Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2000-01-07
2002-08-06
Utech, Benjamin L. (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S710000, C438S712000
Reexamination Certificate
active
06429137
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates in general to an improved thermal switch, and in particular to an improved solid state thermal switch for selectively controlling heat transfer to and from a thermoelectric device. Still more particularly, the present invention relates to a system and method for the manufacturing of solid state thermal switches utilizing integrated circuit manufacturing techniques.
2. Description of the Related Art
Conventional cooling systems, such as found in a refrigerator, utilize vapor compression refrigeration cycles to provide heat transfer. Vapor compression cooling requires significant moving hardware, including at a minimum, a compressor, a condenser, an evaporator, and related coolant transfer plumbing. Miniature vapor compression cooling is not available for small cooling applications. However, small cooling applications are highly desirable.
Semiconductors and superconductors have enhanced performance at lower temperatures. CMOS logic can operate materially faster at lower temperatures. For example, if CMOS logic devices are operated at −50° C., their performance is improved by 50 percent over room ambient temperature. Liquid nitrogen cooling of CMOS logic to −196° C. has shown a 200 percent improvement in speed.
Similar benefits have been shown for integrated circuit wiring. Wiring resistances decrease by a factor of two for integrated circuits operated at −50° C. in comparison to room ambient temperature operation.
Thus, sub-ambient temperature operation of integrated circuit logic devices, such as field effect transistors, as well as the interconnect wiring can materially improve integrated circuit performance. However, accomplishing such cooling in the confines of ever decreasing areas poses new challenges.
Thermoelectric cooling is one alternative that has found some utilization given the compact size of Peltier devices. Peltier device thermoelectric cooling is very reliable because such devices are solid state. The utilization of thermoelectric devices in industry has, to date, been restricted to very specialized applications. Due to inefficiencies, very few applications can effectively utilize thermoelectric effects. The undesirable properties of thermoelectric devices, such as high cost and low efficiency, are out weighed by the desirable properties of thermoelectric devices. Recently, there have been significant advances in material technology, many attributable to advances made by the semiconductor industry. The inefficiency of thermoelectric devices is a key negative aspect of implementing a thermoelectric cooling design. A Peltier device cooling system typically has an efficiency in the range of 20 percent for a relatively nominal temperature differential between the hot sink and ambient temperature conditions.
Utilizing a Peltier cooling system to cool at a rate of one watt and attain a sub-ambient temperature of 0° C. requires that the system be powered with five watts. As the amount of heat to be transferred increases, the total power to be dissipated into the ambient mandates large convection devices. Large power supply circuits must also be utilized.
Therefore, Peltier device thermoelectric cooling has not been considered a broadly applicable technology for cooling integrated circuits and improving integrated circuit performance. However, the introduction of an effective solid state thermal switch could boost the efficiency of thermoelectric coolers when utilized in novel configurations as disclosed in cross referenced copending patent applications referred to in the cross reference section of this patent application. The cross-referenced copending patent applications, disclose novel switching for interrupting thermal conduction to and from a Peltier device.
Peltier cooling devices are typically on the order of a few microns in dimension. Micron sized mechanical switches for connecting and disconnecting to thermoelectric devices provides a less than perfect solution. Construction of mechanical micro-miniature thermal switches is not a well developed art. Mechanical micro-miniature switch assemblies would require the manufacture and assembly of contacts, wipers and actuation mechanism which are microscopic. Mechanical micro-miniature thermal switches are costly. Further micro-miniature switch assemblies are unreliable and have short lifetimes.
Generally, moving contacts have a very limited life in comparison to solid state devices. The life of a electro-mechanical switch is measured in cycles. The useful life of a switch might be on the order of a few million cycles. If an electro-mechanical switch must be cycled at a kilohertz, the short lifetime of the switch severely limits practical applications.
The importance of thermal switching of Peltier devices can be explained by classical equations. In operation, a Peltier device transports electrons from a cold source at temperature T
cold
to a hot sink at temperature T
hot
in response to an electric field placed across the Peltier device.
q=&agr;T
cold
I
−1/2
I
2
R−K&Dgr;T
Equation 1
The net heat energy transported by a Peltier device is composed of three elements. In equation 1, the first element represents the Peltier effect (thermoelectric) contribution, the second element defines negative Joule heating or resistive effects, and the third element defines negative conductivity effects of the heat. The thermoelectric component is composed of the Seebeck coefficient, the temperature of operation (T
cold
) and the current through the (TE) device.
Approximately one half of the Joule heating produced by the bias current is conducted to the cold source and the remainder to the hot sink. Lastly, the negative element attributable to thermal conduction represents the heat flow or heat conduction through the Peltier device. K is the thermal conductivity of the Peltier device from the hot sink to the cold source. Selective interruption of the heat transfer between a Peltier device and a heat sink has proven superior results as discussed in the copending patent applications referenced above. However, the thermal switch must have low thermal conductivity in the “OFF” state.
In equation 1, the thermoelectric component of the heat transport increases linearly with the current through the Peltier device and the Joule heating increases in proportion to the square of the current. Alternately described, the resistive heating exponentially increases due to the current through the Peltier device while the cooling effect linearly increases with increased current flow. The thermal conduction is also in direct proportion to the temperature differential between the cold source and the hot sink. Equation 1 clearly reflects how quickly a Peltier device in a classical configuration becomes inefficient as the cold source and hot sink diverge in temperature.
Equation 2 below defines a coefficient of performance for a Peltier device. The coefficient of performance is the ratio of the net heat energy transported at low temperature to the power consumed by the Peltier device. For a typical Peltier device made from bismuth telluride material, the coefficient of performance is less than 0.3.
η
=
heat transport
power consumption
=
α
T
cold
I
-
1
/
2
I
2
R
-
K
Δ
T
I
2
R
+
α
I
Δ
T
Equation
2
Note that the numerator of equation 2 represents the net cooling capability of the Peltier device. The denominator of equation 2 represents the total energy provided by an external D.C. power supply. The individual elements of the numerator were described in reference to equation 1. The first element in the denominator is the total Joule heating, while the second element is the heat energy transport work done by the Peltier device in moving energy from the T
cold
source to the T
hot
sink. Based upon this relationship, the maximum coefficient of performance possible in the
International Business Machines - Corporation
Salys Casimer K.
Vinh Lan
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