Fully dielectric acoustic Bragg mirror of a-SiOCN:H and a-SiC:H layers for bulk acoustic wave resonators

Abstract

A typical bulk acoustic wave-solidly mounted resonator (BAW-SMR) in filters and duplexers for today's radio frequency front end utilizes an acoustic mirror to trap the acoustic wave energy coupled into the device by the electric signal. The exploited Bragg reflection in the acoustic mirror is created by an alternation structure of low and high acoustic impedance (low-Z and high-Z) material thin films, as the ratio between the paired acoustic impedances directly correlates with the desired mirror reflectivity. Preferably, low-Z thin films consist of silicon dioxide (SiO₂) with ∼13 MRayl, paired with high-Z material layers of tungsten (W), tantalum (Ta), or molybdenum (Mo) in between, featuring up to >100 MRayl for the acoustic impedance (in the case of W) and leading to a high impedance ratio of 8:1. However, an unwanted effect arises from metals as high-Z thin films, requiring photolithographic structuring steps to confine them within the active resonator regions and avoid electromagnetic feedthrough, which is detrimental to filter selectivity outside the desired passband. Hence, fully dielectric acoustic Bragg reflectors have been proposed by various groups, replacing metal thin films in acoustic Bragg reflectors with dielectric high-Z material systems. By the development of a-SiOCN:H as a low-Z material system with only 7.1 MRayl, pairing with a dielectric high-Z thin film of amorphous SiC (a-SiC, a-SiC:H), featuring 22.5 MRayl and, thus, an impedance ratio of 3:1, also becomes applicable. In this work, we present the design and manufacturing of this fully dielectric acoustic Bragg mirror with a-SiOCN:H and a-SiC:H, fabricated by an alternation of deposition parameters within a plasma-enhanced chemical vapor deposition process and reaching a state-of-the-art coupling coefficient of more than 6.6% in an AlN based BAW-SMR device.