A 32.768 kHz MEMS Resonator with +/-20 ppm Tolerance in 0.9 mm x 0.6 mm Chip Scale Package

Abstract

A MEMS-based 32 kHz resonator is designed, simulated, and manufactured. The chip scale package (CSP) dimensions are 0.9 mm x 0.6 mm which is more than a 50% reduction in comparison to the smallest quartz crystal tuning fork resonator available today. The load capacitances required for the oscillation circuit are integrated within the CSP, which offers the smallest size of any 32 kHz solution. Despite the size reduction, the MEMS resonator is electronically equivalent to the quartz crystal tuning fork resonator. The resonator equivalent series resistance is less than $70 \mathrm{k}\Omega$. To meet the stringent timing accuracy requirements without electronic compensation, the resonators are trimmed to +/-20 ppm accuracy. As no active circuit based compensation is required, the presented resonator offers power and oscillator circuit area savings in comparison to prior MEMS oscillators that require phase locked loop (PLL) compensation. The functionality of the MEMS resonator is demonstrated by using the oscillator functionality in a common microcontroller ASIC.