It took the combined experiences of several types of CMOS silicon designers to come up with the smallest high-performance low-cost microelectromechanical-system (MEMS) oscillator on the market.
This feat was accomplished by SiTime, a startup fabless integrated circuit company based in Sunnyvale, Calif. The firm developed a vacuum-sealed MEMS First process that allowed the novel on-chip integration of the MEMS sensing element and temperature-compensation circuitry within industry-standard packages.
The process created a tiny, vibrating mechanical resonator (see the figure). MEMS First promises to revolutionize timing for a wide range of products. Its initial markets will include cell phones, digital cameras, digital TVs, DVD players, and electronic games. Until now, the venerable quartz crystal oscillator dominated these consumer applications.
Clocks lie at the heart of every electronic product. Therefore, being able to produce low-cost integrated MEMS transducers on a standard CMOS process will have a wide effect on the industry. SiTime achieved this low cost by using batch fabrication on an 8-in. (200-mm) CMOS wafer. This wafer format can hold some 50,000 MEMS First resonators, compared with 200 to 300 on an AT-cut (high-frequency) quartz crystal and about 500 on a tuning-fork (32.768-Hz) quartz crystal.
SiTime's initial product families, the SiT11xx fixed-frequency and SiT8002 programmable oscillators, employ 0.18- m design rules. They're based on the proprietary MEMS First process developed at Germany-based Robert Bosch GmBH. Other MEMS sensor technologies pioneered by Bosch include the deep-trench process for surface micromachined accelerometers and gyros, as well as the use of bulk micromachining for air-flow mass sensors and integrated pressure sensors. The MEMS First process achieves a high-Q in-situ vacuum sealing under high-temperature processing.
The SiT8002 is the industry's smallest programmable oscillator, according to the company, with performance that exceeds comparable quartz products. The device is housed in industry-standard plastic packages. It supports cost-effective solutions that compete with and replace other crystal oscillator types. Unlike quartz crystal technology, silicon-based resonators can be integrated easily. As a result, MEMS First oscillators can incorporate many additional time-reference functions on-chip.
Silicon MEMS oscillators have been around for many years, using mechanical vibrating resonators to perform timing. MEMS pioneer and entrepreneur Kurt Petersen, SiTime's chief executive officer and founder, first proposed the use of silicon as a mechanical material back in the 1970s. This later led to many MEMS device developments, including resonating-beam oscillators.1
Yet issues like silicon's 30-ppm/ C temperature coefficient, the demand for complex temperature-compensation circuitry, and the need to use metal or ceramic packages hindered the commercial viability of silicon oscillators. Quartz oscillators also require more expensive metal or ceramic packaging, but they have a natural high stability over temperature when processed properly (see the table).
The MEMS First process, which provides long-term stability, solves the temperature-stability problem in silicon resonators. During its first year of operation (usually the worst year in terms of aging and stability), the MEMS First silicon oscillator achieved long-term stability of 0.05 ppm, limited only by the measurement capability available. That compares with 3 ppm for quartz and 30 to 100 ppm for other silicon MEMS oscillators.
A team led by Bernhard Boser, SiTime's chief scientist and a professor at the University of California at Berkeley, developed the chip's crucial temperature-compensation circuitry. This circuitry consists of the MEMS resonator, an oscillator, a phase-locked loop, digital temperature compensation, an analog-to-digital converter (ADC), and a temperature sensor.
MEMS First resonators have been tested-over 1000 full-scale, industrial temperature-range cycles, from -50°C to 80°C for 6000 hours with 0 ppm of frequency change related to thermal hysteresis. MEMS First also features excellent immunity to shock and vibration compared to quartz crystals.
The key breakthrough in the development of the MEMS First oscillator was its high-temperature sealing system. Above 1100 C, epitaxial silicon is used to encapsulate the device. This innovation came from the minds of Markus Lutz, SiTime's chief operating officer, and Aaron Partridge, SiTime's chief technology officer, while working at Robert Bosch.
"The packaging, encapsulation, and vacuum processing we use make the MEMS First device far more stable than other types of oscillators," says John McDonald, SiTime's vice president of marketing.
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.
- 1. K.E. Petersen, "Silicon as a Mechanical Material," Proceedings of the IEEE, vol. 70, no. 5, pp. 420-457, 1982).