A Temperature-Stable and Low Impedance Piezoelectric MEMS Resonator for Drop-in Replacement of Quartz Crystals

Abstract

In this letter, we report a highly temperature-stable and low impedance scandium doped aluminum nitride (Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> AI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N, x=9.5%) on silicon piezoelectric microelectromechanical system (MEMS) resonator. Passive temperature compensation techniques, including heavy doping and composite structure using oxide layers, are employed to minimize the frequency drifts over temperature of the piezoelectric MEMS resonator. The turnover point is adjusted by changing the in-plane crystal orientation of the (100) doped device silicon wafer. A frequency variation of ±21.5 ppm over the industry temperature range from -40 °C to 85 °C is achieved when the crystal orientation of the device silicon wafer is arranged at around 22.5° from the <;110> direction. The measured loaded quality factor ( Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</sub> ) is 10017 with a motional impedance of 28 Ω at its series resonant frequency of 24.44 MHz. The deduced unloaded quality factor ( Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">un</sub> ) is above 45000. Thanks to the excellent temperature stability and low impedance, the reported piezoelectric MEMS resonator shows great potential to directly replace the classical quartz crystal resonators without any active temperature compensation.