Frequency references are the beating heart of all modern electronics. A frequency reference provides the pulse of the computer processor, the radio channel for the mobile phone, and the timekeeping for the GPS navigation unit. However, modern frequency references are made from quartz crystals, not silicon, so they have been unable to benefit from the exponential advances in silicon-based electronic technology of the past three decades. A single quartz part now requires the same area as 1 million silicon transistors! This work describes the design and demonstration of the first stable silicon MEMS resonator suitable for high-precision frequency reference applications, with frequency stability approaching 1 part in 10 million and power consumption of less than 20 mW. At this level of precision the primary obstacle is the temperature sensitivity of the resonator. This performance is achieved with new designs that enable "micro-ovenized" resonators- resonators in a micro-scale insulated enclosure with heating and sensing elements for temperature control. Using wafer-scale silicon encapsulation technology ("epi-seal"), these devices have been fabricated in silicon volumes of less than 0.3 mm3. The resonator temperature is stabilized with closed-loop control using a novel method of temperature sensing based on the resonator quality factor. The frequency stability performance of these CMOS-compatible silicon MEMS resonators is comparable with commercially available quartz devices, while at the same time they are several orders of magnitude smaller and more power efficient.
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