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Description  |
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BACKGROUND OF INVENTION
This invention relates to an apparatus for depositing thin films of
material and, in particular, to an apparatus for continuous deposition of
thin films by vacuum evaporation. Such apparatus is particularly useful
for the continuous manufacture of thin film solar cells.
Thin film solar cells possess many potential advantages for use in
converting solar energy to electrical power. Thin film cells employing a
film of cadmium sulfide converter and a copper sulfide absorber are
flexible, light of weight and can be made with commercially acceptable
levels of conversion efficiency. Moreover, as contrasted with silicon PN
junction solar cells which must be fabricated by inefficient batch
processes, there is considerable promise that thin film solar cells can be
commercially manufactured using continuous processing techniques.
A key step in the continuous manufacture of thin film cadmium sulfide solar
cells is the vacuum evaporation of the cadmium sulfide collector onto a
temperature-controlled moving substrate. In large-scale production, cost
considerations require that the cadmium sulfide be substantially uniformly
deposited on the substrate with relatively little wastage of the
evaporated material. In addition, economic considerations also require
that large substrate areas be continuously coated without breaking vacuum.
Moreover, the rate of deposition must be controlled in the direction of
substrate movement and be uniform perpendicular to this direction in order
to obtain the required mechanical and electrical properties in the film.
Typical prior art vacuum evaporation arrangements are not well-suited to
this task. Such arrangements typically feed a wire or ribbon of the
material to be coated to an evaporator. The rate of evaporation is
controlled by the evaporator temperature and the rate of wire feed. To
obtain uniform deposition coating, the evaporated material is applied
through a large area orifice through a relatively long source-to-substrate
distance.
Such arrangements are not suitable for the large-scale manufacture of
cadmium sulfide solar cells. Cadmium sulfide is a sublimable powder and
cannot be readily formed into wire or ribbon. Since cadmium sulfide cannot
be fed to the evaporator in the form of a liquid or wire, prior solutions
involved loading cadmium sulfide powder, pellets or sintered cakes into
crucibles. The need to periodically reload the crucibles either precludes
continuous operation or necessitates mechanically complex loading and
transfer machinery. Thus, the conventional technique for continuous feed
of source material is not easily applicable. Control of evaporation rate
from a multitude of crucibles by maintaining the temperature of each
crucible is difficult and costly. Thus the conventional technique for rate
control is inapplicable. Moreover, systems using large source-to-substrate
distances are very wasteful of the evaporated material because much of the
material misses the substrate and coats the walls of the chamber.
SUMMARY OF INVENTION
In accordance with the present invention, an apparatus for deposition by
vacuum evaporation comprises one or more evaporation chambers connected
through a rate control orifice to a manifold comprising an array of small
diameter nozzles. The rate control orifice, whose dimensions together with
the pressure differential across the orifice, determines the rate at which
evaporated material passes from the evaporation chamber to the manifold,
and the dimensions and spacings of the nozzles are chosen to achieve
efficient utilization of evaporant material and substantially uniform
deposition on an adjacent moving substrate. In a preferred embodiment, a
plurality of valve connected evaporation chambers are used in order to
permit continuous coating from at least one evaporator while the source
material in another is replenished.
BRIEF DESCRIPTION OF THE DRAWINGS
The nature, advantages and various additional features of the invention
will appear more fully upon consideration of the illustrative embodiments
now to be described in detail in connection with the accompanying drawings
wherein:
FIG. 1 is a schematic cross section of a preferred vacuum evaporation
apparatus in accordance with the invention; and
FIG. 2 is a plan view partly in section of the apparatus of FIG. 1.
DETAILED DESCRIPTION
Referring to the drawings, FIGS. 1 and 2 illustrate a vacuum evaporation
apparatus comprising a pair of evaporation chambers 10 and 11 for
enclosing respective batches 12 and 13 of material to be evaporated such
as cadmium sulfide or zinc cadmium sulfide. The chambers are defined by
walls 14 and 15, respectively, and disposed in thermal contact with
suitable heating sources 16 and 17 to evaporate the material to be
deposited. Removable closures (not shown) are provided to permit loading
of source material into the chambers.
Each chamber communicates via conduit having a separately controllable
valve 20 and 21, respectively, and a common rate control orifice 22, with
a manifold 23. As will be described in greater detail hereinbelow, the
area of the opening in orifice 22 can be used to control the rate of
effusion of the source material into the manifold.
FIG. 2 illustrates the utilization of a plurality of identical manifolds
23. If desired, however, only a single manifold might be used.
The manifold 23 comprises an open chamber 24 defined by walls 25 having a
plurality of nozzles 26 disposed in an array. Preferably a heat shield 28
is disposed about the manifold walls. As will be discussed in greater
detail hereinbelow, the nozzle diameter, d, the nozzle length, l, and the
center-to-center spacing, s, between adjacent nozzles are chosen to
provide a substantially uniform coating upon a moving substrate 27 passing
adjacent to the nozzles in a vacuum coating chamber 29 a short distance,
D, therefrom.
The drawings illustrate substrate 27 to move transverse to the direction of
manifolds 23. If desired, however, substrate 27 may move parallel to
manifolds 23.
In a preferred embodiment for depositing cadmium sulfide, the walls
defining the evaporation chamber, the communication ducts and the manifold
comprise a suitable material meeting the requirements of high temperature
durability, chemical inertness, and thermal conductivity and emissivity
consistent with efficient heat transfer, such as graphite or boron
nitride; and orifice 22 can be made of the same material. The heat shield
is comprised of thin foils of chemically inert, high emissivity metals
such as tantalum.
In operation, batches of source material are placed in each of the two
evaporation chambers and at least one chamber, e.g., 10, is heated to
evaporate or sublime the source material and connected to the manifold by
opening of its valve 20. The other chamber 11 can be kept in reserve by
maintaining valve 21 closed. Evaporated source material passes through
orifice 22, at a rate controlled by its diameter and the pressure
differential between evaporation chamber 10 and manifold 23, into manifold
23. From the manifold, the evaporated source material passes through
nozzles 26 onto the moving substrate where it deposits as a thin film.
As the source material in chamber 10 approaches exhaustion, chamber 11 can
be heated and applied to the manifold. Chamber 10 can then be withdrawn
from the system by closing valve 20 and its source material can be
replenished without breaking vacuum in coating chamber 29.
When the source material is a single species, the rate of effusion E
through orifice 22 is approximately given by the relation:
##EQU1##
where A is the orifice area in square centimeters, P is the equilibrium
pressure of the species in millimeters of mercury at temperature T, M is
the molecular weight of the species, k is Boltzman's constant and a is the
effective evaporation coefficient of the species.
In the particular case of cadmium sulfide source material effusing at high
rates, the mass effusion rate is more accurately given by the relation:
F=.alpha.T.sup.-1/2 KAM'[GP'--G.sub.o P'.sub.o ]
where K is a correction factor for the decrease in rate due to the finite
thickness of the orifice, G is a correction constant for the increase in
rate due to intermolecular interactions at elevated pressure; M' is the
effective molecular weight of Cd+S.sub.2 corrected for non-stoichiometric
vapor in the chamber at steady state, P' is the total vapor pressure in
the chamber, and .alpha. is a constant. The subscript (o) refers to the
flow from the manifold back into the evaporation chamber.
Continuous control of rate with a given orifice area is obtained by
maintaining a constant pressure difference between evaporation chamber 10
and manifold 23.
In a preferred embodiment the pressure difference is determined by the
pressure in chamber 10. The preferred method for controlling rate involves
measurement of pressure in chamber 10 by means of a transducer, not shown
in FIG. 1, whose electrical output signal, when processed by known
electronic means, is proportional to the pressure. The electrical signal
equivalent of pressure is further electronically processed to control the
heat input to evaporation chamber 10. In another embodiment of the
principle of pressure control, automated application and withdrawal of
alternating evaporation chambers 11 and 10, upon exhaustion of material in
chamber 10 is thereby facilitated. As material in chamber 10 nears
exhaustion, the temperature required to maintain the preferred pressure
will rise markedly. When the temperature in chamber 10 rises to a
predetermined value signaling near exhaustion, a sequence leading to
automatic application of chamber 11 to the manifold is initiated.
Temperature of the chamber is sensed by a thermocouple or infrared
detector.
All of the above rate control and automation benefits made possible by the
invention are further enhanced by adaption of techniques and equipment
known in the technology of digital process control.
The following tables show design parameters for deposition apparatus in
accordance with the invention of dimensions suitable for two different
solar cell manufacturing plants. The smaller pilot plant is designed to
produce annually a total area of solar cells sufficient to generate 1000
peak kilowatts of electricity. The commercial scale plant is designed to
produce annually sufficient area of solar cells capable of generating 10
megawatts of electric power.
Table I shows the basic design parameters used in each plant.
TABLE I
______________________________________
Yearly Production 1000 KW 10 MW
(peak solar electric
generating capacity
at insolation = 0.1
watt/cm.sup.2, efficiency
= 10%)
Production (hrs/yr)
7200 7200
Total Cell Area (m.sup.2)
10,000 100,000
Film Thickness (microns)
10 4
Strip Width (m) 0.25 0.50
Linear Velocity (cm/sec)
0.15 0.77
Exposure Time (sec)
400 160
______________________________________
Table II details the design specifications and approximate size of the
evaporation chamber for each plant.
TABLE II
______________________________________
Yearly Production 1000 KW 10 MW
Operation/Charge (hrs)
24 24
Minimum Sublimation Rate
1.12 11.2
(100% util) (gms/min)
Expected Sublimation Rate
1.40 14.0
(80% util) (gms/min)
Total Sublimation per 24 hrs
5.2 52
(80% until) (kg)
Sublimation Temperature (.degree.K)
1250-1450 1250-1450
Rate Orifice Diameter (mm)
0.5-3 1.-6
Approximate Chamber Size (m.sup.3)
0.06 0.6
Approximate CdS Surface Area (cm.sup.2)
260 1600
______________________________________
Table III gives the nozzle sizes and distribution needed to control
uniformity of deposition.
TABLE III
______________________________________
Yearly Production
1000 KW 10 MW
Overall Dimensions
0.6 m = 0.3 m
1.2m .times. 0.6m
Temperature (.degree.K)
1250-1450 1250-1450
Number of Nozzles
4 to 6 rows 8 to 12 rows
3 to 5 noz ea
6 to 10 noz ea
Nozzle Diameter (mm)
1.3-5.1 1.3-5.1
Nozzle Length (mm)
6.4`13 6.4-13
Source to Substrate
50-200 50-200
Distance (mm)
______________________________________
The following analytical discussion is included for those desiring to use
the apparatus in applications different from those described above.
Deposition rates, uniformity of deposition and effectiveness of source
utilization can all be calculated from the vapor pressure in the sparger
manifold, the mean free path of molecules in the manifold and the nozzle
size and location. The proper nozzle geometry and placement to produce
uniform film thickness with high source utilization can be obtained by
repeated calculations until a design meeting specifications is found. The
total deposition at any location on the substrate is the integrated value
of the fluxes from each nozzle as the substrate moves through the
deposition zone.
The vapor pressure in the sparger manifold P can be measured empirically or
calculated.
The mean free path of molecules .lambda. can be calculated from the vapor
pressure P and temperature in the manifold T in accordance with the
approximate relation:
##EQU2##
where k is Boltzmann's constant and D is the diameter of the molecules.
The angular distribution of vapor out of the nozzles can be empirically
measured or ascertained from published data. See, for example, Stickney et
al, A Journal of Vacuum Science & Technology, Vol. 4, No. 1, page 10
(1967). Curve fitting can then be used to obtain an analytic expression
for the normalized distribution F for each nozzle, as a function of the
angle .phi. from the nozzle axis, the ratio of the mean free path to the
nozzle diameter .lambda./D and the ratio of the nozzle length to the
nozzle diameter L/D, e.g.,
F=F(.phi., .lambda./D, L/D)
The rate R, in units of mass flux, at which vapor builds upon the substrate
for a nozzle with angular distribution F is
##EQU3##
where: m is the rate of effusion,
.alpha. is the angle between the normal of the substrate and the source,
dS is the differential receiving arc,
C is the normalization constant of distribution,
and
r is the distance from the source to dS.
C can be evaluated by integrating over a hemisphere and setting the
integrated arrival rate equal to the total amount evaporated. Thus,
##EQU4##
This constant must be evaluated for each nozzle after the pressure and
mean free path are known.
For a substrate moving through the deposition zone from y.sub.0 to y at a
velocity V, the terminal thickness of a film with density .rho. at a fixed
position on the substrate is given by the relation:
##EQU5##
This equation can be analytically or numerically integrated and the
contributions for each nozzle summed to obtain the total deposition onto
the position.
While the invention has been described in connection with a small number of
specific embodiments, it is to be understood that these are merely
illustrative of the many possible specific embodiments which can represent
applications of the principles of the invention. Numerous and varied
arrangements can be made by those skilled in the art without departing
from the spirit and scope of the invention. Thus, for example, the
invention may also be practiced for vacuum deposition of various materials
such as zinc, zinc phosphide (Zn.sub.3 P.sub.2), zinc sulfide (ZnS),
aluminum, and silicon oxide (SiO), as well as the previously noted cadmium
sulfide and zinc cadmium sulfide.
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