Apparatus and method for substrate temperature control

An apparatus and a method for controlling the temperature of a substrate onto which thin films of semiconductor materials are vapor deposited. The apparatus contains a platen contacting a surface of said substrate over the entire length of the deposition zone; said platen having at least one cavity therein and a rounded edge where said substrate first contacts said platen of the beginning of said deposition zone.

BACKGROUND OF THE INVENTION 
This invention relates to an apparatus and a method for controlling the 
temperature of a substrate onto which a material is to be deposited. More 
specifically, the invention relates to a differential temperature control 
method and an apparatus for maintaining a specified temperature profile of 
a substrate during the formation of thin film semiconductor materials by 
processes which are carried out at low pressures or in vacuum. These 
processes are generally known in the semiconductor art and include, for 
example, vacuum evaporation as described in U.S. Pat. No. 3,531,335, 
plasma deposition as described in U.S. Pat. No. 4,064,521 or chemical 
vapor deposition as described in U.S. Pat. No. 2,671,739. 
The properties of semiconductor materials deposited upon a substrate vary 
widely depending upon such factors as deposition temperature of the 
semiconductor material, temperature of the substrate, the rate of 
deposition and the like. Highly uniform semiconductor layers are required 
in the fabrication of photovoltaic cells which exhibit minimal 
batch-to-batch variations. If photovoltaic cells are to play an important 
role in meeting future energy needs, then large area quantities of 
photovoltaic devices will be needed which can be fabricated cheaply and 
effectively in a continuous process. The ability to uniformly control the 
substrate temperature and thus control a parameter which effects the final 
device characteristics is necessary if photovoltaic cells are to meet 
projected cost and efficiency goals in order to play a role in electrical 
power generation in the coming years. 
To reduce the cost of photovoltaic cells and increase their distribution 
and use, researchers and manufacturers are investigating the fabrication 
of thin film semiconductor solar cells, such as amorphous silicon, 
CuInSe.sub.2 /CdS, Cu.sub.x S/CdS photovoltaic cells and the like on thin 
flexible substrates. The thin flexible substrates can, for example, be 
copper foil or high-temperature polymeric materials which have an 
electrically conductive metallized surface. The flexible substrates are on 
the order of about 25 micrometers in thickness. The thinness of the 
substrate presents problems in maintaining a desired temperature profile 
or a uniform temperature of the substrate. In addition, thin film metallic 
substrates such as copper, are very reflective, i.e. having low 
emissivity, which makes accurate determination of the substrate 
temperature difficult. A continuously moving substrate in an automated 
continuous process complicates the measurement of the actual substrate 
temperature; therefore, temperature control is made difficult. 
The closest art of which we are aware that relates to the deposition of 
uniform semiconductor layers having the desired properties for 
photovoltaic devices is concerned with batch-type operations in which 
small areas or multiple small area pieces are prepared. For example, in 
U.S. Pat. No. 3,531,335 the temperature of a small area substrate can be 
controlled by affixing a thermocouple to the substrate and using the 
thermocouple in conjunction with a proportional temperature controller 
which drives heaters that provide radiant heat to the substrate. This 
method, and related methods which are generally used in batch or 
laboratory scale processes are not readily applicable to moving substrate. 
Furthermore, these methods do not provide the necessary spatial 
temperature control required for the low cost manufacture of large area, 
semiconductor devices such as solar cells. 
Methods and apparatus for the continuous coating of metals and 
semiconductors on moving substrates such as metal foils and polymer films 
are known. However, the art known to us teaches means for merely avoiding 
overheating of the substrate. Thus, for example, in U.S. Pat. .No. 
4,026,787, a polymer film substrate is drawn over a steel drum through 
which coolant is circulated during deposition of a cadmium sulfide 
semiconductor. The apparatus and method of U.S. Pat. No. 4,026,787 does 
not, however, provide control of substrate temperature sufficient to 
produce semiconductor layers having spatially uniform desired properties 
for photovoltaic devices. In particular, the aforementioned method and 
apparatus cannot maintain a uniform substrate temperature or desired 
temperature profile when the emissive properties of the substrate change, 
said change being inevitable as the semiconductor layer being deposited 
grows in thickness. 
Thus, it would be highly desirable to have an apparatus which can heat and 
maintain the temperature of a stationary or continuously moving substrate 
at a constant temperature or with a desired temperature profile during the 
deposition of a thin film semiconductor material. It is also essential to 
have a method of determining the temperature of the substrate in order to 
control the properties of the deposited semiconductor film. 
SUMMARY OF THE INVENTION 
We have invented an apparatus for heating a thin film substrate onto which 
a semiconductor material is deposited and a method for controlling the 
temperature of a stationary or a moving substrate onto which said 
semiconductor film is deposited thereon. The apparatus is a novel platen 
which incorporates internal differential temperature control sensors to 
precisely determine the temperature of the substrate and drive radiative 
heaters facing the surface of the substrate onto which the semiconductor 
material is deposited to measure and control the temperature of the 
substrate.

DETAILED DESCRIPTION OF THE INVENTION 
The apparatus and method of our invention will be more clearly illustrated 
by referring to the Figures. FIG. 1 illustrates an apparatus for carrying 
out the method of our invention. The Figure illustrates a platen 10. The 
platen 10 incorporates a plate 12 fabricated from a rectangular piece of 
metal. The plate 12 can be fabricated of any material such as steel, 
copper, aluminum, iron, and the like which is stable at the deposition 
temperature, easy to shape and does not adversely react. In a particularly 
preferred embodiment, the plate 12 is fabricated from copper and coated 
with a material, known as hard chrome, to prevent scratching of a 
continuously moving copper or zinc-coated copper thin foil substrate. 
The plate 12 has strip heaters 14 attached to the side of the plate 
opposite to the side which contacts the substrate. An example of a 
suitable heater is an Acra Electric model #TEEM 3" by 13" 500 watts 42 VAC 
with Stainless Steel sheathed. The heaters 14 heat the plate 12. The 
platen 10 further incorporates a bar 16 connected to the plate 12. The bar 
16 initiates contact with the substrate and permits the substrate to bend 
around same and contact the heated plate 12. The bar 16 can be fabricated 
from the same materials as the plate 12. In a preferred embodiment, the 
platen and bar are fabricated to be sufficiently convex on the side 
contacting said substrate to prevent kinking of a substrate coated with 
the semiconductor material. A second bar, not illustrated, can be attached 
to the opposite end of the plate 12 to help the transition of the 
substrate out of a semiconductor deposition zone. 
The plate 12 incorporates temperature sensors 18, such as thermocouples, to 
monitor the temperature of the platen 10. An example of a suitable 
thermocouple is a 20 or a 24 gauge chromel-alumel thermocouple, a product 
of Omega Engineering, Inc. Holes are provided in the plate 12 for the 
temperature sensors 20 which monitor the temperature of the substrate. 
The installation of temperature sensors 18 and 20 is more clearly 
illustrated in FIG. 2. The plate temperature sensor is fixed in the plate 
12 with a ceramic cement 19 or other suitable material. The substrate 
temperature sensor 20 is inserted through the hole in plate 12 to be in 
thermal communication with the substrate 50. The temperature sensor 20 is 
a shielded thermocouple which is extremely thin on the order of about 
0.025 cm, so as to minimize deformation of the substrate 50. As 
illustrated in FIG. 2, sensor 20 is in physical contact with plate 12 and 
substrate 50. This configuration is satisfactory. However, it is preferred 
to have a portion of plate 12 hollowed out, or to otherwise dispose sensor 
20, such that sensor 20 is not in physical contact with substrate 50, but 
is in thermal communication with substrate 50. 
The operation of temperature control is more clearly illustrated by 
referring to FIG. 3. FIG. 3 illustrates a section of the platen and a 
method of controlling the temperature of the continuously moving 
substrate. The substrate 50 contacts the plate 12. The temperature of the 
plate 12 is controlled by a controller 30 connected to the strip heaters 
14. An example of a suitable controller is a model 
919/PAP/K/0.degree.-999.degree. C./P10/DVT/115 VAX, a product of the 
Eurotherm International Co. The temperature sensor 18 is connected to an 
input of a differential temperature controller 32. An example of a 
suitable differential temperature controller is a model 
919/PAP/N/0.+-.1.99 mV/P10/DVT/115 VAX controller, a product of Eurotherm 
International Co. The other input is connected to temperature sensor 20. 
The controller heats the platen to a temperature T.sub.1 which is measured 
by temperature sensor 18. The substrate 50 will be at some lower 
temperature T.sub.2. The magnitude of T.sub.1 -T.sub.2 depends upon the 
resistance to heat transfer at the substrate-platen interface, and source 
temperature, and radiative loss from the surface of the substrate upon 
which the thin film semiconductor is deposited. Temperature sensor 20 
situated in the cavity in the plate will come to a steady state 
temperature T.sub.3 intermediate between T.sub.1 and T.sub.2. The 
differential controller 32 compares the temperature of sensors 20 and 18 
and drives facia heaters 36 such that the difference between sensors 18 
and 20 is substantially zero. A suitable example of facia heaters are 
radiative heaters model RTU-2063AX35, 120 V 400 watts heaters, products of 
Chromalox Sales Co. 
With differential temperature control, if temperature of plate 12 is at 
temperature T.sub.1 and the temperature in the cavity T.sub.3 is equal to 
T.sub.1, then T.sub.2 must equal T.sub.1. The differential temperature 
controller 32 senses T.sub.1 -T.sub.3 and this difference signal 34 is 
used to drive the facia heaters 36. If temperature profiling through the 
deposition zone is desired where emissive properties vary with film 
thickness, the platen and the facia heaters can be divided into separately 
controlled zones to provide temperature profiling of the substrate as it 
moves over the deposition zone. Alternatively, a plurality of platens can 
be employed to provide temperature profiling of the substrate. In addition 
to the heaters 36, the source nozzles 38, for the deposition of the 
semiconductor film, can be configured so as to provide a calculated amount 
of radiative heating of the substrate. 
The differential temperature control provides means for temperature 
measurement of highly reflective substrates and is more accurate than 
pyrometric sensing means for measuring the temperature. Fabricating the 
plate 12 from a material having a high thermal conductivity, such as 
copper, provides improved spatial uniformity of the substrate temperature 
and the differential temperature control provides substantially more 
accurate temperature control regardless the variations in the property of 
the substrate, such as surface roughness, emissivity, and reflectivity. 
The utility and novelty of the invention will be better appreciated by 
recognizing that there is a tendency for the temperature of the surface of 
certain areas of a moving substrate to deviate from the desired value. 
These deviations are due, for example, to localized variations in the 
properties of the substrate which affect the rate heat transfer, such as 
emissivity. Deviations are also likely from local variations in heat input 
to the substrate such as inhomogeneous or fluctuating radiation from the 
heaters 36. This problem is especially accute with substrates which are 
thin or have low thermal conductivity. Since said areas are likely to be 
located away from the temperature sensor 20, the deviation could otherwise 
go uncorrected. It would be impractical to dispose such a plurality of 
sensors to overcome this problem. Thus, a particular advantage of this 
invention is thermal communication between the substrate 50 and the plate 
12 which automatically corrects localized deviations in the temperature of 
the substrate. Thermal communication between the substrate 50 and the 
plate 12 may be by radiation, by convection, or preferably, by conduction. 
It is therefore desirable to have the substrate 50 in intimate contact 
with the plate 12 of the platen 10. 
The platen and method of temperature control permits the uniform 
maintenance or temperature profiling of the substrate to within plus or 
minus 5.degree. at operating temperatures on the order of 180.degree. to 
300.degree. C. and preferably within the range of 210.degree. to 
250.degree. C. for the deposition of cadmium-rich cadmium sulfide films in 
the fabrication of cadmium sulfide/copper sulfide solar cells. 
The placement of the platen in the fabrication of semiconductor films is 
more clearly illustrated by referring to FIG. 4. FIG. 4 is a simplified 
illustration of a deposition apparatus 60, such as a High Vacuum Roll 
Coater A500 BI/5, a product of Leybold-Heraeus Vacuum System Inc., 
modified to contain our invention. The vacuum chamber 62 contains the 
apparatus for depositing the semiconductor films on a substrate which 
incorporates a plurality of platens 10, 11, and 13, for the temperature 
profiling of the substrate. However, a more simple embodiment of the 
invention in the vacuum chamber 62 would contain only one platen the size 
of platens 10, 11, and 13. A substrate source 52 feeds the substrate 50 
over rollers 44 to contact the platens over a deposition zone illustrated 
as line 40. The substrate 50 can be a continuous thin film such as a 
copper foil or a continuous support which contains discrete pieces thereon 
of a suitable substrate material for solar cells. The deposition zone 40 
is between the substrate 50 and a deposition source 41 which incorporates 
radiative heaters 36, not illustrated, between and among semiconductor 
source nozzles 38, also not illustrated. The substrate 50 bends around bar 
16 on the platen 10 and contacts the platens. The platen 13 contains a bar 
at the opposite end of the platen to facilitate the exiting of the 
substrate from the deposition zone. Precise temperature control is 
maintained in accordance with the differential temperature control 
operating procedure outlined with respect to the discussion of FIG. 3. The 
additional platens 11 and 13 enable the temperature of the substrate to be 
temperature profiled, if desired, during the deposition of the 
semiconductor film. A rate monitor 42 monitors the rate of evaporation. An 
example of a suitable source for a semiconductor material such as CdS is 
the sparger system disclosed in British Patent Application No. 8,030,657 
filed Sept. 23, 1980, said application incorporated herein by reference. 
However, any deposition source known in the art, such as a plurality of 
heated crucibles, is suitable. Thereafter, take-up reels 46 transfer the 
substrate 50 to a take-up spool 54. 
For example, the apparatus can be used to deposit cadmium sulfide film on a 
zinc plated copper substrate by evacuating chamber 60 to about 10.sup.-5 
torr and heating semiconductor source 41 to a sufficient temperature to 
vapor deposit cadmium sulfide on the substrate 50. The substrate 50 is 
heated to a preferred temperature on the order of about 210.degree. to 
250.degree. C. while the substrate moves across the deposition zone at a 
rate of from about 0.3 to about 12 cm per minute, preferably 0.6 to 6 cm 
per minute. The length of time in the deposition zone, as well as the rate 
of evaporation from the source, determines the thickness of the film. 
It should be understood that the invention is not meant to be limited 
solely to the details described herein. The invention is useful for any 
process involving the formation of semiconductor, insulator or conductor 
layers in large areas on a substrate, including substrates which are 
essentially rigid such as, for examples, glass or ceramic. Modifications 
which should be obvious to one of ordinary skill in the vacuum coating 
art, are contemplated to be within the scope of the invention.