The invention pertains to the application of a Micro-Electro Mechanical Systems (MEMS) based micro vapor compression refrigeration system to thermally control electronic or photonic devices for improved performance and lower cost.
Recent advances in fiber optics and photonics have resulted in a vast increase in the volume of information that can be transmitted optically. This has occurred in two fundamental ways. The speed of modulation of the optical signal has increased to upwards of 40 Gb/s and the wavelength spacing between adjacent channels is only a few tenths of a nanometer. To maintain this performance, the temperature of the photonic device must be held to within less than one degree Celsius of the design temperature1. This temperature is usually less than the ambient temperature surrounding the device requiring active refrigeration. The technology available for refrigerating the device is limited to either a thermoelectric cooler which utilizes the Peltier effect, or some type of large external refrigeration system with coolant piped to the component. Commercial photonic devices utilize the thermoelectric cooler almost exclusively since this is the only way an independently mountable component can be accomplished.
1 Hecht, Jeff, Understanding Fiber Optics, Second Edition, Prentice Hall, 2002.
Recent trends in electronic and especially microprocessor technology continually increase the density of active logic as circuit elements get smaller. One of the most significant limitations preventing further reduction in size is the need to dissipate thermal energy2,1. Current technology utilizes fans, heat pipes, active liquid cooling and multiphase heat transfer techniques to minimize the thermal resistance from device junctions to surrounding ambient. While it may be possible to further increase the active junction density by utilizing an external refrigeration system, this approach is not practical for desktop or laptop computing applications. If a micro-refrigeration technology could be developed, highly localized refrigeration or sub-ambient cooling for the most thermally troublesome parts of the electronic circuitry, even within the microprocessor or integrated circuit chip, could be used to dramatically shrink the circuit elements and increase functionality while reducing cost of the system.2
1 Hecht, Jeff, Understanding Fiber Optics, Second Edition, Prentice Hall, 2002.
2 Yeh, L. T., Chu, RC., Thermal Management of Microelectronic Equipment, ASME Press, 2002.
Other applications of a micro-refrigeration system include micro-sensors, such as IR cameras and miniature chemical systems on a chip, cryogenic photonics such as quantum cascade laser devices where the micro-refrigerator could operate in conjunction with a thermoelectric cooler to achieve extremely low cryogenic temperatures, biomedical devices where thermal control of the drug for delivery into the human body is needed, and many others.
Over the last several years, the application of integrated circuit processing techniques to the design and construction of mechanical, thermal and chemical systems has been developed. This branch of technology is commonly known as Micro-Electro-Mechanical Systems or MEMS. The advantages of this approach to the manufacture of ultra small machines are that the individual devices can be made with many to a single silicon wafer just as in microelectronic integrated circuits and the devices can be made with characteristic dimensions of the order of several microns. This allows the development of machines with tolerances that are much more precise than conventional machining and can be said to be analogous for mechanical devices to the miniaturization revolution that was achieved in microelectronics with the invention of the integrated circuit.
A micro-vapor compression refrigeration system on a MEMS chip is invented that maintains the temperature and optical or electrical performance of a photonic or electronic device. This micro-refrigerator operates on the standard vapor compression refrigeration cycle similar to a home refrigerator or air conditioner with choice of working fluid adapted to the application. It is envisioned that the MEMS refrigerator would be fabricated on a submount which would accommodate the photonic or electronic device and provide in-situ refrigeration to the device at temperatures below the surrounding ambient and which would enable integration with other functions such as optical alignment, high speed RF electronic tuning, or optical wavelength monitoring and control directly on the submount.
It is important to note that the state of the refrigerant fluid as it enters the evaporator section is partially liquid and partially gaseous as shown by the state 4 under the refrigerant vapor dome in
It is possible to operate the cycle in reverse in case heating is needed for the photonic or electronic chip. In this case the compressor operates in reverse and compresses the fluid from state 1 to state 2, which flows to the evaporator cavity. Here the fluid condenses giving up heat to the photonic or microelectronic chip and reaches state 3 at the entrance to the expander valve. The refrigerant then expands across the expander to state 4 and travels through the piping to the condenser where it is heated to state 1 and the cycle repeats. This type of reverse operation may not be as thermodynamically efficient as the normal operation described above, but it is possible.
An example of one possible embodiment of the compressor section 102 is illustrated in
An example of one possible embodiment for the evaporator section is shown in
3 Kovacs, Gregory T. A., Micromachined Transducers Sourcebook, McGraw Hill, 1998.
The advantage of this configuration is that it can be fabricated and assembled in wafer form using two wafers bonded together to provide a hermetic seal for the refrigerant. In the illustrated preferred embodiment, the two wafers containing many individual refrigerators are bonded together in an environment of refrigerant maintained at an appropriate pressure so that the final assemblies are hermetically sealed with the refrigerant inside.
Interconnecting piping 308 will be etched into the wafers and the expansion orifice 105 is just a narrowed etched portion of piping 312 near the evaporator cavity 309 entrance.
Many potential compressor technologies are available. The exact configuration chosen will depend on the particular application and could be piezoelectric, magnetic, or thermally actuated.
Overall the size of the entire refrigerator will be less than approximately 10 mm×10 mm by 1.5 mm in the preferred embodiment. This is smaller than previous patents4 by an order of magnitude.
4 Beebe, D., Bullard, C., Philpott, M., Shannon, M., “Active compressor vapor compression cycle integrated heat transfer device,” U.S. Pat. No. 6,148,635, November 2000.
One of the advantages of the MEMS implementation of a vapor compression refrigerator is the fact that the refrigerant is in a multiphase gaseous and droplet state as it leaves the expansion orifice. This fact has been shown to greatly enhance the heat transfer and reduce the required surface area thereby allowing a much smaller refrigerator for the same thermal performance5.
5 Schlager, L. M., Pate, M. B., Bergies, A. E., “Evaporation and Condensation Heat Transfer and Pressure Drop in Horizontal, 12.7 mm Microfin Tubes With Refrigerant 22,” Journal of Heat Transfer, Volume 112, pp. 1041-1047, 1990.
Typical microelectronic integrated circuits do not have a uniform heat generation over the surface of the chip but often have localized regions of much greater thermal dissipation corresponding to very dense and very active circuit elements with the IC chip. The greatly enhanced thermal performance of droplet spray multiphase cooling over a small area may allow local cooling of hot spots within the integrated circuit chip. An arrangement of refrigerant jets allows cooling of the hottest spots permitting much denser circuits and enabling improved electrical performance. A preferred embodiment of this principle is shown in
Number | Date | Country | |
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60533572 | Jan 2004 | US |