This paper, the first of two parts, reports the design and fabrication of a fully integrated oven controlled microelectromechanical oscillator (OCMO). This paper begins by describing the limits on oscillator frequency stability imposed by the thermal drift and electronic properties (Q, resistance) of both the resonant tank circuit and feedback electronics required to form an electronic oscillator. An OCMO is presented that takes advantage of high thermal isolation and monolithic integration of both micromechanical resonators and electronic circuitry to thermally stabilize or ovenize all the components that comprise an oscillator. This was achieved by developing a processing technique where both silicon-on-insulator complementary metal-oxide-semiconductor (CMOS) circuitry and piezoelectric aluminum nitride, AlN, micromechanical resonators are placed on a suspended platform within a standard CMOS integrated circuit. Operation at microscale sizes achieves high thermal resistances (∼10 °C/mW), and hence thermal stabilization of the oscillators at very low-power levels when compared with the state-of-the-art ovenized crystal oscillators, OCXO. A constant resistance feedback circuit is presented that incorporates on platform resistive heaters and temperature sensors to both measure and stabilize the platform temperature. The limits on temperature stability of the OCMO platform and oscillator frequency imposed by the gain of the constant resistance feedback loop, placement of the heater and temperature sensing resistors, as well as platform radiative and convective heat losses are investigated. [2015-0035].
This paper, the second of two parts, reports the measurement and characterization of a fully integrated oven controlled microelectromechanical oscillator (OCMO). The OCMO takes advantage of high thermal isolation and monolithic integration of both aluminum nitride (AlN) micromechanical resonators and electronic circuitry to thermally stabilize or ovenize all the components that comprise an oscillator. Operation at microscale sizes allows implementation of high thermal resistance platform supports that enable thermal stabilization at very low-power levels when compared with the state-of-the-art oven controlled crystal oscillators. A prototype OCMO has been demonstrated with a measured temperature stability of -1.2 ppb/°C, over the commercial temperature range while using tens of milliwatts of supply power and with a volume of 2.3 mm3 (not including the printed circuit board-based thermal control loop). In addition, due to its small thermal time constant, the thermal compensation loop can maintain stability during fast thermal transients (>10 °C/min). This new technology has resulted in a new paradigm in terms of power, size, and warm up time for high thermal stability oscillators. [2015-0036].
We have developed a MEMS based thin (<100 μm), temperature stable (< 1 parts-per-billion per degree Celsius (ppb/°C)), low power (<10 mW), frequency reference. Traditional high stability oscillators are based on quartz crystals. While a mature technology, the large size of quartz crystals presents important mission barriers including reducing oscillator thickness below 400 μm, and low power temperature stabilization (ovenizing). The small volume microresonators are 2 μm thick compared to 100’s of microns for quartz, and provide acoustic/thermal isolation when suspended above the substrate by narrow beams. This isolation enables a new paradigm for ovenizing oscillators at revolutionary low power levels <10 mW as compared to >300 mW for oven controlled quartz oscillators (OCXO). The oven controlled MEMS oscillator (OCMO) takes advantage of high thermal isolation and CMOS integration to ovenize the entire oscillator (AlN resonator and CMOS) on a suspended platform. This enables orders of magnitude reductions in size and power as compared with today's OCXO technology.
Recently reported narrow bandwidth, <;2%, aluminum nitride microresonator filters in the 100-500 MHz range offer lower insertion loss, 100x smaller size, and elimination of large external matching networks, when compared to similar surface acoustic wave filters. While the initial results are promising, many microresonators exhibit spurious responses both close and far from the pass band which degrade the out of band rejection and prevent the synthesis of useful filters. This paper identifies the origins of several unwanted modes in overtone width extensional aluminum nitride microresonators and presents techniques for mitigating the spurious responses.
An AlN MEMS resonator technology has been developed, enabling massively parallel filter arrays on a single chip. Low-loss filter banks covering the 10 MHz--10-GHz frequency range have been demonstrated, as has monolithic integration with inductors and CMOS circuitry. The high level of integration enables miniature multi-bandm spectrally aware, and cognitive radios.
This work presents a new type of MEMS resonator based on launching an acoustic wave around a ring. Its maximum frequency is set by electrode spacing and can therefore provide a means for developing resonators with center frequencies in the GHz. In addition since the center frequency is dependent on the average radius it is not subject to lithographic process variations in ring width. We have demonstrated several Ring Waveguide (RWG) Resonators with center frequencies at 484 MHz and 1 GHz. In addition we have demonstrated a 4th order filter based on a RWG design.