MEMS oscillators: Better alternative to crystal oscillators?

July 16, 2012 — Since the early 19th century, quartz crystals have been the core frequency reference for oscillators. However, due to the manufacturing complexity and reliability challenges posed by crystal oscillators, a great deal of work is going on in the industry and academia to find alternate frequency reference solutions. To overcome the known technical challenges for quartz crystals, including limitations on higher native frequencies, activity dips, aging, vibration sensitivity, etc., IDT recently introduced piezoelectric micro electro mechanical system (pMEMS) resonator-based oscillators. pMEMS resonators have a higher native frequency (~100 MHz) than quartz crystals and enable better-performing MEMS oscillators (sub-ps jitter), especially for high-performance communications, consumer, cloud, and industrial applications.

As part of higher reliability over quartz, MEMS oscillators demonstrate semiconductor-grade shock and vibration resistance. Standard quartz devices are fragile, since the crystal is housed within a metal or a ceramic package, allowing the crystal to be fractured by a shock of 50-100g. Manufacturers have to implement specific storage, packing, and shipping protocol for crystal devices to avoid damage. MEMS oscillators, on the other hand, sport a 50,000g shock resistance without special construction, packaging, and transportation techniques.

Figure 1. Wafer level packaged pMEMS die stacked on an IC Die in a plastic QFN package.
Figure 2. A typical quartz oscillator in a ceramic package.

Since the MEMS resonators are wafer-level packaged, these oscillators can use low-cost plastic packages, which provide an economical yet reliable timing component.

Another known issue for crystal oscillators — activity dips — can cause intermittent failures. These failures affect both the frequency and the resistance (i.e., the Q) of crystal resonators. Activity dips are usually caused by interfering modes (e.g., by high overtone flexure modes) and are strongly influenced by the crystal’s drive level and load reactance. These activity dips are not present with MEMS oscillators since the resonators are designed to suppress undesired modes over these temperature and process variations that can impair crystal-based oscillators.

pMEMS oscillators have also demonstrated aging comparable with crystal oscillators at room temperature (25°C) and significantly better aging than crystal oscillators under burn-in conditions (125°C).

Other advantages of MEMS-based products include natural compatibility with surface-mount assembly processes and short lead times; this enables suppliers and users (electronic manufacturers) to maintain a smaller device inventory with reduced risk of supply shortages.

IDT’s MEMS oscillators support low-voltage differential signaling (LVDS) and low-voltage positive emitter-coupled logic (LVPECL) outputs at frequencies of up to 625MHz, which is required in most communications, networking and high-performance computing applications.

Conclusion

There have been a lot of improvements in MEMS oscillators over the years, and the recent upgraded products, like IDT’s 4M MEMS oscillators, are ready to provide the performance and accuracy required in the high-performance communications, consumer, cloud computing, and industrial applications. In the foreseeable future, researchers, designers, and manufacturers will continue to work together to enhance the MEMS oscillators to deliver more accurate, cost-effective, and higher performance frequency reference products.

Harmeet Bhugra, managing director, Integrated Device Technology Inc., is responsible for the vision, growth and general management of the MEMS business. Bhugra holds a Bachelor of Engineering degree from University of Victoria, Canada, Masters in Systems Engineering and MBA degrees (Magna Cum Laude) from San Jose State University and Managing Technical Organization certifications from MIT.

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