Freestanding CVD diamond elaborated by pulsed-microwave-plasma for ZnO/diamond SAW devices

Abstract

In previous work, the feasibility of developing high performance AlN/diamond surface acoustic wave (SAW) devices has been demonstrated using the unpolished nucleation side of freestanding CVD diamond. This process shows the advantage of avoiding the diamond polishing, which is a tedious and a time-consuming technological step. As alternative, we propose the use of a diamond substrate obtained by two stages of growth in pulsed MPACVD to constitute two superposed layers. The first one was deposited at optimal conditions with 2% CH 4 in H 2 gas mixture, leading to a high diamond quality. The second diamond layer is used as a support in order to get a freestanding diamond. It has a lower quality because it's obtained by increasing the CH 4 content, thus the growth rate. The first diamond layer combined with piezoelectric film defines the SAW properties. Its thickness is chosen as a function of the acoustic wave penetration depth in order to limit their propagation only in the first diamond layer. Aluminium inter-digital transducers (IDTs) were deposited by conventional contact UV photolithography on the nucleation side of the freestanding diamond. Piezoelectric ZnO film was then deposited by reactive magnetron sputtering to obtain the ZnO/IDT /diamond structure. Electrical characterization of the SAW device exhibits high filtering properties. The following parameters are obtained with 32 μm wavelength, 3 μm ZnO and 25 μm diamond first layer thickness: acoustic phase velocity: v ϕ=9696 m/s, electromechanical coupling coefficient: K 2=0.75%, temperature coefficient of frequency: TCF=29 ppm/°C.