There is a growing demand for heat-transfer expertise to aid with the design of microfabricated transistors, sensors, and actuators for the integrated circuits and MicroElectroMechanical Systems industries. However, the mechanical engineering curriculum provides students with little exposure to thermal phenomena in microdevices. To help remedy this situation, the Microscale Thermosciences Teaching Laboratory (MTTL) offers conduction and convection experiments using micromachined electronic structures. The goal is to teach classical heat transfer through experiments on high-technology devices of relevance for silicon valley companies.

The first microdevice experiment offered as part of the required undergraduate heat-transfer class examines the self heating of VLSI circuit metallization. This experiment introduces students to classical thermal conduction, electrical probing, electrical-resistance thermometry, and calibration. The laboratory also demonstrates the impact of large current densities on the temperature rise in the metallization and shows how this temperature rise depends on the thermal conductivity of the underlying silicon dioxide. The students use the probe stations shown in the facilities pages to sustain a varying electrical currents in interconnect bridges, such as that shown in Figure 1, and measure the resulting temperature rise through the interconnect electrical resistance. A temperature controller and thermal chuck are used for calibration. The students analyze one-dimensional conduction normal to the passivation and consider two-dimensional spreading using a shape factor.

Many micro-sensors and actuators use heat transfer to induce or detect motion. One such device is a thermally-actuated microvalve, shown in Figure 2, which is the subject of the second microdevice experiment. The valve uses thermal expansion of a stationary working fluid to deflect a silicon membrane against an orifice, which increases the impedance for fluid flow through the valve. The working fluid is heated by current sustained in a platinum electrical resistor. The students develop a transient thermal circuit model to interpret and predict the temperature distribution in the valves. The thermal resistances in the circuit are extracted through experiments using the platinum heater as an electrical-resistance thermometer, and the extracted values are compared with theory considering conduction and internal and external convection. The students determine the time constant of the valve and the power required for actuation, both of which are figures of merit. One goal of this experiment is to give students practical experience with the modeling of heat flow in a working device.

Because the probing and visualization equipment at MTTL can be applied to a variety of microdevices, the experiments that can be pursued are not limited to those discussed here. Many of MEMS that make use of heat transfer, such as thermal transducers and sensors, can be studied by large numbers of undergraduates.

Recent Publications

Schuder, R.G., and Goodson, K.E., 1997, "Integrated Circuits and MicroElectroMechanical Systems in the Heat Transfer Teaching Laboratory," ASME Proceedings of the 32nd National Heat Transfer Conference, Baltimore, Maryland, August 7-10, in Innovations in Heat Transfer Education, HTD-Vol. 344, M. Bianchi et al., eds., pp. 83-92. Submitted to IEEE Transactions on Education.

Sponsorship

Stanford University School of Engineering
Stanford University Mechanical Engineering Department
Redwood Microsystems
NSF Grant CTS-9624696 (CAREER Award)