MEMS thin silicon gauge for strain measurement of structural elements

Abstract

In this thesis, a thin poly-silicon strain gauge is proposed for the measurement of strain in the surface of metal structures. Metal-foil strain gauges are commonly used for such measurements even though poly-silicon strain gauges have better sensitivity. However, the proposed poly-silicon strain gauge can be applied to structural elements because the strain gauge element is separated from the silicon wafer, due to the small size and low thickness of the proposed gauge. To realize the proposed poly-silicon strain gauge, a MEMS fabrication process and a thinning process were established. Then, thin poly-silicon strain gauges were glass-frit bonded onto a metal cantilever beam and their performance was evaluated. It is difficult to bond a silicon strain gauge onto a metal cantilever beam using glass frit because the high-temperature aging process during the glass bonding process causes cracking due to the Coefficient of Thermal Expansion (CTE) mismatch between the silicon gauge and metal cantilever beam. The CTE mismatch problem can be solved by using a poly-silicon strain gauge with thin thickness as well as thin width since it is then more expandable than a thick silicon strain gauge. To determine the most appropriate fabrication conditions, poly-silicon strain gauges having various shapes and various concentrations of boron were fabricated. The performance of these strain gauges before and after heat treatment along with the conventional aluminum annealing process and the glass-frit bonding process was evaluated to investigate effect of the glass-frit bonding process. The gauge factors of the poly-silicon strain gauges that were not heat treated were determined by multiplying the piezoresistance coefficient and resistivity. Further, the piezoresistance coefficients were determined from the concentration of boron implanted into the poly-silicon layers. After heat treatment, the resistivity and the gauge factors of the poly-silicon strain gauges heat treate...