High frequency bandpass filter using polycrystalline diamond

Abstract Because of its high Young's modulus, diamond has the highest acoustic velocity of all materials, and thus for surface acoustic wave (SAW) devices diamond can be expected to be one of the candidates for passive component applications. Polycrystalline diamond 40 μm thick was deposited on a silicon substrate by hot-filament chemical vapour deposition. After the diamond surface had been polished, 100 nm of Al was deposited and fabricated into interdigital transducers, utilizing conventional photolithography and etching processes. This was followed by the deposition of a ZnO thin film, which served as a piezoelectric material to generate SAWs. Finally, an SAW device using a ZnO/diamond system was fabricated for the first time. With the high SAW velocity of the ZnO/diamond system being up to 8600 m s, −1 a 1.07 GHz bandpass filter was successfully demonstrated using 2 μm line and space interdigital transducers; additionally −25 dB insertion loss was observed. This preliminary result suggests that, by utilizing diamond, high frequency (2–5 GHz) bandpass filters can be made available, and broad applications in the optical and personal communication systems can be expected.