Sensors and Signal Conditioning, 2nd Edition

1.9   MICROSENSOR TECHNOLOGY

Microsensor materials are prepared according to their nature, the desired sensing principle, and the intended application. There is an increasing interest in applying integrated circuit (IC) technology and micromachining, because they yield small, reliable sensors produced in large amounts leading to low cost.

1.9.1   Thick-Film Technology

Thick-film technology uses pastes or “inks” with fine particles (5 μm in average diameter) of common or noble metals dispersed in an organic vehicle, along with a glass frit that binds them. Depending on the dispersed particles, the paste can be conductive, resistive, or dielectric. Those pastes are screen-printed on a substrate according to a predefined pattern [19] involving width lines from 10 μm to 200 μm. The printed film is dried by heating at about 150 °C to remove the organic solvent that provided the low viscosity needed for the paste to squeeze through the open areas in the screen. The substrate with the deposited film is then fired on a conveyor belt furnace, usually in air atmosphere, so that the metal powder sinters and the glass frit melts, thereby bonding the film to the substrate. The printing, drying, and firing sequence is repeated for each paste used according to predetermined thermal cycles. The result is a 10 μm to 25 μm thick film, impermeable to many substances but relatively porous for specific chemical or biological agents. Thick-film components have a printed tolerance from ±10% to ±20%, ...