Diamond heat sink
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.
This application is a divisional application of prior U.S. patent application Ser. No. 10/114,601 filed on Apr. 2, 2002, which is hereby incorporated herein by reference, and to which this application claims priority.
FIELD OF THE INVENTIONThis invention relates to a novel heat sink made of natural or polycrystalline diamond.
BACKGROUND OF THE INVENTIONState 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).
SUMMARY OF THE INVENTIONIt 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.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGSOther 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:
As delineated in the Background of the Invention section above, a prior state of the art in integrated cooling systems is shown in
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 0.32 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,
As shown in
A comparison of
The microchannels cut in the diamond submount may be forced to intersect one another if the laser cutter is so programmed.
As shown in
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
In
In
Heat sink 40,
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:
Claims
1. 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.
2. The method of claim 1 further including growing multiple diamond layers and securing the multiple diamond layers together to form a substrate with discrete layers before the channels are cut.
3. The method of claim 2 in which cutting includes cutting channels in all the layers.
4. The method of claim 2 in which cutting includes cutting channels in a subset of the layers.
5. The method of claim 1 in which growing includes chemical-vapor-deposition of diamond.
6. The method of claim 1 in which the channels are cut to be 150 um or less in width to form microchannels.
7. The method of claim 1 in which the channels are cut to extend in one direction to form straight fins.
8. The method of claim 1 in which the channels are cut to extend in two different directions to form pin fins.
9. The method of claim 1 further including the addition of a metalization layer to the substrate.
10. The method of claim 1 in which the substrate is polished before it is cut.
11. 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.
12. The heat sink of claim 11 in which the diamond is chemical-vapor-deposited diamond or diamond-like-carbon.
13. The heat sink assembly of claim 11 in which the fins extend continuously along the substrate.
14. The heat sink assembly of claim 11 in which the fins are pin fins.
15. The heat sink assembly of claim 11 in which the substrate and the fins are monolithic.
16. The heat sink assembly of claim 11 in which the substrate has a plurality of layers.
17. The heat sink assembly of claim 16 in which the fins are cut in all of the layers.
18. The heat sink assembly of claim 16 in which the fins are cut in a subset of all the layers.
19. The heat sink assembly of claim 11 in which the fins form microchannels in the substrate.
20. The heat sink assembly of claim 11 in which the substrate has opposing top and bottom planar surfaces, and the fins are formed in one said planar surface.
21. The heat sink assembly of claim 11 further including metalization on the support and metalization on the substrate.
22. The heat sink assembly of claim 11 further including a heat source mounted on the support.
23. The heat sink assembly of claim 22 further including metalization on the heat source.
24. 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.
25. 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.
26. 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.
Type: Application
Filed: Jun 11, 2007
Publication Date: Feb 21, 2008
Inventors: Leo Paradis (Chelmsford, MA), Matthew DeBenedictis (Lynn, MA), Stephen LeBlanc (Sratham, NH), Richard Miller (Franklin, MA)
Application Number: 11/811,490
International Classification: F28D 21/00 (20060101);