MEMS Resonator Based on Gallium Nitride Maintains Stability at High Temperature

A MEMS resonator that achieves operational stability under high temperatures by regulating the strain of imparted heat from gallium nitride has demonstrated qualities that show its promise as a highly sensitive oscillator device in the pursuit of enhanced 5G communication. Liwen Sang, an independent scientist at the International Center for Materials Nanoarchitectonics and the National Institute for Materials Science and a JST PRESTO (Japan Science and Technology Agency Precursory Research for Embryonic Science and Technology) researcher, developed the device.

To improve on temporal performance, Sang fabricated a high-quality GaN epitaxial film on a silicon substrate, using metal organic chemical vapor deposition to fabricate the resonator itself. A thermal and lattice mismatch between the gallium nitride and silicon enabled Sang to achieve a strain necessary to allow the growing of gallium nitride directly on the silicon surface without a strain-removal layer.

By also refining and optimizing the method of temperature decrease in the chemical vapor deposition process, the uncracked gallium nitride exhibited a crystalline quality comparable to that obtained via a superlattice strain-removal layer — a more conventional method.

(1) The as-grown GaN epitaxial film on Si substrate. Except for the AlN buffer layer, no strain-removal layer is used. (2) Spin coating of the photoresist on the GaN-on-Si sample. (3) Laser lithography to define the pattern for the double clamped bridge configuration. (4) Plasma etching to remove the GaN layer without photoresist. (5) Chemical etching to release Si under the GaN layer. Therefore, the air gap is formed. (6) The final device structure of the double clamped bridge resonator. The laser doppler method is used to measure the frequency shift and resolution under different temperatures. Courtesy of Liwen Sang.

5G communication systems operating at high speed and high capacity are contingent on highly precise synchronization. A high-performance frequency reference oscillator capable of balancing the temporal stability and temporal resolution is a necessary time-keeping device that generates signals on a fixed cycle. A quartz resonator functioning as the oscillator, though conventional to such systems, features poor integration qualities, limiting its applicability.

Further, though a silicon-based MEMS resonator achieves high temporal resolution, and with small phase noise and a more desirable integration capability, it is unstable at higher temperatures.

The new device operated stably at up to 600 K, exhibiting only a slight shift in/to frequency (influencing temporal stability) as the temperature increased, according to a release from JST. That shift derived from the internal thermal strain compensating the frequency shift and cutting the dissipation of energy.

(a) The temperature coefficient of frequency (TCF) of the GaN resonator at different temperature. (b) The quality factor of the GaN resonator at different temperature. The temporal stability of a resonator is defined by TCF, which indicates a change of the resonance frequency with changing temperature. For the Si MEMS resonator, its intrinsic TCF is ~ −30 ppm/K. Several methods were proposed to reduce the TCF of Si resonator, but the quality factors of the system were greatly degraded. The quality factor of a resonator in the system can be used to determine the frequency resolution. A high-quality factor is required for the accurate frequency reference. The developed GaN resonator in this work can simultaneously achieve a low TCF and high quality factor up to 600 K. The TCF is as low as −5 ppm/K. The quality factor is more than 105, which is the highest one ever reported in a GaN system. Courtesy of Liwen Sang.

The compact and highly sensitive device can be integrated with CMOS technology. Beyond its potential to support 5G communication, it may prove useful with an IoT timing device, on-vehicle applications, and driver assistance systems.

“Self-Temperature-Compensated GaN MEMS Resonators through Strain Engineering Up to 600K” was presented at the 2020 IEEE International Electron Devices Meeting (IEDM2020) Dec. 12-18. The research was supported by JST PRESTO.