MEMS Resonators with Increased Quality Factor

University of California System: University of California, Berkeley
posted on 02/01/2012

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On-chip capacitively transduced vibrating polysilicon micromechanical resonators have achieved quality factor Q’s over 160,000 at 61 MHz and larger than 14,000 at about 1.5 GHz — making them suitable for on-chip frequency selecting and setting elements for filters and oscillators in wireless communication applications. However, there are applications — such as software-defined cognitive radio, that require even higher Q’s at RF to enable low-loss selection of single channels (instead of bands) to reduce power consumption down to levels conducive to battery-powered handheld devices.

To address those higher Q RF applications, researchers at UC Berkeley have invented design improvements to MEMS resonators that reduce energy loss and in turn increase resonator Q. In reducing energy loss to the substrate while supporting all-polysilicon UHF MEMS disk resonators, the Berkeley design improvements enable quality factors as high as 56,061 at 329 MHz and 93,231 at 178 MHz — that are values in the same range as previous disk resonators using multiple materials with more complex fabrication processes. Measurements confirm Q improvements of 2.6X for contour modes at 154 MHz, and 2.9X for wine glass modes around 112 MHz over values achieved by all-polysilicon resonators with identical dimensions. The results not only demonstrate an effective Q-enhancement method with minimal increase in fabrication complexity, but also provide insights into energy loss mechanisms that have been largely responsible for limiting Q’s attainable by all-polysilicon capacitively transduced MEMS resonators.

• Filters for RF channel-selecting that can enable practical software-defined cognitive radio;
• Ultra-low noise oscillators for timing, radar, navigation, and communication — where higher Q correlates with lower noise and therefore wider application range;
• Gyroscopes, accelerometers, and inertial measurement devices — that use resonating elements such as MEMS-based gyroscopes in automobiles, mobile handsets and gaming wands;
• Sensors that use resonating elements (see aforementioned inertial measurement devices), and also sensors for gas, temperature, pressure, motion, and stress, etc;
• High Q tank circuits used in communications as well as other integrated circuits.