Turnover Temperature Point in Extensional-Mode Highly Doped Silicon Microresonators

This paper demonstrates the existence of a local zero temperature coefficient of frequency (i.e., turnover point) in extensional-mode silicon microresonators, fabricated on highly n-type-doped substrates and aligned to the [100] crystalline orientation. It is shown through both theoretical analysis and finite-element simulation that the turnover point in thin-film piezoelectric-on-silicon (TPoS) resonators is a function of doping concentration and orientation. Moreover, the turnover point can be adjusted by changing the thickness ratio of Si and the piezoelectric film (e.g., AlN) in the resonant structure. In order to experimentally validate this result, similar resonators are fabricated on silicon-on-insulator substrates, and the temperature variation of frequency is measured. An overall temperature-induced frequency variation of less than 245 ppm is measured over the range of -40 °C-85 °C for an ~ 25-MHz TPoS resonator aligned to the [100] plane. This is more than a 15-times reduction with respect to the uncompensated conventional silicon resonators reported before. This work is a significant step toward strengthening silicon's position as an alternative resonator technology in the quartz-dominated stable oscillator market.