U.S. patent application number 10/656130 was filed with the patent office on 2004-03-11 for process for forming a microcrystalline silicon series thin film and apparatus suitable for practicing said process.
Invention is credited to Higashikawa, Makoto, Sano, Masafumi.
Application Number | 20040045505 10/656130 |
Document ID | / |
Family ID | 12855650 |
Filed Date | 2004-03-11 |
United States Patent
Application |
20040045505 |
Kind Code |
A1 |
Higashikawa, Makoto ; et
al. |
March 11, 2004 |
Process for forming a microcrystalline silicon series thin film and
apparatus suitable for practicing said process
Abstract
A process for forming a microcrystalline silicon series thin
film by arranging a substrate in a vacuum chamber so as to oppose
an electrode provided in said vacuum chamber and while transporting
said substrate in a longitudinal direction, causing glow discharge
between said electrode and said substrate to deposit said
microcrystalline silicon series thin film on said substrate,
wherein a plurality of bar-like shaped electrodes as said electrode
are arranged such that they are perpendicular to a normal line of
said substrate and their intervals to said substrate are all or
partially differed, and said glow discharge is caused using a high
frequency power with an oscillation frequency in a range of from 50
MHz to 550 MHz, whereby depositing said microcrystalline series
thin film on substrate. An apparatus suitable for practicing said
process.
Inventors: |
Higashikawa, Makoto;
(Kyotanabe-shi, JP) ; Sano, Masafumi; (Soura-gun,
JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
12855650 |
Appl. No.: |
10/656130 |
Filed: |
September 8, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10656130 |
Sep 8, 2003 |
|
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|
09261499 |
Mar 3, 1999 |
|
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6645573 |
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Current U.S.
Class: |
118/723E ;
118/718 |
Current CPC
Class: |
C23C 16/545 20130101;
C23C 16/509 20130101; C23C 16/24 20130101 |
Class at
Publication: |
118/723.00E ;
118/718 |
International
Class: |
C23C 016/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 1998 |
JP |
050321/HEI.10 |
Claims
What is claimed is:
1. An apparatus for forming a microcrystalline silicon series thin
film on a substrate, having a portion in which said substrate is
arranged to oppose to an electrode in a vacuum chamber, wherein
said apparatus has a plurality of bar shaped electrodes as said
electrode which are arranged such that they are perpendicular to a
normal line of said substrate and their intervals to said substrate
are all different or in part different and a high frequency power
source for causing said glow discharge using a high frequency power
with an oscillation frequency in a range of from 50 MHz to 550 MHz
whereby a glow.
2. An apparatus according to claim 1, wherein said plurality of bar
shaped electrodes are arranged such that they are in parallel to
each other.
3. An apparatus according to claim 1, wherein said plurality of bar
shaped electrodes are arranged such that they are perpendicular to
a transportation direction of the substrate.
4. An apparatus according to claim 1, wherein said plurality of bar
shaped electrodes are arranged such that their intervals to the
substrate are widened in an upper side of a transportation
direction of the substrate and narrowed in a down side thereof.
5. An apparatus according to claim 1, wherein said plurality of bar
shaped electrodes are arranged such that their intervals to the
substrate are periodically changed to a transportation direction of
the substrate.
Description
[0001] This application is a division of co-pending Application
Ser. No. 09/261,499, filed Mar. 3, 1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a process for forming a
microcrystalline silicon series thin film (this film will be
hereinafter referred to as ".mu.c-silicon series thin film" or
".mu.c-Si series thin film") and an apparatus suitable for
practicing said process. More particularly, the present invention
relates to a process and an apparatus which enable one to form a
highly reliable .mu.c-Si series thin film having a large area and a
high energy conversion efficiency which is usable in the production
of semiconductor devices such as electrophotographic light
receiving members (or electrophotographic photosensitive members),
image input line sensors, image pickup devices, photovoltaic
devices (including solar cells), and the like.
[0004] 2. Related Background Art
[0005] Hitherto, solar cells comprising a photovoltaic element
which converts sunlight into electric energy have been widely using
as a small power source in daily appliances such as electronic
calculators, wrist watches, and the like. Such a technique of such
solar cell is expected to provide a practically usable power
generation source which can replace the power generation source
based on fossil fuels such as petroleum.
[0006] Incidentally, in a solar cell the photoelectromotive force
of a pn junction is used in the functional portion. In general, the
pn junction is constituted by a semiconductor material such as a
semiconductor silicon material or a semiconductor germanium
material. The semiconductor functions to absorb sunlight and
generate photocarriers of electrons and holes, where the
photocarriers drift due to an internal electric field of the pn
junction, followed by being outputted to the outside.
[0007] Now, in view of the efficiency of converting light energy
into electricity, it is preferred to use a single crystalline
silicon material. However, crystalline silicon materials including
a single crystalline silicon material have an indirect optical end,
and therefore, they are small in light absorption. In this
connection, in the case of a solar cell in which a single
crystalline silicon is used (this solar cell will be hereinafter
referred to as "single crystal solar cell"), it is necessary for
the single crystal solar cell to have a thickness of at least 50
.mu.m in order for the solar cell to sufficiently absorb incident
sunlight. In this case, if the single crystalline silicon material
is replaced by a polycrystalline silicon material in order to
diminish the production cost of the solar cell, the problem of the
above indirect optical end cannot be solved unless the thickness is
increased. The polycrystalline silicon material has problems such
as grain boundaries and others.
[0008] In view of attaining a large area and a reasonable
production cost for a solar cell, a so-called thin film silicon
solar cell which is represented by an amorphous silicon solar cell
having a semiconductor layer comprising an amorphous silicon thin
film produced by way of CVD (chemical vapor phase deposition) has
been evaluated as being more advantageous. In fact, currently,
amorphous silicon solar cells have been widely used as a small
power source in daily appliances. However, in order for an
amorphous silicon solar cell to be used as an ordinary power
generation source, the photoelectric conversion efficiency must be
improved and the performance stabilized.
[0009] A solar cell in which a microcrystalline silicon (a
.mu.c-Silicon) as a carrier generation layer has been proposed
(see, A. Shah et al., 23th IEEE Photovoltaic Specialist Conf.
(1993), p. 839).
[0010] The most popular film-forming method for depositing such
.mu.c-silicon series thin film or amorphous silicon thin film is a
plasma CVD process. In the plasma CVD process, the formation of a
.mu.c-silicon series thin film or an amorphous silicon thin film is
conducted, for instance, in the following manner. That is, a
film-forming raw material gas such as silane (SiH.sub.4) or
disilane (Si.sub.2H.sub.6) is introduced into a reaction chamber in
which a substrate on which a film is to be deposited is arranged,
if necessary, while being diluted by hydrogen gas (H.sub.2), a high
frequency power with an oscillation frequency of 13.56 MHz in an RF
band region is supplied in the reaction chamber to generate plasma
whereby decomposing the film-forming raw material gas to produce
reactive active species, resulting in depositing a .mu.c-silicon
thin film or an amorphous silicon thin film on the substrate. In
the case where the film formation is conducted by mixing a doping
gas such as phosphine (PH.sub.3), diborane (B.sub.2H.sub.6) or
boron fluoride (BF.sub.3) to the film-forming raw material gas, it
is possible to form a doped .mu.c-siicon thin film whose
conductivity being controlled to n-type or p-type.
[0011] However, such .mu.c-silicon thin film has a disadvantage
such that the photoelectric conversion efficiency of a solar cell
in which such .mu.c-silicon thin film is used is lower than that of
a crystalline series solar cell. In addition, for the .mu.c-silicon
thin film, there is also a disadvantage in that the deposition rate
thereof is low.
[0012] In general, the formation of a .mu.c-silicon thin film is
conducted by using RF glow discharge. However, the .mu.c-silicon
thin film thus formed has an indirect optical end as well as in the
case of a crystalline silicon thin film, and therefore, its light
absorption is small. In this connection, in the case of a
.mu.c-silicon solar cell in which a .mu.c-silicon thin film is
used, it is necessary for the .mu.c-silicon solar cell to have a
thickness of about 5 .mu.m, and therefore, a lot of time is
required to produce the .mu.c-silicon solar cell.
[0013] Shah describes that the formation of a .mu.c-silicon thin
film is conducted using a high frequency power with an oscillation
frequency of 70 MHz. The deposition rate in this case is about 1
.ANG./sec. which is smaller.
[0014] With respect to the formation of an amorphous silicon (a-Si)
thin film by way of RF plasma CVD, for the high frequency discharge
in the RF band region raising the oscillation frequency has been
discussed. Particularly, in Applied Physics-related joint lecture
meetings of 1990 Autumn and 1991 Spring (28p-MF-14 and 28p-S-4),
Oda et al. of Tokyo Institute of Technology have reported that
amorphous silicon thin films were formed by conducting glow
discharge using a high frequency power with an oscillation
frequency of 144 MHz (which is of VHF (very high frequency) band
region) and the amorphous silicon thin films were evaluated.
[0015] Additionally, U.S. Pat. No. 4,400,409 discloses a process of
continuously preparing a photovoltaic element by using a continuous
plasma CVD apparatus of a roll-to-roll system. This document
describes that a plurality of glow discharge regions are separately
arranged along the path of a sufficiently long flexible substrate
having a desired width which is continuously transported to pass
through each of said glow discharge regions, and while forming a
desired semiconductor layer on the substrate in each glow discharge
region, the substrate is continuously transported, whereby a
photovoltaic element having a desired semiconductor junction can be
continuously formed.
[0016] In the case of forming a .mu.c-silicon series thin film by
RF glow discharge using a high frequency power with an oscillation
frequency of 13.56 MHz as in the foregoing prior art, the following
problems need to be solved or improved.
[0017] (1) There are such disadvantages as will be described in
semiconductor devices in which such .mu.c-siicon thin film, because
of the basic property of the thin film, there are such
disadvantages as will be described. That is, in the case of a thin
film transistor, the carrier mobility is small. In the case of a
photo sensor, its S/N ratio defined by a ratio between light
conductivity and that dark conductivity. In the case of a solar
cell, its photoconductivity (.sigma.p) is small.
[0018] (2) With respect to production yield, in the case of a large
area semiconductor device in which such .mu.c-silicon series thin
film is used, a decrease in the yield is caused due to the
distributions and the like of device characteristics which are
based on the distributions of film thickness and film quality.
[0019] (3) With respect to production cost, in the case of forming
a high quality .mu.c-siicon thin film usable in a thin film
semiconductor device, the productivity cannot be increased because
the deposition rate is small, resulting in an increase in the
production cost.
[0020] (4) It is difficult for the .mu.c-silicon thin film to have
a desired property controlled in the film thickness direction.
[0021] Eventually, in order to produce a large area .mu.c-silicon
thin film solar cell having improved device characteristics at a
high yield and at a reasonable production cost, it is necessary to
form a .mu.c-silicon thin film at a high deposition rate while
improving the basic property thereof. In addition, it is necessary
to realize a method which permits control of properties in the film
thickness direction.
[0022] In order to attain this object, in the plasma CVD process of
13.56 MHz, improvements in the production conditions such as flow
rate of raw material gas, pressure upon film formation, power
applied and the like have been generally tried. However, problems
are liable to occur as will be described in the following. That is,
when the deposition rate is increased, a deposited film becomes
amorphous (that is, the film becomes to be an a-Si film), the
amount of in-film hydrogen which is presumed to deteriorate the
property of a .mu.c-silicon thin film, is increased, or foreign
matter which causes a reduction in the yield is generated.
Specifically, for instance, as the deposition rate is increased,
the photoconductivity op as the basic property of the .mu.c-silicon
thin film is decreased. In this connection, in this process of
forming a .mu.c-silicon thin film, the deposition rate capable of
maintaining desirable device characteristics is in a range of from
about 0.2 to about 2 .ANG./sec.
[0023] In the RF glow discharge process, the range for controllable
parameters capable of forming a .mu.c-silicon thin film having a
good quality is narrow, where it is difficult to control the
property of the .mu.c-silicon thin film as desired.
[0024] The RF discharge process of 13.6 MHz has an advantage in
that film formation on a large area can be readily conducted.
However, it has disadvantages such that the deposition rate is
small and ion damage to a substrate or a .mu.c-silicon thin film
itself deposited thereon is large. In this connection, there is
occasionally used a triode process in which a third electrode is
provided between an anode and a cathode. However, this process is
not suitable in terms of industrial production of a .mu.c-silicon
thin film, because the utilization efficiency of raw material gas
is undesirably small and the maintenance efficiency is not
satisfactory. In this respect, this triode process is used only for
research purposes. In addition, in the case of forming a
.mu.c-silicon thin film by the triode process, it is difficult to
control the property thereof as desired. In the case of forming a
.mu.c-silicon thin film by means of a microwave discharge process
of 2.54 GHz, although there are advantages such that the deposition
rate is relatively large and there is no ion damage to the
substrate, there are disadvantages because it is difficult to
continuously maintain the glow discharge according to the current
technique, and the process controllability is not good. In
addition, there are other disadvantage such that gas decomposition
at a microwave introduction position is great and therefore, it is
difficult to conduct uniform deposition. In the case of a photo CVD
process, there are disadvantages such that the quality of a
.mu.c-silicon thin film deposited is not good and it is difficult
to deposit a .mu.c-silicon thin film on a large area. In addition,
the photo CVD process is an undeveloped film-forming technique. For
an ECR-CVD process, since it is possible to freely control the ion
damage to the substrate, there is a possibility of forming a high
quality .mu.c-silicon thin film. But because a magnetic field is
used, it is difficult to deposit .mu.c-silicon thin film in an
essentially uniform state.
[0025] As above described, it is difficult to effectively produce a
.mu.c-silicon thin film semiconductor device with good
reproducibility by any of the conventional techniques, because for
a .mu.c-silicon thin film formed, it is difficult make the
.mu.c-silicon thin film have a high quality to effectively produce
a desirable semiconductor device having satisfactory device
characteristics; it is difficult to make the .mu.c-silicon thin
film have a property controlled in the film thickness direction;
and it is difficult to stably and repeatedly form a high quality
.mu.c-silicon thin film having a desired property at a high
deposition rate and with good reproducibility. In addition to
these, the conditions for causing microcrystallization in the prior
art are severe, and therefore, it is difficult to stably and
repeatedly form a desired .mu.c-silicon thin film.
[0026] In the foregoing Applied Physics documents and Japanese
Unexamined Patent Publication No. 64466/1991 which describes a
similar technique, discussion is made only of amorphous silicon
(a-Si) thin film but no discussion is made of microcrystalline
silicon (.mu.c-Si) thin films. Shah does not touch on optimum
conditions for the formation of a .mu.c-Si thin film and no
discussion is made about the control of the property in the film
thickness direction. The technique described in Shah is not
effective in solving the problem which the present invention is
intended to solve.
SUMMARY OF THE INVENTION
[0027] A principal object of the present invention is to eliminate
the foregoing problems in the prior art and to provide an improved
process and apparatus which permits ready and efficient formation
of a high quality microcrystalline silicon series (.mu.c-Si series)
thin film having a desired property.
[0028] Another object of the present invention is to provide a
process and apparatus which permits ready and efficiently forming a
.mu.c-Si series thin film having a desired property capable of
producing a high quality semiconductor device, at an improved
film-forming raw material gas utilization efficiency and at a
reasonable production cost.
[0029] A further object of the present invention is to provide a
process for forming a .mu.c-Si series thin film film in which the
property thereof in the film thickness direction can be readily
controlled while maintaining the film property, so that a high
quality .mu.c-Si series thin film having a graded film property in
the film thickness direction can be readily produced.
[0030] A further object of the present invention is to provide a
process and apparatus which permits production of a high quality
.mu.c-Si series thin film semiconductor device at a reasonable
production cost.
[0031] A further object of the present invention is to provide a
process for forming a .mu.c-Si series thin film by arranging a
substrate in a vacuum chamber to oppose an electrode provided in
said vacuum chamber and while transporting said substrate in the
longitudinal direction, causing glow discharge between said
electrode and the substrate to deposit said .mu.c-Si series thin
film on the substrate, wherein a plurality of bar-like shaped
electrodes as said electrode are arranged such that they are
perpendicular to a normal line of said substrate and their
intervals to said substrate are all or partially differed. Said
glow discharge is caused using a high frequency power with an
oscillation frequency in a range of from 50 MHz to 550 MHz, whereby
depositing said .mu.c-Si series thin film on the substrate.
[0032] A further object of the present invention is to provide an
apparatus capable forming a .mu.c-Si series thin film on a
substrate, having a portion in which said substrate is arranged to
oppose to an electrode in a vacuum chamber, wherein while
transporting said substrate in the longitudinal direction, glow
discharge is caused between the electrode and the substrate to
deposit said .mu.c-Si series thin film on the substrate, wherein
said apparatus has a plurality of bar-like shaped electrodes as
said electrode which are arranged such that they are perpendicular
to a normal line of said substrate and their intervals to said
substrate are all or partially differed. A high frequency power
source for causing said glow discharge using a high frequency power
with an oscillation frequency in a range of from 50 MHz to 550 MHz
is used.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a schematic diagram illustrating an example of a
film-forming apparatus of a roll-to-roll system.
[0034] FIG. 2 is a view showing an example of distribution of
absorption coefficient in a width direction.
[0035] FIG. 3 is a view showing an example of distribution of
average grain size in a width direction.
[0036] FIG. 4 is a view showing an example of distribution of
crystal volume fraction in a width direction.
[0037] FIG. 5 is a view showing an example of distribution of
hydrogen content in a width direction.
[0038] FIG. 6 is a graph showing an example of a relationship
between absorption coefficient to light having a wavelength of 800
nm and an interval between a substrate and an electrode.
[0039] FIG. 7 is a graph showing an example of a relationship
between average crystal grain size and an interval between a
substrate and an electrode.
[0040] FIG. 8 is a graph showing an example of a relationship
between crystal volume fraction and an interval between a substrate
and an electrode.
[0041] FIG. 9 is a graph showing an example of a relationship
between hydrogen content and an interval between a substrate and an
electrode.
[0042] FIG. 10 is a graph showing an example of relationship
between a thickness of an amorphous layer formed and an interval
between a substrate and an electrode.
[0043] FIGS. 11, 12, 13, 16, 17, 18 and 20 are schematic
cross-sectional slant views respectively for explaining an example
of arranging relationship of a substrate and an electrode.
[0044] FIGS. 14(a) and 14(b) are schematic cross-sectional views
respectively for explaining an example of a cross section of a
.mu.c-Si series thin film.
[0045] FIGS. 15, 19 and 21 are graphs respectively for explaining
an example of hydrogen content in a depth (thickness) direction of
a deposited film.
DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
[0046] The present invention attains the foregoing objects.
[0047] The present invention provides a process and apparatus which
permit forming a .mu.c-Si series thin film having a desired
property which is capable of producing a high quality semiconductor
device, at an improved film-forming raw material gas utilization
efficiency and at a reasonable production cost.
[0048] As above described, a typical embodiment of the process is a
process for forming a .mu.c-Si series thin film by arranging a
substrate in a vacuum chamber so as to oppose an electrode provided
in said vacuum chamber and while transporting said substrate in the
longitudinal direction, causing glow discharge between said
electrode and the substrate to deposit said .mu.c-Si series thin
film on the substrate. A plurality of bar-like shaped electrodes as
said electrode are arranged such that they are perpendicular to a
normal line of said substrate and their intervals to said substrate
are all or partially differed. The glow discharge is caused by
using a high frequency power with an oscillation frequency in a
range of from 50 MHz to 550 Mhz, to deposit said .mu.c-Si series
thin film on the substrate.
[0049] A typical embodiment of the apparatus of the present
invention is an apparatus for forming a .mu.c-Si series thin film
on a substrate, having a portion in which said substrate is
arranged opposite to an electrode in a vacuum chamber. While
transporting said substrate in the longitudinal direction, glow
discharge occurs between the electrode and the substrate to deposit
said .mu.c-Si series thin film on the substrate. The apparatus has
a plurality of bar-like shaped electrodes as said electrode which
are arranged such that they are perpendicular to a normal line of
said substrate and their intervals to said substrate are all or
partially differed and a high frequency power source for causing
said glow discharge using a high frequency power with an
oscillation frequency in a range of from 50 MHz to 550 MHz.
[0050] The above-described process according to the present
invention includes the following embodiments with respect to the
arrangement of the bar-like shaped electrodes.
[0051] (I) The bar-like shaped electrodes are arranged such that
they are in parallel to each other.
[0052] (ii) The bar-like shaped electrodes are arranged such that
they are perpendicular to the transportation direction of the
substrate.
[0053] (iii) The bar-like shaped electrodes are arranged such that
their intervals to the substrate are widened in the upper side of
the transportation direction of the substrate and narrowed in the
down side thereof.
[0054] (iv) The bar-like shaped electrodes are arranged such that
their intervals to the substrate are periodically changed to the
transportation direction of the substrate.
[0055] In the following, description will be made of the
constitution and action of the present invention.
[0056] In the present invention, because of using a frequency band
in a range of from 50 MHz to 550 MHz which is between RF (radio
frequency) and MW (microwave), it is possible to form a high
quality .mu.c-Si series thin film at an increased deposition rate,
specifically, of more than 10 .ANG./sec.
[0057] Particularly, the frequency in this frequency band is higher
than RF, and because of this, the self bias of plasma is decreased
to diminish ion damage to a film deposited on a substrate. Further,
in comparison with the MW plasma CVD process, the introduction of a
power is easier and the controllable range is widened, where it is
possible to form a high quality thin film. In addition, as a result
of experimental studies by the present inventors, there was
obtained a finding. That is, when film deposition on a substrate is
conducted using a high frequency power of said frequency band, in
the case of a low power density (that is, in a region where the
deposition rate is increased in proportion to a power), the
property of a film deposited differs depending on a difference in
the interval between the electrode (the counter electrode) and the
substrate.
[0058] In the case of the above frequency band, in comparison with
RF, the maintenance of discharge under low pressure is easier, the
electron temperature is lower, and the electron density is higher,
and because of this, it is advantageous for the generation of long
life radicals (SiH.sub.3*, H*). At the same electron density, in
the case of RF, the vapor phase reaction proceeds to produce
polysilane. However, in the case of the above referenced band, even
under conditions of producing polysilane when using RF, a desirable
deposited film can be formed without producing such polysilane. In
addition, from the observation the discharge, a positive column
which is observed in the plasma in the case of RF is not seen,
however, emission is observed in the vicinity of the cathode. In
this connection, it is considered such that dissociation proceeds
in the vicinity of the cathode. Radicals generated in the vicinity
of the cathode, are considered isotropically diffused, and during
the diffusion, in a process of colliding with the parent molecules
(SiH.sub.4, H.sub.2), the ion species and the short life radicals
contained therein. Depending on a difference in the interval
between the substrate and the electrode, the density and component
of the radicals reaching the substrate are changed, and this
situation results in a difference in the property of a film
deposited.
[0059] Especially, in the case of a microcrystalline silicon
(.mu.c-Si), H* plays an important role for relaxation and
rearrangement of the lattice, and a microcrystal is grown by way of
combination of this radial with a stable site by SiH.sub.3* which
is likely to be surface-diffused. Hence, the ratio of these
radicals and the like are important factors.
[0060] Therefore, in the case of the above-described frequency
band, the magnitude of a power applied, the pressure, and the
interval between the substrate and the electrode are important
parameters in order to control the property of a film deposited. By
properly controlling these parameters, the grain size, crystal
deposition rate, hydrogen content, stress, absorption coefficient
and the like can be changed. Further, for a compound such as SiGe:H
or SiC:H, it is possible for the composition ratio thereof to be
controlled based on a difference in the decomposition energy or the
lifetime of a radical.
[0061] Further in the case of the above-described frequency band,
the wavelength is shortened to be equal to the electrode length.
When the electrode comprises a flat electrode, a standing wave is
present in the in-plane, where a node in which the power is weak
and an antinode in which the power is strong. The positions of
these node and antinode are different depending on the form of the
electrode, the power introduction position, the state of plasma
generated, or the like, and because of this, it is difficult for
them to be controlled as desired in the case of the flat
electrode.
[0062] Occurrence of this problem can be prevented by making the
flat electrode to comprise a plurality of bar-like shaped
electrodes.
[0063] Based on the above-described factors, in the roll-to-roll
film-forming system, by providing a plurality of bar-like shaped
electrodes and properly adjusting their intervals to a substrate
which is continuously transported, the property of a microcrystal
grown on said substrate in the thickness direction can be desirably
controlled.
[0064] In the present invention, by arranging the bar-like shaped
electrodes in parallel to each other, the way of the power
introduced by each electrode in a width direction to the
transportation direction of the substrate is equalized. As a
result, occurrence of film unevenness is further diminished.
Further, by arranging the bar-like shaped electrodes to be
perpendicular to the transportation direction of the substrate, the
film deposition position by each electrode is equalized to the
transportation distance of the substrate and as a result,
occurrence of film unevenness due to the form of the vacuum chamber
used or the passage of gas is further diminished.
[0065] As above described, the interval between the substrate and
the electrode influences the property of film deposited. In view of
a semiconductor device to be produced, a .mu.c-Si series thin film
is deposited on an amorphous substrate comprising glass or the like
or on a polycrystalline substrate comprising TCO (transparent
electrically conductive oxide) or the like. In this case, there are
such occasions that in the vicinity of the interface between the
substrate and the .mu.c-Si series thin film, an amorphous material
of several tens to several hundreds nm is present or a layer of a
small grain size is present. In many cases, being different from an
amorphous silicon of a device grade, such layer is inferior in
quality. Particularly, in the case of a semiconductor device such
as a solar cell in which an electron is longitudinally mobilized,
such layer is liable to deteriorate its electric characteristics.
Therefore, it is desired to make sure that no such layer is present
or when it should be present, its thickness is diminished.
[0066] Now, when a microcrystal having a large grain size and good
quality is to be deposited from an initial deposition stage, the
following are important factors.
[0067] (1) Generation of a homogeneous crystalline nucleus of low
density.
[0068] (2) Increase in crystal grain size.
[0069] (3) High speed growth in the film thickness direction.
[0070] For factor (1), when a highly dense crystalline nucleus is
generated, though it is considered that coalescing of grains would
occur, the grain size is substantially limited to an area occupied
by one crystalline nucleus. Therefore, in the crystalline nucleus
generation process, it is desired to employ conditions capable of
generating a crystalline nucleus of low density without causing
deposition of an amorphous layer.
[0071] For factor (2), ideally, it is desired to employ such
conditions that the crystalline nucleus is grown crosswise without
causing a new crystalline nucleus. This way a large grain size from
the initial deposition stage can be obtained.
[0072] For factor (3), in the case of using a microcrystal in a
semiconductor device, a thickness of about 1 dun is necessitated.
In order to shorten the time required in the preparation of the
semiconductor device, a high speed deposition rate is necessitated.
On a layer comprising a crystal of a .mu.c-Si series thin film,
crystallinity of good quality can be maintained by taking over the
crystallinity of the under layer even when the deposition rate is
increased to a certain extent.
[0073] Therefore, to grade the intervals of the bar-like shaped
electrodes to the substrate such that they are wide in the upper
side of the transportation direction of the substrate which is
corresponding to the initial deposition stage and narrow in the
down side is preferred in a viewpoint that the above process can be
realized.
[0074] Besides, in the case of a semiconductor device such as a
solar cell into which light is impinged from above, when it is
constituted such that short wavelength light is absorbed in a
region near the light incident side and long wavelength light is
absorbed in a region remote from the light incident side, the
semiconductor device can effectively absorb light. Therefore, the
absorption coefficient is preferred to be such that it is decreased
as the deposition proceeds. In this connection, by grading the
intervals of the bar-like shaped electrodes to the substrate such
that they are wide in the upper side of the transportation
direction of the substrate which is corresponding to the initial
deposition stage and narrow in the down side, the preparation of a
semiconductor device having such constitution as above described
can be realized.
[0075] Further, as above described, the property of a film
deposited can be changed by changing the intervals of the
electrodes to the substrate. Therefore, by arranging the bar-like
shaped electrodes such that their intervals to the substrate are
periodically changed to the transportation direction of the
substrate, the property of a film deposited can be periodically
changed. By this, the absorption coefficient is periodically
changed. In the case of forming a solar cell using such deposited
film whose absorption coefficient being periodically changed, there
can be together attained large Voc and Jsc which are greater than
those in the case of using a deposited film having a simple
absorption coefficient. In addition, the stress of the film is
changed so that the film is hardly peeled due a difference between
the stress of the substrate and that of the film. Further, the
resultant becomes desirably endurable upon curving or bending
processing.
[0076] In the present invention, a film whose property is
periodically changed and which has such advantages as above
described can be readily realized in one deposited film.
[0077] By using the previously described apparatus, such processes
above described can be readily conducted.
[0078] In the following, the process for forming a .mu.c-Si series
thin film according to the present invention will be detailed.
[0079] .mu.c-Si Series Thin Film:
[0080] Description will be made of a .mu.c-Si series thin film
according to the present invention which can be desirably used in a
photovoltaic element.
[0081] As the constituent of the .mu.c-Si series thin film, there
can be mentioned materials containing Si element as a matrix.
Specific examples of such material are group IV alloys such as
SiGe, SiC, SiSn and the like.
[0082] As the .mu.c-Si series thin film, there can be used
appropriate microcrystalline semiconductor materials such as group
IV microcrystalline semiconductor materials and group IV alloy
series microcrystalline semiconductor materials. Preferable
specific examples are .mu.c-Si:H (hydrogenated microcrystalline
silicon), .mu.c-Si:F, .mu.c-Si:H:F, .mu.c-SiGe:H, .mu.c-SiGe:F,
.mu.c-SiGe:H:F, .mu.c-SiC:H, .mu.c-SiC:F, and .mu.c-SiC:H:F.
[0083] Incidentally, for a semiconductor layer, valence electron
control or forbidden band width control can be conducted. The
control of these items can be conducted by separately introducing a
raw material compound containing an element capable of being a
valence electron controlling agent into a film-forming space upon
forming a semiconductor layer or by mixing said raw material
compound with a film-forming raw material gas or a dilution gas,
followed by introduction into the film-forming space.
[0084] In the present invention, in the case where a .mu.c-Si
series thin film according to the present invention is subjected to
valence electron control by means a valence electron controlling
agent, at least part thereof is doped to be p-type and n-type,
whereby forming at least a pin conjunction. And by stacking a
plurality of pin junctions, there can be formed a stacked cell
structure.
[0085] In the case of using a .mu.c-Si series thin film according
to the present invention as a power generation layer (a
semiconductor photoactive layer) of a photovoltaic element, the
average grain size is preferred to be in a range of from 10 nm to 1
pm, the crystal deposition rate is preferred to be in a range of
from 10% to 99%, and the hydrogen content is preferred to be in a
range of from 0.1 atomic % to 40 atomic %.
[0086] Formation of .mu.c-Si Series Thin Film:
[0087] In the process of forming a .mu.c-Si series thin film
according to the present invention, by means of a high frequency
power with an oscillation frequency in a range of from 50 MHz to
550 MHz, plasma is generated to dissociate and decompose a raw
material gas, to cause film deposition on a substrate.
[0088] Specifically, starting gases such as a film-forming raw
material gas, dilution gas and the like are introduced into a
deposition chamber (a vacuum reaction chamber) whose inside can be
maintained under reduced pressure. The inner pressure of the
deposition chamber is made to be constant while evacuating the
inside of the deposition chamber by means of a vacuum pump. A high
frequency power with a desired oscillation frequency in the
foregoing range from a high frequency power source is supplied to
bar-like shaped electrodes arranged in the deposition chamber
through a cable and a waveguide to generate plasma of said gases
and decompose said gases, whereby forming a desired .mu.c-Si series
thin film on a substrate. According to the process of the present
invention, it is possible to form a desirable .mu.c-Si series thin
film usable in a photovoltaic element under a wide range of
deposition conditions.
[0089] The substrate temperature in the deposition chamber upon the
film formation is preferred to be in a range of from 100 to
500.degree. C. Similarly, the inner pressure is preferred to be in
a range of from 1.3.times.10.sup.-1 to 1.3.times.10.sup.2 Pa. For
the wattage of the high frequency power, it is preferred to be in a
range of from 0.05 to 50 W/cm.sup.2.
[0090] For the bar-like shaped electrodes used in the process of
forming a .mu.c-Si series thin film, each of them the length longer
than a diameter or a longest side of the cross section. Each of the
bar-like shaped electrodes may be of a cross section in a round
form, an ellipsoidal form, a square form or a rectangular form.
Each of the bar-like shaped electrodes is constituted by a material
which is not melted with heat and which does not cause reactions.
Specific examples of such material are stainless steel and the
like.
[0091] All the bar-like shaped electrodes are not necessary to be
uniform with respect to their forms, but their forms may be varied.
For the number of the bar-like shaped electrodes, it is properly
determined depending on the size of a vacuum chamber (a deposition
chamber) used, the deposition rate and the property for a .mu.c-Si
series thin film to be formed.
[0092] As described in Experiments 2 and 3 which will be described
later, by adjusting the interval between the substrate and the
electrode, the property of a .mu.c-Si series thin film to be formed
can be changed. Therefore, the bar-like shaped electrodes are
desired to be arranged such that their intervals to the substrate
(the substrate) are all or partially varied, so that a desirable
.mu.c-Si series thin film can be formed. In a more preferred
embodiment in this case, the intervals of the bar-like shaped
electrodes to the substrate are graded such that they are wide in
the upper side of the transportation direction of the substrate
which is corresponding to the initial deposition stage and narrow
in the down side. In order to form a desirable .mu.c-Si series thin
film having different properties, it is preferred to arrange the
bar-like shaped electrodes such that their intervals to the
substrate (the substrate) are partially periodically changed. [The
intervals of the bar-like shaped electrodes to the substrate (the
substrate) will be hereinafter simply referred to as "interval
between the substrate and the electrode"].
[0093] The interval between the substrate and the electrode is
different depending on related film-forming conditions for forming
a desired .mu.c-Si series thin film. However, in view of a
deposition rate which can be industrially employed in practice and
also in terms of stability in the glow discharge, it is preferred
to be in a range of from about 0.5 cm to about 30 cm.
[0094] The supply of a high frequency power to each of the bar-like
shaped electrodes may be conducted by a manner of divergently
supplying a high frequency power to each of the bar-like shaped
electrodes from a single high frequency power source.
Alternatively, it may be conducted by using a plurality of high
frequency power sources. In this case, these high frequency power
sources are desired to be complete with respect to their stability
and conformity of oscillation frequency. If necessary, it is
possible to conform their oscillation frequency by using a phase
shifter or the like.
[0095] As the film-forming raw material gas suitable for forming a
.mu.c-Si series thin film of the present invention, there can be
representatively mentioned silicon (Si)-containing compounds which
are in the gaseous state at room temperature or can be easily
gasified, germanium(Ge)-containing compounds which are in the
gaseous state at room temperature or can be easily gasified, carbon
(C)-containing compounds which are in the gaseous state at room
temperature or can be easily gasified, and mixtures of these
compounds.
[0096] Such Si-containing compound can include chain or cyclic
silane compounds. Specific examples are SiH.sub.4, Si.sub.2H.sub.6,
SiF.sub.4, SiFH.sub.3, SiF.sub.2H.sub.2, SiF.sub.3H,
Si.sub.3H.sub.8, SiD.sub.4, SiHD.sub.3, SiH.sub.2D.sub.2,
SiH.sub.3D, SiFD.sub.3, SiF.sub.2D.sub.2, Si.sub.2D.sub.3H.sub.3,
(SiF.sub.2).sub.5, (SiF.sub.2).sub.6, (SiF.sub.2).sub.4,
Si.sub.2F.sub.6, Si.sub.3F.sub.8, Si.sub.2H.sub.2F.sub.4,
Si.sub.2H.sub.3F.sub.3, (SiCl.sub.2).sub.5, SiBr.sub.4,
(SiBr.sub.2).sub.5, Si.sub.2Cl.sub.6, SiHCl.sub.3,
SiH.sub.2Br.sub.2, SiH.sub.2Cl.sub.2, and
Si.sub.2Cl.sub.3F.sub.3.
[0097] Specific examples of such Ge-containing compound are
GeH.sub.4, GeD.sub.4, GeF.sub.4, GeFH.sub.3, GeF.sub.2H.sub.2,
GeF.sub.3H, GeHD.sub.3, GeH.sub.2D.sub.2, GeH.sub.3D,
Ge.sub.3H.sub.6, and Ge.sub.2D.sub.6.
[0098] Specific examples of such C-containing compound are
CH.sub.4, CD.sub.4, C.sub.nH.sub.2n+2 (with n being an integer),
CH.sub.2n, (with n being an integer), C.sub.2H.sub.2,
C.sub.6H.sub.6, CO.sub.2, and CO.
[0099] Other than the above-mentioned raw materials, it is possible
to use nitrogen (N)-containing raw material gas and oxygen
(O)-containing raw material gas. Specific examples of such
N-containing raw material gas are N.sub.2, NH.sub.3, ND.sub.3, NO,
NO.sub.2, and N.sub.2O. Specific examples of such O-containing raw
material gas are O.sub.2, CO, CO.sub.2, NO, N.sub.2O,
CH.sub.3CH.sub.2OH, and CH.sub.3OH.
[0100] As above described, a .mu.c-Si series thin film of the
present invention may be controlled so as to have a conductivity of
p-type or n-type by incorporating an appropriate valence electron
controlling element (that is, a dopant) thereinto. Such element can
include elements belonging to group IIIb of the periodic table
which provide a p-type conductivity (these elements will be
hereinafter referred to as group IIIb elements) and elements
belonging group Vb of the periodic table which provide an n-type
conductivity (these elements will be hereinafter referred to as
group Vb elements).
[0101] In order for the .mu.c-Si series thin film to contain such
dopants, an appropriate raw material capable of supplying a group
Ib or Vb element is used in addition to the foregoing film-forming
raw material.
[0102] Such group IIIb or Vb element-supplying raw material can
include raw materials capable of supplying a group IIIb or Vb
elements, which are in the gaseous state at room temperature or can
be easily gasified at least under the condition for the formation
of the .mu.c-Si series thin film.
[0103] Such group IIIb element-supplying gaseous or gasifiable raw
material can include boron hydrides such as B.sub.2H.sub.6,
B.sub.4H.sub.10, B.sub.5H.sub.9, B.sub.5H.sub.11, B.sub.6H.sub.10,
B.sub.6H.sub.12, and B.sub.6H.sub.14; and boron halides such as
BF.sub.3, BCl.sub.3, and BBr.sub.3. Besides, AlCl.sub.3,
GaCl.sub.3, Ga(CH.sub.3).sub.3, InCl.sub.3, and T1Cl.sub.3 are also
usable. Of these, B.sub.2H.sub.6 and BF.sub.3 are particularly
preferable.
[0104] Such group Vb element-supplying gaseous or gasifiable raw
material can include phosphorous hydrides such as PH.sub.3 and
P.sub.2H.sub.4; and phosphorous halides such as PH.sub.4I,
PF.sub.3, PF.sub.5, PCl.sub.3, PCl.sub.5, PBr.sub.3, PBr.sub.5 and
PI.sub.3. Besides, AsH.sub.3, AsF.sub.3, AsCl.sub.3, AsBr.sub.3,
AsF.sub.5, SbH.sub.3, SbF.sub.3, SbF.sub.5, SbCl.sub.3, SbCl.sub.5,
BiH.sub.3, BiCl.sub.3, and BiBr.sub.3 are also usable. Of these,
PH.sub.3 and PF.sub.3 are particularly preferable.
[0105] Any of these group IIIb or Vb element-supplying raw
materials may be diluted with an appropriate gas such as H.sub.2
gas or inert gas such as Ar, He, Ne, Xe or Kr, if necessary.
[0106] In the following, description will be made of experiments
which were conducted by the present inventors during the process to
accomplish the present invention.
Experiment 1
[0107] First, description will be made of a film-forming apparatus
of the roll-to-roll system used in this experiment.
[0108] Specifically, there was used a film-forming apparatus having
such constitution as shown in FIG. 1. In FIG. 1, reference numeral
101 indicates a vacuum chamber (or a deposition chamber) which is
provided with an exhaust pipe 107 connected to a vacuum pump (not
shown) though a conductance valve 108 (comprising a butterfly
valve). The exhaust pipe 107 is provided with a vacuum gauge 109.
The inner pressure in the inside of the vacuum chamber 101 can be
adjusted to a desired pressure by evacuating the inside of the
vacuum chamber through the exhaust pipe by actuating the vacuum
pump while properly regulating the conductance valve 108 and
observing the vacuum gauge 109.
[0109] Reference numeral 102 indicates a web substrate (a
substrate) on which a film is to be deposited. The web substrate
102 is wound on a pay-out roll 111 provided in a load chamber 114
capable of being vacuumed. The web substrate 102 is paid out from
the pay-out roll 111, and it passes through a gas gate 113a which
is provided at a side wall (on the left side in the figure) of the
vacuum chamber and enters into the vacuum chamber 101.
Successively, the web substrate 102 passes through a gas gate 113b
which is provided at a side wall (on the right side in the figure)
of the vacuum chamber and enters into an unload chamber 115 capable
of being vacuumed, where it is taken up by a take-up roll 112
provided in the unload chamber 115, followed by being wound on the
take-up roll. Here, the pay-out roll side is an upper side and the
take-up roll side is a down side in relation to the transportation
direction of the web substrate.
[0110] Specifically, by rotating the pay-out roll 111 by means of a
driving motor (not shown), from the pay-out roll 111, the web
substrate 102 is continuously supplied, continuously moved in the
vacuum chamber 101 in the longitudinal direction, and wound on the
take-up roll 112. That is, the web substrate 102 is transported
from the left side in the figure toward the right side in the
figure.
[0111] Each of the gas gates 113a and 113b is structured so that
the web substrate 102 can be transported without breaking the
vacuum and a gate gas can be blown therein. The gas gate 113a
serves to communicate between the load chamber 114 and the vacuum
chamber 101 while maintaining each of the two chambers under
reduced pressure. Similarly, the gas gate 113b serves to
communicate between the vacuum chamber 101 and the unload chamber
115 while maintaining each of the two chambers under reduced
pressure.
[0112] The gas gates 113a and 113b serve also as a communication
means when the vacuum chamber 101 is connected to other vacuum
chamber.
[0113] Reference numeral 110 indicates gas supply pipes which are
open into the vacuum chamber 101. The gas supply pipes 110 are
extended from a raw material gas supply system having a plurality
of gas reservoirs each containing a given raw material gas (not
shown in the figure) and they serve to introduce desired raw
material gas into the vacuum chamber 101. The raw material gas
introduced into the vacuum chamber 101 is exhausted through the
exhaust pipe 107.
[0114] On rear side (which is not opposed to plasma 106) of the web
substrate 102 in the vacuum chamber 101, there is provided a lamp
heater unit 103 capable of radiating heat to heat the web substrate
102. Reference numeral 104 indicates a bar-like shaped electrode
which is electrically connected to a high frequency power source
105 having a matching box installed therein through a cable or a
waveguide (not shown).
[0115] The formation of a .mu.c-Si series thin film using the
film-forming apparatus shown in FIG. 1 is conducted in the
following manner.
[0116] The web substrate 102 from the pay-out roll 111 is passed
through the vacuum chamber 101 and fixed to the take-up roll 112.
The inside of the vacuum chamber 101 is evacuated to a desired
vacuum through the exhaust pipe 107 by actuating the vacuum pump
(not shown). Through the gas supply pipes 110, prescribed raw
material gas and dilution gas are introduced into the vacuum
chamber 101. The lamp heater unit 103 is actuated, and the take-up
roll 112 is rotated by means of a driving motor (not shown) to
continuously move the web substrate 102. By this, the web substrate
102 is continuously transported while passing through the vacuum
chamber 101, where the web substrate 102 situated in the vacuum
chamber 101 is heated by the lamp heater unit 103. Then, the high
frequency power source 105 is switched on to supply a high
frequency power with a desired oscillation frequency to the
bar-like shaped electrode, where glow discharge is generated to
produce plasma 106, whereby the raw material gas is decomposed by
the action of the plasma to deposit a deposited film (a .mu.c-Si
series thin film) on the web substrate which is continuously
moving.
[0117] Description will be made of experiments conducted by the
present inventors.
[0118] Using the film-forming apparatus shown in FIG. 1, a .mu.c-Si
series thin film was formed on a web substrate 102 which is
continuously moving, in accordance with the above-described
manner.
[0119] As the web substrate 102, there was used a stainless steel
SUS430BA web 0.2 mm thick, 20 cm in width and 50 m in length. As
the vacuum pump of the vacuum pump, there were used a rotary pump,
a mechanical booster pump and a turbo-molecular pump.
[0120] Specifically, the inside of each of the load chamber 114
containing the pay-out roll 111, the unload chamber 115 containing
the take-up roll 112, and the vacuum chamber 101 was roughly
evacuated by means of the rotary pump. Successively, the inside of
each of the load chamber 114 containing the pay-out roll 111, the
unload chamber 115 containing the take-up roll 112, and the vacuum
chamber 101 was evacuated by means of the mechanical booster pump
until the pressure became about 1.3.times.10.sup.-1 Pa. While
maintaining the surface of the web substrate at 300.degree. C. by
means of the lamp heater unit 103, the inside of the vacuum chamber
101 is evacuated to a vacuum of about 2.7.times.10.sup.-3 Pa by
means of the turbo-molecular pump. When the inner pressure of the
vacuum chamber 101 became stable at about 2.7.times.10.sup.-3 Pa,
H.sub.2 gas of 400 sccm as a purging gas from the gas supply system
(not shown) was introduced into the vacuum chamber through a mass
flow controller (not shown) and the gas supply pipes 110 and a
conduit (not shown) connected to the vacuum chamber 101.
Successively, the inside of the vacuum chamber 101 was evacuated by
means of the turbo-molecular pump for 2 hours while regulating the
butterfly valve 108 provided at the exhaust pipe 107 so that the
reading on the vacuum gauge 109 became 6.7.times.10.sup.-1.
[0121] Thereafter, SiH.sub.4 gas and H.sub.2 gas from the gas
supply system (not shown) were introduced into the vacuum chamber
101 through mass flow controllers (not shown) and the gas supply
pipes 110 under conditions described below, where the butterfly
valve 108 was regulated so that the reading on the vacuum gauge 109
became to be in a range 1.3.times.10.sup.-1 to 1.3.times.10.sup.2
Pa. Particularly, the above gases were first flowed for 30 minutes.
Then, while the gases continued flowing, from the high frequency
power source 105, a high frequency power with an oscillation
frequency in a range of 13.56 MHz to 1000 MHz in terms of an
effective value was supplied to the bar-like shaped electrode 104
to produce plasma 106, whereby a deposited film was formed on the
web substrate 102 over its length of 40 m. In this case, the web
substrate 102 was transported at a transportation speed of 14
cm/minute.
[0122] The film-forming conditions in this case are as follows.
[0123] Film-Forming Conditions:
[0124] raw material gas (SiH.sub.4): 50 sccm
[0125] dilution gas (H.sub.2): 2000 sccm
[0126] oscillation frequency: 13.56 to 1000 MHz
[0127] high frequency power: 0.05 to 50 W/cm.sup.2
[0128] inner pressure in the deposition chamber:
1.3.times.10.sup.-1 to 1.3.times.10.sup.2 Pa
[0129] interval between substrate and electrode: 0.5 to 30 cm
substrate temperature: 300.degree. C.
[0130] deposited film thickness: 7 .mu.n
[0131] deposition rate: 10 .ANG./sec.
[0132] electrode form: 5 cm (diameter).times.25 cm (length)
[0133] After the formation of the deposited film, the web substrate
102 was cooled to room temperature and taken out from the
film-forming apparatus.
[0134] In this way, there were obtained a plurality of samples each
comprising the web substrate 102 having the deposited film formed
thereon (each sample will be hereinafter referred to as "web
substrate sample").
[0135] For each web substrate sample, with respect to its width
direction, a part thereof was cut to obtain an experimental sample.
Thus, there were obtained a plurality of experimental samples.
[0136] For these experimental samples prepared by changing the
oscillation frequency and adjusting the wattage of the high
frequency power, the inner pressure in the vacuum chamber, and the
interval between the substrate and the electrode so that the
deposition rate was constant for the purpose of examining
influences of the oscillation frequency, the following evaluations
were conducted. The evaluated results with respect to distribution
of the film property in the width direction of the web substrate
102 are graphically shown in FIGS. 2 to 5.
[0137] Evaluation Contents:
[0138] 1. Visual examination and examination by an optical
microscope.
[0139] 2. Evaluation of absorption coefficient:
[0140] Wavelength dependency of the absorption coefficient of a
.mu.c-Si series thin film was examined by measuring its
transmittance using a spectrophotometer U4000 type (produced by
Hitachi Ltd.).
[0141] 3. Evaluation of average grain size:
[0142] This evaluation was conducted in a manner wherein a cross
section of the crystal is observed by means of a transmission
electron microscope (TEM) JEM-4000EX (produced by JEOL Ltd.),
respective crystal grain boundaries are determined by way of image
processing, and based on the resultant images, an average grain
size in the vicinity of the surface and in a direction parallel to
the substrate is obtained.
[0143] 4. Evaluation of Crystal Volume Fraction:
[0144] This evaluation was conducted in a manner wherein a Raman
scattering spectrum is measured by means of a laser. Raman
spectrophotometer NRS200C (produced by Nihon Bunko Kabushiki
Kaisha), followed by obtaining an intensity ratio between a strong
signal from a crystal near 520 cm.sup.-1 and a broad signal from an
amorphous material near 480 cm.sup.-1, whereby obtaining a crystal
volume fraction.
[0145] 5. Evaluation of hydrogen content:
[0146] This evaluation was conducted in a manner wherein an
infrared absorption spectrum is measured by means of a FTIR method
and based on the absorption near 2000 cm.sup.-1, a hydrogen content
is obtained.
[0147] As a result, in the case of the oscillation frequency being
less than 50 MHz, even when film deposition was conducted at a
desired deposition rate by properly changing the wattage of the
high frequency power, the interval between the substrate and the
electrode, and the inner pressure in the vacuum chamber, there was
deposited an amorphous silicon film only. As a result of observing
its surface by way of visual examination and examination by means
of an optical microscope, it was found that the surface is
white-clouded and is in a roughened state. In addition, there was
observed the presence of polysilane in the inside of the vacuum
chamber. In this respect, it is considered that such polysilane is
incorporated into the film and as a result, such film resulted.
[0148] In the case of the oscillation frequency being beyond 550
MHz, sudden disappearance of the glow discharge was first observed.
And even when film deposition was conducted at a desired deposition
rate by properly changing the wattage of the high frequency power,
the interval between the substrate and the electrode, and the inner
pressure in the vacuum chamber, there was observed unevenness (of
more than 10%) for the film property in the width direction. In
addition, the situation of causing such unevenness was different
with respect to the longitudinal direction of the substrate. Hence,
it was found that in the case of forming a .mu.c-Si series thin
film on a substrate, it is difficult to maintain a desired film
property.
Experiment 2
[0149] Using the same apparatus used in Experiment 1 and following
the procedures in Experiment 1, how the properties of the resulting
.mu.c-Si series thin films are changed by changing the interval
between the substrate and the electrode was examined. The
film-forming conditions were made as follows.
[0150] Film-Forming Conditions:
[0151] raw material gas (SiH.sub.4): 50 sccm
[0152] dilution gas (H.sub.2): 2000 sccm
[0153] oscillation frequency: 200 MHz
[0154] high frequency power: 10 W/cm.sup.2
[0155] inner pressure in the deposition chamber: 2.7.times.10.sup.1
Pa
[0156] interval between substrate and electrode: 0.5 to 30 cm
[0157] substrate temperature: 300.degree. C.
[0158] deposited film thickness: 7 .mu.m
[0159] electrode form: 5 cm (diameter).times.25 cm (length)
[0160] And following the evaluation procedures in Experiment 1,
evaluation was conducted.
[0161] As a result, it was found that when the interval between the
substrate and the electrode is 0.5 cm, there is formed a deposited
film apparently of amorphous silicon. And when the interval between
the substrate and the electrode was beyond 30 cm, the film
deposition rate became slow, and it was substantially difficult to
deposit a desired thickness.
[0162] The results obtained in this experiment are graphically
shown in FIGS. 6 to 9. As these figures illustrate, it was found
that the property of the resulting .mu.c-Si series thin film is
varied by changing the interval between the substrate and the
electrode.
Experiment 3
[0163] Using the same apparatus used in Experiment 1 and following
the procedures in Experiment 1, how the properties of the resulting
.mu.c-Si series thin films are changed depending on a given
condition at the initial film deposition stage by a manner of
changing only the interval between the substrate and the electrode
at the initial deposition stage and conducting successive film
deposition under the same film-forming conditions was examined. For
this purpose, there was formed a pin junction type photovoltaic
element. Particularly, an n-type a-Si layer, an i-type .mu.c-Si
layer and a p-type .mu.c-Si layer were sequentially formed on the
web substrate. The formation of the n-type layer and the p-type
layer was conducted using the apparatus shown in FIG. 1 except for
changing the high frequency power source 105 to an RF power source
(13.56 MHz).
[0164] The respective conditions were made as will be described
below.
[0165] The Conditions at the Initial Film Deposition Stage:
[0166] raw material gas (SiH.sub.2): 50 sccm
[0167] dilution gas (H.sub.2): 2000 sccm
[0168] oscillation frequency: 200 MHz
[0169] high frequency power: 50 W/cm.sup.2
[0170] inner pressure in the deposition chamber: 2.7.times.10.sup.1
Pa
[0171] interval between substrate and electrode: 0.5 to 30 cm
[0172] substrate temperature: 300.degree. C.
[0173] deposited film thickness: 300nm
[0174] The Same Conditions Thereafter:
[0175] raw material gas (SiH.sub.4): 50 sccm
[0176] dilution gas (H.sub.2): 2000 sccm
[0177] oscillation frequency: 200 MHz
[0178] high frequency power: 50 W/cm.sup.2
[0179] inner pressure in the deposition chamber: 2.7.times.10.sup.1
Pa
[0180] interval between substrate and electrode: 5 cm
[0181] substrate temperature: 300.degree. C.
[0182] deposited film thickness: 6.7 .mu.m
[0183] electrode form: 5 cm (diameter).times.25 cm (length)
[0184] The Conditions for the Formation of the n-Type layer:
[0185] SiH.sub.4: 5 sccm
[0186] H.sub.2: 50 sccm
[0187] PH.sub.3/H.sub.2 (5%): 0.1 sccm
[0188] RF power: 0.1 W/cm.sup.2
[0189] inner pressure in the deposition chamber: 1.1.times.10.sup.2
Pa
[0190] substrate temperature: 300.degree. C.
[0191] deposited film thickness: 30 nm
[0192] The Conditions for the Formation of the p-Type Layer:
[0193] SiH.sub.4: 5 sccm
[0194] H.sub.2: 200 sccm
[0195] BF.sub.3/H.sub.2 (5%): 0.1 sccm
[0196] RF power: 3 W/cm.sup.2
[0197] inner pressure in the deposition chamber: 2.0.times.10.sup.2
Pa
[0198] substrate temperature: 200.degree. C.
[0199] deposited film thickness: 20 nm
[0200] For the photovoltaic element thus formed, its crystal cross
section was observed by means of a transmission electron microscope
(TEM) JEM-4000EX (produced by JEOL Ltd.), and the thickness of a
portion considered to comprise an a-Si from an initially deposited
portion was evaluated. The results obtained are graphically shown
in FIG. 10. Further, for the photovoltaic element, evaluation of
its initial photoelectric conversion efficiency and light
degradation test were conducted. The results obtained are
collectively shown in Table 1.
[0201] The evaluation of initial photoelectric conversion
efficiency was conducted by placing a solar cell prepared using the
photovoltaic element under the irradiation of pseudo sunlight of
AM-1.5 (100 mW/cm.sup.2) and measuring its V-I characteristics.
[0202] The light degradation test was conducted by placing the
solar cell having been subjected to the evaluation of the initial
photoelectric conversion efficiency in an environment with a
humidity of 50% and a temperature of 25 .degree. C. while
irradiating pseudo sunlight of AM-1.5 thereto for 500 hours,
measuring its photoelectric conversion efficiency under the
irradiation of pseudo sunlight of AM-1.5 (100 mW/cm.sup.2) in the
same manner as above and calculating a deteriorated rate of the
initial photoelectric conversion efficiency.
[0203] Based on the results shown in Table 1, there were obtained
such findings as will be described in the following. That is, by
changing the interval between the substrate and the electrode, it
is possible to prevent the deposition of an amorphous layer at the
initial film deposition stage and a layer whose constituent crystal
grains being of small size. And the presence of these layers
deteriorates the characteristics of a semiconductor device such as
a photovoltaic element.
[0204] In the following, the present invention will be described in
more detail with reference to examples. It should be understood
that these examples are only for illustrative purposes and are not
intended to restrict the scope of the present invention.
EXAMPLE 1
[0205] As the bar-like shaped electrode 104 in the film-forming
apparatus shown in FIG. 1, as shown in FIG. 11 (which is a
conceptual diagram which is not always in agreement with actual
experimental conditions), a plurality of bar-like shaped electrodes
204 each being the same as the bar-like shaped electrode 104 used
in Experiment 1 were arranged such that they are perpendicular to a
normal line of a substrate 102 and their intervals to the substrate
differ from each other.
[0206] Using the apparatus thus constituted, a .mu.c-Si series thin
film was formed under such conditions as will be described
below.
[0207] As the substrate 102, there was used a stainless steel
SUS430BA web of 0.2 mm in thickness, 20 cm in width and 50 m in
length having a 1 .mu.m thick zinc oxide (ZnO) film formed thereon.
The formation of .mu.c-Si series thin film was conducted in
accordance with the procedures in Experiment 1, except for changing
the film-forming conditions in Experiment 1 to those below
mentioned.
[0208] Film-Forming Conditions:
[0209] raw material gas (SiH.sub.4): 50 sccm
[0210] dilution gas (H.sub.2): 2000 sccm
[0211] oscillation frequency: 200 MHz
[0212] high frequency power: 10 W/cm.sup.2
[0213] inner pressure in the deposition chamber:
2.7.times.10.sup.-1 Pa intervals of the electrodes to the substrate
(from the upper side of the substrate transportation
direction):
[0214] 3 cm interval: 10 bar-like shaped electrodes,
[0215] 5 cm interval: 10 bar-like shaped electrodes,
[0216] 10 cm interval: 10 bar-like shaped electrodes,
[0217] 5 cm interval: 10 bar-like shaped electrodes, and
[0218] 4 cm interval: 10 bar-like shaped electrodes
[0219] interval between each adjacent electrodes (the distance from
the center on the central axis of one bar-like shaped electrode to
that of the other bar-like shaped electrode):
[0220] 10 cm
[0221] substrate temperature: 300.degree. C.
[0222] deposited film thickness: 10 .mu.m
COMPARATIVE EXAMPLE 1-1
[0223] Using a flat plate type electrode 1004 as shown in FIG. 12
(which is a conceptual diagram which is not always in agreement
with actual experimental conditions), the formation of .mu.c-Si
series thin film was conducted under film-forming conditions
similar to those in Example 1.
[0224] The flat plate type electrode 1004 herein is of a length of
5 m. The interval between the substrate and the electrode was
graded to be from 3 cm to 5 cm from the upper side of the substrate
transportation direction. Here, reference numeral 1007 in FIG. 12
schematically shows a plasma high density region.
COMPARATIVE EXAMPLE 1-2
[0225] Using a plurality of bar-like shaped electrode 104 as shown
in FIG. 13 (which is a conceptual diagram which is not always in
agreement with actual experimental conditions), the formation of
.mu.c-Si series thin film was conducted under film-forming
conditions similar to those in Example 1.
[0226] In this example, 50 bar-like shaped electrodes were spacedly
arranged so as to have an equal interval of 3 cm to a substrate
102. And the interval between each adjacent electrode (the distance
in a horizontal direction from the center on the central axis of
one bar-like shaped electrode to that of the other bar-like shaped
electrode) was made to be 10 cm.
[0227] In each of Example 1 and Comparative Examples 1-1 and 1-2,
plasma discharge was maintained for 10 hours, where the state of
plasma produced was observed by way of visual examination. As a
result, in each of Example 1 and Comparative Example 1-2, the state
of the plasma was stable all the time. But in Comparative Example 1
- 1, such a localized plasma region that is schematically shown as
a plasma high density region 1007 in FIG. 12 or a partially dense
plasma region occurred. In addition, it was observed that the
discharging position was suddenly changed during the plasma
discharge.
[0228] As a result of having measured a film thickness distribution
to the prescribed 10 .mu.m, the film thickness distribution in each
of Example 1 and Comparative Example 1-2 was found to fall within
15% for both the width direction and the longitudinal direction of
the substrate. On the other hand, in Comparative Example 1-1, it
was found to be beyond 15%.
[0229] As a result of having evaluated a distribution with respect
to each of absorption coefficient, crystal volume fraction, and
hydrogen content, the distribution in each of Example 1 and
Comparative Example 1-2 was found to fall within 15% with respect
to the width direction and the longitudinal direction of the
substrate. On the other hand, in Comparative Example 1-1, amorphous
portions were present, and the distribution was found to be beyond
15%.
[0230] In each of Example 1 and Comparative Example 1-2, evaluation
was conducted with respect to profile of each of absorption
coefficient, crystal volume fraction, and hydrogen content in the
depth direction in the following manner.
[0231] Evaluation of Absorption Coefficient:
[0232] In order to examine a variation in a band profile of
absorption coefficient for a thin film sample, a variation in the
absorption coefficient in the depth direction is evaluated in a
manner of polishing a predetermined thickness of the thin film
sample by means of CMP (chemical-mechanical polishing), and
measuring the transmittance thereof by means of a spectrophotometer
U4000 type (produced by Hitachi Ltd.). The absorption coefficient
to corresponds an average value thereof when the absorption
coefficient is considered to be distributed with the film.
Therefore, it is possible to estimate a distribution of the
absorption coefficient by observing how the absorption coefficient
is changed along with a change in the film thickness.
[0233] Evaluation of Average Grain Size:
[0234] This evaluation is conducted in a manner wherein a crystal
cross section of a thin film sample is observed by means of a
transmission electron microscope (TEM) JEM-4000EX (produced by JEOL
Ltd.), respective crystal grain boundaries are determined by way of
image processing, and based on the resultant images, a variation in
an average grain size in the depth direction in a direction
parallel to the substrate is examined.
[0235] Evaluation of Crystal Deposition Rate:
[0236] This evaluation is conducted as follows. That is, for a thin
film sample (which is formed on the ZnO (of polycrystal) of the
substrate), when the substrate is subjected to plastic deformation
at an acute angle of more than 150.degree., there is afforded a
cross section of the .mu.c-Si layer together with a wall interface
of the ZnO polycrystal. For said cross section, by using a laser
Raman spectrophotometer NRS200C (produced by Nihon Bunko Kabushiki
Kaisha) while throttling the spot of the laser beam, a Raman
scattering spectrum is measured with a resolving power of 1 .mu.m,
followed by obtaining an intensity ratio between a strong signal
from a crystal near 520 cm.sup.-1 and a broad signal from an
amorphous material near 480 cm.sup.-1, and based on the intensity
ratio, a variation in the crystal deposition rate in the depth
direction is evaluated.
[0237] Evaluation of Hydrogen Content:
[0238] This evaluation is conducted by subjecting a thin film
sample to hydrogen content analysis by way of SIMS (secondary ion
mass spectrometry) to obtain a hydrogen content profile in the
depth direction.
[0239] In the following, description will be made of the evaluated
results.
[0240] With Respect to the Absorption Coefficient:
[0241] The absorption coefficient in Comparative Example 1-2 was
found to be simply decreased. Particularly, the absorption
coefficient of a portion (which is about 200 nm thick)
corresponding to the initial deposition stage is small, and for the
absorption thereafter, it coincides well with a model in which a
substantially constant absorption coefficient is assumed.
[0242] On the other hand, the absorption coefficient in Example 1
was found to be increased and decreased. Particularly, it was found
that a layer having a large absorption coefficient and a layer
having a small absorption coefficient are stacked in the thickness
direction.
[0243] With Respect to the Average Grain Size:
[0244] The evaluated results of this evaluation item are shown
Table 2.
[0245] In Comparative Example 1-2, there was obtained a finding
that although an amorphous layer is present and the average grain
size is gradually increased until near 200 nm from the substrate,
the average grain size thereafter is substantially constant until
near the surface.
[0246] In Example 1, there was obtained a finding that the state at
the initial deposition stage in Example 1 is around the same as
that in Comparative Example 1-2, but the average grain size
thereafter is varied such that it is increased and decreased in the
depth direction.
[0247] In FIG. 14(a), there is shown a part of the cross section by
TEM in Comparative Example 1-2. In FIG. 14(b), there is shown a
part of the cross section by TEM in Example 1. It is understood
that portion where substantially no grain size-changed portion is
present in the cross section of Comparative Example 1-2 as shown in
FIG. 14 (a), but in the cross section of Example 1, as shown in
FIG. 14(b), there are present grain size-changed portions A-A' and
B-B'.
[0248] With Respect to the Crystal Volume Fraction:
[0249] The evaluated results of this evaluation item are shown in
Table 3. Based on the results shown in Table 3, the crystal volume
fraction in Comparative Example 1-2 was found to be substantially
constant until near the surface. On the other hand, the crystal
volume fraction in Example 1 was found to be varied such that it is
first increased and then decreased.
[0250] With Respect to the Hydrogen Content:
[0251] The evaluated results of this evaluation item are
graphically shown in FIG. 15. Based on the results shown in FIG.
15, the following facts are understood. That is, in Comparative
Example 1-2, a layer having a large hydrogen content is present
until near 200 nm from the substrate, and although the hydrogen
content thereafter is decreased, the successive hydrogen content is
substantially constant until near the surface. On the other hand,
in Example 1, although the state at the initial deposition stage is
around the same as that in Comparative Example 1-2, the hydrogen
content thereafter is varied such that it is increased and then
decreased.
[0252] As will be understood from the above description, it is
understood that according to the method of Example 1, it is
possible to readily form a .mu.c-Si series thin film whose
properties are desirably controlled in the thickness direction.
EXAMPLE 2
[0253] As the bar-like shaped electrode 104 in the film-forming
apparatus shown in FIG. 1, as shown in FIG. 16 (which is a
conceptual diagram which is not always in agreement with actual
experimental conditions), a plurality of bar-like shaped electrodes
104 each being the same as the bar-like shaped electrode 104 used
in Experiment 1, were arranged in parallel to each other such that
they are perpendicular to a normal line of a substrate 102 and
their intervals to the substrate are partially differed.
[0254] Using the apparatus thus constituted, a .mu.c-Si series thin
film was formed under such conditions as will be described
below.
[0255] As the substrate 102, there was used a stainless steel
SUS430BA web of 0.2 mm in thickness, 20 cm in width and 50 m in
length having a 1 .mu.m thick zinc oxide (ZnO) film formed thereon.
The formation of .mu.c-Si series thin film was conducted in
accordance with the procedures in Experiment 1, except for changing
the film-forming conditions in Experiment 1 to those below
mentioned.
[0256] Film-Forming Conditions:
[0257] raw material gas (SiH.sub.4): 50 sccm
[0258] dilution gas (H.sub.2): 2000 sccm
[0259] oscillation frequency: 200 MHz
[0260] high frequency power: 10 W/cm.sup.2
[0261] inner pressure in the deposition chamber:
2.7.times.10.sup.-1 Pa
[0262] intervals of the electrodes to the substrate (from the upper
side of the substrate transportation direction):
[0263] 3 cm interval: 10 bar-like shaped electrodes,
[0264] 5 cm interval: 10 bar-like shaped electrodes,
[0265] 5 cm interval: 10 bar-like shaped electrodes,
[0266] 5 cm interval: 10 bar-like shaped electrodes, and
[0267] 5 cm interval: 10 bar-like shaped electrodes
[0268] interval between each adjacent electrodes (the distance in a
horizontal direction from the center on the central axis of one
bar-like shaped electrode to that of the other bar-like shaped
electrode):
[0269] 10 cm
[0270] substrate temperature: 300.degree. C.
[0271] deposited film thickness: 10 .mu.m
EXAMPLE 3
[0272] The procedures of Example 2 were repeated, except that the
arrangement of the bar-like shaped electrodes was changed to be not
in parallel to each other, to form a .mu.c-Si series thin film.
[0273] Evaluation
[0274] Evaluation with respect to film thickness distribution in
the width direction of the substrate was conducted in the same
manner as in Example 1. As a result, the film thickness
distribution in Example 2 was found to fall within 10%, but that in
Example 3 was found to be 13%.
[0275] As a result of having evaluated a distribution with respect
to each of absorption coefficient, crystal deposition rate, and
hydrogen content respectively in the width direction of the
substrate, the distribution in Example 2 was found to fall within
10%,but that in Example 3 was found to be 13%.
[0276] Based on the above results, it is understood that by
arranging the bar-like shaped electrodes in parallel to each other
as in Example 2, it is possible to diminish unevenness in the film
thickness in the width direction of the substrate.
EXAMPLE 4
[0277] As the bar-like shaped electrode 104 in the film-forming
apparatus shown in FIG. 1, as shown in FIG. 17 (which is a
conceptual diagram which is not always in agreement with actual
experimental conditions), a plurality of bar-like shaped electrodes
104 each being the same as the bar-like shaped electrode 104 used
in Experiment 1, were arranged in parallel to each other such that
they are perpendicular to a normal line of a substrate 102, they
are perpendicular to the direction for the substrate to be
transported, and their intervals to the substrate are partially
differed.
[0278] Using the apparatus thus constituted, a .mu.c-Si series thin
film was formed under such conditions as will be described
below.
[0279] As the substrate 102, there was used a stainless steel
SUS430BA web of 0.2 mm in thickness, 20 cm in width and 50 m in
length having a 1 .mu.m thick zinc oxide (Zno) film formed thereon.
The formation of .mu.c-Si series thin film was conducted in
accordance with the procedures in Experiment 1, except for changing
the film-forming conditions in Experiment 1 to those below
mentioned.
[0280] Film-Forming Conditions:
[0281] raw material gas (SiH.sub.4): 50 sccm
[0282] dilution gas (H.sub.2): 2000 sccm
[0283] oscillation frequency: 200 MHz
[0284] high frequency power: 10 W/cm.sup.2
[0285] inner pressure in the deposition chamber:
2.7.times.10.sup.-1 Pa
[0286] intervals of the electrodes to the substrate (from the upper
side of the substrate transportation direction):
[0287] 3 cm interval: 10 bar-like shaped electrodes,
[0288] 5 cm interval: 10 bar-like shaped electrodes,
[0289] 5 cm interval: 10 bar-like shaped electrodes,
[0290] 5 cm interval: 10 bar-like shaped electrodes, and
[0291] 5 cm interval: 10 bar-like shaped electrodes
[0292] angle of each electrode to the direction for the substrate
to be transported: 90.degree. interval between each adjacent
electrodes (the distance in a horizontal direction from the center
on the central axis of one bar-like shaped electrode to that of the
other bar-like shaped electrode):
[0293] 10 cm
[0294] substrate temperature: 300.degree. C.
[0295] deposited film thickness: 10 .mu.m
[0296] Evaluation
[0297] Evaluation with respect to film thickness distribution in
the width direction of the substrate was conducted in the same
manner as in Example 1. As a result, the film thickness
distribution in Example 4 was found to fall within 8%, which is
superior to Example 2 in terms of uniformity.
[0298] And, as a result of having evaluated a distribution with
respect to each of absorption coefficient, crystal deposition rate,
and hydrogen content respectively in the width direction of the
substrate, the distribution in Example 4 was found to fall within
8%, which is superior to Example 2 in terms of uniformity.
[0299] Based on the above results, it is understood that by
arranging the bar-like shaped electrodes to be perpendicular to the
transportation direction of the substrate as above described, it is
possible to diminish unevenness in the film thickness in the width
direction of the substrate.
EXAMPLE 5
[0300] As the bar-like shaped electrode 104 in the film-forming
apparatus shown in FIG. 1, as shown in FIG. 18 (which is a
conceptual diagram which is not always in agreement with actual
experimental conditions), a plurality of bar-like shaped electrodes
104 each being the same as the bar-like shaped electrode 104 used
in Experiment 1 were arranged such that they are perpendicular to a
normal line of a substrate 102 and their intervals to the substrate
are widened in the upper side of the transportation direction of
the substrate and narrowed in the down side thereof.
[0301] Using the apparatus thus constituted, a .mu.c-Si series thin
film was formed under such conditions as will be described
below.
[0302] As the substrate 102, there was used a stainless steel
SUS430BA web of 0.2 mm in thickness, 20 cm in width and 50 m in
length having a 1 .mu.m thick zinc oxide (ZnO) film formed thereon.
The formation of .mu.c-Si series thin film was conducted in
accordance with the procedures in Experiment 1, except for changing
the film-forming conditions in Experiment 1 to those below
mentioned.
[0303] Film-Forming Conditions:
[0304] raw material gas (SiH.sub.4): 50 sccm
[0305] dilution gas (H.sub.2): 2000 sccm
[0306] oscillation frequency: 200 MHz
[0307] high frequency power: 10 W/cm.sup.2
[0308] inner pressure in the deposition chamber:
2.7.times.10.sup.-1 Pa
[0309] intervals of the electrodes to the substrate (from the upper
side of the substrate transportation direction):
[0310] 15 cm interval: 5 bar-like shaped electrodes,
[0311] 12 cm interval: 5 bar-like shaped electrodes,
[0312] 10 cm interval: 5 bar-like shaped electrodes,
[0313] 8 cm interval: 10 bar-like shaped electrodes,
[0314] 6 cm interval: 10 bar-like shaped electrodes, and
[0315] 4 cm interval: 15 bar-like shaped electrodes angle of each
electrode to the
[0316] transportation direction of the substrate: 90.degree.
[0317] interval between each adjacent electrodes (the distance in a
horizontal direction from the center on the central axis of one
bar-like shaped electrode to that of the other bar-like shaped
electrode):
[0318] 10 cm
[0319] substrate temperature: 300.degree. C.
[0320] deposited film thickness: 10 .mu.m
COMPARATIVE EXAMPLE 2
[0321] The procedures of Example 5 were repeated, except that all
the intervals of the 50 bar-like shaped electrodes to the substrate
were made to be 4 cm, to form a .mu.c-Si series thin film.
[0322] Evaluation
[0323] For each of Example 5 and Comparative Example 2, evaluation
was conducted with respect to profile of each of absorption
coefficient, crystal volume fraction, and hydrogen content in depth
direction in the same manner as in Example 1.
[0324] In the following, description will be made of the evaluated
results.
[0325] With Respect to the Absorption Coefficient:
[0326] The absorption coefficient in Comparative Example 2 was
found to be simply decreased. Particularly, the absorption
coefficient of a portion (which is about 200 nm thick)
corresponding to the initial deposition stage is small, and for the
absorption thereafter, it well coincides with a model in which
substantially constant absorption coefficient is assumed.
[0327] On the other hand, the absorption coefficient in Example 5
was found to be gradually increased. Specifically, it was found
that the absorption coefficient is gradually enlarged as the film
deposition proceeds.
[0328] With Respect to the Average Grain Size:
[0329] The evaluated results of this evaluation item are shown
Table 4.
[0330] The following facts were found. That is, in Comparative
Example 2, although an amorphous layer is present and the average
grain size is gradually increased until near 200 nm from the
substrate, the average grain size thereafter is substantially
constant until near the surface.
[0331] On the other hand, in Example 5, substantially no amorphous
layer is present in the state at the initial deposition stage, and
the average grain size thereafter is gradually decreased toward
near the surface.
[0332] With Respect to the Crystal Volume Fraction:
[0333] The evaluated results of this evaluation item are shown in
Table 5. Based on the results shown in Table 5, it is understood
that the crystal volume fraction in Comparative Example 2 is
substantially constant until near the surface, but the crystal
volume fraction in Example5 is gradually decreased toward near the
surface.
[0334] With Respect to the Hydrogen Content:
[0335] The evaluated results of this evaluation item are
graphically shown in FIG. 19. Based on the results shown in FIG.
19, the following facts are understood. That is, in Comparative
Example 2, a layer having a large hydrogen content is present until
near 200 nm from the substrate, and although the hydrogen content
thereafter is decreased, the successive hydrogen content is
substantially constant until near the surface. On the other hand,
in Example 5, although the hydrogen content in the state at the
initial deposition stage is substantially constant, the hydrogen
content thereafter is gradually increased toward near the
surface.
[0336] Separately, in each of Example 5 and Comparative Example 2,
there was prepared a photovoltaic element as well as in Experiment
3, wherein the formation of the p-type layer and the n-Type layer
was conducted under the same conditions employed for the formation
of these layers in Experiment 3. For the resultant photovoltaic
elements, evaluation with respect to initial photoelectric
conversion efficiency and light degradation test were conducted in
the same manner as in Experiment 3.
[0337] The results obtained are collectively shown in Table 6. Each
value for Example 5 is a value relative to the corresponding value
of Comparative Example 2 which is set at 1.00.
[0338] Based on the results shown in Table 6, it is understood that
by using a .mu.c-Si series thin film formed by using a plurality of
bar-like shaped electrodes arranged such that their intervals to a
substrate are widened in the upper side of the transportation
direction of the substrate and narrowed in the down side thereof,
it is possible to prepare a photovoltaic element which is superior
in photovoltaic element characteristics.
EXAMPLE 6
[0339] As the bar-like shaped electrode 104 in the film-forming
apparatus shown in FIG. 1, as shown in FIG. 20 (which is a
conceptual diagram which is not always in agreement with actual
experimental conditions), a plurality of bar-like shaped electrodes
104 each being the same as the bar-like shaped electrode 104 used
in Experiment 1 were arranged such that they are perpendicular to a
normal line of a substrate 102 and their intervals to the substrate
are partially periodically changed to the transportation direction
of the substrate.
[0340] Using the apparatus thus constituted, a .mu.c-Si series thin
film was formed under such conditions as will be described
below.
[0341] As the substrate 102, there was used a stainless steel
SUS430BA web of 0.2 mm in thickness, 20 cm in width and 50 m in
length having a 1 .mu.m thick zinc oxide (ZnO) film formed thereon.
The formation of .mu.c-Si series thin film was conducted in
accordance with the procedures in Experiment 1, except for changing
the film-forming conditions in Experiment 1 to those below
mentioned.
[0342] Film-Forming Conditions:
[0343] raw material gas (SiH.sub.4): 50 sccm
[0344] dilution gas (H.sub.2): 2000 sccm
[0345] oscillation frequency: 200 MHz
[0346] high frequency power: 10 W/cm.sup.2
[0347] inner pressure in the deposition chamber:
2.7.times.10.sup.-1 Pa
[0348] intervals of the electrodes to the substrate (from the upper
side of the substrate
[0349] transportation direction):
[0350] 15 cm interval: 5 bar-like shaped electrodes,
[0351] 12 cm interval: 5 bar-like shaped electrodes,
[0352] 4 cm interval: 10 bar-like shaped electrodes,
[0353] 8 cm interval: 10 bar-like shaped electrodes,
[0354] 4 cm interval: 10 bar-like shaped electrodes, and
[0355] 8 cm interval: 10 bar-like shaped electrodes
[0356] angle of each electrode to the transportation direction of
the substrate: 90.degree. interval between each adjacent electrodes
(the distance in a horizontal direction from the center on the
central axis of one bar-like shaped electrode to that of the other
bar-like shaped electrode):
[0357] 10 cm
[0358] substrate temperature: 300.degree. C.
[0359] deposited film thickness: 10 .mu.m
[0360] Evaluation
[0361] For Example 6, evaluation was conducted with respect to
profile of each of absorption coefficient, crystal volume fraction,
and hydrogen content in depth direction in the same manner as in
Example 1.
[0362] In the following, description will be made of the evaluated
results. With respect to the absorption coefficient:
[0363] The absorption coefficient in Example 6 was found to be
periodically changed such that its increase and decrease are
repeated. Specifically, it was found that the absorption
coefficient is periodically changed as the film deposition
proceeds.
[0364] With Respect to the Average Grain Size:
[0365] The evaluated results of this evaluation item are shown
Table 7.
[0366] In Example 6, substantially no amorphous layer was present
in the state at the initial deposition stage, and the average grain
size thereafter was periodically changed such that it was increased
and decreased toward near the surface.
[0367] With Respect to the Crystal Volume Fraction:
[0368] The evaluated results of this evaluation item are shown in
Table 8. Based on the results shown in Table 8, it is understood
that the crystal volume fraction in Example 6 is periodically
changed such that it is increased and decreased toward near the
surface.
[0369] With Respect to the Hydrogen Content:
[0370] The evaluated results of this evaluation item in Example 6
are graphically shown in FIG. 21, in which the evaluated results of
this evaluation item obtained in Comparative Example 2 are together
shown. Based on the results shown in FIG. 21, the following facts
are understood. That is, in Example 6, although the hydrogen
content in the state at the initial deposition stage is
substantially constant, the hydrogen content thereafter is
periodically changed such that it is increased and decreased toward
near the surface.
[0371] From the above results, it is understood that according to
the process of Example 6, it is possible to readily form a .mu.c-Si
series thin film whose absorption coefficient is periodically
changed toward the film thickness direction.
[0372] Separately, in Example 6, there was prepared a photovoltaic
element as well as in Experiment 3, wherein the formation of the
p-type layer and the n-type layer was conducted under the same
conditions employed for the formation of these layers in Experiment
3. For the resultant photovoltaic element, evaluation with respect
to initial photoelectric conversion efficiency and light
degradation test were conducted in the same manner as in Experiment
3.
[0373] The results obtained are collectively shown in Table 9, in
which the evaluated results of the photovoltaic element in
Comparative Example 2 are together shown. Each value for Example 6
is a value relative to the corresponding value of Comparative
Example 2 which is set at 1.00.
[0374] Based on the results shown in Table 9, it is understood that
by using a .mu.c-Si series thin film formed by using a plurality of
bar-like shaped electrodes arranged such that their intervals to a
substrate are periodically changed to the transportation direction
of the substrate, it is possible to prepare a photovoltaic element
which is superior in photovoltaic element characteristics.
[0375] As will be understood from the above description, according
to the process of the present invention, it is possible to readily
control the properties of a .mu.c-Si series thin film in the film
thickness direction by a simple manner. And it is possible to
readily form a high quality .mu.c-Si series thin film excelling in
properties and which has a graded film property in the film
thickness direction. In addition, by using such .mu.c-Si series
thin film, it is possible to produce a high quality microcrystal
semiconductor device at a reasonable production cost.
1TABLE 1 interval between substrate initial photoelectric and
electrode (cm) conversion efficiency degradation test 0.5 0.80 0.85
0.8 0.90 0.95 1.0 0.95 0.97 2.0 0.97 0.99 5.0 1.00 1.00 8.0 1.05
1.03 10 1.05 1.03 20 1.05 1.03
[0376]
2 TABLE 2 depth direction (from substrate) (.mu.m) average grain
size 1 3 5 7 9 Example 1 1.00 1.10 1.20 1.10 1.05 Comparative
Example 1-2 1.00 1.00 1.00 1.00 1.00 normalized values when the
value of the 1 .mu.m thick depth is set at 1.00
[0377]
3 TABLE 3 depth direction (from substrate) (.mu.m) crystal volume
fraction 1 3 5 7 9 Example 1 1.00 1.08 1.20 1.10 1.6 Comparative
Example 1-2 1.00 1.00 1.00 1.00 1.00 normalized values when the
value of the 1 .mu.m thick depth is set at 1.00
[0378]
4 TABLE 4 depth direction (from substrate) (.mu.m) average grain
size 1 3 5 7 9 Example 5 1.00 0.96 0.92 0.90 0.84 Comparative
Example 2 1.00 1.00 1.00 1.00 1.00 normalized values when the value
of the 1 .mu.m thick depth is set at 1.00
[0379]
5 TABLE 5 depth direction (from substrate) (.mu.m) crystal volume
fraction 1 3 5 7 9 Example 5 1.00 0.97 0.93 0.89 0.83 Comparative
Example 2 1.00 1.00 1.00 1.00 1.00 normalized values when the value
of the 1 .mu.m thick depth is set at 1.00
[0380]
6 TABLE 6 initial photoelectric conversion efficiency degradation
test Example 5 1.12 1.09 Comparative Example 2 1.00 1.00
[0381]
7 TABLE 7 depth direction (from substrate) (.mu.m) average grain
size 1 3 5 7 9 Example 6 1.00 0.80 0.93 0.82 0.94 normalized values
when the value of the 1 .mu.m thick depth is set at 1.00
[0382]
8 TABLE 8 depth direction (from substrate) (.mu.m) crystal volume
fraction 1 3 5 7 9 Example 6 1.00 0.84 0.90 0.79 0.92 normalized
values when the value of the 1 .mu.m thick depth is set at 1.00
[0383]
9 TABLE 9 initial photoelectric conversion efficiency degradation
test Example 6 1.05 1.11 Comparative Example 2 1.00 1.00
* * * * *