Highly Reliable MEMS Temperature Sensors for 275 $^{ \circ}\hbox{C}$ Applications—Part 1: Design and Technology

Abstract

This paper presents the design, fabrication, packaging, and experimental characterization of the first temperature-sensitive MEMS capacitors for health monitoring applications up to 275 °C. The capacitive sensor chip is 2 × 2 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> fabricated on a 500-μm -thick Si wafer and comprises an array of 220 individual 10 × 250 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> inherently robust bimorphs. This first part focuses on the fabrication and packaging, thermomechanical design, electromagnetic modeling, and technology of the unpackaged and packaged bimorph beams. On the thermomechanical part, the measured profiles match theoretically predicted profiles from 20 °C to 250 °C within 4% and 3% at the beam midpoints and tips, respectively. Similarly, experimentally obtained capacitance changes for five separate packaged devices show a mean error of less than 3% and a maximum error of less than 5% from the theoretical model up to 165 °C. The packaged sensors simultaneously achieve for the first time: 1) high temperature operation; 2) monotonic capacitance-temperature response from 20 °C to 300 °C; 3) hermetic sealing; 4) miniature size of 5.4 × 5.4 × 3.6 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ; and 5) capacitance quality factor of over 5000 at 20 °C and 1000 at 160 °C. Experimental results underline the tradeoffs between beam arrangement, attachment method to the package, and package to the quality factor of the device.