An Investigation of Using n-Si Piezoresistive Behavior to Develop a Three-Dimensional Stress Sensing Rosette

This work involves the design, microfabrication, calibration, and testing of a new silicon-based piezoresistive rosette capable of extracting the three-dimensional (3D) stresses in the silicon upon deformation. A new 10-element piezoresistive rosette was devised to extract all stress components with temperature compensation compared to the 8-element rosette developed by previous researchers, which delivers partially temperature-compensated stress output. The proposed rosette is made up of either dual- or single-polarity sensing elements through utilizing the unique behavior of the shear piezoresistive coefficient (pi44) in n-Si with impurity concentration. An analytical study was conducted to investigate the feasibility of the new approach. The analysis is based on solving the determinants of the coefficients of the matrices describing the resistance change versus stress and temperature for the sensing elements. The calculated determinants over a range of impurity concentrations showed non-zero regions, thus indicating the feasibility of the approach.;On the other hand, the second prototype, named test chip, using ion implantation doping was used for testing of the rosette after conducting a full calibration process involving uni-axial, thermal, and hydrostatic loads. The testing of the test chip was conducted by applying a four-point bending of a chip-on-beam specimen at room temperature. Three chip orientations and three rosette-sites were used to induce five stress components in a controlled manner, while the out-of-plane normal stress was not tested independently due to its low sensitivity in the current microfabrication run. A finite element model (FEM) of the chip-on-beam loading was developed to compare to the stress output from the experimental testing. The five stress components were extracted from the three rosette-sites and showed good correlation with the FEM.;A full experimental study including the microfabrication, calibration and testing of the 10-element single-polarity rosette was conducted to demonstrate the actual behavior of the rosette in extraction of the 3D stresses. An early prototype, named POC chip, using diffusion doping was used to calibrate the piezoresistive coefficients and temperature coefficient of resistance and calculate the determinants to support the analytical study. The calibration process involved applying uni-axial and thermal loads on the sensing elements. The resulting determinants from the calibration process indicated non-zero values, thus verified the experimental feasibility of the approach.