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BACKGROUND OF THE INVENTION
1 Field of the Invention
The invention relates to a thin film transistor (TFT) and a method of
producing the same, and also to a semiconductor circuit having a plurality
of thin film transistors (TFTs) and a method of producing the same. A thin
film transistor produced in accordance with the invention may be formed on
either of an insulating substrate such as glass, and a semiconductor
substrate such as single crystal silicon. Particularly, the invention
relates a semiconductor circuit comprising a matrix circuit which operates
at a low speed, such as a monolithic active matrix circuit (which may be
used in a liquid crystal display or the like), and a peripheral circuit
which drives the active matrix circuit and operates at a high speed. The
invention relates also to a thin film transistor which is produced by
crystallization and activation according to thermal annealing.
2. Description of the Prior Art
Recently, researches are conducted on insulated gate semiconductor devices
having a thin film active layer (also referred to as an active region).
Particularly, thin film insulated gate semiconductor devices, or so-called
thin film transistors (TFTs) are vigorously studied. Such thin film
transistors are formed on a transparent insulating substrate so as to be
used for controlling pixels or in a driver circuit in a liquid crystal
display device, etc. having a matrix structure. Thin film transistors are
classified into amorphous silicon TFTs and crystalline silicon TFTs,
depending on the kind and crystal state of a used semiconductor material.
A semiconductor of an amorphous state has generally a low electric field
mobility, and therefore cannot be used in a TFT which must operate at a
high speed. Since P-type amorphous silicon has a very low electric field
mobility, moreover, it is impossible to produce a P-channel TFT (PMOS
TFT). Consequently, it is impossible to produce a complementary MOS
circuit (CMOS) which consists of combinations of a P-channel TFT and an
N-channel TFT (NMOS TFT).
In contrast, a crystalline semiconductor has a higher electric field
mobility than that of an amorphous semiconductor, and therefore can
operate at a high speed. When using crystalline silicon, since not only an
NMOS TFT but also a PMOS TFT can be produced in the same manner, it is
possible to produce a CMOS circuit. For example, a known liquid crystal
display device of the active matrix type has a so-called monolithic
structure in which not only the active matrix region but also a peripheral
circuit (including a driver circuit, etc.) are constructed by CMOS
crystalline TFTs. From these reasons, researches and developments on TFTs
using crystalline silicon are vigorously conducted.
In an example of a method of obtaining crystalline silicon, amorphous
silicon is crystallized by irradiating it with a laser or intense light
equivalent to a laser. Because of instability of the laser output and
instability due to a very short process period, however, this method
offers no prospect for mass production or practical use.
Now, a method which may be practically employed is the one in which
amorphous silicon is thermally crystallized. According to this method, it
is possible to obtain crystalline silicon with reduced variation between
batches. However, this method has a problem.
Generally, the formation of crystalline silicon requires annealing of a
long period and at about 600.degree. C., or annealing at a high
temperature of 1,000.degree. C. or higher. In the case where the latter
method is employed, a selectable substrate is limited to one made of
quartz, resulting in that the cost of a substrate is very high. In the
case where the former method is employed, a substrate can be selected from
a wide range, but there arises another problem.
When an inexpensive alkalifree glass substrate (for example, No. 7059
produced by Corning Co. Ltd.) is used, a conventional process of producing
a TFT is roughly conducted in the following manner:
(1) formation of an amorphous silicon film;
(2) crystallization of the amorphous silicon film (600.degree. C. or
higher, and 24 hours or longer);
(3) formation of a gate insulating film;
(4) formation of a gate electrode;
(5) Introduction of doping impurities (by the ion implantation method or
ion doping method);
(6) activation of the doping impurities (600.degree. C. or higher, and 24
hours or longer);
(7) formation of a layer insulator; and
(8) formation of source and drain electrodes.
In the process, steps (2) and (6) have problems. Many kinds of alkalifree
glass have a distortion temperature of about 600.degree. C. (593.degree.
C. in the case of Corning 7059). Therefore, a process at such a
temperature causes problems such as a shrinkage and bend of a substrate.
In step (2) which is a first annealing process, a shrinkage of a substrate
does not cause a serious problem because a patterning process has not yet
been conducted. In step (6), however, a patterning process has been
conducted already. When the substrate shrinks in step (6), therefore, the
mask alignment in the following steps cannot be properly conducted,
thereby forming a major cause for impairing the yield. Consequently, it is
desired to conduct the process of step (2) at a temperature lower than the
distortion temperature of the substrate, and to conduct the process of
step (6) at a further lower temperature (preferably, at a temperature
lower than the distortion temperature of glass by 50.degree. C. or lower,
and, more preferably, at a temperature lower than the maximum process
temperature of step (2) by 50.degree. C. or lower).
In order to meet the requirements, a method in which a laser or the like is
used as described above may be employed. In addition to the problem of
instability of a laser output, however, it was observed there arises
another problem in that a stress is generated owing to a difference in
temperature rise between the portion to which a laser beam is irradiated
(the source and drain regions) and that to which a laser beam is not
irradiated (the active region, i.e., the region below the gate electrode),
thereby impairing the reliability.
On the other hand, a TFT made of an amorphous semiconductor has a feature
that the OFF current is low. Therefore, such a TFT is employed in a use
such as a transistor for a pixel circuit of an active matrix of a liquid
crystal display device which does not require a very high operation speed,
in which only a single conductivity type is sufficient, and which requires
a TFT having a high ability of retaining charges. However, such a TFT
cannot be employed in a peripheral circuit which must operate at a high
speed.
In a crystalline silicon TFT, a leak current flowing when no voltage is
applied to the gate (i.e., during the nonselected period) is larger than
that in an amorphous silicon TFT. When a crystalline silicon TFT is to be
used in a liquid crystal display device, measures that an auxiliary
capacitor for compensating the leak current is disposed, and that two TFTs
are connected in series are taken so as to reduce the leak current.
FIG. 5 shows a block diagram of an active matrix circuit used in a liquid
crystal display device. As peripheral circuits, a column decoder 101 and a
row decoder 102 are disposed on a substrate 107. Pixel circuits 104 each
consisting of a transistor and a capacitor are formed in a matrix region
103. The matrix region and the peripheral circuits are connected to each
other through wirings 105 and 106. TFTs used in the peripheral circuits
are required to have a property of a high speed, and TFTs used in the
pixel circuits are required to have a property of a low leak current.
Although these properties are physically contradictory to each other, it
is required to form the both kinds of TFTs on the same substrate and by
the same process.
Generally, the formation of crystalline silicon requires annealing of a
long period and at about 600.degree. C., or annealing at a high
temperature of 1,000.degree. C. or higher. For example, it is impossible
to construct a structure in which a high OFF resistance of amorphous
silicon TFTs is utilized and these TFTs and a peripheral circuit
consisting of polysilicon TFTs which have a high mobility are formed on
the same substrate, because amorphous silicon is crystallized in the
above-mentioned annealing process.
SUMMARY OF THE INVENTION
In the view point of mass production, therefore, it is difficult to employ
the method using a laser in the production of a TFT. On the other hand, it
is the state of the art that no other effective method cannot be found.
The invention has been conducted in order to solve these difficult
problems. It is an object of the invention to solve the problems while
maintaining the mass productivity.
The invention has been conducted in order to solve these difficult
problems. However, it is not desirable that an improved configuration
produces a complicated process, a lowered yield, and an increased cost. It
is an object of the invention to allow two kinds of TFTs, i.e., TFTs in
which a high mobility is required, and TFTs in which a low leak current is
required, to be selectively produced in an easy manner by changing the
process at a minimum while maintaining the mass productivity.
The result of study of this inventors of the present application made clear
that addition of a catalytic material of a small amount to an amorphous
silicon film enhanced crystallization, lowered temperature of
crystallization, and shortened time of crystallization. As the catalytic
material, simple substance like nickel (Ni), iron (Fe), cobalt (Co),
platinum (Pt), or compound such as silicide thereof is appropriate.
Concretely, the amorphous silicon film can be crystallized by forming a
film, grain, or cluster having these catalytic elements under or on the
amorphous silicon film, or by introducing these catalytic elements in the
amorphous silicon film by a method like ion injection; followed by
thermally annealing the amorphous silicon film at an appropriate
temperature, typically at 580.degree. C. or less.
In the case of forming an amorphous silicon film by a chemical vapor
deposition method (CVD method), these catalysts can be added in a material
gas; in the case of forming an amorphous silicon film by a physical gas
phase method like sputtering, these catalysts can be added in a deposition
material like target or deposition source. Though it is a natural result,
the higher the temperature of annealing is, the shorter the time of
crystallization is. Moreover, the higher the concentration of nickel,
iron, cobalt, platinum is, the lower the temperature of crystallization is
and the shorter the time of crystallization is. By the study of this
inventors of the present application, to enhance crystallization, it was
found that concentration of at least one element should be
1.times.10.sup.17 cm.sup.-3 or more, preferably 5.times.10.sup.18
cm.sup.-3 or more.
Furthermore, what should be noticed is that a region which does not have
catalytic material like this does not enhance crystallization at all, and
can keep condition of amorphous. For example, crystallization of amorphous
with concentration of 1.times.10.sup.17 cm.sup.-3 or less, preferably
1.times.10.sup.16 cm.sup.-3 or less, starts at 600.degree. C. or more, but
is not enhanced at all at 580.degree. C. or less. However, because
hydrogen needed in neutralizing dangling bonds in amorphous silicon is
released in an atmosphere of 300.degree. C. or more, it is desirable that
annealing is performed in hydrogen atmosphere to obtain appropriate
semiconductor character.
In the present invention, characteristic of crystallization with above
mentioned catalytic material is utilized to form an amorphous silicon
film. A part of it is selectively crystallized and is utilized as crystal
silicon TFT in a peripheral circuit of an active matrix circuit. Another
part in amorphous condition is utilized as amorphous silicon TFT in a
matrix region (pixel circuit). As a result, a circuit having transistor
with contradicting characters of low leak current and fast action can be
formed simultaneously on the same substrate.
Because either of above mentioned catalytic material is a material
undesirable for silicon, it is desirable that concentration of them is as
low as possible. In the study of this inventor, to utilize it particularly
as an active region, it is desirable that concentration of these catalytic
material is not more than 10.sup.20 cm.sup.-3 in total, in order to obtain
satisfactory reliability and characteristics. On the other hand, it was
made clear that even if relatively a lot of catalytic material exists in
source, drain and the like, that is not a problem.
This inventors of the present application found that above mentioned
problems can be solved by taking attention to the effect of this catalytic
element and by utilizing it. Manufacturing process of TFT of this
invention is briefly as the following:
1) Deposition of an amorphous silicon film
1') Introduction of a catalytic element (by an ion injection or an ion
doping method)
2) Crystallization of amorphous silicon film (at 600.degree. C. or less,
within 8 hours)
3) Deposition of a gate insulating film
4) Formation of a gate electrode
5) Introduction of doping impurities (by an ion injection or an ion doping
method)
5') Deposition of a material having catalytic element to a silicon film)
6) Activation of a doping impurity (600.degree. C. or less, within 8 hours)
7) Formation of interlayer insulator
8) Formation of source, drain electrode or,
1) Deposition of an amorphous silicon film
1') Introduction of a catalytic element (by an ion injection or an ion
doping method)
2) Crystallization of an amorphous silicon film (600.degree. C. or less,
within 8 hours)
3) Deposition of a gate insulating film
4) Formation of a gate electrode
5) Introduction of a doping impurity (by an ion injection or an ion doping
method)
5') Introduction of a catalytic element (by ion injection or an ion doping
method)
6) Activation of a doping impurity (600.degree. C. or less, within 8 hours)
7) Formation of an interlayer insulator
8) Formation of source, drain electrode
In the processes of these, order of 5) and 5') can be reversed. The process
of 1') can be replaced as "A process of adhering a film and the like
having catalytic element on or under an amorphous silicon film". From the
standpoint of controlling concentration of catalytic element precisely, a
method like ion injection is desirable. However, from the standpoint of
simplifying processes and suppressing plant investment, if characteristic
of TFT obtained is satisfactory, such process can be utilized.
In the present invention, the catalytic element introduced to an amorphous
silicon film by said process 1') enhances its crystallization a lot. On
the other hand, the catalytic element introduced mainly to the source,
drain region by 5') enhances recrystallization of that region a lot.
Therefore, for crystallization and activation, temperature of 600.degree.
C. or less, typically 550.degree. C. or less is enough. Time of annealing
within 8 hours, typically within 4 hours is enough, too. Especially if the
catalytic element is initially distributed equally by an ion injection
method or an ion doping method, crystallization was very easy to proceed.
In this invention, because a gate electrode exists on an active region by
adopting either process, the catalytic element will not be adhered or
injected to an active region directly in the process of 5'). Therefore, it
is possible to change concentration of catalytic element in an active
region and an impurity region. For example, by relatively decreasing
concentration of catalytic element to be added to an active region, bad
effect to be given to characteristic and reliability of TFT is reduced to
the utmost. By increasing concentration of catalytic element to be added
to an impurity region relatively and lowering temperature of activation,
shrinkage and distortion of the substrate is suppressed and yield can be
increased. Reliability and characteristic of TFT is rarely lost.
In the present invention, an amorphous silicon film with thickness of 1000
.ANG. or less is also crystallized by the cause of catalytic element. Such
amorphous silicon film will not be crystallized by the usual heat
annealing. From the standpoint of preventing pinhole and defects of
insularity in the step portion of TFT, and preventing disconnection of the
gate electrode, thickness of the crystal silicon film is demanded as 1000
.ANG. or less, preferably 500 .ANG. or less. This is not achieved by a
method other than laser crystallization, but the present invention
achieved this by heat annealing even at a low temperature. This naturally
contributes to further improvement of yield. The present invention will be
explained more in detail further, by utilizing embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(A) to 1(E) are section views showing steps of the production
process conducted in Embodiment 1;
FIGS. 2(A) to 2(E) are section views showing steps of the production
process conducted in Embodiment 2;
FIGS. 3(A) to 3(E) are section views showing steps of the production
process conducted in Embodiment 3;
FIGS. 4(A) to 4(E) are section views showing steps of the production
process conducted in Embodiment 4; and
FIG. 5 is a diagram showing an example of a monolithic active matrix
circuit.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment 1
FIG. 1 is a section view showing steps of the production process conducted
in the embodiment. First, a foundation film 11 made of silicon oxide and
of a thickness of 2,000 .ANG. was formed on a substrate (Corning 7059) 10
by the sputtering method. Then, an intrinsic (I-type) amorphous silicon
film 12 was deposited by the plasma CVD method in a thickness of 500 to
1,500 .ANG., for example, 1,500 .ANG.. Nickel ions were implanted into the
amorphous silicon film by the ion implantation method in a dose of
1.times.10.sup.13 to 5.times.10.sup.14 cm.sup.-2, for example,
5.times.10.sup.13 cm.sup.-2. As a result nickel ions existed in a
concentration of about 5.times.10.sup.18 cm.sup.-3 in the amorphous
silicon film (FIG. 1(A)).
Next, an annealing process in an atmosphere of nitrogen was conducted at
550.degree. C. for 4 hours to crystallize the silicon film. Then, the
silicon film was patterned to form a silicon island region 13. As a gate
insulating film, a silicon oxide film 14 of a thickness of 1,000 .ANG. was
deposited by the sputtering method. In this sputtering process, silicon
oxide was used as a target, and the substrate temperature was 200.degree.
to 400.degree. C., for example, 250.degree. C. The sputtering process was
conducted in an atmosphere of oxygen and argon in which the ratio of argon
to oxygen was 0 to 0.5, for example, 0.1 or less.
Thereafter, a silicon film (containing 0.1 to 2% of phosphorus) was
deposited by the reduced pressure CVD method in a thickness of 3,000 to
8,000 .ANG., for example, 6,000 .ANG.. It is preferable to conduct in
succession the steps of forming the silicon oxide and the silicon film.
The silicon film was then patterned to form a gate electrode 15 (FIG.
1(B)).
Then, an impurity (phosphorus) was implanted into the silicon region as a
mask by the plasma doping method with using the gate electrode. In this
process, phosphine (PH.sub.3) was used as a doping gas, the acceleration
voltage was 60 to 90 kV, for example, 80 kV, and the dose was
1.times.10.sup.15 to 8.times.10.sup.15 cm.sup.-2, for example,
2.times.10.sup.15 cm.sup.-2. As a result, N-type impurity regions 16a and
16b were formed (FIG. 1(C)).
Next, the silicon oxide film 14 on the impurity regions was etched away to
expose the impurity regions 16. By the sputtering method, a nickel
silicide film (having the formula of NiSi.sub.x wherein
0.4.ltoreq.x.ltoreq.2.5, for example, x=2.0) 17 was uniformly formed in a
thickness of 5 to 200 .ANG., for example, 20 .ANG. on the whole area as
shown in FIG. 1(D). When the film was formed in such a reduced thickness
of about 20 .ANG., it was not continuous, and exhibited an outlook of an
aggregate of particles. However, these did not cause any problem in the
embodiment (FIG. 1(D)).
Thereafter, in an atmosphere of nitrogen, an annealing process was
conducted for 4 hours at a temperature of 480.degree. C. (which is lower
by 70.degree. C. than that of the above-mentioned crystallizing process),
thereby activating the impurity. In this annealing process, first, nickel
diffused into the N-type impurity regions 16a and 16b from the nickel
silicide film covering the impurity regions. Therefore, the annealing
caused the recrystallization to easily progress. In this way, the impurity
regions 16a and 16b were activated.
Then, a silicon oxide film 18 of a thickness of 6,000 .ANG. was formed as a
layer insulator by the plasma CVD method, and contact holes were formed in
the insulator. Electrodes/wirings 19a and 19b were formed in the source
and drain regions of a TFT by a multilayer film of metal materials such as
titanium nitride and aluminum. Finally, in an atmosphere of hydrogen of 1
atm., an annealing process was conducted at 350.degree. C. for 30 minutes.
As a result of the above-mentioned process, a thin film transistor was
completed (FIG. 1(E)).
The concentration of nickel in the active region (the region below the gate
electrode) of the thus produced TFT was measured by the secondary ion mass
spectrometry (SIMS), with the result that it was about 1.times.10.sup.18
to 5.times.10.sup.18 cm.sup.-3. The concentration of nickel in the
impurity regions 16 was about 1.times.10.sup.19 to 5.times.10.sup.19
cm.sup.-3.
Embodiment 2
FIG. 2 is a section view showing steps of the production process conducted
in the embodiment. First, a foundation film 21 made of silicon oxide and
of a thickness of 2,000 .ANG. was formed on a substrate (Corning 7059) 20
by the sputtering method. Then, an intrinsic (I-type) amorphous silicon
film 22 was deposited by the plasma CVD method in a thickness of 500 to
1,500 .ANG., for example, 1,500 .ANG., and a silicon oxide film 23 of a
thickness of 200 .ANG. was deposited by the sputtering method. Nickel ions
were implanted into the amorphous silicon film by the ion implantation
method in a dose of 5.times.10.sup.13 cm.sup.-2 (FIG. 2(A)).
Next, an annealing process in an atmosphere of nitrogen was conducted on
the amorphous silicon film at 550.degree. C. for 8 hours to crystallize
the amorphous silicon film. Thereafter, the silicon film was patterned to
form a silicon island region 24.
Using tetraethoxysilane (Si(OC.sub.2 H.sub.5).sub.4, TEOS) and oxygen as
materials, a silicon oxide film 25 of a thickness of 1,000 .ANG. was
formed by the plasma CVD method as a gate insulating film of a crystalline
silicon TFT. In addition to these material gases, trichloroethylene
(C.sub.2 HCl.sub.3) was further used as one of the materials. Before the
film formation, 400 SCCM of oxygen was passed through a chamber, and
plasma was generated under the conditions that the substrate temperature
was 300.degree. C., the total pressure was 5 Pa, and the RF power was 150
W. This state was maintained for 10 minutes. Thereafter, 300 SCCM of
oxygen, 15 SCCM of TEOS, and 2 SCCM of trichloroethylene were introduced
into the chamber to conduct the film formation of the silicon oxide film.
In this film formation, the substrate temperature, the RF power, and the
total pressure were 300.degree. C., 75 W, and 5 Pa, respectively. After
the completion of the film formation, hydrogen of 100 Torr was introduced
into the chamber, and hydrogen annealing was conducted at 350.degree. C.
for 35 minutes.
Thereafter, a tantalum film was deposited by the sputtering method in a
thickness of 3,000 to 8,000 .ANG., for example, 6,000 .ANG.. In place of
tantalum, titanium, tungsten, molybdenum, or silicon may be used. However,
the material is required to have sufficient heat resisting properties so
that it can withstand the subsequent activation process. It is preferable
to conduct in succession the steps of forming the silicon oxide 25 and the
tantalum film. Then, the tantalum film was patterned to form a gate
electrode 26 of a TFT. The surface of the tantalum wiring was anodically
oxidized to form an oxide layer 27 thereon. The anodic oxidation was
conducted in a 1 to 5% ethylene glycol solution of tartaric acid. The
obtained oxide layer had a thickness of 2,000 .ANG. (FIG. 2(B)).
By the plasma doping method, an impurity (phosphorus) was then implanted
into the silicon region with using the gate electrode as a mask. In this
process, phosphine (PH.sub.3) was used as a doping gas, the acceleration
voltage was 80 kV, and the dose was 2.times.10.sup.15 cm.sup.-2. As a
result N-type impurity regions 28a and 28b were formed. Because of the
anodic oxide, the gate electrode 26 was displaced from the impurity
regions 28. (FIG. 2(C)).
Using the gate electrode as a mask, nickel ions were implanted into the
silicon region by the ion implantation method in a dose of
1.times.10.sup.14 to 2.times.10.sup.15 cm.sup.-2, for example,
5.times.10.sup.14 cm.sup.-2. As a result, the concentrations of the N-type
impurity regions 28a and 28b were about 5.times.10.sup.19 cm.sup.-3 (FIG.
2(D)).
Thereafter, in an atmosphere of nitrogen, an annealing process was
conducted for 4 hours at 450.degree. C., thereby activating the impurity.
In this annealing process, since nickel ions were implanted into the
N-type impurity regions 28a and 28b, the annealing caused the
recrystallization to easily progress. In this way, the impurity regions
28a and 28b were activated.
Then, a silicon oxide film 29 of a thickness of 2,000 .ANG. was formed as a
layer insulator by the plasma CVD method in which TEOS was used as a
material, and contact holes were formed in the insulator. Source and drain
electrodes/wirings 30a and 30b were formed by a multilayer film of metal
materials such as titanium nitride and aluminum. As a result of the
above-mentioned process, a semiconductor circuit was completed (FIG.
2(E)).
In the thus produced thin film transistor, the field effect mobility was 70
to 100 cm.sup.2 /Vs at a gate voltage of 10 V, the threshold voltage was
2.5 to 4.0 V, and the leak current was 10.sup.-13 .ANG. or less when a
voltage of -20 V was applied to the gate.
Embodiment 3
In the embodiment, crystalline silicon TFTs and an amorphous silicon TFT
were formed on the same substrate by a substantially same process. FIG. 3
is a section view showing steps of the production process conducted in the
embodiment. First, a foundation film 111 made of silicon oxide and of a
thickness of 2,000 .ANG. was formed on a substrate (Corning 7059) 110 by
the sputtering method. Then, an intrinsic (I-type) amorphous silicon film
112 was deposited by the plasma CVD method in a thickness of 500 to 1,500
.ANG., for example, 1,500 .ANG., and in succession a nickel silicide film
(having the formula of NiSi.sub.x wherein. 0.4.ltoreq.x.ltoreq.2.5, for
example, x=2.0) 113 was selectively formed by the sputtering method in a
thickness of 5 to 200 .ANG., for example, 20 .ANG. (FIG. 3(A)).
Then, in an atmosphere of hydrogen reduction (preferably, the partial
pressure of hydrogen is from 0.1 to 1 atm.), an annealing process was
conducted at 500.degree. C. for 4 hours to perform crystallization. As a
result, the amorphous silicon film below the nickel silicide film 113 was
crystallized to become a crystalline silicon film 112a. In contrast, the
silicon film in the region where the nickel silicide film did not exist
remained in the amorphous state as indicated by 112b (FIG. 3(B)).
The thus obtained silicon film was patterned by the photolithography method
to form a silicon island region 114a (crystalline silicon region) and
another silicon island region 114b (amorphous silicon region). As a gate
insulating film, a silicon oxide film 115 of a thickness of 1,000 .ANG.
was deposited by the sputtering method. In this sputtering process,
silicon oxide was used as a target, and the substrate temperature was
200.degree. to 400.degree. C., for example, 350.degree. C. The sputtering
process was conducted in an atmosphere of oxygen and argon in which the
ratio of argon to oxygen was 0 to 0.5, for example, 0.1 or less.
Thereafter, a silicon film (containing 0.1 to 2% of phosphorus) was
deposited by the reduced pressure CVD method in a thickness of 6,000 to
8,000 .ANG., for example, 6,000 .ANG.. It is preferable to conduct in | | |
