Field replaceable cryocooled computer logic unit

A cryocooled field-replaceable logic unit for use with a cryogenic cold head such as that of a cryocooler comprises a thermally conductive plate adapted to be disposed in thermal contact with the cold head, a thermally insulating enclosure portion cooperating with the conductive plate to form a sealed enclosure detachable from the cold head, and one or logic chips mounted in the enclosure in thermal contact with the conductive plate. Preferably the conductive plate is attached to the thermally insulating portion through a resilient mounting which forms a recess for receiving the cold head and which is tension-loaded when the logic unit is mounted on the cold head to urge the plate into intimate thermal contact with the cold head. In one embodiment the enclosure receives from an external source a supply of gaseous nitrogen which is liquified by contact with the conductive plate to form a pool of liquid nitrogen in which the logic chips are immersed. In another embodiment plural logic units are detachably mounted on passive cold heads immersed in a cryostat.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a cryocooled logic unit for use with a cryocooler 
cold head and more particularly to a cryocooled logic unit of a digital 
computer. 
2. Description of the Related Art 
Cryocooled computers, in which certain logic components are cryogenically 
cooled to increase their speed of operation, are well known in the art. In 
a cryocooled computer, the logic components may be cooled by being in 
direct contact with the cryocooler cold head and insulated from the 
external environment, preferably by a vacuum vessel. To accommodate a 
logic or cryocooler failure, the vacuum vessel would have to be made so 
that it can be taken apart to separate the logic components from the 
cryocooler. This would be time consuming, since the cold head would have 
to be given time to come to room temperature before the service person 
could take the vacuum vessel apart to remove the logic components. The 
vacuum vessel would then have to be reassembled and a vacuum pump would 
have to be employed to recreate the vacuum in the space around the cold 
head. Even with all this, there would be no easy way for the service 
person to check for leaks. 
SUMMARY OF THE INVENTION 
In general, the present invention contemplates a cryocooled logic unit for 
use with a cryocooler cold head comprising a thermally conductive 
enclosure portion adapted to be disposed in thermal contact with the cold 
head, a thermally insulated enclosure portion cooperating with the 
conductive portion to form an enclosure detachable from the cold head, and 
a semiconductor logic circuit mounted in the enclosure in thermal contact 
with the conductive portion. Preferably, the enclosure is formed with a 
recess, preferably a cylindrical recess, for receiving the cold head. 
Preferably, the enclosure comprises a vacuum vessel, while the thermally 
insulating portion comprises either a lining of insulating material or an 
outer wall and an inner wall defining a region which is also evacuated. 
Preferably, means are provided for urging the conductive portion of the 
enclosure into engagement with the cold head. 
With this invention, the logic components can be assembled and tested in 
the vacuum vessel at the factory as a field-replaceable unit (FRU). If the 
logic fails, the vacuum vessel unit can be quickly removed and can be 
replaced by a spare, and the failing unit sent back to the factory for 
repair. The same is true for a cryocooler failure. If the latter failure 
occurs, the vacuum vessel unit can be easily removed and reassembled onto 
a new cryocooler.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first to FIG. 1, a system 10 incorporating the present invention 
includes a cryocooler field-replaceable unit (FRU) 12 and a logic FRU 14. 
Logic unit 14, which is detachable from the cryocooler unit 12, comprises 
a generally dome-shaped vacuum vessel 16, the thermally insulating portion 
of which comprises an outer wall 18 and an inner wall 20. Walls 18 and 20 
are joined by any suitable means to an annular bottom wall 22. Bottom wall 
22 supports a cylindrical recess-forming portion 24 comprising a side wall 
26 and an upper wall comprising a thermally conductive plate 28. 
Conductive plate 28 of recess 24 supports a chip carrier 30, which in turn 
supports one or more computer logic chips 32, each of which comprises a 
semiconductor integrated circuit. Typically the semiconductor material may 
be CMOS or gallium arsenide, while the circuits may comprise either 
computer processors or computer memories, especially cache memories 
associated with processors. Preferably, chip carrier 30 comprises a 
material, such as silicon or ceramic, having a coefficient of expansion 
approximating that of chips 32. Electrical cables 34 extending between 
chips 32 and one or more cable connectors 36 carried by bottom wall 22 
provide an electrical connection between chips 32 and the external 
environment. Preferably, both the exterior region 38 between outer and 
inner walls 18 and 20 and the interior region 40 within inner wall 20 are 
evacuated. At a minimum, however, there should be at least one vacuum 
barrier insulating the logic chips 32 from the ambient environment. 
Conductive plate 28 preferably comprises a highly thermally conductive 
material such as copper. The remaining walls 18, 20, 22 and 26 of vacuum 
vessel 16 may comprise a suitable structural material such as stainless 
steel. 
Cryocooler unit 12 comprises a generally cylindrical cryocooler cold head 
42, the upper or expander portion of which fits within the recess 24 of 
logic unit 14. Cryocooler cold head 42 is formed with a bore 44 within 
which is disposed an annular regenerator 46 and an annular cooler 48 
beneath regenerator 46. A cylindrical sleeve 50 disposed inside of 
regenerator 46 and cooler 48 in turn houses a reciprocating displacer 52. 
The upper portion of cryocooler cold head 42 is formed with a reduced 
diameter to define a shoulder 54 which abuts bottom wall 22 of logic unit 
14 when the upper surface of cryocooler cold head 42 is pressed against 
the conduction plate 28 of logic unit 14. Except for the attachment 
modifications to be discussed below, cryocooler unit 12 is a conventional 
unit of a type well known in the art. As is known in the art, a 
temperature gradient exists along the surface of cryocooler cold head 42, 
with the uppermost portion adjacent conduction plate 28 the coldest and 
the lower portion adjacent shoulder 54 substantially at room temperature. 
The interface 58 between the upper portion of the cryocooler cold head 42 
and the conduction plate 28 of logic unit 14 represents the thermal gap 
between the cryocooler unit 12 and the vacuum vessel 16 that is part of 
the logic unit. The vacuum vessel 16 insulates the cryocooler cold head 42 
thermally from the ambient air. The thermal resistance at interface 58 
should be kept as low as possible. The particular method used to attach 
the vacuum vessel 16 to the cryocooler cold head 42 will have a great 
effect on the thermal resistance at interface 58. There are several ways 
of attaching the vacuum vessel 16 to the cryocooler cold head 42. Thus, 
some or all of the interface 60 between cryocooler cold head 42 and side 
wall 26 of recess 24 can be threaded and the two parts can be screwed 
together. Alternatively, studs, like the one shown at 62 in FIG. 2, 
protruding from the bottom wall 22 of the vacuum vessel 16 could fit into 
holes in a flange 64 extending around the expander portion of cryocooler 
cold head 42 in the area of the cooler 48. Nut 66 such as on the stud 62 
could then be used to bring the parts together tightly. As a further 
alternative, the bottom wall 22 of the vacuum vessel 16 could have a 
quick-disconnect, twist-on ramp fastening mechanism (not shown) that would 
meet with the cryocooler cold head 42 in the area of the cooler 48. This 
mechanism would be similar to a pressure cooker lid. Still other methods 
may alternatively be used to attach the two parts. Additionally, or 
alternatively, interface 58 may contain certain thermal conductivity 
enhancers such as copper wire mesh, copper "fingers", corrugated copper 
foil or the like. 
Interface 68 between the chip carrier 30 and the conduction plate 28 of 
recess 24 represents another thermal gap whose resistance must be 
minimized. Any one of several methods that will minimize the thermal 
resistance may be used to attach the chip carrier 30 to the conduction 
plate 28. Thus, the chip carrier 30 may be attached directly to the 
conduction plate 28 using screws threaded into the vacuum vessel 16 and 
going through clearance holes in the chip carrier 30. Alternatively, the 
chip carrier 30 may be held against the conduction plate 28 by a system of 
clamps. As a further alternative, the chip carrier 30 may be soldered, 
brazed or glued directly to the plate 28. Still other methods may be used 
to attach the chip carrier 30 to the vacuum vessel 16. 
Interface 70 between bottom wall 22 of vacuum vessel 16 and shoulder 54 of 
cryocooler cold head 42 should contain a seal, such as the O-ring seal 56 
shown in FIG. 1, to prevent room air from circulating in the small space 
between the expander portion of the cryocooler cold head 42 and the vacuum 
vessel 16. If air circulation is allowed in this space, water may condense 
and freeze above the regenerator 46, and air may condense and liquify 
further up on the cryocooler cold head 42. 
The region 40 above the logic components 32 and the vacuum vessel 16 also 
cannot contain room air, since such air contains moisture that will 
condense and freeze on walls 26 and 28, chip carrier 30 and the logic 
component 32, since the cryocooler cold head 42 operates at a temperature 
that will liquify nitrogen. Evacuating the region 40 prevents this from 
happening and, as noted above, thermally insulates the thermally 
conductive portion of the vessel 16 comprising cold plate 28 from walls 18 
and 20 and the ambient atmosphere. 
FIG. 3 shows a modified logic unit in which the rigid side wall 26 of FIGS. 
1 and 2 has been replaced by a compliant spring member to improve the 
reliability of contact between the conduction plate and the cryocooler 
cold head of the cryocooler unit. More particularly, the cryocooler 
assembly 72 shown in FIG. 3 comprises a cryocooler FRU 74 and a detachable 
logic FRU 76. Logic unit 76 comprises a vacuum vessel 78 having an outer 
wall 80 comprising a suitable structural material such as stainless steel. 
Outer wall 80 has its inner surface lined with one or more layers of 
suitable insulating material 82 such as aluminized Mylar (trademark) 
polyester. Outer wall 80 is joined by any suitable means to a bottom wall 
84, which may also comprise stainless steel. The cold head of cryocooler 
unit 74 is adapted to be inserted through a center aperture formed in 
bottom wall 84, into an upwardly extending recess defined by a conduction 
plate 90 at the upper end of the plate 90 to bottom wall 84. As in the 
previous embodiment, conduction plate 90 may comprise a suitable highly 
thermally conductive material such as copper, while compliant member 88 
may comprise a relatively thin sheet of stainless steel. Conduction plate 
90 supports a logic carrier 92, which may be similar to carrier 30 and 
which supports the cryocooled logic 94. Respective cables 96 couple logic 
94 electrically to connectors 98 carried by bottom wall 84, which 
connectors 98 are in turn attachable to external connectors 100 carried at 
the ends of external cables 102. Bolts 108 extending upwardly through a 
flange 106 carried by cryocooler unit 74 into threaded bores formed on the 
underside of bottom wall 84 detachably secure logic unit 76 to cryocooler 
unit 74. A resilient O-ring seal 104 of any suitable material extending 
around cryocooler unit 74 between flange 106 and bottom wall 84 of unit 76 
prevents air from entering the region between compliant member 88 and 
cryocooler unit 74. 
Compliant member 88 is loaded in tension by the attachment of logic unit 76 
to cryocooler unit 74. This load condition creates a deflection which is 
sufficient to compensate for such imperfections as non-parallel mating 
between the conduction plate 90 and the adjacent surfaces of cryocooler 
unit 74 and differential thermal expansion and contraction in the assembly 
72 over the large temperature excursions experienced. The loading of 
compliant member 88 upon attachment of logic unit 76 to cryocooler unit 74 
lowers the thermal resistance at the interface between conduction plate 90 
and the cold head of cryocooler unit 74 by allowing the conduction plate 
to align itself to a range of cryocooler cold head surface tilt angles and 
by maintaining contact pressure along that interface as the materials 
contract when brought down to cryogenic temperatures. This allows the 
logic unit 76 to be installed on the cryocooler unit 74 while the parts 
are at room temperature without the need for retorquing the bolts 108 (or 
other attachment means) when the parts are at low temperature. This makes 
field replacement faster and easier. Also, the ability of the conduction 
plate 90 to conform to a variety of cold head tilt angles provides relief 
on tolerances required to govern the mating surface angles. 
Compliant member 88 is designed to minimize the thermal conductance in an 
axial direction by keeping its cross sectional area small and its axial 
dimension relatively large, while not exceeding the material elastic 
limits when it is loaded in tension. As noted above, the circulation of 
ambient air is precluded by seal 104, which becomes compressed between the 
flange 106 and bottom wall 84 when the assembly 72 is bolted together. The 
seal compression is maintained by the ability of the compliant member 88 
to maintain preload on the bolts 108 over the temperature excursions 
experienced. Seal 104, being compliant, also provides relief on the 
tolerances for the mating parts on flange 106 and bottom wall 84. 
Preferably, assembly 72 includes 8 bolts 108, which are torqued to a 
standard torque that provides the required loading at the interface 
between conduction plate 90 and cryocooler unit 74. Other mounting methods 
such as those pointed out in conjunction with the embodiment shown in FIG. 
1 may also be used. 
FIG. 4 shows a variant of the embodiment shown in FIG. 3 in which the cable 
connectors extend out the side of the logic unit rather than through the 
bottom as in FIGS. 1 through 3. More particularly, a logic unit 110 shown 
in FIG. 4 comprises a vacuum vessel 112 having an upper wall 114 and a 
cylindrical side wall 116 joined together by any suitable means. An 
outwardly extending flange 118 carried at the bottom of wall 116 is formed 
with bores at spaced locations about its periphery for receiving bolts 120 
for attaching the upper portion of unit 110, comprising flange 118 and the 
associated wall portions 114 and 116, to a lower portion 122 having both a 
side wall and an apertured lower wall thereof. Walls 114 and 116 are lined 
along their inner surfaces by one or more insulating layers 124 similar to 
the layers 82 shown in FIG. 3. A cryocooler unit such as the unit 74 shown 
in FIG. 3 is attached to logic unit 110 by inserting it upwardly through 
the aperture in lower portion 122 into a recess 126 defined by a compliant 
member 128 similar to member 88 of FIG. 3 and a conduction plate 130 
similar to plate 90 of FIG. 3. 
Conduction plate 130 supports a logic module 132, which may be CMOS or 
gallium arsenide, as disclosed above, or other cryogenically enhanced 
technology. Respective cables 140, secured to logic module 132 through a 
cable retainer 134, couple module 132 electrically to connectors 142 
carried by the side wall portion of lower vacuum vessel portion 122. As 
shown in FIG. 4A, module 132 fits inside of a mounting frame 138 secured 
to conduction plate 130. Module 132 is sandwiched between plate 130 and a 
clamp frame 136 secured to mounting frame 138 over the module. Clamp frame 
136 also supports cable retainer 134. 
Connectors 142 form a hermetic seal with wall portions 122. Logic unit 110 
is secured to cryocooler unit 74, in a manner similar to that of logic 
unit 76 shown in FIG. 3, with bolts 108 being inserted into apertures 143. 
Owing to the location of the connectors 142 on the side of the vacuum 
vessel 112, there is less interference between the electrical couplings 
through connector 142 and the mechanical coupling to the flange 106 and 
bolts 108 of the cryocooler unit 74. 
Flange 118 and wall portions 114 and 116 supported by the flange may be 
separated from lower vacuum vessel portion 122 after removing bolts 120 to 
access logic module 132 for servicing or replacement. Preferably an O-ring 
seal 121 is disposed along the interface between flange 118 and portion 
122 to ensure the integrity of the enclosure. 
FIG. 5 shows a modified assembly in which the logic unit is inverted in 
order to provide for the immersion of the logic chips in a cryogenic pool 
confined to the logic unit. More particularly, the assembly 144 shown in 
FIG. 5 comprises the cryocooler unit 12 of FIG. 1 and a logic unit 146 
detachably mounted thereon. Logic unit 146 comprises a vacuum 148 having 
an outer wall 150 similar to outer wall 18 of unit 14 and an inner wall 
152 similar to inner wall 20 of unit 14, but provided with a recess or 
well 154 for accommodating a self-contained supply of cryogenic liquid. 
Vacuum vessel 148 also has, in a manner similar to that of vacuum vessel 
16 of FIG. 1, a recess forming portion 156 for receiving cryocooler unit 
12, the recess forming portion 156 being formed from a cylindrical side 
wall 158 and a highly thermally conductive conduction plate 160. A chip 
carrier 162 mounted within well 154 supports the logic chips 164 to be 
cryogenically cooled. Cables 166 couple chips 164 electrically to 
connectors 168 carried by upper wall 153. The exterior region 170 between 
walls 152 and 150 is preferably evacuated, hile the interior region 172 
receives a liquifiable gas such as nitrogen from a supply 174 by way of a 
hose 176 and connector 178 carried by upper wall 153. 
Conduction plate 160 is ground flat and smooth to make good thermal contact 
with the cold head of cryocooler unit 12, which is also ground flat and 
smooth. The attachment mechanism for the logic unit 146 (not shown in FIG. 
5), which may be similar to those shown in FIGS. 2 and 3, will press these 
two surfaces tightly together when the unit 146 is attached to the 
cryocooler unit 12. As in the embodiment shown in FIG. 1, a suitable 
thermal conduction enhancer may be used to fill in any imperfections of 
the surfaces; such enhancers include, for example, a soft thin metal foil, 
thermal grease, a resilient matted conduction material, and the like. 
Further, wall 158 may be replaced by a thin, relatively compliant member 
as in the embodiments shown in FIGS. 3 and 4. 
The temperature rise across the thickness of conduction plate 160 will be 
low. This being the case, the temperature on the inside surface of this 
plate (the lower surface in FIG. 5) can be brought to 77.degree. K. or 
slightly lower with the use of a sufficiently cold cryocooler unit 12. At 
this temperature, nitrogen from the supply 174 will condense on the plate 
160 or on fins (not shown in FIG. 5) protruding from the plate. The liquid 
nitrogen thus formed will drip into the well 154 to immerse the logic 
chips 164 in a pool 180. The logic chips 164 will now operate at slightly 
above 77.degree. K. 
Logic unit 146 is preferably charged with nitrogen gas when it is built. 
When it is mounted on the cryocooler 12, hose 176 from supply 174 is 
attached to the unit 146. The cryocooler unit 12 is then turned on to 
start the liquification of the nitrogen gas within the logic unit 146. As 
the nitrogen within the logic unit 146 is liquified, the pressure within 
the unit drops, drawing in more nitrogen gas from the supply 174. This 
continues until the logic chips are immersed in a liquid nitrogen pool 180 
as shown in FIG. 5. Power is then brought up on the logic circuits and 
processing begins. 
Logic unit 146 preferably includes a relief valve (not shown) to allow 
nitrogen gas to escape when pressure inside the unit increases. This will 
happen when power is turned off or there is a logic failure which requires 
the unit to be changed. The pressure will increase as the liquid nitrogen 
becomes nitrogen gas. 
If more logic is required, the well can be made larger and various 
arrangements of packages can be immersed. Thus, referring now to FIG. 6, a 
logic unit 184 otherwise similar to logic unit 146 of FIG. 5 includes a 
modified inner wall 186 formed with an enlarged well 188 near the bottom 
of which is disposed a card 190. Respective chip carriers 194 plug into 
respective connectors 192 carried by card 190 so as to stand vertically in 
the liquid nitrogen pool as shown in FIG. 6. Connectors 192 support chip 
carriers 194 mechanically as well as providing electrical connections 
therebetween. Each chip carrier 194 carries one or preferably a plurality 
of logic chips 196. Respective cables 198 provide an electrical connection 
between chip carriers 194 and the external environment. 
FIG. 7 shows an alternative embodiment of the present invention in which a 
plurality of logic units may be detachably secured to respective cold 
heads immersed in a cryostat. More particularly, the system 200 shown in 
FIG. 7 comprises a plurality of field-replaceable logic units 202 which 
are detachably secured onto the cold heads (not shown in FIG. 7) of a 
cryostat 204. Since cryostats are well known in the art, the structure and 
operation of the cryostat 204 will not be described in detail except for 
the modifications that are specific to its use in conjunction with logic 
units 202. A cryogenically insulated duct 206 connects cryostat 204 to the 
similarly cryogenically insulated cold head 208 of a cryogenic cooler 212 
having an expander portion 210 and a compressor 214 driven by a motor 216. 
Like cryostat 204, cryocooler 212 is of a type well known in the art, and 
will hence not be described in detail herein. Referring now to FIG. 8, 
cryostat 204 includes an outer wall 218 and an inner wall 220 formed of 
any suitable material, such as stainless steel. Preferably, the region 
between outer and inner walls 218 and 220 is evacuated to minimize heat 
transfer through the cryostat wall. Cryostat 204 contains a pool 222 of 
cryogenic liquid such as liquid air or, preferably, liquid nitrogen which 
is cooled by cryogenic cooler 212. Respective cryostat cold heads 224 
mounted in apertures formed on the upper surface of cryostat 204 have fins 
226 (either straight fins or pin fins) extending downwardly into pool 222 
to maintain the upper portions of heads 224 at a cryogenic temperature. 
Preferably, a thermally insulating seal 228 is used to seat each cryostat 
cold head 224 relative to the upper wall of cryostat 204. Respective 
blocks 230 carried on the underside of outer wall 218 beneath each 
cryostat cold head 224 are threaded through a portion of their thickness 
to receive mounting bolts (not shown) countersunk beneath the upper 
surface of the cryostat cold head 224. 
Each logic unit 202 has a dome-shaped upper portion comprising an outer 
wall 232 and an inner wall 234, the region between which is preferably 
evacuated to minimize heat transfer. Alternatively, inner wall 234 may be 
replaced with a layer of insulating material as in FIGS. 3 and 4. The 
lower rim portions of walls 232 and 234 are joined to an outwardly 
extending solid base or flange 236 which mates with the outer wall 218 of 
the cryostat 204. Blocks 238 similar to blocks 230 carried on the 
underside of outer wall 218 are threaded through a portion of their 
thickness to receive mounting bolts 240 extending through apertures formed 
in flange 236, thereby to detachably secure each logic unit 202 to the 
cryostat 204 above a cryostat cold head 224. Preferably, at least one 
O-ring seal 242 is disposed along the circumferential interface between 
flange 236 and outer wall 218 to minimize heat transfer by convection 
along the interface as well as to exclude ambient air. A resilient annular 
member 244 secured to the inner surface of each flange 236 extends 
upwardly to receive a conduction plate 246 comprising a highly thermally 
conductive material such as copper. As in the embodiment shown in FIG. 3, 
resilient member 244 is tension loaded when logic unit 202 is secured onto 
cryostat cold head 224, thereby urging the lower surface of plate 246 into 
intimate thermal contact with the upper surface of cryostat cold head 224. 
In a manner similar to the embodiment described above, conduction plate 
246 supports a chip carrier 248 comprising a suitable thermally conductive 
material. Chip carrier 248 in turn supports one or more logic chips 250. 
FIG. 9 shows a possible cabling scheme for the logic units 202 shown in 
FIG. 8. In the modification shown in FIG. 9, an electrical cable 254 
extending from chip carrier 248 passes through a hermetic seal 256 carried 
by flange 236 to receive a connector 258. Connector 258 is located in a 
well 260 formed in outer wall 218 of cryostat 204. Depending on the number 
of cables 254 and seals 256, well 260 may comprise either a cylindrical 
recess or a channel extending circumferentially around the axis of 
cryostat cold head 224. Connector 258 mates with a connector 262 coupled 
to a cable group 264. Cable group 264 passes downwardly through a hermetic 
seal 272 extending between the bottom wall of well 260 and inner wall 220 
of cryostat 204. Cable group 264 may comprise a first cable (or group of 
cables) 266 which extend upwardly through an adjacent seal 272 for 
connection to the chip carrier 248 of the corresponding logic unit 202. In 
addition, cable group 264 may comprise one or more cables 268 which pass 
upwardly through a hermetic seal 274, extending between inner and outer 
walls 220 and 218 between adjacent logic units 202, to receive a connector 
270 to provide an external electrical connection. 
Cable group 264 comprising cables 266 and 268 and the corresponding 
connectors are associated with cryostat 204, while cables 254 are 
associated with the corresponding logic units 202. When logic units 202 
are mounted on the cryostat 204, they are first connected electrically by 
inserting their connectors 258 into the corresponding well 260 and 
coupling them to connectors 262. After the necessary electrical 
connections have been made, the logic unit 202 is secured to cryostat 204 
using bolts 240. Cable 254 is provided with a sufficient amount of slack 
between seal 256 and connector 258 to permit this to be done. Preferably, 
to isolate wells 260 both from the external environment and from the 
interior of cryostat 204, a second O-ring seal 252 is disposed coaxially 
inside of seal 242, between the mating surfaces of flange 236 and outer 
wall 218, radially inwardly of well 260. 
While the invention has been particularly shown and described with 
reference to preferred embodiments, it will be understood by those skilled 
in the art that various changes in form and details may be made therein 
without departing from the spirit and scope of the invention.