are formed in the n-type well 151.
[0264] Next, as shown in FIG. 76, a silicon oxide film 417 with a
film thickness of about 50 to 100 nm is deposited and side wall
spacers 417s are then formed on side walls of the gate electrodes
412 and 413 by dry-etching the silicon oxide film 417 on the
peripheral circuit region. Thereafter, silicide layers 418 made of
Co silicide are formed on respective surfaces of the gate
electrodes 412 and 413, the n-type diffusion layer 415, and the
p-type diffusion layer 416 of the CMOS peripheral circuit.
[0265] Thus, the n-type diffusion layer 415 and the p-type
diffusion layer 416 of the CMOS peripheral circuit are formed
deeper than the n-type diffusion layer 424 of the memory cell,
whereby the silicide layer 418 can be prevented from penetrating
them. Therefore, a memory cell realizing a hole injection from a
gate at a high efficiency and a CMOS peripheral circuit realizing a
high-speed operation can be manufactured on the same silicon
substrate 100.
Twelfth Embodiment
[0266] In the present embodiment, a manufacturing method of forming
a split gate type memory cell and a CMOS peripheral circuit as
shown in the eighth embodiment on the same silicon substrate will
be explained.
[0267] As shown in FIG. 77, first, after a gate oxide film 406 with
a film thickness of about 3 nm is formed on a surface of a silicon
substrate 100 on which a device isolation region 150, an n-type
well 151, and a p-type well 152 are formed, a non-doped polysilicon
film 407a with a film thickness of about 150 nm is then deposited
on the gate oxide film 406.
[0268] Next, as shown in FIG. 78, phosphorus is ion-implanted into
the non-doped polysilicon film 407a on a memory cell region and the
non-doped polysilicon film 407a on an n-MOS transistor formation
region to form n-type polysilicon films 407n, and boron is
ion-implanted into the non-doped polysilicon film 407a on a p-MOS
formation region to form a p-type polysilicon film 407p. The
implantation energy of phosphorus is set to about 10 KeV and the
dose amount thereof is set to about 6.times.10.sup.15/cm.sup.2. The
implantation energy of boron is set to about 5 KeV and the dose
amount thereof is set to about 4.times.10.sup.15/cm.sup.2.
[0269] Next, as shown in FIG. 79, a selection gate 421 is formed by
patterning the n-type polysilicon film 407n and the gate oxide film
406 on the memory cell region. Then, a gate electrode 412 of the
n-MOS transistor and a gate electrode 413 of the p-MOS transistor
are formed by patterning the n-type polysilicon film 407n, the
p-type polysilicon film 407p, and the gate oxide film 406 on the
peripheral circuit region.
[0270] Next, as shown in FIG. 80, arsenic is ion-implanted into the
p-type well 152 and boron is ion-implanted into the n-type well
151. The implantation energy of arsenic is set to about 40 KeV and
the dose amount thereof is set to about 4.times.10.sup.15/cm.sup.2.
The implantation energy of boron is set to about 10 KeV and the
dose amount thereof is set to about 2.times.10.sup.15/cm.sup.2. At
this time, a region where a memory gate is formed at a later step
is covered with a photoresist film 425.
[0271] Next, activation treatment for the above-mentioned
impurities is performed by RTA. Thereby, n-type diffusion layers
424 (source and drain) of the memory cell are formed in the p-type
well 152 on the memory cell region, as shown in FIG. 81. Also, the
n-type diffusion layers 415 (source and drain) of the n-MOS
transistor are formed in the p-type well 152 on the peripheral
circuit region, and p-type diffusion layers 416 (source and drain)
of the p-MOS transistor are formed in the n-type well 151.
[0272] Next, as shown in FIG. 82, after a spacer-like memory gate
422 is formed by following the steps shown in FIG. 68 to FIG. 72 in
the eleventh embodiment, as shown in FIG. 83, silicide layers 418
are formed on respective surfaces of the gate electrodes 412 and
413, the n-type diffusion layer 415, and the p-type diffusion layer
416 of the CMOS peripheral circuit by following the step shown in
FIG. 76.
[0273] Thus, by forming the n-type diffusion layer 415 and the
p-type diffusion layer 416 of the CMOS peripheral circuit deeply,
the silicide layer 418 can be prevented from penetrating.
Therefore, a memory cell realizing a hole injection from a gate at
a high efficiency and a CMOS peripheral circuit realizing a
high-speed operation can be manufactured on the same silicon
substrate 100.
[0274] According to the present embodiment, since the n-type
diffusion layer 424 of the memory cell is formed before the memory
gate 422 is formed, even if the impurity concentration of the
n-type diffusion layer 424 is made high, no pn junction is formed
in the memory gate 422. Accordingly, it is also made possible to
form a silicide layer 418 on the surface of the n-type diffusion
layer 424.
Thirteenth Embodiment
[0275] In the tenth embodiment and the twelfth embodiment,
impurities are introduced into the non-doped polysilicon film 407a
for a gate electrode on the CMOS peripheral circuit prior to the
step of forming the gate insulating film 408 including a charge
accumulation layer and a barrier film.
[0276] In the step of forming the gate insulating film 408, there
is a process of performing high temperature treatment such as
anneal in NO atmosphere in order to improve reliability of films.
However, when the high temperature treatment is performed, there is
a possibility that n-type impurities and p-type impurities in the
non-doped polysilicon film 407a are thermally diffused and locally
mixed. In the present embodiment, therefore, a manufacturing method
which can prevent such a drawback will be explained. Here, an
example of a one-transistor type (NROM type) memory cell will be
explained.
[0277] As shown in FIG. 84, first, a gate oxide film 406 with a
film thickness of about 3 nm is formed on a surface of a silicon
substrate 100 in which a device isolation region 150, an n-type
well 151, and a p-type well 152 are formed. Then, a non-doped
polysilicon film 407a with a film thickness of about 150 nm is
deposited on the gate oxide film 406. After that, the non-doped
polysilicon film 407a and the gate oxide film 406 on a memory cell
region are removed. Next, a gate insulating film 408 is formed. The
gate insulating film 408 is, for example, a laminated film
comprising a silicon oxide film with a film thickness of 4 nm, a
silicon oxide film with a film thickness of 5 nm, and a silicon
oxynitride film with a film thickness of 5 nm. But it may be made
of other materials such as shown in the ninth and tenth
embodiments.
[0278] Next, as shown in FIG. 85, the gate insulating film 408 on
the peripheral circuit region is removed and the gate insulating
film 408 on the memory cell region is patterned so that the gate
insulating film 408 is left on a region where a gate electrode
(411) for the memory cell is formed at a later step. Subsequently,
phosphorus is ion-implanted into the non-doped polysilicon film
407a on the n-MOS transistor formation region to form an n-type
polysilicon film 407n and boron is ion-implanted into the non-doped
polysilicon film 407a in the p-MOS transistor formation region to
form a p-type polysilicon film 407p by following the steps shown in
FIG. 54 and FIG. 55 in the ninth embodiment. The implantation
energy of phosphorus is set to about 10 KeV and the dose amount
thereof is set to about 6.times.10.sup.15/cm.sup.2. The
implantation energy of boron is set to about 5 KeV and the dose
amount thereof is set to about 4.times.10.sup.15/cm.sup.2.
[0279] Next, a gate electrode 412 of the n-MOS transistor and a
gate electrode 413 of the p-MOS transistor are formed by patterning
the n-type polysilicon film 407n, the p-type polysilicon film 407p,
and the gate oxide film 406 by dry etching.
[0280] Next, as shown in FIG. 86, the gate insulating film 408, the
gate electrodes 412 and 413, and their periphery are covered with
photoresist films 426, respectively. Then, arsenic is ion-implanted
into the p-type well 152, and boron is ion-implanted into the
n-type well 151. The implantation energy of arsenic is set to about
40 KeV and the dose amount thereof is set to about
4.times.10.sup.15/cm.sup.2. The implantation energy of boron is set
to about 10 KeV and the dose amount thereof is set to about
2.times.10.sup.15/cm.sup.2. A region to be ion-implanted with these
impurities is a region where a silicide layer (18) is formed at a
later step.
[0281] Next, as shown in FIG. 87, activation treatment of the
impurities is performed by RTA. Thereby, deep n-type diffusion
layers 427 (source and drain) are formed in the p-type well 152 on
the memory region. Also, deep n-type diffusion layers 428 (source
and drain) are formed in the p-type well 152 on the peripheral
circuit region and deep p-type diffusion layers 429 (source and
drain) are formed in the n-type well 151.
[0282] Next, as shown in FIG. 88, after an insulating film 430 with
a film thickness of about 50 to 100 nm is deposited, the insulating
film 430 on the memory region is removed. The insulating film 430
is a protective film for protecting the peripheral circuit region
when the memory cell region is processed at the next step. The
insulating film 430 comprises, for example, a silicon oxide film, a
laminated film made of a silicon oxide film and a polysilicon film,
or a laminated film made of a silicon oxide film and a silicon
nitride film.
[0283] Next, a non-doped amorphous film 409a or an amorphous
silicon film 409a with a low impurity concentration is deposited.
Then, thermal treatment is performed at a temperature of
900.degree. C. for about 30 minutes in N.sub.2 atmosphere.
Thereafter, a p-type polysilicon film 410p with a film thickness of
150 nm is deposited on the amorphous silicon film 409a. It is
desirable that the impurity concentration of the amorphous silicon
film 409a is one digit lower than that of the p-type polysilicon
film 410p like the ninth embodiment. It is also desirable that the
film thickness of the amorphous silicon film 409a is 6 nm or less.
Next, the p-type polysilicon film 410p and the amorphous silicon
film 409a on the peripheral circuit region are removed.
[0284] Next, as shown in FIG. 89, a gate electrode 411 for the
memory cell comprising a laminated film made of an amorphous
silicon film 409a and a p-type polysilicon film 410p is formed by
patterning the p-type polysilicon film 410p and the amorphous
silicon film 409a on the memory cell region. Subsequently, the
insulating film 430 on the peripheral circuit region is
removed.
[0285] Next, as shown in FIG. 90, after arsenic is ion-implanted
into a p-type well 152 on the memory cell region and p-type well
152 on the peripheral circuit region, and boron is ion-implanted
into the n-type well 151 on the peripheral circuit region in order
to form respective diffusion layers of the memory cell, the n-MOS
transistor, and the p-MOS transistor, these impurities are
activated, for example, by laser anneal at a temperature of
1200.degree. C. for 800 .mu.sec.
[0286] By doing this, an n-type diffusion layer 431 of the memory
cell is formed in the p-type well 152 on the memory cell region. An
n-type diffusion layer 432 of the n-MOS transistor is formed in the
p-type well 152 on the peripheral circuit region and a p-type
diffusion layer 433 of the p-MOS transistor is formed in the n-type
well 151.
[0287] Next, as shown in FIG. 91, a silicon oxide film (not shown)
with a film thickness of about 50 to 100 nm is deposited and side
wall spacers 417s are then formed on respective side walls of the
gate electrodes 411, 412, and 413 by dry-etching the silicon oxide
film. Thereafter, silicide films 418 made of Co silicide are formed
on respective surfaces of the gate electrodes 411, 412, and 413,
the n-type diffusion layers 431 and 432, and the p-type diffusion
layer 433.
[0288] Thus, according to the manufacturing method of the present
embodiment, since impurities are introduced into the non-doped
polysilicon film 407a for a gate electrode on the CMOS peripheral
circuit after the gate insulating film 408 is formed, such a
drawback can be prevented that n-type impurities and p-type
impurities in the non-doped polysilicon film 407a are thermally
diffused and mixed.
[0289] According to the manufacturing method of the present
embodiment, by performing the activation process by laser anneal at
the step shown in FIG. 90 after ion-implantation of impurities,
shallow diffusion layers (431, 432, 433) functioning as extension
layers of the source and the drain can be formed.
Fourteenth Embodiment
[0290] In the thirteenth embodiment, the example of a
one-transistor type memory cell (NROM type) has been explained.
Then, in the present embodiment, an example of a split gate type
memory cell will be explained.
[0291] As shown in FIG. 92, first, a gate oxide film 406 with a
film thickness of about 3 nm is formed on a surface of a silicon
substrate 100 in which a device isolation region 150, an n-type
well 151, and a p-type well 152 are formed. A non-doped polysilicon
film 407a with a film thickness of about 150 nm is then deposited
on the gate oxide film 406. Thereafter, phosphorus is ion-implanted
into the non-doped polysilicon film 407a on the memory cell region
to form an n-type polysilicon film 407n. The implantation energy of
phosphorus is set to about 10 KeV and the dose amount thereof is
set to about 6.times.10.sup.15/cm.sup.2.
[0292] Next, a selection gate 421 is formed by patterning the
n-type polysilicon film 407n and the gate oxide film 406 on the
memory region.
[0293] Next, as shown in FIG. 93, a gate insulating film 408 of the
memory cell having a charge accumulation portion and a barrier film
is deposited. The gate insulating film 408 is, for example, a
laminated film comprising a silicon oxide film with a film
thickness of 4 nm, a silicon oxide film with a film thickness of 5
nm, and a silicon oxynitride film with a film thickness of 5 nm. In
addition, it may be made of other materials as shown in the ninth
and tenth embodiments.
[0294] Next, after the gate insulating film 408 on the peripheral
circuit region is removed, phosphorus is ion-implanted into the
non-doped polysilicon film 407a on the n-MOS transistor formation
region to form an n-type polysilicon film and boron is
ion-implanted into the non-doped polysilicon film 407a on the p-MOS
transistor formation region to form a p-type polysilicon film. The
implantation energy of phosphorus is set to about 10 KeV and the
dose amount thereof is set to about 6.times.10.sup.15/cm.sup.2. The
implantation energy of boron is set to about 5 KeV and the dose
amount thereof is set to about 4.times.10.sup.15/cm.sup.2.
[0295] Next, a gate electrode 412 of the n-MOS transistor and a
gate electrode 413 of the p-MOS transistor are formed by patterning
the n-type polysilicon film, the p-type polysilicon film, and the
gate oxide film 406 by dry-etching. Next, an insulating film 430
with a film thickness of about 50 to 100 nm is deposited. The
insulating film 430 comprises, for example, a silicon oxide film, a
laminated film made of a silicon oxide film and a polysilicon film,
and a laminated film made of a silicon oxide film and a silicon
nitride film.
[0296] Next, as shown in FIG. 94, arsenic is ion-implanted into the
p-type well 152 and boron is ion-implanted into the n-type well
151. Since impurities must be introduced into the wells through the
insulating film 430, these ion-implantations are conducted with
implantation energy higher than the implantation energy shown in
the ninth to thirteenth embodiments. Ion-implantation may be
performed after spacer-like insulating films 430 are left in a
space-like shape on side walls of the gate electrodes (421, 412,
413) by etching the insulating films 430 in advance.
[0297] Next, an activation process of the impurities is performed
by RTA, whereby, as shown in FIG. 95, deep n-type diffusion layers
427 (source and drain) are formed in the p-type wall 152 on the
memory region. Also, deep n-type diffusion layers 428 (source and
drain) are formed in the p-type well 152 on the peripheral circuit
region and deep p-type diffusion layers 429 (source and drain) are
formed in the n-type well 151.
[0298] Next, after the insulating film 430 on the memory cell
region is removed, as shown in FIG. 96, a spacer-like memory gate
422 is formed by following the steps shown in FIG. 68 to FIG. 72
regarding the eleventh embodiment. In these steps, a film deposited
on the peripheral circuit region is removed by using a conventional
cleaning process, as needed.
[0299] Next, in order to form respective diffusion layers of the
memory cell, the n-MOS transistor, and the p-MOS transistor,
arsenic is ion-implanted into a p-type well 152 on the memory cell
region and a p-type well 152 on the peripheral circuit region and
boron is ion-implanted into the n-type well 151 on the peripheral
circuit region, and then, these impurities are activated, for
example, by laser anneal at a temperature of 1200.degree. C. for
800 .mu.sec.
[0300] By doing this, an n-type diffusion layer 431 of the memory
cell is formed in the p-type well 152 on the memory cell region. An
n-type diffusion layer 432 of the n-MOS transistor is formed in the
p-type well 152 on the peripheral circuit region and a p-type
diffusion layer 433 of the p-MOS transistor is formed in the n-type
well 151.
[0301] Next, as shown in FIG. 97, after a silicon oxide film (not
shown) with a film thickness of about 50 to 100 nm is deposited and
side wall spacers 434 are then formed on respective side walls of
the gate electrodes (421, 422, 412, 413) by dry-etching the silicon
oxide film, silicide films 418 made of Co silicide or other
materials are formed on respective surfaces of the gate electrodes
(421, 422, 412, 413), the n-type diffusion layers 431 and 432, and
the p-type diffusion layers 433.
[0302] Thus, according to the manufacturing method in the
embodiment, since impurities are introduced into the non-doped
polysilicon film 407a for a gate electrode on the CMOS peripheral
circuit after forming the gate insulating film 408, such a drawback
can be prevented that n-type impurities and p-type impurities in
the non-doped polysilicon film 407a are thermally diffused and
mixed.
[0303] According to the manufacturing method of the embodiment,
since the activation process is performed by laser anneal after the
ion-implantation of impurities, shallow diffusion layers (431, 432,
433) serving as extension layers for the source and the drain can
be formed.
[0304] On the peripheral circuit region, impurities for formation
of a diffusion layer may be ion-implanted before the gate
insulating film 408 is deposited. At this time, an extension can be
formed on the peripheral circuit region by a multi-stage
implantation in which after impurities are once ion-implanted,
other impurities are ion-implanted once again after a side wall is
formed.
Fifteenth Embodiment
[0305] The embodiments described above have realized memory cells
having a laminated structure comprising a p-type polysilicon film
and a non-doped polysilicon film or comprising a p-type polysilicon
film and a polysilicon film with a lower impurity concentration
than that of the p-type polysilicon film in order to inject holes
from a gate at a high efficiency.
[0306] As described above, there are advantages, that is, a hole
injection causes less damage to an interface between a substrate
and an insulating film compared to a hot hole junction from a
substrate; suppression of fluctuation in threshold can be achieved
because of non-local injection, in which the fluctuation is caused
by redistribution of charges in a charge accumulation layer after
injecting the charges; a wider difference in threshold between a
writing state and an erasing state can be realized by a bipolar
operation system injecting electrons from a gate and also holes
from a substrate compared to, for example, a unipolar operation
system injecting only electrons into a charge accumulation layer
and withdrawing them therefrom. Especially, since holes are largely
injected and a threshold is reduced at an erasing time, a large
read current can be obtained at a read time, whereby a whole module
including a memory can operate at a high speed.
[0307] Naturally, a writing and erasing operation system by
unipolar carriers, using only electrons, may be adopted depending
on applications. The laminated gate electrode according to the
present invention is useful for this operation system. In this
case, as the laminated structure, a laminated structure made of a
non-doped polysilicon film and type polysilicon film, or made of a
non-doped polysilicon film and a polysilicon film with a low
impurity concentration is adopted.
[0308] It is desirable that the film thickness of the non-doped
polysilicon film or the polysilicon film with a low impurity
concentration is 6 nm or less like the embodiments previously
described. Even if the film thickness is more than 6 nm, an effect
can be obtained, but when the film thickness is excessively thick,
depletion of a gate at a read time is grown, so that read
characteristics is degraded. It is also desirable that the
polysilicon film with a low impurity concentration is one or more
orders of magnitude lower than the n-type polysilicon film
laminated thereon. Besides, the manufacturing method for a single
body of a memory cell and a manufacturing in consideration of
matching a peripheral circuit are as described in the
above-mentioned embodiment. At this time, a advantage in the
present structure lies in that, for example, when n-type impurities
are ion-implanted into a diffusion layer on a memory cell portion,
such a problem that a gate portion becomes a pn junction, which is
described as a problem in the above-mentioned embodiments, does not
occur because a gate electrode is also an n-type polysilicon
film.
[0309] Since a potential barrier of an insulating film to electrons
is lower than that to holes, writing and erasing operations that
electrons are injected into a charge accumulation layer from a gate
by applying a negative voltage to the gate and electrons are
withdrawn to the gate by applying a positive voltage to the gate
can be performed in this structure at a high efficiency.
Especially, when electrons are injected from the gate, since a
voltage is applied to a non-doped polysilicon film or a polysilicon
film with a low impurity concentration in a gate like the case that
holes are injected in the above-mentioned embodiments, a potential
barrier of an insulating film viewed from electrons in the gate
becomes lower effectively, so that electrons can be injected at a
high efficiency. Since the present operation is not a charge
injection system using hot electrons and hot holes from a
substrate, damage to an insulating film on a substrate side can be
suppressed, so that excellent characteristic can be obtained at a
read time and a charge retention time. When further speed-up at a
write time is required, an operation that a hot electron injection
from a substrate is performed and electrons are withdrawn to a gate
can be performed in the present structure.
[0310] The invention invented by the present inventors has been
concretely explained according to the embodiments, but the present
invention is not limited to the above-mentioned embodiments, and
needless to say, it can be variously modified without departing
from the gist of the present invention.
[0311] The present invention can be applied to a semiconductor
storage device having a non-volatile memory.
* * * * *