GB2045910A - Miniature cryogenic refrigerator and device and method of making same - Google Patents

Abstract

A microminiature cryogenic device includes a miniature refrigerator in which a heat exchanger 22 is defined in the surface of a first member 12 such as a silicon wafer by means of lithographic masking and etching techniques. A second member 10 having a confining surface abuts the etched surface of the first member whereby refrigerant lines 14, 16 are formed. in an open cycle system, input and output ports 18, 20 are connected to the refrigerant lines, which are fed with cryogenic fluid through miniature tubes 36, 38 e.g. hypodermic needle type stainless steel tubing and a restricted line portion 26 forms a capillary structure. Other types of refrigeration systems may be used with the device. <IMAGE>

Description

SPECIFICATION Miniature cryogenic refrigerator and device and method of making same This invention relates generally to cryogenic refrigerators, and more particularly the invention relates to miniature cryogenic refrigerators and method of making same.

During the past decade new electronic devices have been developed which are based on the Josephson effect in super-conductors. These include supersensitive magnetometers, gradiometers, voltage standards, bolometers and logic elements. The power dissipated by these devices in the cryogenic environment is typically of the order of micro-watts.

Traditionally the low temperature environment has been provided by a bath of liquid helium. This is not particularly convenient and an effort has thus been made in recent years to use small closed-cycle refrigerators for this purpose. However, these refrigerators typically have a capacity on the order of watts and are thus poorly matched to the minute refrigeration requirements of the devices. In addition the refrigerators are noisy, they vibrate and their moving parts modulate the local magnetic field.

Open cycle miniature refrigerators are available and offer improved operating features when compared to the closed-cycle refrigerators. However, these refrigerators utilize finned tubing and soldered or welded construction which is difficult and expensive to build. Further, the cooling capacity of these refrigerators exceed the requirements of many electronic devices.

Accordingly, an object of the present invention is an improved miniature refrigerator.

Another object of the invention is an improved method of manufacturing a miniature refrigerator.

Yet another object of the invention is a method of making a miniature refrigerator using conventional photolithographic and etching techniques.

Another object of the invention is a microminiature cryogenic device.

Briefly, the miniature refrigerator in accordance with the invention includes a substrate or body having a confining surface on which heat exchanger line or coils are mounted. The heat exchanger comprises a unitary body having a surface in which grooves are formed by photolithographic definition and chemical etching. The surface of the body is bonded to the confining surface of the substrate which cooperatively defines the heat exchanger.

In a microminiature cryogenic device, input and output ports for the refrigerant lines may be provided on the substrate in an open cycle refrigeration system. Additionally, filter means may be included in the interconnection between the lines and the input and output ports. A liquid reservoir is connected to the lines, and a superconductive device is mounted in abutment to the reservoir for cooling.

In a preferred embodiment, the refrigerator com prises a semiconductor body such as silicon in which the grooves are formed by photoresist masking and chemical etching techniques.

These and other objects and features of the invention will be more readily apparent from the following detailed description and appended claims when taken with the drawing.

In the drawing, Figure 1 is an exploded perspective view of one embodiment of a microminiature cryogenic device and refrigerator in accordance with the present invention.

Figures 2A-2E are cross section views illustrating the method of fabricating a miniature refrigerator in accordance with one embodiment of the invention.

Referring now to the drawing, Figure 1 is a perspective exploded view of one embodiment of a microminiature cryogenic device and refrigerator in accordance with the present invention and includes a supporting substrate 10 upon which a second body 12 is mounted. Preferably, the body 12 is a portion of a silicon wafer and substrate 10 is a glass body such as Pyrex (Registered Trade Mark) having a coefficient of thermal expansion which is compatible with the coefficient of thermal expansion of silicon.

Etched in the surface of the silicon body 12 which is mounted to the glass substrate 10 are parallel grooves 14 and 16 which interconnect an output port 18 and an input port 20, respectively, to a reservoir 20. A filter 21 comprising a plurality of etched, fine lines may be provided between port 20 and coil 16.

The grooves 14 and 16 define cooling lines which run in juxtaposition for an initial length and define a heat exchanger shown generally at 22. Past the heat exchanged portion 22 the line 16 is defined by a narrower groove which is more closely positioned and, as will be described further hereinbelow, comprises a capillary expansion line 26 for a refrigerant gas.

The end of the expansion line 26 is connected to the reservoir 20, and the output line 14 extends from the reservoir 20 back through the heat exchanger to the output port 18.

Mounted on the bottom surface of glass plate 10 is an interface unit 30 of suitable material such as Kovar having holes 32 and 34 extending therethrough and communicating with the ports 18 and 20, respectively, of the semiconductor body 12.

Welded to the innerface unit 30 are a pair of miniaturn tubes 36 and 38 which communicate a cryogenic fluid to and from the miniature device.

Preferably, the tubes 36 and 38 may comprise stain less steel material as used in hypodermic needles.

The Kovar unit 30 is attached to the glass substrate 1 by a suitable epoxy sealant.

Mounted on the top surface of the semiconductor body 12 in abutment with the reservoir 20 is a cryogenic device 40. The device may be any one of a number of devices operated at superconductive temperatures such as, for example, supersensitive magnetometers, gradiometers, bolometers, and other like devices which are based on the Josephson effect and are well known in the art. The entire assembly is contained in a dewar vessel to reduce the heat loss to the cryogenic parts.

The illustrated microminiature cryogenic device is a Joule-Thomson, open-cycle refrigeration system in which line 38 is connected to a container of highly compressed refrigerant gas such as nitrogen, hyd rogen, or helium. The highly compressed gas enters through port 20 and passes through the heat exchanger 22 where the gas is cooled by supercooled gas exciting the device through coil 14, port 18, and line 36. The high pressure gas exits the heat exchanger 22 and passes through the capillary expander 26 where the drop in pressure reduces the temperature of the gas which enters the reservoir 20 as a supercooled fluid or cryogenic. The low temperature reservoir 20 in turn cools the device 40 mounted in abutmenttheretoo, and heat absorbed from the device 40 causes the liquid in reservoir 20 to vaporize and flow through line 14 to the exhaust port 18.

In addition to an open-cycle system as described above, it will be appreciated that the refrigerator can be a closed-cycle system such as the Servel, Gifford-McMahon, pulsed tube and Vuilleumier systems. Additional ports can be included in the device with coils interconnecting the additional ports as described and an additional reservoir for further cooling of a cryogenic device. For example, one input port and line may be provided for nitrogen and an additionall port and line may be provided for hydrogen to further reduce the operating temperature of the mounted device.

Referring now to Figures 2A-2E, the method of fabricating the refrigerator by etching grooves in a silicon wafer is illustrated. The techniques to be described are well known in the manufacture of semiconductor devices, such as integrated circuits, and include conventional photoresist masking and etching techniques. In particular, by using silicon material having a surface crystalline orientation on the r1001 plane, anistropic etching can be employed to form V-shaped grooves in the silicon wafer surface.

In Figure 2A a portion of a silicon wafer 50 is shown in cross section and a silicon oxide layer 52 is provided on one major surface thereof. The wafer is on the order of 300 microns in thickness and oxide layer 52 is approximately 9,000 angstroms in thickness. The oxide layer may be formed by heating the silicon wafer in a wet oxygen atmosphere. Photoresist is applied to the surface of silicon oxide 52 and is exposed under a photomask having a configuration as shown in Figure 1. The photoresist in the defined pattern is removed, and the exposed oxide is etched in the pattern shown in Figure 2B. The silicon oxide acts as a mask and the exposed silicon is etched using an anistropic etchant, such as ethylene diamine, resulting in the V-grooves 54 shown in Figure2C.

Upon completion of the etching of the V-grooves, the oxide layer 52 is removed from the silicon wafer and the wafer is cleaned. An optically flat Pyrex plate 56 is then bonded to the surface are of silicon wafer 50, as shown in Figure 2D. The bonding of the glass to the silicon surface is preferably by known field assisted or anodic bonding. Thereafter, as shown in Figure 2E, the silicon wafer 50 is etched from the backside to reduce the thickness and hence the thermal conductance of the bulk wafer.

Input and output lines are then drilled or etched in the reverse side of the glass substrate 56, and the stainless steel hypodermic tubing gas lines are then bonded to the reverse side of the glass plate by means of epoxy.

By using photolithographic definition and chemical etching, the entire refrigerator including gas manifold, particulate filter, heat exchanger, expansion nozzle, and liquid collector can be formed in one step. While in the described embodiment the refrigerator is defined in a wafer of silicon, other material such as copper or stainless steel films may be utilized in fabricating the refrigerator. While the.

described refrigerator is of the open-cycle type, as above indicated closed-cycle refrigerators may be fabricated using the techniques in accordance with the present invention. Further, while the confining surface is a planar substrate, it will be appreciated that the refrigerant lines can be formed on the external surface of a tube with the tube placed in abutment with the confining internal surface of another tube. Electron beam or X-ray lithography can be employed rather than photolithography; and electrolytic and plasma etching can be employed as well as chemical etching.

Thus, while the invention has been described with reference to a specific embodiment, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended

Claims (17)

claims. CLAIMS

1. A miniature refrigerator comprising a first member having a confining surface, a second member having a surface including a recessed pattern formed therein, said pattern defining a heat exchanger and a reservoir interconnected with said heat exchanger, said first and second members being cooperatively positioned with said recessed pattern abutting said confining surface thereby forming refrigerant lines.

2. A miniature refrigerator as defined by Claim 1 and further including an input port and an output port interconnected with said heat exchanger, and means for applying a refrigerant to said input port.

3. A miniature refrigerator as defined by Claim 2 wherein said first member comprises a supporting substrate and said second member comprises a planar member.

4. A miniature refrigerator as defined by Claim 3 wherein said means for applying a refrigerant includes a block mounted on a surface of said substrate opposite to said planar member, a pair of lines connected to said block, and wherein said substrate includes a pair of holes extending therethrough and interconnecting said pair of lines to said input port and said output port.

5. A miniature refrigerator as defined by Claim 4 wherein said substrate comprises glass, said planar member comprises silicon, said block comprises Kovar, and said lines comprise stainless steel.

6. A miniature refrigerator as defined by Claim 3 wherein said substrate comprises glass and said planar member comprises silicon.

7. A miniature refrigerator as defined by Claim 6 wherein said recessed pattern is defined by lithographic means and formed by etching.

8. A miniature refrigerator as defined by Claim 7 wherein said major surface of said silicon planar member has (1,0,0) crystalline orientation and said etching forms V-shaped grooves by anisotropic etching.

10. A miniature refrigerator as defined by Claims 1,3 or 8 further including a cryogenic device mounted to said second member in abutment with said reservoir.

10. The method of fabricating a miniature refrigerator including a heat exchanger and a liquid reservoir comprising the steps of etching a major surface of a first member to form a recessed area in a desired configuration, and bonding a confining surface of a second member to said major surface to form passages for the flow of a refrigerant through said recessed area.

11. The method as defined by Claim 10 further including the steps of forming holes through said second memberto interconnect said passages to input and output refrigerant lines.

12. The method as defined by Claim 10 wherein said first member comprises a semiconductor material and said step of etching a major surface includes forming a layer of etchant resistant material on said major surface of said semiconductor material, defining said desired configuration in said etchant resistant material by lithographic masking and selective etching to thereby expose underlying semiconductor material, and applying an etchant of said semiconductor material to said exposed semiconductor material.

13. The method as defined by Claim 12 wherein said semiconductor material is silicon and said etchant anisotropically etches said silicon.

14. The method as defined by Claims 10 or 13 wherein said second member is glass and said bonding step includes anodically bonding said glass to said first member.

15. A miniature refrigerator as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

16. A method of fabricating a miniature refrigerator as claimed in claim 1 O 10 substantially as hereinbefore described with reference to the accompanying drawings.

17. A miniature refrigerator whenever prepared by the method claimed in any one of claims 10 to 14 or claim 16.