Experimental Characterization of the Influence of Transverse Prestrain on the Piezoresistive Coefficients of Heavily Doped n-Type Silicon

Strain has been integrated into many silicon devices, as it has an essential impact on carrier mobility and crystal symmetry. Those parameters respond differently under both biaxial and uniaxial, so their effect needs to be quantified to successfully employ the strain engineering in different silicon applications. As an extended step toward utilizing strain in enhancing the sensitivity and the temperature independency of a 3-D stress sensor, the effect of uniaxial transverse strain onto the piezoresistive (PR) coefficients of heavily doped n-type silicon will be experimentally characterized. Anew design was developed to apply the transverse tensile and compressive uniaxial stresses onto the silicon substrate using a highly compressive nitride layer. This stressing technique was integrated with six PR elements rosette to fully calibrate the PR coefficients, where unstrained, tensile, and compressive strained PRs are fabricated within the same chip to accurately quantify the strain impact. Four-point bending, stress-free temperature, and hydrostatic test were used to typically measure the PR coefficients. Strain values of 0.065% and 0.083% ε were achieved locally using both the tensile and compressive stressors, respectively. Under this level of strain, the typical result shows opposite impact for both the tensile and compressive transverse strains on the longitudinal and transverse PR coefficients. Moreover, an increase up to 80% can be achieved for the pressure coefficient of heavily doped n-type silicon due to the compressive transverse strain.