U.S. patent application number 13/957380 was filed with the patent office on 2013-11-28 for gan epitaxy with migration enhancement and surface energy modification.
This patent application is currently assigned to Intermolecular, Inc.. The applicant listed for this patent is Intermolecular, Inc.. Invention is credited to Boris Borisov, Thai Cheng Chua, Philip A. Kraus, Sandeep Nijhawan, Yoga Saripalli.
Application Number | 20130313566 13/957380 |
Document ID | / |
Family ID | 48695134 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130313566 |
Kind Code |
A1 |
Kraus; Philip A. ; et
al. |
November 28, 2013 |
GaN Epitaxy With Migration Enhancement and Surface Energy
Modification
Abstract
Methods and apparatus for depositing thin films incorporating
the use of a surfactant are described. Methods and apparatuses
include a deposition process and system comprising multiple
isolated processing regions which enables rapid repetition of
sub-monolayer deposition of thin films. The use of surfactants
allows the deposition of high quality epitaxial films at lower
temperatures having low values of surface roughness. The deposition
of Group III-V thin films such as GaN is used as an example.
Inventors: |
Kraus; Philip A.; (San Jose,
CA) ; Borisov; Boris; (Edina, MN) ; Chua; Thai
Cheng; (Cupertino, CA) ; Nijhawan; Sandeep;
(Los Altos, CA) ; Saripalli; Yoga; (Sunnyvale,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Intermolecular, Inc. |
San Jose |
CA |
US |
|
|
Assignee: |
Intermolecular, Inc.
San Jose
CA
|
Family ID: |
48695134 |
Appl. No.: |
13/957380 |
Filed: |
August 1, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13339695 |
Dec 29, 2011 |
8524581 |
|
|
13957380 |
|
|
|
|
Current U.S.
Class: |
257/76 ; 257/615;
438/478 |
Current CPC
Class: |
C23C 16/303 20130101;
B01J 2219/0059 20130101; H01L 21/0254 20130101; H01L 21/0262
20130101; C23C 16/45551 20130101; H01L 21/02436 20130101; C30B
25/14 20130101; C30B 29/403 20130101; B01J 2219/00756 20130101;
C23C 16/45534 20130101 |
Class at
Publication: |
257/76 ; 257/615;
438/478 |
International
Class: |
H01L 21/02 20060101
H01L021/02 |
Claims
1. A Group III-V film comprising one or more Group III elements and
a Group V element, wherein the film further comprises a surfactant
layer with thickness between one and five monolayers.
2. The Group III-V film of claim 1 wherein the thickness of the
surfactant layer is between one and two monolayers.
3. The Group III-V film of claim 1 wherein the surfactant comprises
hydrogen, chlorine, bromine, iodine, arsenic, phosphorus, bismuth,
indium, antimony, lead, tin, germanium, selenium, tellurium,
magnesium, cadmium, zinc, mercury, or combinations thereof.
4. The Group III-V film of claim 1 wherein the surfactant comprises
indium, antimony, or combinations thereof.
5. The Group III-V film of claim 1 wherein the Group III element
comprises Al, Ga, In, or combinations thereof.
6. The Group III-V film of claim 1 wherein the Group V element
comprises nitrogen.
7. A semiconductor device comprising the Group III-V film of claim
1.
8. A Group III-V thin film prepared by a method comprising: a)
providing a deposition chamber comprising a plurality of processing
regions wherein the processing regions are not in fluid contact
with one another; b) exposing a substrate to one or more Group III
precursors and one or more surfactant precursors in a first
processing region, wherein the exposure to one or more surfactant
precursors forms a surfactant layer with thickness between one and
five monolayers; c) exposing the substrate to one or more Group V
precursors in a second processing region; d) exposing the substrate
to one or more Group III precursors in a third processing region;
e) exposing the substrate to one or more Group V precursors in the
second processing region; and repeating any sequence of steps b)
through e) until a desired thickness of the film is reached.
9. The Group III-V film of claim 8, wherein the thickness of the
surfactant layer is more than one monolayer and less than two
monolayers.
10. The Group III-V film of claim 8, wherein less than one
monolayer of the Group III metal is deposited when the substrate is
exposed to the one or more Group III precursors in the first or
third processing region.
11. The Group III-V film of claim 8, wherein the surfactant
precursor comprises hydrogen, chlorine, bromine, iodine, arsenic,
phosphorus, bismuth, indium, antimony, lead, tin, germanium,
selenium, tellurium, magnesium, cadmium, zinc, mercury, or
combinations thereof.
12. The Group III-V film of claim 8, wherein the surfactant
precursor comprises indium, antimony, or combinations thereof.
13. The Group III-V film of claim 8, wherein the Group III
precursor gases include one or more of Al-containing gases,
Ga-containing gases, In-containing gases, or combinations
thereof.
14. The Group III-V film of claim 8, wherein the Group V precursor
comprises a nitrogen-containing species.
15. The Group III-V film of claim 14, wherein the
nitrogen-containing species comprises active neutral species of
nitrogen.
16. The Group III-V film of claim 15, wherein the active neutral
species of nitrogen comprises nitrogen species having the lowest
excited state of molecular nitrogen
(A.sup.3.SIGMA..sub.u.sup.+).
17. The Group III-V film of claim 8, wherein the substrate is
exposed to the one or more Group V precursors for a time period no
longer than that required to form a Group III-V thin film at
sub-monolayer coverage.
18. The Group III-V film of claim 8, wherein the growth of the film
occurs at a temperature between about 500.degree. C. and about
800.degree. C.
19. The Group III-V film of claim 8, wherein the film is one of
AlN, GaN, InN, AlGaN, AlInN, InGaN, or InGaAlN.
20. The Group III-V film of claim 8, further comprising contacting
the substrate with a hydrogen-containing species before the
substrate is exposed to the one or more Group V precursors.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation claiming priority to U.S.
patent application Ser. No. 13/399,695 filed 29 Dec. 2011, which is
entirely incorporated by reference herein for all purposes.
FIELD OF THE INVENTION
[0002] The present disclosure relates generally to methods and
apparatus for forming metal nitride films, such as epitaxial
gallium nitride (GaN) films, and in particular to methods and
apparatus for forming GaN films with migration enhancement and
surface energy modification.
BACKGROUND OF THE INVENTION
[0003] The growth of high-quality crystalline semiconducting thin
films is a technology of significant industrial importance, with a
variety of microelectronic and optoelectronic applications,
including light emitting diodes and lasers. The current
state-of-the-art deposition technology for gallium nitride (GaN),
indium nitride (InN) and aluminum nitride (AlN) thin films, their
alloys and their heterostructures (collectively "InGaAlN" herein)
is metal-organic chemical vapor deposition (MOCVD), in which a
substrate is held at high temperature and gases which contain the
elements comprising the thin film flow over and are incorporated
into the growing thin film at the surface of the wafer. In the case
of GaN, the state-of-the-art may include growth temperatures of
approximately 1050.degree. C. and the simultaneous use of ammonia
(NH.sub.3) and a Group III alkyl precursor gas (e.g.,
trimethylgallium, triethylgallium).
[0004] While methods exist for forming InGaAlN films, there are
limitations associated with current methods. First, the high
processing temperature involved in MOCVD may require complex
reactor designs and the use of refractory materials and only
materials which are inert at the high temperature of the process
can be used in the processing volume. Second, the high temperature
involved may restrict the possible substrates for InGaAlN growths
to substrates which are chemically and mechanically stable at the
growth temperatures and chemical environment, typically sapphire or
silicon carbide substrates. Notably, silicon substrates, which are
less expensive and are available in large sizes for economic
manufacturing, may be less compatible. Third, the expense of the
process gases involved as well as their poor consumption ratio,
particularly in the case of ammonia, may be economically
unfavorable for low cost manufacturing of InGaAlN based devices.
Fourth, the use of carbon containing precursors (e.g.,
trimethylgallium) may result in carbon contamination in the InGaAlN
film, which may degrade the electronic and optoelectronic
properties of the InGaAlN based devices. Fifth, MOCVD reactors may
have a significant amount of gas phase reactions between the Group
III and the Group V containing process gases. The gas phase
reactions may result in undesirable deposition of the thin film
material on all surfaces within the reaction volume, and in the
undesirable generation of particles. The latter may result in a low
yield of manufactured devices. The former may result in a number of
practical problems, including reducing the efficacy of in-situ
optical measurements of the growing thin film due to coating of the
internal optical probes and lens systems, and difficulty in
maintaining a constant thermal environment over many deposition
cycles as the emissivity of reactor walls will change as deposition
builds up on the reactor walls. These problems may be common to all
the variants of MOCVD, including plasma enhanced MOCVD and
processes typically referred to as atomic layer deposition (ALD) or
atomic layer epitaxy (ALE).
[0005] Other methods for forming InGaAlN thin films include
plasma-assisted molecular beam epitaxy (PAMBE), in which fluxes of
evaporated Ga, In, or Al are directed in high vacuum at a heated
substrate simultaneously with a flux of nitrogen radicals (either
activated molecular nitrogen, atomic nitrogen, or singly ionized
nitrogen atoms or molecules) from a nitrogen plasma source. The
method may be capable of producing high quality InGaAlN thin films
and devices, but the method may suffer from a tendency to form
metal agglomerations, e.g., nano- to microscopic Ga droplets, on
the surface of the growing film. See, for example, "Homoepitaxial
growth of GaN under Ga-stable and N-stable conditions by
plasma-assisted molecular beam epitaxy", E. J. Tarsa et al., J.
Appl. Phys 82, 11 (1997), which is entirely incorporated herein by
reference. As such, the process may need to be carefully monitored,
which may inherently result in a low yield of manufactured
devices.
[0006] Other methods employed to make GaN films include hydride
vapor phase epitaxy, in which a flow of HCl gas over heated gallium
results in the transport of gallium chloride to a substrate where
simultaneous exposure to ammonia results in the growth of a GaN
thin film. The method may require corrosive chemicals to be used at
high temperatures, which may limit the compatible materials for
reactor design. In addition, the byproducts of the reaction are
corrosive gases and solids, which may increase the need for
abatement and reactor maintenance. While the method may produce
high quality GaN films at high growth rates (tens to hundreds of
microns per hour have been demonstrated, exceeding those commonly
achieved with MOCVD), the reactor design and corrosive process
inputs and outputs are drawbacks.
[0007] One approach being considered to lower the cost of
manufacturing of InGaAlN thin films and devices is to develop
methods that lower the energy barrier for surface diffusion (herein
referred to as the "surface energy") of species involved in the
growth of the film, thereby allowing high quality films to be
formed at lower temperatures. Therefore, there is a need to develop
methods and apparatus for delivering beneficial species that lower
the surface energy for adatom diffusion on the growth front of
InGaAlN thin films in a cost effective manner.
SUMMARY OF THE DISCLOSURE
[0008] The following summary of the invention is included in order
to provide a basic understanding of some aspects and features of
the invention. This summary is not an extensive overview of the
invention and as such it is not intended to particularly identify
key or critical elements of the invention or to delineate the scope
of the invention. Its sole purpose is to present some concepts of
the invention in a simplified form as a prelude to the more
detailed description that is presented below.
[0009] In some embodiments of the present invention, surfactants
are disposed on the surface during the growth of a thin film
wherein the surfactants lower the surface energy and increases the
lateral diffusion of species involved in the growth of the
film.
[0010] The deposition of a Group III-V film such as GaN is used as
a non-limiting example. In some embodiments of the present
invention, apparatus are provided that allow the investigation of
the influence of the surfactant on the growth of the film in a
combinatorial manner.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the figures. The drawings are not to scale and
the relative dimensions of various elements in the drawings are
depicted schematically and not necessarily to scale.
[0012] The techniques of the present invention can readily be
understood by considering the following detailed description in
conjunction with the accompanying drawings, in which:
[0013] FIG. 1 illustrates a schematic diagram for implementing
combinatorial processing and evaluation using primary, secondary,
and tertiary screening.
[0014] FIG. 2 is a simplified schematic diagram illustrating a
general methodology for combinatorial process sequence integration
that includes site isolated processing and/or conventional
processing in accordance with some embodiments of the present
invention.
[0015] FIG. 3 presents a flow chart for the deposition of a film
according to some embodiments.
[0016] FIG. 4 illustrates a schematic diagram illustrating four
processing regions and a substrate according to some
embodiments.
[0017] FIG. 5 presents a graph of the calculated average time
needed for a Ga atom to diffuse 5 nm as a function of temperature
illustrating the benefits of using a surfactant.
[0018] FIG. 6 presents a graph of the calculated reflectivity for a
bare sapphire substrate, a GaN layer, and a GaN layer with 2 ML of
In.
[0019] FIG. 7 illustrates a schematic diagram illustrating four
processing regions, and example of a combinatorial configuration,
and a substrate according to some embodiments.
[0020] FIG. 8 illustrates a schematic diagram illustrating four
processing regions, and example of a combinatorial configuration,
and a substrate according to some embodiments.
[0021] FIG. 9 illustrates a schematic diagram illustrating four
processing regions, and example of a combinatorial configuration,
and a substrate according to some embodiments.
DETAILED DESCRIPTION
[0022] A detailed description of one or more embodiments is
provided below along with accompanying figures. The detailed
description is provided in connection with such embodiments, but is
not limited to any particular example. The scope is limited only by
the claims and numerous alternatives, modifications, and
equivalents are encompassed. Numerous specific details are set
forth in the following description in order to provide a thorough
understanding. These details are provided for the purpose of
example and the described techniques may be practiced according to
the claims without some or all of these specific details. For the
purpose of clarity, technical material that is known in the
technical fields related to the embodiments has not been described
in detail to avoid unnecessarily obscuring the description.
[0023] The manufacture of integrated circuits (IC), semiconductor
devices, flat panel displays, optoelectronics devices, data storage
devices, magneto-electronic devices, magneto-optic devices,
packaged devices, and the like entails the integration and
sequencing of many unit processing steps. As an example, IC
manufacturing typically includes a series of processing steps such
as cleaning, surface preparation, deposition, lithography,
patterning, etching, planarization, implantation, thermal
annealing, and other related unit processing steps. The precise
sequencing and integration of the unit processing steps enables the
formation of functional devices meeting desired performance metrics
such as speed, power consumption, and reliability.
[0024] As part of the discovery, optimization and qualification of
each unit process, it is desirable to be able to i) test different
materials, ii) test different processing conditions within each
unit process module, iii) test different sequencing and integration
of processing modules within an integrated processing tool, iv)
test different sequencing of processing tools in executing
different process sequence integration flows, and combinations
thereof in the manufacture of devices such as integrated circuits.
In particular, there is a need to be able to test i) more than one
material, ii) more than one processing condition, iii) more than
one sequence of processing conditions, iv) more than one process
sequence integration flow, and combinations thereof, collectively
known as "combinatorial process sequence integration", on a single
monolithic substrate without the need of consuming the equivalent
number of monolithic substrates per material(s), processing
condition(s), sequence(s) of processing conditions, sequence(s) of
processes, and combinations thereof. This can greatly improve both
the speed and reduce the costs associated with the discovery,
implementation, optimization, and qualification of material(s),
process(es), and process integration sequence(s) required for
manufacturing.
[0025] Systems and methods for High Productivity Combinatorial
(HPC) processing are described in U.S. Pat. No. 7,544,574 filed on
Feb. 10, 2006, U.S. Pat. No. 7,824,935 filed on Jul. 2, 2008, U.S.
Pat. No. 7,871,928 filed on May 4, 2009, U.S. Pat. No. 7,902,063
filed on Feb. 10, 2006, and U.S. Pat. No. 7,947,531 filed on Aug.
28, 2009 which are all herein incorporated by reference. Systems
and methods for HPC processing are further described in U.S. patent
application Ser. No. 11/352,077 filed on Feb. 10, 2006, claiming
priority from Oct. 15, 2005, U.S. patent application Ser. No.
11/419,174 filed on May 18, 2006, claiming priority from Oct. 15,
2005, U.S. patent application Ser. No. 11/674,132 filed on Feb. 12,
2007, claiming priority from Oct. 15, 2005, and U.S. patent
application Ser. No. 11/674,137 filed on Feb. 12, 2007, claiming
priority from Oct. 15, 2005 which are all herein incorporated by
reference.
[0026] FIG. 1 illustrates a schematic diagram, 100, for
implementing combinatorial processing and evaluation using primary,
secondary, and tertiary screening. The schematic diagram, 100,
illustrates that the relative number of combinatorial processes run
with a group of substrates decreases as certain materials and/or
processes are selected. Generally, combinatorial processing
includes performing a large number of processes during a primary
screen, selecting promising candidates from those processes,
performing the selected processing during a secondary screen,
selecting promising candidates from the secondary screen for a
tertiary screen, and so on. In addition, feedback from later stages
to earlier stages can be used to refine the success criteria and
provide better screening results.
[0027] For example, thousands of materials are evaluated during a
materials discovery stage, 102. Materials discovery stage, 102, is
also known as a primary screening stage performed using primary
screening techniques. Primary screening techniques may include
dividing substrates into coupons and depositing materials using
varied processes. The materials are then evaluated, and promising
candidates are advanced to the secondary screen, or materials and
process development stage, 104. Evaluation of the materials is
performed using metrology tools such as electronic testers and
imaging tools (i.e., microscopes).
[0028] The materials and process development stage, 104, may
evaluate hundreds of materials (i.e., a magnitude smaller than the
primary stage) and may focus on the processes used to deposit or
develop those materials. Promising materials and processes are
again selected, and advanced to the tertiary screen or process
integration stage, 106, where tens of materials and/or processes
and combinations are evaluated. The tertiary screen or process
integration stage, 106, may focus on integrating the selected
processes and materials with other processes and materials.
[0029] The most promising materials and processes from the tertiary
screen are advanced to device qualification, 108. In device
qualification, the materials and processes selected are evaluated
for high volume manufacturing, which normally is conducted on full
substrates within production tools, but need not be conducted in
such a manner. The results are evaluated to determine the efficacy
of the selected materials and processes. If successful, the use of
the screened materials and processes can proceed to pilot
manufacturing, 110.
[0030] The schematic diagram, 100, is an example of various
techniques that may be used to evaluate and select materials and
processes for the development of new materials and processes. The
descriptions of primary, secondary, etc. screening and the various
stages, 102-110, are arbitrary and the stages may overlap, occur
out of sequence, be described and be performed in many other
ways.
[0031] This application benefits from High Productivity
Combinatorial (HPC) techniques described in U.S. patent application
Ser. No. 11/674,137 filed on Feb. 12, 2007 which is hereby
incorporated for reference in its entirety. Portions of the '137
application have been reproduced below to enhance the understanding
of the present invention. The embodiments described herein enable
the application of combinatorial techniques to process sequence
integration in order to arrive at a globally optimal sequence of
semiconductor devices, thin film photovoltaic (TFPV) modules,
optoelectronic devices, etc. manufacturing operations by
considering interaction effects between the unit manufacturing
operations, the process conditions used to effect such unit
manufacturing operations, hardware details used during the
processing, as well as materials characteristics of components
utilized within the unit manufacturing operations. Rather than only
considering a series of local optimums, i.e., where the best
conditions and materials for each manufacturing unit operation is
considered in isolation, the embodiments described below consider
interactions effects introduced due to the multitude of processing
operations that are performed and the order in which such multitude
of processing operations are performed when fabricating a
semiconductor devices, TFPV modules, optoelectronic devices, etc.
device. A global optimum sequence order is therefore derived and as
part of this derivation, the unit processes, unit process
parameters and materials used in the unit process operations of the
optimum sequence order are also considered.
[0032] The embodiments described further analyze a portion or
sub-set of the overall process sequence used to manufacture
semiconductor devices, TFPV modules, optoelectronic devices, etc.
Once the subset of the process sequence is identified for analysis,
combinatorial process sequence integration testing is performed to
optimize the materials, unit processes, hardware details, and
process sequence used to build that portion of the device or
structure. During the processing of some embodiments described
herein, structures are formed on the processed substrate that are
equivalent to the structures formed during actual production of the
semiconductor devices, TFPV modules, optoelectronic devices, etc.
For example, such structures may include, but would not be limited
to, contact layers, buffer layers, absorber layers, or any other
series of layers or unit processes that create an intermediate
structure found on semiconductor devices, TFPV modules,
optoelectronic devices, etc. While the combinatorial processing
varies certain materials, unit processes, hardware details, or
process sequences, the composition or thickness of the layers or
structures or the action of the unit process, such as cleaning,
surface preparation, deposition, surface treatment, etc. is
substantially uniform through each discrete region. Furthermore,
while different materials or unit processes may be used for
corresponding layers or steps in the formation of a structure in
different regions of the substrate during the combinatorial
processing, the application of each layer or use of a given unit
process is substantially consistent or uniform throughout the
different regions in which it is intentionally applied. Thus, the
processing is uniform within a region (inter-region uniformity) and
between regions (intra-region uniformity), as desired. It should be
noted that the process can be varied between regions, for example,
where a thickness of a layer is varied or a material may be varied
between the regions, etc., as desired by the design of the
experiment.
[0033] The result is a series of regions on the substrate (or a
series of substrates for reactors with isolated processing regions)
that contain structures or unit process sequences that have been
uniformly applied within that region and, as applicable, across
different regions. This process uniformity allows comparison of the
properties within and across the different regions such that the
variations in test results are due to the varied parameter (e.g.,
materials, unit processes, unit process parameters, hardware
details, or process sequences) and not the lack of process
uniformity. In the embodiments described herein, the positions of
the discrete regions on the substrate can be defined as needed, but
are preferably systematized for ease of tooling and design of
experimentation. In addition, the number, variants and location of
structures within each region are designed to enable valid
statistical analysis of the test results within each region and
across regions to be performed.
[0034] FIG. 2 is a simplified schematic diagram illustrating a
general methodology for combinatorial process sequence integration
that includes site isolated processing and/or conventional
processing in accordance with one embodiment of the invention. In
one embodiment, the substrate is initially processed using
conventional process N. In one exemplary embodiment, the substrate
is then processed using site isolated process N+1. During site
isolated processing, an HPC module may be used, such as the HPC
module described in U.S. patent application Ser. No. 11/352,077
filed on Feb. 10, 2006. The substrate can then be processed using
site isolated process N+2, and thereafter processed using
conventional process N+3. Testing is performed and the results are
evaluated. The testing can include physical, chemical, acoustic,
magnetic, electrical, optical, etc. tests. From this evaluation, a
particular process from the various site isolated processes (e.g.
from steps N+1 and N+2) may be selected and fixed so that
additional combinatorial process sequence integration may be
performed using site isolated processing for either process N or
N+3. For example, a next process sequence can include processing
the substrate using site isolated process N, conventional
processing for processes N+1, N+2, and N+3, with testing performed
thereafter.
[0035] It should be appreciated that various other combinations of
conventional and combinatorial processes can be included in the
processing sequence with regard to FIG. 2. That is, the
combinatorial process sequence integration can be applied to any
desired segments and/or portions of an overall process flow.
Characterization, including physical, chemical, acoustic, magnetic,
electrical, optical, etc. testing, can be performed after each
process operation, and/or series of process operations within the
process flow as desired. The feedback provided by the testing is
used to select certain materials, processes, process conditions,
and process sequences and eliminate others. Furthermore, the above
flows can be applied to entire monolithic substrates, or portions
of monolithic substrates such as coupons. In some embodiments, a
set of monolithic substrates are grouped for processing together as
a unit through the above flows.
[0036] Under combinatorial processing operations the processing
conditions at different regions can be controlled independently.
Consequently, process material amounts, reactant species,
processing temperatures, processing times, processing pressures,
processing flow rates, processing powers, processing reagent
compositions, the rates at which the reactions are quenched,
deposition order of process materials, process sequence steps,
hardware details, etc., can be varied from region to region on the
substrate. Thus, for example, when exploring materials, a
processing material delivered to a first and second region can be
the same or different. If the processing material delivered to the
first region is the same as the processing material delivered to
the second region, this processing material can be offered to the
first and second regions on the substrate at different
concentrations. In addition, the material can be deposited under
different processing parameters. Parameters which can be varied
include, but are not limited to, process material amounts, reactant
species, processing temperatures, processing times, processing
pressures, processing flow rates, processing powers, processing
reagent compositions, the rates at which the reactions are
quenched, atmospheres in which the processes are conducted, an
order in which materials are deposited, hardware details of the gas
distribution assembly, etc. It should be appreciated that these
process parameters are exemplary and not meant to be an exhaustive
list as other process parameters commonly used in semiconductor
device, TFPV module, optoelectronic device, etc. manufacturing may
be varied.
[0037] Methods and systems provided herein enable formation of
Group III-V thin films while reducing, if not eliminating, problems
associated with current systems and methods. In some embodiments,
methods are provided for forming Group III-V thin films at reduced
temperatures. Thin films formed according to methods and systems
provided herein may be used in various applications, such as, for
example, photovoltaic solar cell and light emitting diode (LED)
devices and systems.
[0038] In some embodiments, systems and methods are provided for
forming Group III-V thin films. In some embodiments, time and space
co-divided systems and methods are described in which a surfactant
precursor and a first precursor are brought in contact with a
substrate in a first reaction space and a second precursor is
brought in contact with the substrate in a second reaction space
that is separate from the first reaction space. In some
embodiments, the substrate is brought into contact with the first
precursor in a first reaction space, a second precursor in a second
reaction space, and a surfactant precursor in a third reaction
space. In some embodiments, the substrate is brought into contact
with the first precursor in a first reaction space, and a second
precursor and a surfactant precursor in a second reaction space. As
used herein, a "surfactant" is an atomic or molecular species that
increases the surface diffusivity of adatoms during the deposition
of a thin film, and wherein the surfactant is not incorporated into
the resulting film in concentrations greater than about 1 atomic
percent. In some embodiments, the surfactant precursor comprises
species such as hydrogen, chlorine, bromine, iodine, arsenic,
phosphorus, bismuth, indium, antimony, lead, tin, germanium,
selenium, tellurium, magnesium, cadmium, zinc, mercury, etc., or
their combinations. In some embodiments, the first precursor is a
Group III metal precursor, and exposure of the substrate to the
Group III metal precursor forms a Group III metal thin film at
sub-monolayer coverage. In some embodiments, the substrate is
brought in contact with one or more Group III metal precursors in
the same reaction space or different reaction spaces, and exposure
of the substrate to the one or more Group III metal precursors is
separated by exposure of the substrate to a Group V precursor, or
other processing, such as, e.g., spectroscopies to determine thin
film quality or composition.
[0039] The term "reaction space", as used herein, may refer to any
environment suitable for deposition of a material film or thin film
on or over a substrate, or the measurement of the physical
characteristics of the material film or thin film. In some
embodiments, a reaction space may include a chamber. In some
embodiments, a reaction space may include a chamber in a system
having a plurality chambers. In some embodiments, a reaction space
may include a chamber in a system having a plurality of fluidically
separated chambers. In some embodiments, a system may include
multiple reactions spaces, wherein each reaction space is
fluidically separated from another reaction space. In some
embodiments, a reaction space may be suitable for conducting
measurements on a substrate or a thin film formed on the substrate
(or target).
[0040] The term "metal nitride", as used herein, may refer to a
material comprising one or more metals or one or more
semiconductors, and nitrogen. In certain embodiments, a metal
nitride (e.g., metal nitride thin film) may have the chemical
formula M.sub.xN.sub.y, wherein `M` designates a metal or a
semiconductor, `N` designates nitrogen, and `x` and `y` are numbers
greater than zero. In some embodiments, `M` may comprise one or
more metals and/or semiconductors. In some embodiments,
M.sub.xN.sub.y may refer to a metal nitride, such as a Group III
metal nitride (e.g., gallium nitride, indium nitride, aluminum
nitride, aluminum gallium nitride, aluminum indium nitride, gallium
indium nitride, indium gallium aluminum nitride). In some
embodiments, a metal nitride film or thin film may comprise other
materials, such as, e.g., chemical dopants. Chemical dopants may
include p-type dopants (e.g., magnesium, zinc) and n-type dopants
(e.g., silicon, oxygen).
[0041] The terms "excited species" and "activated species", as used
herein, may refer to radicals, ions and other excited (or
activated) species generated via application (or coupling) of
energy to a reactant gas or vapor. In some embodiments, energy may
be applied via a variety of methods, such as, e.g., ultraviolet
radiation, microwave radiation, inductive coupling and capacitive
coupling, such as with the aid of a plasma generator. The plasma
generator may be a direct plasma generator (i.e., direct plasma
generation) or a remote plasma generator (i.e., remote plasma
generation). In the absence of coupling energy, plasma generation
is terminated. For remote plasma generation, plasma-excited species
of a particular vapor phase chemical (e.g., nitrogen-containing
plasma species) may be formed in a plasma generator in fluid
communication with (or fluidically coupled to) a reaction space
having a substrate to be processed. In some embodiments, energy may
be applied by exposure of a species of gas to hot (or heated)
surfaces or wires, where the interaction of the gas with the heated
surfaces or wires generates excited (or activated) species of the
gas. Details of methods and systems benefitting from the use of hot
wire activation may be found in U.S. patent application Ser. No.
13/302,001 entitled "Method and Apparatus for Atomic Hydrogen
Surface Treatment During GaN Epitaxy" filed on Nov. 22, 2011 which
is herein incorporated by reference.
[0042] The term "nitrogen-containing species", as used herein, may
include, without limitation, nitrogen radicals, nitrogen ions, and
excited (or active) neutral species of nitrogen. In some
embodiments, the gaseous source of nitrogen-containing species may
include, without limitation, N.sub.2, NH.sub.3, and/or hydrazine.
In some embodiments, the gaseous source of nitrogen-containing
species may include mixtures of N.sub.2 and H.sub.2 gases. In some
embodiments, the gaseous source of nitrogen-containing species may
include mixtures of N.sub.2 and inert gases (any of He, Ne, Ar, Kr,
or Xe). In some embodiments, excited nitrogen-containing species
may be provided via remote plasma generation or direct plasma
generation. In some embodiments, excited nitrogen-containing
species may be provided by the thermal disassociation of
nitrogen-containing species by exposure to hot surfaces or wires.
Coupling energy to a mixture of N.sub.2 and H.sub.2 gases may
generate excited molecular NH.sub.x, wherein `x` is a number
greater than or equal to 1.
[0043] The term "hydrogen-containing species", as used herein, may
include, without limitation, hydrogen radicals, hydrogen ions, and
excited (or active) neutral species of hydrogen (H.sub.2). In some
embodiments, hydrogen-containing species includes H.sub.2. In some
embodiments, the gaseous source of hydrogen-containing species may
include, without limitation, H.sub.2, NH.sub.3, and/or hydrazine.
In some embodiments, the gaseous source of hydrogen-containing
species may include mixtures of H.sub.2 and N.sub.2 gases. In some
embodiments, the gaseous source of nitrogen-containing species may
include mixtures of H.sub.2 and inert gases (any of He, Ne, Ar, Kr,
or Xe). In some embodiments, excited hydrogen-containing species
may be provided via remote plasma generation or direct plasma
generation. In some embodiments, excited hydrogen-containing
species may be provided by the thermal disassociation of
hydrogen-containing species by exposure to hot surfaces or wires.
It will be appreciated that excited hydrogen-containing species may
include neutral hydrogen-containing species, such as H.sub.2.
[0044] The term "adsorption", as used herein, may refer to chemical
or physical attachment of atoms or molecules on a surface, such as
a substrate surface or a surface of a film or thin film on or over
a substrate.
[0045] The term "substrate", as used herein, may refer to any
workpiece on which film or thin film formation is desired.
Substrates may include, without limitation, silicon, silica,
sapphire, zinc oxide, SiC, AlN, GaN, Spinel, coated silicon,
silicon on oxide, silicon carbide on oxide, glass, gallium nitride,
indium nitride or aluminum nitride, or combinations (or alloys)
thereof.
[0046] The term "monolayer", as used herein, may refer to a single
layer of atoms or molecules. In some embodiments, a monolayer
includes a monatomic monolayer (ML) having a thickness of one
atomic layer. In some embodiments, a monolayer includes the maximum
coverage of a particular species on a surface. In such a case, all
individual members of the surface adsorbed species may be in direct
physical contact with the surface of the underlying substrate, thin
film or film. The term "sub-monolayer coverage", as used herein,
may refer to a layer of a particular species at a coverage less
than one monatomic monolayer. In some embodiments, a layer of a
particular species at sub-monolayer coverage may permit additional
adsorption of the species or another species. In some embodiments,
sub-monolayer coverage may be referred to as "pre-wetting"
coverage. For example, a layer of a Group III metal, such as
gallium (Ga), indium (In) or aluminum (Al), may include Ga, In or
Al atoms collectively having a coverage of about 0.5 ML on a
surface, which may be provided with respect to the maximum
collective coverage of Ga, In or Al atoms on the surface. In some
embodiments, the maximum coverage of a species on a surface is
determined by the attractive and repulsive interaction between
adsorbed species on the surface. In some embodiments, a layer of a
species at a coverage of one monolayer cannot permit additional
adsorption of the species in the layer. In some embodiments, a
layer of a particular species at a coverage of one monolayer may
permit the adsorption of another species in the layer.
[0047] The term "exposure", as used herein, may refer to the
product of pressure (P) and time (t), (i.e., P.times.t), wherein
`P` and `t` are provided in units of torr and seconds,
respectively. For example, a substrate exposed to a Group III metal
precursor at a pressure of 1.times.10.sup.-6 torr for a period of
60 seconds is contacted with the Group III metal precursor at an
exposure (or dosage) of 1.times.10.sup.-6 torr.times.60 seconds, or
60.times.10.sup.-6 torr*s, or 60 Langmuir (L). One Langmuir (L) is
defined as 1.times.10.sup.-6 torr*s.
[0048] The term "precursor", as used herein, may refer to a liquid
or vapor phase chemical having a species of interest for deposition
on a substrate surface. A Group III metal precursor may include a
chemical compound that includes one or more Group III metal atoms,
such as one or more of Ga, In and Al. A Group V precursor may
include a chemical that includes one or more Group V atoms, such as
one or more of nitrogen, arsenic and phosphorus. Upon interaction
between a substrate surface and a Group III precursor or a Group V
precursor, the Group III precursor or the Group V precursor may
dissociate to yield a Group III chemical (or adatoms of the Group
III atom) or a Group V chemical (or adatoms of the Group V atom) on
the substrate surface.
[0049] In one aspect of the invention, a method for forming a Group
III-V thin film comprises providing a substrate in a first reaction
space, contacting the substrate in the first reaction space with a
surfactant precursor and a Group III metal precursor (also "Group
III precursor" herein) to form a Group III metal thin film, moving
the substrate to a second reaction space, and contacting the
substrate in a second reaction space with a Group V precursor,
thereby forming a Group III-V thin film. In some embodiments,
contacting the substrate in the first reaction space with the
surfactant precursor and the Group III precursor forms a surfactant
layer and a Group III metal thin film, where the Group III metal
thin film is at sub-monolayer coverage.
[0050] In some embodiments, prior to contacting the substrate with
the Group V precursor, the substrate, including the surfactant
layer on the substrate and the Group III metal thin film on the
substrate, is contacted with a hydrogen-containing species. In some
embodiments, the hydrogen-containing species includes H.sub.2. In
some embodiments, the hydrogen-containing species includes excited
species of hydrogen, including one or more of hydrogen cations,
hydrogen anions and hydrogen radicals. In some embodiments, excited
species of hydrogen may be provided via remote plasma generation or
direct plasma generation. In some embodiments, excited species of
hydrogen may be provided by the thermal disassociation of molecular
hydrogen by exposure of H.sub.2 gas to hot surfaces or wires.
[0051] In some embodiments, the substrate may be contacted with a
hydrogen-containing species in the first reaction space. In such a
case, the first reaction space may be evacuated with the aid of
purging and/or pumping prior to contacting the substrate with the
hydrogen-containing species.
[0052] In some embodiments, prior to contacting the substrate in
the second reaction space with the Group V precursor, the substrate
is moved to a third reaction space and contacted with a
hydrogen-containing species in the third reaction space. The third
reaction space may be disposed in-between the first and second
reaction spaces. In some embodiments, contacting a Group III metal
thin film with excited species of hydrogen may reduce multi-layer
Group III islands to islands of monatomic height (or
thickness).
[0053] In some embodiments, following the second reaction space,
the substrate is moved to the first reaction space. In other
embodiments, following the second reaction space, the substrate is
moved to a third reaction space. In some embodiments, in the third
reaction space the substrate is contacted with a Group III metal
precursor. In some embodiments, in the third reaction space the
substrate is contacted with a Group III metal precursor that is
different from the Group III metal precursor exposed to the
substrate in the first reaction space. In some embodiments,
following the third reaction space, the substrate is moved to one
or more additional reaction spaces for separate exposure to a Group
III metal precursor and a Group V precursor. In some embodiments,
in-between exposure to a Group III or Group V precursor, the
substrate may be exposed to other species, such as
hydrogen-containing species or surfactant precursors, which may aid
in maintaining film stoichiometry, in reducing thin film
contamination, and in reducing the surface energy for adatom
diffusion.
[0054] In some embodiments, after forming a Group III-V thin film
on the substrate, the substrate, including the Group III-V thin
film on the substrate, is contacted with a hydrogen-containing
species. In some embodiments, the substrate is contacted with a
hydrogen-containing species in the second reaction space. In such a
case, the second reaction space may be evacuated with the aid of
purging and/or pumping prior to contacting the substrate with a
hydrogen-containing species. In some embodiments, the substrate is
moved to a third reaction space and contacted with a
hydrogen-containing species in the third reaction space. The third
reaction space may be disposed in-between the first and second
reaction spaces. In some embodiments, the third reaction space is
different from the first and second reaction spaces.
[0055] Methods of some embodiments of the invention may be used to
form Group III-V thin films, such as thin films including nitrogen
and one or more of aluminum (Al), gallium (Ga) and indium (In). In
some embodiments, methods are provided for forming GaN thin films.
In some embodiments, methods are provided for forming AlN thin
films. In some embodiments, methods are provided for forming InN
thin films. In some embodiments, methods are provided for forming
AlGaN thin films. In some embodiments, methods are provided for
forming AlInN thin films. In some embodiments, methods are provided
for forming GaInN thin films. In some embodiments, methods are
provided for forming InGaAlN thin films.
[0056] In some embodiments, in the first reaction space the
substrate is contacted with a surfactant precursor and a single
Group III metal precursor. In some embodiments, in the first
reaction space the substrate is contacted with a surfactant
precursor and multiple Group III metal precursors, such as one or
more of Al, Ga and In-containing precursors. In some embodiments,
the substrate is contacted with a surfactant precursor and
different Group III metal precursors in separate reaction spaces.
In some embodiments, the substrate is contacted with a Group V
precursor between exposures to Group III metal precursors. In some
embodiments, the substrate is contacted with a surfactant precursor
between exposures to Group III metal precursors and Group V
precursors.
[0057] In some embodiments, the first reaction space is different
from the second reaction space. In some embodiments, the first
reaction space is fluidically separated from the second reaction
space. In some embodiments, the first reaction space and the second
reaction space are separate pressure-regulated environments. In
some embodiments, gases from the first reaction space are prevented
from entering the second reaction space, and gases from the second
reaction space are prevented from entering the first reaction
space.
[0058] With reference to FIG. 3, a method for forming a Group III-V
thin film is illustrated, in accordance with an embodiment of the
invention. In a first step 100, with the substrate disposed in a
first reaction space, the substrate is contacted with a surfactant
precursor and a Group III metal precursor (e.g., trimethyl gallium)
to form a surfactant layer and a Group III metal thin film on the
substrate. The substrate may be disposed in the first reaction
space by moving the substrate (e.g., moving or rotating a susceptor
having the substrate) to the first reaction space or moving the
first reaction space (e.g., rotating the first reaction space) to
the substrate. In some embodiments, the substrate is contacted for
a time period no more than that required to form a surfactant layer
of between about 0 and about 5 monolayers (ML) and a Group III
metal thin film at sub-monolayer coverage. Advantageously, the
thickness of the surfactant layer is maintained at about 2 ML. In
some embodiments, contacting the substrate in the first reaction
space with the surfactant precursor and the Group III metal
precursor forms a layer of a Group III metal having a thickness
less than about 1 ML. In some embodiments, contacting the substrate
in the first reaction space with the surfactant precursor and the
Group III metal precursor forms a layer of a Group III metal having
a thickness up to about 1 ML. In some embodiments, contacting the
substrate in the first reaction space with the surfactant precursor
and the Group III metal precursor forms a layer of a Group III
metal having a thickness greater than about 1 ML. In some
embodiments, contacting the substrate in the first reaction space
with the surfactant precursor and the Group III metal precursor
forms a layer of a Group III metal at sub-monolayer coverage.
[0059] In some embodiments, an exposure of the Group III metal
precursor may be selected to provide a Group III metal thin film at
a predetermined coverage. Coverage may be assessed by a variety of
spectroscopy tools (see below), such as x-ray photoelectron
spectroscopy (XPS). In achieving a predetermined coverage of a
Group III metal thin film, it will be understood that a
trial-and-error approach may be used to find an exposure of the
Group III metal precursor corresponding to the predetermined
coverage. For example, in one system it may be determined that
achieving a Group III metal thin film coverage at 0.3 ML coverage
requires exposure of a substrate (also "target" herein) to a Group
III metal precursor at about 1 L.
[0060] Next, in step 302, the substrate is disposed in a second
reaction space. In some embodiments, the substrate may be moved
(i.e. rotated) to the second reaction space. In other embodiments,
the second reaction space may be moved (e.g., rotated) to the
substrate. In some embodiments, movement of the substrate to the
second reaction space comprises rotating a susceptor from the first
reaction space to the second reaction space.
[0061] Next, in step 304, with the substrate in the second reaction
space, the substrate, including the Group III metal thin film on
the substrate, is contacted with a Group V precursor to form a
Group III-V-containing thin film. In some embodiments, the Group
III-V-containing thin film includes Group III-V species. In some
embodiments, prior to contacting the substrate with the Group V
precursor, the substrate may be contacted with hydrogen-containing
species (e.g., H.sub.2, excited species of hydrogen). The substrate
may be contacted with hydrogen-containing species in the first
reaction space, the second reaction space, or a third reaction
space. In some embodiments, prior to contacting the substrate with
hydrogen-containing species, a reaction space having the substrate
may be evacuated with the aid of purging and/or pumping.
[0062] In some embodiments, the substrate is exposed to a
hydrogen-containing species. In some embodiments, the
hydrogen-containing species includes plasma-excited species of
hydrogen. In some embodiments, the hydrogen-containing species
includes active neutral species of hydrogen.
[0063] In some embodiments, the Group V precursor includes a
nitrogen-containing species. In some embodiments, the Group V
precursor includes plasma-excited species of nitrogen. In some
embodiments, the Group V precursor includes active neutral species
of nitrogen. In some embodiments, the Group V precursor includes
active neutral species of nitrogen comprises nitrogen species
having the lowest excited state of molecular nitrogen
(A.sup.3.SIGMA..sub.u.sup.+). In some embodiments, the substrate is
contacted for a time period no more than that required to form a
Group III-V thin film at sub-monolayer coverage. In some
embodiments, contacting the substrate in the second reaction space
with the Group V precursor forms a Group III-V thin film having a
thickness less than about 1 ML. In some embodiments, contacting the
substrate in the second reaction space with the Group V precursor
forms a Group III-V thin film having a thickness up to about 1 ML.
In some embodiments, contacting the substrate in the first reaction
space with the Group V metal precursor forms a layer of a III-V
thin film having a thickness greater than about 1 ML. In some
embodiments, contacting the substrate in the second reaction space
with the Group V precursor forms a Group III-V thin film at
sub-monolayer coverage.
[0064] In some embodiments, plasma-excited species of nitrogen may
include a nitrogen and hydrogen-containing species formed by
providing energy to a mixture of N.sub.2 and H.sub.2 gases,
NH.sub.3, a mixture of N.sub.2 and NH.sub.3, hydrazine
(N.sub.2H.sub.4), and/or a mixture of N.sub.2 and N.sub.2H.sub.4.
In some embodiments, plasma-excited species of nitrogen include
NFL, wherein `x` is a number greater than or equal to 1. For
example, plasma-excited species of nitrogen may include one or more
of NH, NH.sub.2 and NH.sub.3, and ions and radicals of such
species, such as, for example, NH.sup.+, NH.sub.2.sup.+,
NH.sub.3.sup.+. In some embodiments, plasma-excited species of
nitrogen are formed by inductively coupling energy to a mixture of
N.sub.2 and H.sub.2 gases having a ratio of N.sub.2 and H.sub.2
flow rates of about 0.5:1, or 1:1, or 2:1, or 3:1, or 4:1, or 5:1.
In some embodiments, plasma-excited species of nitrogen are formed
by inductively coupling energy to a mixture of N.sub.2 and H.sub.2
gases having an H.sub.2 flow rate that is about 1/3 (or 0.333) of
the total N.sub.2 and H.sub.2 flow rate.
[0065] In some embodiments, the substrate may be contacted with
Group III and Group V precursors until a Group III-V-containing
thin film of predetermined thickness is formed on the
substrate.
[0066] With continued reference to FIG. 3, in step 306, the
substrate is moved to the first reaction space or a third reaction
space. In some embodiments, the substrate is moved to the first
reaction space and contacted with a surfactant precursor and a
Group III metal precursor. In some embodiments, the substrate is
moved to a third reaction space that is different from the first
and second reaction spaces. In the third reaction space, the
substrate may be contacted with surfactant precursor, a Group III
metal precursor, a Group V precursor, or another species. For
example, the substrate may be contacted with vapor phase chemical
to aid in removing contaminants (e.g., carbon) from a Group III-V
thin film formed on the substrate.
[0067] In some embodiments, the substrate is moved to successive
reaction spaces (see below) until a thin film of predetermined
thickness and composition is formed.
[0068] In some embodiments, the substrate may be moved to the first
reaction space to repeat steps 300 to 306, as described above.
[0069] In some embodiments, after the second reaction space, the
substrate may be moved to one or more environments that include one
or more thin film diagnostic tools, such as one or more thin film
spectroscopy tools, to aid in assessing the physical
characteristics (e.g., conductivity, thickness, long-range
periodicity, composition) and/or quality of a thin film formed on
the substrate. Spectroscopy tools may include reflection-absorption
infrared spectroscopy (RAIRS), low-energy electron diffraction
(LEED) spectroscopy, x-ray photoelectron spectroscopy (XPS), Auger
electron spectroscopy (AES), scanning probe microscopy (STM, AFM),
near edge x-ray absorption fine structure (NEXAFS), spectral
reflectance and transmission, single wavelength reflectance and
transmission, optical pyrometry (single wavelength, dual
wavelength, or using spectral radiometry), emissometry,
ellipsometry, surface light scattering, and optical
polarimetry.
[0070] In another aspect of the invention, an apparatus for
depositing a Group III-V thin film on a substrate is provided. The
apparatus comprises a first reaction space and a second reaction
space, the first reaction space fluidically separated from the
second reaction space. The apparatus further includes a susceptor
or substrate holder for bringing a substrate in contact with each
of the first and second reaction spaces. In some embodiments, the
apparatus includes a controller (see, e.g., FIG. 4) for directing a
surfactant precursor and a Group III precursor into the first
reaction space at a first exposure and a Group V precursor into the
second reaction space at a second exposure. In some embodiments,
the first exposure is for providing a surfactant layer and layer of
a Group III metal. In some embodiments the Group III metal is at
sub-monolayer (or pre-wetting) coverage. Is some embodiments, the
thickness of the surfactant layer is more than one monolayer and
less than two monolayers.
[0071] In some embodiments, a number of the practical problems with
the state of the art for the deposition of binary semiconducting
thin films may be mitigated by separating the environment (or
reaction space) in which the Group III metal atoms are deposited
onto a substrate from the environment in which these adsorbed metal
atoms are exposed to a Group V-containing precursor gas to form the
Group III-V thin film or compound. In some embodiments, the
substrate is moved between deposition reaction spaces (or
environments) cyclically to deposit a film of a predetermined
thickness. FIG. 4 is an example of a system of linked sequential
processing environments, which may be constructed for this
purpose.
[0072] FIG. 4 illustrates a schematic diagram illustrating a
processing apparatus, 400, comprising four processing regions and
illustrating a substrate according to some embodiments. A similar
apparatus has been described in U.S. application Ser. No.
13/025,046 filed on Feb. 10, 2011, entitled "METHODS AND SYSTEMS
FOR FORMING THIN FILMS" and is incorporated herein by reference. A
deposition chamber (not shown) comprises a reaction zone wherein
there are four independent processing regions. The independent
processing regions are fluidically isolated from one another.
Although four independent processing regions are illustrated, those
skilled in the art will appreciate that any number of independent
processing regions can be envisioned, limited only by the number of
desired processing steps in the sequence and practical physical
limitations. For example, Zone-D can be omitted, and the process
sequence comprises Zone-A, Zone-B and Zone-C. Alternatively, both
Zone-C and Zone-D are omitted, and the process sequence comprises
Zone-A and Zone-B. Further, the processing order, e.g., Zone-A to
Zone-B to Zone-C to Zone-D, are shown for illustrative purposes,
and those skilled in the art will appreciate that the zones can be
arranged in any order. For example, Zone-C can be positioned before
Zone-B to form a process sequence comprises Zone-A, Zone-C, Zone-B,
and Zone-D. Although a circular configuration is illustrated, those
skilled in the art will appreciate that any suitable and practical
configuration may be used. Examples might include linear
configurations, parallel configurations, oval configurations,
square or rectangular configurations, etc.
[0073] In FIG. 4, substrate, 402, is held on a substrate holder,
414. Although one substrate is illustrated in FIG. 4, those skilled
in the art will appreciate that substrate holder, 414, will
generally accommodate multiple substrates resulting in improved
processing efficiency. The use of processing apparatus, 400, will
be described by using the deposition of an InGaAlN film as an
example. Those skilled in the art will appreciate that the features
of processing apparatus, 400, can be applied to many different
types of processing sequences, including, but not limited to, the
growth of the variety of III-V materials listed above, etc.
[0074] In Zone-A, 404, the substrate is exposed to the deposition
precursor gases. For this example, the gases might comprise
surfactants such as hydrogen, chlorine, bromine, iodine, arsenic,
phosphorus, bismuth, indium, antimony, lead, tin, germanium,
selenium, tellurium, magnesium, cadmium, zinc, mercury, etc., or
combinations thereof and Group III metal organic precursors of
aluminum, gallium, indium, or combinations thereof. Zone-A
generally does not include the use of plasma energy. The absence of
plasma energy in Zone-A reduces the frequency of undesirable gas
phase reactions that lead to the formation of particulate matter
that is deposited on the substrate rather than forming the desired
film through well controlled surface reactions. The precursor gases
will adsorb onto the surface, diffuse across the surface, and
interact with reactive surface sites. In the example illustrated in
FIG. 4, processing apparatus, 400, has a generally circular shape
and substrate holder, 414, and substrate, 402, can be rotated about
a central axis, 412, to repeatedly expose the substrate to the
sequence of processing regions. The exposure dosage of the gases or
reactive species to the substrate in the various processing zones
can be influenced through combinations of the gas concentration,
gas flow rate, pressure, and rotation speed. In this manner,
effective growth rates from less than 1 monolayer per cycle to
several monolayers per cycle can be achieved.
[0075] In Zone-B, 406, the substrate may be exposed to excited
species of hydrogen, such as those generated from a hydrogen
plasma. The exposure to excited species of hydrogen can be used to
control the amount of hydrogen on the surface and therefore
influence the incorporation and diffusion of Ga across the surface
as discussed previously. The exposure to excited species of
hydrogen in this zone will minimize the tendency for Ga to
aggregate at higher Ga concentrations by assisting the
incorporation of the Ga into the film growth front.
[0076] In Zone-C, 408, the substrate may be exposed to excited
species of nitrogen, such as those generated from a nitrogen
plasma. The advantageous aspects of the nitrogen plasma are
previously described. The reaction of the Group III thin film on
the substrate with the excited species of nitrogen forms a Group
III-N thin film, where the thickness of the Group III-N thin film
deposited may be less than one monolayer per cycle to several
monolayers per cycle.
[0077] In Zone-D, 410, in-situ metrology techniques may be employed
to monitor and control the growth of the GaN as the substrate is
repeatedly cycled through the processing regions Zone-A, Zone-B,
Zone-C, etc. Examples of suitable metrology techniques comprise
reflectometers and ellipsometers for measuring thickness, Fourier
Transform Infra-Red (FTIR) spectrometers for measuring Ga and In
surface concentrations, optical methods for measuring surface
roughness, pyrometers for measuring substrate temperature, etc.
Data collected and analyzed from the metrology systems may be used
to control the progress of the film growth.
[0078] In some embodiments, the substrate is exposed to excitations
of an inert gas, such as those generated from an inert gas plasma.
Alternately, laser irradiation may be used to excite the inert gas.
In some embodiments, the exposure of the substrate to excitations
of an inert gas may be done in Zone B. Examples of suitable inert
gases comprise one or more of He, Ne, Ar, Kr, Xe, or combinations
thereof. The plasma will generate excited neutral species with
internal energies above about 4 eV. This energy may be used to
facilitate the decomposition of the Group III metal organic species
and the desorption of the by-products. The plasma conditions are
chosen such that the ion density at the substrate surface is
negligible and ion bombardment of the surface is minimized to
prevent damage.
[0079] In some embodiments, this technique may mitigate the gas
phase reaction between Group III and Group V-containing precursor
gases, because the Group III and Group V precursor gases will not
be present in significant amounts in the same reaction volume due
to flow and pressure control of each individual reaction space.
Maintaining separate environments further allows for optimization
of the environment for the delivery of Group III adatoms to the
surface, and a separate environment (or reaction space) where the
delivery of the Group V precursor is optimized for surface reaction
to form the desired III-V compound. In the case of InGaAlN thin
films, the Group III metal delivery may be optimized for use of
metal organic precursors such as (here described for the case of
gallium; similarly for other Group III metals) trimethylgallium,
triethylgallium, diethylgallium chloride, and coordinated gallium
hydride compounds, (e.g., dimethylgallium hydride), etc.; thermal
evaporation of the Group III material; or gaseous Group III
chlorides or Group III halides, among other methods. In some
embodiments, in the case of InGaAlN thin films, the nitridation
environment may be optimized for various methods to deliver active
nitrogen to the substrate surface, including plasma excitation or
thermal disassociation of nitrogen-containing species.
[0080] The separate deposition environments for Group III and Group
V elements may enable the isolation of incompatible processes in
each of the independent environments. For example, different gas
flows and pumping speeds may be used in the Group III and Group V
environments. Additionally, mechanisms useful for the creation of
reactive Group V species but detrimental to the delivery of Group
III precursors, for example ionization and disassociation via
plasma excitation, may be used without constraints due to the
absence of Group III precursors in the Group V reaction
environment.
[0081] Methods of some embodiments of the invention may enable the
use of lower substrate temperatures than is typical in the state of
the art MOCVD due to the improved crystalline quality resulting
from the increased surface migration of metal adatoms. For example,
Sugihara et al. (Phys. Stat. Sol. (a) 176, 323 (1999), which is
entirely incorporated herein by reference) report 750.degree. C.
for their GaN growths, compared to approximately 1050.degree. C.
for typical MOCVD. Reduced processing temperatures relative to the
state of the art enable the use of less costly substrates and a
simplified reactor design.
[0082] Separation of the Group III and Group V deposition
environments necessarily creates a situation where the surface of
the substrate will have an excess of Group III atoms for a portion
of the deposition cycle. Group III metals, in particular Ga and In,
may form droplets on the substrate surface when the Group III
element is in excess of the stoichiometric amount of the Group V
element needed to form the desired III-V compound. For example see
D. Sugihara et al., and "Epitaxial Growth of GaN on Sapphire (0001)
Substrates by Electron Cyclotron Resonance Molecular Beam Epitaxy,"
S. H. Cho et al., Jpn. J. Appl. Phys. 34, L236 (1995), which are
entirely incorporated herein by reference. Group III metal droplets
which do not dissolve during processing and remain in the film may
damage the optoelectronic material properties and may be
incompatible with device manufacture.
[0083] Methods of some embodiments of the invention may enable
Group III metal droplet formation to be mitigated, if not
eliminated, by depositing a sub-monolayer coverage of the Group III
metal during each cycle. At this coverage, the Group III metal
thickness may be less than the wetting layer thickness, and metal
droplet formation may be suppressed. Additionally, sub-monolayer
coverage of metal adatoms reduces the number of interactions
between the adatoms, reducing their self-scattering and increasing
the adatom surface mobility.
[0084] In some embodiments, hydrogen-containing species may be used
during the formation of the Group III-V thin film on a substrate.
The hydrogen-containing species may be maintained in a separate
environment from the Group III and Group V reaction environments,
and may be employed after Group III environment, after Group V
environment, or both. In some embodiments, hydrogen-containing
species may be brought in contact with a substrate, a Group III
metal film, or a Group III-V film on the substrate in a reaction
space that is separate from a reaction space having a Group III or
Group V precursor. In an embodiment, an environment comprising
hydrogen-containing species may be employed sequentially after a
Group V reaction space, where excess Group III atoms or droplets
will be converted to Group III halides, e.g., GaH.sub.3. Group III
halides may be volatile, and droplets will be removed from the
surface each cycle. This allows the growth surface to be
stoichiometric at the beginning of each deposition cycle. In some
embodiments, an environment comprising hydrogen-containing species
may be employed sequentially after a Group III reaction space,
where the excited species of hydrogen may reduce multi-layer Group
III islands to islands of monatomic height (or thickness).
[0085] In some embodiments, an additional benefit of exposing the
film to hydrogen-containing species is that activated species of
hydrogen may scavenge (or getter) carbon atoms and/or hydrocarbon
groups, which are due to residual contamination from the alkyl
groups present in metal organic precursors. See, for example,
Bornscheuer et al., "Production of Atomic Hydrogen and its Use for
the Growth of GaN with Low Carbon Level," Phys. Stat. Sol. (a) 159,
133 (1997), and Kim et al., "Effect of Hydrogen on GaN Growth by
Remote Plasma-Enhanced Metal-Organic Chemical Vapor Deposition,"
Phys. Stat. Sol. (a) 176, 337 (1999), which are entirely
incorporated herein by reference. In some embodiments, excited
hydrogen-containing species, created by plasma excitation or by
thermal disassociation, may be used to scavenge and remove one or
more impurity species selected from carbon, sulfur and oxygen. This
may advantageously minimize, if not eliminate, further
contamination of the substrate or thin film over the substrate.
[0086] In some embodiments, an additional environment may be
maintained where thin film properties may be measured in-situ. In
this environment no deposition occurs and the optical viewports,
lens systems, spectroscopies, and other ports needed to make stable
and accurate measurements may be maintained. Measurements which
could be performed in this environment include, but are not limited
to, reflection-absorption infrared spectroscopy (RAIRS), low-energy
electron diffraction (LEED) spectroscopy, x-ray photoelectron
spectroscopy (XPS), Auger electron spectroscopy (AES), scanning
probe microscopy (STM, AFM), near edge x-ray absorption fine
structure (NEXAFS), spectral reflectance and transmission, single
wavelength reflectance and transmission, optical pyrometry (single
wavelength, dual wavelength, or using spectral radiometry),
emissometry, ellipsometry, surface light scattering, and optical
polarimetry. Thin film data gathered from measurements, such as
those made with the aid of tools (e.g., spectrometers) provided
herein, may include thickness, dielectric constant, conductivity
(or resistivity), optical attenuation constant, temperature, and
emissivity. Such data may be used in a real-time closed loop
control system, where aspects of the deposition environments may be
actuated to maintain the measured parameters within a specified
tolerance. This arrangement may provide for thin film property
optimization, device performance optimization, thin film deposition
parameter optimization (e.g., optimization of reaction space
pressures, deposition temperatures, and flow rates), run-to-run
repeatability, system-to-system repeatability and matching, and
improved yield of manufactured goods.
[0087] In some embodiments of the invention, controllers and
systems are provided for controlling and regulating Group III-V
thin film formation. In some embodiments, a control system is
provided to control various process parameters, such as, for
example, substrate and/or substrate holder (or susceptor)
temperature, reactor pressure, reaction space pressure, reaction
chamber pressure, plasma generator pressure, the flow rate of gas
(e.g., N.sub.2) into a plasma generator, the flow rate of gas
(e.g., metalorganic species) into a reaction space, the rate at
which the substrate is moved from one reaction space to another,
the rate at which the substrate rotates during thin film formation,
power to a plasma generator (e.g., direct current or radio
frequency power), and a vacuum system in fluid communication with
the reaction chamber. The vacuum system may comprise various pumps
configured to provide vacuum to the reaction chamber, such as,
e.g., one or more of a turbomolecular ("turbo") pump, a cryopump,
an ion pump and a diffusion pump, in addition to a backing pump,
such as a mechanical pump.
[0088] In some embodiments of the present invention, surfactant
species are deposited on the surface of the substrate either
before, after, or concurrently with the deposition of the Group III
metal precursor. An important physical process in the deposition of
MOCVD films is the diffusion of the metal atoms across the surface
to a reactive site such as an edge of a step or island. The energy
barrier for this diffusion can be significant. One method to
overcome this barrier is to increase the temperature of the
substrate. This is one reason why typical MOCVD processes require
deposition temperatures of about 1000.degree. C. and higher.
Another method to overcome this barrier involves the use of
surfactant species to lower the surface energy for diffusion of the
metal atom. The surfactant species will cover the substrate surface
and facilitate the diffusion of the arriving metal atoms across the
growth front and the incorporation of arriving metal atoms into the
growth front at a highly-coordinated surface site. Typically, the
surfactant species is chosen to be an atom that segregates easily
to the surface of the growing film. This ensures that the
surfactant species is not incorporated into the film. Examples of
surfactant species for the growth of III-V films comprise hydrogen,
chlorine, bromine, iodine, arsenic, phosphorus, bismuth, indium,
antimony, lead, tin, germanium, selenium, tellurium, magnesium,
cadmium, zinc, mercury, etc., or their combinations.
[0089] FIG. 5 presents a graph of the calculated average time in
seconds needed for a Ga atom to diffuse 5 nm as a function of
temperature illustrating the benefits of using a surfactant. The
calculation assumed that the surfactant decreased the surface
energy by about 20%. Note that this decrease in the surface energy
of about 20% is equal to increasing the temperature by about
300.degree. C. The graph was calculated from the expression
contained in Eqn. 1.
.tau. = .lamda. s 2 D s , 0 - E A , s / k B T Eqn . 1
##EQU00001##
[0090] Eqn. 1 is adapted from Koleske et al. (Journal of Applied
Physics, vol. 84, no. 4, pages 1998-2008, Aug. 15, 1998), which is
herein incorporated by reference, where .tau. is the time constant
to diffuse a distance .lamda..sub.S for Ga atoms on a GaN surface,
D.sub.s,0 is the diffusion coefficient, E.sub.A,s is energy barrier
to diffusion, k.sub.B is Boltzmann's constant, and T is the
substrate temperature in degrees Kelvin. In FIG. 5, .lamda..sub.S
is taken to be 5 nm; similar figures can be produced at various
values of .lamda..sub.S. In FIG. 5, the value of D.sub.s,0, is
taken to be 0.007 cm.sup.2s.sup.-1 and the value of E.sub.A,s is
taken to be 2.48 electron volts (eV) for the upper curve 510, where
the values are taken from Koleske et al. and curve 510 is
representative of the diffusion on a Ga adatom on GaN with no
surfactant. In the lower curve 520, E.sub.A,s is taken to be 2.0 eV
in the lower curve to demonstrate the surfactant effect. As
mentioned with respect to FIG. 5, a decrease in the surface energy
will allow a Ga adatom to diffuse the same distance in the same
amount of time, but at a lower temperature. MOCVD films grown at
lower temperatures (i.e. about 720.degree. C.) have increased
surface roughness when compared to MOCVD films grown at higher
temperatures (i.e. about 1050.degree. C.). This is largely due to
the exponential decrease in the adatom rate of diffusion on the
surface at the lower temperatures. To highlight the benefits of
using a surfactant species, an MOCVD GaN film of 1 .mu.m was grown
on sapphire at 720.degree. C. without a surfactant species. Atomic
force microscopy (AFM) was used to measure the surface roughness
over a 20 nm by 20 nm area. The root mean square (rms) roughness
was determined to be about 32.3 nm. A smaller area of 1 nm by 1 nm
was measured and the rms roughness was determined to be about 2.7
nm. A second MOCVD GaN film of 1 .mu.m was grown on sapphire at
720.degree. C. using a surfactant species comprising about 2 ML of
In. AFM was used to measure the surface roughness over a 20 nm by
20 nm area. The rms roughness was determined to be about 9.4 nm. A
smaller area of 1 nm by 1 nm was measured and the rms roughness was
determined to be about 1.0 nm. The surface roughness of the sample
prepared using the surfactant had a surface roughness that was
about 3 times smaller than the sample prepared without the
surfactant species. This highlights the benefits of using the
surfactant.
[0091] FIG. 6 presents a graph of the calculated reflectivity as a
function of wavelength from about 400 nm to about 1100 nm for three
cases: a bare sapphire substrate, a GaN layer, and a GaN layer with
2 ML of In. The reflectivity of the bare sapphire substrate is, in
comparison to the other two cases, constant over this wavelength
region. The reflectivity of the sample with 500 nm GaN grown on the
sapphire substrate exhibits oscillations due to interference from
the front and rear surfaces of the film. The reflectivity of the
sample with 500 nm GaN grown on the sapphire substrate and
including 2 ML of In on the surface also exhibits oscillations due
to interference from the front and rear surfaces of the film.
However, the calculation including the 2 ML of In also exhibits
increased reflectivity, especially at longer wavelengths. Although
small, this difference in reflectivity is significant and can be
used as an in-situ monitor during the process to measure the
surface coverage of the surfactant (i.e. In) and to ensure that the
desired amount of about 2 ML of the surfactant is maintained on the
surface.
[0092] FIG. 7 illustrates a schematic diagram illustrating four
processing regions, and example of a combinatorial configuration,
and a substrate according to some embodiments. FIG. 7 illustrates
an adaptation of the system illustrated in FIG. 4 wherein one of
the processing environments has been adapted to provide
combinatorial processing of the substrates. The identification
numbers for FIG. 7 correspond to similar items as described in FIG.
4 (i.e. substrate, 702, corresponds to substrate 402, etc.). For
brevity, the complete description will not be repeated for items
that are similar. Processing environment, 704, is illustrated as
having four processing regions, 716A-716D that can be used to vary
a set of processing parameters in a combinatorial manner. The
configuration illustrated in FIG. 7 would result in the deposition
of four materials on the substrate in a site isolated manner
wherein each site isolated region would appear as an arc on the
surface of the substrate. Examples of processing parameters that
may be varied using the configuration illustrated in FIG. 7
comprise, gas flow composition (i.e. a different material may be
deposited in each site isolate region), gas flow concentration
(i.e. a similar material may be deposited in each site isolate
region, but the concentration of the precursor may be varied by
adjusting a ratio of the precursor to an inert carrier gas), the
presence or absence of a surfactant, etc. This configuration is
advantageous during the development of thin films using a
surfactant as discussed previously. The benefits of the surfactant
will be influenced by the relative surface coverage of the
surfactant on the surface during the thin film deposition process.
The relative surface coverage of the surfactant will be influenced
by parameters such as temperature (a global process parameter--the
same for all regions of the surface during a particular
deposition), the pressure (a global process parameter), the flux of
surfactant precursor to the surface, the time that the substrate is
exposed in the processing environment, the composition of the
surfactant, etc. When a surfactant is used, thin films may be
formed using the techniques disclosed herein in a temperature range
between about 200.degree. C. and about 800.degree. C., and
advantageously between about 500.degree. C. and about 800.degree.
C. Similarly, each of the processing regions, 716A-716D, can be
used to vary the deposition parameters for the deposition of the
thin film material.
[0093] FIG. 8 illustrates a schematic diagram illustrating four
processing regions, and example of a combinatorial configuration,
and a substrate according to some embodiments. FIG. 8 illustrates
an adaptation of the system illustrated in FIG. 4 wherein one of
the processing environments has been adapted to provide
combinatorial processing of the substrates. The identification
numbers for FIG. 8 correspond to similar items as described in FIG.
4 (i.e. substrate, 802, corresponds to substrate 402, etc.). For
brevity, the complete description will not be repeated for items
that are similar. Processing environment, 804, is illustrated as
having two processing regions, 816A and 816B, that can be used to
vary a set of processing parameters in a combinatorial manner. The
configuration illustrated in FIG. 8 would result in the deposition
of two materials on the substrate in a site isolated manner wherein
each site isolated region would appear as an arc on the surface of
the substrate. Examples of processing parameters that may be varied
using the configuration illustrated in FIG. 8 comprise, gas flow
composition (i.e. a different material may be deposited in each
site isolate region), gas flow concentration (i.e. a similar
material may be deposited in each site isolate region, but the
concentration of the precursor may be varied by adjusting a ratio
of the precursor to an inert carrier gas), the presence or absence
of a surfactant, etc. This configuration is advantageous during the
development of thin films using a surfactant as discussed
previously. The benefits of the surfactant will be influenced by
the relative surface coverage of the surfactant on the surface
during the thin film deposition process. The relative surface
coverage of the surfactant will be influenced by parameters such as
temperature (a global process parameter--the same for all regions
of the surface during a particular deposition), the pressure (a
global process parameter), the flux of surfactant precursor to the
surface, the time that the substrate is exposed in the processing
environment, the composition of the surfactant, etc. When a
surfactant is used, thin films may be formed using the techniques
disclosed herein in a temperature range between about 200.degree.
C. and about 800.degree. C., and advantageously between about
500.degree. C. and about 800.degree. C. Similarly, each of the
deposition regions, 816A and 816B, can be used to vary the
deposition parameters for the deposition of the thin film
material.
[0094] FIG. 9 illustrates a schematic diagram illustrating four
processing regions, and example of a combinatorial configuration,
and a substrate according to some embodiments. FIG. 9 illustrates
an adaptation of the system illustrated in FIG. 4 wherein one of
the processing environments has been adapted to provide
combinatorial processing of the substrates. The identification
numbers for FIG. 9 correspond to similar items as described in FIG.
4 (i.e. substrate, 902, corresponds to substrate 402, etc.). For
brevity, the complete description will not be repeated for items
that are similar. Processing environment, 904, is illustrated as
having three processing regions, 916A-916C that can be used to vary
a set of processing parameters in a combinatorial manner, and two
purge regions, 918A and 918B that can be used to improve the
isolation between processing regions, 916A-916C. The configuration
illustrated in FIG. 9 would result in the deposition of three
materials on the substrate in a site isolated manner wherein each
site isolated region would appear as an arc on the surface of the
substrate. Examples of processing parameters that may be varied
using the configuration illustrated in FIG. 9 comprise, gas flow
composition (i.e. a different material may be deposited in each
site isolate region), gas flow concentration (i.e. a similar
material may be deposited in each site isolate region, but the
concentration of the precursor may be varied by adjusting a ratio
of the precursor to an inert carrier gas), the presence or absence
of a surfactant, etc. This configuration is advantageous during the
development of thin films using a surfactant as discussed
previously. The benefits of the surfactant will be influenced by
the relative surface coverage of the surfactant on the surface
during the thin film deposition process. The relative surface
coverage of the surfactant will be influenced by parameters such as
temperature (a global process parameter--the same for all regions
of the surface during a particular deposition), the pressure (a
global process parameter), the flux of surfactant precursor to the
surface, the time that the substrate is exposed in the processing
environment, the composition of the surfactant, etc. When a
surfactant is used, thin films may be formed using the techniques
disclosed herein in a temperature range between about 200.degree.
C. and about 800.degree. C., and advantageously between about
500.degree. C. and about 800.degree. C. Similarly, each of the
deposition regions, 916A-916C, can be used to vary the deposition
parameters for the deposition of the thin film material.
[0095] Although the foregoing examples have been described in some
detail for purposes of clarity of understanding, the invention is
not limited to the details provided. There are many alternative
ways of implementing the invention. The disclosed examples are
illustrative and not restrictive.
* * * * *