Wide bandgap (AlN, GaN, SiC) RF-MEMS devices for extreme environment applications

A. Qamar, M.-R. Zadeh
University of Michigan,
United States

Keywords: waveband gap semiconductors, RF-MEMS, SAW, BAW

Summary:

The talk will focus on the development of unprecedented, robust, and highly sensitive piezoelectric resonators with high-quality factors (Qs) and improved electromechanically coupling coefficient using AlN/ScAlN, GaN, and SiC wide bandgap semiconductors. Among different piezoelectric materials, AlN/ScAlN is a much suitable choice for many RF applications due to its fabrication simplicity and reproducibility as well as its high thermal conductivity and compatibility with CMOS fabrication. The main objective is to investigate all the possible loss mechanisms in thin-film piezoelectric resonators and ultimately rectify the losses to produce ultra-high Q resonators for applications in radio frequency (RF) filters, integrated high-frequency oscillators, and low power resonant sensors. The losses are investigated by analytical modeling, COMSOL and experimentally with different heterostructures stacks of Al/AlN/Si and Al/AlN/SiC, etc. Analysis of surface acoustic wave (SAW) resonators and bulk acoustic wave (BAW) resonators using the wideband semiconductors will be presented, especially focusing on how to improve the figure of merit by different material stacks and techniques. For Super-High-Frequency (SHF) range (3-30GHz), the inter-digitated-transducer size reduces to submicron size, which seriously affects the figure of merit of SAW devices. In this talk, we will focus on how to enhance the figure of merit at SHF range and the ways to get the ultra-high Q SAW devices, which can be used, in harsh environment applications, especially for planetary exploration. Another wide bandgap material Gallium nitride (GaN), which is less explored for piezoelectric MEMS devices, will also be discussed. GaN has a high acoustic velocity (comparable to silicon) and large piezoelectric coefficients (comparable to that of aluminum nitride (AlN)). However, what is unique about GaN is that it is both a semiconductor and a piezoelectric material, unlike silicon and AlN. The action of piezoelectricity within a semiconductor allows for a family of versatile and sensitive transducers in GaN, amongst which, this research focuses on developing GaN resonators and RB-HEMTs. Along with new device development, this research aims at understanding the phenomena resulting from the simultaneous presence of piezoelectricity and conductivity in the same crystal. The acoustic wave propagating in such a crystal will be accompanied by a piezoelectric electric field, which in turn, acts on the mobile charge carriers (e.g., electrons). The effect of such interactions between electrons and phonons can be undesirable or very interesting, depending on the conductivity, and the crystalline properties of the material, as well as the acoustic wave frequency. To fully unlock the potential of GaN and realize an all-GaN MMIC, it is essential to co-integrate passive devices (such as resonators and filters), sensors (such as temperature and gas sensors), and other more-than-Moore functional devices with GaN ICs. This study will provide a platform for all-GaN MMIC devices and systems. Silicon Carbide (SiC) on the other hand is a piezoresistive material, which can be used for pressure/force/strain sensing in the harsh environment and when combined with piezoelectric materials (AlN, GaN), it can provide a platform for the coexistence of low loss passive devices (resonators, filters) and sensors (pressure, stress, strain, temperature).