Achieving < ±25 ppb Frequency Stability With a ±0.125 °C Oven Control on a Si Interposer for an AlScN-on-Si Shear-BAW Resonator

A major challenge of long-term clock stability is frequency drift due to temperature variations. This paper describes the design of a proportional, integral, derivative (PID) control system for external ovenization of an AlScN-on-Si Shear-BAW Resonator (S³R), which has a fixed turnover temperature where the $1{^{\text {st}}}$ order temperature coefficient of frequency is $\approx 0$ ppm/°C. The control system provides $\pm ~0.125^{\circ }$ C temperature stability and assists in achieving better than $\pm ~25$ ppb frequency stability over a temperature range of 15-40°C by maintaining resonator operation near the turnover temperature, where the $2{^{\text {nd}}}$ order temperature coefficient of frequency drift is -62.71ppb/°C². The robust and adaptive PID algorithm (programmed on an external microcontroller unit connected to the interposer) ensures continuous ovenization by configuring the duty cycle of a compact heat actuator (powerMOS) that is placed in <3.5mm proximity of the 2mm $\times 2$ mm resonator and a complementary to absolute temperature sensor (implemented as a 1mm $\times 1$ mm, 65nm integrated circuit), that are all held on a thermally conductive 7mm $\times$ 7mm Si interposer.