For quite some time, an accurate oscillator using a silicon resonator remained elusive, as research revealed its potential as an alternative to quartz oscillators without actually proving its worth. Quartz crystal resonators have been the standard in the frequency-control arena because of their piezoelectric and mechanical properties. There have been other contenders, such as ceramic, silicon, and RLC resonators, but none could match the temperature stability, thermal hysteresis, and long-term stability of quartz resonators. But quartz crystal resonators do have some limitations. They cannot suitably be integrated onto silicon complementary metal oxide semiconductor (CMOS) wafers, their cost significantly increases when their package volume decreases, and they are vulnerable to performance degradation when subjected to more severe levels of shock and vibration.
Amid growing demand—in 2007 more than an estimated 10 billion quartz crystals and oscillators were manufactured—the electronics industry has studied microelectromechanical systems (MEMS) for decades with the hope of developing frequency-control products that in some applications could replace quartz crystal oscillators without sacrificing quality or performance.
Ecliptek, a supplier of frequency-control products to the electronics industry since 1987, is just one company to have embraced the fast-growing MEMS resonator timing market, including the EMO product family of programmable oscillators. The oscillators include 12 different product series encompassing four industry-standard package sizes and three supply voltages (1.8-, 2.5-, and 3.3-V dc), providing frequency stabilities of ±50 ppm maximum over an operating temperature range of -40 to +85°C (-40 to +185°C).
The MEMS clock oscillators provided by Ecliptek contain a MEMS resonator, an oscillator stage, frequency-temperature compensation, a low noise phase-locked loop, and a tri-state output buffer stage. A 200-mm (7.9-in) CMOS wafer fabrication process for reduced lot-to-lot MEMS resonator variation, industry standard QFN (Quad Flat No-Lead) packaging, and a COL (Chip On Lead) assembly process improves reliability while reducing assembly costs. This outcome compares well to quartz, which cannot take advantage of these manufacturing techniques and processes due to its mechanical structure.
Reduced inventory carrying costs and time-to-market are the key reasons why the use of MEMS oscillators is on the rise, says Ecliptek. Manufacturers can now offer MEMS oscillators with a shorter lead time than either quartz fixed frequency oscillators or quartz programmable oscillators. By narrowing the delivery window, a company limits its liability from a time and financial perspective.
Some industry insiders say it may be premature to laud MEMS-based oscillators, says Ecliptek. However, others believe the future of MEMS will be sealed as they increasingly get integrated directly with CMOS. Integration will yield benefits in continuous size reduction, better performance, and less manufacturing complexity. The size advantage of MEMS solutions will provide an avenue for further size reductions in wireless nodes, multi-chip modules, and lead to the elimination of quartz from the printed circuit board altogether.