U.S. patent number RE36,836 [Application Number 08/510,422] was granted by the patent office on 2000-08-29 for semiconductor device for driving a light valve.
This patent grant is currently assigned to Agency of Industrial Science and Technology, Seiko Instruments Inc.. Invention is credited to Yutaka Hayashi, Masaaki Kamiya, Yoshikazu Kojima, Hiroaki Takasu.
United States Patent |
RE36,836 |
Hayashi , et al. |
August 29, 2000 |
Semiconductor device for driving a light valve
Abstract
The invention provides a semi-conductor light valve device and a
process for fabricating the same. The device comprises a composite
substrate having a supporte substrate, a light-shielding thin film
formed on said supporte substrate and semiconductive thin film
disposed on the light-shielding thin film with interposing an
insulating thin film. A switching element made of a transistor and
a transparent electrode for driving light valve are formed on the
semiconductive thin film, and the switching element and the
transparent electrode are connected electrically with each other.
The transistor includes a channel region in the semiconductive thin
film and a main gate electrode for controlling the conduction in
the channel region, and the light-shielding thin film layer is so
formed as to cover the channel region on the side opposite to said
channel region, so as to prevent effectively a back channel and
shut off the incident light.
Inventors: |
Hayashi; Yutaka (Tsukuba,
JP), Kamiya; Masaaki (Tokyo, JP), Kojima;
Yoshikazu (Tokyo, JP), Takasu; Hiroaki (Tokyo,
JP) |
Assignee: |
Agency of Industrial Science and
Technology (JP)
Seiko Instruments Inc. (JP)
|
Family
ID: |
17583540 |
Appl.
No.: |
08/510,422 |
Filed: |
August 2, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
771756 |
Oct 4, 1991 |
05233211 |
Aug 3, 1993 |
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Foreign Application Priority Data
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Oct 16, 1990 [JP] |
|
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2-277436 |
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Current U.S.
Class: |
257/72; 257/347;
349/44; 349/43; 257/659 |
Current CPC
Class: |
H01L
29/78648 (20130101); H01L 29/78645 (20130101); H01L
21/86 (20130101); G02F 1/1368 (20130101); H01L
27/1259 (20130101); H01L 29/78633 (20130101); H01L
29/78654 (20130101); G02F 1/13454 (20130101); H01L
27/1203 (20130101); G02F 1/136209 (20130101); H01L
29/66772 (20130101); H01L 21/84 (20130101); H01L
27/1214 (20130101); Y10S 438/977 (20130101); Y10S
148/012 (20130101); G02F 1/133512 (20130101); Y10S
148/15 (20130101); G02F 2202/105 (20130101); G02F
1/13613 (20210101) |
Current International
Class: |
G02F
1/1362 (20060101); G02F 1/1368 (20060101); G02F
1/13 (20060101); H01L 27/12 (20060101); H01L
29/786 (20060101); H01L 21/336 (20060101); H01L
21/84 (20060101); H01L 21/86 (20060101); H01L
29/66 (20060101); H01L 21/70 (20060101); H01L
21/02 (20060101); G02F 1/1335 (20060101); H01L
029/04 (); H01L 027/01 () |
Field of
Search: |
;257/347,659,72
;359/54,59,62,82,87,88,42,43,44 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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304824 |
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Mar 1989 |
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EP |
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57-167655 |
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Oct 1982 |
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JP |
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59-126639 |
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Jul 1984 |
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JP |
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59-224165 |
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Dec 1984 |
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JP |
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60-081869 |
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May 1985 |
|
JP |
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60-143666 |
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Jul 1985 |
|
JP |
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62-005661 |
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Jan 1987 |
|
JP |
|
1-259565 |
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Oct 1989 |
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JP |
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2-154232 |
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Jun 1990 |
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JP |
|
Primary Examiner: Tran; Minh-Loan
Attorney, Agent or Firm: Adams & Wilks
Claims
What is claimed is:
1. A semiconductor device comprising:
a composite substrate having a stacked-layer structure comprising a
support substance composed of a light-transmitting electrically
insulating material, a light-shielding thin film, and a
semiconducting thin film composed of single crystal silicon having
a lattice defect density smaller than 500 defects/cm.sup.2 and
thermo-compression bonded on the support substrate with the
light-shielding thin film interposed therebetween;
a transparent electrode formed on a portion of said composite
substrate where at least said light-shielding thin film is removed;
and
a switching element connected electrically to said transparent
electrode and comprising a transistor having a channel region
formed in said semiconducting thin film and a main gate electrode
for controlling the conduction in said channel region, wherein the
light-shielding thin film layer is disposed on the side opposite to
said main gate electrode with respect to said channel region.
2. A semiconductor device according to claim 1, wherein said
support substrate is made of aluminum oxide.
3. A semiconductor device according to claim 1, wherein the
light-shielding thin film is formed on the support substrate, and
an insulating film is formed on at least the light-shielding thin
film.
4. A semiconductor device according to claim 3, including a ground
film interposed between the support substrate and the
light-shielding thin film.
5. A semiconductor device according to claim 10, wherein said
ground film is made of oxynitrile.
6. A semiconductor device according to claim 4, wherein said ground
film is made of silicon oxide, and said support substrate is made
of quartz consisting mainly of silicon oxide.
7. A semiconductor device according to claim 3, wherein said
insulating film is made of silicon nitride.
8. A semiconductor device according to claim 3, wherein said
insulating film is made of silicon oxide.
9. A semiconductor device according to claim 3, wherein said
insulating film is a multi-layer film consisting of silicon nitride
and silicon oxide.
10. A semiconductor device according to claim 1, including an
insulating film formed on the semiconducting thin film at the side
of the support substrate, the light-shielding thin film being
formed on the insulating film.
11. A semiconductor device according to claim 1, wherein said
light-shielding thin film is made of an electrically conductive
material.
12. A semiconductor device according to claim 11, wherein said
light-shielding thin film is made of polysilicon.
13. A semiconductor device according to claim 1, wherein said
light-shielding thin film contains at least one of germanium,
silicon-germanium, and silicon.
14. A light valve device comprising:
a composite structure having a stacked-layer structure comprising a
support substrate comprised of a light-transmitting electrically
insulating material, a light-shielding thin film, and a
semiconducting thin film composed of a single crystal silicon
having a lattice defect density smaller than 500 defects/cm.sup.2
and thermo-compression bonded on the support substrate with the
light-shielding thin film interposed therebetween;
a pixel electrode formed on .[.said semiconducting thin film.].
.Iadd.a portion of said composite substrate where at least said
light-shielding
thin film is removed .Iaddend.and a switching element formed
.[.on.]. .Iadd.in .Iaddend.said semiconducting thin film for
energizing said pixel electrode;
an opposed substrate disposed opposite to said composite substrate
at a predetermined gap; and
an electro-optical material filling up said gap for undergoing
optical change in accordance with energization of said pixel
electrode.
15. A light valve device according to claim 14, wherein said
switching element comprises a transistor having a channel region
formed in said semiconductor thin film and a main gate electrode
for controlling the conduction of said channel region, and said
light-shielding thin film is provided on the side opposite to said
main gate electrode with respect to said channel region.
Description
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to light valve devices of a flat
plate type, such as active matrix liquid crystal panels used for
display apparatus of a direct viewing type or display apparatus of
a projection type. More specifically, the invention relates to a
semiconductor device that is incorporated as a substrate of the
liquid crystal panel and that has electrodes and switching elements
to directly drive the liquid crystal.
2. Description of the related background Art
The active matrix device is based on a simple principle in which
each pixel is equipped with a switching element and when a
particular pixel is to be selected, the corresponding switching
element is made conductive and when it is not selected, the
switching element is made nonconductive. The switching elements are
formed on a glass substrate which forms part of the liquid crystal
panel. Therefore, the technology for thinning and miniaturizing the
switching elements is very important. Thin-film transistors are
ordinarily used as such switching elements.
In an active matrix device, so far, thin-film transistors have been
formed on the surface of a thin silicon film that is deposited on
the glass substrate. Such transistors are generally of a field
effect insulated gate type. A transistor of this type is
constituted by a channel region formed in the thin silicon film and
a gate electrode which is so formed as to cover the channel region.
A predetermined voltage is applied to the gate electrode to control
the conductance of the channel region and to carry out the
switching operation.
The conventional insulated gate-type thin film transistors have a
problem in that a leakage current flows into the channel region
through the back
side of the thin film even when the channel region is made
nonconductive by controlling the gate voltage. That is, so-called
back channel takes place impairing the proper operation of the
active matrix device. Namely, in order to operate the pixels at
high speeds by a line sequence system, the conductance ratio of the
switching elements between the conductive state and the
nonconductive state must be greater than 10.sup.6. In fact,
however, the back channel makes it difficult to obtain the required
switching performance.
Even if the back channel were extinguished, the channel region of
the thin-film transistor exhibits an increased conductance if light
from an external source falls thereon since the semiconductor
device is used under the illuminated condition, causing leakage
current in the drain and source in the nonconductive state.
Further, the ratio of this leakage current to the leakage current
when the semiconductor device is not illuminated increases with
increase in the quality of the semiconductor thin film such as
single crystal that forms the channel regions, causing another
problem.
In view of the above-mentioned conventional problems, the object of
the present invention is to provide a semiconductor device for
driving light valves of a flat plate type having thin film
transistors of a structure which is capable of effectively
preventing the back channel and shutting off the incident
light.
SUMMARY OF THE INVENTION
In order to accomplish the above object, a semiconductor device
according to the present invention is formed by using a substrate
having a stacked structure which includes an insulating support
substrate made of a light-transmitting material, a light-shielding
thin film provided on the support substrate, and a semiconductor
thin film provided on the light-shielding thin film via an
insulating film. Transparent electrodes for driving the light
valve, i.e., pixel electrodes, are arranged on the support
substrate. Furthermore, switching elements are formed to
selectively excite the pixel electrodes. Each of the switching
elements consists of a field effect insulated gate type transistor
having a channel region and a main gate electrode for controlling
the conductance of the channel region. The channel region is formed
in the semiconductor thin film, and the main gate electrode is so
formed as to cover the channel region. A light-shielding layer is
formed separately from the main gate electrode. The light-shielding
layer is constituted by a light-shielding thin film and is provided
on the side opposite to the main gate electrode with respect to the
channel region. That is, the channel region of the transistor is
sandwiched between the main gate electrode and the light-shielding
layer vertically.
Preferably, both the main gate electrodes disposed on the
transistor channel region and sub-gate electrodes under the
transistor channel region are made of a light-shielding material
and together with the light-shielding layer nearly completely shuts
off light entering the channel region from the outside.
More preferably, the light-shielding layer is made of an
electrically conductive material to eliminate the back channel. It
is further possible to supply an.. electric current to the
light-shielding layer in order to control the back channel.
Further, the channel region of the transistor is preferably formed
in a semiconducting thin film of single silicon crystal and can be
processed on the order of sub-microns using an ordinary LSI
technology.
According to the present invention, the conductance in the channel
region of each transistor constituting a switching element is
controlled by a couple of main gate and sub-gate electrodes on both
surfaces of the semiconducting thin film via insulating films.
Therefore, there develops no back channel unlike the structure
controlled by one gate electrode on one surface thereof only as in
a conventional thin film transistor. In other words, the sub-gate
electrodes according to the present invention are provided in order
to suppress the back channel.
In addition, since the channel region is covered by a couple of
light-shielding gate electrodes from the upper and lower sides,
light incident or the light valve device passes through the pixel
electrode but is nearly completely shut off in the channel region,
making it possible to effectively prevent the generation of
photoelectric current.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial sectional view which schematically illustrates
the structure of a flat-type semiconductor device for driving light
valves;
FIG. 2 is a partial sectional view which schematically illustrates
the structure of a composite substrate used for fabricating the
above semiconductor device;
FIGS. 3(A)-3(H) and 4(A)-4(J) are schematic diagrams illustrating
different steps for fabricating the flat-type semiconductor device
for driving light valves; and
FIG. 5 is a perspective exploded view which schematically
illustrates the structure of the light valve device of a flat
plate-type constituted by using the semiconductor device of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of the present invention will now be
described in detail with reference to the drawings. FIG. 1 is a
schematic partial sectional view of a semiconductor device for
driving light valves of a flat plate-type. The device is of a
laminated layer structure 2 formed on a support substrate 1. The
laminated layer structure 2 includes a light-shielding thin film
and a semiconducting thin film formed on the light-shielding thin
film via an insulating film. A transparent electrode for driving a
respective light valve, i.e., pixel electrode 3 is formed on the
surface of a composite substrate which is of a stacked-layer
structure 2. Further, in the stacked-layer structure 2 is
correspondingly formed a switching element 4 to selectively excite
the pixel electrode 3. The switching element 4 has a drain region 5
and a source region 6 that are spaced formed in a semiconducting
thin film and are spaced apart from each other. The drain region 5
is connected to a signal line 12 and the source region 6 is
connected to the corresponding pixel electrode 3. Moreover, a
channel region 7 is provided between the drain region 5 and the
source region 6. A main gate electrode 9 is formed on the front
surface side of the channel region 7 via a gate insulating film 8.
The main gate electrode 9 is connected to a scanning line that is
not shown and controls the conductance of the channel region 7 in
order to turn the switching element 4 on or off. A light-shielding
layer 11 is arranged on the back surface side of the channel region
7 via an insulating layer 10. That is, the light-shielding layer 11
is arranged on the side opposite to the main gate electrode 9 with
respect to the channel region 7. The light-shielding layer 11
consists of the aforementioned light-shielding thin film. When the
light-shielding thin film is electrically conductive, the
light-shielding layer 11 also works as a sub-gate electrode that
controls back channels.
The pair of main and sub-gate electrodes 9 and 11 arranged on both
sides of the channel region 7 are made of light-shielding material
and, hence, completely shut off the light incident on the channel
region 7.
In this embodiment, furthermore, the channel region 7 is formed of
a semiconducting thin film of a single silicon crystal to which
ordinary LSI processing technologies can be directly adapted.
Therefore, the channel length can be shortened to the order of
submicrons.
FIG. 2 is a schematic partial sectional view of a composite
substrate used for fabricating a semiconductor device for driving
light valves of a flat plate-type according to the present
invention. The composite substrate consists of the support
substrate 1 and the stacked-layer structure 2 formed thereon as
shown. First, the support substrate 1 is composed of a
light-transmitting insulating material such as a heat-resistant
quartz consisting chiefly of silicon oxide or aluminum oxide.
Aluminum oxide exhibits a coefficient of thermal expansion which is
close to that of silicon and gives advantage in regard to
suppressing stress. Furthermore, since single crystals of aluminum
oxide can be formed, it is possible to heteroepitaxially grow a
single crystalline semiconductor film thereon. Next, the laminated
layer structure 2 has a light-shielding thin film 21 arranged on
the support substrate 1, an insulating film 22 arranged on the
above light-shielding thin film 21, and a semiconducting thin film
23 that is arranged on the insulating film 22 and is composed of a
single crystalline material adhered to the support substrate 1. The
light-shielding thin film 21 is composed of an electrically
conductive material such as polysilicon. Alternatively, it is also
possible to use a single layer film of germanium, silicon-germanium
or silicon, or a multi-layer film of silicon including at least one
layer of germanium or silicon-germanium instead of the single layer
film of polysilicon. It is further possible to use a metal film
such as of silicide or aluminum instead of the above semiconductor
materials. When aluminum oxide, i.e., sapphire, is used as the
support substrate, furthermore, a single silicon crystal can be
heteroepitaxially grown thereon to form a light-shielding film.
The stacked-layer structure 2 may include a ground film 24 that is
interposed between the support substrate 1 and the light-shielding
thin film 21. The ground film 24 is provided to improve the
adhesion between the support substrate 1 and the stacked-layer
structure 2. For instance, when the support substrate 1 is of
quartz that consists mainly of silicon oxide, there can be used
silicon oxide as the ground film 24. When it is desirable for the
ground film 24 to block the infiltration of impurities from the
support substrate 1, the ground film should be composed of a layer
of silicon nitride or oxynitride, or a multilayer film of at least
either one of them and silicon oxide. In particular, the oxynitride
is useful since it is capable of adjusting the stress.
Next, the insulating film 22 is used as a gate insulating film for
the sub-gate electrode which consists of the light-shielding thin
film 21, and is composed of, for instance, silicon oxide or silicon
nitride. Or, the insulating film 22 may be constituted by a
multi-layer film of silicon nitride and silicon oxide.
The semiconducting thin film 23 positioned at the top of the
stacked-layer structure 2 is made, for example, of silicon. This
silicon may be of a single crystalline form, polycrystalline form
or amorphous form. The amorphous silicon thin film or the
polycrystalline silicon thin film can be easily deposited on a
glass substrate by chemical vapor deposition, and is adaptable to
manufacture of an active matrix device having a relatively large
screen. When such an amorphous silicon thin film is used, it is
possible to make a active matrix liquid crystal device having a
screen size of about thee inches to ten inches. In particular, the
amorphous silicon thin film can be formed at a temperature as low
as 350.degree. C. or less, and is suitable for liquid crystal
panels of large areas. When a polycrystalline silicon thin film is
used, it is possible to make small liquid crystal panels of about
two inches.
In the case of using a polycrystalline silicon thin film, however,
the reproducibility of element constant is poor and variations
increase when a transistor is formed having a channel length on the
order of submicrons by adopting fine semiconductor processing
technology. When amorphous silicon is used, furthermore, a
high-speed switching cannot be realized even though the submicron
processing technology is employed. When a semiconducting thin film
of a single crystal of silicon is used, on the other hand, it is
possible to directly adopt fine semiconductor processing technology
to greatly increase the density of the switching elements and to
obtain an ultra-fine valve device.
Even though the switching element has a channel length on the order
of microns, a high channel mobility makes it possible to realize a
high-speed operation. It is further possible to integrate
peripheral circuits for controlling the switching elements on the
same support substrate in a large scale and to control the array of
switching elements at high speeds, so that the light valve device
manufactured by use of semiconductor thin film of silicon single
crystal is indispensable for displaying highly fine moving
pictures.
Described below in detail with reference to FIG. 3 is a method of
fabricating a semiconductor device according to the present
invention. First, a composite substrate is prepared in a process
shown in FIG. 3(A). That is, the ground film 24 of silicon oxide is
formed on the support substrate 1 of a polished quartz plate by a
chemical vapor deposition method of sputtering. The light-shielding
thin film 21 made of polysilicon is deposited on the ground film 24
by a chemical vapor deposition method. Then, the insulating film 12
made of silicon oxide is formed on the light-shielding thin film 21
by a thermal oxidation method or chemical vapor deposition method.
Lastly, the semiconductor thin film 23 of a single silicon crystal
is formed on the insulating film 22. The semiconductor thin film 23
is obtained by adhering the semiconducting substrate of a single
silicon crystal to the insulating film 22 and then polishing it
until its thickness becomes several .mu.m. The single crystalline
silicon semiconductor substrate to be used is preferably a silicon
wafer of high quality that is used for manufacturing LSIs having
uniform crystal orientation within a range of [100]0.0.+-.1.0 and a
single crystal lattice defect density of smaller than 500
defects/cm.sup.2. The surface of the silicon wafer having such
physical properties is, first, finished precisely and smoothly.
Then, the smoothly finished surface is superposed on the insulating
film 22 followed by heating to adhere the silicon wafer to the
support substrate 1 by thermocompression bonding. By
thermocompression bonding, the silicon wafer and the support
substrate 1 are firmly adhered to each other. In this state, the
silicon wafer is polished until a desired thickness is obtained.
Here, it is possible to carry out an etching instead of the
polishing. The thus obtained single crystalline silicon
semiconductor thin film 23 substantially keeps the quality of the
silicon wafer; therefore a semiconductor substrate material is
obtained having a very excellent uniformity in the crystal
orientation and a low lattice defect density.
By current technologies, the surface of the silicon wafer that is
adhered by thermocompression bending has electrical defects to some
extent, and the following process is further preferable. SiO.sub.2
is formed in the single crystalline wafer by thermo oxydation or
CVD. Then, polysilicon is formed by CVD and the surface is polished
as required. Thereafter, SiO.sub.2 is formed by thermal oxidation
or CVD, silicon nitride film is formed by CVD and SiO.sub.2 is
formed by thermal oxidation or CVD in the order mentioned. The
silicon wafer is adhered by thermocompression bonding onto a quartz
support substrate or a quartz support substrate coated with
SiO.sub.2 by CVD and is, then, polished.
Next, in a step shown in FIG. 3(B), the stacked-layer structure 2
is etched in order except the ground film 24 in order to form the
light-shielding layer 11 of the light-shielding thin film 21 on the
undermost layer, namely, the ground film 24. At the same time, the
gate oxide film 10 of an insulating film 22 is formed on the
light-shielding layer 11. The light-shielding layer 11 is formed by
applying a photosensitive film 26 onto the whole surface of the
composite substrate followed by patterning into a desired shape
and, then, effecting an etching selectively using the patterned
photosensitive film 26 as a mask.
Then, in a step shown in FIG. 3(C), an element region 25 is formed
on the two-layer structure consisting of the patterned
light-shielding layer 11 and the gate oxide film 10. The element
region 25 is obtained by selectively etching only the
semiconducting thin film 23 into a desired shape. The
semiconducting thin film 23 is selectively etched by using, as a
mask, the photosensitive film 26 that is patterned to the shape of
the element region.
In a step shown in FIG. 3(D), furthermore, the photosensitive film
26 is removed and, then, a thermally oxidized film is formed on the
whole surface inclusive of the surface of the semiconducting thin
film 23 that is exposed. As a result, a gate oxide film 8 is formed
on the surface of the semiconducting thin film 23.
Then, in a step shown in FIG. 3(E), a polycrystalline silicon film
is
deposited by a chemical vapor deposition method to cover the
element region 25. The polycrystalline silicon film is selectively
etching using a photosensitive film (not shown) that is patterned
into a desired shape in order to form the main gate electrode 9.
The main gate electrode 9 is positioned above the semiconducting
thin film 23 via the gate oxide film 8.
In a step shown in FIG. 3(F), impurity ions are injected through
the gate oxide film 8 using the main gate electrode 9 as a mask, in
order to form the drain region 5 and the source region 6 in the
semiconducting thin film 23. As a result, there is formed a
transistor channel region 7 that contains no impurity under the
main gate electrode 9 between the drain region 5 and the source
region 6.
Next, in a step shown in FIG. 3(G), a protective film 27 is formed
to cover the element region. As a result, the switching element
that includes the light-shielding layer 11 and the main gate
electrode 9 is buried under the protective film 27.
Lastly, in a step shown in FIG. 3(H), the gate oxide film 8 on the
source region 6 is partly removed to form a contact hole, and a
transparent pixel electrode 3 is so formed as to cover this
portion. The pixel electrode 3 is made of a transparent material
such as ITO. In addition, the protective film 27 provided on the
lower side of the pixel electrode 3 is made, for example, of
silicon oxide and is transparent. Moreover, the support substrate 1
of quartz glass on the lower side thereof is transparent, too.
Therefore, the three-layer structure consisting of the pixel
electrode 3, protective film 27 and quartz glass support substrate
1, is optically transparent and can be utilized for a light valve
device of a transmission type.
The pair of main gate and sub-gate electrodes 9 and 11 that
vertically sandwich the channel region 7 are made of polysilicon
which is optically opaque in contrast to the above three-layer
structure, and shut off the entering light and prevent leakage
current from flowing into the channel region. The incident light
can be perfectly shut off by using a material having a low band gap
such as silicon, germanium or the like.
In the manufacturing method shown in FIG. 3 as described above, the
semiconductor thin film 23 of high-quality single crystalline
silicon is processed at a temperature higher than 600.degree. C.,
followed by a photolithoetching technique with high resolution and
by ion implantation, making it possible to form a field-effect
insulated gate transistor having a size of the order of microns or
submicrons. The single crystalline silicon film of a very high
quality is used, and, hence, the obtained insulated gate transistor
exhibits excellent electric characteristics. At the same time, the
pixel electrode 3 can be formed is a size of the order of microns
by the miniaturization technology, making it possible to fabricate
a semiconductor device for active matrix liquid crystal having a
high density and fine structure.
FIG. 3 shows the embodiment in which the single crystalline
semiconductor film 23 is formed by thermocompression bonding.
Referring to FIG. 4, another embodiment is shown where the single
crystalline semiconductor film is formed by an epitaxial method
instead of the thermocompression bonding. First, a transparent
aluminum oxide 101 such as sapphire is used as the support
substrate as shown in FIG. 4(A). Next, as shown in FIG. 4(B), a
single crystalline silicon film 102 is heteroepitaxially grown
using the crystal of aluminum oxide 101 as a seed. In the
polycrystalline form, aluminum oxide has a coefficient of thermal
expansion that is closer to that of silicon than that of quartz.
When polycrystalline aluminum oxide is used as the support
substrate of the embodiment shown in FIG. 3, the thermal stress is
small and the crystallinity of the single crystalline silicon film
formed thereon can be maintained even after the semiconductor
processing at a high temperature. In FIG. 4 showing a process where
single crystalline aluminum oxide is used, it is possible to
hetero-epitaxially grow the single crystalline silicon film 102 as
shown in FIG. 4(B). Next, the single crystalline silicon film 102
that is grown is patterned as shown in FIG. 4(C) to form a
light-shielding film 111. Then, an insulating film 110 is formed as
shown in FIG. 4(D), a hole 112 is made in a portion thereof, so
that the surface of the single crystalline silicon film 111 is
partially exposed as shown in FIG. 4(E). Next, an amorphous or
polycrystalline semiconductor film 123 is formed as shown in FIG.
4(F). The single crystalline silicon film 111 and the semiconductor
film 123 are in contact with each other in the hole 112. If the
heat treatment is carried out in this state at a high temperature,
the semiconductor film 123 grows laterally and epitaxially with the
single crystalline silicon film 111 in the hole as a seed. As shown
in FIG. 4(G), therefore, the regions 123A close to the hole is
transformed into a single crystal. The region 123B that is not
transformed into a single crystal is kept in a polycrystalline
form. Though FIG. 4(F) illustrates an example in which the
polycrystalline semiconductor film 123 is, first, formed and is
then grown laterally and epitaxially by a heat treatment, it should
be noted that the single crystalline semiconductor film can also be
formed, as shown in FIG. 4(G), even by gas-source epitaxy or
liquid-phase epitaxy from the state shown in FIG. (E). Use can be
made of a silicon film or a GaAs film as a semiconducting film.
Next, as shown in FIG. 4(H), a region 124, that serves as the
substrate of a transistor is patterned. Next, as shown in FIG.
4(I), a gate insulating film 108 is formed and, finally, a
transistor is formed in which a transparent electrode 103 is
connected with a drain region 106 as shown in FIG. 4(J). The
conductance of a channel region 107 between the source region 105
and the drain region 106 is controlled through the gate electrode
125 and the light-shielding film 111. FIG. 4(J) illustrates an
example in which the light-shielding film 111 is connected to the
source region 105 which, however, is not necessary. In FIG. 4(G),
the single crystalline region is grown laterally and epitaxially so
long as 3 to 5 .mu.m, enabling a single crystalline transistor to
be formed on the insulating film as shown in FIG. 4(J).
Lastly, described below with reference to FIG. 5 is an optical
valve device that is assembled by using the semiconductor device of
the present invention. As shown, the light valve device is
constituted by a semiconductor device 28, a facing substrate 29
facing the semiconductor device 28, and an electrooptical material
layer such as a liquid crystal layer 30 between the semiconductor
device 28 and the facing substrate 29. In the semiconductor device
28 are formed pixel electrodes or drive electrodes 3 that define
pixels and switching elements 4 for exciting the drive electrodes 3
in response to predetermined signals.
The semiconductor device 28 consists of the support substrate 1
composed of quartz glass and the laminated layer structure 2 formed
on the support substrate 1. In addition, a polarizer plate 31 is
adhered to the back surface of the support substrate 1. The
switching elements 4 are formed in a single crystalline silicon
semiconductor thin film included in the stacked-layer structure 2.
The switching elements 4 are constituted by a plurality of
field-effect insulated gate transistors arranged in a matrix. The
source region of each transistor is connected to a corresponding
pixel electrode 3, the main gate electrode is connected to the
scanning line 32, and the drain electrode is connected to the
signal line 7.
The semiconductor device 28 further includes an X-driver 33
connected to the signal lines 7 arranged in columns, and includes a
Y-driver 34 connected to the scanning lines 32 arranged in rows.
The facing substrate 29 is constituted by a glass substrate 35, a
polarizing plate 36 adhered to the outer surface of the glass
substrate 35, and a facing electrode or a common electrode 37
formed on the inner surface of the glass substrate 35.
Though not diagramed, the light-shielding layer or the sub-gate
electrode included in each switching element 4 is preferably
connected to the scanning line 32 together with the main gate
electrode. The above connection makes it possible to effectively
prevent leakage current from flowing into the channel region of the
transistor that constitutes the switching element. Or, the
light-shielding layer can be connected to the source region or the
drain region of the corresponding transistor. In any way, a
predetermined voltage is applied to the light-shielding layer to
effectively prevent leakage current from flowing due to the back
channel. By controlling the voltage applied to the light-shielding
layer, furthermore, the threshold voltage of the channel region can
be set to a desired value.
Operation of the thus constituted light valve device will now be
described in detail with reference to FIG. 5. The main gate and
sub-gate electrodes of each switching element 4 are connected in
common to the scanning line 32, and are supplied with scanning
signals from the Y-driver 34; therefore the turn-on/off of the
switching elements 4 is controlled line by line. A data signal
output from the X-driver 33 is applied, via signal line 7, to a
selected switching element 4 that has been turned on. The applied
data signal is transmitted to the corresponding pixel electrode 3
to excite it and acts upon the liquid crystal layer 30, so that its
transmission factor becomes substantially 100%. When not selected,
on the other hand, the switching element 4 is left nonconductive
and maintains the data signal previously written on the pixel
electrode as electric charge. Here, the liquid crystal layer 30 has
a large resistivity and usually operates a capacitive element.
An on/off current ratio is used to represent the switching
performance of the switching elements 4. The current ratio
necessary for operating the liquid crystal can be easily found from
the write time and the holding time. For instance, when the data
signal is a television signal, more than 90% of the data signal
must be written within about 60 .mu.sec of one scanning period. On
the other hand, more than 90% of the electric charge must be
retained for a period of one field which is about 16 msec.
Therefore, the current ratio must be at least on the order of ten
thousand. In this respect according to the present invention, the
conductance of the channel region is controlled from both surfaces
thereof by the main gate and sub-gate electrodes, and the leakage
current is substantially perfectly eliminated during the off
period. The on/off ratio of the switching elements having such a
structure is on the order of a hundred thousand or greater. It is
therefore possible to obtain a light valve device of an active
matrix type having a very fast signal response characteristic.
According to the present invention as described above, a pair of
main gate and sub-gate electrodes arranged on both sides of the
transistor channel region are formed using a light-shielding
material such as polysilicon, and whereby the light entering the
channel region from outside is effectively shut off, making it
possible to effectively prevent the occurrence of leakage current
by the photo-electric effect. Moreover, since the transistor
channel region formed in the semiconductor thin film is controlled
from the upper and lower sides thereof by the main gate electrode
and by the light-shielding layer, i.e., the sub-gate electrode
composed of an electrically conductive material, the so-called back
channel is effectively prevented, and a thin-film transistor having
a very excellent on/off ratio is obtained. As a result, there can
be obtained a semiconductor device for driving light valves of a
flat plate-type featured by very high response characteristics and
free from erroneous operation. In addition, by forming the
switching elements consisting of field-effect insulated gate
transistors in the semiconducting thin film of single crystalline
silicon, it is possible to obtain a flat-type semiconductor device
for driving light valves that has a very fine and very large scale
integration structure.
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