U.S. patent application number 14/948652 was filed with the patent office on 2016-03-17 for semiconductor device and manufacturing method thereof.
The applicant listed for this patent is Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Takuya HANDA, Mitsuo MASHIYAMA, Kenichi OKAZAKI, Masahiro WATANABE.
Application Number | 20160079433 14/948652 |
Document ID | / |
Family ID | 47742328 |
Filed Date | 2016-03-17 |
United States Patent
Application |
20160079433 |
Kind Code |
A1 |
WATANABE; Masahiro ; et
al. |
March 17, 2016 |
SEMICONDUCTOR DEVICE AND MANUFACTURING METHOD THEREOF
Abstract
To provide a semiconductor device including an oxide
semiconductor, which has stable electric characteristics and has
high reliability. To provide a method for manufacturing the
semiconductor device. The semiconductor device includes a gate
electrode, a gate insulating film formed over the gate electrode,
an oxide semiconductor film formed over the gate insulating film, a
source electrode and a drain electrode formed over the oxide
semiconductor film, and a protective film. The protective film
includes a metal oxide film, and the metal oxide film has a film
density of higher than or equal to 3.2 g/cm.sup.3.
Inventors: |
WATANABE; Masahiro;
(Tochigi, JP) ; MASHIYAMA; Mitsuo; (Oyama, JP)
; HANDA; Takuya; (Tochigi, JP) ; OKAZAKI;
Kenichi; (Tochigi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Semiconductor Energy Laboratory Co., Ltd. |
Atsugi-shi |
|
JP |
|
|
Family ID: |
47742328 |
Appl. No.: |
14/948652 |
Filed: |
November 23, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13592870 |
Aug 23, 2012 |
9252279 |
|
|
14948652 |
|
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Current U.S.
Class: |
257/43 |
Current CPC
Class: |
H01L 29/7869 20130101;
H01L 29/78606 20130101; H01L 29/24 20130101; H01L 29/41733
20130101 |
International
Class: |
H01L 29/786 20060101
H01L029/786; H01L 29/417 20060101 H01L029/417; H01L 29/24 20060101
H01L029/24 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2011 |
JP |
2011-189717 |
Claims
1. (canceled)
2. A semiconductor device comprising: an oxide semiconductor film
over an insulating surface; a source electrode and a drain
electrode over the oxide semiconductor film; an oxide insulating
film over and in contact with the oxide semiconductor film, the
source electrode, and the drain electrode; and a first metal oxide
film over the oxide insulating film, wherein the first metal oxide
film has a film density of higher than or equal to 3.2 g/cm.sup.3,
wherein the source electrode overlaps with a first end of the oxide
semiconductor film, and wherein the drain electrode overlaps with a
second end of the oxide semiconductor film.
3. The semiconductor device according to claim 2, further
comprising a second metal oxide film in contact with the oxide
semiconductor film.
4. The semiconductor device according to claim 2, wherein the first
metal oxide film comprises aluminum oxide.
5. The semiconductor device according to claim 2, wherein the first
metal oxide film comprises Ga--Zn-based oxide.
6. The semiconductor device according to claims 2, wherein the
oxide semiconductor film comprises at least one oxide of indium,
zinc, gallium, zirconium, tin, gadolinium, titanium, and
cerium.
7. The semiconductor device according to claim 2, wherein the oxide
insulating film comprises a region in which the oxygen content is
higher than the stoichiometric proportion.
8. A semiconductor device comprising: a gate electrode; a gate
insulating film over the gate electrode; an oxide semiconductor
film over the gate insulating film; a source electrode and a drain
electrode over the oxide semiconductor film; and a protective film
over the oxide semiconductor film, the source electrode, and the
drain electrode, wherein the protective film is a stack in which a
first metal oxide film is provided over an oxide insulating film,
wherein the oxide insulating film is in contact with the oxide
semiconductor film, wherein the first metal oxide film has a film
density of higher than or equal to 3.2 g/cm.sup.3, wherein the
source electrode overlaps with a first end of the oxide
semiconductor film, and wherein the drain electrode overlaps with a
second end of the oxide semiconductor film.
9. The semiconductor device according to claim 8, wherein the first
metal oxide film comprises aluminum oxide.
10. The semiconductor device according to claim 8, wherein the
first metal oxide film comprises Ga--Zn-based oxide.
11. The semiconductor device according to claim 8, further
comprising a conductive film in contact with the first metal oxide
film of the protective film.
12. The semiconductor device according to claim 11, wherein the
conductive film includes at least one of zinc oxide, indium tin
oxide, titanium oxide, aluminum, and titanium.
13. The semiconductor device according to claim 8, wherein the
oxide semiconductor film includes at least one oxide of indium,
zinc, gallium, zirconium, tin, gadolinium, titanium, and
cerium.
14. The semiconductor device according to claim 8, further
comprising a second metal oxide film over and in contact with the
gate insulating film.
15. The semiconductor device according to claim 8, further
comprising a base insulating film under and in contact with the
gate electrode, wherein the base insulating film includes a third
metal oxide film in contact with the gate electrode, and wherein a
film density of the third metal oxide film is higher than or equal
to 3.2 g/cm.sup.3.
16. The semiconductor device according to claim 8, wherein the
oxide insulating film comprises a region in which the oxygen
content is higher than the stoichiometric proportion.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a semiconductor device and
a manufacturing method thereof.
[0003] In this specification, a semiconductor device generally
means any device which can function by utilizing semiconductor
characteristics, and an electro-optic device, a semiconductor
circuit, and an electronic appliance are all included in the
category of the semiconductor device.
[0004] 2. Description of the Related Art
[0005] A technique by which transistors are formed using
semiconductor thin films formed over a substrate having an
insulating surface has attracted attention. The transistor is
applied to a wide range of electronic devices such as an integrated
circuit (IC) and an image display device (display device). A
silicon-based semiconductor material is widely known as a material
for a semiconductor thin film applicable to the transistor; in
addition, an oxide semiconductor has attracted attention as another
material.
[0006] For example, a transistor whose semiconductor thin film
includes an amorphous oxide containing indium (In), gallium (Ga),
and zinc (Zn) and having an electron carrier concentration of less
than 10.sup.18/cm.sup.3 is disclosed (for example, see Patent
Document 1).
REFERENCE
Patent Document
[0007] [Patent Document 1] Japanese Published Patent Application
No. 2006-165528
SUMMARY OF THE INVENTION
[0008] However, when water or hydrogen, which forms an electron
donor, enters an oxide semiconductor or oxygen is released from an
oxide semiconductor film in a manufacturing process of a device,
the electrical conductivity of the oxide semiconductor may change.
Such a phenomenon causes variation in the electric characteristics
of a transistor including the oxide semiconductor.
[0009] In view of the above problems, an object is to provide a
semiconductor device including an oxide semiconductor, which has
stable electric characteristics and high reliability. Another
object is to provide a method for manufacturing the semiconductor
device.
[0010] In a semiconductor device including an oxide semiconductor
film, a metal oxide film is used for a protective film in contact
with the oxide semiconductor film. The metal oxide film is formed
using a material including an element of the same group as that of
an element other than oxygen included in the oxide semiconductor
film. The metal oxide film can suppress entry and diffusion of
water or hydrogen into the oxide semiconductor film. In addition,
release of oxygen from the oxide semiconductor film can be
suppressed. Further, with a structure in which the metal oxide film
is in contact with the oxide semiconductor film, the interface
between the metal oxide film and the oxide semiconductor film has
extremely stable interface characteristics because the metal oxide
film and the oxide semiconductor film include metal elements of the
same group.
[0011] In addition, when a bottom-gate transistor is formed in the
semiconductor device including the oxide semiconductor film, the
film under the oxide semiconductor film is preferably a metal oxide
film. Since the metal oxide film is provided in contact with the
oxide semiconductor film, the interface characteristics are
extremely stable, and a more excellent semiconductor device can be
provided.
[0012] According to one embodiment of the present invention, a
semiconductor device includes a gate electrode, a gate insulating
film formed over the gate electrode, an oxide semiconductor film
formed over the gate insulating film, a source electrode and a
drain electrode formed over the oxide semiconductor film, a
protective film formed over the oxide semiconductor film, the
source electrode, and the drain electrode. The protective film is a
stack in which a metal oxide film is formed over an oxide
insulating film, and the metal oxide film has a film density of
higher than or equal to 3.2 g/cm.sup.3.
[0013] According to another embodiment of the present invention, a
semiconductor device includes a gate electrode, a gate insulating
film formed over the gate electrode, an oxide semiconductor film
formed over the gate insulating film, a source electrode and a
drain electrode formed over the oxide semiconductor film, and a
protective film formed over the oxide semiconductor film, the
source electrode, and the drain electrode. The protective film is a
stack in which a metal oxide film is formed over an oxide
insulating film. The metal oxide film is a film including aluminum
oxide and has a film density of higher than or equal to 3.2
g/cm.sup.3.
[0014] In the above structure, it is preferable that a conductive
film be formed in contact with the metal oxide film of the
protective film.
[0015] In the above structure, the conductive film preferably
includes at least one of zinc oxide, indium tin oxide, titanium
oxide, aluminum, and titanium.
[0016] In the above structure, the oxide semiconductor film
preferably includes at least one of oxides of indium, zinc,
gallium, zirconium, tin, gadolinium, titanium, and cerium.
[0017] In the above structure, it is preferable that the
semiconductor device include a base insulating film under and in
contact with the gate electrode, the base insulating film include a
metal oxide film on a surface in contact with the gate electrode,
and the film density of the metal oxide film be higher than or
equal to 3.2 g/cm.sup.3.
[0018] According to another embodiment of the present invention, a
method for manufacturing a semiconductor device includes the
following steps: forming a gate electrode; forming a gate
insulating film over the gate electrode; performing heat treatment
after the gate insulating film is formed; forming an oxide
semiconductor film over the gate insulating film; forming a source
electrode and a drain electrode over the oxide semiconductor film;
and forming a protective film after the source electrode and the
drain electrode are formed. The protective film is a stack in which
a metal oxide film is formed over an oxide insulating film, and the
metal oxide film has a film density of higher than or equal to 3.2
g/cm.sup.3.
[0019] According to another embodiment of the present invention, a
method for manufacturing a semiconductor device includes the
following steps: forming a gate electrode; forming a gate
insulating film over the gate electrode; performing heat treatment
after the gate insulating film is formed; forming an oxide
semiconductor film over the gate insulating film; forming a source
electrode and a drain electrode over the oxide semiconductor film;
and forming a protective film after the source electrode and the
drain electrode are formed. The protective film is a stack in which
a metal oxide film is formed over an oxide insulating film. The
metal oxide film is a film including aluminum oxide or a
Ga--Zn-based oxide film and has a film density of higher than or
equal to 3.2 g/cm.sup.3.
[0020] In the above method, a conductive film is preferably formed
in contact with the metal oxide film of the protective film. The
metal oxide film is preferably an aluminum oxide film or a
Ga--Zn-based oxide film.
[0021] In the above method, it is preferable that a metal oxide
film be formed over and in contact with the gate insulating film,
and the metal oxide film have a film density of higher than or
equal to 3.2 g/cm.sup.3.
[0022] A semiconductor device including an oxide semiconductor,
which has stable electric characteristics and high reliability, can
be provided. Further, a method for manufacturing the semiconductor
device can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] In the accompanying drawings:
[0024] FIG. 1 illustrates a cross section of one embodiment of a
semiconductor device;
[0025] FIGS. 2A to 2C illustrate one embodiment of a method for
manufacturing a semiconductor device;
[0026] FIGS. 3A and 3B illustrate one embodiment of a method for
manufacturing a semiconductor device;
[0027] FIG. 4 illustrates a cross section of one embodiment of a
semiconductor device;
[0028] FIGS. 5A to 5C each illustrate a plane of one embodiment of
a semiconductor device;
[0029] FIG. 6 illustrates a cross section of one embodiment of a
semiconductor device;
[0030] FIG. 7 illustrates a cross section of one embodiment of a
semiconductor device;
[0031] FIGS. 8A to 8F each illustrate an electronic appliance;
[0032] FIGS. 9A and 9B each illustrate an example of a metal oxide
film of Example;
[0033] FIG. 10 shows measurement results of film density of an
aluminum oxide film;
[0034] FIGS. 11A and 11B each illustrate an example of a metal
oxide film of Example;
[0035] FIGS. 12A and 12B show measurement results of SIMS
analysis;
[0036] FIGS. 13A and 13B each illustrate an example of a metal
oxide film of Example; and
[0037] FIGS. 14A and 14B show measurement results of TDS
analysis.
DETAILED DESCRIPTION OF THE INVENTION
[0038] Hereinafter, embodiments of the present invention will be
described in detail with reference to the accompanying drawings.
However, the present invention is not limited to the description
below, and it is easily understood by those skilled in the art that
modes and details disclosed herein can be modified in various ways.
Therefore, the present invention is not construed as being limited
to description of the embodiments.
[0039] Further, in the embodiments described below, the same parts
are denoted with the same reference numerals throughout the
drawings. Note that components illustrated in the drawings, that
is, a thickness or a width of a layer, a region, or the like, a
relative position, and the like are exaggerated in some cases for
clarification in description of the embodiments.
[0040] In this specification and the like, ordinal numbers such as
"first", "second", and "third" are used in order to avoid confusion
among components, and the terms do not limit the components
numerically.
[0041] In addition, in this specification and the like, the term
such as "electrode" or "wiring" does not limit a function of a
component. For example, an "electrode" is sometimes used as part of
a "wiring", and vice versa. Further, the term "electrode" or
"wiring" can also mean a combination of a plurality of "electrodes"
and "wirings", for example.
[0042] Functions of a "source" and a "drain" are sometimes replaced
with each other when a transistor of opposite polarity is used or
when the direction of current flowing is changed in circuit
operation, for example. Thus, the terms "source" and "drain" can be
replaced with each other in this specification and the like.
[0043] In this specification and the like, average surface
roughness (R.sub.a) is obtained by three-dimensional expansion of
arithmetic mean surface roughness that is defined by JIS B
0601:2001 (ISO4287:1997) so as to be applied to a curved surface,
and is an average value of the absolute values of deviations from a
reference surface to a specific surface.
[0044] When the specific surface is expressed as Z.sub.0=f(x, y),
the average surface roughness (R.sub.a) is an average value of the
absolute values of deviations from the reference surface to the
specific surface and is shown by the following formula (1).
R a = 1 S 0 .intg. y 1 y 2 .intg. x 1 x 2 f ( x , y ) - Z 0 x y ( 1
) ##EQU00001##
[0045] Here, the specific surface is a surface which is a target of
roughness measurement, and is a quadrilateral region which is
specified by four points represented by the coordinates (x.sub.1,
y.sub.1, f(x.sub.1, y.sub.1)), (x.sub.1, y.sub.2, f(x.sub.1,
y.sub.2)), (x.sub.2, y.sub.1, f(x.sub.2, y.sub.1)), and (x.sub.2,
y.sub.2, f(x.sub.2, y.sub.2)). S.sub.0 represents the area of a
rectangle which is obtained by projecting the specific surface on
the xy plane, and Z.sub.0 represents the height of the reference
surface (the average height of the specific surface). The average
surface roughness (R.sub.a) can be measured using an atomic force
microscope (AFM).
Embodiment 1
[0046] In this embodiment, a semiconductor device including a
transistor according to one embodiment of the present invention and
a method for manufacturing the semiconductor device will be
described with reference to FIG. 1, FIGS. 2A to 2C, and FIGS. 3A
and 3B.
<Structure of Semiconductor Device of Embodiment 1>
[0047] FIG. 1 is a cross-sectional view of a semiconductor device
including an oxide semiconductor film. A transistor 150 in FIG. 1
includes a gate electrode 106 formed over a substrate 102 having an
insulating surface over which a base insulating film 104 is
provided, a gate insulating film 108 formed over the base
insulating film 104 and the gate electrode 106, an oxide
semiconductor film 110 formed over the gate insulating film 108, a
source electrode 112a and a drain electrode 112b formed over the
gate insulating film 108 and the oxide semiconductor film 110, and
a protective film 114 formed over the oxide semiconductor film 110,
the source electrode 112a, and the drain electrode 112b. The
protective film 114 is a stack of an oxide insulating film 114a and
a metal oxide film 114b.
[0048] The metal oxide film 114b is formed using a material which
contains an element of Group 12 or Group 13, or an element of Group
3 having a property similar to that of the element of Group 13,
which is the same group as one of the elements included in the
oxide semiconductor film 110. For example, in the case where the
oxide semiconductor film 110 is formed of an oxide semiconductor
material including oxides of indium (In) and zinc (Zn), the metal
oxide film 114b is preferably an insulating metal oxide film
containing an element belonging to the same group as zinc, namely,
Group 12, an element belonging to the same group as indium, namely,
Group 13, or an element of Group 3 having a property similar to
that of the element of Group 13. An oxide film including a
lanthanoid-based element, as the element of Group 3, such as cerium
(Ce) or gadolinium (Gd) may be used. As the metal oxide film 114b,
an aluminum oxide film, a gallium oxide film, a zinc oxide film, or
a Ga--Zn-based oxide film can be selected as a favorable
example.
[0049] Further, it is preferable that an aluminum oxide film with a
film density of higher than or equal to 3.2 g/cm.sup.3, preferably
higher than or equal to 3.6 g/cm.sup.3 be used as the metal oxide
film 114b. When the aluminum oxide film is used as the metal oxide
film 114b and the film density thereof is within the above range,
water or hydrogen can be prevented from entering and diffusing into
the oxide semiconductor film. In addition, release of oxygen from
the oxide semiconductor film can be suppressed.
<Manufacturing Method of Semiconductor Device of Embodiment
1>
[0050] A method for manufacturing the transistor 150 will be
described with reference to FIGS. 2A to 2C and FIGS. 3A and 3B.
[0051] First, the base insulating film 104 is formed over the
substrate 102.
[0052] For the substrate 102, a glass material such as
aluminosilicate glass, aluminoborosilicate glass, or barium
borosilicate glass is used. In mass production, a mother glass with
the following size is preferably used for the substrate 102: the
eighth generation (2160 mm.times.2460 mm); the ninth generation
(2400 mm.times.2800 mm, or 2450 mm.times.3050 mm); the tenth
generation (2950 mm.times.3400 mm); or the like. High process
temperature and a long period of process time drastically shrink
the mother glass. Thus, in the case where mass production is
performed with the use of the mother glass, the preferable heating
temperature in the manufacturing process is lower than or equal to
700.degree. C., preferably lower than or equal to 450.degree. C.,
more preferably lower than or equal to 350.degree. C.
[0053] The base insulating film 104 is formed by a plasma CVD
method or a sputtering method to have a thickness greater than or
equal to 50 nm and less than or equal to 600 nm with the use of one
of a silicon oxide film, a gallium oxide film, an aluminum oxide
film, a silicon nitride film, a silicon oxynitride film, an
aluminum oxynitride film, and a silicon nitride oxide film or a
stack of any of these films. The base insulating film 104 can
prevent entry of impurities from the substrate 102 side. In the
case where the base insulating film 104 is unnecessary, e.g., in
the case where the amount of moisture adsorbed on a surface of the
substrate 102 and the amount of moisture included in the substrate
102 are small, the base insulating film 104 is not necessarily
provided.
[0054] Further, it is preferable to provide a metal oxide film in
contact with the gate electrode 106 to be formed later. In
particular, it is preferable to use an aluminum oxide film with a
film density of higher than or equal to 3.2 g/cm.sup.3, preferably
higher than or equal to 3.6 g/cm.sup.3. The aluminum oxide film has
a thickness greater than or equal to 30 nm and less than or equal
to 150 nm, preferably greater than or equal to 50 nm and less than
or equal to 100 nm. When the film density of the aluminum oxide
film is within the above range, water or hydrogen can be prevented
from entering and diffusing into the oxide semiconductor film. In
addition, release of oxygen from the oxide semiconductor film can
be suppressed.
[0055] Note that in this specification, "oxynitride" such as
silicon oxynitride contains more oxygen than nitrogen.
[0056] Further, in this specification, "nitride oxide" such as
silicon nitride oxide contains more nitrogen than oxygen.
[0057] Next, after a conductive film is formed over the base
insulating film 104, the gate electrode 106 is formed through a
photolithography step and an etching step (see FIG. 2A). The gate
electrode 106 can be formed by a sputtering method or the like as a
single layer or a stacked layer using a metal material such as
molybdenum, titanium, tantalum, tungsten, aluminum, copper,
neodymium, or scandium, or an alloy material containing any of
these materials.
[0058] Next, the gate insulating film 108 is formed over the base
insulating film 104 and the gate electrode 106 (see FIG. 2B). In
this embodiment, a silicon oxynitride film can be used as the gate
insulating film 108.
[0059] The silicon oxynitride film can be formed by a plasma CVD
apparatus in vacuum, following the formation of the gate
electrode.
[0060] The silicon oxynitride film can be formed using a gas such
as SiH.sub.4, N.sub.2O, NH.sub.3, or N.sub.2.
[0061] The gate insulating film 108 preferably includes a region in
which the oxygen content is higher than the stoichiometric
proportion. In that case, the oxygen content is higher than the
stoichiometric proportion of the gate insulating film 108. For
example, in the case of using a silicon oxide film whose
composition is expressed by SiO.sub.x (x>0), the stoichiometric
proportion of silicon oxide is Si:O=1:2; therefore, a silicon oxide
film including an oxygen-excess region, in which x is greater than
2, is preferably used. Such an oxygen-excess region exists in part
(including an interface) of the silicon oxide film.
[0062] When the gate insulating film 108 in contact with the oxide
semiconductor film 110 to be formed later includes a region in
which the oxygen content is higher than the stoichiometric
proportion, transfer of oxygen from the oxide semiconductor film
110 to the gate insulating film 108 in contact therewith can be
suppressed and oxygen can be supplied from the gate insulating film
108 in contact with the oxide semiconductor film 110 to the oxide
semiconductor film 110.
[0063] Next, heat treatment is performed on the substrate 102 over
which the silicon oxynitride film is formed, in order to remove
moisture, hydrogen, or the like.
[0064] For the heat treatment, an electric furnace or a device for
heating an object by heat conduction or heat radiation from or a
heating element such as a resistance heating element can be used.
For example, a rapid thermal annealing (RTA) apparatus such as a
lamp rapid thermal annealing (LRTA) apparatus or a gas rapid
thermal annealing (GRTA) apparatus can be used. An LRTA apparatus
is an apparatus for heating an object to be processed by radiation
of light (an electromagnetic wave) emitted from a lamp such as a
halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc
lamp, a high pressure sodium lamp, or a high pressure mercury lamp.
A GRTA apparatus is an apparatus for heat treatment using a
high-temperature gas. As the high-temperature gas, an inert gas
which does not react with an object to be processed by heat
treatment, such as nitrogen or a rare gas like argon, is used.
[0065] For example, as the heat treatment, GRTA treatment may be
performed as follows. The object to be processed is put in a heated
inert gas atmosphere, heated for several minutes, and taken out of
the inert gas atmosphere. The GRTA treatment enables
high-temperature heat treatment for a short time. Moreover, the
GRTA treatment can be employed even when the temperature exceeds
the upper temperature limit of the object to be processed. Note
that the inert gas may be changed during the treatment to a gas
including oxygen. The heat treatment is performed in an atmosphere
including oxygen, whereby the defect density in the film can be
decreased.
[0066] Note that as the inert gas atmosphere, an atmosphere that
contains nitrogen or a rare gas (e.g., helium, neon, or argon) as
its main component and does not include moisture, hydrogen, or the
like is preferably used. For example, the purity of nitrogen or a
rare gas such as helium, neon, or argon introduced into the heat
treatment apparatus is higher than or equal to 6N (99.9999%),
preferably higher than or equal to 7N (99.99999%) (that is, the
impurity concentration is lower than or equal to 1 ppm, preferably
lower than or equal to 0.1 ppm).
[0067] In the case where the mother glass is used as the substrate
102, high process temperature and a long period of process time
drastically shrink the mother glass; therefore, the temperature of
the heat treatment is higher than or equal to 200.degree. C. and
lower than or equal to 450.degree. C., preferably higher than or
equal to 250.degree. C. and lower than or equal to 350.degree.
C.
[0068] Impurities such as water or hydrogen in the silicon
oxynitride film can be removed by the heat treatment. In addition,
the defect density in the film can be decreased by the heat
treatment. Impurities in the silicon oxynitride film are removed or
the defect density in the film is decreased, so that the
reliability of the semiconductor device can be improved. For
example, deterioration of the semiconductor device due to a
negative bias stress test with light irradiation, which is one of
reliability tests for semiconductor devices, can be suppressed.
[0069] The above heat treatment can be referred to as dehydration
treatment, dehydrogenation treatment, or the like because of its
advantageous effect of removing moisture, hydrogen, or the like.
Such dehydration treatment or dehydrogenation treatment may be
performed once or plural times.
[0070] Next, the oxide semiconductor film 110 is formed (see FIG.
2C).
[0071] As an oxide semiconductor for forming the oxide
semiconductor film 110, an intrinsic (i-type) or substantially
intrinsic (i-type) oxide semiconductor obtained by removing
impurities to highly purify the oxide semiconductor so that
impurities which are carrier donors besides main components do not
exist in the oxide semiconductor as much as possible, is used.
[0072] The oxide semiconductor film 110 may be either single
crystal or non-single-crystal. In the latter case, the oxide
semiconductor film may be either amorphous or polycrystal. Further,
the oxide semiconductor film may have either an amorphous structure
including a portion having crystallinity or a non-amorphous
structure.
[0073] It is relatively easy to make a surface of an amorphous
oxide semiconductor film flat. Thus, when a transistor is
manufactured with the use of the oxide semiconductor film,
interface scattering can be reduced, and relatively high mobility
can be obtained relatively easily.
[0074] In an oxide semiconductor film having crystallinity (a
crystalline oxide semiconductor film), defects in the bulk can be
further reduced, and mobility higher than that of an amorphous
oxide semiconductor film can be obtained when a surface flatness is
improved. In order to improve the surface flatness, the oxide
semiconductor film is preferably formed on a flat surface.
Specifically, the oxide semiconductor film is preferably formed on
a surface with an average surface roughness (R.sub.a) of less than
or equal to 1 nm, preferably less than or equal to 0.3 nm, more
preferably less than or equal to 0.1 nm. Note that the average
surface roughness (R.sub.a) is preferably close to 0.
[0075] The crystals in the crystalline oxide semiconductor film may
have crystal axes oriented in random directions or in a certain
direction.
[0076] As the oxide semiconductor film, a c-axis aligned
crystalline oxide semiconductor (CAAC-OS) film can be used.
[0077] The CAAC-OS film is not completely single crystal nor
completely amorphous. The CAAC-OS film is an oxide semiconductor
film with a crystal-amorphous mixed phase structure where crystal
parts and amorphous parts are included in an amorphous phase. Note
that in most cases, the crystal part fits inside a cube whose one
side is less than 100 nm. From an observation image obtained with a
transmission electron microscope (TEM), a boundary between an
amorphous part and a crystal part in the CAAC-OS film is not clear.
Further, with the TEM, a grain boundary in the CAAC-OS film is not
found. Thus, in the CAAC-OS film, a reduction in electron mobility,
due to the grain boundary, is suppressed.
[0078] In each of the crystal parts included in the CAAC-OS film, a
c-axis is aligned in a direction parallel to a normal vector of a
surface where the CAAC-OS film is formed or a normal vector of a
surface of the CAAC-OS film, triangular or hexagonal atomic
arrangement which is seen from the direction perpendicular to the
a-b plane is formed, and layers including metal atoms and oxygen
atoms are arranged to have a layered structure (Note that normal
vectors of the layers are the c-axis direction.). Note that, among
crystal parts, the directions of the a-axis and the b-axis of one
crystal part may be different from those of another crystal part.
In this specification, a simple term "perpendicular" includes a
range from 85.degree. to 95.degree.. In addition, a simple term
"parallel" includes a range from -5.degree. to 5.degree..
[0079] In the CAAC-OS film, distribution of crystal parts is not
necessarily uniform. For example, in the formation process of the
CAAC-OS film, in the case where crystal growth occurs from a
surface side of the oxide semiconductor film, the proportion of
crystal parts in the vicinity of the surface of the oxide
semiconductor film is higher than that in the vicinity of the
surface where the oxide semiconductor film is formed in some cases.
Further, when an impurity is added to the CAAC-OS film, the crystal
part in a region to which the impurity is added becomes amorphous
in some cases.
[0080] Since the c-axes of the crystal parts included in the
CAAC-OS film are aligned in the direction parallel to a normal
vector of a surface where the CAAC-OS film is formed or a normal
vector of a surface of the CAAC-OS film, the directions of the
c-axes may be different from each other depending on the shape of
the CAAC-OS film (the cross-sectional shape of the surface where
the CAAC-OS film is faulted or the cross-sectional shape of the
surface of the CAAC-OS film) Note that when the CAAC-OS film is
formed, the direction of c-axis of the crystal part is the
direction parallel to a normal vector of the surface where the
CAAC-OS film is formed or a normal vector of the surface of the
CAAC-OS film. The crystal part is formed by deposition or by
performing treatment for crystallization such as heat treatment
after deposition.
[0081] With the use of the CAAC-OS film in a transistor, change in
electric characteristics of the transistor due to irradiation with
visible light or ultraviolet light is small. Thus, the transistor
has high reliability.
[0082] For example, the CAAC-OS film is formed by a sputtering
method with a polycrystalline oxide semiconductor sputtering
target. When ions collide with the sputtering target, a crystal
region included in the sputtering target may be separated from the
target along an a-b plane; in other words, a sputtered particle
having a plane parallel to an a-b plane (flat-plate-like sputtered
particle or pellet-like sputtered particle) may flake off from the
sputtering target. In that case, the flat-plate-like sputtered
particle reaches a substrate while maintaining their crystal state,
whereby the CAAC-OS film can be formed.
[0083] For the deposition of the CAAC-OS film, the following
conditions are preferably used.
[0084] By reducing the amount of impurities entering the CAAC-OS
film during the deposition, the crystal state can be prevented from
being broken by the impurities. For example, the concentration of
impurities (e.g., hydrogen, water, carbon dioxide, or nitrogen)
which exist in the deposition chamber may be reduced. Furthermore,
the concentration of impurities in a deposition gas may be reduced.
Specifically, a deposition gas whose dew point is -80.degree. C. or
lower, preferably -100.degree. C. or lower is used.
[0085] By increasing the substrate heating temperature during the
deposition, migration of a sputtered particle is likely to occur
after the sputtered particle reaches a substrate surface.
Specifically, the substrate heating temperature during the
deposition is higher than or equal to 100.degree. C. and lower than
or equal to 740.degree. C., preferably higher than or equal to
200.degree. C. and lower than or equal to 500.degree. C. By
increasing the substrate heating temperature during the deposition,
when the flat-plate-like sputtered particle reaches the substrate,
migration occurs on the substrate surface, so that a flat plane of
the flat-plate-like sputtered particle is attached to the
substrate.
[0086] Furthermore, it is preferable that the proportion of oxygen
in the deposition gas be increased and the power be optimized in
order to reduce plasma damage at the deposition. The proportion of
oxygen in the deposition gas is 30 vol. % or higher, preferably 100
vol. %.
[0087] As an example of the sputtering target, an In--Ga--Zn-based
oxide target is described below.
[0088] The In--Ga--Zn-based oxide target, which is polycrystalline,
is made by mixing InO.sub.X powder, GaO.sub.Y powder, and ZnO.sub.Z
powder in a predetermined molar ratio, applying pressure, and
performing heat treatment at a temperature higher than or equal to
1000.degree. C. and lower than or equal to 1500.degree. C. Note
that X, Y, and Z are given positive numbers. Here, the
predetermined molar ratio of InO.sub.X powder to GaO.sub.Y powder
and ZnO.sub.Z powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1,
4:2:3, or 3:1:2. The kinds of powder and the molar ratio for mixing
powder may be determined as appropriate depending on the desired
sputtering target.
[0089] The oxide semiconductor film 110 can have a thickness
greater than or equal to 1 nm and less than or equal to 200 nm,
preferably greater than or equal to 15 nm and less than or equal to
30 nm and can be formed by a sputtering method, a molecular beam
epitaxy (MBE) method, a pulsed laser deposition method, an atomic
layer deposition (ALD) method, or the like as appropriate. The
oxide semiconductor film 110 may be formed using a sputtering
apparatus which performs deposition with surfaces of a plurality of
substrates set substantially perpendicular to a surface of a
sputtering target.
[0090] It is preferable that impurities such as hydrogen, water, a
hydroxyl group, or hydride in a deposition chamber be removed by
heating and evacuation of the deposition chamber before deposition
of the oxide semiconductor. It is particularly important to remove
such impurities adsorbed on an inner wall of the deposition
chamber. Here, the heat treatment may be performed at a temperature
higher than or equal to 100.degree. C. and lower than or equal to
450.degree. C., for example. Evacuation of the deposition chamber
is preferably performed with a rough vacuum pump such as a dry
pump, and a high vacuum pump such as a sputter ion pump, a turbo
molecular pump, or a cryopump, in appropriate combination. The
turbo molecular pump has an outstanding capability in evacuating a
large-sized molecule, whereas it has a low capability in evacuating
hydrogen or moisture. Further, combination with a cryopump having a
high capability in evacuating moisture or a sputter ion pump having
a high capability in evacuating hydrogen is effective. At this
time, when the impurities are removed while an inert gas is
introduced, the rate of desorption of moisture or the like, which
is difficult to desorb only by evacuation, can be further
increased. Removal of impurities in the deposition chamber by such
treatment before the deposition of the oxide semiconductor can
prevent hydrogen, water, a hydroxyl group, hydride, or the like
from entering the oxide semiconductor film 110.
[0091] An oxide semiconductor to be used preferably contains at
least indium (In) or zinc (Zn). In particular, In and Zn are
preferably contained. As a stabilizer for reducing variation in
electric characteristics of a transistor including the oxide
semiconductor, gallium (Ga) is preferably additionally contained.
Tin (Sn) is preferably contained as a stabilizer. Hafnium (Hf) is
preferably contained as a stabilizer. Aluminum (Al) is preferably
contained as a stabilizer. Titanium (Ti) is preferably contained as
a stabilizer. Zirconium (Zr) is preferably contained as a
stabilizer.
[0092] As another stabilizer, one or plural kinds of lanthanoid
such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium
(Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb),
dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium
(Yb), or lutetium (Lu) may be contained.
[0093] As the oxide semiconductor, for example, a single-component
metal oxide such as indium oxide, tin oxide, or zinc oxide, a
two-component metal oxide such as an In--Zn-based oxide, a
Sn--Zn-based oxide, an Al--Zn-based oxide, a Zn--Mg-based oxide, a
Sn--Mg-based oxide, an In--Mg-based oxide, or an In--Ga-based
oxide, a three-component metal oxide such as an In--Ga--Zn-based
oxide (also referred to as IGZO), an In--Al--Zn-based oxide, an
In--Sn--Zn-based oxide, a Sn--Ga--Zn-based oxide, an
Al--Ga--Zn-based oxide, a Sn--Al--Zn-based oxide, an
In--Hf--Zn-based oxide, an In--La--Zn-based oxide, an
In--Ce--Zn-based oxide, an In--Pr--Zn-based oxide, an
In--Nd--Zn-based oxide, an In--Sm--Zn-based oxide, an
In--Eu--Zn-based oxide, an In--Gd--Zn-based oxide, an
In--Tb--Zn-based oxide, an In--Dy--Zn-based oxide, an
In--Ho--Zn-based oxide, an In--Er--Zn-based oxide, an
In--Tm--Zn-based oxide, an In--Yb--Zn-based oxide, or an
In--Lu--Zn-based oxide, or a four-component metal oxide such as an
In--Sn--Ga--Zn-based oxide, an In--Hf--Ga--Zn-based oxide, an
In--Al--Ga--Zn-based oxide, an In--Sn--Al--Zn-based oxide, an
In--Sn--Hf--Zn-based oxide, or an In--Hf--Al--Zn-based oxide can be
used.
[0094] Note that here, for example, an In--Ga--Zn-based oxide means
an oxide containing In, Ga, and Zn as its main component, and there
is no limitation on the ratio of In:Ga:Zn. Further, the
In--Ga--Zn-based oxide may contain a metal element other than In,
Ga, and Zn.
[0095] Alternatively, a material represented by
InMO.sub.3(ZnO).sub.m (m>0, m is not an integer) may be used as
the oxide semiconductor. Note that M represents one or more metal
elements selected from Ga, Fe, Mn, and Co. Alternatively, as the
oxide semiconductor, a material represented by
In.sub.2SnO.sub.5(ZnO).sub.n (n>0, n is an integer) may be
used.
[0096] For example, an In--Ga--Zn-based oxide with an atomic ratio
of In:Ga:Zn=1:1:1 or In:Ga:Zn=2:2:1, or any of oxides whose
composition is in the neighborhood of the above can be used.
Alternatively, an In--Sn--Zn-based oxide with an atomic ratio of
In:Sn:Zn=1:1:1, In:Sn:Zn=2:1:3, or In:Sn:Zn=2:1:5, or any of oxides
whose composition is in the neighborhood of the above may be
used.
[0097] However, the composition is not limited to those described
above, and a material having an appropriate composition may be used
depending on needed semiconductor characteristics (such as
mobility, threshold voltage, and variation). In order to obtain
needed semiconductor characteristics, it is preferable that the
carrier concentration, the impurity concentration, the defect
density, the atomic ratio of a metal element to oxygen, the
interatomic distance, the density, and the like be set to
appropriate values.
[0098] For example, it is relatively easy to obtain high mobility
with an In--Sn--Zn-based oxide. However, mobility can be increased
by reducing the defect density in a bulk also in the case of using
an In--Ga--Zn-based oxide.
[0099] Note that for example, the expression "the composition of an
oxide containing In, Ga, and Zn at an atomic ratio, In:Ga:Zn=a:b:c
(a+b+c=1), is in the neighborhood of the composition of an oxide
containing In, Ga, and Zn at an atomic ratio, In:Ga:Zn=A:B:C
(A+B+C=1)" means that a, b, and c satisfy the following relation:
(a-A).sup.2+(b-B).sup.2+(c-C).sup.2.ltoreq.r.sup.2, and r may be
0.05, for example. The same applies to other oxides.
[0100] In the case where an In--Ga--Zn-based oxide is used for the
oxide semiconductor, a target having a composition of
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:x:y [molar ratio] (x is
greater than or equal to 0, y is greater than or equal to 0.5 and
less than or equal to 5) is preferably used. For example, a target
having a composition of In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:2
[molar ratio], or the like can be used. It is also possible to use
a target with a composition of
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:1 [molar ratio] or a target
with a composition of In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:4
[molar ratio].
[0101] In the case where an In--Sn--Zn-based oxide is used for the
oxide semiconductor, an atomic ratio of metal elements in a target
to be used may be In:Sn:Zn=1:2:2, In:Sn:Zn=2:1:3, In:Sn:Zn=1:1:1,
In:Sn:Zn=20:45:35, or the like.
[0102] In the case where an In--Zn-based oxide is used for the
oxide semiconductor, an atomic ratio of metal elements in a target
to be used is In:Zn=50:1 to 1:2 (In.sub.2O.sub.3:ZnO=25:1 to 1:4 in
a molar ratio), preferably In:Zn=20:1 to 1:1
(In.sub.2O.sub.3:ZnO=10:1 to 1:2 in a molar ratio), further
preferably In:Zn=15:1 to 1.5:1 (In.sub.2O.sub.3:ZnO=15:2 to 3:4 in
a molar ratio). For example, in a target used for formation of an
In--Zn-based oxide which has an atomic ratio of In:Zn:O=X:Y:Z, the
relation of Z>1.5X+Y is satisfied.
[0103] Note that it is preferable that the oxide semiconductor film
110 be formed under a condition that much oxygen is contained
during deposition (e.g., deposited by a sputtering method in an
atmosphere where the proportion of oxygen is 100%), so that a film
containing much oxygen (preferably including a region where the
oxygen content is higher than the stoichiometric composition of the
oxide semiconductor in a crystalline state) is formed.
[0104] The deposition atmosphere may be a rare gas (typically
argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere
containing a rare gas and oxygen. Here, when the volume of oxygen
is larger than the volume of a rare gas at the time of the
deposition, supply of oxygen into the oxide semiconductor film 110
can be facilitated and oxygen vacancies in the oxide semiconductor
film 110 can be reduced. In order to prevent hydrogen, water, a
hydroxyl group, hydride, or the like from entering the oxide
semiconductor film 110, an atmosphere of a high-purity gas from
which impurities such as hydrogen, water, a hydroxyl group, or
hydride are sufficiently removed is preferable.
[0105] After the deposition of the oxide semiconductor film 110,
heat treatment may be performed on the oxide semiconductor film 110
in order to remove excess hydrogen (including moisture and a
hydroxyl group) (to perform dehydration or dehydrogenation). The
heat treatment can further remove hydrogen atoms or substances
including hydrogen atoms in the oxide semiconductor film 110. The
heat treatment is performed in an inert gas atmosphere at higher
than or equal to 250.degree. C. and lower than or equal to
700.degree. C., preferably higher than or equal to 450.degree. C.
and lower than or equal to 600.degree. C., and lower than the
strain point of the substrate. The inert gas atmosphere is
preferably an atmosphere which contains nitrogen or a rare gas
(e.g., helium, neon, or argon) as its main component and does not
contain moisture, hydrogen, or the like. For example, the purity of
nitrogen or a rare gas such as helium, neon, or argon introduced
into a heat treatment apparatus is higher than or equal to 6N
(99.9999%), preferably higher than or equal to 7N (99.99999%) (that
is, the concentration of the impurities is lower than or equal to 1
ppm, preferably lower than or equal to 0.1 ppm).
[0106] For example, after the substrate is introduced into an
electric furnace including a resistance heating element or the
like, the heat treatment can be performed at 450.degree. C. in a
nitrogen atmosphere for one hour.
[0107] The heat treatment apparatus is not limited to the electric
furnace and may be an apparatus for heating an object to be
processed by thermal conduction or thermal radiation from a medium
such as a heated gas. For example, a rapid thermal annealing (RTA)
apparatus such as a lamp rapid thermal annealing (LRTA) apparatus
or a gas rapid thermal annealing (GRTA) apparatus can be used. An
LRTA apparatus is an apparatus for heating an object to be
processed by radiation of light (an electromagnetic wave) emitted
from a lamp such as a halogen lamp, a metal halide lamp, a xenon
arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high
pressure mercury lamp. A GRTA apparatus is an apparatus for
performing heat treatment using a high-temperature gas. As the gas,
an inert gas which does not react with an object to be processed by
heat treatment, such as nitrogen or a rare gas like argon, is used.
Note that in the case where a GRTA apparatus is used as the heat
treatment apparatus, the substrate may be heated in an inert gas
heated to a high temperature of 650.degree. C. to 700.degree. C.
because the heat treatment time is short.
[0108] The above heat treatment has an advantageous effect of
removing moisture, hydrogen, or the like and can be referred to as
dehydration treatment, dehydrogenation treatment, or the like. The
heat treatment can be performed at the timing, for example, after
the oxide semiconductor film is processed into an island shape.
Such dehydration treatment or dehydrogenation treatment may be
performed once or plural times.
[0109] After the above heat treatment is performed on the oxide
semiconductor film 110, heat treatment for supply of oxygen
(supplying oxygen to an oxide semiconductor film; the same can be
applied to the description hereinafter) may be performed in the
same furnace. The heat treatment may be performed at a temperature
higher than or equal to 200.degree. C. and lower than the strain
point of the substrate in a heat treatment apparatus to which a
high-purity oxygen gas, a high-purity N.sub.2O gas, or ultra dry
air (with a moisture content of 20 ppm (-55.degree. C. by
conversion into a dew point) or less, preferably 1 ppm or less,
further preferably 10 ppb or less, in the case where measurement is
performed with the use of a dew point meter of a cavity ring down
laser spectroscopy (CRDS) system) is introduced. The heat treatment
is performed preferably at a temperature higher than or equal to
250.degree. C. and lower than or equal to 450.degree. C. It is
particularly preferable that moisture, hydrogen, or the like be not
contained in these gases. The purity of the oxygen gas or the
N.sub.2O gas that is introduced into the same furnace is preferably
greater than or equal to 6N, further preferably greater than or
equal to 7N (i.e., the impurity concentration is preferably less
than or equal to 1 ppm, further preferably less than or equal to
0.1 ppm). By the action of the oxygen gas or the N.sub.2O gas,
oxygen that is a main component of the oxide semiconductor and that
has been reduced through the steps for removing impurities by
dehydration or dehydrogenation treatment can be supplied. Through
this step, an oxygen vacancy generated by dehydration or
dehydrogenation treatment can be compensated.
[0110] Note that the above heat treatment has an advantageous
effect for compensating an oxygen vacancy generated in the oxide
semiconductor by dehydration treatment or dehydrogenation
treatment; thus, the heat treatment can also be referred to as
oxygen supplying treatment or the like. The heat treatment can be
performed at the timing, for example, after the oxide semiconductor
film is processed into an island shape. Such oxygen supplying
treatment may be performed once or plural times.
[0111] Next, a conductive film is formed over the gate insulating
film 108 and the oxide semiconductor film 110 and is subjected to a
photolithography step and an etching step, whereby the source
electrode 112a and the drain electrode 112b are formed (see FIG.
3A).
[0112] As the conductive film used for the source electrode 112a
and the drain electrode 112b, for example, a metal film containing
an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W or a metal
nitride film containing any of the above elements as a component (a
titanium nitride film, a molybdenum nitride film, or a tungsten
nitride film) can be used. A high-melting-point metal film of Ti,
Mo, W, or the like or a metal nitride film of any of these elements
(a titanium nitride film, a molybdenum nitride film, or a tungsten
nitride film) may be stacked on one of or both a bottom side and a
top side of a metal film of Al, Cu, or the like.
[0113] Next, the protective film 114 is formed over the oxide
semiconductor film 110, the source electrode 112a, and the drain
electrode 112b (see FIG. 3B). In this embodiment, a stack of the
oxide insulating film 114a and the metal oxide film 114b can be
used as the protective film 114.
[0114] The oxide insulating film 114a preferably includes a region
in which the oxygen content is higher than the stoichiometric
proportion. When the oxide insulating film 114a includes a region
in which the oxygen content is higher than the stoichiometric
proportion, transfer of oxygen from the oxide semiconductor film
110 to the oxide insulating film 114a in contact therewith can be
suppressed and oxygen can be supplied from the oxide insulating
film 114a in contact with the oxide semiconductor film 110 to the
oxide semiconductor film 110. Further, for example, in the case
where a silicon oxide film is used as the oxide insulating film
114a, a sputtering method or a plasma CVD method can be
employed.
[0115] The metal oxide film 114b is preferably formed using a
material which contains an element of Group 12 or Group 13, or an
element of Group 3 having a property similar to that of the element
of Group 13, which is the same group as one of the elements
included in the oxide semiconductor film 110. For example, in the
case where the oxide semiconductor film 110 includes oxides of
indium (In) and zinc (Zn), it is preferable to use the insulating
metal oxide film 114b formed using an element belonging to the same
group as zinc, namely, Group 12, an element belonging to the same
group as indium, namely, Group 13, or an element of Group 3 having
a property similar to that of the element of Group 13. As the
element of Group 3, an oxide film including a lanthanoid-based
element such as cerium (Ce) or gadolinium (Gd) may be used. As the
metal oxide film 114b, an aluminum oxide film, a gallium oxide
film, or a zinc oxide film can be selected as a favorable
example.
[0116] The metal oxide film 114b can be formed by a sputtering
method using a metal oxide target or a metal target. As an
atmosphere in sputtering, an inert gas atmosphere, an oxygen gas
atmosphere, a mixed atmosphere containing an inert gas and an
oxygen gas, or the like can be used. Examples of a sputtering
method include an RF sputtering method in which a high-frequency
power source is used as a sputtering power source, a DC sputtering
method in which a direct-current power source is used, and an AC
sputtering method in which an alternating-current power source is
used. Alternatively, a pulsed DC sputtering method in which a bias
is applied in a pulsed manner can be used. The metal oxide film
114b is preferably formed by an RF sputtering method or an AC
sputtering method because the metal oxide film 114b can be dense.
The metal oxide film 114b is preferably formed while the substrate
is heated because the metal oxide film 114b can be dense.
[0117] It is preferable that heat treatment be performed in a
pre-heating chamber of a sputtering apparatus as pre-treatment for
the metal oxide film 114b and impurities such as water or hydrogen
be removed and evacuated so that the metal oxide film 114b includes
impurities such as water or hydrogen as little as possible in
formation of the metal oxide film 114b. As an evacuation unit
provided in the pre-heating chamber, a cryopump is preferable.
[0118] Further, as the metal oxide film 114b, it is particularly
preferable to use an aluminum oxide film with a film density of
higher than or equal to 3.2 g/cm.sup.3, preferably higher than or
equal to 3.6 g/cm.sup.3. The aluminum oxide film has a thickness
greater than or equal to 30 nm and less than or equal to 150 nm,
preferably greater than or equal to 50 nm and less than or equal to
100 nm. When an aluminum oxide film is used as the metal oxide film
114b and has a film density within the above range, entry and
diffusion of water or hydrogen into the oxide semiconductor film
can be suppressed. In addition, release of oxygen from the oxide
semiconductor film can be suppressed.
[0119] Heat treatment may be performed after the metal oxide film
114b is formed. By the heat treatment, oxygen can be supplied to
the oxide semiconductor film 110 and micro-defects in the film and
defects at the interface between stacked layers can be repaired.
Thus, the oxide semiconductor film 110 can be highly purified to
become i-type (intrinsic). The temperature of the heat treatment is
preferably higher than or equal to 300.degree. C. and lower than or
equal to 350.degree. C.
[0120] Note that a planarization insulating film may be formed over
the protective film 114 in order to reduce the unevenness of the
transistor 150. For the planarization insulating film, an organic
material such as a polyimide-based resin, an acrylic resin, or a
benzocyclobutene-based resin can be used.
[0121] A single layer selected from zinc oxide (ZnO), indium tin
oxide (ITO), titanium oxide (TiO.sub.x), aluminum (Al), and
titanium (Ti) or a stack of any of them may be provided in contact
with the metal oxide film 114b.
[0122] The above metal oxide or metal is provided in contact with
the metal oxide film 114b, whereby charges accumulated between the
oxide insulating film 114a and the aluminum oxide film serving as
the metal oxide film 114b can be released and charge accumulation
on a surface of the protective film 114 can be suppressed.
[0123] As described above, in the semiconductor device including
the oxide semiconductor, which is described in this embodiment, the
film including the metal oxide film is provided as the protective
film in contact with the oxide semiconductor film. The metal oxide
film can suppress entry and diffusion of water or hydrogen into the
oxide semiconductor film. In addition, release of oxygen from the
oxide semiconductor film can be suppressed.
[0124] Therefore, a semiconductor device including an oxide
semiconductor, which has more stable electric characteristics and
high reliability, can be provided. Further, a method for
manufacturing the semiconductor device can be provided.
[0125] This embodiment can be combined with any of the other
embodiments as appropriate.
Embodiment 2
[0126] In this embodiment, a semiconductor device including a
transistor according to one embodiment of the present invention and
a method for manufacturing the semiconductor device will be
described with reference to FIG. 4.
<Structure of Semiconductor Device of Embodiment 2>
[0127] FIG. 4 is a cross-sectional view of a semiconductor device
including an oxide semiconductor film. A transistor 250 in FIG. 4
includes the gate electrode 106 formed over the substrate 102
having an insulating surface over which the base insulating film
104 is provided, a gate insulating film 208a formed over the base
insulating film 104 and the gate electrode 106, a metal oxide film
208b formed over the gate insulating film 208a, the oxide
semiconductor film 110 formed over the metal oxide film 208b, the
source electrode 112a and the drain electrode 112b formed over the
metal oxide film 208b and the oxide semiconductor film 110, and the
protective film 114 formed over the oxide semiconductor film 110,
the source electrode 112a, and the drain electrode 112b. The
protective film 114 is a stack of the oxide insulating film 114a
and the metal oxide film 114b.
[0128] This embodiment is different from Embodiment 1 in that the
metal oxide film is provided over the gate insulating film.
<Manufacturing Method of Semiconductor Device of Embodiment
2>
[0129] Next, a method for manufacturing the transistor 250 will be
described.
[0130] First, the base insulating film 104 is formed over the
substrate 102, and the gate electrode 106 is formed over the base
insulating film 104.
[0131] Embodiment 1 can be referred to for the formation methods,
materials, and the like of the substrate 102, the base insulating
film 104, and the gate electrode 106.
[0132] Next, the gate insulating film 208a is formed over the base
insulating film 104 and the gate electrode 106.
[0133] Embodiment 1 can be referred to for the formation method,
material, and the like of the gate insulating film 208a.
[0134] Next, the metal oxide film 208b and the oxide semiconductor
film 110 are formed over the gate insulating film 208a.
[0135] The metal oxide film 208b and the oxide semiconductor film
110 can be successively formed in vacuum by a multi-chamber
sputtering apparatus.
[0136] In the case where heat treatment is performed before the
metal oxide film 208b is formed, heat treatment, formation of the
metal oxide film 208b, and formation of the oxide semiconductor
film 110 can be successively formed in vacuum by a multi-chamber
sputtering apparatus.
[0137] The metal oxide film 208b is preferably formed using a
material including an element of Group 12, Group 13, or Group 3,
which is the same group as one of the elements included in the
oxide semiconductor film 110 because the metal oxide film 208b is
in contact with the oxide semiconductor film 110. For example, in
the case where the oxide semiconductor film 110 includes oxides of
indium and zinc, it is preferable to use the insulating metal oxide
film 208b formed using an element belonging to the same group as
zinc, namely, Group 12, an element belonging to the same group as
indium, namely, Group 13, or an element of Group 3 having a
property similar to that of the element of Group 13. As the element
of Group 3, an oxide film including a lanthanoid-based element such
as cerium (Ce) or gadolinium (Gd) may be used. As the metal oxide
film 208b, an aluminum oxide film, a gallium oxide film, or a zinc
oxide film can be selected as a favorable example.
[0138] The metal oxide film 208b can be formed by a sputtering
method using a metal oxide target or a metal target. As an
atmosphere in sputtering, an inert gas atmosphere, an oxygen gas
atmosphere, a mixed atmosphere containing an inert gas and an
oxygen gas, or the like can be used. Examples of a sputtering
method include an RF sputtering method in which a high-frequency
power source is used as a sputtering power source, a DC sputtering
method in which a direct-current power source is used, and an AC
sputtering method in which an alternating-current power source is
used. Alternatively, a pulsed DC sputtering method in which a bias
is applied in a pulsed manner can be used. The metal oxide film
208b is preferably formed by an RF sputtering method or an AC
sputtering method because the metal oxide film 208b can be dense.
The metal oxide film 208b is preferably formed while the substrate
is heated because the metal oxide film 208b can be dense.
[0139] The following steps are preferably performed so that
moisture or hydrogen is included in the metal oxide film 208b and
the oxide semiconductor film 110 as little as possible in formation
of the metal oxide film 208b and the oxide semiconductor film 110:
heat treatment is performed on the substrate 102 over which the
gate insulating film 208a is formed in a pre-heating chamber of a
sputtering apparatus, i.e., in vacuum, as pre-treatment for the
metal oxide film 208b; and impurities such as water or hydrogen
adsorbed on the substrate 102 and the gate insulating film 208a are
removed and evacuated. As an evacuation unit provided in the
pre-heating chamber, a cryopump is preferable.
[0140] Embodiment 1 can be referred to for the material and the
like of the oxide semiconductor film 110.
[0141] Next, a conductive film is formed over the metal oxide film
208b and the oxide semiconductor film 110 and is subjected to a
photolithography step and an etching step, whereby the source
electrode 112a and the drain electrode 112b are formed.
[0142] Next, the protective film 114 is formed over the oxide
semiconductor film 110, the source electrode 112a, and the drain
electrode 112b. Through the above steps, the transistor 250 is
formed.
[0143] Embodiment 1 can be referred to for the materials and the
like of the source electrode 112a, the drain electrode 112b, and
the protective film 114.
[0144] Further, as the metal oxide film 114b of the protective film
114, it is preferable to use an aluminum oxide film with a film
density of higher than or equal to 3.2 g/cm.sup.3, preferably
higher than or equal to 3.6 g/cm.sup.3. The aluminum oxide film has
a thickness greater than or equal to 30 nm and less than or equal
to 150 nm, preferably greater than or equal to 50 nm and less than
or equal to 100 nm. When an aluminum oxide film is used as the
metal oxide film 114b and the film density of the aluminum oxide
film is within the above range, entry and diffusion of water or
hydrogen into the oxide semiconductor film can be suppressed. In
addition, release of oxygen from the oxide semiconductor film can
be suppressed.
[0145] Heat treatment may be performed after the metal oxide film
114b is formed. By the heat treatment, oxygen can be supplied to
the oxide semiconductor film 110 and micro-defects in the film and
defects at the interface between stacked layers can be repaired.
Thus, the oxide semiconductor film 110 can be highly purified to
become i-type (intrinsic). The temperature of the heat treatment is
preferably higher than or equal to 300.degree. C. and lower than or
equal to 350.degree. C.
[0146] Note that a planarization insulating film may be formed over
the protective film 114 in order to reduce the unevenness of the
transistor 250. For the planarization insulating film, an organic
material such as a polyimide-based resin, an acrylic resin, or a
benzocyclobutene-based resin can be used.
[0147] A single layer selected from zinc oxide (ZnO), indium tin
oxide (ITO), titanium oxide (TiO.sub.x), aluminum (Al), and
titanium (Ti) or a stack of any of them may be provided in contact
with the metal oxide film 114b.
[0148] The above metal oxide or metal is provided in contact with
the aluminum oxide film, whereby charges accumulated between the
oxide insulating film 114a and the aluminum oxide film serving as
the metal oxide film 114b can be released and charge accumulation
on a surface of the protective film 114 can be suppressed.
[0149] As described above, in the semiconductor device including
the oxide semiconductor, which is described in this embodiment, the
film including the metal oxide film is provided as the protective
film in contact with the oxide semiconductor film. With a structure
in which the metal oxide film is in contact with the oxide
semiconductor film, The interface characteristics are extremely
stable because the metal oxide film is formed using a material
including an element of the same group as that of an element other
than oxygen included in the oxide semiconductor film. Further, the
metal oxide film can suppress entry and diffusion of water or
hydrogen into the oxide semiconductor film. In addition, release of
oxygen from the oxide semiconductor film can be suppressed.
[0150] Since the metal oxide film and the oxide semiconductor film
are successively formed in vacuum without exposure to the air, the
interface between the metal oxide film and the oxide semiconductor
film can be kept clean.
[0151] Therefore, a semiconductor device including an oxide
semiconductor, which has more stable electric characteristics and
high reliability, can be provided. Further, a method for
manufacturing the semiconductor device can be provided.
[0152] This embodiment can be combined with any of the other
embodiments as appropriate.
Embodiment 3
[0153] A semiconductor device having a display function (also
referred to as a display device) can be manufactured using the
transistor described in Embodiment 1. Moreover, part or all of a
driver circuit which includes the transistor can be formed over a
substrate where a pixel portion is formed, whereby a
system-on-panel can be obtained.
[0154] In FIG. 5A, a sealant 405 is provided so as to surround a
pixel portion 402 provided over a first substrate 401, and the
pixel portion 402 is sealed by using a second substrate 406. In
FIG. 5A, a signal line driver circuit 403 and a scan line driver
circuit 404 which are formed using a single crystal semiconductor
film or a polycrystalline semiconductor film over a substrate
separately prepared are mounted in a region that is different from
the region surrounded by the sealant 405 over the first substrate
401. Various signals and potentials are supplied to the signal line
driver circuit 403 and the scan line driver circuit 404 which are
separately formed and the pixel portion 402 from flexible printed
circuits (FPCs) 418a and 418b.
[0155] In FIGS. 5B and 5C, the sealant 405 is provided so as to
surround the pixel portion 402 and the scan line driver circuit 404
which are provided over the first substrate 401. The second
substrate 406 is provided over the pixel portion 402 and the scan
line driver circuit 404. Consequently, the pixel portion 402 and
the scan line driver circuit 404 are sealed together with a display
element, by the first substrate 401, the sealant 405, and the
second substrate 406. In FIGS. 5B and 5C, the signal line driver
circuit 403 which is formed using a single crystal semiconductor
film or a polycrystalline semiconductor film over a substrate
prepared separately is mounted in a region that is different from
the region surrounded by the sealant 405 over the first substrate
401. In FIGS. 5B and 5C, various signals and potentials are
supplied to the signal line driver circuit 403 which is separately
formed, the scan line driver circuit 404, and the pixel portion 402
from an FPC 418.
[0156] Although FIGS. 5B and 5C each illustrate an example in which
the signal line driver circuit 403 is formed separately and mounted
on the first substrate 401, one embodiment of the present invention
is not limited to this structure. The scan line driver circuit may
be separately formed and then mounted, or only part of the signal
line driver circuit or part of the scan line driver circuit may be
separately formed and then mounted.
[0157] Note that a connection method of a separately formed driver
circuit is not particularly limited, and a chip on glass (COG)
method, a wire bonding method, a tape automated bonding (TAB)
method, or the like can be used. FIG. 5A illustrates an example in
which the signal line driver circuit 403 and the scan line driver
circuit 404 are mounted by a COG method. FIG. 5B illustrates an
example in which the signal line driver circuit 403 is mounted by a
COG method. FIG. 5C illustrates an example in which the signal line
driver circuit 403 is mounted by a TAB method.
[0158] In addition, the display device includes a panel in which
the display element is sealed, and a module in which an IC or the
like including a controller is mounted on the panel.
[0159] Note that the display device in this specification means an
image display device, a display device, or a light source
(including a lighting device). Furthermore, the display device also
includes the following modules in its category: a module to which a
connector such as an FPC, a TAB tape, or a TCP is attached; a
module having a TAB tape or a TCP at the tip of which a printed
wiring board is provided; and a module in which an integrated
circuit (IC) is directly mounted on a display element by a COG
method.
[0160] The pixel portion 402 and the scan line driver circuit 404
provided over the first substrate 401 include a plurality of
transistors, and the transistor described in Embodiment 1 can be
applied thereto.
[0161] As the display element provided in the display device, a
liquid crystal element (also referred to as a liquid crystal
display element) or a light-emitting element (also referred to as a
light-emitting display element) can be used. The light-emitting
element includes, in its category, an element whose luminance is
controlled by a current or a voltage, and specifically includes, in
its category, an inorganic electroluminescent (EL) element, an
organic EL element, and the like. Furthermore, a display medium
whose contrast is changed by an electric effect, such as electronic
ink, can be used.
[0162] One embodiment of the semiconductor device is described with
reference to FIG. 6 and FIG. 7. FIG. 6 and FIG. 7 correspond to
cross-sectional views taken along line Q-R in FIG. 5B.
[0163] As illustrated in FIG. 6 and FIG. 7, the semiconductor
device includes a connection terminal electrode layer 415 and a
terminal electrode layer 416. The connection terminal electrode
layer 415 and the terminal electrode layer 416 are electrically
connected to a terminal included in the FPC 418 through an
anisotropic conductive film 419.
[0164] The connection terminal electrode layer 415 is formed using
the same conductive film as a first electrode layer 430, and the
terminal electrode layer 416 is formed using the same conductive
film as source and drain electrode layers of transistors 410 and
411.
[0165] The pixel portion 402 and the scan line driver circuit 404
which are provided over the first substrate 401 include a plurality
of transistors. In FIG. 6 and FIG. 7, the transistor 410 included
in the pixel portion 402 and the transistor 411 included in the
scan line driver circuit 404 are illustrated as an example. In FIG.
6, a protective film 420 is provided over the transistors 410 and
411, and in FIG. 7, a protective film 424 and an insulating film
421 are further provided. Note that an insulating film 423 is an
insulating film serving as a base film.
[0166] In this embodiment, the transistor described in Embodiment 1
can be applied to the transistor 410 and the transistor 411.
[0167] The transistor 410 and the transistor 411 are each a
transistor including an oxide semiconductor film in which formation
of an oxygen vacancy and entry of water or hydrogen are suppressed.
Therefore, variation in the electric characteristics of the
transistors 410 and 411 is suppressed, and the transistors 410 and
411 are electrically stable.
[0168] As described above, highly reliable semiconductor devices
can be provided as the semiconductor devices of this embodiment
illustrated in FIG. 6 and FIG. 7.
[0169] The transistor 410 provided in the pixel portion 402 is
electrically connected to a display element to form a display
panel. A variety of display elements can be used as the display
element as long as display can be performed.
[0170] An example of a liquid crystal display device using a liquid
crystal element as a display element is described in FIG. 6. In
FIG. 6, a liquid crystal element 413 which is a display element
includes the first electrode layer 430, a second electrode layer
431, and a liquid crystal layer 408. An insulating film 432 and an
insulating film 433 which function as alignment films are provided
so that the liquid crystal layer 408 is provided therebetween. The
second electrode layer 431 is provided on the second substrate 406
side, and the first electrode layer 430 and the second electrode
layer 431 are stacked with the liquid crystal layer 408 provided
therebetween.
[0171] A columnar spacer 435 is obtained by selective etching of an
insulating film and is provided in order to control the thickness
(a cell gap) of the liquid crystal layer 408. Alternatively, a
spherical spacer may be used.
[0172] In the case where a liquid crystal element is used as the
display element, a thermotropic liquid crystal, a low-molecular
liquid crystal, a high-molecular liquid crystal, a polymer
dispersed liquid crystal, a ferroelectric liquid crystal, an
anti-ferroelectric liquid crystal, or the like can be used. Such a
liquid crystal material exhibits a cholesteric phase, a smectic
phase, a cubic phase, a chiral nematic phase, an isotropic phase,
or the like depending on a condition.
[0173] Alternatively, a liquid crystal exhibiting a blue phase for
which an alignment film is unnecessary may be used. A blue phase is
one of liquid crystal phases, which is generated just before a
cholesteric phase changes into an isotropic phase while temperature
of cholesteric liquid crystal is increased. Since the blue phase
appears only in a narrow temperature range, a liquid crystal
composition in which several weight percent or more of a chiral
agent is mixed is used for the liquid crystal layer in order to
improve the temperature range. The liquid crystal composition which
includes a liquid crystal exhibiting a blue phase and a chiral
agent has a short response time, and has optical isotropy, which
makes the alignment process unneeded and viewing angle dependence
small. In addition, since an alignment film does not need to be
provided and rubbing treatment is unnecessary, electrostatic
discharge damage caused by the rubbing treatment can be prevented
and defects and damage of the liquid crystal display device can be
reduced in the manufacturing process. Thus, productivity of the
liquid crystal display device can be increased. A transistor that
includes an oxide semiconductor film has a possibility that the
electric characteristics may vary significantly by the influence of
static electricity and deviate from the designed range. Therefore,
it is more effective to use a liquid crystal material exhibiting a
blue phase for a liquid crystal display device including a
transistor which includes an oxide semiconductor film.
[0174] The specific resistivity of the liquid crystal material is
greater than or equal to 1.times.10.sup.9 .OMEGA.cm, preferably
greater than or equal to 1.times.10.sup.11 .OMEGA.cm, more
preferably greater than or equal to 1.times.10.sup.12 .OMEGA.cm.
The value of the specific resistivity in this specification is
measured at 20.degree. C.
[0175] The size of a storage capacitor formed in the liquid crystal
display device is set considering the leakage current of the
transistor provided in the pixel portion or the like so that charge
can be held for a predetermined period. The size of the storage
capacitor may be set considering the off-state current or the like
of the transistor. By using the transistor including the oxide
semiconductor film which includes an oxygen-excess region, it is
enough to provide a storage capacitor having a capacitance that is
1/3 or less, preferably 1/5 or less of a liquid crystal capacitance
of each pixel.
[0176] In the transistor used in this embodiment, which includes an
oxide semiconductor film in which formation of an oxygen vacancy is
suppressed, the current in an off state (the off-state current) can
be made small. Accordingly, an electrical signal such as an image
signal can be held for a long period, and a writing interval can be
set long in an on state. Accordingly, the frequency of refresh
operation can be reduced, which leads to an effect of suppressing
power consumption.
[0177] The transistor used in this embodiment, which includes an
oxide semiconductor film in which formation of an oxygen vacancy is
suppressed, can have relatively high field-effect mobility and thus
can operate at high speed. For example, when such a transistor
which can operate at high speed is used for a liquid crystal
display device, a switching transistor in a pixel portion and a
driver transistor in a driver circuit portion can be formed over
one substrate. That is, since a semiconductor device formed of a
silicon wafer or the like is not additionally needed as a driver
circuit, the number of components of the semiconductor device can
be reduced. In addition, by using a transistor which can operate at
high speed in a pixel portion, a high-quality image can be
provided.
[0178] For the liquid crystal display device, a twisted nematic
(TN) mode, an in-plane-switching (IPS) mode, a fringe field
switching (FFS) mode, an axially symmetric aligned micro-cell (ASM)
mode, an optical compensated birefringence (OCB) mode, a
ferroelectric liquid crystal (FLC) mode, an antiferroelectric
liquid crystal (AFLC) mode, or the like can be used.
[0179] A normally black liquid crystal display device such as a
transmissive liquid crystal display device utilizing a vertical
alignment (VA) mode may be used. Some examples are given as the
vertical alignment mode. For example, a multi-domain vertical
alignment (MVA) mode, a patterned vertical alignment (PVA) mode, or
an advanced super view (ASV) mode can be used. Furthermore, this
embodiment can be applied to a VA liquid crystal display device.
The VA mode is a mode for controlling alignment of liquid crystal
molecules of a liquid crystal display panel. In the VA mode, liquid
crystal molecules are aligned in a vertical direction with respect
to a panel surface when no voltage is applied. Moreover, it is
possible to use a method called domain multiplication or
multi-domain design, in which a pixel is divided into some regions
(subpixels) and molecules are aligned in different directions in
their respective regions.
[0180] In the display device, a black matrix (a light-blocking
layer), an optical member (an optical substrate) such as a
polarizing member, a retardation member, or an anti-reflection
member, and the like are provided as appropriate. For example,
circular polarization may be obtained by using a polarizing
substrate and a retardation substrate. In addition, a backlight, a
side light, or the like may be used as a light source.
[0181] As a display method in the pixel portion, a progressive
method, an interlace method, or the like can be employed. Further,
color elements controlled in a pixel at the time of color display
are not limited to three colors of R, G, and B (R, G, and B
correspond to red, green, and blue, respectively). For example, R,
G, B, and W (W corresponds to white), or R, G, B, and one or more
of yellow, cyan, magenta, and the like can be used. Further, the
sizes of display regions may be different between respective dots
of the color elements. The disclosed invention is not limited to
the application to a display device for color display but can also
be applied to a display device for monochrome display.
[0182] Alternatively, as the display element included in the
display device, a light-emitting element utilizing
electroluminescence can be used. Light-emitting elements utilizing
electroluminescence are classified depending on whether a
light-emitting material is an organic compound or an inorganic
compound. In general, the former is referred to as an organic EL
element, and the latter is referred to as an inorganic EL
element.
[0183] In an organic EL element, by application of voltage to a
light-emitting element, electrons and holes are injected from a
pair of electrodes into a layer containing a light-emitting organic
compound, and current flows. The carriers (electrons and holes) are
recombined, and thus, the light-emitting organic compound is
excited. The light-emitting organic compound returns to a ground
state from the excited state, thereby emitting light. Owing to such
a mechanism, this light-emitting element is referred to as a
current-excitation light-emitting element.
[0184] The inorganic EL elements are classified depending on the
element structure into a dispersion-type inorganic EL element and a
thin-film inorganic EL element. A dispersion-type inorganic EL
element has a light-emitting layer where particles of a
light-emitting material are dispersed in a binder, and its light
emission mechanism is donor-acceptor recombination type light
emission that utilizes a donor level and an acceptor level. A
thin-film inorganic EL element has a structure where a
light-emitting layer is sandwiched between dielectric layers, which
are further sandwiched between electrodes, and its light emission
mechanism is localized type light emission that utilizes
inner-shell electron transition of metal ions. Note that an example
of an organic EL element as a light-emitting element is described
here.
[0185] In order to extract light emitted from the light-emitting
element, at least one of a pair of electrodes has a
light-transmitting property. The transistor and the light-emitting
element are provided over the substrate. The light-emitting element
can have a top emission structure in which light is extracted
through the surface opposite to the substrate; a bottom emission
structure in which light is extracted through the surface on the
substrate side; or a dual emission structure in which light is
extracted through the surface opposite to the substrate and the
surface on the substrate side.
[0186] An example of a light-emitting device in which a
light-emitting element is used as a display element is illustrated
in FIG. 7. A light-emitting element 453 which is a display element
is electrically connected to the transistor 410 provided in the
pixel portion 402. A structure of the light-emitting element 453 is
not limited to the stacked-layer structure including the first
electrode layer 430, an electroluminescent layer 452, and the
second electrode layer 431, which is illustrated in FIG. 7. The
structure of the light-emitting element 453 can be changed as
appropriate depending on a direction in which light is extracted
from the light-emitting element 453, or the like.
[0187] A partition wall 451 is formed using an organic insulating
material or an inorganic insulating material. It is particularly
preferable that the partition wall 451 be formed using a
photosensitive resin material to have an opening over the first
electrode layer 430 so that a sidewall of the opening has a tilted
surface with continuous curvature.
[0188] The electroluminescent layer 452 may be formed using a
single layer or a plurality of layers stacked.
[0189] A protective film may be formed over the second electrode
layer 431 and the partition wall 451 in order to prevent entry of
oxygen, hydrogen, water, carbon dioxide, or the like into the
light-emitting element 453. As the protective film, a silicon
nitride film, a silicon nitride oxide film, a diamond like carbon
(DLC) film, or the like can be formed. In addition, in a space
which is formed with the first substrate 401, the second substrate
406, and the sealant 405, a filler 454 is provided for sealing. It
is preferable that a panel be packaged (sealed) with a protective
film (such as a laminate film or an ultraviolet curable resin film)
or a cover material with high air-tightness and little
degasification so that the panel is not exposed to the outside air,
in this manner.
[0190] As the filler 454, an ultraviolet curable resin or a
thermosetting resin can be used as well as an inert gas such as
nitrogen or argon. For example, polyvinyl chloride (PVC), an
acrylic resin, a polyimide-based resin, an epoxy resin, a silicone
resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA) can
be used. For example, nitrogen is used for the filler.
[0191] In addition, if needed, an optical film, such as a
polarizing plate, a circularly polarizing plate (including an
elliptically polarizing plate), a retardation plate (a quarter-wave
plate or a half-wave plate), or a color filter, may be provided as
appropriate for a light-emitting surface of the light-emitting
element. Further, the polarizing plate or the circularly polarizing
plate may be provided with an anti-reflection film. For example,
anti-glare treatment by which reflected light can be diffused by
surface roughness so as to reduce the glare can be performed.
[0192] Note that in FIG. 6 and FIG. 7, a flexible substrate as well
as a glass substrate can be used as the first substrate 401 and the
second substrate 406. For example, a plastic substrate having a
light-transmitting property can be used. As plastic, a
fiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride
(PVF) film, a polyester film, or an acrylic resin film can be used.
In addition, a sheet with a structure in which an aluminum foil is
sandwiched between PVF films or polyester films can be used.
[0193] In this embodiment, a silicon oxide film is used as the
protective film 420, and an aluminum oxide film is used as the
protective film 424 in FIG. 7. The protective film 420 and the
protective film 424 can be formed by a sputtering method or a
plasma CVD method.
[0194] The aluminum oxide film provided over the oxide
semiconductor film as the protective film 424 preferably has a film
density of higher than or equal to 3.2 g/cm.sup.3, more preferably
higher than or equal to 3.6 g/cm.sup.3. Thus, a high blocking
effect, which is not permeable to either oxygen or impurities such
as hydrogen or water, can be obtained.
[0195] Therefore, during the manufacturing process and after the
manufacture, the aluminum oxide film functions as a protective film
for preventing entry of impurities such as hydrogen or water, which
causes a change, into the oxide semiconductor film and release of
oxygen, which is a main component material of the oxide
semiconductor film, from the oxide semiconductor film.
[0196] The silicon oxide film provided as the protective film 420
in contact with the oxide semiconductor film has a function of
supplying oxygen to the oxide semiconductor film. Therefore, the
protective film 420 is preferably an oxide insulating film
containing much oxygen.
[0197] The transistor 410 and the transistor 411 each include an
oxide semiconductor film which is highly purified and in which
formation of an oxygen vacancy is suppressed. In the transistor 410
and the transistor 411, a gate insulating film is formed using a
silicon nitride oxide film, a silicon oxynitride film, and a metal
oxide film. With such a structure of the gate insulating film,
variation in characteristics is suppressed and the transistors are
electrically stable.
[0198] The insulating film 421 serving as a planarization
insulating film can be formed using an organic material having heat
resistance, such as an acrylic resin, a polyimide-based resin, a
benzocyclobutene-based resin, a polyamide resin, or an epoxy resin.
The insulating film may be formed by stacking a plurality of
insulating films formed of these materials.
[0199] There is no particular limitation on the method for forming
the insulating film 421, and the insulating film 421 can be formed,
depending on the material, by a method such as a sputtering method,
an SOG method, spin coating, dipping, spray coating, a droplet
discharge method (e.g., an inkjet method), or a printing method
(e.g., screen printing or offset printing), or with a tool
(equipment) such as a doctor knife, a roll coater, a curtain
coater, or a knife coater.
[0200] The display device displays an image by transmitting light
from a light source or a display element. Therefore, the substrate
and the thin films such as the insulating film and the conductive
film provided for the pixel portion where light is transmitted all
have a light-transmitting property with respect to light in the
visible-light wavelength range.
[0201] The first electrode layer and the second electrode layer
(each of which may be called a pixel electrode layer, a common
electrode layer, a counter electrode layer, or the like) for
applying voltage to the display element may have light-transmitting
properties or light-reflecting properties, which depends on the
direction in which light is extracted, the position where the
electrode layer is provided, and the pattern structure of the
electrode layer.
[0202] The first electrode layer 430 and the second electrode layer
431 can be formed using a light-transmitting conductive material
such as indium oxide containing tungsten oxide, indium zinc oxide
containing tungsten oxide, indium oxide containing titanium oxide,
indium tin oxide containing titanium oxide, ITO, indium zinc oxide,
indium tin oxide to which silicon oxide is added, or graphene.
[0203] The first electrode layer 430 and the second electrode layer
431 can be formed using one or more kinds of materials selected
from metals such as tungsten (W), molybdenum (Mo), zirconium (Zr),
hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium
(Cr), cobalt (Co), nickel (Ni), titanium (Ti), platinum (Pt),
aluminum (Al), copper (Cu), and silver (Ag); alloys thereof; and
nitrides thereof.
[0204] A conductive composition containing a conductive high
molecule (also referred to as a conductive polymer) can be used for
the first electrode layer 430 and the second electrode layer 431.
As the conductive high molecule, a so-called .pi.-electron
conjugated conductive polymer can be used. For example, polyaniline
or a derivative thereof, polypyrrole or a derivative thereof,
polythiophene or a derivative thereof, and a copolymer of two or
more of aniline, pyrrole, and thiophene or a derivative thereof can
be given.
[0205] Since the transistor is easily broken owing to static
electricity or the like, a protective circuit for protecting the
driver circuit is preferably provided. The protective circuit is
preferably formed using a non-linear element.
[0206] As described above, by using any of the transistors
described in the above embodiments, a semiconductor device having a
variety of functions can be provided.
[0207] As described above, the aluminum oxide film having a film
density of higher than or equal to 3.2 g/cm.sup.3, more preferably
higher than or equal to 3.6 g/cm.sup.3, whereby entry and diffusion
of water or hydrogen into the oxide semiconductor film can be
suppressed in the semiconductor device including the transistors
and having a display function. Therefore, variation in the electric
characteristics of the transistors is suppressed and the
transistors are electrically stable. Thus, by using the
transistors, a highly reliable semiconductor device can be
provided.
[0208] This embodiment can be combined with any of the other
embodiments as appropriate.
Embodiment 4
[0209] A semiconductor device disclosed in this specification can
be applied to a variety of electronic appliances (including game
machines). Examples of electronic appliances are a television set
(also referred to as a television or a television receiver), a
monitor of a computer or the like, a camera such as a digital
camera or a digital video camera, a digital photo frame, a mobile
phone handset (also referred to as a mobile phone or a mobile phone
device), a portable game console, a portable information terminal,
an audio reproducing device, a large-sized game machine such as a
pachinko machine, and the like. Examples of electronic appliances
each including the semiconductor device described in any of the
above embodiments will be described.
[0210] FIG. 8A illustrates a laptop personal computer, which
includes a main body 3001, a housing 3002, a display portion 3003,
a keyboard 3004, and the like. By applying the semiconductor device
described in any of the above embodiments to the display portion
3003, a highly reliable laptop personal computer can be
provided.
[0211] FIG. 8B is a personal digital assistant (PDA), which
includes a main body 3021 provided with a display portion 3023, an
external interface 3025, operation buttons 3024, and the like. A
stylus 3022 is included as an accessory for operation. By applying
the semiconductor device described in any of the above embodiments
to the display portion 3023, a highly reliable personal digital
assistant (PDA) can be provided.
[0212] FIG. 8C illustrates an example of an electronic book reader.
For example, the electronic book reader includes two housings,
i.e., a housing 2701 and a housing 2703. The housing 2701 and the
housing 2703 are combined with a hinge 2711 so that the electronic
book reader can be opened and closed with the hinge 2711 as an
axis. With such a structure, the electronic book reader can operate
like a paper book. In addition, the electronic book reader can be
more highly resistant to external impact. The housing 2701 and the
housing 2703 can be separated from each other by detaching the
hinge 2711.
[0213] A display portion 2705 and a display portion 2707 are
incorporated in the housing 2701 and the housing 2703,
respectively. The display portion 2705 and the display portion 2707
may display one image or different images. In the structure where
different images are displayed on different display portions, for
example, the right display portion (the display portion 2705 in
FIG. 8C) can display text and the left display portion (the display
portion 2707 in FIG. 8C) can display images. The semiconductor
device described in any of the above embodiments is applied to the
display portion 2705 and the display portion 2707, whereby a highly
reliable electronic book reader can be provided. In the case of
using a transflective or reflective liquid crystal display device
as the display portion 2705, the electronic book reader may be used
in a comparatively bright environment; therefore, a solar cell may
be provided so that power generation by the solar cell and charge
by a battery can be performed. When a lithium ion battery is used
as the battery, there are advantages of downsizing and the
like.
[0214] FIG. 8C illustrates an example in which the housing 2701 is
provided with an operation portion and the like. For example, the
housing 2701 is provided with a power switch 2721, operation keys
2723, a speaker 2725, and the like. With the operation keys 2723,
pages can be turned. Note that a keyboard, a pointing device, or
the like may also be provided on the surface of the housing, on
which the display portion is provided. Furthermore, an external
connection terminal (an earphone terminal, a USB terminal, or the
like), a recording medium insertion portion, and the like may be
provided on the back surface or the side surface of the housing.
Moreover, the electronic book reader may have a function of an
electronic dictionary.
[0215] The electronic book reader may have a structure capable of
wirelessly transmitting and receiving data. Through wireless
communication, desired book data or the like can be purchased and
downloaded from an electronic book server.
[0216] FIG. 8D illustrates a mobile phone, which includes two
housings, i.e., a housing 2800 and a housing 2801. The housing 2801
includes a display panel 2802, a speaker 2803, a microphone 2804, a
pointing device 2806, a camera lens 2807, an external connection
terminal 2808, and the like. In addition, the housing 2800 includes
a solar cell 2810 having a function of charge of the mobile phone,
an external memory slot 2811, and the like. An antenna is
incorporated in the housing 2801. By applying the semiconductor
device described in any of the above embodiments to the display
panel 2802, a highly reliable mobile phone can be provided.
[0217] Further, the display panel 2802 is provided with a touch
panel. A plurality of operation keys 2805 which is displayed as
images is illustrated by dashed lines in FIG. 8D. Note that a
boosting circuit by which a voltage output from the solar cell 2810
is increased to be sufficiently high for each circuit is also
provided.
[0218] On the display panel 2802, the display direction can be
appropriately changed depending on a usage pattern. Further, the
mobile phone is provided with the camera lens 2807 on the same
surface as the display panel 2802, and thus it can be used as a
video phone. The speaker 2803 and the microphone 2804 can be used
for videophone calls, recording and playing sound, and the like as
well as voice calls. Furthermore, the housings 2800 and 2801 which
are developed as illustrated in FIG. 8D can overlap with each other
by sliding; thus, the size of the mobile phone can be decreased,
which makes the mobile phone suitable for being carried.
[0219] The external connection terminal 2808 can be connected to an
AC adapter and various types of cables such as a USB cable, and
charging and data communication with a personal computer are
possible. Moreover, a large amount of data can be stored by
inserting a storage medium into the external memory slot 2811 and
can be moved.
[0220] Further, in addition to the above functions, an infrared
communication function, a television reception function, or the
like may be provided.
[0221] FIG. 8E illustrates a digital video camera which includes a
main body 3051, a display portion A 3057, an eyepiece portion 3053,
an operation switch 3054, a display portion B 3055, a battery 3056,
and the like. By applying the semiconductor device described in any
of the above embodiments to the display portion A 3057 and the
display portion B 3055, a highly reliable digital video camera can
be provided.
[0222] FIG. 8F illustrates an example of a television set. In the
television set, a display portion 9603 is incorporated in a housing
9601. The display portion 9603 can display images. Here, the
housing 9601 is supported by a stand 9605. By applying the
semiconductor device described in any of the above embodiments to
the display portion 9603, a highly reliable television set can be
provided.
[0223] The television set can be operated by an operation switch of
the housing 9601 or a separate remote controller. Further, the
remote controller may be provided with a display portion for
displaying data output from the remote controller.
[0224] Note that the television set is provided with a receiver, a
modem, and the like. With the use of the receiver, general
television broadcasting can be received. Moreover, when the
television set is connected to a communication network with or
without wires via the modem, one-way (from a sender to a receiver)
or two-way (between a sender and a receiver or between receivers)
data communication can be performed.
[0225] This embodiment can be combined with any of the other
embodiments as appropriate.
Example
[0226] In this example, as an example of a metal oxide film which
was able to be used in a semiconductor device according to one
embodiment of the present invention, an aluminum oxide film was
evaluated. Description is given with reference to FIGS. 9A and 9B,
FIG. 10, FIGS. 11A and 11B, FIGS. 12A and 12B, FIGS. 13A and 13B,
and FIGS. 14A and 14B. As the evaluation methods, X-ray
reflectometry (XRR), secondary ion mass spectrometry (SIMS), and
thermal desorption spectrometry (TDS) were used.
[0227] First, evaluation results of XRR are shown. FIGS. 9A and 9B
each illustrate a structure of a sample used for the XRR.
[0228] The sample illustrated in FIG. 9A was obtained by forming a
metal oxide film 512a over a glass substrate 502. The sample
illustrated in FIG. 9B was obtained by forming a metal oxide film
512b over the glass substrate 502.
[0229] As the metal oxide film 512a, an aluminum oxide film was
formed with a sputtering apparatus. The formation conditions of the
metal oxide film 512a were as follows: the substrate temperature
was room temperature; O.sub.2=50 sccm (O.sub.2=100%); the power was
6 kW (pulsed DC power source, pulse=300 kHz); and the pressure was
0.4 Pa. Note that the film thickness was 100 nm. An aluminum target
was used as a sputtering target.
[0230] As the metal oxide film 512b, an aluminum oxide film was
formed with a sputtering apparatus. The formation conditions of the
metal oxide film 512b were as follows: the substrate temperature
was 150.degree. C.; O.sub.2=300 sccm (O.sub.2=100%); the power was
30 kW (AC power source); and the pressure was 0.7 Pa. Note that the
film thickness was 100 nm. An aluminum target was used as a
sputtering target.
[0231] The samples having the structures illustrated in FIGS. 9A
and 9B are referred to as Samples 1 and 2, respectively. The film
density of the aluminum oxide films of the samples was evaluated by
the XRR. In the XRR, each of the samples was measured at three
points. Further, the film density was evaluated on the assumption
that the composition of the aluminum oxide film was an ideal
composition: Al.sub.2O.sub.3 (Z/A=0.4882, (Z=atomic number, A=mass
number)).
[0232] FIG. 10 shows the measurement results. As shown in FIG. 10,
the film density of the aluminum oxide film of Sample 1 was
approximately 3.0 g/cm.sup.3, and the film density of the aluminum
oxide film of Sample 2 was approximately 3.8 g/cm.sup.3.
[0233] Next, evaluation by SIMS analysis is described. FIGS. 11A
and 11B each illustrate a structure of a sample used for the SIMS
analysis.
[0234] The sample for evaluation illustrated in FIG. 11A was
obtained by forming a silicon oxide film 504a over the glass
substrate 502 and forming a metal oxide film 513a over the silicon
oxide film 504a. The sample for evaluation illustrated in FIG. 11B
was obtained by forming a silicon oxide film 504b over the glass
substrate 502 and forming a metal oxide film 513b over the silicon
oxide film 504b. Note that the samples having the structures
illustrated in FIGS. 11A and 11B are referred to as Samples 3 and
4, respectively.
[0235] The formation conditions of the silicon oxide film 504a were
as follows: a sputtering method is used; the substrate temperature
was 200.degree. C.; O.sub.2=50 sccm (O.sub.2=100%); the power was 6
kW (pulsed DC power source, pulse=300 kHz); and the pressure was
0.4 Pa. Note that the film thickness was 100 nm.
[0236] The formation conditions of the silicon oxide film 504b were
as follows: a sputtering method is used; the substrate temperature
was room temperature; O.sub.2=300 sccm (O.sub.2=100%); the power
was 6 kW (AC power source); and the pressure was 0.7 Pa. Note that
the film thickness was 400 nm.
[0237] As the metal oxide film 513a, an aluminum oxide film was
formed with a sputtering apparatus. The formation conditions of the
metal oxide film 513a were as follows: the substrate temperature
was room temperature; O.sub.2=50 sccm (O.sub.2=100%); the power was
6 kW (pulsed DC power source, pulse=300 kHz); and the pressure was
0.4 Pa. Note that the film thickness was 50 nm. An aluminum target
was used as a sputtering target.
[0238] As the metal oxide film 513b, an aluminum oxide film was
formed with a sputtering apparatus. The formation conditions of the
metal oxide film 513b were as follows: the substrate temperature
was 150.degree. C.; O.sub.2=300 sccm (O.sub.2=100%); the power was
30 kW (AC power source); and the pressure was 0.7 Pa. Note that the
film thickness was 100 nm. An aluminum target was used as a
sputtering target.
[0239] Note that the film density of the aluminum oxide film of the
metal oxide film 513a was set to 3.0 g/cm.sup.3, and the film
density of the aluminum oxide film of the metal oxide film 513b was
set to 3.8 g/cm.sup.3.
[0240] The structures of Samples 3 and 4 described above are shown
in Table 1.
TABLE-US-00001 TABLE 1 Film density of aluminum oxide Structure
film [g/cm.sup.3] Sample 3 glass\silicon oxide (100 nm)\ 3.0
aluminum oxide (50 nm) Sample 4 glass\silicon oxide (400 nm)\ 3.8
aluminum oxide (100 nm)
[0241] A pressure cooker test (PCT) was performed on Samples 3 and
4 shown in Table 1. In the PCT in this example, Samples 3 and 4
were held for 100 hours under the following conditions: the
temperature was 130.degree. C.; the humidity was 85% (the volume
ratio of water to deuterated water of water vapor contained in a
gas is H.sub.2O (water):D.sub.2O (deuterated water)=3:1); and the
pressure was 2.3 atm (0.23 MPa).
[0242] In this example, a "D atom", which is expressed by deuterium
water or the like, expresses a hydrogen atom with a mass number of
2 (.sup.2H).
[0243] As SIMS analysis of Samples 3 and 4 after the PCT, substrate
side depth profile (SSDP) SIMS was used to measure the
concentrations of hydrogen (H) atoms and deuterium (D) atoms in the
films. The measurement results of Samples 3 and 4 are shown in
FIGS. 12A and 12B, respectively.
[0244] It is known that it is difficult to obtain accurate data in
the proximity of a surface of a sample or in the proximity of an
interface between stacked films formed using different materials by
the SIMS analysis in measurement principle. Thus, in the case where
distributions of the concentrations of hydrogen (H) atoms and
deuterium (D) atoms in the film in the thickness direction are
analyzed by SIMS, an average value in a region where the film is
provided, the value is not greatly changed, and an almost constant
level of strength can be obtained is employed as the concentrations
of hydrogen (H) atoms and deuterium (D) atoms.
[0245] FIG. 12A shows that the concentrations of hydrogen (H) atoms
and deuterium (D) atoms in the silicon oxide film 504a of Sample 3
are 1.4.times.10.sup.21 atoms/cm.sup.3 and 2.9.times.10.sup.20
atoms/cm.sup.3, respectively.
[0246] FIG. 12B shows that the concentration of hydrogen (H) atoms
in the silicon oxide film 504b of Sample 4 is 2.2.times.10.sup.19
atoms/cm.sup.3 and the concentration of deuterium (D) atoms in the
silicon oxide film 504b of Sample 4 is lower than or equal to a
lower limit of detection. Note that the lower limit of detection of
the concentration of deuterium (D) atoms by SIMS analysis in this
example is 1.0.times.10.sup.16 atoms/cm.sup.3.
[0247] Note that all the results of SIMS analysis in this example
were quantified using a standard sample of a silicon oxide
film.
[0248] FIGS. 12A and 12B show that in Sample 3 which includes the
aluminum oxide film of which a film density was set to 3.0
g/cm.sup.3, hydrogen (H) atoms and deuterium (D) atoms pass through
the aluminum oxide film and are diffused into the silicon oxide
film. On the other hand, in Sample 4 which includes the aluminum
oxide film of which a film density was set to 3.8 g/cm.sup.3,
diffusion of hydrogen (H) atoms and deuterium (D) atoms is
suppressed in the aluminum oxide film. From Sample 4, in hydrogen
(H) atoms and deuterium (D) atoms, the concentrations are
drastically decreased in the aluminum oxide film at a depth of
around 30 nm; thus, it can be said that the diffusion of hydrogen
(H) atoms and deuterium (D) atoms can be suppressed even when the
thickness of the aluminum oxide film is 50 nm, as in Sample 3.
[0249] As described above, it is founded that barrier properties of
the aluminum oxide film to hydrogen (H) atoms and deuterium (D)
atoms of the aluminum oxide film are varied depending on the film
density of the aluminum oxide film.
[0250] Next, evaluation by TDS analysis is described. FIGS. 13A and
13B each illustrate a structure of a sample used for the TDS
analysis.
[0251] The sample illustrated in FIG. 13A was obtained by forming a
silicon nitride film 505 over the glass substrate 502. The sample
illustrated in FIG. 13B was obtained by forming the silicon nitride
film 505 over the glass substrate 502 and forming an aluminum oxide
film over the silicon nitride film 505 as a metal oxide film
514.
[0252] The formation conditions of the silicon nitride film 505
were as follows: a plasma CVD apparatus was used; the substrate
temperature was 220.degree. C.; and SiH.sub.4=270 sccm,
H.sub.2=4000 sccm, and N.sub.2O=2700 sccm. Note that the film
thickness was 100 nm.
[0253] For the metal oxide film 514, an aluminum oxide film was
formed with a sputtering apparatus. The formation conditions of the
aluminum oxide film were as follows: the substrate temperature was
150.degree. C.; O.sub.2=300 sccm (O.sub.2=100%); the power was 30
kW (AC power source); and the pressure was 0.7 Pa. Note that the
film thickness was 100 nm.
[0254] Note that the samples having the structures illustrated in
FIGS. 13A and 13B are referred to as Samples 5 and 6, respectively,
and TDS analysis was performed on Samples 5 and 6. FIG. 14A shows
TDS results of the samples when m/z (m=mass, z=charge)=2 (H.sub.2),
and FIG. 14B shows TDS results of the samples when m/z=18
(H.sub.2O). In FIGS. 14A and 14B, the horizontal axis represents
substrate temperature and the vertical axis represents detection
intensity.
[0255] FIG. 14A shows that Sample 5 has a peak at around
350.degree. C. and hydrogen (H.sub.2) is released. The released
hydrogen is regarded as H.sub.2 contained in the silicon nitride
film 505. On the other hand, the intensity distribution in Sample 6
is substantially flat in the measurement range, and hydrogen
(H.sub.2) is not prominently detected. Accordingly, it is
considered that the metal oxide film 514 suppresses release of
H.sub.2 contained in the silicon nitride film 505 to the
outside.
[0256] FIG. 14B shows that Samples 5 and 6 each have a peak of
H.sub.2O at around 50.degree. C. to 100.degree. C., and this peak
occurs because of absorbed moisture on a surface of the sample.
When Sample 5 is compared with Sample 6, the amount of released
H.sub.2O in Sample 6 is smaller than that in Sample 5. It is
indicated that there is a possibility that the amount of absorbed
moisture on the surface of the sample was reduced when the metal
oxide film 514 is an outermost surface.
[0257] As described above, a metal oxide film is formed over a
silicon nitride film, so that even when moisture, hydrogen, or the
like is contained in the silicon nitride film, release thereof can
be suppressed by the metal oxide film.
[0258] This example can be combined with any of the embodiments as
appropriate.
[0259] This application is based on Japanese Patent Application
serial no. 2011-189717 filed with Japan Patent Office on Aug. 31,
2011, the entire contents of which are hereby incorporated by
reference.
* * * * *