US 20030183368 A1
A heat sink made of a natural or polycrystalline diamond substrate with fins formed thereon. Diamond is grown to form a substrate and a laser is used to cut channels in the substrate to form the fins.
1. A heat sink comprising:
a natural or polycrystalline diamond substrate with fins formed thereon.
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12. An integrated cooling system comprising:
a heat source;
a heat sink made of a natural or synthetic diamond substrate with fins formed thereon mounted to the heat source;
a metalization layer at the interface between the heat source and the heat sink; and
a bonding layer between the metalization layer and the heat source for securing the heat source to the heat sink.
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26. An optical device comprising:
a reflective surface; and
a heat sink adjacent the optical surface, the heat sink including a natural or polycrystalline diamond substrate with fins formed thereon.
27. A window comprising:
a natural or a polycrystalline diamond substrate with upper and lower surfaces; and
fins formed in at least one edge of the substrate.
28. An integrated cooling system comprising:
an integrated electronic or optical device; and
a natural or polycrystalline diamond substrate mated on one surface with the device and including fins formed on the substrate for cooling the device.
29. A method of manufacturing a diamond heat sink, the method comprising:
growing diamond to form a substrate; and
using a laser to cut channels in the substrate to form fins thereon.
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39. A heat sink assembly comprising:
a natural or polycrystalline diamond substrate with fins formed thereon; and
a natural or polycrystalline diamond support attached to the substrate.
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 This invention relates to a novel heat sink made of natural or polycrystalline diamond.
 State of the art cooling systems with integral natural or polycrystalline diamond heat spreaders include a heat source such as a RF power amplifier chip, a diode chip or chip array that may be light emitting, or a power regulator chip attached to a diamond submount, which serves as a heat spreader. The thermal purpose of the diamond heat spreader is to reduce the intensity level of the heat flux emanating from the heat source, thereby making it more amenable to transfer to more conventional heat sink materials such as copper or aluminum which possess poorer thermal transport properties than diamond. Copper or aluminum materials are formed into heat sinks for the purpose of further reducing the heat flux density thereby allowing its efficient introduction into the heat rejection medium which might be gaseous or liquid, or even a solid thermal mass. Thus, heat dissipated in the electrical component flows through a complex mechanical assembly encountering several interfaces along the way.
 Unfortunately, each interface resists heat flow which must be overcome by increasing the temperature in the assembly and ultimately, at the source. Special precautions are taken at each interface in order to reduce the resistance to heat flow. The bottom of the heat source and the diamond heat spreader are plated with special materials that enhance their affinity to low resistance interface materials including solders such as gold/tin eutectic. However, high temperatures persist at the source leading to premature electrical failure of the power-dissipating device and causing system failure and downtime and increasing system-operating expense. Alternatively, complex and bulky refrigeration systems are required to lower device temperatures to acceptable levels. Frequently, these systems are incapable of dramatically reducing device temperature.
 For example, one arrangement consists of an RF power amplifier chip soldered to a diamond submount, which in turn is soldered to chip carrier made of copper molybdenum. The carrier is adhesively bonded to an amplifier package, which in turn is bonded to an aluminum heat sink. A refrigerated anti-freeze solution flows over the fin-like surfaces of the heat sink picking up the dissipated heat and carrying it away from the heat source for ultimate rejection to the environment.
 As solid state electrical devices are made smaller and smaller and yet at the same time designed to process more power and thus more heat, researchers are continuously looking for ways to lower the thermal resistance for heat transfer from the active regions of the device to the environment.
 In response, those skilled in the art have attempted to etch microchannels in the base of silicon devices and to mount laser diode arrays on silicon in which the microchannels have been etched. See U.S. Pat. No. 5,548,605. Another approach uses epitaxial lift-off (ELO) and grafting which yield epitaxial GaAs films of thickness as thin as 200 Å on diamond substrates. See Goodson et al., “Improved Heat Sinking for Laser-Diode Arrays using Microchannels in CVD Diamond”, IEEE Transactions on Components, Packaging, and Manufacturing Technology—Part B, vol. 20, No. 1, February 1997, incorporated herein by this reference.
 In this article, the authors theorized that microchannels could be formed in diamond instead of silicon to lower the thermal boundary resistance since diamond is the best heat conductor known. The idea of forming microchannels in diamond, however, was only notional and the authors provided only a theoretical basis for unexplained future experimental work: “future experimental work needs to include several technological innovations that make the proposed cooling system ready for practical implementations.” Id. page 108 (emphasis added).
 It is therefore an object of this invention to provide a heat sink made of natural or, more typically, polycrystalline diamond suitable for practical implementations.
 It is a further object of this invention to provide such a heat sink which greatly reduces resistance to heat transfer from a heat source such as a power amplifier chip or a laser diode array to the environment.
 It is a further object of this invention to provide such a heat sink which eliminates many of the interfaces between the power dissipating device and the environment.
 This invention results from the realization that the thermal resistance to heat transfer from a heat source such as a power amplifier chip or a semiconductor laser-diode array to the environment can be improved and numerous interfaces between the power dissipating chip and the heat sink eliminated by using a laser to cut microchannels in the diamond submount previously used as a lateral heat spreader thereby converting the diamond submount into a heat sink with the remaining diamond material acting as heat transfer surfaces or fins and defining microchannels between the fins.
 This invention features a heat sink comprising a natural or polycrystalline diamond substrate with fins formed preferably via laser cutting operations thereon. In the preferred embodiment, the diamond is chemical-vapor deposited diamond but it may also be diamond-like-carbon. The fins typically extend continuously along the substrate but may instead be pin fins. In some embodiments, the substrate and the fins are monolithic. In other embodiments, substrate has a plurality of layers and the fins are cut in all of the layers or instead only a subset of all the layers.
 In the preferred embodiment, the fins form microchannels in the substrate. Typically, the substrate has opposing top and bottom planar surfaces and the fins are formed in one said planar surface. In other embodiments, however, the fins are formed in an edge of the substrate.
 An integrated cooling system in accordance with the subject invention includes a heat source, a heat sink made of a natural or synthetic diamond substrate with fins formed thereon mounted to the heat source, a metalization layer at the interface between the heat source and the heat sink, and a bonding layer between the metalization layer and the heat source for securing the heat source to the heat sink. In the preferred embodiment, the metalization layer is a gold layer formed on the heat sink substrate and there is also a metalization layer formed on the heat source and mated with the bonding layer. The bonding layer is typically solder, but may also be braze, or formed by compression bonding.
 An optical device in accordance with the subject invention includes a reflective surface and a heat sink adjacent the optical surface, the heat sink including a natural or polycrystalline diamond substrate with fins formed thereon.
 A window in accordance with this invention includes a natural or a polycrystalline diamond substrate with upper and lower surfaces and fins formed in at least one edge of the substrate.
 A method of manufacturing a diamond heat sink according to this invention includes growing diamond to form a substrate and using a laser to cut channels in the substrate to form fins thereon. In some embodiments, multiple diamond plates are grown and secured together to form a substrate with discrete layers before the channels are cut. The channels may be cut in all the layers or only in a subset of the layers.
 Chemical-vapor-deposition of diamond is the preferred technique for growing the diamond and the channels are preferably cut to be 150 um or less in width to form microchannels.
 The channels may be cut to extend in one direction to form straight fins or instead cut to extend in two different directions to form pin fins. A metalization layer may be added to the substrate and substrate polished before it is cut by the laser.
 In another embodiment, the heat source is mounted to a diamond support or strong back which is attached to the diamond heat sink.
 Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:
FIG. 1 is a schematic cross-sectional view of a prior state-of-the-art integrated cooling system using microchannel flow passages in an aluminum heat sink;
FIG. 2 is a schematic cross-sectional view of an integrated cooling system incorporating a diamond heat sink in accordance with the subject invention;
FIG. 3 is a schematic view of a rectangular cross-section pin fin embodiment of the diamond heat sink of the subject invention;
FIG. 4 is a top view of a parallelogram cross-section pin fin embodiment of the diamond heat sink of the subject invention;
FIG. 5 is a photograph of a diamond heat sink with deepcut microchannels manufactured in accordance with the subject invention;
FIG. 6 is a photograph of a diamond heat sink with shallow cut microchannels manufactured in accordance with the subject invention;
FIG. 7 is a schematic cross sectional view of a diamond structural strongback embodiment of the subject invention;
FIG. 8 is a schematic cross sectional view of a laminated diamond structural strongback embodiment of the subject invention;
FIG. 9 is a photograph of an integrated assembly manufactured in accordance with the subject invention;
FIG. 10 is a schematic view of an edge cooled electromagnetic or light window embodiment of the diamond heat sink of the subject invention;
FIG. 11 is a schematic top view of a diamond optical or electromagnetic energy reflector with integral cooling channels in accordance with the subject invention;
FIG. 12 is a bottom view of the reflective device shown in FIG. 11; and
FIG. 13 is a schematic view of a RF power amplifier mounted in a radar embodiment of the diamond heat sink in accordance with the subject invention.
 As delineated in the Background of the Invention section above, a prior state of the art in integrated cooling systems is shown in FIG. 1. Heat source 10 is an electrical device which may be a power amplifier chip or a laser diode array with a gold adherence promoting metalization layer 12 plated on the bottom side thereof as shown. A heat spreader in the form of diamond submount 18 is metalized on both sides with adherent 16 and 20. AuSn solder layer 14 secures heat source 10 to diamond submount 18. Another AuSn solder layer 22 secures this subassembly to copper molybdenum carrier 24. Solder layer 26 is used to secure carrier 24 to aluminum silicon carbide (AlSiC) package 28. Package 28 is adhesive bonded as shown by layer 30 to aluminum microchannel heat sink 36. Antifreeze coolant flows between fins 32 in channels 34 to remove heat emanating at source 10.
 In one embodiment, heat source 10 is a power amplifier chip 6.6 mm by 4.9 mm in area including a GaN layer 2 um thick and a SiC layer 100 um thick. Gold adherent layers 12 and 16 are 5 um thick and AuSn solder layer 14 is 5 um thick. Diamond submount 18 is 380 um thick. Gold adherent layer 20 is 5 um thick and AuSn solder layer 22 is 5 um thick. CuMo carrier 24 is 1 mm thick and approximately 25 mm by 25 mm in area. Solder layer 26 is 50 um thick. AlSiC package 28 is 1 mm thick. Adhesive layer 30 is 250 um thick. Heat sink 36 comprises a 1 mm face plate with a fin pitch of .32 mm, and 150 um thick aluminum fin stock 2 mm high. Fins 32 interface with a coolant such as an ethylene glycol/water composition at a 20° C. inlet temperature. When the coolant which flows through the microchannels 34 (e.g., 150 um or less in width) between fins 32 is at this temperature, the temperature of the active regions of the power amplifier chip 10 (determined by computer modeling) can be maintained at 233° C.
 Still, as devices are made smaller and smaller and yet at the same time process more power and thus generate more heat, researchers have searched for ways to lower the resistance to heat transfer from the active regions of device 10 to the environment as represented by the coolant flowing between the microchannels 34 of heat sink 36.
 This invention results in part from the realization that if diamond submount 18 is made thicker and is cut with a laser to form microchannels, it will then perform two functions: lateral spreading of heat from the active regions of chip 10 and the heat transfer function previously supplied by aluminum heat sink 36. Thus, aluminum heat sink 36 can be eliminated and at the same time many undesirable interfaces which impede heat transfer are also eliminated (e.g., solder layer 22, carrier 24, solder layer 26, and adhesive layer 30). In addition, manufacturing process steps are eliminated including two soldering steps, one adhesive bonding operation and two gold plating steps.
 Accordingly, the subject invention features diamond heat sink 40, FIG. 2. Heat sink 40 is made by cutting channels 50 in diamond submount 52 to form fins 54. Submount 52 is typically polycrystalline chemical-vapor-deposition (CVD) diamond but could also be diamond-like-carbon or even natural diamond. Fins 54 typically extend continuously along one surface (typically the bottom) of the diamond submount but if channels 50 are cut in two directions, pin fins 53, FIG. 3 may be formed. In FIG. 4, pin fins 53′ have a parallelogram cross-section.
 As shown in FIG. 2, the top surface of heat sink 40 typically includes gold adherent plated layer 16 which is attached via AuSn solder layer 14 to gold adherent layer 12 of heat source 10. Other types of solder or brazing materials or even compression bonding techniques may be used to secure heat sink 40 to heat source 10.
 A comparison of FIGS. 1 and 2 reveals the advantages of the diamond heat sink of the subject invention: in FIG. 2, the following components of FIG. 1 are eliminated: adherent 20, solder 22, carrier 24, adhesive 26, AlSic package 28, SnPb solder layer 30, and aluminum heat sink 36. In addition, heat sink 40, FIG. 2 performs the function of diamond submount 18, FIG. 1 (lateral heat spreading) and yet also acts as the cooling interface by virtue of fins 54, FIG. 2 and channels 50. Furthermore, in FIG. 1, when the coolant was at 20° C., the maximum temperature of heat source 10 was 233° C. In contrast, in accordance with the design of FIG. 2, when the coolant was at 20° C., the maximum temperature of heat source 10 (determined by computer modeling) was much lower—162° C. As such, novel diamond heat sink 40 can result in the elimination of the refrigeration subsystems of the prior art for a given device temperature, while reducing the temperature of the heat source thus improving electrical performance and increasing the useful life of the heat source. The elimination of the refrigeration system reduces system weight, space, power consumption and maintenance requirements.
 The microchannels cut in the diamond submount may be forced to intersect one another if the laser cutter is so programmed. FIG. 3 shows a rectangular cross-section ‘pin-fins’ 53 that results from channels cut along orthogonal axes. The microchannels may also be formed to have a parallelogram cross-section by cutting along non-orthogonal axes, as shown in FIG. 4. These channels forming approaches may be beneficial to reduce coolant hydrodynamic boundary layer build-up along the coolant flow through the microchannels, and thus enhance heat transfer, or reduce coolant pressure drop. In either case, the basic stack-up of FIG. 2 is maintained, with the power dissipating device 10 attached via gold adherent 12 to solder 14 and another gold adherent layer 16 on the heat sink 40.
 As shown in FIGS. 5 and 6, microchannels 50 cut by a laser in a CVD submount 52 maybe 150 um in width or less, maybe 0.5 mm (FIG. 6) to 0.8 mm (FIG. 5) in depth or greater, and may have walls contoured to correspond to the reduced fin cross sectional area required to conduct heat at fin tips. The latter may reduce coolant pressure drop and subsequent pumping power requirements.
FIG. 7 is a schematic representation of diamond microchannel heat sink 63, which has been attached to a diamond structural support or strongback 66. In this embodiment, the power dissipating device 10 is plated with gold adherent 12 and attached to diamond strongback 66 via AuSn solder 14. Strongback 66 has been plated top and bottom with gold adherent 16, thereby allowing the heat sink 63 to be soldered to strongback 66. Strongback 66 provides mechanical support for heat sink 63 and the electrical devices as well as providing an electrical ground plane for those devices. In addition, the strongback serves as a carrier for the electrical devices as well as providing a mounting frame for bonding into higher assemblies. Another feature of strongback 66 is that it can emulate the heat spreader function of the current diamond and, as such, can be made of higher quality diamond than heat sink 63, which can reduce overall assembly costs.
FIG. 8 is a schematic representation of a laminated diamond microchannel heat sink, which has been attached to a strongback similar to that shown in FIG. 7. Heat sink 63′ is fabricated from a lamination of thinner diamond submounts 64 and 60 that are plated with gold adherent 101 and 103 and attached to each other using AuSn solder 102. After being joined, microchannels 104 are cut using the laser in accordance with the subject invention. The lamination approach allows thinner and thus lower cost diamond material to be fabricated into thicker heat sinks. It also permits the use of poorer quality diamond material with lower thermal conductivities at distances further removed from the heat source where heat flux is reduced. Laminations consisting of three plies have been successfully fabricated in the laboratory. In some embodiments, channels 104 may not extend through all the plies as shown at 104′.
FIG. 9 is a view of a diamond microchannel heat sink 40, which has been attached, via gold adherent and AuSn solder to a diamond strongback 66. Four power dissipating resistors have been gold adherent plated and soldered to the strongback, and the diamond microchannel shown in FIG. 6 has been soldered to the bottom of the strongback in accordance with the subject invention.
 Thus far, the fins have been shown to be formed in the top or bottom planar surface of the diamond plate. This, however, is not a necessary limitation of the subject invention as shown in FIG. 10 where diamond plate 70 has channels 72 cut in edge surface 74 so as not to interfere with top and bottom planar surfaces 76 and 78 thus rendering diamond plate 70 suitable for use as a window—i.e., a window between the environment and an infrared radiation detection subsystem in a missile.
 In FIGS. 11-12, top surface 80 of diamond substrate 82 is rendered optically reflective by a gold coating, for example, and channels 84 are cut in bottom surface 88 to form an optical device with integral cooling channels.
 In FIG. 2, no system packaging is shown. In contrast, in FIG. 13, there are two diamond heat sinks 40 each with heat source 10 mounted thereto and this subassembly is mounted in package 90 which includes a vacuum brazed aluminum manifold 92 which drives coolant 94 in the microchannels of each heat sink.
 Heat sink 40, FIGS. 5 and 6 was manufactured as follows. A CVD diamond blank approximately 1 mm thick and 125 mm diameter was grown using a microwave assisted CVD diamond reactor at Raytheon Company's Advanced Materials Laboratory, Lexington, Mass., the assignee of the subject application, and delivered to the Mechanical and Materials Engineering Laboratory diamond cutting and polishing facility to be cut by a laser into the desired rectangular shape and then ground to a near-optical quality finish. The blanks were then cut into pieces measuring 18 mm by 8 mm and placed in the laser cutting facility where a YAG laser was programmed to transverse the diamond material is an X-Y plane with various material feed rates along the X-direction to provide the channel cuts at the approximate desired depth. At the completion of each X-direction pass, the material was indexed approximately 50 um inches in the Y-direction and the pass repeated in the opposite direction. Upon completing each channel cut, which required 3 passes, the material was indexed 150 um in the Y-direction, and the channel cutting resumed. Channel depth, spacing and contours are controlled by virtue of the programming loaded into the piece drive controls, the laser pulse duty cycle, and the number of passes. This process resulted in producing channels 50 with a depth of 800 um and a width of 150 um.
 Accordingly, in accordance with the subject invention, the resistance to heat transfer from a heat source such as power amplifier chip or semi-conductor laser-diode array to the environment is greatly improved and numerous interfaces between the power dissipating chip and the heat sink eliminated by using a laser to cut microchannels in the diamond submount previously used as a lateral heat spreader to turn the diamond submount into a heat sink with fins and microchannels.
 The diamond microchannel heat sink in accordance with the subject invention exhibits the capability to accommodate high heat flux levels (3,200 W/cm2)—an order of magnitude above current technology. As stated above, the diamond heat sink of the subject invention performs two functions: heat spreading and heat dissipation. The life expectancy of GaAs type chips is expected to increase by at least a factor of 2 per Mil-HDBK-217F for a 25° C. reduction.
 In full production runs, a diamond wafer up to 125 mm in diameter is grown, cut to a convenient size and polished. Many heat sinks may be manufactured at once by laser cutting the microchannels and then laser cutting the plurality of heat sinks from the wafer. Such heat sinks can be used in conjunction with many different types of heat sources such as power amplifiers, laser diode chips, integrated electronic devices (ASICs), optical devices, and the like.
 Therefore, although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments.
 Other embodiments will occur to those skilled in the art and are within the following claims: