Researchers at NXP Semiconductors have described what they say is a demonstrable, scalable piezoresistive MEMS resonator built in silicon and operating at a record 1.1 GHz.
The team devised a novel transduction scheme in which an electrostatic force excites the silicon resonator and the mechanical motion is detected using the piezoresistive properties of silicon, the group said in a presentation at the International Electron Devices Meeting (IEDM) here.
This transduction scheme, which can be implemented using simple processing, allows for resonators with a low effective impedance that is, in principle, insensitive to geometric scaling. It thus allows for the realization of miniature, high-frequency MEMS resonators and oscillators without significant performance reduction, the group said. The effective impedance at resonance is said to be orders of magnitude lower than that obtained using the more-common capacitive or field-effect readout.
"The potential for on-chip integration that can be achieved with gigahertz MEMS resonators opens exceptional possibilities for creating miniature-scale precision oscillators and filters for wireless communications," said Reinhout Woltjer, department head for microsystems technology at NXP Semiconductors' Corporate I&T Research operation. "The minuscule mass of gigahertz MEMS resonators, combined with their high quality factor, also provides them with unprecedented potential for mass sensing."
Although gigahertz MEMS resonators have been demonstrated, their impedance is extremely high due to their small size. As a result, signal levels at resonance are barely detectable.
Several approaches have been explored to lower the impedance of silicon MEMS resonators. They include decreasing the gap width and aspect ratio of the transduction gap, filling the transduction gap with a material with high dielectric permittivity, or using piezoelectric instead of capacitive transduction.
Unfortunately, all these concepts still lead to a high impedance when the size of the resonator is further decreased, said Woltjer. "Furthermore, all of these concepts increase manufacturing complexity and have limited process compatibility to standard CMOS," he said.
In the NXP device, the resonator layout was reactive-ion-etched into a 1.5-micron-thick, n-type, silicon-on-insulator layer down to the buried-oxide layer. Isotropic etching of the buried-oxide layer releases the resonator. Subsequent layouts are scaled in-plane by a factor of approximately 4x.
The resonator detailed at IEDM was shown scaled to resonate at 18, 74, 290 and 1,094 MHz.