An Outlook on Potentialities and Limits in Using Epitaxial Polysilicon for MEMS Real-Time Clocks

Epitaxial polysilicon is a cheap material that has been used in the microelectromechanical system (MEMS) industry for a long time. Its relatively large -30 ppm/K temperature coefficient of frequency (TCf) has prevented its widespread use in real-time clock (RTC) applications. This article investigates the possibility to use this material to build resonators to be used in RTC applications, starting from the assumptions that the TCf of polysilicon is large but very repeatable. This would allow a large manufacturer to use a time-saving, thus inexpensive, family compensation, where the calibration procedure is computed for a very small fraction of the population of produced RTCs and then applied to the entire production output. This article presents a novel design of capacitive resonators for RTCs and demonstrates that the spread of the part-to-part temperature dependence is low enough to allow an electronic family compensation to within ±20 ppm of the output frequency.