Next generation Acoustic Wave Filter Platform

Acoustic wave devices exploit the higher speed of sound in solid materials than air by converting electrical energy into acoustic energy by piezoelectricity. Devices such as Surface Acoustic Wave (SAW) devices and Bulk Acoustic Wave (BAW) devices can filter high frequencies in the acoustic domain and form the backbone of mobile telephony base stations and radar.

Unfortunately, piezoelectrics do not exhibit the highest acoustic wave velocities, and high velocity materials such as diamond do not exhibit piezoelectricity. This limits incumbent SAW technologies to around 2GHz. Piezoelectrics also have very limited thermal conductivity which means that operation at high powers is not possible.

There is an increasing drive to create new materials combinations to realise higher frequency devices as evidenced by Murata's "Incredibly High Performance" (IHP) SAW filter which combines the piezoelectric LiTO3 and silicon. Diamond would be a natural extension of this technology with the highest of all acoustic wave velocities as well as the highest thermal conductivity of any electrical insulator. Unfortunately, the coupling between diamond and most piezoelectrics is relatively weak which leads to high insertion loss.

This project aims to alleviate this issue with a Surface Activated Wafer Bond (SAWB), which also circumvents the high temperature and harsh environment of diamond growth which can significantly damage piezoelectric materials. By bonding at room temperature, it will be possible to combine high performance piezoelectric single crystals with diamond over large areas for unrivalled performance. The superlative acoustic wave velocity of diamond provides for high frequency operation whilst the thermal conductivity simultaneously unlocks high power operation currently unavailable to any other SAW platform (piezoelectrics are inherently of low thermal conductivity). This platform will have multiple applications from 5G base station transceivers to quantum memories.