GaN Micromechanical Resonators with Meshed Metal Bottom Electrode.

Azadeh Ansari,Hao-Chung Kuo,Che-Yu Liu,Mina Rais-Zadeh,Pei-Cheng Ku,Chien-Chung Lin

doi:10.3390/ma8031204

Azadeh Ansari, Hao-Chung Kuo + Show 4 more

Open Access

https://doi.org/10.3390/ma8031204

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Abstract

This work describes a novel architecture to realize high-performance gallium nitride (GaN) bulk acoustic wave (BAW) resonators. The method is based on the growth of a thick GaN layer on a metal electrode grid. The fabrication process starts with the growth of a thin GaN buffer layer on a Si (111) substrate. The GaN buffer layer is patterned and trenches are made and refilled with sputtered tungsten (W)/silicon dioxide (SiO2) forming passivated metal electrode grids. GaN is then regrown, nucleating from the exposed GaN seed layer and coalescing to form a thick GaN device layer. A metal electrode can be deposited and patterned on top of the GaN layer. This method enables vertical piezoelectric actuation of the GaN layer using its largest piezoelectric coefficient (d33) for thickness-mode resonance. Having a bottom electrode also results in a higher coupling coefficient, useful for the implementation of acoustic filters. Growth of GaN on Si enables releasing the device from the frontside using isotropic xenon difluoride (XeF2) etch and therefore eliminating the need for backside lithography and etching.

Highlights

Gallium nitride (GaN), typically grown on SiC, sapphire, or Si (111), is a piezoelectric material and is used as the transduction layer—sandwiched between a top and a bottom electrode—in bulk acoustic wave (BAW) resonators
GaN layer using BCl3/Cl2 plasma etch, (c) the trenches are filled with sputtered W and (d) evaporated SiO2, (e) GaN device layer is regrown on the W/SiO2 mesh, starting from the bottom GaN seed layer, (f) trenches are made through the GaN layer to access the Si substrate, (g) the top metal is deposited on the device area, (h) the resonator is released with XeF2 isotropic etch
In [13], we have shown GaN micromechanical resonators with a 400 nm thick blanket SiO2 on the top surface to reduce the value of temperature coefficient of frequency (TCF) to ~−15 of ppm/K

Summary

Introduction

Gallium nitride (GaN), typically grown on SiC, sapphire, or Si (111), is a piezoelectric material and is used as the transduction layer—sandwiched between a top and a bottom electrode—in bulk acoustic wave (BAW) resonators. Unlike aluminum nitride (AlN), low-temperature sputtering of GaN on metals is not established, restricting its deposition or growth on specific substrates and making the fabrication of a metal-GaN-metal structure challenging. Because of such issues, different approaches have been taken to implement GaN-based piezoelectric transducers. 20–30 nm below the surface of the structure due to restriction of lattice-mismatched epitaxial growth, considerably limiting the thickness of the active piezoelectric layer compared to the resonant stack and making it inefficient as the actuator. This work seeks a different solution using embedded bottom electrodes for piezoelectric actuation of GaN resonators This technique enables frontside release of the resonant structure using xenon difluoride (XeF2), eliminating the DRIE step.

Regrowth of GaN on Embedded Metal Grids

Experimental Results

Resonator Fabrication Overview

Conclusions