Mini CMOS MEMS Oscillator Marks A New Era For Timing

PERFECTING THE SACRIFICIAL PROCESS
SiTime uses deep-trench etching of silicon formed with a photolithographic process to create a very narrow electrode-to-beam spacing of just 0.4 µm. This feature combined with a controlled-release etch generates the high performance. Other CMOS MEMS processes are limited and constrained by design rules for post metal processing, which become problematic in terms of oscillator performance.

The MEMS First process begins on a silicon-on-insulator (SOI) wafer, on which deep reactive ion etching (DRIE) creates 10-µm deep and 0.4-µm narrow trenches that form the beam. After the release step, the beam is free to vibrate horizontally to the wafer's surface. Next, the epitaxial layer is grown to encapsulate the mechanical structure in an ultraclean environment. Then the resonator beam is released using hydrogen fluoride (HF) and annealed, and a final thick polysilicon cap is grown. Lastly, contacts to the resonator beam and top aluminum contacts are created, and aluminum is deposited as the interconnect to the CMOS drive circuitry.

Unlike quartz crystals and oscillators, SiTime's oscillators are built on 200-mm standard silicon-on-insulator (SOI) CMOS-compatible wafers and come in standard IC QFN-type (quad flat nolead) packages. This improves yields and reliability while lowering costs. When taken together with the inherent electrostatic-discharge, electromagnetic compatibility, vibration, and shock immunity offered by MEMS resonators, designers can improve virtually every product and lower manufacturing cost.

According to the company, its oscillators are priced competitively with small surface-mount device quartz crystals. As such, they require no load capacitors, they're easier to design in, they have no interference problems, and startup is guaranteed. Customers who desire ultrafast service can choose the SiT8002, which is a programmable version of the SiT11xx family that's available for prototypes and quick-turn production. Its performance is similar to other SiT11xx family members.

Package sizes for the SiT11xx family and SiT8002 measure 2.0 by 2.5 mm, 2.5 by 3.2 mm, and 3.2 by 5.0 mm—all having a total thickness of 0.85 mm. The SiT11xx, which includes the 32.768-Hz SiT 1564 version, costs less than $0.49 each in lots of 1 million units/month. The SiT8002 goes for less than $0.69 each in lots of 1 million units/month. SiTime says the product's cost will drop even further once it can integrate even more functions on the same silicon chip.

SiTime Inc.
www.sitime.com

Reference:

1. 1. K.E. Petersen, "Silicon as a Mechanical Material," Proceedings of the IEEE, vol. 70, no. 5, pp. 420-457, 1982).