Thick Film Sensors

What Are Thick Film Sensors?

Thick film sensors are sensing devices fabricated by screen printing functional pastes onto ceramic substrates and firing the deposited layers at high temperature to form stable, adherent sensing elements. The technique produces resistive, piezoelectric, or chemically active films that respond to mechanical, thermal, or chemical stimuli by changing their electrical properties in a measurable way. Alumina ceramic is the most common substrate material, providing mechanical robustness, chemical inertness, and thermal stability that allows sensors to operate in demanding industrial and automotive environments where polymer-based sensors would fail.

The approach belongs to the broader field of thick film microelectronics and draws on the same screen printing and firing infrastructure used to fabricate thick film resistor networks and hybrid circuits. Sensing functionality is tailored through paste composition: ruthenium oxide blends yield piezoresistive sensing layers, metal oxide formulations produce gas-sensitive films, and platinum or nickel oxide pastes form resistive temperature detectors.

Piezoresistive Sensing Elements

Piezoresistive thick film sensors measure force and pressure by monitoring the resistance change in a printed ruthenium oxide resistor as it deforms under applied load. The resistors are arranged as a Wheatstone bridge, typically printing four matched elements directly onto the surface of a ceramic diaphragm or beam. When pressure deflects the diaphragm, the bridge arm resistances shift in proportion to the induced strain, producing a differential output voltage. Research on ceramic substrates for piezoresistive thick film sensor applications has examined how substrate strength and surface finish influence long-term sensor reliability and repeatability. Compared with bonded foil strain gauge designs, the directly printed approach eliminates the adhesive layer and its associated creep and temperature sensitivity, improving stability in continuous monitoring applications.

Temperature and Chemical Sensors

Thick film technology is well suited to fabricating resistance temperature detectors (RTDs) by printing platinum paste conductors in a precisely metered serpentine pattern, exploiting the predictable and stable temperature coefficient of resistance of platinum to produce sensors traceable to the ITS-90 temperature scale. Metal oxide thick films, including tin oxide (SnO2), zinc oxide (ZnO), and lanthanum-based perovskites, change resistance by several orders of magnitude in the presence of specific gases such as carbon monoxide, hydrocarbons, or nitrogen dioxide, enabling printed chemical gas sensors on alumina substrates. Ceramic gas sensor technology based on zirconia additionally measures oxygen partial pressure at elevated temperatures by exploiting ionic conductivity, an operating mode used widely in automotive exhaust sensing.

Fabrication and Substrate Technology

The manufacturing sequence for thick film sensors follows the standard thick film process: artwork generation, screen preparation, paste deposition by squeegee action, infrared drying, and belt furnace firing at temperatures from 850 to 1000 degrees Celsius. Low temperature cofired ceramic (LTCC) substrates allow multilayer sensor structures with embedded channels for microfluidic or thermal management functions. For demanding high-temperature applications, zirconia-toughened alumina (ZTA) substrates increase fracture toughness and allow sensors to survive higher mechanical and thermal shock. After firing, laser trimming adjusts individual resistor values to calibration tolerances, and final encapsulation protects the sensing elements in service. WIKA's sensor product documentation illustrates how this fabrication process is scaled to high-volume industrial pressure transducer manufacturing.

Applications

Thick film sensors have applications across a broad range of measurement and control fields, including: