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Concept and Manufacturing of a Pressure Sensor Based on a LTCC Thin Film Strain Gauge
Keywords: thin film strain gauge, pressure-assisted sintering, silver sintering
Amorphous carbon thin films that contain nickel nanoparticels (Ni:a-C:H) show a significant change of electrical resistivity under mechanical load. These thin films are promising resistors for highly sensitive strain and force sensors. In the paper, the concept and manufacturing of a pressure sensor with Ni:a-C:H thin film resistor is presented. The pressure sensor consists of a metal diaphragm with a central boss that pushes on a low temperature co-fired ceramic (LTCC) strain gauge with Ni:a-C:H coating. The LTCC for the strain gauge substrate, a composite of lead free glass and 35 wt% quartz, is designed to have a low modulus of elasticity of 82 GPa, and a high coefficient of thermal expansion of 9.4 ppm/K. Substrates are prepared by multilayer technology, and sintered under axial pressure of 1 MPa. A thin film capable surface quality of the fired substrate is achieved by the use of a glassy carbon setter instead of alumina release tape in the sintering process. The Ni:a-C:H resistor layer is deposited on the substrate by sputtering of a Ni target during the PECVD of the amorphous carbon. The resistor layer is structured by lift-off technique with a metal sacrificial layer. To protect the Ni:a-C:H resistor against environmental influences, and to provide good electrical contact and solderability, additional layers are deposited. After singulation, the LTCC strain gauges are joined to the sensor base body using silver sintering paste and a low temperature pressure-assisted sintering process at 280 °C and 20 MPa. Output signals of first generation sensors at nominal load of 25 MPa exceed 4.0 mV/V. Higher signal levels are possible with increased resistor layer gauge factor. The linearity error amounts to 0.44 % full scale. Compared to sensors joined with tin-gold solders, the overload capacity of the silver sintered sensors is thrice as high.
Bjoern Brandt,
BAM Federal Institute for Materials Research and Testing
Berlin, Berlin

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