GaN based MEMS resonator operates stably at high temperature

January 18, 2021 //By Jean-Pierre Joosting
GaN based MEMS resonator operates stably at high temperature
Promising as a highly sensitive oscillator for 5G as well as IoT timing devices, in-vehicle applications, and advanced driver assistance systems.

Liwen Sang, independent scientist at International Center for Materials Nanoarchitectonics, National Institute for Materials Science (also JST PRESTO researcher) has developed a MEMS resonator that stably operates even under high temperatures by regulating the strain due to the lattice mismatch and thermal mismatch between GaN and Si.

High-precision synchronization is required for 5G, in conjunction with high speed and capacity. To that end, a high-performance frequency reference oscillator that can balance the temporal stability and temporal resolution is necessary as a timing device to generate signals in a fixed cycle. The conventional quartz resonator as the oscillator has poor integration capability and its application is limited. Although a micro-electromechanical system (MEMS) resonator can achieve a high temporal resolution with small phase noise and superior integration capability, silicon (Si)-based MEMS devices suffer from bad stability at higher temperatures.

In this study, a high-quality GaN epitaxial film was fabricated on a Si substrate using metal organic chemical vapor deposition (MOCVD) to make the GaN resonator. Strain engineering was proposed to improve the temporal performance. Strain was achieved by utilizing the lattice mismatch and thermal mismatch between the GaN and the Si substrate. Consequently, GaN was directly grown on Si without any strain-removal layer. By optimizing the temperature decrease method during MOCVD growth, no cracking was observed on the GaN and its crystalline quality remained comparable to that obtained by the conventional method of using a superlattice strain-removal layer.

The device processing for the double-clamped GaN bridge resonator on Si substrate: (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. We use the laser doppler method to measure the frequency shift and resolution under different temperatures. Image courtesy of Liwen Sang.

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