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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and a method for
manufacturing semiconductor devices which use semiconductor material
having crystallinity.
2. Description of the Related Art
A thin film transistor (hereinafter referred to as "TFT") using thin film
semiconductor material has been well known. The TFT is constructed by
forming thin film semiconductor on a substrate and using the thin film
semiconductor. The TFT has been used for various integrated circuits,
especially an electrooptical device, and particularly much attention is
paid to the TFT as a switching device which is provided to each pixel of
an active matrix type of liquid crystal display device, or a driver device
formed in its peripheral circuits.
Amorphous silicon film is most easily used as thin film semiconductor for
the TFT, however, it has a problem that its electric characteristics is
not good. In order to improve the characteristics of the TFT, it is a
better way to use a silicon film having crystallinity as thin film
semiconductor. The crystalline silicon film is known as polycrystal
silicon, polysilicon or microcrystal silicon or the like. In order to
obtain this crystalline silicon, an amorphous silicon film is first formed
and then crystallized by heating.
However, the crystallization by heating requires a heating temperature of
600.degree. C. or more and a heating time above 10 hours, so that it is
difficult to use a glass substrate as a substrate. For example, a glass
strain temperature of Corning 7059 glass which is used for an active
matrix type liquid crystal display device is equal to 593.degree. C.
Therefore, the heating temperature exceeding 600.degree. C. causes some
problem when a large-scale substrate is required to be used.
SUMMARY OF THE INVENTION
In order to solve the above problems, the same inventors as this
application previously proposed a method of manufacturing semiconductor
devices as disclosed in Japanese patent Application No. Hei-5-294633. In
the method as disclosed in this patent application, a crystalline silicon
film was obtained by adding catalysts, especially nickel to an amorphous
silicon film with solution and performing a heating treatment at a low
temperature and for a short time. The present invention has been made to
improve the apparatus and method of manufacturing semiconductor devices
according to the Japanese patent Application No. Hei-5-294633 which was
previously proposed by the inventors of this application, and through
various studies on the above apparatus and method, the inventors of this
application have proposed the following two crystallizing methods. In one
method, an area where crystal growth is made in a substantially
perpendicular direction to the substrate in a region which is directly
doped with catalysts (hereinafter referred to as "longitudinal growth
area") is used as a device-forming area. On the other hand, in the other
method, catalysts are selectively added, and an area where crystal growth
is made in a substantially horizontal direction to the substrate in a
peripheral region of the catalysts-doped region (hereinafter referred to
as "lateral growth area) is used as a device-forming area.
Through various studies of these two crystallizing methods, it has been
concluded that the latter method using the lateral crystal growth process
is more preferable on the characteristics of completed devices, and the
inventors have made a further consideration on this lateral-growth
crystallizing method.
An object of the present invention is to improve the apparatus and method
disclosed in the Japanese Patent Application No. Hei-5-294633, and it is
to provide an apparatus and a method of manufacturing thin film
semiconductor having crystallinity by a heating treatment at a temperature
of 600.degree. C. or less using catalysts, which has higher
controllability, larger process margin and higher productivity than the
Japanese patent Application No. Hei-5-294633.
In order to attain above object, according to a first aspect of the present
invention, a method of manufacturing a semiconductor device, comprises a
step of forming a silicon oxide film and an amorphous silicon film on a
substrate having an insulating surface, a step of performing a heat
treatment on the film-formed substrate sequentially to the film-forming
step without exposing the film-formed substrate to atmospheric air to
thereby remove hydrogen, a step of forming a silicon nitride film on the
hydrogen-removed substrate sequentially to the hydrogen-removing step, a
step of patterning the silicon nitride film to selectively expose the
amorphous silicon film, a step of doping metal elements so as to be
contacted with the exposed amorphous silicon film to promote crystal
growth of the exposed amorphous silicon film, and a step of performing a
heat treatment on the amorphous silicon film to crystallize the amorphous
silicon film in a direction parallel to the substrate from an area in
which the metal elements are doped.
According to another aspect of the present invention, an apparatus for
manufacturing semiconductor devices, comprises a first treatment chamber
for forming a silicon oxide film and an amorphous silicon film on a
substrate having an insulating surface, a second treatment chamber for
performing a heat treatment on the film-formed substrate to remove
hydrogen therefrom sequentially to the film-forming operation in the first
treatment chamber without exposing the substrate to atmospheric air, a
third treatment chamber for forming a silicon nitride film on the
hydrogen-removed substrate sequentially to the hydrogen-removing operation
in the second treatment chamber, and a common chamber which commonly
intercommunicates with the first treatment chamber, the second treatment
chamber and the third treatment chamber, wherein the first, second and
third treatment chambers are designed in a hermetic structure, and the
common chamber has means for feeding a substrate or sample.
According to another aspect of the present invention, a method of
manufacturing semiconductor devices, comprises a step of forming a silicon
oxide film and an amorphous silicon film on a substrate having an
insulating surface, a step of performing a heat treatment on the
film-formed substrate sequentially to the film-forming step without
exposing the film-formed substrate to atmospheric air to thereby remove
hydrogen, a step of forming a silicon nitride film on the hydrogen-removed
substrate sequentially to the hydrogen-removing step, a step of patterning
the silicon nitride film in the form of an active layer to selectively
expose the amorphous silicon film, a step of doping metal elements so as
to be contacted with the exposed amorphous silicon film to promote crystal
growth of the exposed amorphous silicon film, a step of performing a heat
treatment on the amorphous silicon film to crystallize the amorphous
silicon film in a direction parallel to the substrate from an area in
which the metal elements are doped, and a step of patterning the
crystallized silicon film using the silicon nitride film as a mask to form
an active layer.
According to another aspect of the present invention, a method of
manufacturing semiconductor devices, comprises a step of forming a silicon
nitride film as a mask to form an active layer on an amorphous silicon
film which is formed on a substrate having an insulating surface, a step
of doping metal elements using the silicon nitride film as a mask to
promote crystal growth of the amorphous silicon film, a step of performing
a heat treatment to crystallize the amorphous silicon film, and a step of
forming an active layer using the silicon nitride film as a mask.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1C are diagrams showing a series of processes of manufacturing
a semiconductor device of an embodiment according to the present
invention;
FIG. 2 shows an apparatus of manufacturing a semiconductor device;
FIG. 3 shows the apparatus of manufacturing a semiconductor device;
FIGS. 4A to 4F are diagrams showing a series of processes of manufacturing
a semiconductor device;
FIGS. 5A to 5D are diagrams showing a series of processes of manufacturing
a semiconductor device; and
FIG. 6 shows a semiconductor device which is manufactured according to the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments according to the present invention will be described
hereunder with reference to the accompanying drawings.
Before describing preferred embodiments according to the present invention,
the lateral-growth crystallizing method as disclosed in the Japanese
Patent Application No. Hei-5-294633 will be briefly described with
reference to FIGS. 1A to 1C.
First, an undercoat film of silicon oxide is formed on a glass substrate
such as a Corning 7059 substrate or the like, and an amorphous silicon
film 12 is formed at a thickness of 100 to 5000 .ANG., preferably from 500
to 800 .ANG. on the substrate 11 by a plasma CVD treatment or a
pressure-reduced heating CVD treatment.
Subsequently, a mask material film 21 which is typically formed of silicon
oxide film is formed on the amorphous silicon film 12, and an opening
portion through which nickel will be added is formed in the mask material
21 so that the amorphous silicon film below the mask material 21 is
exposed to the opening portion. Thereafter, the surface of the amorphous
silicon film which is exposed to the opening portion is oxidized thinly if
occasion demands (it is represented by reference numeral 20 in FIG. 1),
and nickel is doped into the amorphous silicon film with solution 14
containing nickel.
The substrate which has been doped with nickel by the method as described
above is subjected to a heat treatment at 450.degree. to 600.degree. C.,
typically at about 550.degree. C. under an atmosphere of inert gas such as
N.sub.2 or the like or under an oxidizing atmosphere to form a crystalline
silicon film 25 whose crystal growth is made in a lateral direction.
An attempt was made to change the mask material from the silicon oxide film
to a silicon nitride film for a series of processes as described above,
and it was experimentally proved that a long-term heat treatment caused
nickel to pass through the silicon oxide film and reach the amorphous
silicon film when the silicon oxide film was used as the mask material.
This induces a phenomenon of longitudinal growth of passed nickel occurs,
and thus it was observed that the lateral growth was disturbed by the
longitudinal growth. On the other hand, the above phenomenon was not
observed when the silicon nitride film is used as the mask material.
However, in this case, it was also observed that the lateral growth degree
was somewhat smaller as compared with the silicon oxide mask. As a result
of an additional experiment, it was found out that the reduction in the
lateral growth degree can be avoided by removing hydrogen in advance. That
is, it is required as a pre-treatment for crystallization to remove
hydrogen from the amorphous silicon film. However, when the silicon
nitride film is used, it is difficult to remove hydrogen from the
amorphous silicon film.
Next, a comparison experiment was made for the following two cases, a case
where the undercoat film, the amorphous silicon film and the mask material
film were sequentially (continuously) formed on the substrate without
being exposed to the atmospheric air (hereinafter referred to as
"sequential film-forming process") and a case where these films were
separately formed on the substrate with being exposed to the atmospheric
air (hereinafter referred to as "separate film-forming process"). Through
this comparison experiment, it was proved that the lateral growth distance
was longer and the crystallinity was higher in the sequential film forming
process than the separate film forming process even though the same film
quality was set in both the cases. This fact would mean that the lateral
growth process that crystal growth is made in a substantially parallel
direction to the substrate is strongly affected by the condition of the
interface.
Accordingly, on the basis of the series of experiments as described above,
the following crystal growth method using the lateral crystal growth
having high reproducibility and high controllability includes; a step of
continuously (sequentially) forming a silicon oxide film and an amorphous
silicon film on a glass substrate, a step of performing a heat treatment
on the film-formed substrate sequentially to the film-forming step without
being exposed to atmospheric air to remove hydrogen from the substrate, a
step of forming a silicon nitride film on the hydrogen-removed substrate
sequentially to the hydrogen-removing step, a step of subjecting the
substrate having the silicon oxide film, the amorphous silicon film and
the silicon nitride film formed thereon to a patterning treatment and an
etching treatment of the silicon nitride film to partially expose the
amorphous silicon film, a step of coating the substrate with solution
containing nickel to doped nickel into the selectively exposed amorphous
silicon film, and a step of performing a heat treatment on the
nickel-coated substrate to crystallize the amorphous silicon film.
In order to achieve a series of processes as described above, an apparatus
in which a silicon oxide film, an amorphous silicon film and a silicon
nitride film can be formed in succession and a heat treatment (hydrogen
removing process) can be performed without exposing these films to
atmospheric air even only once is required as a multipurpose substrate
treatment device.
Specifically, the multipurpose substrate treatment includes plural
pressure-reducible treatment chambers, and a common pressure-reducible
chamber through which the plural treatment chambers intercommunicate with
one another, and which has substrate feeding means for feeding the
substrate between the common chamber and each treatment chamber, wherein
at least one of the treatment chambers is capable of forming a silicon
oxide film using a plasma CVD method, at least one of the treatment
chambers is capable of forming silicon nitride film using a plasma CVD
method, at least one of the treatment chambers is capable of forming an
amorphous silicon film using a plasma CVD method, and at least one of the
treatment chambers is capable of performing a heat treatment at
400.degree. C. or more on plural substrates at the same time.
The apparatus thus constructed is shown in FIGS. 2 and 3.
The apparatus shown in FIG. 2 can be used for multipurpose, and it can be
constructed by combining the desired number of treatment chambers which
are used to perform desired film forming processes and annealing
processes. As the substrate to be treated in the apparatus shown in FIG. 2
may be used a glass substrate, a silicon substrate, an insulating
substrate, a semiconductor substrate or the like. Namely, any substrate
may be used insofar as it has an insulating surface. For example, a glass
substrate which is inexpensive is generally used for an electrooptical
device such as an active matrix type liquid crystal display device, an
image sensor or the like.
For example, in the case of FIG. 2, the apparatus may be constructed as
follows. That is, a chamber 301 is used as a substrate carry chamber which
corresponds to the common chamber, chambers 306 and 307 of treatment
chambers in which various kinds of treatments are performed are used as
spare chambers, one of which is used for feed-in of the substrate and the
other of which is used for feed-out of the substrate, a chamber 302 is
used as a plasma CVD apparatus for forming an insulation film, a chamber
303 is used as a plasma CVD apparatus for forming an amorphous silicon
film, a chamber 304 is used as a plasma CVD apparatus for forming a
silicon nitride film, and a chamber 305 is used as a heat treatment
furnace to remove hydrogen. In the apparatus thus constructed, only the
heat treatment process needs a long treatment time of several hours and it
becomes the main factor of reducing the total throughput of this process.
Therefore, it is important to construct the apparatus so that plural
substrates 322 are simultaneously heated by a heater 310, carried to a
substrate feeding position by a stage 315 if necessary, and then carried
to a next process while fed by a robot arm 314. Here, the spare chambers
may be also called as treatment chambers in the meaning that these
chambers have functions of feeding in and out the substrate. The
respective treatment chambers are partitioned from one another by gate
valves 308-313, and they can be independently and individually
decompressed by vacuum pumps 319-321 respectively, so that gas occurring
at a treatment time can be prevented from contaminating into another
treatment chamber. The substrates 322 are carried by the robot arm 314,
and the throughput can be improved by the multitask.
Any combination like the combination shown in FIG. 2 can be freely
performed. As the elements to be combined may be used plasma CVD,
pressure-reduced heat CVD (hereinafter referred to as "LPCVD"), photo CVD,
microwave CVD, heating furnace, anneal furnace by light irradiation,
sputtering, plasma anneal, plasma etching (isotropic or anisotropic). In
order to construct the apparatus of the present invention, at least the
elements as described above are required.
In the present invention, the most remarkable effect can be obtained when
nickel is used as catalyst. Another material usable as catalyst may be
preferably used one or several kinds of elements selected from Ni, Pd, Pt,
Cu, Ag, Au, In, Sn, Pd, Sn, Pd, P, As, Sb.
Next, preferred embodiments according to the present invention will be
described.
(Embodiment 1)
In this embodiment, a silicon nitride film of 500 .ANG. is selectively
formed and nickel is selectively doped using this silicon nitride film as
a mask.
A series of processes in the manufacturing method of this embodiment will
be briefly described with reference to FIG. 1.
First, a silicon oxide film of 2000 .ANG. and an amorphous silicon film 12
of 100-1500 .ANG. are sequentially formed on a glass substrate (Corning
7059, 10 cm square) by the plasma CVD method using the apparatus shown in
FIGS. 2 and 3. In this case, the amorphous silicon film 12 is formed at a
thickness of 1000 .ANG.. A film forming condition of the silicon oxide
film is as follows: film forming pressure of 0.1-1 torr (0.3 torr in this
embodiment), the ratio of TEOS:O.sub.2 is set to 1:10, RF power of 1-500 W
(300 W in this embodiment) and a substrate temperature of
100.degree.-500.degree. C. (400.degree. C. in this embodiment). A film
forming condition of the amorphous silicon film is as follows: film
forming pressure of 0.1-1 torr (0.3 torr in this embodiment), film forming
gas of monosilane, RF power of 1-100 W (35 W in this embodiment) and a
substrate temperature of 100.degree.-300.degree. C. (160.degree. C. in
this embodiment). (FIG. 1(A))
Subsequently, the substrate is fed to the heat treatment chamber 305
without exposing the substrate to atmospheric air, and a heat treatment at
350.degree.-550.degree. C., 400.degree. C. in this case, for one hour
under N.sub.2 atmosphere is performed on the substrate to remove hydrogen
from the amorphous silicon film 12 which is formed by the plasma CVD
method.
Thereafter, the substrate 322 is fed to the treatment chamber 304 without
exposing the substrate to the atmospheric air, and a silicon nitride film
21 serving as a mask is formed at a thickness of 200 .ANG. or more, at 500
.ANG. in this case. A film forming condition is as follows: film forming
pressure of 0.1-1 torr (0.3 torr in this embodiment), the ratio of
monosilane:ammonia=1:4, RF power of 100-500 W (300 W in this embodiment)
and the substrate temperature of 200.degree.-500.degree. C. (400.degree.
C. in this embodiment). On the basis of the experiments which had been
performed by the inventors of this application, it was confirmed that no
problem occurred even when the silicon nitride film 21 was equal to 100
.ANG., and thus it is expected that the thickness of the silicon nitride
may be smaller if film quality is fine.
Thereafter, the silicon nitride film 21 is patterned in a desired pattern
by an ordinary photolitho-patterning process, and a thin silicon oxide
film 20 is formed by irradiation of uv rays under an oxygen atmosphere.
The formation of the silicon oxide film 20 is performed by irradiation of
uv rays for 5 minutes under the oxygen atmosphere. The thickness of the
silicon oxide film 20 is guessed to be about 20-50 .ANG. (FIG. 1A). With
respect to the silicon oxide film to improve wettability, an adequate
doping can be performed with only hydrophilic property of the silicon
oxide film when the solution and the pattern size are matched to each
other. However, such a case is very rare, and it is generally safety to
use the silicon oxide film 20.
In this condition, 5 ml of acetate solution containing 100 ppm nickel is
dropped (in case of 10 cm square substrate). At this time, the solution
can be prevented from leaking to the back surface of the substrate by
coating the substrate with solution while rotating a spinner at 150 rpm.
After keeping the substrate in the above state for 5 minutes, a spin dry
is performed at 2000 rpm for 60 sec spinning (FIG. 1(B)).
Thereafter, a heat treatment is performed for 8 hours at 550.degree. C.
(nitrogen atmosphere) to crystallize the amorphous silicon film 12. At
this time, the crystal growth is made in a lateral (horizontal) direction
by about 40 .mu.m from a nickel-doped region 22 to a nickel-undoped region
as indicated by an arrow 23. In FIG. 1(C), reference numeral 24 represents
the region in which nickel was directly doped and thus crystallization
occurred, and reference numeral 25 represents the region in which the
lateral crystallization occurred. It was confirmed that the crystal growth
was made substantially along <111> axis in the region 25.
In this embodiment, the nickel concentration in the nickel-doped region
where nickel was directly doped can be controlled to be in the range from
1.times.10.sup.16 atoms cm.sup.-3 to 1.times.10.sup.19 atoms cm.sup.-3 by
changing the solution concentration and the keeping time. Likewise, the
concentration in the lateral growth region can be also controlled to be
below the above range.
It is necessary to exfoliate the mask material if a device is afterwards
formed on the substrate thus formed. In this case, when the silicon oxide
mask is used like the prior art, the glass substrate and the undercoat
silicon oxide would be greatly damaged because hydrofluoric acid etchant
or a dry etching with fluoride gas must be used. On the other hand, when
the silicon nitride mask is used, heated phosphoric acid can be used as an
etchant, and it little damages the crystalline silicon film, the silicon
oxide film and the glass substrate.
As described above, the concentration of the catalysts is small and the
degree of crystallization is excellent in the region where the lateral
crystal growth is made, and thus it is useful to utilize this region as an
active region for a semiconductor device. For example, it is remarkably
effective to use this region as a channel-forming region for a thin film
transistor.
›EMBODIMENT 2!
In this embodiment, an electronic device is formed using a region which was
selectively doped with nickel in the same manner as the embodiment 1 and
in which the crystal growth was performed in a lateral direction (a
direction parallel to the substrate) from a nickel-doped area. When such a
construction is adopted, the nickel concentration can be reduced to a
lower level, and thus this construction is remarkably preferable in view
of electric stability and reliability of the device.
This embodiment relates to a process of manufacturing TFTs which are used
to control active matrix pixels. FIG. 4 shows a series of manufacturing
processes of this embodiment. First, a substrate 201 is cleaned, and then
an undercoat film 202 of silicon oxide is formed at a thickness of 2000
.ANG. by a plasma CVD method using TEOS (tetraethoxysilane) and oxygen as
source gas in the multipurpose substrate treatment apparatus shown in
FIGS. 2 and 3. Sequentially to the above process, an intrinsic (type I)
amorphous silicon film 203 of 500-1500 .ANG. in thickness, for example
1000 .ANG., is formed by the plasma CVD method. Thereafter, a heat
treatment at 450.degree. C. for one hour is performed using a heat
treatment furnace 305 to remove hydrogen. After that, an silicon nitride
film 205 of 500-2000 .ANG. thickness, for example 1000 .ANG., is
sequentially formed in the same apparatus by the plasma CVD method.
Subsequently, the silicon nitride film 205 is selectively etched to form a
region 205 in which amorphous silicon is exposed. If the patterning
treatment of this region 205 is performed so that the silicon nitride film
remains on a region in which islands will be afterwards formed, it is very
useful in the process because the patterning treatment of the amorphous
silicon film can be performed using the silicon nitride film as a mask
after the crystallizing process.
Thereafter, solution (acetate solution in this embodiment) containing
nickel which is catalyst prompting crystallization is coated on the
substrate according to the method as described in the embodiment 1. The
nickel concentration in the acetate solution is set to 100 ppm. The other
processes and conditions are the same as those of the embodiment 1.
Thereafter, a heat annealing treatment at 500.degree.-620.degree. C., for
example, at 550.degree. C. under nitrogen atmosphere is performed for four
hours to crystallize a silicon film 303. The crystallization starts from
an area where nickel and silicon film are contacted with each other, and
progresses in parallel to the substrate as shown by an arrow. In FIGS. 4A
to 4F, the region 204 represents a portion where nickel was directly doped
and crystallized, and the region 203 represents a portion where the
crystallization progressed in the lateral direction. It is confirmed that
the crystallization in the lateral direction as represented by reference
numeral 203 is extended by about 25 microns, and the direction of the
crystal growth is substantially in parallel to the direction of <111>
axis. (FIG. 4A).
Subsequently, the crystalline silicon film 204 is subjected to the dry
etching treatment to form islands using the silicon nitride film 205 as a
mask. Through this process, an etch-off treatment can be performed on the
directly-doped region 206 having high nickel concentration. As a result,
in this embodiment, the region having high nickel concentration was
designed not to be overlapped with a channel-forming region in an active
layer 208. Thereafter, the silicon nitride film 208 is etched with heated
phosphoric acid to form the islandish active layer region 208.
Subsequently, the active layer (silicon film) 208 is left under an
atmosphere of 10 atms containing 100 vol. % water vapor at
500.degree.-600.degree. C., typically 550.degree. C. to oxidize the
surface of the active layer and form a silicon oxide film 209. The
thickness of the silicon oxide film is set to 1000 .ANG.. After the
silicon oxide film 209 is formed by the heat oxidization, the substrate is
kept at 400.degree. C. temperature under an ammonia atmosphere (1 atm,
100%). In this state, infrared rays having a peak in the range 0.6-4
.mu.m, for example 0.8-1.4 .mu.m is irradiated to the substrate for 30-180
seconds, and a nitridation treatment is performed on the silicon oxide
film 209. In this case, 0.1-10% HCl may be mixed into the atmosphere.
A halogen lamp is used as the infrared ray source. The intensity of the
infrared ray is controlled so that the temperature on a single crystal
silicon wafer which is used as a monitor is in the range of
900.degree.-1200.degree. C. Specifically, the temperature of a thermo
couple which is buried in the silicon wafer is monitored, and fed back to
the infrared ray source. In this embodiment, a temperature increasing
(heating) rate is set to be constant, 50.degree.-200.degree. C./sec, and a
temperature decreasing (cooling) rate is set to be a naturally cooling
rate, 20.degree.-100.degree. C./sec. The silicon film is selectively
heated by the infrared ray irradiation, so that it can suppress the
heating of the glass substrate to the minimum. (FIG. 4B).
Subsequently, an aluminum (containing 0.01-0.2% of scandium) film of
3000-8000 .ANG. in thickness, for example 6000 .ANG., is formed by a
sputtering method. The aluminum film is subjected to the patterning
treatment to form a gate electrode 210 (FIG. 2C).
Subsequently, an oxide layer 211 is formed on the surface of the aluminum
electrode by performing anodic oxidization on the surface. The anodic
oxidization is performed in ethylene glycol solution containing 1-5% of
tartaric acid. The thickness of the oxide layer 211 is equal to 2000
.ANG.. The thickness of the oxide layer 211 corresponds to an offset gate
region which will be formed in a subsequent ion-doping process, and thus
the length of the offset gate region can be determined in the above anodic
oxidation process (FIG. 4D).
Subsequently, impurities (in this case, phosphorus) which can provide
N-type conductivity is doped in self-alignment into an active region
(constructing source/drain and channel regions) using a gate electrode
portion, that is, a gate electrode 210 and its peripheral oxide layer 211
as a mask by an ion-doping method (called as a plasma doping method).
Phosphine (PH.sub.3) is used as doping gas, and an acceleration voltage is
set to 60-90 kV, for example 80 kV. A dose amount is set to
1.times.10.sup.15 -8.times.10.sup.15 cm.sup.-2, for example
4.times.10.sup.15 cm.sup.-2. As a result, N-type impurity regions 212 and
213 can be formed. As is apparent from the figures, the impurity region
and the gate electrode are kept in such an offset state as to be away from
each other at a distance x. The offset state like this is particularly
effective to reduce a leak current (called as "off current") occurring
when a reverse voltage (minus voltage for N channel TFT) is applied to the
gate electrode. Especially, it is effective to provide the offset because
the leak current is desired to be low so that no charges stored in pixel
electrodes leak to obtain a good image in the case of TFTs which control
active matrix pixels like this embodiment.
Thereafter, an anneal process is performed by laser irradiation. A KrF
excimer laser (wavelength 248 nm, pulse width 20 nsec) is utilized as a
laser light source, but another type of laser may be used. The condition
of laser irradiation is as follows: the energy density of irradiated laser
is 200-400 mJ/cm.sup.2, for example 250 mJ/cm.sup.2, and irradiation is
repeated at 2-10 times, for example 2 times every irradiation target
point. In this case, the effect could be enhanced if the substrate is
heated at 200.degree.-450.degree. C. during the laser irradiation process
(FIG. 4E).
Subsequently, a silicon oxide film 214 of 6000 .ANG. thickness is formed as
a layer insulator by the plasma CVD method. Further, a transparent
polyimide film 215 is formed by a spin coating method, and the surface
thereof is flattened. A transparent electroconductive film (ITO film) of
800 .ANG. thickness is formed on the surface of the polyimide film thus
formed by a sputtering method, and then patterned to form a pixel
electrode 216.
Thereafter, contact holes are formed in layer insulators 214, 215, and
electrodes/wires 217 and 218 of the TFTs are formed of metal material, for
example, a multilayer film of titanium nitride and aluminum. Finally, an
anneal treatment is performed at 350.degree. C. for 30 minutes under a
hydrogen atmosphere of 1 atom to complete an active matrix pixel circuit
having the TFTs (FIG. 4F).
›EMBODIMENT 3!
FIGS. 5A to 5D are cross-sectional views showing a series of manufacturing
processes in a third embodiment of the present invention. First, an
undercoat film 102 of silicon oxide is formed at a thickness of 2000 .ANG.
on a glass substrate (Corning 7059) 501 by the sputtering method. In a
case where the substrate is annealed at a temperature higher than the
distortion temperature and then gradually cooled to a temperature below
the distortion temperature at 0.1.degree.-1.0.degree. C./minute before or
after the undercoat film is formed, a mask alignment work can be
facilitated because contraction of the substrate can be suppressed in a
subsequent process containing a temperature increasing step (containing a
thermal oxidization process and subsequent thermal anneal process of this
invention). For the Corning 7059 substrate, it is preferable to anneal the
substrate at 620.degree.-660.degree. C. for 1-4 hours, then gradually cool
the substrate at 0.03.degree.-1.0.degree. C./min, preferably
0.1.degree.-0.3.degree. C./min and then take out the substrate at a time
when the temperature is reduced to 400.degree.-500.degree. C.
Subsequently, a silicon oxide film, an amorphous silicon film and a silicon
nitride film are sequentially formed by the plasma CVD method in the same
manner as the embodiment 2. Thereafter, the amorphous silicon film is
crystallized by the same method as the embodiment 2, and then the
annealing treatment is performed under a nitrogen atmosphere (1 atm) at
600.degree. C. for 48 hours to crystallize the silicon film. Thereafter,
the silicon film is subjected to the patterning treatment into
10.degree.-1000 .mu.m square parts to form islandish silicon films (active
layers of TFTs) 503 (FIG. 5A).
Subsequently, an oxygen atmosphere of 1 atm, 500.degree.-750.degree. C.,
typically 600.degree. C. containing 70-90% of water vapor was formed by a
pyrogenetic reaction method at hydrogen/oxygen ratio of 1.5-1.9. The
substrate is kept in this atmosphere for 3 to 5 hours to oxidize the
surface of the silicon surface and form a silicon oxide film 504 of
500-1500 .ANG., for example, 1000 .ANG. in thickness. it should be noted
that the thickness of the surface of the initial silicon film is reduced
by 50 .ANG. or more by the oxidization, so that contamination on the
uppermost surface of the silicon film does not extend to the
silicon-silicon oxide interface, that is, clean silicon-silicon oxide
interface can be obtained. The thickness of the silicon oxide film is set
to be twice as large as that of the silicon film to be oxidized, and thus
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