MEMS resonator and oscillator circuitry integrated onto single IC

June 27, 2013 // By EDN
Silicon Labs, using MEMS technology it acquired when it bought Silicon Clocks, has introduced a wide-frequency-range oscillator that uses a MEMS resonator fabricated on the same die as the oscillator and frequency synthesiser.

The company's Si50x CMEMS oscillators represent a “leap ahead of quartz-based timing devices with superior frequency stability, reliability and programmability” Silabs asserts. Designed to replace general-purpose crystal oscillators (XOs) in cost-sensitive, low-power and high-volume industrial, embedded and consumer electronics applications the oscillators are based on Silicon Labs’ patented CMEMS technology – the first to enable MEMS structures to be built directly on top of standard CMOS wafers in high-volume fabs, resulting in fully integrated, highly reliable monolithic “CMOS+MEMS” IC solutions. The company first fabricates the oscillator and frequency synthesiser circuitry, then planarises the wafer, deposits silicon on top, and fabricates the MEMS section in that silicon.

A single MEMS resonator reference frequency is used – 25 MHz – and the mode of resonance is in the plane of the silicon die – in this device, MEMS flexing is in the X/Y directions only, not along the Z axis. Temperature stability is enhanced by combining silicon and SiO2 patterns in the MEMS element; these material have opposing responses to changes in temperature and increase the inherent stability of the MEMS resonator's frequency by passive compensation.

The oscillator family achieves smaller size, higher reliability, better aging and higher integration than existing frequency control solutions; Silabs adds that;

• CMEMS oscillators are manufactured in a high-volume CMOS fab with unified foundry lines that support wafer probing of complete oscillator systems for state-of-the-art quality and process control.

• CMEMS technology enables guaranteed data sheet performance with 10 years of frequency stability including solder shift, load pulling, VDD variation, operating temperature range, vibration and shock. This guaranteed operating life performance is 10 times longer than typically offered by comparable crystal and MEMS oscillators.

• CMEMS oscillators tightly couple the MEMS resonator with CMOS temperature sensor and compensation circuitry, ensuring a highly stable frequency output in the face of thermal transients and over the full industrial temperature range. The end result is a predictable, reliable frequency