Paul M. Hagelin

A 3 ppm 1.5 × 0.8 mm 2 1.0 µA 32.768 kHz MEMS-Based Oscillator

This paper describes the first 32 kHz low-power MEMS-based oscillator in production. The primary goal is to provide a small form-factor oscillator (1.5 × 0.8 mm 2 ) for use as a crystal replacement in space-constrained mobile devices. The oscillator generates an output frequency of 32.768 kHz and its binary divisors down to 1 Hz. The frequency stability over the industrial temperature range (-40 °C to 85 °C) is ±100 ppm as an oscillator (XO) or ±3 ppm with optional calibration as a temperature compensated oscillator (TCXO). Supply currents are 0.9 μA for the XO and 1.0 μA for the TCXO at supply voltages from 1.4 V to 4.5 V. The MEMS resonator is a capacitively-transduced tuning fork at 524 kHz. The circuitry is fabricated in 180 nm CMOS and includes low power sustaining circuit, fractional-N PLL, temperature sensor, digital control, and low swing driver.

We know that MEMS is replacing quartz. But why? And why now?

MEMS (Microelectromechanical) oscillators were first introduced commercially six years ago. Now MEMS is making significant incursions into what was once the exclusive territory of quartz oscillators. Why is this transition happening now, rather than ten years ago or ten years from now? The transition has been paced by the MEMS community developing the necessary high stability, high frequency, high Q, and high output resonators. Also, MEMS oscillators rely on advanced circuit architectures, particularly leveraging fractional-PLLs and precision temperature sensors. These specialized circuits have only recently become sufficiently small, low noise, power efficient, and accurate.

2-die wafer-level chip scale packaging enables the smallest TCXO for mobile and wearable applications

Real-time clocking for space-constrained mobile and wearable applications require low-power 32.768 kHz references with small form-factor and tight frequency stability, at a competitive price built in an ultra-high volume capable manufacturing process. Legacy 32 kHz quartz-based technology has reached the limits of miniaturization, performance and cost. In this work, a temperature compensated 32 kHz MEMS-based oscillator (TCXO), in a 1.55 mm × 0.85 mm × 0.55 mm form factor, with ±5 ppm frequency stability over -40°C to 85°C, will be presented. The combination of wafer-level chip scale packaging (WL-CSP) and silicon MEMS technology has enabled the smallest and best-in-class 32 kHz clocking solution for very high volume applications. The underlying MEMS system packaging and test technologies will be presented along with the electrical and reliability results.

A MEMS TCXO with Sub-PPM Stability

This paper introduces a MEMS-based TCXO that delivers <1 ppm (parts per million) frequency stability from −40 C to +85 C. Its system architecture, MEMS resonator, and key circuit blocks are described. The oscillator achieves a phase noise of −134dBc/Hz at 1 kHz and −142dBc/Hz at 10 kHz from a 26 MHz carrier, with a far phase noise of −158dBc/Hz. Moreover, its integrated jitter is 0.5 ps from 12 to 20Mhz. The oscillator’s frequency is programmable from 1 to 220 MHz and it draws 32 mA from 1.8 to 3.3 V supply at 26 MHz. The transition from quartz- to MEMS-based oscillators is also discussed, with a review of the oscillator architecture and accompanying benefits, e.g. programmability, improved reliability and robustness, and decreased sensitivity to vibration and EMI.