Cooling performance of integrated thermoelectric microcooler
Introduction
Thermoelectric coolers (Peltier devices) are employed widely in microelectronics to stabilise the temperature of solid-state lasers, to cool infrared detectors and charge-coupled devices (CCD) and to increase the operating speed and reduce unwanted noise in integrated circuits. A conventional thermoelectric cooler (Fig. 1) usually consists of a number of n- and p-type bulk semiconductor thermoelements connected electrically in series by copper strips and sandwiched between two electrically insulating but thermally conducting ceramic plates. The dimensions of commercially available thermoelectric coolers vary from about 50×50×5 mm to a lower limit of around 4×4×3 mm. Generally, the cooler is operated with the electronic devices mounted in piggy-back fashion. Accompanying the increasing reduction in device size is a requirement for smaller thermoelectric coolers. Although, in principle, the dimension of commercial coolers can be reduced further, their manufacture involves bulk technology and is incompatible with microelectronics fabrication processes.
Reductions in the dimensions of a thermoelectric generators and sensors can be achieved using standard silicon integrated circuit technology [1]. In principle, this technology can also be used to make thermoelectric coolers [2] but, in practice, the substrate is very thick compared with that of thermoelements. Consequently, the cooling achieved by thin film thermocouples, when operated in the Peltier mode, is adversely offset by the substrate's thermal bypass and heat exchange with surroundings due to convection and radiation.
Recently, progress has been made in improving the figure-of-merit of thin film thermoelectric materials [3] and a significant improvement in thermoelectric figure-of-merit may also be achieved by employing superlattice or quantum well structures [4]. This improvement in materials' performance is accompanied by progress in micro-machining which facilitates the fabrication of ultra thin substrates with very low thermal conductivity. It has been reported that a very thin SiC membrane with an accompanying low thermal conductivity can be deposited on a thick Si substrate [5], [6]. This structure was identified as suitable for fabrication of a thin film thermoelectric cooler with sufficient cooling power for microelectronic device applications and a practical microcooler was proposed [7]. Preliminary calculation, based on a simplified theoretical model which neglects contact effects, indicated that a maximum temperature difference of about 40 K could be achieved using a 1 μm thick SiC substrate. In this paper an improved theoretical model which includes contacts effects is formulated and used to provide guidelines to optimise the microcooler's design and performance.
Section snippets
Thermoelectric microcooler configuration
Fig. 2(a) and (b) are schematics of the proposed thin film thermoelectric cooler, which can be fabricated as follows: a very thin amorphous SiC film is `laid down' on a silicon substrate using conventional thin film deposition and a `membrane' formed by removing the silicon substrate over the desired regions using micro-machining. N- and p-type thermoelements are then deposited on the membrane to form thermocouples using conventional thin film deposition and patterning techniques. Thermocouples
Thermoelectric cooling performance
The cooling performance of a thermoelectric cooler is generally expressed in terms of the following characteristics: the maximum temperature difference established across the cold and hot sides, ΔTmax, coefficient of performance, η, and the heat pumping capacity, Qc. A conventional thermoelectric cooler is a free-standing and has no thermal bypass (Fig. 1) and its performance characteristics ΔTmax, η0, and Qc, are given by Refs. [9], [10], [11], [12],
Effect of thermal bypass of substrate
When the effect of the thermal bypass of the substrate, together with convection and radiation heat transfer over the central to-be-cooled area, is taken into consideration, the heat pumping rate at the cold side, Qc, can be written as,where I is the electrical current; h the convection heat transfer coefficient; ε the emissivity of the central to-be-cooled region; σ the Stefan–Boltzmann constant; b the width of thin film thermoelements; L2 the
Effect of electrical contact resistance
The electrical contact resistance between the thermoelements and metallic contact layer affects the thermocouple's figure-of-merit, ZD, which results in a decrease in both the maximum temperature difference and the coefficient of performance of the cooler. In practice, this effect is inevitable and has to be taken into consideration. The total resistance of one thermocouple when the electrical contact resistance is taken into consideration can be written aswhere A=bd is the
Effects of thermal contact resistance
In a conventional free-standing thermoelectric cooler, the heat flow is essentially one dimensional and the effect of thermal contact on the cooling performance can be described by a one-dimensional model. However, the thermal contact configuration of a thermoelectric microcooler is complicated as shown schematically in Fig. 3. Although rigorous calculation of thermal contact effects requires solving a complex two-dimensional heat transfer problem, a simplified model based on one-dimensional
Results and discussions
In Fig. 4, Fig. 5, Fig. 6 ΔTmax, η and Qc are presented, respectively, as a function of thermoelement length. The results were calculated using , , and assuming that z=2.8×10−3 K−1 for both n- and p-type thermoelectric materials, λ=1.5 W/mK, λs=2 W/mK, d=2 μm, ds=1 μm, L=1 mm, Th=300 K, 4εσTh3=6.12 W/m2K, and h≈0 when operating in vacuum. The dotted-lines are based on experimental data n≈0.1 mm and r≈0.2 mm which are obtained from conventional thermoelectric module [14]. In general, the
Conclusions
The necessary technology is readily available to make an integrated thermoelectric microcooler which can be integrated into the fabricating sequence of current microelectronic device such as laser diode or infrared detector. Theoretical analysis indicates that the necessary temperature difference and cooling capacity required for improved operation of microelectronic components would be achieved employing current IC technology and a typical microcooler configuration reported in this paper. The
Acknowledgements
This work is supported by the New Energy and Industrial Technology Development Organisation (NEDO), the Energy Conversion Centre, Japan. Dr. S.G.K. Williams is thanked for providing drawings in Fig. 1.
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