Resonators for real-time clocks based on epitaxial polysilicon process: A feasibility study on system-level compensation of temperature drifts

Abstract

A statistical campaign on the frequency temperature coefficient (TCf) of resonators fabricated in a standard epitaxial polysilicon MEMS process, currently used for inertial sensors, is presented. The frequency variation is dominated by the elastic modulus temperature coefficient (TCE) and is not compensated at process level. The campaign examines four real-time clock (RTC) resonators, differing in geometry, resonance frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) and quality factor (Q). An accurate characterization on about 40 samples highlights the possibility to obtain oscillators with frequency stability within ±50 ppm and ±20 ppm with purely electronic compensation to 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> - and 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> -order respectively, using family compensation and thus with no need for part-to-part TCf characterization.