The thermal applications of electrically-heated polycrystalline-silicon microbridges are explored. An electrothermal model based on the balance of heat dissipation and heat losses is developed which determines the microbridge electrical characteristics. The model has been adapted for inclusion into the SPICE3 circuit simulator. Complementary to the model, all relevant thermal properties of the microbridge mechanical materials have been studied and measured using newly developed methods. Various microbridge electrical biasing schemes have been considered to determine the most desirable circuit interface.
The microbridge heat losses, hence its electrical characteristics, are dependent on its environment. Thus this device is useful as a transducer. We constructed and tested a gas flow sensor with a microbridge sensor 270 X 3 X 1 um3 suspended 3 um off the substrate and a constant-temperature bias circuit. The microbridge was enclosed in a silicon microfabricated package assembly with a 300 um-high channel that constrained the flow. The circuit yielded a 35 mV signal for a nitrogen flow of 100 sccm at a microbridge temperature of 200C.
A thermal absolute pressure sensor of the heated microbridge type has been integrated with an active constant-resistance bias circuit and an 8-bit successive approximation register A/D converter. The chip, which contains more than 1000 MOSFETs, is sensitive to variations in absolute gas pressure between 10 and l0^4 Pa, and it is implemented in a 14-mask, 4-um NMOS technology merged with the microsensor process. The average sensitivity of the circuit was 2 V/decade for a 400 x 3 x 1 um^3 microbridge operating at an average temperature of 100C above the substrate. Silicon-nitride-coated microbridges were used which reduce oxidation induced signal drifts. This circuit demonstrates the integration of microbridges with MOS circuits to form monolithic measurement systems.
As a last thermal application, a silicon-filament vacuum-sealed incandescent light source has been fabricated using IC technology and subsurface micromachining. The incandescent source consists of a heavily-doped p+ polysilicon filament coated with silicon nitride and enclosed in a vacuum-sealed (= 80 mT) cavity in the silicon chip surface. The filament is formed beneath the surface and later released using sacrificial etching resulting in a microstructure that is protected from the external environment. The filament is electrically heated to reach incandescence at a temperature near 1400 K. The power required to achieve this temperature for a filament 510 x 5 x 1 um^3 is 5 mW yielding a total optical power of 250 uW with a peak distribution wavelength near 2.5 um. The energy-conversion efficiency is 5 %.
December 31, 1990
Mastrangelo, C. H. (1991). Thermal Applications of Microbridges. United States: University of California, Berkeley.