Differentially pumped temperature controller for low pressure thin film fabrication process

The temperature of a substrate on which a thin film structure is to be fabricated in a low pressure environment is controlled by supporting the substrate on a heater block having a rim which defines first and second regions between the substrate and the heater block. The first region is inside the rim. The second region is outside the rim, surrounding the first region. Heat is applied to the heater block and an inert gas having good thermal conductivity is pumped through the block into the first region at a first controlled pressure. Gas is pumped away from the second region at a second pressure substantially less than the first pressure. A pressure differential is thus maintained between the two regions. This prevents the inert gas from contaminating the thin film fabrication environment by escaping past the substrate into the fabrication environment.

FIELD OF THE INVENTION 
This application pertains to a method and apparatus for controlling the 
temperature of a thin film fabrication substrate by maintaining a pressure 
differential between a first region into which gas passes through a 
substrate-supporting heater block to heat the substrate and a second 
region surrounding the first region from which gas is withdrawn. 
BACKGROUND OF THE INVENTION 
Thin film fabrication techniques are becoming essential aspects of many 
advanced technology manufacturing operations. In particular, thin film 
semiconductor fabrication techniques have been used for decades. Newer 
techniques employing gallium arsenide ("GaAs") and other semiconductor 
materials are being developed to exploit the high speed and other superior 
performance properties of such materials. Recent advances in 
superconducting devices made from high critical temperature ("T.sub.c ") 
ceramic compounds have necessitated development of new processes for 
fabricating, annealing and testing exotic materials. Such processes 
include pulsed laser ablation, single target sputtering, multi-element 
co-sputtering, metallo-organic chemical vapour deposition ("MOCVD"), 
molecular beam epitaxy ("MBE"), electron beam co-evaporation, reactive 
sputtering and etching, and various other processes. 
Many of these processes involve chemical reactions, partial pressures, etc. 
which are highly temperature dependant. Accordingly, the temperature of 
the thin film fabrication process must be carefully controlled. For 
example, thin films are conventionally fabricated at operating 
temperatures ranging from about 600.degree. C. to about 900.degree. C., 
with the operating temperature being ideally controlled within a tolerance 
range of about .+-.1.degree. C. to .+-.5.degree. C. In practice however, 
the absolute operating temperature cannot economically be measured with 
the desired precision, so a relative temperature is used instead. Thus, 
the temperature is often controlled by reference to a "tracking" variable 
such as a thermocouple readout, with the optimal process temperature being 
determined empirically by evaluating the quality of successive batches of 
finished thin films and iteratively adjusting the associated value of the 
tracking variable until acceptable quantities of thin films of acceptable 
quality are produced. 
A further problem is that it is difficult to reliably obtain a good thermal 
connection between the thermocouple and the substrate on which the thin 
film is fabricated. Consequently, the absolute value of the temperature of 
the substrate may be known only to within about .+-.10.degree. C. Thus, 
even if it is possible to control the process temperature to within 
.+-.1.degree. C., such accurate temperature control resolution can not be 
applied effectively. More importantly however, the fabrication of uniform 
quality thin films over the entire substrate requires that the temperature 
of the substrate be uniformly controlled over the entire substrate. The 
larger the substrate, the more stringent this control must become and the 
more difficult it is to attain. 
In most epitaxial deposition processes currently in use, the substrate on 
which thin film structures are to be fabricated is mounted in a support 
holder which is attached to a thermally massive heater block. A good 
thermal connection to the block is achieved by using a heat conducting 
medium such as indium solder or silver paste to bond the substrate to the 
block. This causes two major problems. First, the bond is often 
non-uniform across the substrate. Second, after fabrication of the thin 
film structures on the substrate, the bond must be broken to free the 
substrate from the heater block (i.e. by physically prying the substrate 
away from the block). 
These problems impose production limitations which inhibit widespread use 
of thin film fabrication techniques in comparison to other techniques such 
as those used to fabricate silicon devices. For example, a non-uniform 
bond between a 50 mm diameter substrate and the heater block may cause 
temperature variations of several degrees across the substrate, making it 
impossible to fabricate thin film structures of uniformly high quality 
over the entire substrate. The fragility of the substrate material results 
in significant breakage loss when the bond is broken to remove the 
substrate from the block. Breakage loss escalates rapidly as the diameter 
of the substrate increases above about 50 mm. Large diameter substrates 
are preferably utilized in order to maximize yield and minimize cost, but 
the breakage factor limits the size of substrates which can practically be 
accommodated in many fabrication processes. 
A further problem is that the mechanical bonding of one side of the 
substrate to the heater block renders that side of the substrate 
unsuitable for thin film fabrication. This precludes double-sided thin 
film fabrication of the sort commonly encountered in microwave device 
fabrication, which greatly constrains the design of the circuits capable 
of fabrication by thin film techniques. 
The foregoing problems have restricted the use of various thin film 
fabrication processes, limited the yields attainable by those processes, 
and added greatly to their complexity and expense. However, if the 
foregoing problems can be overcome, many of these processes offer 
significant potential benefits in the fabrication of thin film 
microcircuits. The present invention is accordingly directed to overcoming 
such problems. 
SUMMARY OF THE INVENTION 
In accordance with the preferred embodiment, the invention provides a 
method of controlling the temperature of a substrate on which a thin film 
structure is to be fabricated in a low pressure or other environment 
requiring the substrate to be held at a controlled temperature. The 
substrate is supported on a heater block having a rim which defines a 
first region between the substrate and the heater block inside the rim, 
and a second region between the substrate and the heater block outside the 
rim. The second region surrounds the first region. Heat is applied to the 
heater block and an inert gas of high thermal conductivity is introduced 
into the first region by allowing the gas to pass through apertures in the 
heater block at a first, controlled pressure. The gas acts as a good 
thermal conductor between the heater block and the substrate. Vacuum is 
applied to withdraw gas from the second region at a second pressure 
substantially less than the first pressure. A pressure differential is 
thus maintained between the first and second regions, preventing the inert 
gas from contaminating the thin film fabrication environment by escaping 
past the substrate into the fabrication environment. 
In some applications of the invention, a higher vacuum may be applied to 
withdraw gas from a third region surrounding the second region at a third 
pressure substantially less than the second pressure. This establishes a 
further pressure differential barrier to prevent the inert gas from 
contaminating the thin film fabrication environment. 
The invention also provides apparatus for controlling the temperature of a 
thin film fabrication substrate. The apparatus incorporates a heater block 
having a rim for supporting the substrate. A first aperture is provided in 
the block to conduct an inert gas of high thermal conductivity through the 
block to the first region mentioned above. A second aperture in the block 
conducts gas through the block away from the second region mentioned 
above. A pressure controlled gas source supplies gas to the first region 
at a first, controlled pressure, while a first pumping means pumps gas 
away from the second region at a second pressure substantially less than 
the first pressure. A third aperture may be provided in the block to 
conduct gas through the block away from the third region mentioned above, 
with a second pumping means being provided to pump gas away from the third 
region at a third pressure substantially less than the second pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As depicted in the drawings, heater block 10 is mounted atop electric 
powered heating element 12. Block 10 is made of a material compatible with 
a thin film semiconductor fabrication environment. The top surface of 
block 10 is milled and lapped to a flatness comparable to that of wafer 
substrate 14 on which thin film structures are to be fabricated by a 
process such as metallo-organic chemical vapour deposition ("MOCVD"), 
molecular beam epitaxy ("MBE"), reactive sputtering and etching, etc. A 
first circular region 16 is machined to a depth of a few microns in the 
top surface of heater block 10. A second annular region 18 is machined in 
the top surface of heater block 10, around first region 16, leaving a rim 
20 between first and second regions 16, 18. Accordingly, when substrate 14 
is clamped atop block 10 by clamp 22, rim 20 supports substrate 14 with 
first and second regions 16, 18 lying between substrate 14 and block 10. 
A first aperture 24 is machined in block 10 to conduct a high thermal 
conductivity inert gas such as helium or hydrogen through block 10 from a 
pressure-controlled gas source (not shown) into gas reservoir chamber 25. 
A large plurality of small apertures 27 in block 10 permit the gas to pass 
from chamber 25 into first region 16 (the drawings illustrate only a few 
such apertures, and greatly exaggerate their size). Apertures 27 ensure 
that the gas pressure within first region 16 (hereafter the "first 
pressure") is uniform and equal to the gas pressure within chamber 25, 
which is in turn governed by the pressure-controlled gas source. The 
thermal conductance of the gas within first region 16 is accordingly 
uniform throughout first region 16. All points on the underside of 
substrate 14 which are exposed to first region 16 are thus maintained at a 
temperature very close to the temperature of heater block 10. 
A second aperture 26 is machined in block 10 to conduct gas away from 
second region 18, through block 10 toward a first high vacuum pumping 
means (not shown) such as a turbopump or a dry pump capable of sustaining, 
for example, pressures on the order of 10.sup.-7 torr. The first high 
vacuum pumping means maintains the pressure within second region 18 at a 
second pressure substantially less than the first pressure within first 
region 16. 
If required in order to meet cleanliness specifications of the thin film 
the fabrication process, a third aperture 28 may be provided in block 10 
to conduct gas through block 10 away from a third region 30 between the 
top surface of block 10 and clamp 22. In such case a second pumping means 
(not shown) is provided to pump gas away from third region 30 at a third 
pressure substantially less than the second pressure within second region 
18. 
The first, second and third pressures may vary over a wide range, depending 
upon the nature of the particular fabrication process carried out on 
substrate 14, but the relative pressure differentials are maintained as 
aforesaid. That is, the second pressure is always substantially less than 
the first pressure; and, if a second pumping means is provided, then the 
third pressure is always substantially less than the second pressure. 
In operation, substrate 14 is laid atop block 10 such that rim 20 supports 
substrate 14 around its lower circumference, with the underside of 
substrate 14 covering first and second regions 16, 18. Clamp 22 is 
positioned over substrate 14 and used to securely clamp substrate 14 in 
place on block 10. Heating element 12 is then activated to raise the 
temperature of block 10 to its normal operating temperature for thin film 
fabrication (typically in the range of about 600.degree. C. to about 
900.degree. C.). 
After the temperature of heater block 10 has stabilized, helium or hydrogen 
gas is introduced into first aperture 24, which conducts the gas through 
heater block 10 as aforesaid, thereby preheating the gas to a temperature 
close to the temperature of block 10. The preheated gas emerges from 
aperture 24 into chamber 25 and then passes through apertures 27 into 
first region 16, as aforesaid. The hot gas in first region 16 forms a thin 
layer between the underside of substrate 14 and the flat upper surface of 
heater block 10. The uniform thermal conductance of the gas in region 16 
maintains substrate 14 at a uniform temperature close to that of heater 
block 10. 
To prevent contamination of the thin film fabrication environment atop 
substrate 14 the gas must not be allowed to escape from first region 16 
into the thin film fabrication environment. This is achieved by 
establishing a series of "barriers" which present progressively higher 
impedance to passage of the gas, while simultaneously providing a series 
of "escape conduits" which present much lower impedance to passage of the 
gas than the corresponding high impedance barrier. These barriers and 
escape conduits constitute a "differential pumping" means capable of 
establishing a large differential between the pressure in each of conduits 
24, 26, 28. 
More particularly, rim 20 presents a first high impedance barrier to gas 
flow, preventing most of the gas from escaping from first region 16 into 
second region 18. If some gas does overcome the first barrier and escape 
into second region 18, it encounters a second barrier 32 where the outer 
edge of substrate 14 contacts heater block 10. Gas escaping past the first 
barrier into second region 18 also encounters a much lower pressure within 
second region 18 than the pressure within first region 16, due to the high 
vacuum applied to second aperture 26 by the first pumping means. 
Accordingly, gas in second region 18 tends to escape from second region 18 
through second aperture 26, which acts as a low impedance "escape 
conduit", rather than escaping past the much higher impedance second 
barrier 32. If any gas escapes past second barrier 32 into third region 
30, it encounters clamp 22 which acts as a third high impedance barrier to 
passage of the gas. Gas escaping past second barrier 32 into third region 
30 also encounters a lower pressure within third region 30 than the 
pressure within second region 18, due to the differential manner of 
operation of the second pumping means, whereby an even higher vacuum is 
applied to third aperture 28 than is applied by the first pumping means to 
second aperture 26. Accordingly, gas in third region 30 tends to escape 
therefrom through third aperture 28, which acts as a low impedance "escape 
conduit", rather than escaping past the much higher impedance barrier 
presented by clamp 22. 
It will be understood that, in order to achieve the desired high impedance 
to gas passage, the top surface of the first barrier (i.e. rim 20) must be 
precisely level with the top surface of second barrier 32. Otherwise, 
substrate 14 (which is itself made flat with a high degree of precision) 
will not properly contact all points along both barriers. Those skilled in 
the art will understand that conventional machining techniques can readily 
be used to make both barriers smooth and level to a tolerance of much less 
than one micron. The actual tolerances required for the surfaces depend 
upon the application, specifically upon the way in which any residual 
escaping gas may affect the particular fabrication process employed. The 
tolerances may also ultimately depend on the purity of the inert gas used, 
because contaminants in the gas may adversely affect the quality of 
fabricated thin films to a much greater extent than the inert gas itself. 
By carefully regulating the pressure of the gas within first region 16, one 
may attain uniformly optimal thermal conductance within first region 16 
and thereby attain correspondingly uniform heat transfer to all points on 
substrate 14 which are exposed to first region 16. Since the hot gas 
reservoir (i.e. chamber 25) is a single large cavity, it is relatively 
easy to regulate the gas pressure and keep it uniform. 
In general, the pressure in the film fabrication region atop substrate 14 
will be much lower than that in first region 16. Under these conditions, a 
relatively large pressure in region 16 will cause substrate 14 to distort 
outwardly (i.e. away from heater block 10). Therefore, it is generally 
preferable to maintain the lowest possible pressure in first region 16 
that will provide good thermal conductance. The optimal pressure is that 
pressure at which one mean free path length for an average molecule or 
atom of the inert gas is equal to the separation between the flat upper 
surface of block 10 and the lower surface of substrate 14. Higher 
pressures may be allowed in cases where a higher pressure (e.g., 0.1 torr) 
processing environment is used. 
The invention eliminates the need for bonding substrate 14 to block 10. 
This in turn eliminates contamination of one side of the substrate with 
bonding material, and also eliminates the use of force to break a bond in 
order to remove substrate 14 from block 10 after fabrication is complete. 
The invention should not limit the diameter of substrate which could be 
accommodated, with the possible exception at some point of flatness 
considerations and wafer flexing. The invention may thus facilitate 
uniform heating of substrates as large as those currently used in silicon 
VLSI device fabrication (diameter.apprxeq.150 mm) without surface 
contamination and with zero loss of yield due to separation breakage or 
processing temperature non-uniformity across the substrate. 
As will be apparent to those skilled in the art in the light of the 
foregoing disclosure, many alterations and modifications are possible in 
the practice of this invention without departing from the spirit or scope 
thereof. Accordingly, the scope of the invention is to be construed in 
accordance with the substance defined by the following claims.