An Investigation of Thick-film Materials for Temperature and Pressure Sensors on Self-constrained LTCC Substrates

For temperature sensors in MEMS (the ceramic micro electro-mechanical systems) thick-film temperature-dependent resistors PTC-5093 (Du Pont) and NTC-4993 (Ferro) resistor materials were evaluated. Thick-film piezo-resistors for strain-gauges were realised with Du Pont 2041 (low noise and very stable resistors) and ESL 341-B (high gauge factor) 10 kohm/sq. resistor materials. The PZT paste was prepared from a pre-reacted PZT powder with the composition near the morphotropic phase boundary (Zr/Ti 53/47). Thick-film PTC and NTC thermistors, "normal" low TCR resistors and PTZ piezoelectric pastes were printed and fired on Heraeus HL-2000 LTCC zero shrink substrates. Results obtained on relatively inert alumina substrates were used for the comparison. In most cases the required characteristics of thick film materials on LTCC substrates were inferior to those obtained on relatively inert alumina substrates. This is attributed to the inter-diffusion of oxides, mainly SiO2, from the rather glassy LTCC substrates into active layers and presumably also to the diffusion of some from oxides from thick fil layers into LTCC substrates.