U.S. patent application number 14/413587 was filed with the patent office on 2015-06-18 for apparatus and method for film formation.
This patent application is currently assigned to Gallium Enterprises Pty Ltd. The applicant listed for this patent is Gallium Enterprises Pty Ltd. Invention is credited to Satyanarayan Barik, Ian Mann, Marie-Pierre Francoise Wintrebert EP Fouquet.
Application Number | 20150167162 14/413587 |
Document ID | / |
Family ID | 49915264 |
Filed Date | 2015-06-18 |
United States Patent
Application |
20150167162 |
Kind Code |
A1 |
Barik; Satyanarayan ; et
al. |
June 18, 2015 |
APPARATUS AND METHOD FOR FILM FORMATION
Abstract
An apparatus and method for forming a thin film on a substrate
by RPCVD which provides for very low levels of carbon and oxygen
impurities and includes the steps of introducing a Group VA plasma
into a first deposition zone of a growth chamber, introducing a
Group IIIA reagent into a second deposition zone of the growth
chamber which is separate from the first deposition zone and
introducing an amount of an additional reagent selected from the
group consisting of ammonia, hydrazine, di-methyl hydrazine and a
hydrogen plasma through an additional reagent inlet into the second
deposition zone such that the additional reagent and the Group IIIA
reagent mix prior to deposition.
Inventors: |
Barik; Satyanarayan;
(Holroyd, AU) ; Wintrebert EP Fouquet; Marie-Pierre
Francoise; (Silverwater, AU) ; Mann; Ian;
(Silverwater, AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Gallium Enterprises Pty Ltd |
Silverwater, New South Wales |
|
AU |
|
|
Assignee: |
Gallium Enterprises Pty Ltd
Silverwater, New South Wales
AU
|
Family ID: |
49915264 |
Appl. No.: |
14/413587 |
Filed: |
July 15, 2013 |
PCT Filed: |
July 15, 2013 |
PCT NO: |
PCT/AU2013/000786 |
371 Date: |
January 8, 2015 |
Current U.S.
Class: |
252/521.5 ;
118/723R; 423/409; 438/508 |
Current CPC
Class: |
C23C 16/452 20130101;
H01L 21/0262 20130101; C23C 16/45536 20130101; H01L 29/2003
20130101; C01B 21/0632 20130101; C23C 16/45551 20130101; H01J
37/3244 20130101; C23C 16/455 20130101; H01L 21/0254 20130101; C23C
16/303 20130101; H01J 37/32357 20130101; H01B 1/06 20130101; H01L
21/0257 20130101; C23C 16/513 20130101 |
International
Class: |
C23C 16/455 20060101
C23C016/455; H01L 29/20 20060101 H01L029/20; H01B 1/06 20060101
H01B001/06; H01L 21/02 20060101 H01L021/02; C23C 16/513 20060101
C23C016/513; C01B 21/06 20060101 C01B021/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 13, 2012 |
AU |
2012903023 |
Aug 10, 2012 |
AU |
2012903455 |
Claims
1. An RPCVD apparatus for forming a film, the apparatus including a
growth chamber comprising: (a) a Group VA plasma inlet located in a
first deposition zone of the growth chamber to introduce a Group VA
plasma thereto; (b) a Group IIIA reagent inlet located in a second
deposition zone of the growth chamber to introduce a Group IIIA
reagent thereto; (c) an additional reagent inlet adjacent the Group
IIIA reagent inlet to introduce an additional reagent selected from
the group consisting of ammonia, hydrazine, di-methyl hydrazine and
hydrogen plasma into the second deposition zone such that the
additional reagent and Group IIIA reagent mix prior to deposition;
and (d) a substrate holder adapted to support one or more
substrates and rotate each substrate between the first and second
deposition zones.
2. The apparatus of claim 1 wherein the additional reagent inlet is
an ammonia inlet.
3. The apparatus of claim 1 or claim 2 wherein the first deposition
zone is substantially isolated from the second deposition zone
4. The apparatus of any one of the preceding claims wherein the
Group VA plasma inlet and/or the Group IIIA reagent inlet open into
the growth chamber at a distance between about 1 cm to about 30 cm
from a growth surface of the one or more substrates.
5. The apparatus of claim 4 wherein the Group IIIA reagent inlet
opens into the growth chamber at a distance between about 1 cm to
about 10 cm from a growth surface of the one or more
substrates.
6. The apparatus of claim 1 wherein at least one of the Group VA
plasma inlet or the Group IIIA reagent inlet end flush with a
ceiling of the growth chamber which is located between about 1 to
about 30 cm vertically above a growth surface of the one or more
substrates.
7. The apparatus of claim 6 wherein the ceiling is located between
about 15 to 30 cm vertically above a growth surface of the one or
more substrates.
8. The apparatus of any one of the preceding claims wherein the
additional reagent inlet opens into the growth chamber
substantially adjacent to the opening of the Group IIIA reagent
inlet to promote mixing of said reagents prior to their contacting
the one or more substrates.
9. The apparatus of any one of the preceding claims wherein the
growth chamber comprises one or more structures associated with the
additional reagent inlet and/or the Group IIIA reagent inlet to
promote mixing of said reagents immediately prior to their
contacting the one or more substrates.
10. The apparatus of any one of the preceding claims wherein there
is a direct flow path between the Group VA plasma inlet and the one
or more substrates.
11. The apparatus of claim 10 wherein the direct flow path between
the Group VA plasma inlet and the one or more substrates extends to
an unimpeded path between a plasma generator for generating the
Group VA plasma and the one or more substrates.
12. The apparatus of claim 1 wherein an opening of the additional
reagent inlet opens into the growth chamber in close proximity to
the one or more substrates.
13. The apparatus of claim 12 wherein the additional reagent inlet
opens into the growth chamber at a distance between about 1 cm to
about 10 cm from a growth surface of the one or more
substrates.
14. The apparatus of claim 12 wherein the additional reagent inlet
extends downwardly from the ceiling of the growth chamber to end in
close proximity to a growth surface of the one or more
substrates.
15. The apparatus of claim 12 wherein the additional reagent inlet
opens into the growth chamber through a side wall thereof at a
height suitable to enable a flow of additional reagent entering
therethrough to have a flow path passing over and substantially
adjacent to a growth surface of the one or more substrates.
16. The apparatus of any one of the preceding claims wherein the
Group VA plasma inlet and the Group IIIA reagent inlet are located
centrally within the growth chamber.
17. The apparatus of claim 16 wherein at least one of the Group VA
plasma inlet and the Group IIIA reagent inlet is provided with a
flow control device to direct the corresponding plasma or reagent
into the appropriate first or second deposition zone.
18. The apparatus of any one of claim 1 to claim 15 wherein the
Group VA plasma inlet and the Group IIIA reagent inlet are located
peripherally within the growth chamber.
19. The apparatus of claim 18 wherein the Group VA plasma inlet and
the Group IIIA reagent inlet are located substantially at opposite
ends of the growth chamber.
20. The apparatus of any one of the preceding claims wherein
rotation of the substrate holder causes the one or more substrates
to pass sequentially from the first deposition zone to the second
deposition zone.
21. The apparatus of any one of the preceding claims further
comprising one or more heating devices to heat the additional
reagent inlet and/or the Group IIIA reagent inlet prior to the
respective reagents entering the growth chamber.
22. A method of forming a thin film on a substrate by RPCVD
including the steps of: (a) introducing a Group VA plasma through a
Group VA plasma inlet into a first deposition zone of a growth
chamber; (b) introducing a Group IIIA reagent through a Group IIIA
reagent inlet into a second deposition zone of the growth chamber,
the second deposition zone being substantially isolated from the
first deposition zone; (c) introducing an additional reagent
selected from the group consisting of ammonia, hydrazine, di-methyl
hydrazine and hydrogen plasma through an additional reagent inlet
into the second, deposition zone such that the additional reagent
and the Group IIIA reagent mix prior to deposition; and (d) moving
the substrate between the first and second deposition zones, to
thereby form a thin film on the substrate.
23. The method of claim 22 wherein the additional reagent is
ammonia.
24. The method of claim 22 or 23 wherein the additional reagent is
introduced into the second deposition zone substantially adjacent
the opening of the Group IIIA inlet.
25. The method of any one of claim 22 to claim 24 additional
reagent and the Group IIIA reagent are preferably being introduced
into the growth chamber simultaneously
26. The method of any one of claim 22 to claim 25 wherein the Group
IIIA reagent is a Group IIIA metal organic reagent.
27. The method of claim 26 wherein the Group IIIA metal organic
reagent is a Group IIIA metal alkyl reagent.
28. The method of claim 27 wherein the Group IIIA metal alkyl
reagent is selected from the group consisting of trimethylgallium,
triethylgallium, trimethylindium and trimethylaluminium.
29. The method of any one of claim 22 to claim 28 wherein the Group
VA plasma is a nitrogen plasma comprising active nitrogen
species.
30. The method of any one of claim 22 to claim 29 further including
the step of promoting the mixing of the Group IIIA reagent and the
additional reagent adjacent the one or more substrates.
31. The method of any one of claim 22 to claim 30 wherein the
additional reagent flow rate is between 15 to 1500 sccm.
32. The method of claim 31 wherein the additional reagent flow rate
is between 30 to 1000 sccm.
33. The method of any one of claim 22 to claim 31 further including
the step of controlling the power of the plasma generator to be
between about 500 to about 4000 W.
34. The method of claim 33 wherein the power of the plasma
generator is between about 500 to about 3000 W.
35. The method of any one of claim 22 to claim 34 wherein the
growth pressure in the growth chamber is between 2-5 torr.
36. The method of any one of claim 22 to claim 35 wherein the
plasma flow is between 2000-3000 sccm.
37. The method of any one of claim 22 to claim 36 further including
the step of controlling the temperature in the growth chamber to be
between about 400 to about 1200.degree. C.
38. The method of claim 37 wherein the temperature in the growth
chamber is between about 500 to about 1000.degree. C.
39. The method of claim 38 wherein the temperature in the growth
chamber is between about 500 to about 800.degree. C.
40. The method of any one of claim 22 to claim 39 further including
the step of isolating the deposition zones to prevent the mixing of
the Group VA plasma and Group IIIA reagent.
41. The method of any one of claim 22 to claim 40 further including
the step of controlling the flow of one or more of the Group VA
plasma or Group IIIA reagent upon exiting the associated inlet to
direct that flow to a desired deposition zone.
42. The method of any one of claim 22 to claim 41 wherein the
additional reagent is introduced into the growth chamber through a
side wall thereof.
43. The method of any one of claim 22 to claim 42 wherein the
additional reagent is introduced into the growth chamber to form a
substantially horizontal flow path passing over and substantially
adjacent to a growth surface of the substrate.
44. The method of any one of claims 22 to 43 claim further
including the step of heating one or more of the reagents prior to
their entering the growth chamber.
45. The method of any one of claims 22 to 44 further include a step
of p-type doping of the growing film.
46. A method of forming a thin film having a carbon impurity
content of less than about 5E+17 atom/cm.sup.3, on a substrate by
RPCVD including the steps of: (a) introducing a Group VA plasma
through a Group VA plasma inlet into a first deposition zone of a
growth chamber wherein a direct flow path is provided between the
Group VA plasma inlet and a substrate located in the first
deposition zone; (b) introducing a Group IIIA reagent through a
Group IIIA reagent inlet into a second deposition zone of the
growth chamber, the second deposition zone being substantially
isolated from the first deposition zone; (c) introducing an
additional reagent selected from the group consisting of ammonia,
hydrazine, di-methyl hydrazine and hydrogen plasma through an
additional reagent inlet into the second deposition zone such that
the additional reagent and the Group IIIA reagent mix prior to
deposition; (d) moving the substrate between the first and second
deposition zones, to thereby form a thin film on the substrate
having a carbon impurity content of less than about 5E+17
atom/cm.sup.3.
47. The method of claim 46 wherein the carbon impurity content is
less than about 3E+17 atom/cm.sup.3.
48. The method of claim 47 wherein the carbon impurity content is
less than about 2E+17 atom/cm.sup.3.
49. The method of claim 48 wherein the carbon impurity content is
less than or about 1E+17 atom/cm.sup.3.
50. The method of claim 46 wherein the oxygen impurity content of
the thin film is less than about 6E+17 atom/cm.sup.3.
51. The method of claim 50 wherein the oxygen impurity content is
less than about 2E+17 atom/cm.sup.3.
52. A film formed by the method of any one of claim 22 to claim
51.
53. Use of the film of claim 52 in a semiconductor device.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an apparatus and method for
the production of films by chemical vapour deposition.
BACKGROUND OF THE INVENTION
[0002] Metal or metalloid containing films, such as gallium nitride
(GaN) films, have applications in a range of devices from light
emitting diodes (LEDs) to ultraviolet detectors to transistor
devices.
[0003] These films have commonly been produced by techniques
including molecular beam epitaxy (MBE), metal organic chemical
vapour deposition (MOCVD) and remote plasma enhanced chemical
vapour deposition (RPECVD or RPCVD). RPECVD has been employed to
produce films of high quality at considerably lower temperatures
than those used in MOCVD which thereby reduces process costs and
allows the use of temperature sensitive preferred substrates for
film deposition.
[0004] One problem which must be addressed during film production
using any chemical vapour deposition (CVD) technique is to obtain
an even and controlled distribution of reagents across the
substrate of the surface onto which the film is to be grown to
thereby achieve uniform thin film growth. At least part of the
solution to this problem may be addressed by the design of the
distribution systems. For example, in RPECVD a shower head or
lattice design may be employed to obtain an even distribution of
metal organic reagent across the substrate while a baffle may be
used to enhance the even distribution of the plasma stream of
active nitrogen species. One such baffle design is disclosed in
WO/2010/091470, the disclosure of which is hereby incorporated in
its entirety, wherein an `inverse pagoda` style of baffle is used
to diffuse and filter the plasma stream.
[0005] Many of these approaches focus on growth of a single film
and so the reagent, e.g. metal organic, distribution lattice and
plasma channel with baffle are generally centred over the substrate
location to thereby provide a generally homogeneous distribution of
both materials over the entire substrate surface. This form of
chamber design is not so effective when it is desired to grow a
plurality of films within the same growth chamber to improve
productivity.
[0006] The use of multiple substrates is particularly desirable in
those film deposition techniques where the growth rate is extremely
slow. For example, atomic layer deposition (ALD) is a useful growth
technique based upon the sequential pulsing of chemical precursor
vapours to thereby achieve one atomic layer per pulse. Due to the
sequential pulsing arrangement of ALD each reagent pulse reacts
with the deposition surface until the reaction is completed. A
purge gas is used to carry away excess reagent and reaction side
products after each pulse in an attempt to minimise impurities
being deposited in the film.
[0007] ALD is of interest due to the ability to produce thin
uniform films with a high degree of control over film thickness and
composition. One of the drawbacks of ALD is the amount of time
required to grow a useful film since only a monolayer may be
deposited in each complete deposition cycle. The time required for
each cycle is limited by the switching speed of the reagent release
valves as well as the time taken to purge after each half cycle and
rotate the substrate into place. This results in each full cycle
taking from 0.5 to a few seconds, further contributing to the slow
production.
[0008] Further, the purging cycle is not entirely effective which
often means that an amount of the metal organic reagent, such as
trimethylgallium (TMG), remains in the growth chamber during
pulsing of the plasma containing the active second reagent, such as
nitrogen. This may result in carbon impurities being incorporated
into the growing film, thereby reducing its quality.
[0009] Minimising the extent of incorporation of both carbon and
oxygen, as impurities, in the growing film is a major challenge in
CVD film production. As well as altering the desired chemical
composition of the film these impurities disrupt the lattice
matching of forming layers thereby causing defects within the film
and negatively impacting on the overall quality of the product.
[0010] MOCVD approaches have been relatively more successful than
certain other CVD techniques at lowering oxygen incorporation into
the growing thin films but levels of carbon incorporation are not
ideal. More particularly, MOCVD often involves growth temperatures
of about 1000.degree. C. to 1200.degree. C., which thereby results
in high equipment costs and rules out the use of temperature
sensitive preferred substrates for film deposition.
[0011] It would thus be desirable to provide for a CVD apparatus
and method which could provide for the advantages in control of
film growth afforded by ALD while minimising the drawbacks of that
technique. Particularly, it would be useful to provide for a CVD
apparatus and method allowing for a reduction in the levels of
incorporation of carbon and oxygen as impurities in the film
product and preferably a CVD apparatus and method which can be run
at lower temperatures than those employed in a standard MOCVD
approach.
SUMMARY OF THE INVENTION
[0012] In a first aspect, although it need not be the only or
indeed the broadest form, the invention resides in an RPCVD
apparatus for forming a film, the apparatus including a growth
chamber comprising: [0013] (a) a Group VA' plasma inlet located in
a first deposition zone of the growth chamber to introduce a Group
VA plasma thereto; [0014] (b) a Group IIIA reagent inlet located in
a second deposition zone of the growth chamber to introduce a Group
IIIA reagent thereto; [0015] (c) an additional reagent inlet
adjacent the Group IIIA reagent inlet to introduce an additional
reagent selected from the group consisting of ammonia, hydrazine,
di-methyl hydrazine and a hydrogen plasma into the second
deposition zone such that the additional reagent and Group IIIA
reagent mix prior to deposition; and [0016] (d) a substrate holder
adapted to support one or more substrates and rotate each substrate
between the first and second deposition zones.
[0017] Preferably, the additional reagent inlet is an ammonia
inlet.
[0018] Preferably, the Group VA plasma inlet, the Group IIIA
reagent inlet and the additional reagent inlet open into the growth
chamber at a distance between about 1 cm to about 30 cm from a
growth surface of the one or more substrates. More preferably,
between about 1 to about 20 cm or 1 to about 10 cm.
[0019] Preferably, a ceiling of the growth chamber is located less
than about 30 cm vertically above the location of the substrates,
more preferably less than about 25 cm, even more preferably less
than about 20 cm, still more preferably less than about 10 cm.
Values of 5 cm and 7.5 cm may be useful with 3 cm to 4 cm as the
lower end values.
[0020] In certain embodiments at least one of the Group VA plasma
inlet, the Group IIIA reagent inlet and the additional reagent
inlet end flush with the ceiling of the growth chamber which is
located between about 1 to about 30 cm, 1 to 20 cm, 1 to 10 cm
vertically above a growth surface of the substrates, preferably,
between 4 to 15 cm, 4 to 10 cm, 4 to 8 cm.
[0021] Suitably, an opening of the additional reagent inlet opens
into the growth chamber in close proximity to the one or more
substrates.
[0022] The additional reagent, inlet may extend downwardly from the
ceiling of the growth chamber to end in close proximity to the
growth surface of the one or more substrates.
[0023] In one embodiment, the additional reagent inlet opens into
the growth chamber through a side wall thereof at a height suitable
to enable a flow of additional reagent entering therethrough to
have a flow path passing over and substantially adjacent to the
growth surface of the one or more substrates.
[0024] In a preferred embodiment there is a direct flow path
between the Group VA plasma inlet and the one or more
substrates.
[0025] Suitably, the direct flow path between the Group VA plasma
inlet and the one or more substrates extends to an unimpeded path
between a plasma generator for generating the Group VA plasma and
the one or more substrates.
[0026] In an embodiment, the Group VA plasma inlet and the Group
IIIA reagent inlet end flush with a ceiling and/or side wall of the
growth chamber through which they extend.
[0027] Preferably, the first deposition zone is substantially
isolated from the second deposition zone.
[0028] Preferably, rotation of the substrate holder causes the one
or more substrates to pass sequentially from the first deposition
zone to the second deposition zone.
[0029] Preferably, the substrate holder is of a turntable design
whereby it rotates around a central pivot and is provided with a
plurality of recesses, each adapted to hold a substrate, around its
periphery.
[0030] The Group VA plasma inlet and the Group IIIA reagent inlet
may be located centrally within the growth chamber.
[0031] When the Group VA plasma inlet and the Group IIIA reagent
inlet are located centrally within the growth chamber one or both
thereof may be provided with a flow control device to direct the
corresponding plasma or reagent into the appropriate first or
second deposition zone.
[0032] The flow control device may be a flow barrier blocking one
or more reagent flow paths within the Group VA plasma inlet or the
Group IIIA reagent inlet or a directing portion, such as a shroud,
continuous with the first or second reagent inlet.
[0033] In one embodiment, the apparatus may further comprise a
baffle associated with the Group VA plasma inlet such that the
plasma substantially passes therethrough.
[0034] The baffle may comprise the flow control device which may be
a flow barrier blocking one or more outlets of the baffle.
[0035] Preferably, the additional reagent inlet opens into the
growth chamber substantially adjacent to the opening of the Group
IIIA reagent inlet to promote mixing of said reagents prior to
their contacting the one or more substrates.
[0036] Suitably, the Group VA plasma inlet is in fluid
communication with a plasma generator producing a Group VA plasma
comprising an active species.
[0037] Preferably, the Group, VA plasma is a nitrogen plasma
comprising active nitrogen species.
[0038] Suitably, the Group IIIA reagent is a Group IIIA metal
organic reagent.
[0039] In one particularly preferred embodiment, the Group VA
plasma inlet and the Group IIIA reagent inlet are located
peripherally within the growth chamber.
[0040] Suitably, the Group VA plasma inlet and the Group IIIA
reagent inlet are located substantially at opposite ends of the
growth chamber.
[0041] The growth chamber may comprise one or more structures
associated with the additional reagent inlet and/or the Group IIIA
reagent inlet to promote mixing of said reagents immediately prior
to their contacting the one or more substrates.
[0042] The apparatus may further comprise one or more heating
devices to heat the additional reagent inlet and/or the Group IIIA
reagent inlet prior to entering the growth chamber.
[0043] In a second aspect the invention resides in a method of
forming a thin film on a substrate by RPCVD including the steps of:
[0044] (a) introducing a Group VA plasma through a Group VA plasma
inlet into a first deposition zone of a growth chamber; [0045] (b)
introducing a Group IIIA reagent through a Group IIIA reagent inlet
into a second deposition zone of the growth chamber, the second
deposition zone being substantially isolated from the first
deposition zone; [0046] (c) introducing an additional reagent
selected from the group consisting of ammonia, hydrazine, di-methyl
hydrazine and hydrogen plasma through an additional reagent inlet
into the second deposition zone such that the additional reagent
and the Group IIIA reagent mix prior to deposition; [0047] (d)
moving the substrate between the first and second deposition zones,
to thereby form a thin film on the substrate.
[0048] Preferably, the additional reagent is ammonia.
[0049] Suitably, the additional reagent is introduced into the
second deposition zone substantially adjacent the opening of the
Group IIIA inlet.
[0050] In one embodiment, the additional reagent is introduced into
the growth chamber through a side wall thereof.
[0051] In one embodiment, the additional reagent is introduced into
the growth chamber to form a substantially horizontal flow path
passing over and substantially adjacent to the growth surface of
the substrate.
[0052] The additional reagent and the Group IIIA reagent are
preferably being introduced into the growth chamber
simultaneously.
[0053] Suitably, the Group IIIA reagent is a Group IIIA metal
organic reagent.
[0054] Preferably, the Group IIIA metal organic reagent is a Group
IIIA metal alkyl reagent.
[0055] Preferably, the Group IIIA metal alkyl reagent is selected
from the group consisting of trimethyigallium, triethylgallium,
trimethylindium and trimethylaluminium.
[0056] The method may further include the step of heating one or
more of the reagents prior to their entering the growth
chamber.
[0057] The method may further include the step of promoting the
mixing of the metal organic reagent and the additional reagent
adjacent the one or more substrates.
[0058] Suitably, the Group VA plasma inlet is in fluid
communication with a plasma generator.
[0059] Preferably, the Group VA plasma is a nitrogen plasma
comprising active nitrogen species.
[0060] The isolation of the deposition zones substantially prevents
the mixing of the Group VA plasma and Group IIIA reagent.
[0061] The method may further include the step of controlling the
flow of one or more of the Group VA plasma or Group IIIA reagent
upon exiting the associated inlet to direct that flow to a desired
deposition zone.
[0062] The method may further include the step of controlling the
temperature to be between about 400 to about 1200.degree. C.,
preferably between about 500 to about 1000.degree. C. (inclusive of
a temperature of about 500.degree. C., 600.degree. C., 700.degree.
C., 800.degree. C., 900.degree. C. or 1000.degree. C.), more
preferably between about 500 to about 850.degree. C.
[0063] In combination with the presence of an additional reagent
gas, preferably ammonia, it has been found that the power of the
plasma generator has an effect on carbon incorporation into the
thin film and so the method may also include the step of
controlling the power of the plasma generator to be between about
500 W to about 5000 W from a single source. This may be combined
with a growth pressure of 2-5 torr and a nitrogen plasma flow of
2000-3000 sccm with an ammonia flow of about 15 to about 1500 sccm,
preferably about 20 to about 200 sccm, preferably about 20 to about
100 sccm, more preferably about 20 to about 50 sccm.
[0064] The growth pressure may be between 2-5 torr, 2-4 torr or
about 3 torr.
[0065] Preferably, the power of the plasma generator is between
about 100 watts to about 3000 watts with a nitrogen flow rate of
1000-3000 sccm extending to 100-20000 sccm in a commercial unit. A
preferred metal organic reagent flow rate is 1200-2000 sccm which
may extend to 100-10000 sccm in a commercial unit. A value for the
plasma generator power of about 500 to 5000 W, 500 to 4000, 500 to
3000, 500 to 2000, 500 to 1000, 500 to 900 W, 500 to 800 W, 600 to
1000 W, 600 to 900 W, 600 to 800 W, 700 to 1000 W, 700 to 900 W and
preferably about 800 W is preferred and each value or range of
which may be independently coupled with an ammonia flow rate of any
one of between 15 to 1500 sccm. For relatively small growth
chambers it is found that ammonia flows of 10 to 75, 10 to 60, 10
to 50 10 to 40, 10 to 30, 15 to 75, 15 to 60 15 to 50, 15 to 40, 15
to 35, 15 to 30, 20 to 75, 20 to 60, 20 to 50, 20 to 40, 20 to 30,
including values of about 15, 20, 25, 30, 35, 40, 45 and 50 sccm
are particularly useful in lowering carbon incorporation, however,
in moving to commercial scales higher powers and multiple plasma
sites are envisaged as being useful.
[0066] In a third aspect the invention resides in a method of
forming a thin film having a carbon impurity content of less than
about 5E+17 atom/cm.sup.3, on a substrate by RPCVD including the
steps of: [0067] (a) introducing a Group VA plasma through a Group
VA plasma inlet into a first deposition zone of a growth chamber
wherein a direct flow path is provided between the Group VA plasma
inlet and a substrate located in the first deposition zone; [0068]
(b) introducing a Group IIIA reagent through a Group IIIA reagent
inlet into a second deposition zone of the growth chamber, the
second deposition zone being substantially isolated from the first
deposition zone; [0069] (c) introducing an additional reagent
selected from the group consisting of ammonia, hydrazine, di-methyl
hydrazine and hydrogen plasma through an additional reagent inlet
into the second deposition zone such that the additional reagent
and the Group IIIA reagent mix prior to deposition; [0070] (d)
moving the substrate between the first and second deposition
zones,
[0071] to thereby form a thin film on the substrate having a carbon
impurity content of less than about 5E+17 atom/cm.sup.3.
[0072] Preferably, the carbon impurity content is less than about
3E+17 atom/cm.sup.3, even more preferably less than about 2E+17
atom/cm.sup.3, yet more preferably less than or about 1E+17
atom/cm.sup.3. A lower limit may be considered to be about the SIMS
detection limit for carbon impurities in such films.
[0073] In one embodiment, the thin film also has an oxygen impurity
content of less than about 8E+17 atom/cm.sup.3, even more
preferably less than about 6E+17 atom/cm.sup.3, yet more preferably
less than about 4E+17 atom/cm.sup.3, still more preferably less
than about 2E+17 atom/cm.sup.3, or even less than or about 1E+17
atom/cm.sup.3. A lower limit may be considered to be about the SIMS
detection limit for oxygen impurities in such films.
[0074] The statements made above in relation to the second aspect
apply equally well to the third aspect.
[0075] In a fourth aspect the invention resides in a film made by
the method of the second or third aspects.
[0076] In a fifth aspect the invention resides in use of a film of
the fourth aspect in a semiconductor device.
[0077] Further features of the present invention will become
apparent from the following detailed description.
[0078] Throughout this specification, unless the context requires
otherwise, the words "comprise", "comprises" and "comprising" will
be understood to imply the inclusion of a stated integer or group
of integers but not the exclusion of any other integer or group of
integers.
BRIEF DESCRIPTION OF THE FIGURES
[0079] In order that the invention may be readily understood and
put into practical effect, preferred embodiments will now be
described by way of example with reference to the accompanying
figures wherein:
[0080] FIG. 1 shows a schematic representation of a typical RPCVD
apparatus for depositing a metal nitride film on a substrate;
[0081] FIG. 2 shows a perspective sectional view of one embodiment
of an apparatus for depositing a metal nitride film on a substrate
when employing an inverse pagoda baffle and multiple
substrates;
[0082] FIG. 3 shows a schematic sectional representation of one
embodiment of an apparatus for forming a film according to the
present invention;
[0083] FIG. 4 shows a schematic sectional representation of one
preferred embodiment of an apparatus for forming a film according
to the present invention;
[0084] FIG. 5 shows a partial perspective sectional view of the
apparatus for forming a film, as represented in FIG. 4;
[0085] FIG. 6 shows a schematic sectional representation of a
highly preferred embodiment of an apparatus for forming a film
according to the present invention;
[0086] FIG. 7 shows a partial perspective sectional view of the
apparatus for forming a film, as represented in FIG. 6;
[0087] FIG. 8 shows a partial perspective sectional view of an
alternative embodiment of the apparatus for forming a film to that
shown in FIG. 7;
[0088] FIG. 9 shows a partial perspective sectional view of an
alternative embodiment of the apparatus for forming a film to that
shown in FIG. 7;
[0089] FIG. 10 shows a schematic representation of an RPCVD
apparatus for depositing a film on a substrate, according to
another embodiment of the invention;
[0090] FIG. 11 shows a schematic representation of an alternative
RPCVD apparatus for depositing a film on a substrate to that shown
in FIG. 10;
[0091] FIG. 12 shows a schematic representation of a further
alternative RPCVD apparatus for depositing a film on a substrate to
that shown in FIG. 10;
[0092] FIG. 13 shows a schematic representation of yet a further
alternative RPCVD apparatus for depositing a film on a substrate to
that shown in FIG. 10;
[0093] FIG. 14 shows a partial perspective sectional view of an
apparatus for forming a film, according to a further embodiment of
the present invention;
[0094] FIG. 15 is a graphical representation of the carbon levels
incorporated into films under varying conditions;
[0095] FIG. 16 is a SIMS graphical analysis of the typical
impurities found in a film produced by a method and apparatus of
the invention and an underlying GaN template;
[0096] FIG. 17 is a SIMS graphical analysis of the level of carbon,
as an impurity, found in a film produced by a method and apparatus
of the invention on an underlying GaN template with varying ammonia
flow rates; and
[0097] FIG. 18 is a SIMS graphical analysis of the level of oxygen,
as an impurity, found in a film produced by a method and apparatus
of the invention on an underlying GaN template with varying ammonia
flow rates.
DETAILED DESCRIPTION OF THE INVENTION
[0098] The present inventors have identified a particular RPCVD
apparatus and process conditions for the production of high quality
films which results in improvements to the film growth rate and
growth control, by comparison to standard ALD techniques and other
CVD processes, and, importantly, which provides for a surprising
level of reduction in oxygen and carbon-based film impurities due
to reagent side reactions.
[0099] The reagents which may be employed with the present
apparatus and method, and hence the nature of the films which can
be formed, are not particularly limited. Although the embodiments
discussed herein generally employ a nitrogen plasma and a metal
organic (typically a gallium containing metal organic such as
trimethylgallium) as the reagents, the utility of the present
invention is not so limited. The Group IIIA (otherwise known as
Group 13 under the current IUPAC system) reagent may comprise an
element which is selected from the group consisting of boron (B),
aluminium (Al), gallium (Ga), indium (In) or thallium (TI). The
Group VA (otherwise known as Group 15 under the current IUPAC
system) plasma may be generated from any suitable reagent
containing a Group VA element selected from the group consisting of
nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb) and
bismuth (Bi).
[0100] The term "deposition zone" as used herein is used to refer
to a distinct region, section or segment of the growth chamber into
which one or more reagents are introduced. Individual depositions
zones, such as first and second deposition zones, are isolated one
from the other such that a substrate or growing film will only be
substantially exposed to a particular reagent introduced only into
one deposition zone when the substrate or growing film actually
enters that deposition zone. The separation or isolation of
deposition zones may be spatial only or may be effected by partial
or complete physical barriers.
[0101] In the embodiments described the reagents employed will be
trimethylgallium, a nitrogen plasma and ammonia but the person
skilled in the art will appreciate the principles disclosed herein
could be applied mutatis mutandis to other reagent
combinations.
[0102] Without wishing to be bound by any particular theory, the
present inventors have postulated that an experimentally observed
dramatic reduction in the levels of oxygen and carbon incorporated
into gallium nitride films produced in an RPCVD apparatus are due
to a choice of conditions, including, primarily, the supply of an
additional gaseous reagent, preferably ammonia gas, to mix with the
Group IIIA reagent and preferably in close proximity to the
substrate, which favours formation of a trimethylgallium:ammonia
Lewis acid:base adduct. This adduct breaks down to form gallium
nitride with the release of methane gas which is not incorporated
into the growing film to the same extent as methyl radicals would
be.
[0103] In typical prior art CVD approaches, and particularly MOCVD
due to the high temperatures employed, a molecule of
trimethylgallium injected into the growth chamber will decompose
thermally to finally produce a gallium atom and three methyl
radicals. The gallium will react with the nitrogen source, which
may be ammonia or a nitrogen plasma, to form the GaN film. The
reactive methyl radicals are often incorporated into the growing
film as an impurity thereby increasing strain and lowering the
overall quality of the film product. Additional hydrogen-containing
reagents, such as ammonia gas, are typically only introduced with
the plasma stream and the benefits produced herein are not seen
with such an approach.
[0104] When the growth temperature is instead kept below the
thermal decomposition point of trimethylgallium and an additional
reagent, preferably ammonia, is introduced into the growth chamber
then the two components form a Lewis acid-base adduct of proposed
formula (CH.sub.3).sub.3Ga:NH.sub.3. It is further proposed that
this adduct reacts to form an intermediate
(CH.sub.3).sub.2Ga:NH.sub.2+CH.sub.4. A further step in this
pathway leads to the formation of an adduct of formula
[(CH.sub.3).sub.2Ga:NH.sub.2].sub.3 with eventual formation, from
that structure, of three molecules of GaN and six molecules of
CH.sub.4 gas. The methane gas is less reactive than a methyl
radical and is easily removed via the exhaust of the growth
chamber.
[0105] The inventors have further postulated that such adduct
formation and preference of the formation of methane over methyl
radicals can be encouraged by minimising the extent of the
reactions which occur in the gas phase i.e. in the upper and
central regions of the growth chamber and instead maximising the
mixing of the reagents only in the immediate vicinity of the
substrates. This can be achieved by introducing the ammonia or
other additional reagent into the growth chamber with the Group IIA
reagent in a manner which causes it to be present or available only
adjacent the substrates and hence the growth surface of the growing
film.
[0106] FIG. 1 shows a schematic representation of a typical RPCVD
apparatus 100 for depositing a Group IIIA nitride film on a
substrate. The apparatus 100 comprises a growth chamber 105 inside
which film growth will occur. Located within the growth chamber 105
is a substrate 110 which is supported by a substrate holder 115
which may include or be connected to a heater to allow the
substrate 110 to be adjusted to growth temperatures. A plasma inlet
120, located at a distance from the substrate 110, allows for entry
of plasma 130 formed in the high frequency generator 125 into the
growth chamber 105. The high frequency generator 125 acts on a
region of the apparatus 100 receiving nitrogen from a nitrogen
source 135. A Group IIIA reagent source which is usually a Group
IIIA metal organic reagent source 140, which is usually also at a
distance from the substrate 110 i.e. not adjacent thereto,
introduces the metal organic into a flow path 145 which delivers
the reagent to a metal organic injector 150 for dispersion into the
growth chamber 105.
[0107] It can be seen that the plasma enters an area of the growth
chamber 105 directly above the metal organic injector 150 and so,
in operation, the plasma containing active neutral nitrogen species
and the metal organic reagent mix and react to form the particular
metal nitride, such as gallium nitride, which is deposited on the
substrate to form the film. Excess reagents, carrier gases,
contaminants etc are removed via a waste outlet 155.
[0108] Carbon and oxygen are inevitably incorporated as impurities
into the film but, these aside, this approach is generally
satisfactory for the formation of a film on a single substrate.
However, it is often desirable to have the capacity to generate a
number of such films at the same time. Hence, an apparatus such as
that shown in FIG. 2 may be useful.
[0109] FIG. 2 shows a perspective sectional view of one embodiment
of an apparatus 200 for depositing a metal nitride film on a
substrate which essentially corresponds to the simple
representation shown in FIG. 1 but with the use of a baffle and
multiple substrates. The apparatus 200 comprises a growth chamber
205 partially formed from outer housing 210.
[0110] A plasma generator 215 receives nitrogen through a nitrogen
inlet 220 and the active nitrogen plasma formed passes through
plasma inlet 225, which once again is remote from the substrates,
and into the growth chamber 205 via a baffle 230, which in the
embodiment shown takes the form of an inverse pagoda style baffle
as described in WO/2010/091470. The plasma passes through the
baffle 230 and is evenly distributed by its concentric ring-like
structure. The distributed plasma flow then passes over a metal
organic injector 235 where the metal organic reagent is introduced
and mixes with the plasma. The metal nitride formed will then
deposit on one or more of substrates 240 located on a substrate
holder 245. The substrate holder 245 may be of a turn table design
and so may be rotating at high speed throughout the deposition
process. Waste is removed via outlet 250.
[0111] It will be appreciated that the central placement of the
plasma inlet 225 will likely result in the bulk of the plasma flow
being focused on the centre of the substrate holder 245, even with
the use of a distribution system such as the inverse pagoda baffle
230. It is critical to quality film growth that the reagents be
distributed evenly across the surface of the appropriate substrate
240 and the deficiency in this apparatus 200 will not be solved by
rotation of the substrate holder 245. This type of apparatus 200
also does not typically provide the advantages of control over film
growth and thickness that is provided by atomic layer deposition
(ALD).
[0112] FIG. 3 shows a schematic representation of one embodiment of
an apparatus 300 for forming a film, according to the present
invention. The actual components of the apparatus 300 are much the
same as those displayed in FIG. 2 but with two notable exceptions
being that one region of the baffle is blocked to plasma flow and a
number of ports of the Group IIA reagent inlet (referred to herein
as the metal organic injector) are either removed or closed to
reagent flow.
[0113] As for FIG. 2, the apparatus 300 shown in FIG. 3 comprises a
growth chamber 305 having a plasma inlet 310 to receive plasma flow
comprising active neutral nitrogen species from a plasma generator
315. Although FIG. 3 is merely a schematic representation, the
plasma inlet 310 in this embodiment will be located physically
closer to the level of the substrates than in the prior art
apparatus. The plasma will flow into a baffle 320 which may be of
any suitable design but in the embodiment shown has an inverse
pagoda shape, as shown in FIG. 2. This time the baffle is provided
with a flow barrier 325 formed around one side of the baffle 320 so
as to prevent plasma from exiting along that side. This will result
in the plasma flow being directed towards the opposite side of the
growth chamber 305 from the side of the baffle 320 bearing the flow
barrier 325.
[0114] The active nitrogen species then pass by a Group IIIA
reagent injector in the form of metal organic reagent (e.g.
trimethylgallium) injector 330. In FIG. 3, those circles which are
black inside represent ports or valves of the metal organic reagent
injector 330 which are open to reagent flow i.e. they are open
ports 335 while those circles which are white inside (not filled)
represent ports or valves of the metal organic reagent injector 330
which are closed to reagent flow i.e. they are closed ports 340. In
reality, the parts of the metal organic reagent injector 330 which
are represented as closed may simply not be present in the
apparatus 300 and so only those regions of the growth chamber 305
having the open ports 335 will actually be provided with a metal
organic reagent injector 330 structure.
[0115] Located beneath the metal organic reagent injector 330 are a
number of substrates 345 which are supported by a substrate holder
350. The substrate holder 350 may hold any desired number of
wafers, for example, from 2 to 20 individual substrates, preferably
3 to 10, more preferably 5, 6 or 7. The substrates may have a
crystal structure that is suitable for growth of the particular
film desired. In particular examples, the substrates 345 may
comprise sapphire, SiC, silica, soda lime glass, borosilicate
glass, Pyrex.RTM., silicon, glass, synthetic sapphire, quartz, zinc
oxide, nitride coated substrates and other materials as are well
known in the art including free standing bulk semiconductor
substrates and nitride templates. As is indicated by the arrow in
FIG. 3, the substrate holder is adapted to rotate relative to the
plasma inlet 310 and metal organic reagent injector 330 thereby
controlling growth and deposition uniformity. Waste materials may
be removed through waste outlet 355.
[0116] The combined effect of the directing action of flow barrier
325 on the plasma pathway and the release of metal organic reagent
from only those open ports 335, as can be seen from FIG. 3, means
that, mixing between the metal organic reagent and plasma species
is minimized. Reduction of dead zones within the chamber could be
accomplished by arranging the reagent inlets to be flush with the
upper surface of the chamber minimising premature mixing of reagent
gasses. The design of the apparatus 300 has thus effected a
physical separation of the regions into which plasma and metal
organic reagent are released into a first deposition zone and a
second deposition zone, respectively, which are substantially
isolated from one another. It will be appreciated that rotation of
the substrate holder 350 causes the substrates 345 to pass
sequentially from the first deposition zone to the second
deposition zone in a repetitive, continuous manner to thereby be
exposed to the plasma and metal organic reagents, one after the
other.
[0117] The sequential exposure of each substrate 345 to the metal
organic reagent and the active nitrogen species will result in
formation of subsequent layers of a film, much in the manner of
ALD. However, the formation of separate deposition zones means that
the delays experienced in ALD in both waiting for valves to be
switched and the removal of one reagent by a purge gas before
introduction of the second reagent, are avoided. Instead, the
growing surface of the film is exposed to each reagent with a
minimum of downtime in between due to the ability of the substrate
holder 350 to rotate at very high speeds. This greatly accelerates
the growth of the films while maintaining control over sample
growth.
[0118] The substrate holder 350 may be adapted to rotate
continuously. Preferably, the substrate holder is capable of
rotating at speeds of between 10-2000 rpm. A preferred rotation
speed may be between 25 to 100 rpm, more preferably about 50 rpm.
The skilled addressee will understand that film growth will be
controlled by a combination of the speed of rotation of the
substrate holder 350 and the reagent flow rates in the deposition
chamber. Higher rotation speeds of the substrate holder 350 will
require a higher flow rate of reagents to ensure an overall
increase in the growth rate of the film is produced.
[0119] It will be appreciated that although only one plasma inlet
310, and associated baffle 320, and one region of open ports 335
have been shown in FIG. 3 the apparatus 300 may in fact comprise
multiples of each component. For example, when looking down on the
growth chamber 305 from above the circular substrate holder 350
could be imagined to be split into quadrants with a plasma inlet
and associated baffle, if required, sitting above two adjacent or
diagonally opposite quadrants and the same relationship for two
distinct regions of reagent injector 330 which are open to release
of metal organic or other reagent.
[0120] It will also be understood that baffle 320 is not an
essential feature but may be preferred, under certain process
conditions, to prevent or reduce etching due to active nitrogen
species which may have relatively high kinetic and/or potential
energies. If the baffle 320 was not present in FIG. 3 then some
form of structure, such as a shroud, could be used in its place to
direct and contain plasma flow to one isolated deposition zone.
When the baffle 320 is employed then it may take a variety of forms
other than the inverse pagoda style shown which are well known in
the art such as a plate with tortured multiple pathways
therethrough shower head design etc. Whatever alternate style of
baffle is ultimately used it may have either closed pathways or
some form of flow barrier or flow directing means to ensure the
plasma passes only into a discrete deposition zone and
substantially avoids mixing with the other reagent.
[0121] The apparatus 300 may further comprise one or more heaters
to heat the growth chamber and/or one of the reagent inlets. This
may be useful to promote increased reaction rates, assist with
quality of the growing film or to break or otherwise activate one
or more of the reagents before exposure to the substrates.
[0122] As was mentioned above, the present apparatus and method are
not particularly limited in the type of reagents suitable for use
therein. Any reagents which are suitable for use in ALD may be
appropriate. A wide range of reagent classes including, nitrogen
plasma, nitrogen/hydrogen plasma, ammonia plasma and metal organics
may be suitable. When a metal organic reagent is used then
preferred examples include alkyl Group IIIA reagents such as but
not limited to one or more of trimethylgallium, trimethylindium,
trimethylaluminium as well as employing various well known Mg, Si
and Zn precursors as dopant sources.
[0123] FIG. 4 shows a schematic representation of one embodiment of
an apparatus 400 for forming a film, according to the present
invention. The majority of the components are as described for FIG.
3 and so will only be referred to briefly. A growth chamber 405 is
provided with a plasma inlet 410, as for FIG. 3 being located
relatively close to the substrates, which is continuous with a
plasma generator 415. Plasma introduced into the growth chamber 405
will pass through a baffle 420, which again in the embodiment shown
is an inverse pagoda style baffle 420, before passing by metal
organic reagent injector 425 which has closed ports 430 (circles
with white inside) and open ports 435 (circles with black inside).
Again, the regions of the metal organic reagent injector 425 having
closed ports 430 may simply not be present. A number of substrates
440 are placed upon a substrate holder 445 which rotates relative
to the growth chamber 405 and unwanted reactants and reaction
products are vented via waste outlet 450.
[0124] The key difference in FIG. 4, compared to the embodiment
shown in FIG. 3, is the physical location of the plasma inlet 410
and associated baffle 420 relative to the open ports 435. Whereas
FIG. 3 showed a modified design whereby a typical central placement
of the plasma inlet 310 was manipulated with the additional feature
of a flow barrier 325 formed around a portion of the border of the
baffle 320, FIG. 4 represents a radical shift in the growth chamber
405 design by comparison to standard ALD set ups.
[0125] When looking at a section of the growth chamber 405, as
represented in FIG. 4, the plasma inlet 410 and baffle 420 have
been shifted to the left hand side of the chamber 405 to form a
discrete first deposition zone which is substantially separate from
the second deposition zone formed under and adjacent the open ports
435 of the reagent injector 425.
[0126] The schematic representation shown in FIG. 4 is reproduced,
in part, in three dimensions in FIG. 5 which shows a partial
perspective sectional view of the apparatus 400. For the sake of
clarity many of the components of the apparatus 500, such as the
housing and high frequency generator, have been removed to focus on
the key relationship between plasma inlet 505, baffle 510 and metal
organic reagent injector 515.
[0127] In the embodiment shown in FIG. 5, as for FIG. 4, the plasma
inlet 505 and associated baffle 510 are located peripherally to sit
more or less directly above, i.e. adjacent, a first substrate 520
which can therefore be described as being within a first deposition
zone receiving active nitrogen species. The baffle illustrated may
be replaced with a showerhead or shroud or similar distribution
system common to the field. The reagent injector 515 is only
disposed on the opposite side of the chamber to the plasma inlet
505, generally above a second substrate 525 and located within a
second deposition zone receiving only metal organic reagent, for
example trimethylgallium and/or trimethylindium. Thus, as substrate
holder 530 rotates the first substrate 520 will have been contacted
with a first reagent (in this case active nitrogen species from the
plasma) before proceeding out of the first deposition zone and
entering the second deposition zone to then be contacted by the
second reagent (in this case the metal organic). The second
substrate 525, and all substrates located upon the substrate holder
530, will undergo a similar cycle of sequential exposure to one
reagent and then the other. This allows epitaxial crystal layers to
be deposited sequentially to build up a film with a high degree of
control. Alternating the exposure of the substrates to reagents by
controlling the speed of rotation of the substrate holder 530
provides for finer control than the rotating, pulsing and purging
arrangement employed in ALD.
[0128] Although in FIGS. 3 to 5 the plasma and metal organic
reagent inlets are shown as being vertically above the substrates
it will be appreciated by the person skilled in the art that this
is not necessarily the case. For example, the plasma inlet may
inject plasma into the growth chamber from the side of the housing
i.e. the plasma is injected parallel to the substrates and then
proceeds to deposit down upon them. References herein to a plasma
inlet or metal organic reagent inlet or additional gas inlet are
meant to address the point at which the plasma or plasma activated
reactant or metal organic reagent or additional gas enters the
reaction chamber proper.
[0129] It will be appreciated that in the embodiments described in
FIGS. 3 to 5 the two streams of reagents do not, to any notable
degree, come into contact with one another. The physical separation
of the reagent inlets assists with minimising reagent mixing such
that the amount of oxygen and carbon-based impurities, which may be
formed by such mixing and incorporated into the growing films, is
reduced compared with a standard ALD or other CVD approach.
[0130] However, even employing the approach discussed above it has
been found that sufficient amounts of oxygen and carbon impurities
are still incorporated in the thin film formed to thereby reduce
its quality. The use of RPCVD approaches, while more convenient in
many ways than a standard MOCVD approach, are generally accepted in
the art as inevitably resulting in moderate levels of oxygen and
carbon impurities in the films produced as compared with high end
MOCVD produced films. Very low impurity limits can be considered as
being at least relatively close to the SIMS detection limits as set
out in table 1.
TABLE-US-00001 TABLE 1 SIMS detection limits of selected elements
in GaN under normal depth profiling conditions O.sub.2/SIMS
C.sub.s/SIMS C.sub.s/SIMS Positive Secondary Ion Negative Secondary
Ion Positive Secondary Ion Detection Detection Detection
(C.sub.sM+) Detection Detection Detection Limit Limit Limit Element
(atoms/cm.sup.3) Element (atoms/cm.sup.3) Element (atoms/cm.sup.3)
Be 1E+14 H* 8E+16-2E+17 Mg 5E+15 Li 1E+14 C* 5E+15-2E+16 Zn 1E+16 B
1E+15 O* 1E+16-3E+16 Na 5E+14 Si 3E+15 Mg 5E+14 As 5E+15 *Varies
with vacuum conditions
[0131] However, the present inventors have found that the levels of
these impurities can be lowered significantly by the use of an
apparatus as shown in FIGS. 6 to 9.
[0132] FIG. 6 shows a schematic sectional representation of one
preferred embodiment of an apparatus 600 for forming a film
according to the present invention while FIG. 7 is a partial
perspective sectional view of the same apparatus 600. The RPCVD
apparatus 600 comprises a growth chamber 605 which is provided with
a plasma inlet 610. It is clear from FIGS. 6 and 7 that the plasma
inlet 610 is physically adjacent to the level of the substrates.
The plasma inlet 610 is continuous with a plasma generator 615
(detail not shown).
[0133] The growth chamber 605 is also provided with a Group IIIA
reagent inlet and, more specifically, in the embodiment discussed
herein, a metal organic reagent inlet 620 and an additional reagent
inlet which may be a hydrazine inlet, a di-methyl hydrazine inlet
or a hydrogen plasma inlet but is preferably an ammonia inlet 625.
As was described for FIG. 4 the plasma inlet 610 and metal organic
reagent inlet 620 are physically distant forming a first and second
deposition zone, respectively, with substrates 630 arranged within
each zone by a substrate holder 635 which rotates relative to the
growth chamber 605. Unwanted reactants and reaction products are
vented via waste outlet 640 to which access is provided by gap 645
provided between the circumference of the substrate holder 635 and
the inner walls of the growth chamber 605. The ammonia inlet 625,
however, is immediately adjacent the metal organic reagent inlet
620 and so the ammonia will be introduced into the second
deposition zone along with the metal organic reagent.
[0134] Plasma introduced into the growth chamber 605 will directly
contact the substrate 630 placed in the first deposition zone as no
baffle, shroud or like blocking or distributing device is in place
in the embodiment shown. The present inventors have found that,
when using such an apparatus under conditions of relatively low
power of the plasma generator (around 500 W to 2500 W) and
temperature (about 700.degree. C. to 800.degree. C.) no significant
degree of etching was observed. The use of this arrangement with an
injection of between about 15 to about 50 sccm ammonia resulted in
a substantial reduction of the levels of oxygen and carbon
incorporated into the film product.
[0135] Further process runs were carried out varying the power
output from the plasma generator. At a power output of about 800 W
the level of carbon incorporated into the film was reduced to
levels approaching the actual detection limit of secondary ion mass
spectrometry (SIMS). Oxygen levels have been reduced to those
observed using MOCVD wherein oxygen is effectively removed as an
impurity of concern. Such a reduction in the levels of oxygen and
carbon in RPCVD produced films has hitherto not been shown.
[0136] It is envisaged that the method may also include the step of
controlling the power of the plasma generator to be between about
500 W to about 5000 W from a single source. This range would be
suitable with a growth pressure of 2-3 torr, a nitrogen plasma flow
of 2000-3000 sccm and an ammonia flow of between about 15-1500
sccm.
[0137] Preferably, the power of the plasma generator is between
about 100 watts to about 5000 watts, preferably about 500 to about
3000 W with a nitrogen plasma flow rate of 1000-3000 sccm extending
to 100-20000 sccm in a commercial unit. A preferred metal organic
reagent flow rate is 1200-2000 sccm which may extend to 100-10000
sccm in a commercial unit. A value for the plasma generator power
of about 500 to 1000 W, 500 to 900 W, 500 to 800 W, 600 to 1000 W,
600 to 900 W, 600 to 800 W, 700 to 1000 W, 700 to 900 W and
preferably about 800 W is preferred.
[0138] Such power levels may be independently coupled with an
ammonia injection (in sccm) of between 15 to 1500 sccm. Ranges of
between about 15 to 200, preferably 15 to 150, 15 to 100, 15 to 75,
15 to 60, 15 to 50, 15 to 40, 15 to 30, 20 to 150, 20 to 100, 20 to
75, 20 to 60, 20 to 50, 20 to 40, 20 to 35, 20 to 30, 25 to 150, 25
to 100, 25 to 75, 25 to 60, 25 to 50, 25 to 40, 25 to 30, including
values of about 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75,
80, 85, 90, 95, 100, 120, 140, 160, 180 and 200 sccm have been
found to be particularly useful in lowering carbon incorporation
for relatively small growth chamber sizes, for example, of
dimensions in the order of 7.times.2''. However, in moving to
commercial scales higher powers and multiple plasma sites are
envisaged as being useful. For growth chambers of larger dimensions
of, for example, 56.times.2'' then ammonia injection flows of
between about 200 to about 1500 sccm, inclusive of 200 to 1300, 200
to 1100, 200 to 1000, 200 to 900, 200 to 800, 200 to 700, 200 to
600, 200 to 500, 200 to 400, 300 to 1500, 300 to 1300, 300 to 1100,
300 to 1000, 300 to 900, 300 to 0.800, 300 to 700, 300 to 600, 300
to 500, 300 to 400, 400 to 1500, 400 to 1300, 400 to 1100, 400 to
1000, 400 to 900, 400 to 800, 400 to 700, 400 to 600, 400 to 500,
500 to 1500, 500 to 1300, 500 to 1100, 500 to 1000, 500 to 900, 500
to 800, 500 to 700, 500 to 600, 600 to 1500, 600 to 1300, 600 to
1100, 600 to 1000, 600 to 900, 600 to 800, 600 to 700, 700 to 1500,
700 to 1300, 700 to 1100, 700 to 1000, 700 to 900, 700 to 800, 800
to 1500, 800 to 1300, 800 to 1100, 800 to 1000, 800 to 900, 900 to
1500, 900 to 1300, 900 to 1100, 900 to 1000, 1000 to 1500, 1000 to
1300, 1000 to 1100, are appropriate.
[0139] It will be appreciated that higher power output from the
plasma generator can be tolerated when a baffle or like device is
employed whereas lower values will be preferred when there is an
unimpeded flow path between the plasma inlet and the substrates to
minimise etching.
[0140] FIGS. 8 and 9 show partial perspective sectional views of
alternative embodiments of the apparatus for forming a film to that
shown in FIG. 7. Like parts have been given like numbers between
FIGS. 7 to 9 and it will be appreciated that the most important
difference is in the placement and/or design of the additional
reagent inlet 625, which is preferably an ammonia gas inlet. FIG. 8
also demonstrates an embodiment of the apparatus 600 wherein the
waste outlet 640 is actually provided within the hollow centre of
the axis about which the substrate holder 635 rotates.
[0141] FIG. 8 shows the ammonia reagent inlet 625 placed behind or
collinear with the metal organic reagent inlet 620. This is a
preferred embodiment as the location of the ammonia reagent inlet
625 is such that the introduced ammonia will be directed more or
less to the centre of the substrates as they rotate on the
substrate holder 635. This ensures good delivery of the ammonia to
the surface of the substrate whereas in the embodiment shown in
FIG. 7 the side by side placement of the ammonia reagent inlet 625
and the metal organic reagent inlet 620 means that the ammonia
reagent inlet 625 is somewhat offset from being directly vertically
above the centre of the rotating substrates.
[0142] FIG. 9 represents a slightly different operational set up in
that the ammonia reagent inlet 625 enters the growth chamber 605
from the side and so is at more or less a right angle to the
substantially vertical metal organic reagent inlet 620. The metal
organic reagent inlet 620 has been cut away in FIG. 9 to better
show the design of the ammonia reagent inlet 625. On approaching
the metal organic reagent inlet 620 the ammonia reagent inlet 625
then has a bend such that its terminal portion finishes vertically
and in a similar position to that shown in FIG. 8. The horizontal
placement of the ammonia reagent inlet 625 may have operational
advantages in use.
[0143] Although not shown in FIGS. 7 to 9 in one embodiment it may
be preferred that the plasma inlet 610, the metal organic reagent
inlet 620 and the ammonia reagent inlet 625 all finish flush with
the ceiling of the growth chamber 605. To maintain the inlets close
to the substrates the ceiling will therefore be located at a lower
level than in a typical RPCVD apparatus. For example, in one
embodiment the ceiling may be located approximately less than 30
cm, preferably less than 25 cm, more preferably less than 20 cm,
still more preferably less than about 10 cm vertically above the
location of the substrates. Values of about 5 cm and 7.5 cm may be
useful with 3 cm to 4 cm as the lower end values.
[0144] As discussed earlier, the present inventors postulate that
it is important for the minimisation of carbon and oxygen
impurities in the final film to minimise reactions which occur in
the gas phase above the substrates. Instead, it is preferable to
encourage the key film forming reactions to occur on or as close to
the actual substrate surface as possible. Directing the reactions
to occur on the substrate surface may improve the scavenging of the
oxygen and carbon impurities.
[0145] Thus the lowering of the growth chamber 605 ceiling, with
reagent inlets formed therein and having their openings flush with
said ceiling, results in delivery of the reagents more quickly and
effectively to the substrate surface.
[0146] Minimising deadspots and, particularly, optimising the flow
of the plasma and reagents in relation to the substrates are
considered in FIGS. 10 to 14 which are further embodiments on the
inventive apparatus and method already discussed.
[0147] FIG. 10 shows a schematic representation of an RPCVD
apparatus 1000 for depositing a film on a substrate, according to
one embodiment of the invention. The apparatus 1000 comprises a
growth chamber 1005 inside which film growth will occur. An exhaust
1010 is provided at a lower extent of the growth chamber 1005 for
the removal of excess reagents and waste products.
[0148] A plasma generator 1015 is located externally to the growth
chamber 1005 which may be a high frequency generator acting upon
nitrogen received from a nitrogen source (not shown). The nitrogen
plasma thereby generated enters the growth chamber 1005 at plasma
inlet 1020 which ends flush with the ceiling of the growth chamber
1005 i.e. the plasma inlet 1020 does not, to any significant
extent, extend into the interior of the growth chamber 1005. The
plasma inlet 1020 may, if required, open into a baffle, shroud,
impeller or the like to modify the flow path and energy of the
plasma.
[0149] This is not an essential component and the need for such a
device will depend upon the power of the radiofrequency generator.
A suitable baffle may be as described in the applicant's prior PCT
publication WO 2010/091470 which is hereby incorporated by way of
reference in its entirety.
[0150] A metal organic reagent source 1025 supplies the metal
organic reagent which, in a preferred embodiment, is
trimethylgallium (TMG) or triethylgallium (TEG). The TMG or TEG
enters the growth chamber 1005 via metal organic reagent inlet 1030
which, in the embodiment shown, is located in a side wall of the
growth chamber 1005 and ends flush therewith i.e. the metal organic
reagent inlet 1030 does not, to any significant extent, extend into
the interior of the growth chamber 1005.
[0151] A hydrogen-containing, additional reagent source 1035
supplies the additional reagent which, in a preferred embodiment,
is ammonia. The ammonia enters the growth chamber 1005 via
additional reagent inlet 1040 which, in the embodiment shown, is
located in a side wall of the growth chamber 1005, beneath the
location of the metal organic reagent inlet 1030, and ends flush
therewith i.e. the additional reagent inlet 1040 does not, to any
significant extent, extend into the interior of the growth chamber
1005.
[0152] Under some conditions it may be preferable that the
additional reagent inlet 1040 enters the growth chamber 1005
through a side wall thereof at a height suitable to enable a flow
of additional reagent entering therethrough to have a flow path
passing over and substantially adjacent to a growth surface of the
substrates 1050. It is also beneficial that the positioning of the
exhaust 1010 is at an opposite end of the growth chamber 1005 to
the additional reagent inlet 1040 which further encourages a flow
path of the additional reagent which passes over the surface of the
substrates 1050. Thus, the region of injection of the additional
reagent and the provision of the exhaust 1010 generally opposite
that create an environment whereby the reagent is in constant
contact with the growth surface of the substrates 1050 and growing
film.
[0153] The provision of the three plasma/reagent inlets all with
ends flush with either the ceiling (the plasma inlet 1020) or the
side walls (the metal organic reagent inlet 1030 and the additional
reagent inlet 1040) or a combination thereof avoids the presence of
`dead spots` within the growth chamber 1005. It is preferred that
the Group IIIA reagent inlet is flush with the chamber ceiling. It
is further preferred that the additional reagent inlet injects the
ammonia, or other gas, through entry points, for example view
ports, which physically surround the additional reagent inlet to
thereby have these two reagents introduced into the growth chamber
together to encourage mixing.
[0154] The presence of inlets which extend into the growth chamber
1005 would result in adjacent regions therein where reagents can
collect and be moved around in vortex like movements, due to the
spinning of substrate holder 1045. Dead spots are considered to be
unwanted volumes within the growth deposition chamber where there
could be depletion or recirculation of gas which does not help with
the growth of the film. This would encourage reaction pathways
other than the desirable adduct formation outlined above and would
result in TMG or TEG degradation with methyl radical
production.
[0155] The substrate holder 1045 may be adapted to support a single
substrate 1050 but it is preferred that it is of a design adapted
to support multiple substrates 1050. Suitably, the substrate holder
1045 is rotatable.
[0156] The design of the apparatus 1000 shown in FIG. 10 is one
preferred embodiment in that the plasma inlet 1020 and metal
organic reagent inlet 1030 are substantially separated to thereby
avoid any potential degradation of the TMG/TEG or other metal
organic by the high energy plasma stream.
[0157] As discussed previously, the ceiling height of the growth
chamber 1005 is preferably lowered with respect to a standard RPCVD
set up. Suitable heights have been set out previously. This helps
minimise undesirable non-adduct forming reactions by minimising the
space in which they can occur due to the placing of the plasma and
reagent inlets close to the substrates 1050.
[0158] Although not shown in FIG. 10, the apparatus 1000 may
further comprise one or more heating devices to heat the additional
reagent inlet 1040 and/or the metal organic reagent inlet 1030
prior to the reagents entering the growth chamber 1005. The heating
devices may take the form of external heaters surrounding the
transport members running between the relevant source and its
inlet. Simple heating coils or heating tape placed around the
piping may suffice. The heating of, particularly, the ammonia
introduced above the substrate 1050 surface means that it is
introduced in an activated state, in anticipation of adduct
formation, in the key reaction zone above the substrates 1050.
[0159] Although not shown for the sake of simplicity, the growth
chamber 1005 may comprise one or more structures associated with
the additional reagent inlet 1040 and/or the metal organic reagent
inlet 1030 to promote mixing of said reagents immediately prior to
their contacting the one or more substrates 1050. Particularly, it
may be desirable to generate some turbulence in the flow path of
the introduced additional reagent, preferably being ammonia. Since
this reagent is introduced to generate a flow path just above the
substrates 1050 this ensures rapid and efficient mixing with the
TMG or TEG to promote adduct formation prior to contact with the
growth surface of the growing film.
[0160] The structures themselves may take the form of a baffle-like
structure, vanes or any shape which promotes flow turbulence. They
may be in direct contact with the corresponding reagent inlets or
may be operatively associated with them such that the reagent must
flow through the structure before passing close to the substrates
1050.
[0161] FIG. 11 shows an alternative schematic representation of an
RPCVD apparatus 200 for depositing a film on a substrate to that
shown in FIG. 10. Similar numbering to the apparatus 1000 of FIG.
10 has been maintained for like parts and it will be apparent that
all of the growth chamber 2005, exhaust 2010, plasma generator
2015, plasma inlet 2020, metal organic reagent source 2025, metal
organic reagent inlet 2030, additional reagent (ammonia) source
2035 and additional reagent inlet 2040 are present to supply
necessary reagents to the substrates 2050 supported on substrate
holder 2045 which rotates around central pivot 2055.
[0162] The key difference between the embodiments of FIGS. 10 and
11 is that in FIG. 11 the plasma inlet 2020, and associated plasma
generator 2015, has been shifted to be closer to the side wall in
which the metal organic reagent inlet 2030 and additional reagent
inlet 2040 are provided. Although less preferred than the
embodiment in FIG. 10 due to the close proximity of TEG or TMG
reagent and plasma, this design of apparatus 2000 may still provide
a significant improvement in the purity of the growing films over
those grown in a typical RPCVD apparatus. There may be advantages
in the directing of the plasma directly onto the region of the
growth chamber 205 in which the TEG or TMG and ammonia are
encouraged to mix. All other components and reagents in the
apparatus 2000 may be as described for apparatus 1000 in FIG.
10.
[0163] FIG. 12 shows a further alternative schematic representation
of an RPCVD apparatus 3000 for depositing a film on a substrate to
that shown in FIG. 10. Once again the components of the apparatus
3000 are substantially the same as those discussed in relation to
FIGS. 10 and 11 and so will not be repeated here. The key
differences between the embodiments of FIGS. 12 and 10 are that,
firstly, the additional reagent source 3035 and associated
additional reagent inlet 3040 are located on the ceiling of the
growth chamber 3005, rather than a side wall as in FIG. 10, and
secondly, to ensure the introduction of additional reagent e.g.
ammonia, into the growth chamber 3005 only at a point close to the
substrates, the additional reagent inlet 3040 is provided with an
extended portion 3060. Although not shown in the figures a
temperature regulating means may be provided generally adjacent
extended portion 3060 to control the reagent temperature before
contact with the growth surface.
[0164] The provision of the extended portion 3060 does result in
the potential generation of one or two "dead spots" as discussed
before but this does not prevent the formation of an improved film
product compared with many prior art RPCVD approaches. The design
of the apparatus 3000 still ensures that the ammonia, or other
additional reagent, is only provided in close proximity to the
growing film such that adduct formation and the production of
methane, as opposed to methyl radicals, is promoted immediately
adjacent said film.
[0165] FIG. 13 shows yet a further alternative schematic
representation of an RPCVD apparatus 4000 for depositing a film on
a substrate to that shown in FIG. 10. Once again like numbering is
employed for like components to those in FIG. 10. In this
embodiment the plasma generator 4015 and associated plasma inlet
4020 are found in a side, wall opposite that in which the
additional reagent inlet 4040 is located. Further, instead of the
representation of a metal organic reagent inlet as a single inlet
it takes the form of an injector framework. The framework may be
operative over the entire area of the growth chamber 4005 occupied
by the substrates 4050 but, preferably, the injector framework will
have open ports 4065 and closed ports 4070. The closed ports 4070
may be those adjacent the plasma inlet 4020 to protect the TEG or
TMG or other metal organic reagent from exposure to high energy
plasma as it exits the plasma inlet 4020.
[0166] FIG. 14 shows a partial perspective sectional view of an
RPCVD apparatus 5000 for forming a film, according to one
embodiment of the present invention. For the sake of clarity not
all components, such as the plasma generator and reagent sources,
have been shown but rather only those components required to convey
the key aspects of the apparatus.
[0167] The growth chamber 5005 is defined, in part, by the ceiling
5010 which, in relative terms, is not very distant from the
substrates 5035 to minimise the chamber mixing space. A plasma
inlet 5015 opens into the growth chamber 5005 through the ceiling
5010 to deliver a plasma, such as a nitrogen plasma. A metal
organic reagent inlet takes the form of an injector framework 5020
(details of the port openings not shown) while a additional reagent
inlet 5025 opens into the growth chamber 5005 through a side wall
thereof at a point underneath the metal organic injector framework
5020 and a height such that a flow path of a additional reagent,
preferably ammonia, is created just above the growth surface of the
substrates 5035 which are rotating with the movement of the
substrate holder 5030.
[0168] The design in FIG. 14 represents a further variation on the
themes discussed in relation to the previous figures and achieves
its advantages in much the same way. The embodiment shown in FIG.
14 does have the advantages of the plasma inlet 5015 and TEG or TMG
injector framework 5020 being separated and, other than the
relatively small size of the injector framework 5020 itself,
minimal structural components being located within the growth
chamber 5005 above the substrates 5035. By these means degradation
of the TEG or TMG and the creation of "dead spots" which might
promote the methyl radical production pathway are minimised.
[0169] It will be appreciated that, in one embodiment, the
additional reagent may only need to supply hydrogen in a reactive
form if the plasma is a nitrogen plasma and so can be used as the
nitrogen source for adduct formation. This would result in the use
of a hydrogen plasma generator and inlet along with a nitrogen
plasma generator and inlet. Due to the possibility of arcing
between these components it would be preferable to physically
separate them as much as possible and so one may be located in the
ceiling at one end of the growth chamber 5005 and the other in a
side wall at an opposite end of the growth chamber 5005.
[0170] Although the discussion herein has been of rotation of the
substrate holder it will be appreciated that it may be possible for
the substrate holder and substrates to remain stationary while the
plasma inlet and reagent inlets spin within the growth chamber.
This will require a design whereby a rotatable connection mates
with each of the plasma inlet, the metal organic reagent inlet and
the additional reagent inlet which will both be operated in pulses
timed to coincide with the rotation speed to ensure that each is
only dispersed where required. Such a design would present greater
challenges in operation over those disclosed in the figures which
have the substrate holder rotate while the plasma inlet and reagent
inlets remain stationary and, hence, is a less preferred
approach.
[0171] The apparatus may also be adapted to allow additional
individual rotations of each substrate relative to the substrate
holder for further improvement of thin film growth uniformity.
[0172] Thus, from the various embodiments described above, it will
be appreciated that the components of the inventive apparatus
described herein may be arranged in a number of different ways
while still achieving a reduced carbon and/or oxygen level in the
growing films compared with that achieved by standard RPCVD
approaches. However, all of the embodiments described share at
least the feature of the additional reagent being introduced to the
growth chamber in close if not immediate proximity to the Group
IIIA reagent introduction point and, preferably, the substrates to
promote formation of the adduct directly above the growing film
surface. This approach has been found to greatly reduce the level
of carbon and/or oxygen incorporation into the film. Further common
features which assist in further reducing the level of carbon
and/or oxygen incorporation include a low chamber ceiling height
and therefore a correspondingly reduced chamber volume along with
provision of reagent inlets which end flush with the ceiling and/or
side walls to minimise dead spots and positioning of the exhaust to
encourage a flow path of additional reagent over the substrate
surface.
[0173] In one highly preferred embodiment of the present invention
any one or more and most preferably all of the Group VA plasma
inlet, the Group IIIA reagent inlet and the additional reagent
inlet all end flush with the ceiling of the growth chamber, as
previously discussed. However, if the Group IIIA reagent inlet and
the additional reagent inlet are to extend into the chamber then,
in one embodiment, it is useful if they extend into the chamber to
be between about 1 to about 10 cm vertically above the substrates
inclusive of between 2 to 9 cm, 3 to 6, cm and 4 to 5 cm.
[0174] The process of film formation as described in relation to
any of the aspects herein may also include a doping step, which may
be necessary for films to be employed in devices such as LEDs and
solar cells. Preferably, the doping step is a p-type doping step.
For p-type doping, the dopants could be Mg or Zn or other suitable
elements. Suitable reagents that contain these elements, such as
diethyl zinc (DEZn), bis(cyclopentadienyl)magnesium (Cp2Mg) can be
selected from those known in the art for p-type doping. p-type
doping is known in the art to be particularly challenging but it
has been found that the present set of process conditions and
apparatus features used to reduce carbon impurities in the growing
film also allow for better p-type doping. Values obtained for
p-type doping (Hall measurements) are: resistivity of 0.9 Ohm-cm,
mobility of 2.7 cm 2/Vs for a carrier concentration of 1.4E18 cm
-3. For n-type doping, the dopants could be Si or oxygen or other
suitable elements. Suitable reagents that contain these elements,
such as silane, disilane, di-tert-butylsilane, oxygen can also be
used for n-type doping.
[0175] It will be appreciated from the foregoing discussion that a
number of other factors can be controlled to further contribute to
the extent of the reduction of impurities achieved in the film
product.
[0176] For example, the method may further include the step of
controlling the temperature to be between about 400 to about
1200.degree. C., preferably between about 500 to about 1000.degree.
C. (inclusive of a temperature of about 500.degree. C., 600.degree.
C., 700.degree. C., 800.degree. C., 900.degree. C. or 1000.degree.
C.), more preferably between about 500 to about 850.degree. C. This
is a relatively low temperature range in comparison to typical
MOCVD and even many RPCVD approaches. The lower temperatures favour
adduct formation over TMG thermal degradation and so reduce methyl
radical reactions at the film surface.
[0177] The method may further include the step of promoting the
mixing of the metal organic reagent and the additional reagent
adjacent the one or more substrates using a flow perturbation
device. Once again, the mixing step is to promote immediate
formation of the adduct in the vicinity of the film/substrate
surface.
[0178] It has been found that the power of the plasma generator has
an effect on carbon incorporation into the thin film and so the
method may also include the step of controlling the power of the
plasma generator to be between about 400 W to about 5000 W from a
single source. Preferably, the power of the plasma generator is
between about 500 to about 3000 W, 500 to 2750 W, 500 to 2500 W,
500 to 1000 W, 500 to 900 W, 500 to 800 W, 600 to 1000 W, 600 to
900 W, 600 to 800 W, 700 to 1000 W, 700 to 900 W and preferably
about 800 W is preferred. A value of about 800 W has been found to
be particularly useful in lowering carbon incorporation, however,
in moving to commercial scales higher powers and multiple plasma
sites are envisaged as being useful such as about 2500 W.
[0179] Although not wishing to be bound by any particular theory,
it is postulated that the surprising results achieved may be as a
result of one or more of the following processes. Firstly, it is
postulated that the injection of ammonia provides additional
available nitrogen to the system and this acts as both a reagent in
film formation and also as a scavenger for oxygen and/or carbon.
Secondly, it is theorized that the improvement in lowering carbon
incorporation into the film when the power output of the plasma
generator is increased could be due to the carbon atoms being
actively removed in favour of nitrogen. Thirdly, and as alluded to
previously, a proposed mechanism for removal of carbon from the
system is that the ammonia assists in the formation of an adduct
with the trimethylgallium, i.e. initially
(CH.sub.3).sub.3Ga:NH.sub.3, which subsequently releases a methane
molecule. The methane is not incorporated into the film as readily
as a CH.sub.3 radical may be. Subsequent decomposition of the
adduct releases further methane until all of the carbon of the
trimethylgallium has been removed as methane and only GaN is left.
It is believed formation of this adduct and subsequent
decompositions are occurring at the surface of the substrate.
Finally, it is possible that the improved lowering of carbon and
oxygen incorporation upon removing the baffle from the system is a
consequence of the energy from the UV light emitted from the plasma
generator/plasma chamber contacting the growing film ejecting
carbon and/or oxygen in favour of nitrogen. In reality, it is
possible that all of these mechanisms may play at least some role
in providing the results achieved.
[0180] In a third aspect the invention resides in a method of
forming a thin film having a carbon content of less than about
5E+17 atom/cm.sup.3, on a substrate by RPCVD including the steps
of: [0181] (a) introducing a Group VA plasma through a Group VA
plasma inlet into a first deposition zone of a growth chamber
wherein a direct flow path is provided between the Group VA plasma
inlet and a substrate located in the first deposition zone; [0182]
(b) introducing a Group IIIA reagent through a Group IIIA reagent
inlet into a second deposition zone of the growth chamber, the
second deposition zone being substantially isolated from the first
deposition zone; [0183] (c) introducing an additional reagent
selected from the group consisting of ammonia, hydrazine, di-methyl
hydrazine and hydrogen plasma through an additional reagent inlet
into the second deposition zone such that the additional reagent
and the Group IIIA reagent mix prior to deposition; [0184] (d)
moving the substrate between the first and second deposition
zones,
[0185] to thereby form a thin film on the substrate having a carbon
content of less than about 5E+17 atom/cm.sup.3.
[0186] Preferably, the carbon impurity content is less than about
3E+17 atom/cm.sup.3, even more preferably less than about 2E+17
atom/cm.sup.3, yet more preferably less than or about 1E+17
atom/cm.sup.3. A lower limit may be considered to be about the SIMS
detection limit for carbon impurities in such films.
[0187] In one embodiment, the thin film also has an oxygen impurity
content of less than about 8E+17 atom/cm.sup.3, even more
preferably less than about 6E+17 atom/cm.sup.3, yet more preferably
less than about 4E+17 atom/cm.sup.3, still more preferably less
than about 2E+17 atom/cm.sup.3, or even less than or about 1E+17
atom/cm.sup.3. A lower limit may be considered to be about the SIMS
detection limit for oxygen impurities in such films.
[0188] The statements made above in relation to the second aspect
apply equally well to the third aspect.
[0189] In a fourth aspect the invention resides in a film made by
the method of the second or third aspects. Such films will have
demonstrably lower levels of oxygen and/or carbon incorporated into
their structure in comparison to similar films made by standard
RPCVD approaches. In one embodiment, films produced by the method
of the present invention may have a carbon content of less than
about 10E+16 atoms/cm.sup.3. Values of 3E+16 atoms/cm.sup.3 have
been attained and it is believed that values of less than 1E+16
atoms/cm.sup.3 are attainable with process optimisation.
[0190] In a fifth aspect the invention resides in use of a film of
the fourth aspect in a semiconductor device.
[0191] The examples set out in further detail the process runs
using the apparatus of the invention and the results thereby
obtained. In the examples nitrogen was used as the Group VA plasma
and trimethylgallium as the Group IIIA reagent.
Examples
Process Runs with Baffle
[0192] An apparatus essentially as set out in FIGS. 6 and 7 was
used employing a stainless steel style shower head baffle located
below the plasma inlet. The power of the plasma generator was
between 500 W to 600 W and a growth temperature of 700.degree. C.
was employed. The films were grown onto a GaN template. An initial
control run was carried out using a nitrogen plasma and
trimethylgallium (TMG) as the organometallic reagent but without
the injection of any ammonia. This produced a film as would be
expected when made via the apparatus of FIG. 2 i.e. with standard
levels of oxygen and carbon impurities.
[0193] A second run was then carried out under essentially similar
conditions but with an injection of a 15 sccm ammonia flow into the
second deposition zone (the organometallic reagent deposition
zone). The ammonia was injected at the same time as injection of
the TMG so that the two mixed together prior to deposition. This
produced a film with a substantial reduction in the levels of both
oxygen and carbon. Specifically, compared with the first run
without the injection of ammonia, the level of carbon decreased
from about 6E+20 atom per cubic centimetre (atom/cc) to about 3E+20
atom/cc while the level of oxygen decreased from about 3E+20
atom/cc to about 1E+17 atom/cc.
[0194] The figure of 1E+17 atom/cc for the oxygen level represents
an extremely surprising result in that it ceases, for practical
purposes, to be a problematic impurity at that level and the result
is comparable to that observed using MOCVD. Although the additional
hydrogen provided by the ammonia may be expected to provide some
benefit in reducing carbon and oxygen impurities it could not have
been predicted, based upon accepted wisdom in the semiconductor
field, that such a small injection of ammonia could result in such
a large reduction in oxygen and carbon impurities.
[0195] This experiment was repeated using different flow rates of
ammonia. The results of these process runs, in terms of carbon
incorporation into the film, are shown in FIG. 15 wherein the
diamond icons (labelled `Short Jar A RPCVD`) indicate the levels of
carbon in the film.
Process Runs without Baffle
[0196] A number of runs were then carried out using the same
apparatus and conditions as described above but with the shower
head baffle removed. Thus, a direct flow path between the plasma
generator, plasma inlet and substrates was established in the first
deposition zone.
[0197] The distance between the plasma inlet and the substrates was
less than 20 cm and no plasma etching was observed. It is
postulated this may be due to the relatively low (500-600 W) power
output from the plasma generator employed while still providing
enough energy to activate the nitrogen.
[0198] The results of this run are indicated on FIG. 15 as the
smaller square icons (labelled as `Short Jar B (no plasma
showerhead) RPCVD`). It was observed that, under the same
conditions as process runs using the shower head baffle and using
the same amounts of ammonia, the levels of carbon incorporated into
the film was greatly reduced.
[0199] A further process run was carried out under identical
conditions (30 sccm ammonia) but with the power of the plasma
generator increased to 800 W. Once again, surprisingly, the
resulting film was not etched to a significant degree. More
surprising, however, was that the level of carbon incorporated into
the film was about 1.7E+17 atom/cc. This result is indicated on
FIG. 15 as a single point larger square icon. The results for the
levels of carbon found in films produced by all of the process runs
described above, and as indicated graphically in FIG. 15, are shown
in table 2.
TABLE-US-00002 TABLE 2 Levels of carbon incorporated into films
grown under a variety of conditions NH3 RPCVD B flow Standard
higher RF Standard (sccm) RPCVD RPCVD A RPCVD B power (800 W) MOCVD
0 4.10E+19 6.00E+20 1.05E+19 2.83E+16 15 2.00E+20 2.83E+16 30
1.40E+20 1.67E+18 1.70E+17 2.83E+16 50 8.50E+19 2.83E+16 70
7.00E+19 2.83E+16 100 9.40E+17 2.83E+16 150 2.44E+18 2.83E+16
[0200] It will be appreciated that there will be an upper limit to
the plasma generator power which can be employed without etching
occurring. If this point is reached and further increases in plasma
generator power are desirable then a baffle may once again be
placed between the plasma inlet and the substrates.
[0201] Further runs designed to optimise the process have resulted
in levels of 3E+16 atom/cc of carbon and 3E+16 atom/cc of oxygen in
the grown GaN films. It may assist the appreciation of how low
these levels are by considering that the SIMS detection limits for
carbon are between about 1-2E+16 and between about 1-3E+16 for
oxygen. At the above quoted optimal results the present films are
approaching the detection limits for carbon and oxygen. SIMS is one
of the most sensitive surface analysis techniques available, being
able to detect elements present in the parts per billion range, and
is the accepted standard for analysis in this field. The low levels
of carbon and oxygen impurities achieved by use of the present
apparatus and method are comparable with those observed in GaN
templates and have never been seen previously using RPCVD.
[0202] The data in table 2 also indicates that ammonia actively
takes part in GaN formation and carbon removal rather than just
scavenging carbon and/or oxygen.
[0203] FIG. 16 is a SIMS graphical analysis of the typical
impurities found in a film produced by a method and apparatus of
the invention as described and grown on a GaN template. Films
produced by RPCVD would usually not be comparable with the purity
levels achieved in a template, however, in the present instance
when formed under the optimal conditions described above it is seen
that the films are essentially of equal quality.
[0204] The first 0-0.5 .mu.m of the depth profile (indicated on the
x axis) represents a film produced by the present apparatus and
method while the 0.5-2.7 .mu.m component represents the underlying
GaN template produced by an MOCVD process. It is clear that the
levels of the various impurities, particularly carbon and oxygen,
are similar. The spikes observed in the traces are representative
of the interfaces between layers or changes in growth conditions
and not an increase in impurity levels.
Triethylgallium Experiments
[0205] A further series of experiments were performed using
triethylgallium (TEG) as the Group IIIA reagent and investigating
different ammonia injection rates. The process conditions used for
these experiments are set out below in table 3. Also noteworthy is
that the plasma inlet, TEG inlet and ammonia inlets all ended flush
with the growth chamber ceiling to reduce recirculation of gases
i.e. deadspot effects. The ceiling was fixed at a height
approximately 5.0-7.5 cm above the substrates and the speed of
rotation of the substrate holder was 1200 rpm.
TABLE-US-00003 TABLE 3 Process conditions for growth runs using TEG
N2 RF Run Time MO PL H2 NH3 Growth Press power # (min) TEG inj. (2
+ 3) Shrd 0.25 L Temp (Torr) (W) 1380 120 120 1600 2500 800 30 720
3.5 2500 uGaN 1386 120 120 1600 2500 800 0 720 3.5 2500 uGaN 1388
120 120 1600 2500 800 100 720 3.5 2500 uGaN N2 PL (2 + 3) is the
nitrogen flow to the plasma inlet. H2 Shrd is the hydrogen flow to
the chamber through an outer shroud. MO inj. is the metal organic
injector flow of hydrogen which carries the metal organic reagent.
NH3 0.25 L is the ammonia flow into the growth chamber in sccm.
[0206] The results of the experiments set out in table 3, in terms
of the carbon and oxygen impurity levels in the grown films, are
shown graphically in the SIMS data in FIGS. 17, for carbon, and 18,
for oxygen. The grown films were approximately 1 um thick and were
grown on top of GaN MOCVD templates. The thickness of the as grown
films means that the SIMS data only needs to be viewed from the
left side of the x-axis on the data plots up until the 1 um depth
point. The spike at this region is due to the interface between the
films and the template.
[0207] It can be seen from FIG. 17, firstly, that the injection of
increasing amounts of ammonia causes a very significant decrease in
the levels of carbon as an impurity in the grown films when
compared with the baseline levels with no ammonia injected (run no.
1386). The injection of 30 sccm of ammonia brings the level of
carbon impurities down to under 10.sup.17 atoms/cm.sup.3 while,
with an injection of 100 sccm of ammonia the level of carbon
impurities is seen to actually be reduced to more or less
correspond to those in the MOCVD generated GaN templates, a result
hitherto unseen when employing RPCVD growth of GaN films.
[0208] FIG. 18 shows that the oxygen impurity levels in the grown
films were on a par with those observed in the MOCVD grown. GaN
template under all conditions.
[0209] It will be appreciated from all of the foregoing that the
use of separate deposition zones in an RPCVD arrangement can be
useful in reducing impurity incorporation into films, such as GaN
films, however these impurities will still be found in the produced
films in noticeable quantities. The use of a growth chamber
arrangement whereby the plasma inlet and/or organometallic reagent
inlet are between about 1 to about 30 cm vertically above a growth
surface of the substrates. However, the introduction of relatively
small quantities of ammonia into the second deposition zone,
simultaneously with the organometallic reagent, has been shown to
dramatically reduce the level of oxygen and, particularly, carbon
in the film. Under similar conditions but with the removal of any
impediment to direct flow between the plasma generator and/or
plasma inlet and the substrates results in an extremely surprising
reduction in the incorporation of, particularly, carbon into the
film. As a further level of control it has been shown that
increasing the power of the plasma generator output can further
lower the level of carbon impurities found in the film.
[0210] Throughout the specification the aim has been to describe
the preferred embodiments of the invention without limiting the
invention to any one embodiment or specific collection of features.
It will therefore be appreciated by those of skill in the art that,
in light of the instant disclosure, various modifications and
changes can be made in the particular embodiments exemplified
without departing from the scope of the present invention.
* * * * *