An Introduction to Microelectromechanical Systems Engineering The Sandbox: Materials for M E M S devices are made of n-type and p-type bismuth telluride elements, and are used to cool high-performance microprocessors, laser diodes, and infrared sensors. Peltier devices have proven difficult to implement as micromachined thin film structures. In the Seebeck effect, named for the scientist who made the discovery in 1822, a temperature gradient across an element gives rise to a measurable electric field that tends to oppose the charge flow (or electric current) resulting from the temperature imbalance. The measured voltage is, to first order, proportional to the temperature difference,. | 36 An Introduction to Microelectromechanical Systems Engineering devices are made of -type and p-type bismuth telluride elements and are used to cool high-performance microprocessors laser diodes and infrared sensors. Peltier devices have proven difficult to implement as micromachined thin film structures. In the Seebeck effect named for the scientist who made the discovery in 1822 a temperature gradient across an element gives rise to a measurable electric field that tends to oppose the charge flow or electric current resulting from the temperature imbalance. The measured voltage is to first order proportional to the temperature difference with the proportionality constant known as the Seebeck coefficient. While in theory a single material is sufficient to measure temperature in practice thermocouples employ a junction of two dissimilar materials. The measurable voltage at the leads AV is the sum of voltages across both legs of the thermocouple Figure . Therefore AV 71 Tcold Thot 72 Thot Tcold 72 Ql Thot Tcold where at and a2 are the Seebeck coefficients of materials 1 and 2 and Thot and Tcoid are the temperatures of the hot and cold sides of the thermocouple respectively. Alternately one may use this effect to generate electrical power by maintaining a temperature difference across a junction Table . Hot Material 1 Material 2 Cold a Figure The basic structure of a thermocouple using the Seebeck effect. The measured voltage is proportional to the difference in temperature. Thermocouples can be readily implemented on silicon substrates using combinations of thin metal films or polysilicon. The Sandbox Materials for MEMS 37 Table The Seebeck Coefficients Relative to Platinum for Selected Metals and for n- and -Type Polysilicon. zV K UV K Bi Ag Ni Cu Pa Zn Pt 0 Au Ta W Al Mo Sn n-poly 30 Q 0 -100 Mg n-poly 2600 Q 0 -450 Ir p-poly 400 Q 0 270 The sheet resistance is given for the .
