Silicon Carbide Micro-Extensometers for Harsh Environments

This dissertation describes the initial development of a microelectromechanical silicon car-bide (SiC) strain sensor with an integrated flexible ceramicencapsulation. Such sensors are well suited for real-time harsh environment structural health monitoring applications. This work covers three major aspects of the sensor development: 1) design of the sense elements, 2) understanding how the sensor substrate transmits the strain to the sense elements, and 3) design of a flexible thin-film wafer-level ceramic encapsulation. It also explores using the sensor substrate to mechanically reduce the temperature sensitivity of the strain sensor.
Strain sensors of two different designs were fabricated in thin-film, surface-micro-machined polycrystalline 3C-SiC (poly-SiC). The first design is a shock-tolerant variation of the comb-driven double-ended tuning fork resonator. This gauge design improves the fracture-limited shock survivability over the standard comb-driven resonator topology by 25% (to 93,000 g) while still achieving a resolution of 0.11με resolution in a 10 Hz to 20 kHz bandwidth, which is equivalent to the state-of-the-art silicon (Si) resonant strain gauge [1]. It also operates at 32degC in air.
The second design is an offset-gap capacitive strain sensor.This sensor is fabricated from 3.5 μm thick poly-SiC and evaluated using commercial integrated circuit (IC) electronics. This sensor exhibits 2.7 με resolution in a 1 Hz bandwidth. A closed-form analytical solution to the optimal spacing of this offset gap array design is derived and shown to match well with numerical solutions.
A model of a Si substrate bonded to a steel beam by a finite-thickness bond isdeveloped and experimentally verified. This model led to a partially trenched substrate concept which improves strain transfer efficiency to 118%, which is a 23% improvement over the untrenched substrate.
A process for producing ribbed or corrugated membrane wafer-level encapsulationis developed. A small-deflection model of a laminated structurally orthotropic plate as well as preliminary encapsulation fabrication results are presented, including a high-yield recipe for obtaining annealed, crack-free 11 μm thick silicon dioxide coatings on a Si wafer.
The poly-SiC resonant strain sensor is fabricated on a Si substrate, which intro-duces a mismatch in rate of thermal expansion between the sense element and the substrate.This mismatch is shown to reduce the temperature sensitivity to 3.6 ppm/degC over a temperature range of 17C to 65C.
Oliver M. O'Reilly
Andrew Neureuther
Publication date: 
May 31, 2007
Publication type: 
Ph.D. Dissertation
Azevedo, R. G. (2007). Silicon Carbide Micro-extensometers for Harsh Environments. (n.p.): University of California, Berkeley.

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