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Silicon micromachining technology can be used to build micromechanical sensors and actuators by using tools derived from standard IC processing. Processes developed for micromachining can also be used to add functions to integrated circuits. One such application is for the thermal isolation of electrical devices. A method was developed to thermally isolate regions of single crystal silicon on an unmodified IC process. Using an anisotropic silicon etching step, portions of circuits are undercut, and suspended over cavities in the substrate by dielectric support beams. Transistors, resistors, and diodes with thermal resistances of tens of thousand degrees per Watt can be fabricated. This enables a wide variety of thermally based measurement systems. The development of the fabrication methods is described, and three applications of thermally based instrumentation systems are presented.

The first is a thermal AC to RMS converter. The system uses the thermal domain to measure the root-mean-square value of AC signals. It has a dynamic range comparable to computational RMS converters, with much higher bandwidth. Control circuitry was integrated with the micromachined thermoelements. The second system is a thermal conductivity vacuum sensor. The sensor measures gas pressure by detecting ambient thermal conductivity changes, and has a sensitive range of over five decades of pressure. The third application is low-power temperature regulation of a bandgap voltage reference. Due to the high thermal isolation of the suspended silicon, very little dissipated power is needed to maintain the circuit at an elevated temperature. By regulating the operating temperature of critical devices in a voltage reference, output voltage drift was improved by more than a factor of 40.

Klaassen, E. H., "Micromachined Instrumentation Systems," Ph.D. Thesis, Stanford University, CA, 1996.