U.S. patent application number 10/194227 was filed with the patent office on 2002-12-26 for method and apparatus for depositing semiconductor film and method for fabricating semiconductor device.
This patent application is currently assigned to MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.. Invention is credited to Ban, Yuzaburo, Harafuji, Kenji, Ishibashi, Akihiko, Ohnaka, Kiyoshi.
Application Number | 20020195054 10/194227 |
Document ID | / |
Family ID | 18688612 |
Filed Date | 2002-12-26 |
United States Patent
Application |
20020195054 |
Kind Code |
A1 |
Harafuji, Kenji ; et
al. |
December 26, 2002 |
Method and apparatus for depositing semiconductor film and method
for fabricating semiconductor device
Abstract
A method for depositing a semiconductor film on a wafer by
making a source gas supplied flow almost horizontally to the
surface of the wafer. When a process condition, e.g., the flow
velocity or pressure of the source gas, should be changed, the
source gas has its velocity and/or pressure changed so that the
source gas is supplied at a substantially constant flow rate.
Inventors: |
Harafuji, Kenji; (Osaka,
JP) ; Ishibashi, Akihiko; (Osaka, JP) ; Ban,
Yuzaburo; (Osaka, JP) ; Ohnaka, Kiyoshi;
(Osaka, JP) |
Correspondence
Address: |
NIXON PEABODY, LLP
8180 GREENSBORO DRIVE
SUITE 800
MCLEAN
VA
22102
US
|
Assignee: |
MATSUSHITA ELECTRIC INDUSTRIAL CO.,
LTD.
Osaka
JP
|
Family ID: |
18688612 |
Appl. No.: |
10/194227 |
Filed: |
July 15, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10194227 |
Jul 15, 2002 |
|
|
|
09884133 |
Jun 20, 2001 |
|
|
|
Current U.S.
Class: |
118/715 ;
117/200 |
Current CPC
Class: |
C23C 16/52 20130101;
C23C 16/45582 20130101; Y10T 117/10 20150115; C30B 25/14
20130101 |
Class at
Publication: |
118/715 ;
117/200 |
International
Class: |
C23C 016/00; C30B
001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 23, 2000 |
JP |
2000-188903 |
Claims
What is claimed is:
1. An apparatus for depositing a semiconductor film on a wafer by
making a source gas supplied flow almost horizontally to the
surface of the wafer, the apparatus comprising: a reactor, in which
the wafer is placed and which has a gas inlet port for supplying
the source gas onto the wafer; velocity control means for
controlling the flow velocity of the source gas; and pressure
control means for controlling the pressure of the source gas in the
reactor, wherein the velocity and pressure control means control
the flow velocity and the pressure in such a manner as to keep the
flow rate of the source gas near the gas inlet port substantially
constant.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method and apparatus for
depositing a semiconductor film on a wafer by making source gases
supplied flow almost horizontally to the surface of the wafer. The
present invention also relates to a method for fabricating a
semiconductor device by using the film deposition method or
apparatus.
[0002] Group II-VI or III-V compound semiconductors are direct
transition type semiconductors with wide bandgap energy, and are
hopefully applicable to emitting light at various wave-lengths that
range from visible through ultraviolet regions of the spectrum.
[0003] Among other things, Group III-V nitride semiconductors,
including gallium (Ga) or aluminum (Al) as a Group III constituent
and nitrogen (N) as a Group V constituent, have attracted much
attention, because those semiconductors exhibit
crystallographically excellent properties. Thus, a method for
depositing a film of a nitride semiconductor just as intended is in
high demand.
[0004] A metalorganic chemical vapor deposition (MOCVD) process has
been researched and developed widely and vigorously as one of
industrially implementable methods of promise.
[0005] Hereinafter, a so-called "horizontal MOCVD reactor", which
is so constructed as to make source gases flow horizontally to the
wafer surface, will be described as a known semiconductor film
deposition apparatus with reference to FIGS. 7A and 7B.
[0006] As shown in FIGS. 7A and 7B, the horizontal reactor 200
includes: reactor body 201; gas inlet tube 202 with a gas inlet
port 221; and susceptor 211 attached to the bottom of the reactor
body 201. In this case, the reactor body 201 and gas inlet tube 202
are made of quartz glass, for example. Also, a gas outlet port 212
is provided at the other end of the reactor body 201 on the
opposite side to the gas inlet tube 202.
[0007] The susceptor 211 holds a wafer 100 thereon to heat the
wafer 100 up to a predetermined temperature.
[0008] A source gas 101, supplied through the gas inlet port 221,
should be a laminar flow with no vortices after the gas 101 enters
the tube 202 through the inlet port 221 and until the gas 101
reaches the space over the susceptor 211. The gas 101 also needs to
flow in such a manner as to show spatially uniform velocity
distribution over the wafer 100 to grow compound semiconductor
crystals of quality.
[0009] However, the opening width of the gas inlet port 221 is
relatively small as defined by its manufacturing standard, and the
gas, supplied through the inlet port 221, should expand to cover an
area equal to or greater in width than that of the susceptor 211.
For that purpose, the gas inlet tube 202 has an expanded portion
222, the width of which gradually increases from the gas inlet port
221 toward the susceptor 211. In this case, if the angle .alpha. of
expansion of the expanded portion 222 is large, then a streamline,
which has flowed along the inner wall surface of the tube 202,
separates from the surface in a velocity boundary layer near the
wall of the expanded portion 222 as shown in FIG. 7A. Then, the
streamline flows backward, i.e., toward the gas inlet port 221, to
turn into a separated streamline (or vortex streamline) 102. Also,
a wake, or a vortex 103, is created inside a curvature formed by
the separated streamline 102. In other words, a backward flow,
moving upstream along the wall surface of the expanded portion 222,
is created and then separated from the wall surface at a separation
point to form the separated streamline 102. In FIG. 7A, only the
streamlines flowing along the wall on the left-hand side of the gas
flow are illustrated. Actually, though, similar streamlines also
flow along the right-hand-side wall surface almost symmetrically to
the illustrated ones about the centerline.
[0010] If the vortex 103 is created in the expanded portion 222,
then the channel width of the gas flow is substantially decreased
or deformed. As a result, the velocity distribution of the gas flow
over the susceptor 211 cannot be spatially uniform anymore. In
addition, the source gas 101 gets partially stuck inside the vortex
103, thus adversely delaying the exchange of one source gas for
another. In that case, even if the semiconductor film being
deposited should have its composition changed, the interfacial
profile cannot be steep enough.
[0011] To solve these problems, G. B. Stringfellow proposed
expanding the sidewalls of the expanded portion 222 gently by
setting the expansion angle .alpha. to 7 degrees or less (see
"Organometallic Vapor-Phase Epitaxy", Second Edition, p. 364,
Academic Press).
[0012] Another solution is disposing a netlike or porous diffuser
223 in the expanded portion 222 of the gas inlet tube 202 as shown
in FIGS. 8A and 8B or 9A and 9B to prevent the vortex from being
created in the expanded portion 222.
[0013] However, the known horizontal reactor 200 has the following
drawbacks. Specifically, if the expansion angle .alpha. of the
expanded portion 222 is set to about 7 degrees or less, then the
distance from the gas inlet port 221 to the gas outlet port 212 of
that reactor 200 becomes very long. Accordingly, it may take an
excessively large area to dispose such a bulky reactor. Or that
long reactor may break very easily, so too much care should be
taken in handling such a reactor.
[0014] On the other hand, if the diffuser 223 is disposed inside
the gas inlet tube 202, then the spatial uniformity in the velocity
distribution of the gas flow improves. Nevertheless, the gas flow
is reflected by the diffuser 223 to create another type of vortex,
thus also delaying the exchange of one source gas for another.
[0015] In addition, if the horizontal reactor 200 should be
redesigned every time some process condition, e.g., the flow
velocity or pressure of a source gas, is changed and optimized,
then the productivity should decline or the costs would increase
disadvantageously.
SUMMARY OF THE INVENTION
[0016] It is therefore an object of the present invention to save
the need for re-designing a horizontal reactor even if some
condition, like the flow velocity or pressure of a source gas, for
a film deposition process to be carried in the reactor has been
changed and optimized.
[0017] To achieve this object, in depositing a semiconductor film,
a gas flow rate is fixed at a predetermined value according to the
present invention by keeping the product of the flow velocity and
pressure of source gases inside the reactor constant.
[0018] The present inventors carried out various types of research
on a process for depositing a compound semiconductor film using a
horizontal reactor. As a result, we found that the spatial
distributions of velocity and temperature of source gases and that
of the thickness of a film to be deposited on a wafer are
substantially controllable in the reactor by the flow rates of the
source gases. The velocity and temperature distributions of
reactant gases, resulting from chemical reaction between the source
gases, were also controllable by the flow rates. As is well known
in the art, the flow rate of a gas is proportional to the product
of the flow velocity and pressure of the gas. Accordingly, each of
those spatial distributions can be kept substantially uniform
during the film deposition process only if the flow velocity or
pressure of the source gases is changed in such a manner as to
maintain a predetermined gas flow rate.
[0019] Specifically, a first inventive film deposition method is
for use to deposit a semiconductor film on a wafer by making a
source gas supplied flow almost horizontally to the surface of the
wafer. In this method, the source gas has its flow velocity and/or
pressure changed so that the source gas is supplied at a
substantially constant flow rate.
[0020] According to the first inventive method, a source gas
supplied has its flow velocity near its inlet port and pressure
inside a reactor changed so that the source gas is supplied onto a
wafer at a substantially constant flow rate. Thus, it is clear from
our findings that even if the flow velocity of the source gas is
changed to deposit a film at a higher rate, the film deposited
still can have its thickness uniformized. So there is no need to
re-design the horizontal reactor each time the process conditions
are changed.
[0021] In one embodiment of the present invention, the pressure of
the source gas is preferably set within a range from about 0.01 atm
and about 2 atm.
[0022] A second inventive film deposition method is also for use to
deposit a semiconductor film on a wafer by making a source gas
supplied flow almost horizontally to the surface of the wafer. The
method includes the step of a) controlling the flow velocity and
pressure of the source gas to find a first flow velocity and a
first pressure that make such a combination as substantially
uniformizing the thickness of the film deposited, and then
determining a reference flow rate for the source gas. The reference
flow rate should meet a predetermined relationship with the product
of the first flow velocity and the first pressure. The method
further includes the step of b) changing the first flow velocity
and the first pressure into a second flow velocity and a second
pressure with the reference flow rate kept constant. And the method
further includes the step of c) supplying the source gas onto the
wafer at the reference flow rate with the flow velocity and
pressure of the source gas set equal to the second flow velocity
and the second pressure, respectively, thereby depositing the film
on the wafer.
[0023] According to the second inventive method, a first flow
velocity and a first pressure of a source gas are changed into a
second flow velocity and a second pressure with a reference flow
rate kept constant. Then, the source gas is supplied onto a wafer
at the reference flow rate with the flow velocity and pressure of
the source gas set equal to the second flow velocity and the second
pressure, respectively, to deposit a film on the wafer. Thus even
if the flow velocity and pressure of the source gas have been
changed, each film deposited can have its thickness uniformized. As
a result, there is no need to re-design the horizontal reactor each
time the process conditions are changed.
[0024] In one embodiment of the present invention, the first flow
velocity is preferably determined in the step a) by setting an
initial value of the first pressure to 1 atm or less. Specifically,
it would be easier to find an optimum reference flow rate by
setting an initial value of the first pressure to 1 atm or less and
then changing the first flow velocity gradually to determine the
best first flow velocity as compared to setting an initial value of
the first pressure to more than 1 atm and then changing the first
flow velocity gradually to determine the best first flow
velocity.
[0025] Also, the first and second pressures are each preferably set
within a range from about 0.01 atm and about 2 atm.
[0026] This invention also provides a method for fabricating a
semiconductor device, including at least first and second
semiconductor films stacked in this order on a wafer, by making at
least first, second and third source gases supplied flow almost
horizontally to the surface of the wafer. The method includes the
step of a) controlling the flow velocity and pressure of the first
source gas to find a first flow velocity and a first pressure that
make such a combination as substantially uniformizing the thickness
of each said film to be deposited, and then obtaining a reference
flow rate for the first source gas. The reference flow rate meets a
predetermined relationship with the product of the first flow
velocity and the first pressure. The method further includes the
step of b) setting a second flow velocity and a second pressure,
which are different from the first flow velocity and the first
pressure, respectively, for the second source gas with the
reference flow rate kept constant. The second source gas has a
viscosity substantially equal to that of the first source gas. The
method further includes the step of c) supplying the second source
gas onto the wafer at the reference flow rate with the flow
velocity and pressure of the second source gas set equal to the
second flow velocity and the second pressure, respectively, thereby
depositing the first film on the wafer. The method further includes
the step of d) setting a third flow velocity and a third pressure,
which are different from the second flow velocity and the second
pressure, respectively, for the third source gas with the reference
flow rate kept constant. The third source gas has a viscosity
substantially equal to that of the first source gas. The method
further includes the step of e) supplying the third source gas onto
the first film at the reference flow rate with the flow velocity
and pressure of the third source gas set equal to the third flow
velocity and the third pressure, respectively, thereby depositing
the second film on the first film.
[0027] According to the present invention, first, a reference flow
rate is determined for a first source gas. Next, with the reference
flow rate kept constant, a second source gas is supplied at a
second flow velocity and a second pressure to deposit a first film.
Then, a third source gas is supplied at a third flow velocity and a
third pressure to deposit a second film on the first film. Thus
each of multiple films for a semiconductor device can have its
thickness uniformized and its quality improved.
[0028] As used herein, the second or third source gas should "have
a viscosity substantially equal to that of the first source gas" in
the following two situations. One of the two situations is that
even though the second or third source gas is made of a molecular
species different from that of the first source gas, the second or
third source gas has a viscosity substantially equal to that of the
first source gas. In the other situation, the second or third
source gas is also made of a different species from that of the
first source gas and the first and second or third source gases are
both diluted with a carrier gas in a huge quantity. In that case,
the first and second or third source gases have their viscosities
determined almost by the viscosity of the carrier gas itself.
[0029] In one embodiment of the present invention, the first and
second films each preferably contain at least one Group III element
and at least one Group V element. The second source gas preferably
contains gallium and indium as Group III element sources and the
second pressure set for the second source gas is preferably about
0.3 atm or more. The third source gas preferably contains gallium
and aluminum as Group III element sources and the third pressure
set for the third source gas is preferably about 1.0 atm or less.
And the second pressure is preferably equal to or higher than the
third pressure.
[0030] Also, the first, second and third pressures are each
preferably set within a range from about 0.01 atm and about 2
atm.
[0031] The present invention further provides an apparatus for
depositing a semiconductor film on a wafer by making a source gas
supplied flow almost horizontally to the surface of the wafer. The
apparatus includes: a reactor, in which the wafer is placed and
which has a gas inlet port for supplying the source gas onto the
wafer; velocity control means for controlling the flow velocity of
the source gas; and pressure control means for controlling the
pressure of the source gas in the reactor. In this apparatus, the
velocity and pressure control means control the flow velocity and
the pressure in such a manner as to keep the flow rate of the
source gas near the gas inlet port substantially constant.
[0032] According to the inventive semiconductor film deposition
apparatus, a film of a uniform thickness can be obtained even if
the process conditions have been changed. Thus there is no need for
re-designing the reactor every time the process conditions are
changed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIGS. 1A and 1B illustrate a situation where temperature,
thickness of a film deposited and gas streamlines are substantially
uniformly distributed in a semiconductor film deposition apparatus
according to a first embodiment of the present invention:
[0034] FIG. 1A is a plan view illustrating the apparatus of the
first embodiment; and FIG. 1B is a cross-sectional view thereof
taken along the line IB-IB shown in FIG. 1A.
[0035] FIG. 2 is a plan view illustrating how the uniformity in the
temperature, thickness and streamline distributions collapses in
the apparatus of the first embodiment.
[0036] FIG. 3 is a plan view illustrating how the temperature,
thickness and streamline distributions have restored its uniformity
in the apparatus of the first embodiment.
[0037] FIG. 4 is a plan view illustrating how the uniformity in the
temperature, thickness and streamline distributions collapses in a
semiconductor film deposition apparatus according to a second
embodiment of the present invention.
[0038] FIG. 5 is a plan view illustrating how the temperature,
thickness and streamline distributions have restored its uniformity
in the apparatus of the second embodiment.
[0039] FIGS. 6A and 6B are cross-sectional views corresponding to
respective process steps for fabricating a semiconductor laser
diode made of Group III-V nitride semiconductors according to a
third embodiment of the present invention.
[0040] FIGS. 7A and 7B are respectively a plan view, and a
cross-sectional view taken along the line VIIB-VIIB shown in FIG.
7A, illustrating a known MOCVD reactor.
[0041] FIGS. 8A and 8B are respectively a plan view, and a
cross-sectional view taken along the line VIIIB-VIIIB shown in FIG.
8A, illustrating another known MOCVD reactor.
[0042] FIGS. 9A and 9B are respectively a plan view and a side view
illustrating still another known MOCVD reactor.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0043] Embodiment 1
[0044] Hereinafter, a first embodiment of the present invention
will be described with reference to the accompanying drawings.
[0045] FIGS. 1A and 1B are respectively a plan view and a
cross-sectional view, taken along the line IB-IB shown in FIG. 1A,
illustrating a semiconductor film deposition apparatus (e.g.,
horizontal MOCVD reactor) according to the first embodiment.
[0046] As shown in FIGS. 1A and 1B, the horizontal reactor 10
includes: reactor body 11; gas inlet tube 12 with a gas inlet port
21; and susceptor 31 for holding a wafer 100 thereon and heating
the wafer 100. The reactor body 11 and gas inlet tube 12 may be
made of quartz glass and the susceptor 31 may be made of carbon,
for example.
[0047] The reactor body 11 includes an opening at the bottom. The
susceptor 31, whose bottom is heated by a heater (not shown), for
example, is fitted in with the opening so that the wafer 100 is
exposed inside the reactor body 11 and that the upper surface of
the susceptor 31 is leveled with the bottom of the reactor body
11.
[0048] A gas outlet port 13 is provided at the other end of the
reactor body 11 on the opposite side to the gas inlet tube 12.
[0049] The gas inlet tube 12 has the gas inlet port 21 with an
opening width smaller than the width of the susceptor 31 as defined
by the gas tube manufacturing standard. The gas inlet tube 12
supplies the source gases 101 onto the wafer 100 substantially
horizontally to the wafer surface. The other end of the gas inlet
tube 12 on the opposite side to the gas inlet port 21 is welded
airtightly to the reactor body 11.
[0050] Also, the gas inlet tube 12 includes an expanded portion 22,
in which the gap between the walls gradually increases from the gas
inlet port 21 toward the susceptor 31. In the illustrated
embodiment, the expansion angle .alpha. formed between each wall of
the expanded portion 22 and the centerline running straight from
the gas inlet port 21 toward the susceptor 31 is set to about 10
degrees.
[0051] Furthermore, a partition 23 for dividing the inner space of
the gas inlet tube 12 into upper and lower channels 12a and 12b is
also disposed inside the gas inlet tube 12.
[0052] In addition, the gas inlet tube 12 includes first pressure
gauge 40A, first current meter 41A and first flowmeter 42A to
respectively monitor the pressure of the source gases 101 inside
the gas inlet tube 12 and the flow velocity and flow rate of the
source gases 101 near the gas inlet port 21.
[0053] Similarly, the reactor body 11 includes second pressure
gauge 40B, second current meter 41B and second flowmeter 42B to
respectively monitor the pressure, flow velocity and flow rate of
the source gases 101 that flow over the susceptor 31.
[0054] It should be noted that the pressure indicated by the first
pressure gauge 40A is normally substantially equal to that
indicated by the second pressure gauge 40B.
[0055] The first current meter 41A acts as a part of velocity
control means and the second pressure gauge 40B acts as a part of
pressure control means.
[0056] In the first embodiment, hydrogen gas and trimethylgallium
(TMG) gas diluted with the hydrogen gas are supplied into the upper
channel 12a to have a velocity of about 6 m/sec near the gas inlet
port 21. Ammonia (NH.sub.3) gas is supplied into the lower channel
12b to have a velocity of about 6 m/sec near the gas inlet port
21.
[0057] The source gases supplied separately into the upper and
lower channels 12a and 12b combine with each other near the
junction between the reactor body 11 and the gas inlet tube 12. The
velocity of the confluent gas over the susceptor 31 is controlled
at about 0.6 m/sec. Also, the internal pressure of the reactor body
11 is controlled at about 0.5 atm.
[0058] As shown in FIG. 1A, streamlines 110 are substantially
uniformly distributed inside the reactor body 11. Also, the
temperature distribution 50, thickness distribution 51 and velocity
distribution of streamlines (not shown) 110 are substantially
uniform spatially over the wafer 100 held on the susceptor 31.
[0059] In FIG. 1A, for the sake of simplicity, the temperature
distribution 50, thickness distribution 51 and streamlines 100 are
partially illustrated only on one side of the axis of symmetry
running in the direction in which the source gases 101 flow.
Actually, though, the same patterns are also formed symmetrically
on the other side of the axis of symmetry. The same statement will
be applicable to FIGS. 2, 3, 4 and 5.
[0060] Next, it will be described what if the internal pressure of
the reactor body is increased from about 0.5 atm to about 2.0 atm
with the velocity of the source gases 101 supplied through the gas
inlet port 21 kept at about 6 m/sec near the gas inlet port 21 to
improve the crystal quality of the film deposited on the wafer
100.
[0061] FIG. 2 illustrates the temperature distribution 50,
thickness distribution 51 and streamlines 110 in the horizontal
reactor 10 where only the gas pressure has been increased.
[0062] As shown in FIG. 2, a streamline 110, which has flowed along
the inner wall surface of the gas inlet tube 12, separates at a
point from the surface in a velocity boundary layer near the wall
of the gas inlet tube 12. Then, the streamline 110 starts to flow
downstream to create a vortex 110a.
[0063] If the vortex 110a is created, then the channel width of the
gas flow is substantially decreased or the streamlines 110 are
deformed. As a result, the velocity distribution of the gas flow
with the streamlines 110 comes to show decreased spatial uniformity
over the susceptor 31. Among other things, part of the gas flow
showing the streamlines 110 around the axis of symmetry increases
its velocity considerably in the reactor body 11. Thus, the gas
temperature does not rise to reach a predetermined value, but a
colder gas is supplied densely onto the center of the susceptor 31.
Accordingly, the temperature distribution 50 and thickness
distribution 51 shown in FIG. 2 have lost much of their uniformity
over the center of the susceptor 31. As a result, it becomes
difficult to keep the film thickness and quality uniform enough
just as intended.
[0064] Furthermore, as described above, if the vortex 110a is
created, the source gases get partially stuck inside the vortex
110a, thus adversely delaying the exchange of one source gas for
another. In that case, even if the semiconductor film being
deposited should have its composition changed, the interfacial
profile cannot be steep enough.
[0065] To solve these problems, the first embodiment utilizes a
proportionality found between the flow rate of a gas and its flow
velocity and pressure. Specifically, in the example shown in FIG.
3, to equalize the resultant flow rate of the gases supplied
through the gas inlet port 21 with the rate in the example shown in
FIG. 1, the gas flow velocity near the gas inlet port 21 is
decreased from about 6 m/sec to about 1.5 m/sec with the internal
pressure of the reactor body 11 kept at about 2.0 atm.
[0066] As described above, the process conditions for the example
shown in FIG. 1 include an internal pressure of about 0.5 atm for
the reactor body 11 and a gas flow velocity of about 6 m/sec near
the gas inlet port 21. On the other hand, the process conditions
for the example shown in FIG. 3 include an internal pressure of
about 2.0 atm for the reactor body 11 and a gas flow velocity of
about 1.5 m/sec near the gas inlet port 21. In both cases, the
product of the internal pressure and the gas flow velocity of the
gas is about 3.0. In the first embodiment, this value is used as a
reference flow rate.
[0067] To find an optimum reference flow rate, it is preferable to
set an initial value of the internal pressure to 1 atm or less and
then change the gas flow velocity gradually.
[0068] As shown in FIG. 3, the vortex 110a created in FIG. 2 has
disappeared, so the temperature distribution 50, thickness
distribution 51 and velocity distribution of the streamlines 110
are substantially uniform spatially over the wafer 100 held on the
susceptor 31. Accordingly, a film of quality can be obtained for
each wafer loaded as in the example illustrated in FIG. 1.
[0069] As can be seen, the quality of a film being deposited can be
improved just as intended by changing the internal pressure and gas
flow velocity in such a manner as to maintain a predetermined
reference flow rate.
[0070] Embodiment 2
[0071] Hereinafter, a second embodiment of the present invention
will be described with reference to the accompanying drawings.
[0072] In the second embodiment, the process conditions for the
example shown in FIG. 1 are modified to improve the crystal quality
and deposition rate of the resultant film. Specifically, it will be
described what if the gas flow velocity near the gas inlet port 21
is increased from about 6 m/sec to about 24 m/sec with the internal
pressure of the reactor body kept at about 0.5 atm.
[0073] FIG. 4 illustrates the temperature distribution 50,
thickness distribution 51 and streamlines 110 in the horizontal
reactor 10 where only the gas flow velocity has been increased.
[0074] As shown in FIG. 4, a streamline 110, which has flowed along
the inner wall surface of the gas inlet tube 12, separates from the
surface. Then, the streamline 110 starts to flow downstream to
create a vortex 110a. If the vortex 110a is created, then the
channel width of the gas flow is substantially decreased or the
streamlines 110 are deformed. As a result, the velocity
distribution of the gas flow with the streamlines 110 comes to show
decreased spatial uniformity over the susceptor 31. Among other
things, part of the gas flow showing the streamlines 110 around the
axis of symmetry increases its velocity considerably in the reactor
body 11. Thus, the gas temperature does not rise to reach a
predetermined value, but a colder gas is supplied densely onto the
center of the susceptor 31. Accordingly, the temperature
distribution 50 and thickness distribution 51 shown in FIG. 4 have
lost much of their uniformity over the center of the wafer 100. As
a result, it becomes difficult to keep the film thickness and
quality uniform enough just as intended.
[0075] The temperature distribution 50, thickness distribution 51
and streamlines 110 shown in FIG. 4 are substantially the same as
those shown in FIG. 2 resulting from the process conditions
including a gas flow velocity of about 6 m/sec and an internal
pressure of about 2.0 atm.
[0076] Furthermore, as described above, if the vortex 110a is
created, the source gases get partially stuck inside the vortex
110a, thus adversely delaying the exchange of one source gas for
another. In that case, even if the semiconductor film being
deposited should have its composition changed, the interfacial
profile cannot be steep enough.
[0077] In view of these problems, the internal pressure of the
reactor body 11 is decreased from about 0.5 atm to about 0.125 atm
with the gas flow velocity near the gas inlet port 21 kept at about
24 m/sec to equalize the flow rate of the gases supplied through
the gas inlet port 21 with the reference flow rate of about 3.0
determined for the example shown in FIG. 1.
[0078] Then, as shown in FIG. 5, the vortex 110a created in FIG. 4
has disappeared, and the temperature distribution 50, thickness
distribution 51 and velocity distribution of the streamlines 110
are substantially uniform spatially over the wafer 100 held on the
susceptor 31. Accordingly, a film of quality can be obtained for
each wafer loaded as in the example illustrated in FIG. 1 or 3.
[0079] As described above, according to the first or second
embodiment, if a reference flow rate is determined in advance in
such a manner as to uniformize a film thickness and then process
conditions are changed to obtain a more preferable internal
pressure, the flow velocity of a gas is controlled so that the
reference flow rate is kept constant. In this manner, the thickness
of the film deposited can be kept uniform.
[0080] Also, if the process conditions are changed to obtain a more
preferable gas flow velocity, the thickness of the film deposited
can also be kept uniform by controlling the internal pressure so
that the reference flow rate is kept constant. In this case, the
internal pressure is preferably set within a range from about 0.01
atm and about 2 atm.
[0081] In addition, the semiconductor films to be deposited do not
have to be made of Group III-V compounds but may be made of Group
II-VI compounds.
[0082] Embodiment 3
[0083] Hereinafter, a third embodiment of the present invention
will be described with reference to the accompanying drawings.
[0084] For the first and second embodiments, where process
conditions for depositing a film on a wafer should be changed to
deposit another film on another wafer, the flow velocity and
pressure of source gases are supposed to be controlled in such a
range as to keep a reference flow rate unchanged.
[0085] In the third embodiment, it will be described how to deposit
at least two films with mutually different compositions on a wafer
with the process conditions changed in accordance with the
compositions of those films.
[0086] FIGS. 6A and 6B illustrate cross-sectional structures
corresponding to respective process steps for fabricating a
violet-light-emitting semiconductor laser diode made of Group III-V
nitride semiconductors according to the third embodiment.
[0087] First, as shown in FIG. 6A, a wafer 60, made of sapphire,
for example, is held on the susceptor 31 of the horizontal reactor
10 shown in FIG. 1. Then, buffer layer 61 of gallium nitride (GaN),
n-type contact layer 62 of GaN, n-type cladding layer 63 of
aluminum gallium nitride (AlGaN), multi-quantum well (MQW) active
layer 64, p-type cladding layer 65 of AlGaN, and p-type contact
layer 66 of GaN are stacked in this order on the wafer 60. The MQW
active layer 64 is formed by alternately stacking multiple barrier
layers 64a and multiple well layers 64b made of AlGaN and indium
gallium nitride (InGaN), respectively.
[0088] In depositing the buffer layer 61 and n- and p-type contact
layers 62 and 66, TMG gas diluted with nitrogen (N.sub.2) gas is
supplied as a Group III source into the upper channel to have a
velocity of about 6 m/sec near the gas inlet port and NH.sub.3 gas
is supplied as a Group V source into the lower channel to have a
velocity of about 6 m/sec near the gas inlet port. The internal
pressure inside the reactor body is controlled at about 0.4
atm.
[0089] In depositing the n- and p-type cladding layers 63 and 65
and barrier layers 64a containing aluminum (Al), TMG and
trimethylaluminum (TMA) gases diluted with N.sub.2 gas are used as
Group III sources and NH.sub.3 gas is used as a Group V source. The
velocity of each of these gases near the gas inlet port is
controlled at about 6 m/sec. The internal pressure inside the
reactor body is controlled at about 0.4 atm.
[0090] In depositing the well layers 64b containing indium (In),
TMG and trimethylindium (TMI) gases diluted with N.sub.2 gas are
used as Group III sources and NH.sub.3 gas is used as a Group V
source. Since In has a high vapor pressure, the internal pressure
should be set higher compared to depositing a film containing no
In. The flow velocity of each of these gases near the gas inlet
port is controlled at about 2 m/sec and the internal pressure of
the reactor body is controlled at about 1.2 atm to keep a reference
flow rate, which is proportional to the product of the gas flow
velocity and the internal pressure of the reactor body, unchanged.
In this embodiment, the reference flow rate is 2.4 (=6
(m/s).times.0.4 (atm)).
[0091] Next, as shown in FIG. 6B, the p-type contact layer 66,
p-type cladding layer 65, MQW active layer 64 and n-type cladding
layer 63 are selectively dry-etched to expose the n-type contact
layer 62. And an n-side electrode 67 is formed as a stack of
titanium (Ti) and aluminum (Al) films on the exposed surface of the
n-type contact layer 62. Then, a ridged p-side electrode 68 is
formed as a stack of nickel (Ni) and gold (Au) films on the p-type
contact layer 66b. The n- and p-side electrodes 67 and 68 may be
formed in reverse order.
[0092] In fabricating a semiconductor laser diode including
semiconductor layers with mutually different compositions, the flow
velocity or pressure of source gases is changed in accordance with
the composition of each of those layers so that the reference flow
rate of the source gases is kept constant. In this manner, each
film deposited can have its thickness kept spatially uniform and
can also have its quality improved.
[0093] If the barrier layers 64a should be made of gallium nitride,
trimethylgallium gas diluted with N.sub.2 gas and NH.sub.3 gas may
be supplied in such a manner as to have a velocity of about 6 m/sec
near the gas inlet port and an internal pressure of about 0.4 atm
in the reactor body. Then, a film of quality can also be
obtained.
[0094] In addition, the resonant cavity structure of the
semiconductor laser diode according to the third embodiment is just
an example, so the MQW active layer 64 may have a single quantum
well structure including a single well layer 64b.
[0095] Further, the MQW active layer 64 may be sandwiched between
n- and p-type light guides, or may include an optical guiding
layer.
[0096] Furthermore, a semiconductor device to be fabricated does
not have to be a semiconductor laser diode but may be a
light-emitting diode, for example.
* * * * *