Batteries: thermoelectric and photoelectric – Thermoelectric – Processes
Reexamination Certificate
1999-12-09
2001-04-24
Gorgos, Kathryn (Department: 1741)
Batteries: thermoelectric and photoelectric
Thermoelectric
Processes
C136S203000, C062S003300, C062S003610, C062S003700
Reexamination Certificate
active
06222113
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to cooling apparatuses and methods for making same. More particularly, the invention is directed to thermoelectric cooling apparatuses attaining high relative efficiency thermoelectric cooling through the use of thermally conducting, electrically insulating semiconductor based substrates.
2. Description of the Related Art
Sub-ambient cooling is conventionally accomplished through gas/liquid vapor phase compression based refrigeration cycles using Freon type refrigerants to implement the heat transfers. Such refrigeration systems are used extensively for cooling human residences, perishable items, and vehicles. Sub-ambient cooling is also often used with major electronic systems such as mainframe, server and workstation computers. Though vapor compression cooling can be very efficient, it does require significant moving hardware. Vapor compression cooling systems, at a minimum, include a compressor, a condenser, an evaporator, and related coolant transfer plumbing. As a result of the complexity and associated high cost, vapor compression cooling has not found material acceptance in small cooling applications, such as personal computers, integrated circuits, etc.
The fact that CMOS logic can operate significantly faster as the temperature decreases has been well known for many years. For example, when CMOS logic devices are operated at −50° C., their performance is improved by 50 percent over room temperature operation. Liquid nitrogen operating temperatures, in the range of −196° C., have shown 200 percent performance improvements. Similar benefits have been shown to accrue for integrated circuit wiring, where metal wiring resistance decreases by a factor of 2 for integrated circuits operated at −50° C. in comparison to room temperature operation. These performance improvements rival the recent technological breakthrough of using copper wiring in integrated circuits to reduce interconnect resistance and thereby effectively increase the operating frequencies attainable. Thus, sub-ambient temperature operation of integrated circuit logic devices, such as field effect transistors, as well as interconnect wiring can improve integrated circuit performance. This performance enhancement then poses the question of how to accomplish such cooling in the confines of the ever decreasing size and materially shrinking cost environment of microelectronics.
FIG. 1
schematically depicts a conventional Peltier type thermoelectric element (TE)
1
with DC power supply
2
creating the electric field across TE
1
while at a load current
3
. The desired heat transfer is from cold sink
4
, at temperature T
cold
, to hot sink
6
, at temperature T
hot
. As indicated in the equation of
FIG. 1
, the net heat energy transported is composed of three elements, the first representing the Peltier effect (thermoelectric) contribution, the second defining negative Joule heating effects, and the third defining negative conductivity effects. The thermoelectric component is composed of the Seebeck coefficient, the temperature of operation (T
cold
) and the current being applied. The Joule heating component reflects that roughly half the Joule heating goes to the cold sink and remainder to the hot sink. Lastly, the negative component attributable to thermal conduction represents the heat flow through the Peltier device, as defined by the thermal conductivity of the Peltier device, from the hot sink to the cold sink. See equation (1).
q=&agr;T
cold
I−(½)I
2
R−K&Dgr;T (1)
High thermal conductivity is a desired characteristic of the substrates in thermoelectric coolers, such as thermal sinks
4
and
6
. This high level of thermal conductivity reduces the thermal resistance of the substrate which, in turn, reduces the temperature drop across the substrate and therefore increases the heat transfer efficiency of the thermoelectric cooler. In addition to providing high levels of thermal conductivity, the substrates must also be electrically insulating. Electrically insulating substrates are used to ensure electrical isolation of the thermoelements in a thermoelectric cooler while providing appreciable thermal conduction. It is also desirable to make the substrates used to fabricate thermoelectric coolers as thin as possible to minimize the temperature drop across the substrate to thereby maximize the efficiency of the thermoelectric cooler.
Conventional thermoelectric coolers utilize berylia (BeO) ceramic substrates which are thermally conducting and electrically insulating. BeO has a high thermal conductivity, approximately 320 W/m-K, however, it is difficult to fabricate and use thin BeO substrates. The brittle nature of a BeO substrate makes it susceptible to fractures when thinned to less than 1 millimeter. Therefore, typical BeO substrates are limited to thicknesses of between 2 and 4 millimeters. As a result, temperature drops of only 25 K to 30 K or greater can be achieved for high heat flux condition. BeO substrates are further limited by their inability to allow the growth of advanced thermoelectric lattices as well as by the fact that the thermal expansion of BeO is not matched with silicon, the material used in the fabrication of most processors and integrated circuits.
Alternatives, such as silicon-on-insulator (SOI) wafers and diamond thin films or silicon-on-diamond (SOD) wafers, have been considered, but with each alternative comes different limitations. For example, the silicon dioxide layer in SOI wafers has a very low, approximately 1 W/m-K, thermal conductivity which translates to a high thermal resistance of the substrate. SOD wafers, on the other hand, have high costs associated with their use. For example, 500 micron diamond films cost around $90 per square centimeter and are expensive to process due to their rough surfaces and graphite formations at the grain boundaries.
There presently exists a need for thin film implementations and minitarization of thermoelectric cooler substrates. Thin film implementations and minitarization of thermoelectric cooler substrates would provide high cooling flux scaling with the smaller geometries to provide cooling in the range of 50 to 100 W/cm
2
with high entropy gradients and lower thermal conductivities. Use of thin film implementations would yield higher reliability in the order of MTBF (mean time between failures) of greater than 10
6
hours, lower cost in the order of less than 10¢/W and ease of constructing of multistage configurations wherein microcoolers can be operated in parallel for large cooling capacity and high efficiency.
Thus present thermoelectric cooler substrates limit the ability to achieve relative low temperature drops and thereby limit the use, scalability and efficiency of thermoelectric coolers and improved substrates are needed which overcome these limitations.
SUMMARY OF THE INVENTION
The present invention overcomes the limitations of presently used thermoelectric cooler substrates by forming and using thermally conductive semiconductor based substrates having doped regions providing electrically insulating properties.
In one form, the invention relates to a thermoelectric cooling apparatus comprising at least one thermal sink, at least one thermoelectric cooling element situated to be coupled to the thermal sink and wherein the thermal sink comprises a semiconductor material having a plurality of doped regions.
In another form, the invention relates to a method of fabricating a thermoelectric cooling apparatus comprising forming at least one thermal sink of a semiconductor material having a plurality of doped regions and coupling a thermoelectric element to thermal sink.
In still another form, the invention relates to a thermal sink situated to be coupled to a thermoelectric element comprising a semiconductor material having a plurality of doped regions.
In still another form, the invention relates to a method of fabricating a thermal sink adapted to be coupled to a thermoelectric ele
Gorgos Kathryn
International Business Machines - Corporation
Parsons Thomas H
Salys Casimer K.
Yee Duke W.
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