Abstract:
A micro-refrigerator is fabricated using Micro-Electro Mechanical Systems processing and is used to thermally control photonic or microelectronic circuits. Temperatures below local ambient are possible due to the refrigeration capability of the device and unwanted parasitic heat such as from the walls or lid of an enclosure is minimized due to the small size of the cooled mounting area for the integrated circuit. Localized cooling is provided by jets of vapor droplet mixture controlled to impinge directly onto the hottest regions of a microelectronic or photonic integrated circuit allowing greater circuit density and thermal dissipation at isolated regions within the integrated circuit and advantageously improving performance. Methods of manufacturing micro-scale refrigerator elements including the compressor, evaporator and condenser are defined. This device is a direct improvement over the commonly used thermoelectric cooler for thermal control of microelectronic or photonic devices.

Description:
FIELD OF THE INVENTION  
       [0001]     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.  
       BACKGROUND OF THE INVENTION  
       [0002]     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 temperature 1 . 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.    
         [0003]     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 energy 2,1 . 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.    
         [0004]     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.  
         [0005]     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.  
       SUMMARY OF THE INVENTION  
       [0006]     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. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0007]      FIG. 1  illustrates the basic principle of the invention.  
         [0008]      FIG. 2  shows the thermodynamic cycle for the invention and relates the refrigerant states to the various entities in  FIG. 1 .  
         [0009]      FIG. 3  is a perspective view of the main parts of the invention in the preferred embodiment.  
         [0010]      FIG. 4  shows the various entities that make up the microrefrigerator including section views with the essential features of each element.  
         [0011]      FIG. 5  illustrates the capability to locally cool the microelectronic or photonic chip using jets of refrigerant. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]      FIG. 1  shows schematically the MEMS vapor compression refrigerator on a submount  100 . The refrigerant fluid  101  is compressed in the micro-compressor section  102  and piped  103  to the condenser  104  where it is condensed to a liquid at a temperature slightly above ambient. The liquid is piped  103  to a pressure reducing orifice expander  105  arranged near the evaporator cavity  106 . The photonic or electronic chip  107  is mounted atop an evaporator cavity  106  that has been etched into the substrate material. Heat is conducted from the photonic or electronic chip into the evaporator cavity where it completes the evaporation of the refrigerant and is carried away by the refrigerant fluid. The warmed refrigerant is piped to the compressor section  102  and the cycle repeats. A thermal sensor  108  such as a thermistor or thermocouple measures the temperature for control purposes. The thermodynamic cycle with the corresponding states of the refrigerant is illustrated schematically in  FIG. 2 . This is a standard vapor compression refrigeration cycle shown on a temperature entropy diagram, that is familiar to those skilled in the art. The refrigerant enters the compressor as mostly or all gaseous vapor at state  1 . It is compressed irreversibly to state  2  by the compressor. State  2   s  illustrates an adiabatic reversible compression process that is isentropic. This would be achieved only in an idealized compressor. Real compressors have less than ideal efficiency resulting in the higher entropy shown in state  2 . The refrigerant then travels through the piping and enters the condenser where it is cooled at constant pressure to state  3  which is all liquid. The liquid travels through the piping to the expander valve and expands adiabatically through the expander valve into the evaporator cavity at state  4 . Heat is withdrawn from the photonic or microelectronic chip causing a constant pressure evaporation of the refrigerant to state  1  and the cycle repeats.  
         [0013]     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  FIG. 2 . One of the most efficient heat transfer mechanisms known is that of multiphase heat transfer wherein a phase change from liquid to vapor or vapor to liquid with associated latent heat transfer and vigorous mixing results in heat transfer per unit contact area that is orders of magnitude higher than the comparable single phase heat transfer. This mechanism is advantageously employed in the evaporator and condenser sections to minimize the overall size of the evaporator and condenser and to allow the overall size of the micro-refrigerator to be as small as possible. The size of the evaporator and condenser cavities are further reduced by employing extended heat transfer surfaces or fins to increase the overall heat transfer area. Thus the size of the mounting pedestal for the photonic or microelectronic integrated circuit chip is as small as possible minimizing any parasitic heat transfer from the surroundings to the component mounting surface as is experienced with thermoelectric coolers or larger conventional refrigeration systems.  
         [0014]     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.  
         [0015]      FIG. 3  shows a  3 D perspective illustration of the MEMS submount with the refrigerator structures as follows: The submount with micro-refrigerator is constructed of two main MEMS parts including the top part and bottom part. Illustrated is the underside view of the top section  301  of the submount showing the evaporator cavity  302 . Not shown in this view are the thermal enhancement fins in the evaporator cavity. The condenser cavity  303  includes the thermal enhancement fins within the cavity  304 . A recess for the compressor mechanism is shown as  305 . A top view of the top section also shown in perspective illustrates the mounting platform  313  for the photonic or microelectronic chip. The bottom section  307  is shown below with the etched piping recesses  308 , and the bottom recesses forming the evaporator  309 , condenser  310  and compressor  311 . The expander valve is shown as a narrow orifice  312  etched near the inlet to the evaporator.  
         [0016]     An example of one possible embodiment of the compressor section  102  is illustrated in  FIG. 4  with details shown in Section II. The flow cavity  311  is covered with a bimorph membrane  401  consisting of a piezoelectric polymer sheet as for example PVDF  403 . The sheet is metalized in stripe regions on top  402  and bottom  404  with non-metalized regions between each metalized stripe such that the metalized regions are perpendicular to the flow direction. The sheet is bonded to a second PVDF polymer sheet  405 . This bimorph assembly  401  is then bonded to the top of the compressor cavity  311  and when voltage is applied at any point along the membrane a change in shape of the bimorph is created, locally, displacing the fluid from the cavity. Successive voltage applications along the length of the cavity create a peristaltic action and produce flow and increasing pressure in the flow direction. This is illustrated by the deformation in the bimorph  401  shown in Section I.  
         [0017]     An example of one possible embodiment for the evaporator section is shown in  FIG. 4 . The evaporator heat exchanger  106  is composed of a cavity  302  etched in preferably silicon or other MEMS material  301 . The shape of the cavity  302  and pedestal  313  is such that air insulates the cavity and prevents parasitic heat transfer to the surrounding material. Glass  314  and air  309  insulate the bottom of the heat exchanger. The entire structure may be manufactured using standard MEMS wafer processing techniques 3 . In this embodiment, the size of the evaporator is approximately 2 mm×2 mm×1 mm high. The evaporator cavity is provided with extended surface heat transfer fins  306  to allow the cavity to be as small as possible. A similar heat exchanger cavity is shown in  FIG. 4  Section III for the condenser. Here the cavities for heat exchange  303  in top part  301  and  310  in bottom part  307  are also provided with extended surface heat transfer fins  304 . The condenser cavity in this embodiment is approximately 2 mm×4 mm in plan area by about 1.5 mm high.    3  Kovacs, Gregory T. A.,  Micromachined Transducers Sourcebook , McGraw Hill, 1998.    
         [0018]     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.  
         [0019]     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.  
         [0020]     Many potential compressor technologies are available. The exact configuration chosen will depend on the particular application and could be piezoelectric, magnetic, or thermally actuated.  
         [0021]     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 patents 4  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.    
         [0022]     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 performance 5 .    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.    
         [0023]     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  FIG. 5 . The integrated circuit microelectronic chip  107  is illustrated mounted on the evaporator  106  pedestal  502  with various hot spots shown by the darker shading  503 . Cooling jets of refrigerated multiphase vapor droplet refrigerant  101  are directed via a plenum chamber  504  with precisely located orifices  505  to the most intense hot regions requiring cooling thereby allowing much greater densification of the circuit elements on the microelectronic chip and potentially faster performance of the electronic device.