CA1170851A - Refrigerators - Google Patents

Abstract

REFRIGERATORS

ABSTRACT OF THE DISCLOSURE
A multilayer miniature low temperature refrigerator wherein a cooling chamber for a device to be continuously cooled is connected to an input and an output by micron sized channels or like passages in interfaces of a laminate of glass or like plates, the passages including a counterflow heat exchanger and a capillary section.

Description

1 1 7~85 1 This invention relates generally to refrigeration and more particularly the invention relates to microminiature refrigerators and methods of making the same.
Certain materials, called superconductors, have the ability to pass electric current without resistance. Since superconductivity is observed only at temperatures close to absolute zero, one of the main obstacles to extensive use of superconductins devices is the need for reliable, continuous refrigeration Superconducting devices, such as supersensitive magnetometers, voltmeters, ammeters, voltage standards, current comparators, etc., require a cryogenic environment to operate.
Traditionally this has been provided by a bath of liquid helium.
The helium is liquified elsewhere and transported to, and transferred to the device Dewar. The labor and complexity of such an operation has severely limited the use of these devices.
Many of the above superconducting devices dissipate only a few microwatts in operatlon whlle the available cryogenic systems provide a refrigeration capacity of watts, thus the devices are poorly matched to the refrigeration.
In addition, ~any devices such as optical microscope stages, x-ray diffraction sample holders, electron microscope cold stages, devices for cryosurgery in the brain, for ECG, ~CG
and EKG measurements, and low noise amplifiers require or benefit from suhambient operating temperatures.
Additionally, there are a number of high speed, high power devices such as VLSI (very large scale integration) chips and transmitters that are small, on the order of a centimeter square, and dissipate large amounts of heat, on the order of *;

1 1 7085 ~
10 to 50 watts. Traditional cooling devices, such as fans for convection cooling, are not capable of dissipating this amount of heat without significant increases in te~perature above a~ient.
Miniature closed cycle refrigerators such as those based on the Gifford-Mc~ahon cycle, Vuilleumier, ~tirling, etc., have been developed. These refrigerators, with capacities in the range of 0.5-13 watts, are convenient and compact but, be-cause of their moving parts, they introduce a large amount of vihration and magnetic noise which interferes with the operation of the devices. ~iniature Joule-Thomson refrigeration systems have been developed which have a cooling capacity typically be-tween O.S-10 watts. mhe design configurations of these compact systems are generally helicallY finned tubes coiled around a mandrel, the high-pressure gas flowing inside the tubes and the low-pressure gas flowing outside the tubes. Such helically finned and coiled heat exchangers are fabricated ~y laborious welding or soldering of the individual components. Because of the intricacy of the device, microminiature refrigerators with milliwatt capacities until now have not been made practically available.
What is needed for many devices is a microminiature refrigerator of approximately 1/2" to 4" in size with a cooling capacity in the milliwatt ran~e. Also needed are microminiature refrigerator fabrication methods which avoid conventional laborious welding or soldering techniques and allow the forma-tion of very small gas lines to operate the heat exchangers in the laminar flow regime and still have an efficient exchange of heat. The consequent absence of turbulence in the gas stream eliminates vibration and noise, both important considerations for superconducting device applications. The miniature size would allow the incorporation of an entire cryogenic system -superconducting sensor as a hybrid component in electronic ~ ~ 7~8~ 1 circuitry. me microminiature refrigeration capacity would allow the match-ing of the refrigeration system to the load. me invention meets these needs.
Also needed are microminiature refrigerators of the same general dimensions as discussed above that can dissipate large amounts of heat, 10--50 watts, generated by certain small devices while maintaining ambient or subambient operating temperatures. And such refrigerators should be easy to manufacture and in configurations that are compatible with standard elec-tronic packaging.
As explained in greater detall below, the microminiature refriger-ator of the present invention comprises, in a unique form and scale a plura-lity of sealed plate-like members which form between them a cooling chamber, heat exchanger capillary passages and fluid passages for conveying incoming high pressure gas successively through the heat exchanger the capillary sec-tion and into the cooling chamber. Return or outflow passages conduct the fluid from the cooling chamber through the heat exchanger in counterflow relation with the incoming gas and then to the exterior of the device.
Such a microminiature refrigerator requires scaling down a conven-tional refrigerator by a factor of about a thousand. The design parameters for a microminiat~re refrigerator of the same efficiency as a conventional refrigerator using turbulent flow are described in "Scaling of Miniature Cryocoolers to Microminiature Size", by W.A. Little, published in NBS
Special Publication in April, 1978.
In summary, the diameter d of the heat exchanger tubing, L the length of the exchanger and _ the cooldown time are related to the capacity which is proportional to _ the mass flow, in the following manner:

. 0.5 d ~ (m) . 0.6 1 ~ (m) . 0.6 t ~ (m) 7~85 ?.

A microminiature turbulent flow refrigerator with a capacity of a few milliwatts should have d = 25~ and 1 a few centimeters.
As the device becomes smaller and smaller, eventually the mass flcw becomes too small to allow turbulent flow of the fluid to occur.
Laminar flow operation then becomes possible without loss of refrigeration efficiency and gives improved performance.
The theoretical basis for designing microminiature refrigerators using laminar flow heat exchangers is discussed in "Design Considerations for Microminiature Refrigerators Using Laminar Flow Heat Exchangers", pre-sented by W.A. Little at the Conference on Refrigeration for CryogenicSensors and Electronic Systems, Eoulder, Colorado, October 6 and 7, 1980.
For microminiature heat exchangers operating in the laminar flow region over the same pressure regime and having the same efficiency, the length of the exchanger (1) should be made proportional to the square of the diameter (d) of the exchanger tubing. For example, a Joule-Thomson exchanger operating with N2 at 120 atmospheres, with a capillary channel passage 5 cm long, 110 microns wide and 6 microns deep should provide approximately 25 milliwatts cooling. Different refrigeration capacities can be obtained by varying the width of the channel with no change of the efficiency. Gne may thus operate under streamlined conditions free of vibration and turbulen oe noise, an advantage, particularly for superconducting d~vices, which require a very low noise envirolment.

l ~7~851 In a Joule-Thomson refrigerator of this type it is normally convenient to use a capillary channel to throttle the compressed gas; however, it is common knowledge that a porous structure such as porous metal, sintered ceramic, etc.
can be used equally well for throttling the gas.
To increase the efficiency of the refrigerator for certain applications one form of the invention provides two capillary sections arranged in series or in parallel and passages for conduting a substantial portion of the gas directly to the outflow passage of the heat exchanger after passage through only one of the capillary sections.
In order to construct microminiature refrigerators, new fabrication techniques are needed for producing heat exchangers and expansion nozzles, a factor of 100 to 1,000 times smaller than those of conventional refrigerators.
Conventional fabrication techniques are ill-suited for microminiaturization since channels of the order to 5-500 microns must be formed accurately and the device must be sealed so as to withstand high pressure of the order of 150-3000 psi for refrigeration efficiency.
Accordingly, the major object of the invention is to provide a novel microminiature refrigerator particularly for cryogenic cooling and mode of assembly.
Another object of the i.nvention is to provide a novel microminiature refrigerator with a cooling capacity ranging from milliwatts up to 50 watts or more.
Another object of the invention is a novel multilayer microminiature refrigerator.
Another object of the invention is a novel single-stage cryogenic microminiature refrigerator.

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Another object of the invention is a novel multistage cryogenic microminiature refrigerator.
Another object of the invention is a novel method of manufactur-ng a microminiature refrigerator.
Yet another object of the invention is a novel method of making a microminiature refrigerator us~ng photolithographic and chemical etching techniaues.
Still another object of the invention is a novel method of making a microminiature refrigerator using a fine-particle san~blastin~ technique.
A further object of the invention is to provide a novel refrigerator of small size comprising two or more plates of a low thermal conductivity material such as glass bonded pressure-tight and containing at one or more plate interfaces micron-sized gas supply and return passages to a chamber that is adapted to continuously cool a superconductor or like device.
Pursuant to rhis object the inlet gas pressures may be in the order of 150-3Q~n pounds per square inch, and the passages may be in the range of 5-500 microns wide and 5-6n microns deep.
Pursuant to the fore~oing object of the ~assages and cooling chamber may be formed by recessing plate surface areas or forming raised channel walls at the interface or interfaces of the plates.
And still another object of the invention is a novel method of making a microminiature refriaerator hy forming raised channel walls.

Another object of the invention is to provide a novel refrigerator assembly having two or more multilayer refrigerators for cascade cooling to reach low cryogenic temperatures.

1 t ~ ~ 8 5 1 A further object of the invention is to provide a novel microminiature refrigerator composed of three or more similar plates of glass or equivalent material assembled in a stack with micron-sized fluid flow high-pressure inlet and low-pressure outlet passages cor.nected to a cooling chamber and being arranged in separate layers at interfaces between adjacent plates.
Further to this object it is contemplated that in a lamination of three plates hish-pressure inlet, heat exchange a~d czpillary e~pansion passage sections are formed in series in one plate surface-bonded to an intermediate plate, one sur-face of which forms one side of the passages and low-pressure pressure return recess means is formed in the surface of the other plate that is bonded to said intermediate plate.
~ nother object of the invention is to provide a novel refrigeration assembly having a special mounting and gas passage defining holder at one end, as for mounting the refriserator to ex.end cantilever fashion within an evacuated enclosure.
Further objects of invention will appear as the description proceeds in connection with the appended claims and the accompanying drawings.
In accordance with the invention, a microminiature refrigerator and method of ma~.ing the same is provided. It should be noted that the microminiature refrigerator can be scaled up both in size and capacity for certain applications.
The refrigerator is novel, irrespective of size; however, it is the miniaturization of the refrigerator that is difficult and therefore this is 2escribed in 2etail here. The turbulent or laminar flow microminiature refr:igerator includes one or more plates on some of which micron-sized gas channels are formed and completed by one or more bonded plates resistant to high pressures. The microminiature refrigerator may be used for, but is not limited to, cryogenic refrigeration.

~-7-1 1 7~85 1 According to a first broad aspect of the present invention, there is provided a multilayer microminiature refrigerator comprising a laminate of three planar-surfaced thin plates bonded pressure-tight at the interfaces between adjacent plates, means providing in one inter-face between two adjacent plates a first continuous inflow passage means extending from an inlet to a cooling chamber and including capillary passage means, means for connecting said inlet to a source of refrigerant gas at high pressure, means providing in the other interface between two adjacent plates a further continuous outflow passage means leading to an outlet, and a passage interconnecting said chamber and said further passage means whereby fluid at reduced pressure from said cooling chamber may pass through said further passage means to said outlet in counter-flow heat exchange relation with fluid in said first passage means for regenerative precooling, each of said passage means being of micron size whereby to promote laminar flow therein.
According to a second broad aspect of the present invention, there is provided a nnethod of making a refrigerator wherein a thin glass plate is surface-bonded pressure-tight to another plate of about the same coefficient of the:rmal expansion, comprising the steps of: a) forming a layer of fine-particlc sandblast-resistant material on sai.d gla.ss plate; b) definillg a. flow path pattern in sa:i.d rcsistant material by lithographic masking and selective etching to thereby expose the underlying surface of said glass plate in said pattern; c) scanning a miniature air abrasive device across said glass plate so as to form recessed channels in said surface by fine-particle blasting to the required depth; and then d) bonding said other plate upon the recessed surface of said glass plate.
~ he invention will now be clescribed in greater detail with reference to the accompanying drawings.

-7a-1 1 7û8~ ~

BP`IEF DE~CRIPTION OF DRAWINGS
-Fig. 1 is an exploded view showing a microminiature refrigerator according to one embodiment of the invention;
Fig. 2 is a generally plan view showing a micromini-ature refrigerator part having a fluid passage pat'ern accord-ing to a further em~odiment;
Fig. 3 is a generally perspective view showing a microminiature refrigerator part e~hi~lting a further modification;
Fig. 4 is a plan view showing a multiple unit module embodiment of a microminiature refrigerator;
Fis. 5 is a cross sectional view showing another embodiment of a microminiature refrigerator;
Fig. 6 is a cross sectional view showing another embodiment of a multiple unit module microminiature refrigerator;
Figs. 7A-7E are cross section views illustrating ste~s in a method of fabricating a microminiature refrigerator;
Figs. 8A-8D are cross section views showing steps in ar.other method of fabricating a microminiature re.frigerator;
Figs. 9A-9C are cross section views sho~!ing steps in a method of fa~ricating a microminiature refrigerator having raised channel walls;
Figs. lOA-lOD are cross section views showing steps in another method of fabricating a microminiature refrigerator;
Fig. 11 is a cross section view showing fabrication of a multiple unit module embodiment.

~ I 7~5 ~
Fig. 12 is a plan view illustrating a use and environ-ment of the refrigerator of the invention;
Fig. 13 is a schematic view illustrating the fluid circuit in the refrigerator of the invention;
Fig. 14 is an illustration section showing the laminated nature of the refrigerator and the multilayer fluid flow paths, details of which are shown in later Figures;
Fig. 15 is a plan view partly broken away showing a refrigerator holder;
Fig. 16 is a section substantially on line 16-16 of Fig. 15;
Figs. 17, 18 and 19 are top plan views of the three (upper, middle and lower) elements that comprise the laminate or sandwich construction of the refrigerator of the invention according to a preferred embodiment particularly illustrating the various fluid flow passages and chambers;
Figs. 20 and 21 are section-s substantially along lines 20-20 and 21-21 in Fig. 17;
Fig. 22 is a section substantially along lines 22-22 of Fig. 18;
Figs. 23 and 2~ are sections substantially along lines 23-23 and 24-24 in Fig. 19;
Fig. 25 is an exploded view showing an embodiment wherein the flow passages are differently located;
Fig. 26 is an exploded view showing an embodimer.t wherein two low-pressure flow paths are provided;
Fig. 27 is a fragmentary view showing a pad of increased thermal conductivity on the cool end of the refrigerator;
Fig. 2~ is an exploded view showing a multilayer refrigerator according to a further embodiment;

. .

1 17~5 .

Fig. 29 is a plan view showing the plate of Fig. 28 in which the low-pressure gas passage means is formed;
Fig. 30 is a plan view of the plate of Fig. 28 in which high-pressure passage means is formed;
Fig. 31 is an illustration view showing the laminated nature of two multilayer refrigerators combined in one unit to provide cascade cooling;
Fig. 32 is a section substantially along line 32-32 of Fig. 31;
Fig. 33 is an exploded view of a further embodiment of the invention; and Fig. 34 is a plan view of one component OL a further embodiment of the invention.

PREFERRED EMBODIMENTS
Referring now to the drawings, Fig. 1 is a perspective exploded view of one embodiment of a microminiature re-refrigerator in accordance with the present invention and includes a body 12 in sealed surface contact with substrate 10. ~he body 12 is of crystalline (e.g., such as silicon), an amorphous (e.g., glass), or metallic (e.g., copper, stainless steel) material; and substrate 10 is a material such as*Pyrex, soda gl&ss or~Kovar, having a coefficient of thermal expansion which is compatible with the coefficient of thermal expansion of the body 12. Preferably, the body 12 and substrate 10 are coextensive thin glass plates.
J~ k ~ 17~85~

The body 12 and substrate 10 must be thic~ and/or strong enough to withstand the pressure of the incoming gas typically of the order of 150-3000 pounds per s~uare inch pressure. For example, a silicon body may be approxi~ately 300 microns (.0118") thick, a glass hody may he approximately 0.010" to .020" thic~s, and a glass substrate is about 0.010"
to 0.020" thick. Formed on or etched in the surface of the body 12 which is mounted on substrate 10 are parallel "serpen-tine" channels 14 and 16, 5-300 microns wide and separated by 150-S00 microns walls 15. Channels 14 and 16 interconnect an outlet port 18 (the low-pressure return) and input port 20 (high-pressure inlet) respectively to a reservoir or cooling chamber 24. The size of reservoir 24 is determined by the de-sired reserve capacity neeced for fluctuating demandsO The foregoing channels and reservoir are formed in the bottom sur-face 13 of body 12 which is bonded flush onto the flat top sur-face 17 which effectively closes them. The interface ccnsist-ing of surfaces 13 and 17 is bonded pressure-tight.
The channels 14 and 16 define respective low-pressure and high-pressure cooling lines which run in juxtaposition for an initial length and thereby define a heat exchanser section shown senerally at 22. A fine channel filter section 21 is provided between the inlet and the heat exchange section.
Beyond the heat exchanger section 22 the input channel 16 be-comes independently sinuous and narrower at 26 of substrate 10 allowing the fluid to drop in pressure and then expand. As an example, channel section 22 is about 250 microns wide and 50 microns deep while channel 26 is about 125 microns wide and 10 microns deep. The end of the expansion line 26 is connected directly into the reservoir 24 and the output channel 14 extends from tne reservoir 24 back through the heat exchanger to the i 1 7~85 ~
output port 18. Reservoir 7~ is preferably about 20-50 microns deep.
~ ounted on the other or lower surface of substrate 10 is an interface unit 30 of suitable metal alloy such as~ ovar which is an alloy of iron, nickel and cobalt with a coefficient of e~pansion about the same as the material of substrate 10 having holes 32 and 34 e~tending therethrough and com~unicatlng through aligred bores in substraie 10 with the ports 18 and 20 respectively of the body 12. Bonded to the interface unit 30 are a pair of miniature tubes 36 and 38 which communicate a fluid to and from the refrigerator. Tubes 36 and 38 may com-pr se stainless steel material as used in hypodermic needles or Teflon tubing. The interface unit 30 is attached pressure-tight to the suhstrate 10 by a suitable sealant such as epoxy.
Suitably mounted on the top surface of body 12 in direct abutment with the wall of the reservoir 24 is a device 40 to be cooled. The device 40 may be any one of a number of devices operated 2t a low temperature (e.g., supersensitive magnetometers, gradiometers, bolometers an~ other like devices which are based on the Josephson effect or other devices which are well known in the art) or devices for operating at higher temperatures (e.g., infra-red detectors, solid state lasers or samples whose physical properties are to be determined).
The entire assembly may be contained in a Dewar or vacuum vessel to reduce the heat transfer to the parts The illustrated microminiature refrigerator is a Joule-Thomson, open-cycle refrigeration system in which tube 38 is connected through a control valve 39 to a container 37 of highly compressed refrigerant gas such as nltrogen, hydrogen or helium.
The highly compressed gas enters at an inlet pressure of approximately 150-3000 pounds per square inch and a flow rate of approximately 5-50 milliliters/sec ~STP) through port 20 and passes through the heat exchanger 22 where the gas is t~ra~ ma~ -12-8 5 ~
cooled by lower-pressure supercooled gas eYiting the device through channel 14, port 18 and tube 36~ The high-pressure gas exits the heat e~changer 22 and passes through the capillary expander 26 where the drop in pressure reduces the temperature of the sas which enters the reservoir 2~ as a supercooled or cryogenic fluid. The low-temperature reservoir 24 in turn cools the device 40 mounted on reservoir 24 and the absorbed heat causes the fluid to vaporize and it flows through channel 14 to the e~haust port 18.
An illustrative microminiature refrigerator whose hody 12 is of 0.020" thick glass 1/2 inches wide by 3 inches long has a refrigeration capacity of 100 milliwatts at 122K;
channel dimensions are of the order o~ 100 microns and the flow rate about 30 milliliters/sec (STP) of nitroaen at an inlet pressure of 1600 pounds per square inch. The substrate 10 is a glass plate of the same size Another illustrative microminiature refrigerator whose body is silicon measures 75 by 12 by 2 millimeters, has a 30-centimeter-lona heat exchanger section and operates from room temperature to 200K using CO2.
A microminiature refrigerator can be appro~.imately 1/2" to 4" in length, 1/2 inches wide, 0.040-0.060 inches thick with typical channel di~ensions between 5-500 microns with separating walls 150-500 microns wide, can have a cooling capacity between 1.0-50,000 milliwatts at temperatures ranging from 2-3Q0K, and can withstand input pressures from 150-3000 pounds per square inch. However, it should be appreciated that the microminiature refrigerator as described could be scaled up or down both in size and capacity for certain applications.
In addition to an open-cycle described above, it will be appreciated that the refrigerator can be a closed-cycle system using a compressor to recompress the gas. In addition, the method of fabrication described here could be used for the construction of parts of refrigerators using other cycles such as the Servel, Gifford-McMahon, pulsed tube and Vuille~mier systems.

1 17~5~
In sore of the laminar flow ~evices the design of the channels may be modified by having them straight as illus-trated in Fig. 2 rather than "serpentine." Fig, 2 shows the bottom surface of such a body~ In this embodiment, high-pressure gas enters at port 42, flows through the parallel heat exchanger channels 43 to the sinuous capillary channel section 44 thence to the reservoir or cooling chamber 45, through the lo~-pressure return 46 to the outlet port 47. An advantage of this design is that the low-pressure return 46 as shown completely surrounds the high-pressure channel lines so that any minor aas leak from a high-pressure line is captured by the low-pressure return and does not escape into the sur-rounding vacuum, ~7hich insulates the refrigerator from the environment. A possible disadvanta~e of this design is the long path for the heat to travel through the alass at the heat exchange section bet~7een the incoming lines 43 and outgoing channel 46. This difficulty may ~e avoided by combining the glass body with a glass substrate 48 shown in Fig. 3 which has highly conducting transverse metal strips or wires ~9 printed or bonded respectively. These stripes or strips may be bonded upon the surface of the glass substrate that form the lnter-face with body 41. These transverse conducting pieces ~9 pro-vide a high thermal conductive path laterally across the refrigerator while maintaining the thermal conductivity length-~ise along the refrigerator at a low value determined by the glass.
The microminiature cryogenic device and refrigerator of this invention also lends itself to multiple u~it config-urations. For example, a plurality of refrigerators, each using a different coolant, will provide cascade cooling of one gas by another and thus produce refrigeration at extremely low temperatures. Additional poxts can be included in the device with channels interconnecting the additional ports as described and additional reservoirs for further cooling of a cryogenic `~ :

1 ~ 7~85 ' device. Fig. 4 shows in perspective a multiple glass unit body module 51 of a microminiature refrigerator in accordance with this phase of the present invention.
There are two reservoirs or coolant chambers 52 and 53. Cha.~er 52 has an input line 54 leading from input port 55 through a sinuous heat~e~change section 56 and a fine capillary section 57 into the chamber, and an output line 58 that has a sinuous heat exchan~e section 5q coe~tensive with section 56 leading to output port 60. Chamber 53 has an input line 61 leading from an input port 62 through a sinuous heat e;~change section 63 and a capillary section 64 into the cham~er, and an output line 65 that has a sinuo~s heat exchange section 66 coextensive with section 63 le2dino to output port ~7. A
source of high-pressure nitrogen is connected to port 55, and a source of high-pressure hs~drogen is attached to port 62. The two circuits thus are partially interactive whereby the fluid going to chamber 53 is precooled more extensively hefore expansion. One circuit uses Joule-Thomson expansion of nitrogen to cool hydrogen to 77K at chamber 52. The other circuit uses Joule-Thomson expansion of the precooled hvdrogen to reach ~1K
at chamber 53. The entire refrigerator as shown comprises two heat exchangers, two expansion sections, two cold licuid reser-voirs, and the input and output ports. The size of the device is approximately 2" to 5". Similarly, a three-stage system using nitrogen, hydrogen and helium will produce cooling at 4.5K.
Fig. 5 is a cross sectional view of another embodiment showing a multilayer microminiature refrigerator where the in-coming and outgoing channel formations 73 and 74 are formed on either side or both sides of a glass body 75 and on either side or both sides of which are bonded two glass substrates or cover plates 76 and 77. High-pressure fluid flows along the channels 73 and returns through channels 74. The channels 73 and 74 may be etched or otherwise formed il the surfaces of body 75 as shown in full lines or etched or other~7ise formed in the :

l ~ 7~85 1 surfaces of either or both glass cover plates 76 and 77 t~at are bonded to the body as shown at 73' and 74' in dotted lines.
The channels 73 and 74 may also be formed with raised channel walls on the body 75 or on the substrates 76 and 77. The channel formations 73 and 74 in each instance consist of inlet sections, heat exchange sections, capillary sections and cool-ing chambers which may be constructed and related as in ~igs.
1-4. The difference between Fig. 5 and Fig. 1 is that in Pig. 1 the incoming and outgoing channels may be described as formed in a single layer- that is, they are formed in the same or an abutting planar surface -whereas in Fig. 5 they are formed in a nonabutting surface. It is conte~plate~ that in Fig. 1 the in-coming ~nd out~oing channels may be formed in the op~osed sur-faces 13 and 17 respectively.
Fig. 6 is a cross sectional view showino another e~bodiment of a multiple unit module microminiature refrigerator.
Channels 80, 81, 82 and 83 are forme~ in facing surfaces of thin glass bodies ~, 85, 86, 87 and 88 by etching, particle blastina or by forming raised channel walls. For example, hiah-pressure nitrogen enters chanr.el 80 ~nd returns via low-pressure channel 81; an~ high-pressure hydrogen enters via channel 82 and is precooled by nitrogen in channel 81. Lo~t-pressure hydrogen exits via channel 83. The inlets, heat exchange sections, expansion lines and reservoirs are included in the respecti~e channels as in the earlier embodiments.

FP~P~ICATION ~ETHODS
Fig. 7A-7E illustrate one method of fabricating the refrigerator by etching channels in glass or silicon ~lates.

The techniquesto be described are to some extent well ~no~n in the manufacture of semiconductor devices, such as integrated circuits, and may include conventional photoresist masking and etching techniques. In one instance, by using a silicon plate 1 17~8~' material having a surface crystalline orientation on the (1,0,0) plane, anisotropic etching can be employed to form V-shaped grooves in the silicon plate surface. ~lternately, vertical walls can be made using a silicon plate with a surface o-ientation of the (l,l,O)plane. In Figure 7A a portion of a silicon plate 9Q is shown in cross section and a silicon oxide layer 91 is provided on one major surface thereof. The plate 90 is on the order of 300 microns in thickness and o~ide layer 91 is approxi-mately 9,000 angstroms in thickness. The oxide layer may be formed by heating the silicon wafer in a wet oxygen atmosphere.
Photoresist 92 is applied to the surface of silicon oxide 91 and is exposed under a photomask having the desired channel pattern. The photoresist is removed and the e~posed o~ide is etched leaving it in the pattern 93 as shown in Figure 7s.
The silicon oxide now acts asa mask and the exposed silicon is etched using an anisotropic etchant, such as ethylene diamine, resulting in the V-grooves 94 shown in Fig. 7C. Upon completion of the etching of the V-grooves, the remaining oxide layer 93 is removed from the silicon plate and the plate is cleaned.
An optically flat Pyrex or equivalent glass plate 95 is then bonded to the etched surface of silicon plate 90, as sho~m in Fig.
7D. The bonding of the glass to the silicon surface is performed by known field assisted or anodic bonding techniques.
Thereafter, as shown in Fig. 7E, the silicon plate 90 is eLched or otherwise cut from the backside to reduce the thic}:ness of the assembly and hence the thermal conductance of the laminated refrigerator structure.
Input and output lines are then drilled or etched in the reverse side of the glass substrate 95, and the tubing gas lines are then bonded to the reverse side of the glass plate by means of epoxy. ~y using photolithographic definition ~ 1 7~85 ' and chemical etching, the entire refrigerator including heat exchanger, expansion line and liquid reservoir channel ormations can be formed in one step. Electron beam or x-ray lithography electrolytic and plasma etching can be employed as well as chemical etching. The foregoing photoresist method may be used where the body and substrate are both glass plates, using csnven-tional materials and techni~ues.
Figs. 8A-8D illustrate another method of fabricating recessed channel formations in a hard, amorphous isotropic material such as glass or crystalline material such as silicon. The method allows good si~e control, improved reso-lution compared to chemical etching, eliminates undercutting and allows the formation of vertical walls. This method is not limited to the manufacture of the microminiature refrigera'or.
In Fig. 8A, a portion of a glass plate lO0 approximately .020" thick is shown in cross section and a resist layer lO1 is provided on one major surface thereof. The purpose of this resist layer is to protect the underlying surface and to provide a pattern for channel layout. The resist may be a photo-sensitive or non-photosensitive resist but must be resilient or tenacious enough to }~e able to withstand fine-particle "sand-blasting" as will be described below. The resist may cover the surface of the entire plate lO0 or be screen-printed on plate lO0 so as to form a pattern If the resist is a photoresist, it is exposed through a conventional photo mask to ultraviolet light in order to define a pattern. A novel photoresist which meets the requirements of being able to withstand fine-particle sand-blasting is comprised of: 7 grams gelatin (e.g., Knox) and l gram ammonia bichromate dissolved in 50 cc hot water.
The resist forms a thick, spongy layer approximately 20-30 times the thickness of conventional resists. The unexposed 1 1 7~85 1 portions of the resist can be removed by hot water, or by using the enzyme, protease, to digest the unexposed portion, and result in the structure ~hown in Fig. 8B. The remaining resist 102 is tough and resilient, able to withstand the abrasive action of fine-particle "sand-blasting" while allowing exposed areas of the glass plate surface be abraded away. A miniature air abrasive device (e.g., Airbrasive Unit, ~odel K; S.S. White~, which entrains a stream of fine alumina particles at 30 pounds ~er square inch acts as a fine particle "sand-blast". The "sand-blast" device is scanned at a constant rate across the resist carrying plate surface of Fig. ~B; and a jet of 17 micron particle abrasive powder can be used to remove approximately 2 microns of material at each pass. Larger powder ~articles (e.g., 27 and 50 microns) etch more rapidly but may give poorer definition.
They may be usea for fabricating larger devices with adequate accuracy.
Channels 103 formed by this particle-blast method, as shown in Fig. 8C, have a precisely controlled depth (2-300 microns), vertical walls and edge definition of approY.imately 5 microns roughness. Vpon completion of the formation of the recessed channels, the remaining photoresist is removed and the plate 100 cleaned. The entire refrigerator cooling chamber and passage system can thus be formed in one step. As shown in Fig. 8D a substrate 104, such as a soda glass cover slide, is bonded to the etched surface of glass plate 100 with an adhesive bond less than 10 microns thic~ but able to withstand 500-3,000 psi.
This is the same mode of bonding used in all embodiments for securing two or more glass plates together to form a permanent pressure-tight assembly. Such a bond can be made with epoxy or ultra-violet curable cement (e.g., Norland's Optical Adhesive). A
micron-thick seal can be made by drawing diluted adhesive into the space between the plate 100 and substrate 104 by capillary -lg-l ~ 7~85 1 action. The re~rigerator is then illuminated with intense ultraviolet radiation until the adhesive polymerizes and forms a bond.
Alternately the cover plate-or suhstrate may be fused to the etched body plate with a thin film of solder glass screen-printed on either plate or both plates, using conventional methods for the fabrication of liquid crystal displays. Input and output lines are drilled or etched in the reverse side of the glass substrate 104, and stainless steel hypodermic or Teflon tubing gas lines are then bonded to the reverse side of the glass plate lO~ as by ~eans of epoxy. By using the fine particle sandblasting technique, a hard amorphous isotropic material such as glass can be used for the body plate lO0 of the refrigerator. The use of such materials avoids the problems associated with the high thermal conductivity of silicon and therefore lower te~peratures can be achieved at the co~d chamber end of the refrigerator.
Figs. 9A-9C illustrate a method of fabricating a microminiat~re refrigerator by forming raised channel walls on a surface as opposed to recessed channels in a surface. The plate 110 may be a crystalline, amorphous or metallic material, prefexably glass on the plane surface 111 of which channels are to be formed. ~aterial 112 such as glass frit powder, epoxy, solder glass, ultraviolet cu^able cement, etc. is screen-printed as a pattern onto the surface 111 as shown in Fig. 9B. Upon firing, the glass frit powder melts and defines the channel w211s. Likewise, as the epoY.y cures, solder glass hardens, or ultraviolet curable ce~ent is exposed to ultraviolet radiation, the channel walls are formed. 5-300 micron spacings are accur-ately made using this technique. Fig. 9C shows a glass sub-strate plate 113 boncled by any of the previously mentioned bonding methods to seal the gas eYchanger lines. As before, input and output lines are then drilled or etched in the ~ l 7~85 1 reverse side of plate 113, and the gas lines are then bonded to the reverse side of plate 113.
Figs. lOA-lOD show a methoc1 for the fabrication of the refrigerator such as that of Fig. 5 where the channels are etched on either or both planar sides of a glass plate 120 on which are to be honded the two glass plates 121 and 122 (Fig. lOD). Particularly for the laminar flow design, this simplifies the design of multistage devices.
To fabricate the device the plate 12~ is screen-printed with a thin continuous layer of glass frit or covered with sol~er glass 123 on each side (the same process may be done on one side only); these layers are then fused, as in the method step shown in Fig. lOA. Then a resist 124 is either printed on the top and bottom surfaces; or a photoresist is used, exposed and developed on each side at 124 (Pig. lOB), so that each side of the plate 120 bears the desire~ pattern, using one of the above disclosed methods, Plate 120 now is abraded and the resist removed, leaving the ~late 120 as sho~n in Fig. lOC. It will be noted that at this point the channel formations 125 and 126 respectively appear in separate layers, and the glass surfaces between them are covered with the fused-on frit 123.
Cover glass plates 121 and 122 are then bonded upon the top and bottom surfaces by a program that may include first heating the entire assembl~ to the softening temperature of the solder glass or frit. This also seals the inlet port and the outlet port to complete the refrigerator (Fig. l~D). Holes through the plate at the cold or cooling chamber end connect the xeservoir, which is connected via the capillary to the high-pressure channels 125, to the low-pressure channels 126.
High-pressure gas passes through channels 125 to the cooling chamber and then back through channels 126. The channel layers may be formed in either or both suhstrate plates 121 and 122 instead of entirely in body plate 12~.

1 ~ 7 ~ 8 ~ 1 Fig. 11 illustrates a method of fabricating a stacked multistage refrigerator 130 such as that of Fig. 6. By this pro-cedure multistage devices may be conveniently constructed where the different gases pass in spaced layers through passages in a bonded stack of plates as illustrated in Fig. 11.
This refrigerator co~prises five bonded glass plates preferably of the same size in a stack. Here the three inter-mediate plates 131, 132 and 133 are ~ormed with surface recesses according to one of the foresoing methods. ~or example, plates 131 and 132 are formed with surface channels 134 and 135 respectively in accord with ~he methods of Figs. 7A-7~, Figs.
8A-8D or Figs. 9A-9C; and plate 133 is formed on opposite sides with surface channels 136 and 137 respectively as by the method of Figs. lOA-lOD. In each case the particle blast mode is preferred whereby the open ends of each channel are spaced by fused frit layers on the glass surface.
In the two stage device illustrated high pressure nitrogen may enter via channel 134 and return via 10W pressure channel 135~ high pressure hydrogen may enter via channel 136 and is precooled by heat exchange with nitrogen in channel 135, The low pressure hydrogen returns to the outlet port via channels 137. The whole assembly is bonded together with solder glass as disclosed earlier. Alternately plates having raised channel walls may be used instead of the recessed channels in this two-stage refrigeration assembly. The communicating ports between the various layers are suitably formed.

1 1 7~8S 1 While in the above~described embodiments, the refrigerator has a crystalline or amorphous body, in some cases the refrigerator can be photoetched in a copper film on the surface of a circuit board or a thin sheet of stainless steel. While the described refrigerator is of the open-cycle type, as indicated above, closed-cycle refrigerators may be fabricated using the techniques in accordance with the present invention.
It should be appreciated that the channels can be also formed as described above on capillary tubing with the tube in abutment with the confining internal surface of another tube to form a cylindrical heat exchanger refrigerator. The sealing of this tube to the inner one may be accomplished by using a heat shrinkable tubing such as Betalloy (Raychem Corp.) for the outer tube.
Channel dimensions, surface bonding techniques, channel forming and other characteristics of the refrigerator may be as described below in connection with the embodiments of Figs. 12-32.
Referring to Fig. 12 which illustrates a typical installation, the refrigerator 211 is mounted at one end in a holder 212 within which it i5 fixed so that the refrigerator and holder usually comprise a unitary assembly indicated at 213.
In the illustrated assembly, the refrigerator contains flow passages that are connected through the holder to fluid inlet and outlet means.
Fig. 12 shows the assembly mounted in a well 214 of a typical boxlike enclosure 215 having a suitable airtight cover indicated at 216. In the enclosure the holder 212 is affixed suitably at the bottom of the well and the refrigerator extends cantilever fashion through the well. Preferably the well is subjected to subatmospheric pressure through the conduit 217 leading to a source of vacuum 218.

The device 219 to be continuously cooled which may be a small superconductor chip or like device is suitably mounted within the well 214 preferably in contact with the coolest region of the refrigerator as indicated in dotted lines in the drawings and any wiring therefrom (not shown) passes through sealed ports in the enclosure. In the illustrated embodiment this device is preferably mounted in direct contact with the refrigerator glass surface that serves as a cover for chamber 224.
Referring to Fig. 13 the fluid path is at least schematically shown. The refrigerator 211 has an inlet port bore 221 for admitting highly compressed gas that flows through a passage having heat exchange section indicated at 222 and a smaller diameter capillary section 223 into a cooling chamber 224.
The device 219 to be cooled is located as closely as possible to chamber 224, which is the coolest part of the refrigerator as will appear, and fluid leaving the chamber 224 returns along a passage 225 extending adjacent and in heat exchange relation to the inlet passage section 222 and leading to an outlet port 226. These passages are micron-sized for very low temperature refrigerations of milliwatt capacity as will appear.
In the invention inlet port 221 and outlet port 226 connect into bores 227 and 228, respectively, in holder 212, and the holder is so mounted in the enclosure as to connect bores 227 and 228 with fluid inlet conduit 229 and fluid outlet conduit 231 projecting from the enclosure. Where the system is an open cycle refrigerator, conduit 229 is connected to a source of pressurized refrigerant gas and conduit 231 is connected to a suitable -exhaust. In a closed cycle system the conduits 229 and 231 are connected through a loop containing a condensor and pressurizing assembly.

~ 37~85~
As shown best in Fig 14 the refrigerator 211 comprises a three-element sandwich consisting essentially of three bonded similar accurately flat glass or o~her similarly low thermally conductive plates 232, 233 and 234 of approximately the same length and widthJ having thickness preferably in the order of 0.020". The middle plate may be thinner than the others to enhance the heat exchange between inflow and outflow channels.
The refrigerator 211 may overall be about 1/2" wide and 2 1/4"
long with a total thickness of about 0.060", this representing a workable embodiment that has been successfully tested. Another workable embodiment is 0.2" wide, 1.0" long and .060" thick.
The refrigerator is of multigas-layer construction.
That is, the fluid supply passages 222 and 223 and the chamber 224 are formed to provide one fluid flow passage layer within sandwich substantially in one plane while the return passage is formed to provide a separate fluid flow passage layer within the sandwich substantially in a spaced plane As will appear these layers are in fluid flow connection through an opening in the chamber wall.
Figs. 17-24 illustrate detail of anotl~er embodiment.
The thin flat glass plates 232, 233 and 234 are of the same size.
The intermediate glass plate 233 is smooth with opposite flat smooth coplanar surfaces 235 and 236 (Fig. 22). The upper plate 232 as illustrated is transparent. As shown in Fig. 17, plate 232 has recessed regions or channels in its bottom surface 237 defining the inlet port 221, the heat exchange passage section 222, the capillary passage section 223 and the chamber 224 connected in series providing a continuous fluid flow path from the inlet port 221 to chamber 224. Port 221 is in the nature of a closed bottom well.

I ~ 7~85 1 Intermediate plate 233 closes one side of the passages in plate 232, and at one end it has a through bore 238 aligned with port 221 of plate 232 in the assembly. Plate 233 at the other end has a through port 239 aligned with chamber 224 in the assembly.
Bottom plate 234 which is formed on its top surface 240 with the low-pressure fluid return path that comprises a generally rectangular large area surface recess 241. A series of spaced ribs 242 and several rows of projections 243 are provided on the bottom of recess to supportingly contact the lower surface 236 of the intermediate plate in the assembly while not interfering appreciably with fluid flow. Plate 234 is formed with a through bore 243 that aligns with bores 238 and 221 in the assembly of Fig. 16. A second bore 244 in plate 234 opens into recess 241. Fluid exhausted from chamber 224 in the assembly passes through port 239 into recess 241 and exhausts through port 244.
In the assembly glass plates 232 and 233 and 234 are bonded in the stack pressure tight at their interfaces. This laminate is placed in holder 212 where one end i6 bonded to the metal holder by adhesive layers 249 and 250 with ports 243 and 244 in alignment with bores 227 and 228, respectively.
This provides support for the refrigerator and sealed leakproof inl.et and outlet connections for the refrigerator flow passages.
The passage sizes are selected for cooling capacities.

... .. . . . . . ... . , . . . . ., . ..... , .. , ... . . . , . . . ... . .. .... _ I 1 7~85 s The passages 222, 223, 225 are micron-sized. The passages 222 and 223 particularly are micron-sized having dimensions in the order of 5-500 microns. In a typical refrigerator these passages may be as shallow as 5-10 microns and as narrow as 150-200 microns. In passage 222 the gas passes in laminar flow, thereby reducing vibration, noise and other problems incident to turbulent flow. Recess 241 is typically about 20-240 microns deep.
In a modification of the foregoing, the inlet and outlet passage means may be recessed or otherwise formed in opposite surfaces of intermediate plate 233, or one of the passage means formed in plate 233, while the other is formed in one of the other two plates.
In operation in Figs. 12-24, compressed gas such as nitrogen or ammonia at ambient temperature (60F - 90F) is introduced through line 229 and port 221. These gas pressures are in the region of about 150-3000 pounds per square inch.
The gas flows through the heat e~change section 222 of the micron-sized passageways and then through the smaller cross section aapillary section 223 where the gas expands and reduces in temperature and enters cooling chamber 224. The fluid in chamber 224 may be supercooled gas, liquid or a mixture, and in any event this is the coolest part of the refrigerator.
Fluid leaving chamber 224 through port 239 flows at reduced pressure through the return passage 225 in heat exchange relation with inlet passage 222 and then to outlet port 226 and line 231. It will be noted that this heat exchange takes place substantially directly through the small uniform thickness of the intermediate glass plate 233 and is efficient and accurate, the cool low-pressure outgoing gas precooling the incoming highly compressed incoming gas.

.. . .. ..

1 17~85 1 In the invention the heat exchange between the heat exchange sections of the high-pressure gas passages and the low-pressure return passages takes place through walls having the dimension of the thickness of respective plates whereby accurate relative location is possible, the heat exchange being controlled by the thickness of plate.
One of the major advantages in providing the laminar flow low-pressure return in a separate layer is that the refrigerator may operate at a lower temperature than refrigerators designed for turbulent flow in the return passage. This results because the laminar flow channels produce lower back pressures and hence lower operating temperatures.
Experience has shown that mutlilayer refrigerators of the foregoing type may be fabricated using up to one-third less material volume than a single-layer refrigerator (single-layer is where inlet and return passages are formed in one interface) of the same cooling capacity, and hence operate more efficiently.
Plates 232, 233 and 234 are preferably of soda lime glass, Pyrex or other similarly low thermally conductive material.
They must be flat, of low thermal conductivity and capable of being worked to form the surface recess passages and chamber above described. This is particularly desirable where cooling down to -50C or below is required, However, when the required temperature is not so low, it is preferable to provide a top plate of a very high conductivity material but about the same coefficient of thermal expansion such as beryllium oxide, silicon or a crystalline aluminum oxide whereby to effect a more efficient exchange of heat.
In some embodiments the high-pressure inlet passage system and the low-pressure passage system may be etched or otherwise recessed into opposite sides of the intermediate plate, while the other two plates of the stack are planar-surfaced to ~ 1 7~85 ' close the recess sides. Also it is within the scope of the invent.ion to provide one layer (high- or low~pressure passageways) in the intermediate plate and the other layer in one of the top or bottom plates.
Fig. 25 discloses an embodiment wherein three similar plates 251, 252 and 253 about 0.020 inches thick provide the micron-sized passages. The plates are bonded in a stack in use, but are here shown in exploded view to better illustrate detail of the refrigerator components.
The high-pressure gas is introduced at inflow port 254 of plate 251 which is a flat glass plate with planar surfaces, and continues through bore 255 in plate 252 to a closed bo~tom well 256 in the upper surface of lower plate 253 which may be the same as plate 232 of Fig. 12-24. The higher pressure circuit continues in plate 253 as a heat exchange section 257 and a capillary expansion section 258 opening into the cooling chamber recess 259.
Intermediate plate 252 which may be the same as plate 234 of Figs. 12-24 is flat and planar on its bottom surface to complete the passages in plate 253, and its upper surface is recessed at 261 to provide the low-pressure return. A
through bore 262 in the bottom of recess 251 connects the low-pressure recess 261 to an external circuit.
Fig. 26 shows an embodiment similar to Fig. 25 but providing two low-pressure returns. Four similar-sized plates are here bonded in a stack, shown exploded in Fi.g. 26 for clarity of disclosure. Plates 251, 252 and 253 are as in Fig. 25, and a fourth plate 264 is added to provide a second low-pressure return. Plate 264 may be a duplicate of plate 252 with the low-pressure return recess being formed in its upper surface. However, a through bore 265 in plate 253 in the bottom of chamber 259 connects the chamber to low-pressure recesses 266 ~ 1 7~85 ~

in the upper surface of plate 264. Also, there is no bore corresponding to bore 262 in recess 266, but the low-pressure gas from recess 266 flows through a through bore 267 in plate 253 and a through bore 268 in plate 252 to join the exhaust gas from recess 261 in passing to outflow port 263.
Thus in this embodiment there are two heat exchange paths providing quicker and lower precooling of the incoming high-pressure gas.
In the embodiment of Fig. 27, the cool end of the refrigerator 211 is modified to the extent that the cooling chamber, instead of being a recess as at 224 in Fig. 14 is a through opening 270 in the cover glass plate 251, and over this opening is bonded pressure-tight a thin flat pad 271 of a material that has very high thermal conductivity. The device 219 to be cooled is mounted directly upon the pad 271. Thus the fluid at its lowest temperature contacts the undersurface of the pad 217.
Preferred materials for pad 271 are silicon, beryllium and sapphire. All have high thermal conductivities that increase dramatically at very low temperatures and can be matched to an appropriate plate material of about the same coefficient of thermal expansion. The preferred material is beryllium oxide. This material combines high hardness with a coefficient of thermal expansion closer to the preferred glass plate material.
Figs. 28-30 illustrate another form of multilayer refrigerator operating on the same principle.
As shown in Fig. 28 the refrigerator laminate comprises three similar thin planar plates 280, 281 and 282 of a material that ~ay be etched. Preferably plates 280 and 281 are glass plates and plate 282 may be of glass for many purposes ~ 1 7~85 ~

but for other purposes may be of a higher thermal conductivity glasslike material such as crystalline aluminum oxide ~sapphire), beryllium or silicon.
Upon the top surface 283 of plate 280 is etched or equivalently formed the high-pressure gas inlet passage means, here consisting essentially of a micron-sized capillary dimension recess 284 that travels from inlet port 285 in a labyrinth to a central recessed cooling chamber 286.
Plate 281 has a continuous planar bottom surface 287 bonded pressure-tight onto plate 280 to complete the inlet passage and chamber and in its upper surface 288 there is a large low-pressure gas return recess 289 connected to chamber 286 by a port 290. A series of raised radial ribs 291 in the recess 289 supportingly engage plate 280 to increase the mechanical strength of the assembly.
The outgoing gas at reduced-pressure flows in recess 289 to an outlet port 292 that continues through one of the plates 280 or 282 to an external device as in the earlier embodiments.
As indicated in Fig. 28 the plate 282 is bonded pressure-tight onto plate 281 to complete the micron-sized deep return passage 289.
On the top surface 293 of plate 282 is directly secured the ckip 294 to be cooled, and a printed or like circuit 295 for the chip extends over plate 282 to suitable external electrical connections. The chip is thus exposed to the cooled fluid at substantially the coldest region of the refrigerator. This mode of attaching the device to be cooled may be employed in all of the embodiments herein.
In the foregoing, the capillary passage exhibits heat exchange relation with the fluid in exhaust recess 289. This arrangement may be preferable for higher temperature refrigeration . , .. . .. , ~ _ . .. . . . .

~ ~ 7~8S ~

where cooling to cryogenic temperatures is not required.
For example, ammonia may be used as the refrigerant to achieve temperatures of -30~C and refrigeration capacities up to 50 watts. As the requirements approach cryogenic cooling a longer heat exchange region for precooling prior to the capillary section is provided. For example, the refrigerant may be Freon introduced at high pressure and which will drop in pressure, expand and cool in the capillary section 284 while in heat exchange relation with the return gas in recess 289 for further cooling. Where the gas is nitrogen for cryogenic cooling as in the embodiment of Fig.s. 12-24, the longer precooling heat exchange region is provided.
The foregoing refrigerator may be of particular value in cooling larger computer chips such as those known as VLSI
(very large scale integration) chips which today are being designed with greater circuit density and increased power capacity, thereby dissipating large amounts of heat, i.e.
10-50 watts. Refrigeration enables such chips to operate at lower temperatures, improves their operating efficiency, speed and reliability, and increases their useful life.
Figs. 31 and 32 illustrate a refrigerator unit consisting of two multilayer refrigerators constructed between five laminated plates. This configuration allows for cascade cooling the precooling of one fluid by another to enable either faster cooldown or lower temperatures. For example, ammonia could be used in the first stage of the cascade, layers 294, to precool nitrogen in the second stage, layers 295. This will decrease the cooldown time for nitrogen by a factor of three or more. As another example, nitrogen could be used in the shorter stage, layers 294, to precool hydrogen which will then cool to 20K. Low boiling point fluids such as hydrogen and helium will not cool in the Joule-Thomson cycle unless they are precooled to the proper temperature in this manner. Refrigerators _~ . .. ... . . .... .. ... . . . . . .

1 17~85 1having three or more stages can be constructed in a similar manner.
Other channel dimensions and characteristics of the refrigerator may be as described in connection with the embodiments of Figs. 1-11. The formation of the channels and the bonding of the plates herein is accomplished by the channel-forming techniques and the bonding material described in connection with said embodiments.
In accordance with another aspect of the invention it has been discovered that there is a trade off between efficiency of the heat exchanger and the minimum temperature which can be reached in the cooling section of the refrigerator.
The minimum temperature is determined by the gas pressure at the point at which the gas exits from the cooling chamber.
The lower the pressure at this point, the lower the temperature.
On the other hand, the heat exchanger efficiency is a function of the pressure drop along the outflow channel, an increase in the pressure drop producing more efficient heat exchange.
Thus to obtain an efficient heat exchange which leads to faster cool down time and/or lower gas consumption rates the pressure of the gas as it leaves the cooling chamber must be relatively high. Conversely, to effect maximum temperature drop in the cooling chamber the gas pressure at the same point should be relatively low.
The embodiments of Figs. 33 and 34 represent an effective solution to this problem. In each of these embodiments two capillary sections are provided and a portion of the incoming gas is by-passed directly to the outflow passage of the heat exchanger after passage through one of the capillary section thus by-passing the cooling chamber.
The embodiment of Fig. 33 comprises a stack of four 21ateS, 300J 302, 304 and 306 which may be of the same materials ~ 17~851 and bonded together in the same manner as in the previously described embodiments. In this embodiment two capillary sections 308 and 310 are provided in series relationship. Between the two capillary sections a small port 312 is provided leading to the upstream end of the heat exchanger outflow passage 314 formed in plate 300. The downstream end of the second capillary section 310 is connected to the cooling chamber 316 which in turn is connected by a port 318 to a second outflow passage 320 formed by the recessed face of plate 304. Ribs 322 are provided in the plate 304 to impart stiffness to the unit and assure uniform spacing between the plates 304 and 306.
In operation, high pressure gas flows through the inflow section of the heat exchanger 324 and expands and drops in pressure as it flows thereafter through the first capillary section 308 to the port 312. At this point the gas flow is divided so that a substantial portion of the gas proceeds through the port 312 directly to the outflow heat exchanger passage 314.
The port 312 and the outflow passage 314 are so dimensioned that a relatively high pressure is maintained at the port 312, typically 10 to 30 atmospheres. Accordingly, a large pressure drop will occur in the outflow section 314 of the heat exchanger producing high efficiency of the heat exchange function.
The remainder of the gas flows through the second capillary section 310 to the cooling chamber 316. Here the gas absorbs heat from the device being cooled and then flows out through the port 318 and to the exterior of the device through the second outflow passage 320, at a relatively low pressure, typically 2 to 3 atmospheres. This low pressure assures achievement of the desired low pressure in the cooling chamber.

~ 1 7~8S 1 It has been found that both heat exchanger efficiency and the desired cooling can be achieved by permitting from 50%
to as much as ~5% of the gas to flow through the port 112 and the outflow heat exchanger passage 314.
Fig. 34 illustrates a plate 324 which may be substituted for the plate 302 in the embodiment of Fig. 33 to achieve similar results. In this form of the invention the two capillary sections, indicated at 326 and 328, are arranged in parallel rather than in series as in the embodiment of Fig. 33.
The incoming gas passes through the inflow section of the heat exchanger 324, through the first capillary section 326 and through the port 312 for passage to the first outflow heat exchanger passage 314. The remainder of the gas passes through the second capillary section 328 into the cooling section 316 and thence into the alternate outflow passage 320. ~s in the previously described embodiment good results may be obtained by by-passing from 50 to 95% of the incoming gas through the port 312.
Other channel dimensions or characteristics of the refrigerator may be as disclosed above. The formation of the channels and the bonding of the plates herein also may be accomplished by the channel-forming techniques and the bonding material described above.
Refrigerators as above described are ideal for a wide range of laboratory and like applications. They provide convenient very low temperature economic operation as an alternative to volatile liquid cryogens. They are of small size and low weight enabling them to be used directly on instruments such as microscope stages. The small size enables them to be used to cool very small devices enabling tools or optical instruments to observe or work directly on the device without interference. Small gas consumption enables days of continuous 1 1 7~85 ' use from a standard pressurized cylinder of gas. Temperature control is simple. The refrigerators are simple in structure and may be constructed and operated relatively simply and safely.

Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A multilayer microminiature refrigerator comprising a laminate of three planar-surfaced thin plates bonded pressure-tight at the inter-faces between adjacent plates, means providing in one interface between two adjacent plates a first continuous inflow passage means extending from an inlet to a cooling chamber and including capillary passage means, means for connecting said inlet to a source of refrigerant gas at high pressure, means providing in the other interface between two adjacent plates a further continuous outflow passage means leading to an outlet, and a passage interconnecting said chamber and said further passage means whereby fluid at reduced pressure from said cooling chamber may pass through said further passage means to said outlet in counterflow heat exchange relation with fluid in said first passage means for regenerative precooling, each of said passage means being of micron size whereby to promote laminar flow therein.

2. The refrigerator defined in claim 1, wherein said plates are thin glass plates of uniform thickness.

3. The refrigerator defined in claim 2, wherein said glass plates are about 0.020 inches thick.

4. The refrigerator defined in claim 1, wherein at least the plates in which said passage means are formed are of a low thermal conductivity material such as glass.

5. The refrigerator defined in claim 1, wherein each of said passage means is recessed into a plate surface.

6. The refrigerator defined in claim 1, wherein said inflow passage means are each about 250 microns wide and about 10 microns deep, said capillary passage means is about 200 microns wide and said about 10 microns deep and outflow passage means is about 15,000 microns wide and about 25 microns deep.

7. The refrigerator defined in claim 1, wherein said laminate is mounted at one end on a holder having bores therethrough connecting with said inlet and outlet.

8. The refrigerator defined in claim 7, in combination with an enclosure that is adapted to be connected to a source of vacuum, and means securing said holder within said enclosure so that the refrigerator has cantilever support in said enclosure.

9. The refrigerator defined in claim 1, wherein said plates are glass and said chamber is a through opening in one of said plates and a pad is bonded over said opening whereby to provide contact with a device to be cooled, said pad being of a material that is of higher thermal conductivity than the plate in which said opening is formed.

10. The refrigerator defined in claim 9, wherein said pad is of crystalline aluminum oxide, silicon or beryllium oxide.

11. The refrigerator defined in claim 1, wherein two adjacent plates are glass and the third plate is of a material having an appreciably higher coefficient of thermal conductivity, said first passage means being formed in the interface between said third plate and the glass plate is bonded to it.

12. A multilayer refrigerator as defined in claim 1, wherein said cooling chamber is substantially centrally located in said laminate and means is provided whereby an electrical device to be cooled may be secured directly on an outer plate substantially directly in line with said cooling chamber, electrical circuit means for said device being formed on the adjacent surface of said outer plate.

13. The refrigerator defined in claim 1, wherein said first passage means comprises a heat exchange section comprising a series of parallel channels extending in heat exchange relation with said further passage means and said capillary passage means comprises a smaller diameter con-voluted section extending from the heat exchange section to said chamber.

14. A method of making a refrigerator wherein a thin glass plate is surface-bonded pressure-tight to another plate of about the same coefficient of thermal expansion, comprising the steps of:
a) forming a layer of fine-particle sandblast-resistant material on said glass plate;
b) defining a flow path pattern in said resistant material by lithographic masking and selective etching to thereby expose the under-lying surface of said glass plate in said pattern;
c) scanning a miniature air abrasive device across said glass plate so as to form recessed channels in said surface by fine-particle blasting to the required depth; and then d) bonding said other plate upon the recessed surface of said glass plate.

15. A method as defined by claim 14 wherein said fine-particle sandblast-resistant material comprises: gelatin, ammonia bichromate and hot water in quantities approximately proportional to 7 gm, 1 gm and 50 cc respectively.