Photolithographic techniques were employed to fabricate small Joule-Thomson refrigerators in laminated substrates. The gas passages of a J-T refrigerator are formed by etching channels as narrow as 50 microns and as shallow as 5 microns in glass plates which are laminated together. Circular refrigerators on the order of 1.5 centimeters in diameter and .75 millimeters thick were produced which cool down to cryogenic temperatures in a few seconds, using Argon or Nitrogen, with no vacuum or radiation insulation. Smaller refrigerators are developed for both faster cooldown and low refrigeration capacity applications. By using this technology, custom refrigerators can be designed to meet specific application requirements

Duboc, R. M., Jr.

English

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LOW COST MICROMINIATURE REFRIGERATORS
FOR LARGE UNIT VOLUME APPLICATIONS
Robert M. Duboc, Jr.
MMR Technologies, Inc.
1400 Stierlin Road, Suite A-5
Mountain View, California 94043
ABSTRACT
Photolithographic techniques have been employed to fab-
ricate small Joule-Thomson refrigerators in laminated
suDstrates. The gas passages of a J-T refrigerator are
formed by etching channels as narrow as 50 microns and as
shallow as 5 microns in glass plates which are then
laminated together. Circular refrigerators on the order
of 1.5 centimeters in diameter and .75 millimeters thick
have been produced which cool down to cryogenic tem-
peratures in a few seconds, using Argon or Nitrogen, with
no vacuum or radiation insulation. Even smaller refrig-
erators are being developed for both faster cooldown and
low refrigeration capacity applications.
By using this technology, custom refrigerators can be
designed to meet specific application requirements. These
refrigerators can then be mass produced in a high yield
process, which is expected to lead to low unit costs for
large volume applications.
431
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INTRODUCTION
Novel photolithographic techniques have been used to
produce miniature cryogenic gas refrigerators. In this
paper, I discuss the significant design capabilities and
inherent low cost nature of this new microminiature
refrigeration technology.
BACKGROUND
In the early 1970's, Little [I] recognized the need for
refrigerators with cooling capacities much smaller than
any then commercially available. Many electronic com-
ponents such as infrared detectors, low noise amplifiers,
solid state lasers, SQUID sensors and Josephson devices
either require or significantly benefit from low tem-
perature operation. Such components typically dissipate
only a few microwatts to a few milliwatts while commer-
cially available refrigerators have capacities ranging
from one watt to many watts. This obvious mismatch
between the refrigerators and the needs of the cryogenic
electronic components suggested the need for a refrigera-
tion cycle which could be scaled down to microminiature
size and capacity [2].
After evaluating the numerous refrigerators which had been
developed to achieve temperatures of 80K and below, Little
focused on the Joule-Thomson refrigerator because it had
no moving parts in its cold stage. This simplified the
scaling problems and had the additional advantage of being
free of vibration at the cold end. The advantages and
disadvantages of the Joule-Thomson refrigerator as well as
the scaling laws subsequently developed by Little have
been presented in a series of articles [3] , [4] , [5] , [6].
These scaling laws indicated that channel dimensions of a
few microns would be required in the lower capacity J-T
coolers, which effectively ruled out the small capillary
tubing used in conventional coolers and provided the
impetus for developing the photolithographic techniques to
produce the micron sized channels.
The first microminiature refrigerators were constructed in
silicon; however, this proved only partially successful
because the silicon wafers used for the heat exchanger had
to be made excessively thin due to the high thermal
conductivity of silicon and proved to be too fragile to
hold the high operating pressures. Glass was then selec-
ted due to its high strength and low thermal conductivity.
432
A set of novel photolithographic techniques have sub-
sequently been developed to etch and bond together the
thin slides of glass to form the present generation of
microminiature Joule-Thomson refrigerators [7].
FABRICATION-TECHNIQUES
To form the heat exchanger, channels of 50 microns width
and i0 microns depth are precisely etched in thin glass
plates by an abrasive etching technique. This is done by
the application of a special photo resist to the glass
slides which is then exposed and developed to define the
pattern of channels for a J-T refrigerator. These chan-
nels are then etched using a fine particle sandblasting
technique, Figure i. Next, the slides are cleaned and
bonded together to form the encapsulated gas passages of
the refrigerator.
In mass production, a step and repeat process is used to
produce a mask of many images of the channel pattern.
This allows one to generate multiple copies of the refrig-
erators on one large sheet of glass. These are then diced
and laminated. Figure 2 shows a plate of multiple slides
for a fast cooldown refrigerator discussed later in this
paper. This fabrication process has proven to be reliable
and consistent, leading to low unit costs for high volume
production runs.
DESIGN CAPABILITIES
Figure 3 shows an early model microminiature refrigerator
produced with the techniques described above. During
operation, high pressure gas, such as N 2 at 1800 psi, is
introduced to this device through one of the holes at the
right. It then flows through the narrow channels in the
counterflow heat exchanger to the single capillary channel
where it drops to a low pressure approaching one atmo-
sphere. This isenthalpic reduction of gas pressure causes
the temperature of the gas to fall a few degrees. The
cooler gas then flows into the boiler cavity at the left
end of the refrigerator, continues through a hole in the
central plate of glass, and finally flows out through the
wide, shallow outflow channel. During initial cooldown,
the outgoing gas in the counterflow heat exchanger causes
regenerative cooling of the incoming gas until lique-
faction occurs in the boiler cavity. The temperature of
this liquid is determined by the pressure at the boiler,
433
which in turn is determined by the pressure drop of the
gas as it flows through the outflow channel. For example,
LN2 at one atmosphere boils at 77K, while at two atmo-
spheres it boils at 84K. Hence, as in all Joule-Thomson
refrigerators, the pressure drop in the outflow channel
limits the minimum temperature.
The scaling laws developed for MicroMiniature Refrig-
erators indicate that as the mass flow through the
channels drop, turbulent flows cannot be maintained
without a significant increase in the pressure drop.
While in conventional heat exchangers, turbulent mixing
of the gas is essential for adequate heat transfer; in
these planar microminiature refrigerators, adequate heat
transfer can occur in relatively wide, shallow outflow
channels through conduction alone because of the small
mass flow.
As an example, one should note that in the refrigerator
of Figure 3 there are twenty narrow inflow channels in
parallel which are bracketed together to feed the single
capillary channel. This type of interconnecting would be
difficult and costly with traditional plumbing techniques,
but with photolithography, the only difficulty lies in
preparing the initial artwork. More advanced models of
microminiature refrigerators have been produced which
rely on complex networks of interconnecting channels,
which accomplish a number of design objectives not
practical in 'standard plumbing' Joule-Thomson refrig-
erators. For instance, most microminiature refrigerators
have redundant inflow channels to minimize the clogging
problems normally associated with J-T refrigerators.
Also, a two J-T stage Linde refrigeration cycle is often
designed into refrigerators which must operate at tempera-
tures near the 1 atmosphere boiling temperature of the
refrigerant [8].
Microminiature refrigerators with Linde cycles, or even
more complex cycles, can be fabricated for only slightly
more cost then ones with single J-T stages.
NEW REFRIGERATOR MODELS
Most microminiature refrigerators produced to date are
similar in size to the one in Figure 3 and are typically
designed to achieve 80K using N 2 at 1800 psi at 1.5 liters
per minute (STP) with a net refrigeration capacity of 250
milliwatts and a cooldown time of 8 to 12 minutes.
434
However, the potential commercial and non-commercial
applications for this technology have stimulated develop-
ment in two directions:
Lower Flowrate Models: These refrigerators are
designed to minimize the gas flow required for a
particular application. Hence, efficiency is
maximized by retaining the long thin design which
isolates the cold end from the warm end by a
narrow counterflow heat exchanger of low thermally
conductive glass. Figure 4 shows the inflow slide
of such a device designed to have 25 milliwatts
capacity using 1800 psi N 2 with a flow of about
0.i liter per minute. The cooldown time will vary
from one to several minutes depending on input
pressure and the mass being cooled.
Figure 5 illustrates a low flowrate refrigerator
cooling an electronic device in a hermetic package.
The planar nature of the refrigerator lends itself
to this type of hybrid packaging. Such packages
are potentially lower in cost than the double
walled glass/metal dewars used with traditional
J-T coolers.
Fast Cooldown Models: In many applications such
as cooling IR detectors in tactical missiles, the
primary objective is rapid cooldown. Efficiency
is of secondary importance. In these cases,
cooldown rate is optimized by changing the refrig-
erator configuration so that the amount of material
being cooled, including that in the refrigerator
itself, is minimized -- while at the same time
maximizing the cooling capacity in the refrigerator.
These objectives are achieved in the fast cooldown
model shown in Figure 6. The cooling in this
model takes place in the central portion of the
disc shaped refrigerator, with the heat exchanger
extending radially outward so that the outer
perimeter is at room temperature. Hence, this
refrigerator can be supported around its peri-
meter, providing a sturdy device capable of
withstanding the high 'g' forces that occur
during launch of tactical missiles. The device in
Figure 6 has a cooldown rate under 4 seconds using
3500 psi Argon with a 9 liter per minute (STP)
flowrate. This performance is in the absence of
any vacuum insulation.
435
One possible way of packaging a fast cooldown
refrigerator for cooling an electronic device is
illustrated in Figure 7. Since no vacuum insula-
tion is required with these devices, this package
could be backfilled with a dry gas which will
yield a longer shelf life package than one with a
vacuum shroud.
CONCLUSION
The use of photolithography to produce microminiature
refrigerators has enabled the production of refrigerators
with cooling capabilities on the order of i0 to I00 times
smaller than those of other commercially available refrig-
erators. A few significant side benefits have also
resulted from this approach. The fabrication process is
ideal for mass production with high yields, leading to low
unit costs for high volume applications. J-T coolers can
now be custom tailored for particular applications and
packaged in low cost hybrid packages.
Open cycle microminiature refrigerators are finding ap-
plications in laboratories, certain scientific equipment
and tactical missile guidance systems, where gas cylinders
or small gas bottles can be used. Also, MMR Technologies
is currently developing a miniature diaphram compressor
under ONR contract N00014-78-C-0514 which will allow for
closed cycle operation of microminiature refrigerators.
The initial compressor prototype is designed to produce 2
liters/minute gas flow at 1400 psi while operating at 2
hz. If a miniature compressor such as this can achieve
reliable life performance in excess of 2,000 hours, then
closed cycle microminiature refrigerator systems are
likely to find broad applications in military and com-
mercial instruments for cooling low heat dissipating
devices and specimens.
Acknowledements
I wish to thank W. Little for his helpful comments on this
paper and his continuing innovative contributions to this
technology. I also wish to thank S. Garvey, R. Wolfe,
S. Logan and the numerous others on the staff of MMR Tech-
nologies who have contributed significantly to the rapid
developments of this technology. We are indebted to the
Office of Naval Research for its early support of the
technology at Stanford University (grant N00014-78-C-0514).
436
Figure I. Scanning electron microscope picture of
portion of etched glass plate used to form the heat
exchanger. The wider grooves are about 200 um in
width.
437
Figure 2. Plate of outflow channels
for fast cooldown refrigerators before
they are etched and laminated to other
etched plates.
438
Figure 3. Original laminar flow glass Joule-Thompson
refrigerator showing high pressure inflow channels,
capillary, boiler and low pressure return.
439
Figure 4. Inflow slide for low flowrate Micromlnlature
Refrigerator.
Figure 5. Possible packageconfiguration for
low flowrate model.
440
Figure 6. Fast cooldownMicrominiature Refrigerator
which cools in its center with heat exchangeoccurring
radially outward.
441
Figure 7. Possible package configuration for fast cooldown
Microminiature Refrigerator with componentbeing cooled
mounted on the center cold spot and the electrical leads
extending radially outward.
442
RE FERENCES
[i]
[21
[31
[4]
[5]
[6]
[7]
[8]
W.A. Little, (private communication).
W.A. Little, MicroMiniature Refrigeration -
"Small is Better", Physica 109 & IIOB
(1982)
2001-2009
W.A. Little, "Scaling of Miniature Cryocooler
to Microminiature Size", Proc. NBS Cryocooler
Conf., J.E. Zimmerman and T.M. Flynn, eds.
Special Publication 508 (April 1978) p. 75.
W.A. Little, "Design and Construction of Micro-
miniature Cryogenic Refrigerators", in
Future Trends in Superconducting Electronics,
B.S. Deaver et al, ed. APS Conf. Proc. 44
(1978) p. 421
R. Hollman and W.A. Little, "Progress in the
Development of Microminiature Refrigerators
Using Photolithographic Fabrication Techniques",
Proc. NBS Conf. Refrigeration for Cryogenic
Sensors and Electronic Systems, J.E. Zimmerman
et al, eds. Special Publication 607 (May 1980)
p. 160.
W.A. Little, "Design Considerations for Micro-
miniature Refrigerators Using Laminar Flow Heat
Exchangers", NBS Conf. Refrig. for Cryogenic
Sensors and Electronic System, J.E. Zimmerman
et al, eds. Special Publication 607 (May 1980)
p. 154.
U.S. and foreign patents are pending on these
techniques and on the refrigerators themselves.
J. G. Daunt in Encyclopedia of Physics, ed
Flugge, Vol. XIV, Low Temperature Physics,
Springer-Verlag, Berlin (1956) p. I.
443


Low cost microminiature refrigerators for large unit volume applications

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