U.S. patent application number 12/151465 was filed with the patent office on 2008-12-04 for zinc oxide film and method for making.
Invention is credited to Isaiah O. Oladeji.
Application Number | 20080299411 12/151465 |
Document ID | / |
Family ID | 40088610 |
Filed Date | 2008-12-04 |
United States Patent
Application |
20080299411 |
Kind Code |
A1 |
Oladeji; Isaiah O. |
December 4, 2008 |
Zinc oxide film and method for making
Abstract
A method for depositing a solid film of ZnO onto a substrate
from a reagent solution includes a reservoir of reagent solution
maintained at a sufficiently low temperature to inhibit homogeneous
reactions within the reagent solution. The reagent solution
contains a source of Zn, a source of O, and multiple ligands to
further control solution stability and shelf life. The chilled
solution is dispensed through a showerhead onto a substrate. The
substrate is positioned in a holder that has a raised structure
peripheral to the substrate to retain or impound a controlled
volume (or depth) of reagent solution over the exposed surface of
the substrate. The reagent solution is periodically or continuously
replenished from the showerhead so that only the part of the
solution directly adjacent to the substrate is heated. A heater is
disposed beneath the substrate and maintains the substrate at an
elevated temperature at which the deposition of a desired solid
phase from the reagent solution may be initiated. The showerhead
may also dispense excess chilled reagent solution to cool various
components within the apparatus and minimize nucleation of solids
in areas other than on the substrate. The deposited film may be
annealed after deposition and may be doped to enhance selected
characteristics. The ZnO films made by the process have distinctive
electrical and optical properties and are suitable for a variety of
electronic and optical devices.
Inventors: |
Oladeji; Isaiah O.; (Gotha,
FL) |
Correspondence
Address: |
ROBERT J. LAUF
998 W. OUTER DRIVE
OAK RIDGE
TN
37830
US
|
Family ID: |
40088610 |
Appl. No.: |
12/151465 |
Filed: |
May 7, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60940797 |
May 30, 2007 |
|
|
|
Current U.S.
Class: |
428/620 ;
257/E29.094; 427/226 |
Current CPC
Class: |
H01L 29/22 20130101;
C23C 18/1279 20130101; H01L 21/02573 20130101; H01L 21/02628
20130101; C23C 18/1275 20130101; C23C 18/06 20130101; C23C 18/1283
20130101; C23C 18/1216 20130101; H01L 21/02554 20130101; H01L
21/02565 20130101; C23C 18/1241 20130101; H01L 21/68785 20130101;
H01L 21/67207 20130101; H01L 21/6838 20130101; Y10T 428/12528
20150115 |
Class at
Publication: |
428/620 ;
427/226 |
International
Class: |
H01L 29/12 20060101
H01L029/12; B05D 3/02 20060101 B05D003/02 |
Claims
1. A method for depositing a solid ZnO film onto a substrate from a
reagent solution comprising the steps of: a. providing a supply of
reagent solution containing a source of Zn, a source of O, and at
least two ligands maintained at a first temperature at which
homogeneous reactions are substantially inhibited within said
reagent solution; b. dispensing a controlled flow of said reagent
solution from a showerhead assembly; c. positioning said substrate
to receive at least a portion of said controlled flow of said
reagent over a selected area of said substrate; d. providing a
raised structure peripheral to said selected area whereby a
controlled volume of said reagent solution may be maintained upon
said substrate and replenished at a selected rate; and, e. heating
said substrate and said controlled volume of said reagent solution
upon said substrate to a second temperature, higher than said first
temperature, whereby deposition of zinc oxide from said reagent
solution may be initiated.
2. The method of claim 1 wherein said second temperature is at
least 60.degree. C. higher than said first temperature.
3. The method of claim 1 wherein said source of Zn comprises a
soluble Zn salt.
4. The method of claim 3 wherein said soluble Zn salt is selected
from the group consisting of: chlorides; nitrates; citrates;
sulfates; and acetates.
5. The method of claim 1 wherein said at least two ligands are
selected from the group consisting of: acetic acid;
triethanolamine; citric acid; potassium nitrilotriacetate; sodium
citrate; ammonium hydroxide; ammonium chloride; and hydantoin.
6. The method of claim 5 wherein said at least two ligands comprise
sodium citrate, ammonium hydroxide, and potassium
nitrilotriacetate.
7. The method of claim 1 further comprising the step of: f.
annealing said coated substrate at a selected temperature in a
selected atmosphere.
8. The method of claim 7 wherein said selected temperature is in
the range of 300 to 1000.degree. C. and said selected atmosphere is
selected from the group consisting of: Ar; N.sub.2; H.sub.2;
O.sub.2; air; and combinations thereof.
9. The method of claim 1 wherein said reagent solution further
comprises a soluble salt of a dopant species having a valence
different from that of Zn(II).
10. The method of claim 9 wherein said soluble salt is selected
from the group consisting of: In(NO.sub.3).sub.3; InCl.sub.3;
Ga(NO.sub.3).sub.3; Al(NO.sub.3).sub.3; Al.sub.2(SO.sub.4).sub.3;
salts of other group III elements; LiNO.sub.3; AgNO.sub.3; salts of
other group I elements; and salts of other group IV elements.
11. The method of claim 1 further comprising the step of: g.
treating said ZnO film by ion implantation of a species selected
from the group consisting of: In; Ga; Al; and N.
12. A multilayer structure comprising: a substrate material; and, a
thin film of ZnO deposited upon said substrate, said ZnO film
having a carrier concentration less than about 10.sup.15 cm.sup.-1
and a resistivity greater than about 600 .OMEGA.-cm.
13. The multilayer structure of claim 12 further comprising at
least two electrodes.
14. The multilayer structure of claim 13 wherein said substrate
material is metallic and said substrate comprises one of said at
least two electrodes.
15. The multilayer structure of claim 12 wherein said ZnO film has
sufficient optical transparency whereby a film 0.4 .mu.m thick
exhibits an average optical transmission of at least 95% over the
wavelength range from 400 to 1000 nm.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of Provisional
Application Ser. No. 60/940,797 filed by the present inventor on
May 30, 2007, the entire disclosure of which is incorporated herein
by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention pertains to a system and method for chemically
coating a variety of surfaces with zinc oxide thin films for
various applications including electronics, and to zinc oxide films
having novel properties.
[0004] 2. Description of Related Art
[0005] Numerous coating processes are commonly employed in
industrial applications, including electroless chemical, chemical
vapor and physical vapor depositions. Physical vapor deposition is
commonly used in semiconductor manufacturing applications, often
employing expensive vacuum techniques in order to sustain a
relatively high film growth rate. Many such processes, while
performed at high temperatures (e.g., greater than 300.degree. C.),
are non-equilibrium, often resulting in non-stoichiometric
proportions. Also, due to the nature of the deposition processes,
the deposited films often include relatively high defect densities.
In the case of semiconducting devices, such high defect levels can
limit electrical performance characteristics. In semiconductor
device fabrication wherein p-n junctions are formed in a partial
vaccuum by depositing one film over a second film or a substrate of
different conductivity type, the conventional evaporative and
sputtering techniques may provide unsatisfactory film qualities. As
an alternative, relatively more expensive techniques such as
Chemical Vapor Deposition (CVD), Molecular Beam Epitaxy (MBE),
pulsed laser deposition, and atomic layer epitaxy, are useful,
especially with formation of III-V compound semiconductor
materials, but satisfactory deposition processes have not been
available for fabrication of thin film III-VI compound
semiconductor materials.
[0006] Chemical bath deposition (CBD) is a low cost, low
temperature technique which, under certain limited conditions, can
provide high quality thin film growth of desired stoichiometry.
However, the technique has provided low growth rates, e.g., 20 to
30 .ANG./minute, and the grown film thickness per bath cycle is
limited to around 1000 .ANG. rendering it non-suitable for volume
manufacture. Generally, for products, such as solar diodes and
light emitting diodes, film thicknesses on the order of 3,000 to
20,000 .ANG. are needed. With traditional chemical bath deposition
techniques, efforts to increase film growth rates typically result
in degradation of film quality. Thicker films of higher quality,
however, can be attained by cyclic deposition of stacked relatively
thin layers. This, of course, is time consuming and generally can
be an expensive endeavor, wherein relatively expensive chemicals
are wasted due to deposition of film materials on unwanted surfaces
such as the equipment.
[0007] Conventional CBD is carried out with the source solution
held at slightly elevated temperature, typically .about.85.degree.
C. [see, for example, Oladeji and Chow, "Optimization of Chemical
Bath Deposited Cadmium Sulfide Thin Films," J. Electrochem. Soc.
144 (7): 2342-46 (1997)]. In this situation, heterogeneous
nucleation of the CdS film on the substrate must compete with
homogeneous nucleation of colloidal CdS particles within the
stirred reactant solution. Particulates represent not only a waste
of reagents but also a source of defects in the deposited film.
[0008] A process for depositing ultra-thin semiconductors is taught
by McCandless et al. in U.S. Pat. No. 6,537,845. The process uses a
premixed liquid containing Group IIB and VIA ionic species and a
complexing agent. The solution is applied to a substrate heated to
a temperature from 55 to 95.degree. C., forming an ultra-thin
(100-500 .ANG.) coating. For thicker coatings, the process can be
repeated. The process taught in '845 suffers from several
noteworthy shortcomings. First, using a single complexing agent
(generally taught to be NH.sub.4OH) prevents adequate process
control: at a low concentration the solution is so unstable that
unwanted homogeneous nucleation can occur, whereas at high
concentration the activation energy required to form the film
becomes so high that the claimed substrate temperature may not be
able to overcome it to cause a film growth. Second, the thickness
of film that can be grown in a single step is very small, so to
grow a film of 0.1 .mu.m or greater, multiple cycles are needed and
this will tend to introduce greater concentrations of defects. This
will also make the process cumbersome and less manufacturing
friendly.
[0009] Films have also been prepared using the flowing liquid film
process as taught by Ito et al. [Preparation of ZnO thin films
using the flowing liquid film method, Thin Solid Films 286: 35-6
(1996)]. The deposition process involved the reaction of zinc
chloride and urea at 70.degree. C. according to the reaction:
ZnCl.sub.2+NH.sub.2CONH.sub.2+2H.sub.2O.fwdarw.ZnO+2NH.sub.4Cl+CO.sub.2.
[0010] The use of NH.sub.2CONH.sub.2 as a single ligand will result
in excessive homogenous reaction, and ZnO film growth takes place
mostly by particulate adsorption. ZnO so formed is less transparent
and of little or no practical use. Furthermore, the side of the
substrate next to the incoming solution gets the full dose of the
solution, hence high growth rate, whereas the side of the substrate
at solution exit receives the least dose, hence low growth rate. As
a result, the film non-uniformity will increase with increasing
substrate size. Since all the flowing solution sees the heat
provided by the substrate, film growth will take place on the
substrate plus any part of the system on contact with the hot
growth solution, leading to material waste. If the particulate
generation happens in the enclosed chamber, these could be trapped
and get adsorbed unto the substrate, leading to poor quality
film.
[0011] In U.S. Pat. App. Pub. 2003/0181040 as taught by Ivanov et
al., films have also been prepared on semiconductor substrates
using a sealed chamber filled with growth solution maintained at
high pressure about 2 atmosphere and temperature at 0 to 25% below
the boiling point of the solution. The use of high pressure is
needed to keep the rigid substrate in place, and the cost
associated with this is not trivial. The relatively large volume of
solution in the chamber is heated completely by the heater located
outside the chamber or in the substrate holder or both. This
arrangement forces the hot growth solution to be in contact with
the substrate and substantial part of the chamber system, and
results in wasteful film growth on unwanted areas. The latter will
also increase the cost of keeping the chamber clean to prevent
particle build up. Subjecting a relatively large volume of growth
solution in chamber at high pressure and temperature will not only
encourage the desired heterogeneous reaction responsible for film
growth but also substantially increase the unwanted homogenous
reaction; the latter will lead to fast depletion and waste of
material, and this will in turn increase the production cost.
[0012] These facts were recognized by Ivanov, who through U.S. Pat.
No. 7,235,483, attempted to minimize the material waste, by having
the substrate heated and cooled instantaneously, and using the high
temperature only during the growth regime when it is needed,
especially during the bulk film growth step. For Cu interconnect,
the focus of that work, where 500 .ANG. or less Cu metal seed and
CoWP Cu capping layer are needed, the bulk deposition step may be
short enough to prevent quick depletion of chemicals. However, for
semiconductor film deposition needed for solar cells and other
optoelectronics applications where the required film thickness is
more than 1000 .ANG., the high temperature bulk deposition step
time will be longer and the instantaneous heating and cooling will
not help. Generally, in electroless semiconductor film deposition
catalyzed substrate surface is not required. One would therefore
expect the growth rate on the substrate as well as other parts of
the chamber system in contact with the hot solution to be about
same; the consequence of this is substantial material waste. This
will make the chamber cleanliness even a much bigger challenge. The
operating cost and waste management in using this system for
semiconductor film manufacturing will be prohibitive. It should
also be noted that this approach is only applicable to rigid
substrates; flexible substrates cannot be used.
[0013] The growth of ZnO films, in particular, is of great
technological interest and a number of techniques have been
reported. Methods include the following: radical-source molecular
beam epitaxy [see R. Hunger et al., Mat. Res. Soc. Symp. Proc. Vol.
668 (2001)]; pulsed laser deposition [see T. Oshima et al., Thin
Solid Films 435: 49-55 (2003); also Y Ryu et al., Appl. Phys.
Letters 83[1]: 7 (2003)]; metalorganic vapor phase epitaxy [see Y.
Ma et al., Journal of Crystal Growth 255: 303-07 (2003)]; and DC
reactive sputtering [see A. Mosbah et al., Surface and Coating
Technology 200: 293-96 (2005)]. In general, films deposited by
these techniques exhibit carrier concentrations ranging from about
5.times.10.sup.15 to 2.times.10.sup.18 and resistivities ranging
from 0.3 to 565 .OMEGA.-cm.
[0014] Objects and Advantages
[0015] Objects of the invention include the following: providing a
bath deposition apparatus capable of depositing ZnO on a substrate
while minimizing homogeneous nucleation of the same or similar
phase within the bulk of the fluid bath; providing a bath
deposition apparatus capable of depositing ZnO films having
improved physical, chemical, optical, or electrical properties;
providing a ZnO bath deposition process that is more easily
controlled; providing a ZnO bath deposition process that uses
reagents more efficiently; providing a ZnO bath deposition process
that allows localized heating of a relatively small volume of the
bath while maintaining the remainder of the bath at a relatively
lower temperature; providing a ZnO film having improved physical,
electrical, and optical properties; and, providing a ZnO film
deposited on a selected substrate whereby the film and substrate
may form part of an electrical or optical device. These and other
objects will become apparent on reading the specification in
conjunction with the accompanying drawings
SUMMARY OF THE INVENTION
[0016] In accordance with one aspect of the invention, a method for
depositing a solid ZnO film onto a substrate from a reagent
solution comprises the steps of:
a. providing a supply of reagent solution containing a source of Zn
and at least two ligands maintained at a first temperature at which
homogeneous reactions are substantially inhibited within said
reagent solution; b. dispensing a controlled flow of the reagent
solution from a showerhead assembly; c. positioning the substrate
to receive at least a portion of the controlled flow of reagent
over a selected area of the substrate; d. providing a raised
structure peripheral to the selected area whereby a controlled
volume of reagent solution may be maintained upon the substrate and
replenished at a selected rate; and, e. heating the substrate and
the controlled volume of reagent solution upon the substrate to a
second temperature, higher than the first temperature, whereby
deposition of zinc oxide from the reagent solution may be
initiated.
[0017] In accordance with another aspect of the invention, a
multilayer structure comprises: a substrate material; and, a thin
film of ZnO deposited upon the substrate, the ZnO film having a
carrier concentration less than about 10.sup.15 cm.sup.-3 and a
resistivity greater than about 600 .OMEGA.-cm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] A clear conception of the advantages and features of the
invention, and of the components and operation of exemplary systems
provided with the invention, will become more readily apparent by
referring to the drawings accompanying and forming part of this
specification, in which like numerals designate like elements in
several views. The features are not necessarily drawn to scale.
[0019] FIG. 1 illustrates schematically the general flow of
reagents in accordance with one aspect of the invention.
[0020] FIG. 2 illustrates typical process inputs and outputs for a
system controller suitable for use with the present invention.
[0021] FIG. 3 illustrates schematically a plan view of a substrate
holder and CBD chamber in accordance with one aspect of the present
invention.
[0022] FIG. 4 illustrates schematically a vertical section along
A-A' of the substrate holder and CBD chamber illustrated in FIG. 3
in accordance with one aspect of the present invention.
[0023] FIG. 5 illustrates schematically a vertical section of the
substrate holder and CBD chamber in accordance with another aspect
of the present invention.
[0024] FIG. 6 presents a process flow diagram for a two-layer film
stack prepared according to one aspect of the invention.
[0025] FIG. 7 illustrates schematically a number of deposition
chambers in accordance with the present invention arranged in a
cluster tool configuration.
[0026] FIG. 8 illustrates schematically another way in which
multiple processing chambers may be arranged.
[0027] FIG. 9 illustrates schematically a plan view (upper) and
cross-sectional view (lower) of an embodiment of the invention
adapted for deposition onto a continuous flexible substrate.
[0028] FIG. 10 compares the transmission spectra of a ZnO film
grown by the inventive method, as grown and after annealing at
400.degree. C.
[0029] FIG. 11 shows the X-ray diffraction pattern of one example
of the inventive ZnO film after annealing at 400.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
[0030] There has been a need for a chemical system that can create
high quality films, e.g., semiconductor films, at high formation
rates, while providing a relatively uniform film thickness across
the entire substrate surface over which the film is formed. In
accordance with several embodiments of the invention, such a high
quality semiconductor film is formed by a replacement reaction
wherein the system provides a continuous or replenishable supply of
chemical processing solution. In several examples, the solution
reacts about the surface of a heated substrate. The substrate
temperature may be controlled to exhibit a substantially uniform
temperature across the surface. That is, temperature differentials
along the surface over which the film is formed are limited in
order to effect a substantially constant reaction rate along the
surface, thereby assuring a relatively uniform film growth rate.
Further, the pH and composition of the solution may be continuously
monitored and maintained to improve the stability of the process
and, hence, the quality of the deposited film, e.g., stoichiometry,
defect density, uniformity and consistency of dopant distribution.
The system may be operated above atmospheric pressure to increase
the rate of film growth.
[0031] FIGS. 1-4 illustrate simplified schematic diagrams of a Film
Growth System (FGS). In some of the examples that follow, the
illustrated films comprise II-VI compound materials but the system
is useful for a wide variety of film compositions and numerous
other embodiments are contemplated in order to effect film growth
according to the principles described herein. The system of FIGS.
1-4 and other embodiments are shown with specific geometries but
these are merely exemplary. For example, the shapes of substrates,
chambers, chamber interiors, shower heads and numerous other
features described herein may be readily modified without departing
from the spirit and scope of the invention. It is also to be
understood that, throughout the written description and the
figures, numerous like components common to different embodiments
are designated with similar names and, in such cases, descriptions
and functions described for a component with reference to one or
more embodiments are applicable to similarly named components of
other embodiments.
[0032] As used herein the term film forming surface means the
surface of a workpiece, e.g., a substrate, over which a desired
film is grown. As described below, the film forming surface may be
the surface of a substrate that faces a showerhead in order to
receive a direct flow or spray of chemical processing solution as
illustrated for several embodiments herein. Further, the
illustrated embodiments are described with reference to an
exemplary orientation wherein the reaction chamber receives the
workpiece or substrate in a mounting such that the workpiece is in
a level position with respect to a major surface, e.g., during the
film growth process the surface over which a film is grown is
level. The illustrated chambers are also depicted as having bases,
showerheads and lids which are positioned in a level orientation
during film growth processes. However, other orientations of the
substrate and various components may be preferred for other
embodiments and applications without departing from the spirit and
scope of the invention.
[0033] The FGS of FIG. 1 includes a reaction chamber 10 and a
chilled solution reservoir 11 for retaining and chilling chemical
processing solution which may be held at a stable, low temperature
preferably less than about 25.degree. C. and more preferably in the
range of 10 to 25.degree. C. or lower. At the preferred
temperatures, homogeneous reactions are substantially inhibited and
the solution has adequate shelf life and minimal homogeneous
precipitation of particulates. The FGS further includes a solution
supply subsystem for moving the chemical processing solution (while
maintained at a low temperature) from the reservoir to the chamber,
and a solution return subsystem for cycling partially spent
solution from the chamber back to the solution reservoir. The
solution supply subsystem includes a supply line filter F2 for
removing particulates from the solution, a supply line pump P2 for
supplying the solution to the reaction chamber, and a liquid
control panel 12 under the direction of a system controller. The
liquid control panel 12 includes a series of conventional valves
and flow controllers for selectably injecting and varying flows of
liquids into the processing chamber 10. For example, the liquid
control panel 12 may deliver the chemical processing solution and
one or more chemicals for chamber cleaning or for cleaning of the
surface of the substrate over which a desired film is grown.
[0034] Chemical processing solution from the reservoir is supplied
to the liquid control panel by the supply line pump after passing
through the supply line filter F2. The solution return subsystem
includes a plurality of valves for controlling flows of various
liquids from the processing chamber, a return line reservoir 13, a
return line filter F1 for removing particulates from the returning
solution, and a return line pump P1 for sending the filtered
solution to the process solution reservoir 11. Proper control and
regulation of the liquid flowing through the liquid control panel
is effected by the flow controllers under the direction of the
system controller. Solution reservoir 11 may optionally include
such familiar components as high and low level switches 14, 15,
temperature sensor 16, pH meter 17, and chemical analyzer 18.
[0035] The reaction chamber 10 illustrated schematically in FIGS.
3-4 includes a substrate holder assembly 31 with clamps 32 arranged
to secure substrate 33 under spacer 35, a delivery system that
includes a showerhead 41 for supplying and distributing processing
solution within the chamber and over a substrate (workpiece) 33.
The substrate holder assembly 31 comprises an insulative housing 43
with a heater block 44 formed therein. The combination of the
heater block and an upper insulative surface (over which a
substrate is placed) is referred to herein as a platen. Various of
these components are illustrated as having circular or rectangular
shapes, but the functionality of the reaction chamber is not
limited to these example geometries. A continuous spacer 35 may be
positioned about the upper, exposed surface of the workpiece, e.g.,
along the periphery thereof or beyond the periphery. In other
embodiments the continuous spacer may be replaced by a spacer
including apertures or slots or may be replaced by several
individual spaced-apart spacers wherein the separation facilitates
exit flow of processing solution from the reaction region adjacent
to the exposed upper substrate surface through the solution return
subsystem to the chilled reservoir 11. The chamber may further
include a pressure gauge, a gas inlet 42, a gas outlet 42', and a
drain line 34 which is part of the return subsystem that directs
partially spent processing solution from the chamber 10 to the
reservoir 11. In this example, to minimize maintenance and extend
useful life of the chamber, all chamber parts that are exposed to
the chemical solution are preferably made of chemically inert
material, such as polytetrafluoroethylene (PTFE) or perfluoroalkoxy
(PFA), or the surfaces of such parts may be coated with a PTFE or
PFA film. For example, the spacer 35 (positioned about the
substrate to create a shallow catch region or substrate opening 49
for retaining flowing portions of the processing solution over the
substrate surface) may be formed of a relatively rigid material
such as PTFE, PFA or PFA-coated or PTFE-coated metal. The
illustrated continuous spacer 35 encloses an area of the substrate
surface that may be continuously, continually or periodically
replenished with processing solution, as solution previously
supplied from an overlying showerhead overflows or pours outward
from the volume enclosed by the spacer 35 and passes into the drain
line 34.
[0036] It will be appreciated that the operation of spacer 35
differs fundamentally from the "containment frame" of McCandless et
al. '845 in that McCandless contemplates a substantially static
containment of a fixed volume of solution, whereas the present
invention relies on continuous or periodic replenishment using
chilled solution from the showerhead. Applicant uses this novel
feature to further cool adjacent hardware components as well as
cool the solution in areas other than the immediate deposition
layer in order to more effectively suppress homogeneous
nucleation.
[0037] To prevent leakage of various process liquids or gases,
seals or O-rings 48 may be placed at various locations where two
components face one another, as is well known in the art.
[0038] Other flow means are contemplated, including provision of
flow lines about the spacer(s) to actively or passively remove
partially spent chemical solution from regions overlying the
substrate. For reactions performed under atmospheric conditions,
processing solution overflowing from the spacer volume above the
substrate is collected at the bottom of the chamber and
continuously drained. The processing chamber may be filled or
continuously purged with filtered air or nitrogen for controlling
air-born particles during deposition. To effect this, a gas supply
valve may be positioned in the gas supply line 42 to control
injection of the air or inert gas into the chamber while an exhaust
valve is positioned in the gas exhaust line 42'.
[0039] A feature of the FIG. 1 embodiment is that the rate of film
formation, e.g., the rate of growing II-VI semiconductor materials,
in the FGS is relatively high while the ratio of the volume of
processing solution about the film formation surface on the
substrate to the area of the film formation surface over which the
film is formed is small, i.e., small relative to the corresponding
ratio present in a conventional, traditional chemical bath. The
foregoing statement is made with the understanding that the
geometry and orientation of the substrate within a traditional
chemical bath is typically different than that of the FGS since the
traditional bath typically immerses the substrate in a heated
vessel containing processing solution while the FGS positions and
heats the substrate to receive a supply of processing solution,
e.g., via a continuous or continual or periodic flow such as from
above. Thus a direct comparison of such ratios is not easily
ascertained. Nonetheless, according to the invention, the
applicable volume-to-surface area ratio is effectively
substantially smaller than that which would be present in a
traditional bath, e.g., when the substrate is positioned in a
horizontal or vertical orientation inside a vessel filled with
processing solution or otherwise oriented with respect to a level
solution surface. For operation in the FGS, the ratio of the volume
of processing solution about the film formation surface of the
substrate to the area of the substrate surface over which the film
is formed is referred to herein as the Volume-to-Surface Area Ratio
(VSAR). When the VSAR is high, a relatively small fraction of the
processing solution is effectively contributing to the film
formation while the remaining portion may result in formation of
colloids in the bulk of the solution. In the FGS the spacer allows
a limited volume of processing solution to be positioned above the
substrate surface. The spacer may appear like a shallow tank, but
actually serves as a temporary, periodically replenished collection
region, as portions of the processing solution remain over the
substrate surface for brief periods in order to effect chemical
reactions along the exposed surface. The exemplary continuous
spacer is secured along the periphery of the substrate with a
multi-damp design (shown in FIGS. 3 and 4) which allows the
solution to flow over the spacer. Thus, the volume of chemical
processing solution retained within the spacer confines is
generally determined by the height of the spacer above the
substrate, ranging, for example, between 0.1 and 10 mm. Depending
on the specific process, setting of process parameters (e.g.,
substrate temperature, flow rate of processing solution,
concentration of reactants in the solution) a spacer with a
suitable or optimum height may be chosen. Due to the complexity of
multiple variables, and the desire to maximize the rate of film
formation, it may be necessary to experimentally determine process
geometries and settings. The FGS may be programmed to deliver a
continuous or periodic flow of processing solution through the
shower head, or may be programmed to deliver metered volumes, to
provide a uniform delivery of the solution over the film formation
surface. With a continuous spacer formed over a uniformly heated
substrate and a continual or periodic replenishment of processing
solution over the reaction region of the substrate, a low VSAR
effects a relatively uniform film growth over the entire region of
film growth at a relatively high growth rate suitable for volume
manufacture. Among the several embodiments disclosed herein,
exemplary VSARs range from 0.1 to 10 mm, with exemplary
corresponding film growth rates ranging from 100 to 1000
.ANG./minute and film thicknesses of satisfactory uniformities
(e.g., less than 10 percent) may exceed several microns.
[0040] Another feature of the invention is that during the film
formation process the film growth surface may be kept at a
relatively high temperature while other surfaces within the
reaction chamber are relatively cool. This feature can minimize or
prevent formation of film on surfaces other than that of the
substrate. By way of example, the temperature differential between
the growth surface and other surfaces in the reaction chamber can
range from 60.degree. C. or 70.degree. C. to 200.degree. C. and
may, for example be 140.degree. C. The chamber pressure, the
solution flow rate, and the substrate temperature may be adjusted
to achieve deposition rates on the order of 500 .ANG./min to
produce a relatively uniform film having a thickness on the order
of one to five microns. The achievable uniformity (i.e., measurable
based on variation in film thickness) over a film growth area of 21
cm.sup.2 is generally less than 10% and in some instances less than
5%. As shown generally in FIG. 1, during operation of the FGS the
processing solution passes from within the volume defined by the
spacer 35 to the solution return line and the associated reservoir
13 in order to be pumped back into the solution reservoir 11 for
chilling and recirculation until such time that the requisite
chemicals are so spent that the solution should be replaced. In the
process of passing into the return line, the processing solution
leaving the volume defined by the spacer may join other portions of
processing solution which are relatively cool. That is, such other
portions of processing solution are injected through the showerhead
and into regions of the chamber outside the region of the heated
film growth surface such that these portions are relatively cool.
The latter, relatively cool solution may mix with portions of the
solution flowing from the spacer volume to facilitate temperature
reduction of the heated solution, thereby limiting formation of
undesirable products in the reaction chamber. The solution
reservoir may include a temperature sensor 16, a chemical analyzer
18, a pH meter 17, and a pair of liquid level switches 14, 15. The
temperature of the solution reservoir is controlled by the chiller
and the reservoir is typically maintained at a temperature in the
range of 10-25.degree. C. A chemical analyzer 18 is connected to
the solution reservoir to monitor the chemical composition of the
reservoir constantly or periodically. When needed, the pre-mixed
processing solution is added to the solution reservoir from a tank
containing pre-mixed solution of desired concentration, although
the solution may be stored in a more concentrated form and diluted
upon or prior to entry into the reservoir. For example, the system
controller may operate valves associated with the liquid control
panel to dispense distilled (DI) water and precursor solutions
directly into the reservoir for mixing in the solution reservoir.
In some chemical processes, deposition rates are significantly
influenced by the pH of the solution. The pH meter 17 monitors the
pH of the solution bath and the pH may be adjusted under direction
of the controller. Liquid level in the reservoir is controllable
with first and second liquid level sensor switches. The first
liquid level sensor switch 15 provides a signal to the controller
when a minimum desired level of solution resides in the reservoir.
The second liquid level sensor switch 14 provides a signal to the
controller 20 when a maximum level of liquid resides in the
reservoir.
[0041] The entire system may operate under direction of the system
controller, which actuates numerous valves and switches in response
to sensor information, e.g., signals provided by level switches.
The system controller controls delivery of processing solution to
the reaction chamber via the solution flowmeter and may also
selectively rotate the showerhead during the chemical process. A
thermocouple is provided to control the platen temperature during
operation. The controller also controls delivery of pre-mixed
processing solution to the solution reservoir to maintain suitable
level of processing solution in the reservoir.
[0042] For periodic maintenance of the processing chamber, a
chemical solution for chamber cleaning may be supplied from a
cleaning chemical tank via a cleaning chemical supply line. DI
water for rinsing the chamber after chamber cleaning may be
supplied from a DI water tank via a DI water supply line. Delivery
of cleaning chemical is controlled by a cleaning chemical supply
line valve. Delivery of DI water is controlled by a DI water supply
line valve. A separate drain under control of a dedicated valve may
be provided to collect the chamber cleaning chemical and rinse
water. Generally, the flow of drain liquid from the chamber is
controlled by a combination of the solution return line valve and a
cleaning chemical return line valve under direction of the
controller. The collected cleaning chemical may be sent to a
chemical recovery processing unit for recovery of chemical
ingredients.
[0043] In the subject chemical film growth processes, the formation
rates may be increased as a function of pressure. It will be
appreciated that chamber 10 may be maintained at elevated pressure
using filtered air or nitrogen injected into the chamber via the
gas inlet 42. Chamber pressure may continuously monitored by the
pressure gauge 19.
[0044] FIG. 3 provides a simplified plan view of the chambers shown
in FIG. 1. The exemplary chamber 10 comprises a circular housing
and a square substrate holder 31 for processing a square substrate
33. In other embodiments, the chamber may comprise a circular
substrate holder and a circular chamber housing for processing a
circular substrate. In still other embodiments, a chamber may
comprise a square substrate holder in a square chamber housing. The
illustrated substrate holder assembly includes a clamping unit
having a plurality of clamps 32 coupled to a continuous spacer 35.
The exemplary clamping structure shown in FIG. 3 includes four
clamps. Each clamp is connected to the chamber bottom or base
region by a substrate holder screw (shown in FIG. 4). In an
automated system, the clamping unit may be operated by one or more
actuator mechanisms for automatic clamping of the spacer onto the
substrate and releasing of the spacer from the substrate. The
actuators may be of any familiar type such as hydraulic, pneumatic,
or electromechanical devices. The substrate holder assembly further
includes a plurality of substrate holder screws which are connected
to the chamber bottom wall (shown in FIG. 4) for securing the
substrate holder assembly. A drain 34 is provided at the chamber
base for recirculation of processing solution to the solution
reservoir and disposal of cleaning chemical and rinse water during
periodic chamber cleaning.
[0045] FIG. 4 depicts a cross-sectional view of the FGS reaction
chamber taken along line A-A' of FIG. 3. The chamber includes a
lower chamber wall having a top surface, a showerhead plate 41
having an upper surface and a lower surface, a chamber lid having
an upper surface and a lower surface configured for placement
against the top surface of the lower chamber wall to effect a
sealing arrangement during operation of the chamber, and a
substrate holder assembly for holding a substrate during
deposition. The showerhead plate 41, having a plurality of openings
in a central region thereof is positioned along a lower surface of
the chamber lid so that it is positioned at a level coincident with
the top surface of the lower chamber wall during chamber
operations. The showerhead may be secured by a showerhead clamping
unit comprising showerhead plate nuts and shower head plate screws
or may be secured with a quick release clamping mechanism for an
automated operation. A showerhead O-ring may be placed between the
lower surface of the showerhead and the top surface of the lower
chamber wall to facilitate creation of an air-tight seal when the
chamber is closed with the lid. The chamber lid is placed over the
showerhead plate and secured by a chamber lid clamping unit
comprising an upper chamber lid nut, a lower chamber lid nut and a
chamber lid screw. The lower chamber wall includes a gas inlet 42,
a gas outlet 42', and a processing solution drain 34.
[0046] Depending on the specific process, the substrate can be
heated to some desired temperature during the deposition process. A
feature of the invention is that the film growth rate in the FGS is
exponentially dependent upon temperature, and it is therefore
beneficial to keep only the substrate at a high temperature while
chamber surfaces are kept at a low temperature for efficient,
selective deposition. In the example embodiments the platen 44 is a
PTFE or PFA coated thermally conductive block (e.g., formed of
copper or aluminum) and is positioned along or near an upper
surface of the insulating substrate holder housing 43 to provide
uniform heating of the substrate while other portions of the
housing are maintained at low temperatures. The substrate holder
housing 43 may be made of PTFE or ceramic coated with PTFE or PFA
or may be an actively cooled metal that is coated with, for
example, PTFE or PFA. Such an active cooling arrangement can assure
that surfaces of the substrate holder housing are kept at a
relatively low temperature to prevent film formation thereover. At
the same time, for example, the substrate may be heated by one or
more heating elements 45 embedded in the thermally conductive
platen 44. The thermally conductive platen 44 can be resistively
heated by applying an electric current from an AC power supply 46
to the heating elements 45. The substrate 33 is, in turn, heated by
the platen. The conductive platen may be made of CVD-SiC or made of
high conductivity metal like Cu or W--Cu alloy or Al coated with
PFA or PTFE to protect the platen from chemical reaction with
process solution. Although not illustrated, the heating elements
used to control the temperature of the film formation surface may
alternatively be formed in a horizontal orientation and may be in
an array of parallel elements or a two dimensional matrix or
mesh-like design to facilitate uniform generation and distribution
of thermal energy and achieve relatively uniform temperature across
the film-forming surface. Radiant heat generation is also
contemplated, e.g., based on positioning of radiant sources above
or adjacent the film forming surface. For the illustrated
embodiments, a temperature sensor 47, such as a thermocouple, is
also embedded in or near the platen to monitor the temperature of
the platen or substrate in a conventional manner. The measured
temperature is used in a feedback loop to control the power
supplied to the heating element 45, whereby the substrate
temperature can be maintained or controlled at a desired
temperature suitable for the particular process application.
[0047] The showerhead 41 which introduces a processing solution
over the substrate surface 33 is located above the substrate holder
assembly 31. The showerhead 41 receives processing solution and
other liquids based on the configuration of the liquid control
panel, which directs the supply of various liquids used in
different steps of the process sequence or used in different steps
of the chamber cleaning sequence. The showerhead allows processing
solutions from the liquid control panel to be uniformly introduced
and distributed over the substrate holder assembly. In the
illustrated examples, the showerhead dimension is sufficiently
larger than the heated platen and the substrate to provide a flow
of cool processing solution along regions about the substrate
holder housing adjacent the platen. Optionally a cooling channel
may be formed inside the substrate holder housing to further limit
temperature elevation beyond the film forming surface of the
substrate. In FIG. 4, an exemplary mechanical clamping unit
comprising an upper clamp nut, a lower clamp nut, and a clamp screw
is also illustrated. The mechanical clamp unit mechanically presses
the spacer against the substrate. In an automated system, the
mechanical clamping unit may be operated by one or more mechanical
actuators.
[0048] FIG. 4 shows an embodiment in which the showerhead 41 is
substantially fixed in position relative to the substrate holder
assembly 31. It will be appreciated that in other contemplated
embodiments, it might be desirable to use a moving, rotating, or
oscillating showerhead as another means of ensuring uniform flow of
process liquid over the entire substrate surface. Skilled artisans
can easily add such modifications without undue experimentation
using electric motors, mechanical linkages and the like as are well
known in the art.
[0049] Embodiments of the reaction chamber 10 have been illustrated
with exemplary fasteners for attaching components to one another
and for attaching the workpiece within the reaction chamber for
processing. It will be appreciated that a vacuum chuck may also be
used to secure the substrate 33 to platen 44. Other designs are
contemplated. The cross sectional view of FIG. 5 illustrates
another exemplary embodiment of an FGS process chamber. In this
embodiment, the substrate holder 31' incorporates a vacuum chuck
including vacuum orifices 54 to secure substrate 33. A ring 51
serves the function of spacer 35 (i.e., defining a volume of
entrapped process liquid above the substrate. Substrate holder 31'
further incorporates a cooling jacket 52 maintained by the flow of
coolant through inlet 53 and outlet 53'. The showerhead assembly
41' further incorporates a temporary solution reservoir 55, from
which process fluid is dispensed through capillary tubes 56 and
microswitch valves 57. Solution is added to temporary reservoir 55
through inlet 58. An outlet 59 provides pressure relief so that the
pressure within reservoir 55 is held constant.
[0050] It will be appreciated that the pattern of openings in the
showerhead may be relatively uniform across the face or it may be
nonuniform. For instance, the showerhead may include two concentric
zones: an inner zone centered over the spacer seal for dispensing
processing solution on to the film growth surface of a substrate;
and an outer zone for dispensing additional processing solution to
cool portions of the processing chamber outside the spacer opening
which receives the substrate. The inner showerhead zone primarily
directs processing solution to the film growth surface to effect
chemical reactions which lead to film growth. Relatively hot,
partially spent solution exits the spacer volume as new cool
solution is dispensed therein. The outer showerhead zone primarily
directs cool solution to regions of the chamber beyond or outside
the spacer opening. This flow of relatively cool solution serves to
cool sides of the platen and portions of the substrate holder
assembly which may be heated incidental to the heating of the
substrate. The same flow can mix with the relatively hot, partially
spent solution that exits the spacer volume. The combined flow
exits the chamber 10 via drain 34 may return to solution reservoir
11 such as shown in FIG. 1. The returning solution may be
pre-cooled prior to entry into the reservoir 11.
[0051] It will be understood that flows through the inner and outer
zones of the showerhead may be under separate control to optimize
process parameters and quality of film growth. The inner zone of
the showerhead may receive chemical processing solution from a
first inlet. The system controller 20 directs the volume or
volumetric flow rate and frequency (if not a continuous flow) for
dispensing of solution into the spacer opening through the inner
showerhead zone based on, for example, specific chamber geometries,
a selected VSAR, the film forming chemistry and the reaction
rate--all to achieve a satisfactory quality of film formation. Flow
through the inner zone is controlled and dispenses the solution
over the spacer seal to react along the film growth surface of a
substrate. The outer zone of the showerhead may receive chemical
processing solution from a second inlet. The system controller
directs the volume or volumetric flow rate and frequency (if not a
continuous flow) for dispensing of solution through the outer
showerhead zone and into regions of the chamber primarily outside
the spacer opening based on, for example, specific chamber
geometries, heat transfer characteristics of the components being
cooled, temperature stability of the partially spent processing
solution. Parameters may be optimized to minimize formation of
undesirable precipitates in solution being cycled back to the
reservoir and to minimize or eliminate formation of film on, for
example, walls or other surfaces in the reaction chamber.
[0052] In one example of process conditions when the chamber is
operated with the dual zone shower head, the inner zone may be
connected to a metering pump which dispenses solution over the
substrate opening every 20 seconds in a volume which is one to two
times the spacer volume. The volume dispensed may be a multiple of
the spacer volume to quickly quench the partially spent solution
and assure complete replenishment of processing solution in the
spacer volume. At the same time, the outer zone may be programmed
to dispense, with the assistance of a pump, a continuous flow of 30
to 100 ml/min (for a chamber designed for coating a 5.times.5 cm
substrate) in regions outside of the spacer opening to cool the
chamber surfaces and the solution which flows over the spacer.
[0053] Other arrangements which provide further optimization of
process conditions include provision of processing fluid from the
second inlet at a lower temperature than processing fluid delivered
from the first inlet so that the outer zone of the showerhead
provides solution with greater cooling capacity while the inner
zone provides solution which requires less heating in order to
effect desired reactions along the film growth surface. That is,
the solution can be cooled to a temperature which assures
sufficient stability while being transferred and dispensed but
which minimizes the amount of heat generated to sustain the growth
surface at a minimum desired temperature. For example, the
temperature of solution exiting the inner zone of the shower head
may be in the range of 20-25.degree. C. while the temperature of
solution exiting the outer shower head may be in the range of 10 to
15.degree. C. or lower. Optionally a cooling channel may be formed
inside the substrate holder housing to further limit temperature
elevation beyond the film forming surface of the substrate. Thus it
can be seen that the inventive apparatus and method provides the
user with wide latitude to adjust the operating conditions for
particular purposes.
[0054] In some applications, more than one layer of semiconductor
material may be formed on a substrate by a sequence of chemical
processes. Referring to FIG. 6, there is shown a flow chart of
exemplary steps that sequentially place multiple layers of material
on the substrate. Formation of two layers in a sequence may include
a clean step for removing contaminants from the substrate surface,
followed by formation of a first layer. Next, a first post-film
formation clean step is performed for removing residual chemicals
from the substrate surface, followed by a step for forming a second
film layer different from the first layer. This is followed by a
second post-film formation clean step for removing residual
chemicals from the substrate surface. In the pre-film formation
clean step, a substrate placed on the substrate holder assembly
inside the chamber is cleaned with a first cleaning solution,
rinsed with de-ionized (DI) water, and dried, for example, by a
spin drying method as is common in semiconductor device
manufacturing. Next, a first semiconductor layer is formed along
the surface of the substrate, followed by a postfilm formation
clean wherein the substrate is cleaned with a second cleaning
solution, rinsed with DI water for a suitable duration and dried.
For multi-layer film formations, a system comprising a plurality of
process chambers and a substrate handling robot can offer high
operation efficiency and throughput resulting in a relatively low
equipment cost per film layer. A single robot may provide substrate
handling for multiple processing chambers.
[0055] FIG. 7 depicts an exemplary multi-chamber system
incorporating one or more of the chambers illustrated in the
foregoing figures and comprising a substrate load/unload unit, an
optional loadlock chamber, a plurality of processing chambers, and
a substrate handling robot. Each processing chamber may be
dedicated to either a reaction or a clean. In the exemplary system
of FIG. 7, the processing chambers include a first clean chamber, a
first reaction chamber, a second reaction chamber, a third reaction
chamber, and a second clean chamber. In operation of the
multi-chamber system of FIG. 7, a substrate cassette loaded with
one or more substrates is placed in the substrate load/unload unit.
The substrate handling robot removes a substrate from the
load/unload unit and places it in a selected process chamber under
direction of a system controller (not shown). After deposition is
complete, the robot picks up the substrate from the processing
chamber and transfers the substrate to another chamber or to the
substrate cassette in the substrate load/unload unit. In other
embodiments, the substrate load/unload unit may handle a plurality
of substrate cassettes for higher operational efficiency or may
simultaneously process wafers in different chambers. To provide
high pressure chemical film growth conditions in the system of FIG.
7, the loadlock chamber may be configured to include or movably
receive the substrate storage cassette. With such an arrangement,
higher system operating efficiencies can result as the frequency of
pressure equalization for substrate transfer between the substrate
load/unload unit and the individual chambers is reduced. In other
embodiments, some chambers or all chambers may be equipped to
perform both chemical processing and cleaning.
[0056] FIG. 8 depicts another exemplary multi-chamber FGS according
to the invention. The multi-chamber system comprises a substrate
load unit, a load chamber, a substrate unload unit, a plurality of
processing chambers, and a substrate handling robot positioned to
move substrates between units and one or more chambers. The
processing chambers include a first dean chamber, a first reaction
chamber, a second reaction chamber, a third reaction chamber, a
fourth reaction chamber, and a second clean chamber, each chamber
being dedicated to either chemical film growth or a cleaning
operation. A substrate cassette loaded with one or more substrates
is placed in the substrate loader. The substrate handling robot in
the load chamber picks up a substrate and places in a selected
process chamber by a system controller (not shown). After
deposition is complete, the robot picks up the substrate from the
processing chamber and transfers the substrate to the substrate
cassette in the substrate unload unit. In other embodiments, the
substrate load unit and unload unit may handle a plurality of
substrate cassettes for higher operational efficiency.
[0057] It will be appreciated that in some instances it is
desirable to deposit a film onto a somewhat continuous, flexible
substrate 33' in a roll-to-roll configuration. The inventive
apparatus may be modified as shown schematically in FIG. 9 by
modifying the spacer assembly 35' to form a thin framework that can
be lowered against a selected portion of the substrate 33' to
secure the substrate against the heater block 44 and define the
volume of solution held over the area of deposition 49. Any
conventional means may be used to provide a fluid seal when the
spacer assembly 35' contacts the film, such as gaskets, O-rings or
various compliant materials. The substrate may be handled by any
conventional roll-to-roll handler, which may preferably include
tensioning rolls and drive mechanisms on either the feed roll or
the take-up reel, or both. Vacuum delivered via one or more
orifices 54 may be used to hold substrate 33' flat against heater
block 44. Insulation 61 may be placed between heater block 44 and
cooling jacket 52.
[0058] The operation of the system illustrated in FIG. 9 is similar
to that of other embodiments, with the following differences:
First, the flexible substrate 33' is fed from the feed roll and
secured to the take-up roll. Second, the spacer assembly 35' is
lowered against the substrate, securing substrate 33' against the
heater block and defining the area upon which film deposition will
occur. Deposition is carried out in the same manner as in the
previously-described embodiments. When a film of adequate thickness
has been deposited, spacer assembly 35' is raised and the substrate
is indexed forward to bring a new area under the spacer assembly
for the next deposition.
[0059] It will be appreciated that downstream operations such as
washing, drying, heat treatment, dicing, etc., may be carried out
in any desired order and may be done while the substrate is still
continuous or may be done after individual "panes" have been diced
from the roll. Heat treatment, in particular, may be carried out by
passing the continuous film through an oven, microwave cavity, or
radiant heater zone, with the final take-up roll located at the
opposite end of the heating apparatus. Conventional means may be
employed to buffer the movement of the film between the deposition
stage and the heat treatment stage if desired. Rinsing and drying
may be carried out in the buffer zone, if desired, so that the film
may enter and move through the heat treatment stage in a uniform
manner without excess moisture.
[0060] The exemplary film growth systems described in FIGS. 1-8 may
be used to form films comprising any of a wide range of materials,
including metals, semiconductors, and insulators on a
temperature-controlled substrate from constituents in a solution by
series of reactions which are performed while controlling one or
more of: the substrate temperature, the chamber pressure, the flow
rate of the processing solution, the pH and the composition of the
solution. The following description of a chemical process suitable
for use in one of the aforedescribed chambers will lead to
formation of an intrinsic II-VI compound semiconductor film. In
this example, a set of chemical reactions provide relatively
uniform and low defect formation of a stoichiometric II-VI compound
film, e.g., CdS. This is to be compared with the film quality
achievable via physical vapor deposition that may involve
sputtering of the Group II and VI species. Generally, a mix of one
or several ligands is combined in an aqueous solution containing a
group II salt having an anion that can act as a supplementary and
somewhat competitive complexing agent of the group II element
cation of the salt to form a complex of low or moderate stability.
The solution further includes an OH-source for pH control, and a
source of a group VI element, all mixed together in the right
proportion in an aqueous bath to grow an intrinsic II-VI compound
film on a substrate. The substrate forms the base of the
replenishable shallow solution bath. The substrate is preferably
hydrophyllic, thereby facilitating accumulation of OH.sup.- bonds
along a surface thereof. If the OH.sup.- is derived from
NH.sub.4OH, then an ammonium salt buffer, NH.sub.4Cl, with the same
anion as that of the group II salt, CdCl.sub.2, is also included in
the solution to avoid depleting OH.sup.- and NH.sub.3. When this
solution is maintained at 5 to 15.degree. C. there will be
substantially no reaction between the OH radicals and the complexes
for more than two weeks (very good shelf life). When the deposition
is performed in the chamber on a 5 cm.times.5 cm substrate
(nominal), the aqueous solution may be formed in the following
proportions:
TABLE-US-00001 CdCl2 0.0015M Ammonium chloride (NH.sub.4Cl) 0.006M
Ammonium hydroxide (NH.sub.4OH) 0.77M Potassium nitrilotriacetate
(NTA or N(CH.sub.2OOK).sub.3) 0.0009M Thiourea
((CS(NH.sub.2).sub.2) 0.003M
Ligands: Cl--, NH.sub.3 (from NH.sub.4OH), NTA
(N(CH.sub.2OOK).sub.3) Group II salt: CdCl.sub.2 Group VI element
source: Thiourea (CS(NH.sub.2).sub.2) OH-- source: NH.sub.4OH
NH.sub.3 buffer: NH.sub.4Cl The constituents dissociate as
follows:
1) NH.sub.4Cl.fwdarw.NH.sub.4+Cl.sup.-
[0061] 2) NH.sub.4OH.fwdarw.NH.sub.4+OH wherein
NH.sub.4.sup.++OH.sup.-.fwdarw.NH.sub.3+H.sub.2O
3) KOH K.sup.++OH.sup.-
4) CdCl.sub.2.fwdarw.Cd.sup.2++2Cl.sup.-
[0062] The NH.sub.4Cl and NH.sub.4OH regulate the OH.sup.-
concentration in the solution. With sufficient OH.sup.-
concentration in the solution, there will be a significant number
of bonds between the OH.sup.- radicals and sites along the
hydrophilic substrate so as to facilitate reaction of complexes
containing the Group II element, e.g., Cd, to form an intermediary
Cd--hydroxyl compound bound to the substrate with the help of
heat.
[0063] With dissociation of CdCl.sub.2 per (4), above, in the
presence of the NH.sub.3 ligand, cadmium ions form first Group II
based complexes (I) according to:
Cd.sup.2++nNH.sub.3.fwdarw.[Cd(NH.sub.3).sub.n].sup.2+ (1)
where n=1, 2, 3, or 4. The stability of this complex depends on the
value of n. The higher the value of n, the more stable the complex
becomes. The potassium nitrilotriacetate, N(CH.sub.2OOK).sub.3, or
NTA also acts as a ligand forming second Group II based complexes
(II) according to:
Cd.sup.2++nN(CH.sub.2OOK).sub.3.fwdarw.[Cd(N(CH.sub.2OOK).sub.3).sub.n].-
sup.2+ (II)
The Cl.sup.- supplied by CdCl.sub.2 and NH.sub.4Cl acts as a third
ligand to form a third Group II based complex according to:
Cd.sup.2++nCl.sup.-[Cd(Cl).sub.n].sup.(2+)+(n-) (III)
The use of multiple ligands can optimize binding of Cd.sup.2+ or
other metallic ions of interest into complexes which stay in
solution and permit better control of the growth of high quality
intrinsic films, allowing for on-demand delivery of the metallic
ion at the reaction site:
OH.sup.-[substrate]+[Cd(L).sub.n].sup.m.fwdarw..sup.heatCd(OH).sup.+[sub-
strate]+nL.sup.p
wherein L represents any of one or more suitable ligands, e.g.,
NH.sub.3, N(CH.sub.2OOK).sub.3, or Cl.sup.-. and
[Cd(L).sub.n].sup.m represents one of the corresponding complexes
containing Cd.sup.2+.
[0064] Furthermore, to complete the formation of CdS principally at
the substrate surface, the thiourea ((NH.sub.2)CS) in the presence
of a sufficient OH.sup.- concentration first undergoes a partial
reaction as shown below to produce HS.sup.-:
(NH.sub.2)CS+OH.sup.-.fwdarw.SH.sup.-+H.sub.2O+H.sub.2CN.sub.2
[0065] With the metastable Cd(OH).sup.+ already bound to the
substrate and with HS present in surrounding solution, it becomes
possible to convert all of the Cd(OH).sup.+ to CdS on the heated
substrate:
Cd(OH).sup.+[substrate]+SH.sup.-.fwdarw..sup.heatCdS[substrate]+H.sub.2O-
.
[0066] Since the Cd.sup.2+ ions are optimally and primarily bound
in a hydroxyl metastable compound at the desired site of
deposition, the spontaneous formation of Cd(OH).sub.2 that will
eventually lead to the precipitation of CdS in the solution tank is
inhibited. To further ensure prevention of precipitate which would
then deposit out of the solution, the solution reservoir containing
the aqueous solution which flows into the chamber is maintained at
low temperature, e.g. 5 to 15.degree. C. That is, generally there
is inhibition of the reaction
Cd.sup.2++OH.sup.-.fwdarw.Cd(OH).sub.2
which would typically be present in a chemical bath deposition
technique that attempts to deposit poor quality CdS directly from
the aqueous solution on to a substrate, e.g.,
Cd.sup.2++S.sup.2-.fwdarw.CdS.
[0067] Instead of depositing CdS as a precipitate in a bath and on
a substrate simultaneously, it is possible to create the CdS on a
hydrophilic substrate surface only. Further, as a substrate surface
becomes completely covered with CdS, the reaction continues as
hydroxyl ions present in the aqueous solution associate with the
newly deposited CdS to continue formation of the Cd(OH).sup.+ bound
to the exposed CdS. With HS.sup.- present in surrounding solution,
it becomes possible to convert all of the Cd(OH).sup.+ to CdS:
Cd(OH)+[CdS]+SH.sup.-.fwdarw..sup.heatCdS[CdS]+H.sub.2O.
[0068] Generally, there is little or no precipitation or true
deposition of the CdS on the substrate. Instead, the CdS is formed
along the growth surface in a replacement reaction. Further, with
the Group II element bound in complexes while in solution, there is
little or no opportunity for formation of undesirable precipitates
of the metallic ion, e.g., Cd(OH).sub.2 when forming CdS. A feature
of the invention is the simultaneous provision of a series of
reactions that prevent deposition of the Group II element
containing compound by precipitation and allow formation of the
Group II element containing intermediary compound bound along the
substrate surface prior to formation of the II-VI compound.
[0069] To facilitate selective growth of the II-VI film by the
aforedescribed replacement technique, the following conditions are
believed to be useful:
a. The substrate is maintained at a temperature ranging between 100
to 200.degree. C., depending on the substrate, preferably
170.degree. C. for a glass substrate. b. The solution is
periodically replenished over the growth area with an injection
which washes away solution which has been present. The interval
time for injecting fresh solution over a substrate surface in the
illustrated chamber may range from 10 to 30 seconds, with 20
seconds having been preferred in some experiments. c. The volume of
the injected solution may be one to two times the volume of the
solution being replaced e.g., for the 5.times.5 cm substrate having
an overlying solution volume of about 3 ml and the replenishing
solution is at least 3 ml uniformly dispensed into the bath to
displace the spent solution. d. The reactions which might otherwise
form precipitates and lead to deposition on surfaces other than
that of the substrate are limited by cooling the outer part of the
heater assembly and the washed solution, e.g., the outer part of
the heater assembly outside the reaction bath area may be
continuously or intermittently cooled with the relatively cold
solution flowing from the reservoir. As this solution accumulates
at the bottom of the chamber, it is pumped back into the solution
reservoir for further cooling and recirculation. For the 5.times.5
cm substrate in the film growth system, the flow rate of this
solution may be about 120 ml/min.
[0070] The exemplary process described above was specifically
directed to the growth of CdS films. It will be appreciated that
many other useful film compositions can benefit from the inventive
apparatus and method. The following table presents the reagent
formulations that Applicant has found suitable for creating films
of InOSe, InOS, CuS, MnO, LiMnO, CdO, ZnO, CdS, Cu, CdSSe, ZnS, and
CDAlS. In each case it can be seen that multiple ligands are
combined in order to achieve a bath solution that exhibits a
desired combination of stability when cool with ease of reaction
when warmed adjacent to the substrate. This combination cannot be
achieved with a single ligand or "complexing agent" as taught, for
example, by McCandless et al '845.
TABLE-US-00002 Concentration Remarks/Comment InOSe InCl.sub.3
0.002M Acetic acid 0.04M Ligand Selenourea 0.08M Ligand and source
of Se Hydrogen peroxide 2.4M InOS InCl.sub.3 0.002M Acetic acid
0.04M Ligand Thiourea 0.08M Ligand and source of S Hydrogen
peroxide 2.4M CuS CuSO.sub.4 0.005M Triethanolamine 0.09M Ligand
Citric Acid 0.125M Ligand Potassium nitrilotriacetate 0.03M Ligand
Thioacetamide 0.008M Ligand and source of S MnO MnCl.sub.2 0.002M
Sodium citrate 0.008M Ligand Potassium nitrilotriacetate 0.01M
Ligand Ammonium hydroxide 0.6M Ligand and source of O LiMnO
MnCl.sub.2 0.0016M LiCl 00004M Sodium citrate 0.008M Ligand
Potassium nitrilotriacetate 0.01M Ligand Ammonium hydroxide 0.6M
Ligand and source of O CdO CdCl.sub.2 0.002M Sodium citrate 0.0056M
Ligand Ammonium hydroxide 0.77M Ligand Potassium nitrilotriacetate
0.0009M Ligand Hydrogen peroxide 3.6M ZnO Zn(NO.sub.3).sub.2
0.0036M Sodium citrate 0.006M Ligand Ammonium hydroxide 0.77M
Ligand Potassium nitrilotriacetate 0.0009M Ligand Hydrogen peroxide
3.6M CdS CdCl.sub.2 0.0015M source of Cd and Cl (another ligand)
Ammonium chloride 0.006M Ligand buffer Ammonium hydroxide 0.77M
Ligand Potassium nitrilotriacetate 0.0009M Ligand Thiourea 0.003M
Cu CuSO.sub.4.cndot.5H.sub.2O 0.0096M Hydantoin 0.107M Ligand
Citric Acid 0.004M Ligand Potassium nitrilotriacetate 0.05M Ligand
Hydrazine 1.6M Potassium hydroxide 0.22M CdAlSSe CdCl.sub.2 0.001M
Al(NO.sub.3).sub.3 0.001M Sodium citrate 0.008M Ligand Ammonium
hydroxide 0.6M Ligand Potassium nitrilotriacetate 0.0009M Ligand
Thiourea 0.004M Sodium selenosulfite 0.004M ZnS Zinc Acetate 0.004M
source of Zn and acetate (another Ligand) Sodium citrate 0.0008M
Ligand Thioacetamide 0.004M Ligand and source of S CdAlS CdCl.sub.2
0.001M Al(NO.sub.3).sub.3 0.001M Sodium citrate 0.008M Ligand
Ammonium hydroxide 0.6M Ligand Potassium nitrilotriacetate 0.0009M
Ligand Thiourea 0.004M
[0071] It will be appreciated that the invention is not limited to
the exemplary formulations and combinations of reagents presented
in the foregoing table. Skilled artisans will notice that there is
wide flexibility in the choice of soluble metal salts with examples
of chlorides, nitrates, citrates, acetates, and sulfates having
been demonstrated. Similarly, there is a wide choice of ligands
including acetic acid, triethanolamine, citric acid, potassium
nitrilotriacetate, sodium citrate, ammonium hydroxide, ammonium
chloride, and hydantoin. Thus, the user may select particular
soluble species and suitable ligands for a particular application
through routine experimentation.
[0072] For some applications, the films deposited by the chemical
method outlined above are typically subject to a heat treatment or
annealing step to develop the desired microstructure,
crystallinity, and other properties for particular applications.
Those skilled in the art will appreciate that the preferred heat
treatment will depend on several factors such as the compound being
formed and the type of substrate. The following table presents some
typical heat treatments, and the skilled artisan can easily adapt
these results to other systems with routine experimentation.
[0073] Exemplary heat treatments in the tube furnace:
TABLE-US-00003 Coating Substrate Anneal T (.degree. C.) Time (min)
Atmosphere CdS glass 300 60 Ar ZnO glass 400 60 Ar CdAlS glass 275
60 Ar CdO glass 400 60 Ar
[0074] It will be appreciated that heat treatment may be carried
out using any suitable means, including but not limited to
convective, conductive, radiative (including flash lamp, laser,
and/or IR heating) and microwave or RF heating. In some cases,
microwave heating may be faster than convective or conductive
heating, for example. Radiative methods may be used when it is
desirable to apply heat quickly to the film while minimizing
heating of the substrate, for instance when the substrate is a
polymeric material. Conductive heating may be desirable when the
substrate is metallic and is therefore a good thermal conductor.
Through routine experimentation the skilled artisan can therefore
select the optimal heat treatment method for a particular
combination of film and substrate.
[0075] Materials made by the inventive process display several
improvements in useful properties. One accepted method for
assessing the adherence of a film is the simple adhesive-tape test.
This test was performed on the CdS films deposited on glass by the
traditional CBD and the one deposited by the current invention with
no post deposition annealing. After three repetitions, the film
deposited by the traditional CBD was successfully removed, whereas
the film grown by the current invention still remained intact after
twenty repetitions. The CdS film also showed superior optical
properties compared to films grown by traditional CBD.
[0076] Chemical Recycling
[0077] Spent solution can be reused in the coating process by
replenishing the aqueous solution with depleted reactants. It will
be appreciated that after several cycles of replacing depleted
reactants, there may be a high build-up of undesirable by-products,
rendering the solution unsuitable for high quality film growth. As
a result, the remaining quantity of Group II metal, e.g., Cd, could
be precipitated out of the solution in the form of CdS by adding
excess KOH, (NH.sub.2)CS to the solution and heating the solution
to about 90.degree. C. The CdS precipitate can then be filtered out
of the mixture and washed. Next, the CdS precipitate can be
dissolved in HCl according to the equation
CdS+HCl.fwdarw.CdCl.sub.2+H.sub.2S.uparw.
[0078] The process is performed under a ventilation hood in the
presence of a carbon filter which will absorb essentially all of
the H.sub.2S gas. The CdCl.sub.2 solution may then be purified,
recrystallized, and dried for reuse.
[0079] Doping of Compound Semiconductor Material During Film
Growth
[0080] In lieu of forming intrinsic CdS as described above, e.g.,
creating the CdS on a hydrophilic substrate surface, the intrinsic
film can be doped in-situ by replacing less than 1% concentration
of the matrix salt with the appropriate dopant salt. For n-type
doping the dopant salt cation needs to be a group III ion and for
p-type doping the dopant salt cation needs to be a group I ion.
Alternately, ex-situ doping of the intrinsic film can be achieved
by ion-implantation of group III ions for n-type conductivity and
group V ions, preferably nitrogen ions, for p-type doping. Ex-situ
doping can also be achieved in a FGS by flowing a low concentration
of a dopant solution on the substrate with the intrinsic film
subjected to a temperature ranging between 80 to 250.degree. C. The
following salts may be used to provide dopants:
Group III salt of interest: AlCl.sub.3, GaSO.sub.4, InCl.sub.3,
etc. Group I salt of interest: AgNO.sub.3, KNO.sub.3,
LiNO.sub.3.
[0081] The invention may be used to form multiple layers of
material including p-n semiconductor junctions. A second exemplary
film that may be formed over CdS is CdSe. An exemplary processing
solution for such film formation is as follows:
TABLE-US-00004 CdCl.sub.2 0.0023M Ammonium chloride (NH.sub.4Cl)
0.005M Ammonium hydroxide (NH.sub.4OH) 0.60M Potassium
nitrilotriacetate (NTA or N(CH.sub.2OOK).sub.3) 0.0009M
Na.sub.2SeSO.sub.3 0.007M Na.sub.2SO.sub.3 0.004M
[0082] It will be understood by those familiar with chemical
processing that in numerous applications such as manufacture of
solar cells numerous other layers may be deposited on a substrate
and a variety of cleans will be performed as intermediate and pre-
and post-processing steps. It will be further understood that when
more than one material is deposited on a substrate, the individual
material layers may be deposited in any desired sequence to create
a multilayer device having a desired functionality.
[0083] Deposition of Intrinsic ZnO Films
[0084] Zinc oxide films were grown according to the recipe shown in
the table of formulations. As grown, the film was amorphous and
highly insulative. After annealing for 1 hour at 400.degree. C. in
Ar, some reduction in resistivity was observed. However, Applicant
discovered, surprisingly, that the resistivity of the inventive
material is higher, and the corresponding carrier concentration is
lower, than what has been reported for undoped ZnO films grown by
prior methods, as summarized in the following table:
TABLE-US-00005 Post 400 to 1000 nm Undoped ZnO deposition Carrier
average Carrier conc. Resistivity Deposition method Annealing type
Substrate Thickness transmission (cm.sup.-3) (.OMEGA.-cm) This
invention Yes n Borofloat glass 0.4 .mu.m 95% 3.54 .times.
10.sup.14 616 Radical-source MBE.sup.a No n Glass and 0.49 .mu.m
90% 1.5 .times. 10.sup.18 0.09 Sapphire Pulsed Laser Yes n Sapphire
0.4 um NR 4.59 .times. 10.sup.15 0.48 Deposition.sup.b Metalorganic
Vapor No n Sapphire NR NR 1.58 .times. 10.sup.16 to 0.3 to 565
Phase Epitaxy.sup.c 6.60 .times. 10.sup.17 Pulsed Laser No n
Sapphire NR NR 2 .times. 10.sup.18 0.09 Deposition.sup.d DC
reactive No NR.sup.f Glass NR 75% NR NR sputtering.sup.e .sup.aR.
Hunger et al., Mat. Res. Soc. Symp. Proc. Vol. 668 (2001) .sup.bT.
Oshima et al., Thin Solid Films 435: 49-55 (2003) .sup.cY. Ma et
al., Journal of Crystal Growth 255: 303-07 (2003) .sup.dY. Ryu et
al., Appl. Phys. Letters 83[1]: 87 (2003) .sup.eA. Mosbah et al.,
Surface and Coating Technology 200: 293-96 (2005) .sup.fNR = not
reported
[0085] The inventive ZnO film as grown is amorphous and has a band
gap much higher than that of bulk ZnO as shown in FIG. 10; although
the exact mechanism for this is not known with certainty, Applicant
speculates that it might be due to quantum shift caused primarily
by the nano-particles constituting the film or to the presence of
ZnO.sub.2 phase. However, after annealing the film recrystallizes,
resulting in band gap of about 3.2 eV, which is similar to that of
bulk ZnO. The x-ray diffraction pattern of the annealed film shown
in FIG. 11 clearly indicates that the film is the hexagonal ZnO
phase with an estimated crystallite size of 22 nm.
[0086] One can also see from the table and FIG. 10 that the optical
transmission of the inventive film is also superior to those of
prior art ZnO films. This shows that the inventive material will be
suitable for many applications where good optical transmission is
required.
[0087] The properties of the inventive material make it extremely
well suited for carrier type engineering. The p- or n-type doping
could be done in situ or ex situ followed by annealing to
recrystallize the film and allow the dopant to naturally occupy the
correct lattice position and become activated. Some exemplary
dopants and their source compounds include the following:
a) For in-situ n-type doping, preferred sources are: In salt
(In(NO.sub.3).sub.3 or InCl.sub.3), Ga salt (Ga(NO.sub.3).sub.3),
or Al salt (Al(NO.sub.3).sub.3 or Al.sub.2(SO.sub.4).sub.3), or
other group III element containing salts. b) For in-situ p-type
doping preferred sources are: Li salt (Li(NO.sub.3), or Ag salt
(Ag(NO.sub.3), or group I or IV element containing salts. c) For
ex-situ n-type doping: a very thin layer of In.sub.2O.sub.3, or
Ga.sub.2O.sub.3, or Al.sub.2O.sub.3 may be deposited on the as
grown ZnO film before annealing. Alternatively, ion implantation of
In, or Ga, or Al may be used. d) For ex-situ p-type doping: N ion
implantation would be a preferred approach.
[0088] All the foregoing doping processes are preferably followed
by annealing at temperatures ranging from 300 to 1000.degree. C.,
depending on the choice of substrate, in Ar, N.sub.2, H.sub.2,
O.sub.2, air, or some combination of these gases in a suitable tube
or muffle furnace or in a rapid thermal annealing system.
[0089] The table below show some preliminary Hall measurement data
of the in-situ n-type doped inventive ZnO using In(NO.sub.3).sub.3
and Ga(NO.sub.3).sub.3 salts. The films were annealed at
400.degree. C. in Ar for one hour before the Hall measurement.
TABLE-US-00006 Zn-salt to carrier dopant-salt carrier concentration
Resistivity Mobility Sample ID dopant ratio type (cm.sup.-3)
(Ohm-cm) (cm.sup.2/Vs) ZnO:Al 80207-2 Al 100:1 n 9.83 .times.
10.sup.15 466 1.36 ZnO:In 80207-3 In 9:1 n 1.31 .times. 10.sup.17
23.3 2.05
[0090] The inventive ZnO film may be deposited on a variety of
substrates and combined with other familiar structures such as
electrodes, etc., to form various useful devices. Some examples
include the following: transparent conducting oxide for current
collection in solar cells, light emitting diodes, and flat panel
displays; ZnO based light emitting diodes; matrix material for soft
magnets used in spintronics; sensors and detectors. It will be
appreciated that the substrate may be rigid (e.g., glass or
ceramic) or flexible (e.g., sheet metal or polymer film); it may
also include one or more electrodes. Electrodes may be provided as
a separate metallic coating on a substantially insulating substrate
(e.g., Cu on polyimide); alternatively, the substrate itself may be
used as an electrode in the case of a sheet metal substrate.
Electrodes may be substantially continuous over the area occupied
by the coating or they may be discontinuous or arranged in any
selected pattern (e.g., in order to define selected active areas,
arrays of pixels, etc.).
[0091] Although numerous embodiments of the invention have been
illustrated and described, the invention is not so limited.
Numerous modifications, variations, substitutions and equivalents
will occur to those skilled in the art without departing from the
spirit and scope of the present invention.
* * * * *