SOI MEMS Resonators for Timing Application
Posted date : May 27, 2009

SiTime has developed a range of ultra-slim, SOI-MEMS based timing devices aimed at replacing bulky crystal quartz products in high-volume portable electronics.

By using an SOI-MEMS solution, SiTime’s products are a fraction of the size of existing quartz crystal based timing products. They are also 10 times better in terms of reliability and shock resistance, while consuming a third of the power. (Courtesy: SiTime)

The timing market can be divided into three segments: passive quartz resonators, active oscillators and timing devices. The annual volume exceeds 15B products sold per year with annual revenue of greater than $5B.

SiTime uses ultra small SOI-MEMS resonators to replace classic quartz crystal based products. Target markets include high-volume portable electronics, such as high capacity SIM cards, smart cards, mobile phone system in package (SIP) modules, digital cameras, LCD TVs, media players, gaming solutions, hard disk drives and USB peripherals.

The advantages of SiTime’s MEMS resonators over quartz are in size, robustness and manufacturability. The large volumes require scalability, reproducibility and reliability from die to die, wafer to wafer, and lot to lot.

SOI’s mechanical advantages

MEMS resonators’ mechanical performance depends strongly on two factors: material properties of the mechanical resonator and cleanliness of the vacuum packaging.

The material requirement is solved by choosing bonded SOI wafers. The mechanical resonators are formed in the upper silicon layer. The SOI layer is made from single crystalline bulk wafers, which guaranties defect free material with well controlled doping levels for conductivity.

From a mechanical perspective the single crystal silicon is perfect: the material properties are the same from lot to lot, and the only impurities are the dopants which account for less than 1ppm of the atoms and therefore do not affect the mechanical parameters.

A high temperature wafer level encapsulation process provides the ultra clean vacuum.

Fig.1 Frequency stability

Performance and yield

The performance enabled by SOI wafer material leads to high manufacturing yield, zero drift, and a performance similar to or better than quartz.

Figure 1 shows the temperature stability of the high-performance, programmable oscillator SiT8102, which is significantly better than the required specification of ±25ppm over the temperature range of -40°C to +85°C.

Standard packaging

SiTime has introduced 11 product families all based on this resonator technology. The resulting wafer level packaged MEMS resonators can be packaged with any standard chip packaging technology such as plastic molding, flip chip, and SIP.

SiTime’s MEMS First™ Fabrication Technology

Fabrication cross section

SEM picture of a cleaved resonator

Using a resonator as an example:
A 10µm SOI substrate is patterned with a Bosch deep reactive ion etch (DRIE) into a resonant structure.
An oxide is deposited and patterned to cover selected parts of the resonator.
A thin epi silicon layer is deposited and patterned with vents.
The oxide over and under the structures is removed through the vents.
The resonator chamber is sealed in an epi environment with SiTime’s EpiSeal process, giving an extremely clean enclosure.
6. The wafer surface is patterned with contact isolation trenches.
7. Insulation oxide, metal interconnections, and a scratch mask are deposited and patterned.

A co-founder of SiTime (a spin-off of the Bosch Research and Technology Center in Palo Alto, CA), Markus Lutz was the initial inventor of the company’s MEMS-First™ integration technology. He started his career at Robert Bosch GmbH, where he invented the world’s first mass-produced silicon-based MEMS gyroscope.

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