Force Sensing by Electrical Contact Resistance in SOI-DRIE MEMS
Rauscher, Scott Gibson
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MEMS force sensors employ microfabricated elements to convert applied external forces to electrical signals, typically by piezoelectric, piezoresistive, or capacitive transduction. While existing force sensors based on these sensing principles have commercial success, system dynamics inherent to displacement and strain-based sensing can limit force and frequency ranges. This work explores an alternative force-sensing principle in silicon-based MEMS devices that exploits changes in electrical contact resistance (ECR) during loading between two silicon surfaces, with the aim to determine if ECR can be used to sense force in SOI-DRIE microsystems containing only Silicon and bond pads. While several analytic models were combined to create an ECR-force model for predicting ECR-force sensitivity in systems containing differing contact geometry, topology, and electrical properties, experimental testing is the focal point of this work. The feasibility of using ECR to sense force in bare DRIE silicon contacts is initially evaluated using force applied by simple thermal actuation, which indicated that ECR behavior during applied cyclic loading was erratic and occasionally nonmonotonic with increasing load, while absolute contact resistance varied significantly chip-to-chip (200 Ω – 15 kΩ) and increased asymptotically as contact was removed. Results from further investigation using manual spring elongation show a consistent pre-load of at least 5 mN is critical to obtaining repeatable ECR-force curves, “break-in” cycling is required prior to consistent ECR-force behavior, and sidewall fracture occurs in 100 µm line contacts with radii less than 50 µm. Results from testing of packaged chips through inertial acceleration of embedded proof masses show that minimizing contact area during line contact loading reduces relative standard deviation (RSD) and increases sidewall fracture. When normalized to initial contact resistance, chips subjected to inertial loading exhibited linearized sensitivities of 2.0 %/mN and 2.1% hysteresis, with 1.6% RSD. The use of DRIE, as opposed to additive poly-Silicon-based fabrication, allows a tailorable force range through proof mass sizing and aspect ratio changes, adjustable pre-load through simple design, and integration of an ECR force sensor into existing systems. The successful use of a proof mass to apply force by acceleration indicates ECR between SOI-DRIE interfaces is a viable method to measure acceleration in the future. As with piezo-sensors, calibration of ECR force sensors is expected to improve chip-to-chip repeatability. Compared to commercially available force sensors, the realized ECR force sensor has several advantages (smaller size, lower force range, and simpler fabrication) that may be further leveraged in future development.