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Determination of Doping and Temperature Dependent Elastic Constants of Degenerately Doped Silicon from MEMS Resonators

Elastic constants c11, c12 and c44 of degenerately doped silicon are studied experimentally as a function of the doping level and temperature. First and second order temperature coefficients of the elastic constants are extracted from measured resonance frequencies of a set of MEMS resonators fabricated on seven different wafers doped with phosphorus (carrier concentrations 4.1, 4.7, and 7.5*10^19 cm^-3), arsenic (1.7, 2.5*10^19 cm^-3), and boron (0.6, 3*10^19 cm^-3), respectively. Measurements cover a temperature range from -40 to +85 C. It is found that that the linear temperature coefficient of the shear elastic parameter c11-c12 is zero at n-type doping level of n~2*10^19 cm^-3, and that it increases to over 40 ppm/K with increasing doping. This observation implies that the frequency of many types of resonance modes, including extensional bulk modes and flexural modes, can be temperature compensated to first order. The second order temperature coefficient of c11-c12 is found to decrease by 40% in magnitude when n-type doping is increased from 4.1 to 7.5*10^19 cm^-3. Results of this study enable calculation of the frequency drift of an arbitrary silicon resonator design with an accuracy of +/- 25 ppm over T=-40 ... 85 C at the doping levels covered in this work. Absolute frequency can be estimated with an accuracy of +/- 1000 ppm.