Surface acoustic waves (SAWs) are widely used and very important phenomena in both research and industry. SAWs are mechanical vibrations propagating along the surface by confining the acoustic energy over a depth of typically one wavelength. In piezoelectric materials, they can be excited through the inverse piezoelectric effect by using interdigital transducers (IDTs): two interlocking comb-shaped metallic electrode pairs. A piezoelectric potential wave accompanies the mechanical wave. The unique SAW properties make them suitable for wide range of applications. Among the most exciting applications of SAWs are acousto-optical modulators as well as the control of excitons, electrons, and spins in semiconductors. For almost all SAW applications, there is a strong demand for higher frequencies, for example to enhance processing speed or to reach the quantum regime. There are two main factors determining the IDT resonant frequencies: the acoustic properties of the substrate, and the IDT period constrained by lithography resolution. In latter case, the resolution of standard photolithography (minimum finger width of approx. 0.5 um) limits the operation frequency to around a few GHz, even for high acoustic-velocity substrates.

In this study, an alternative lithography technique, “step-and-flash nanoimprint lithography” (SFIL), with a novel material system for lift-off was used to reach very high resolution and higher reproducibility. Hydrogen silsequoxane (HSQ) was used as a planarization layer and an excellent etching mask to get inverted replication of the IDT features on the SFIL template. A sufficient undercut profile of the electrode features, which is essential for metal lift-off at nanoscale, was successfully achieved. Very high critical dimension (CD) control has been obtained for the electrode dimensions from 125nm down to 65 nm. While this method has the advantage of EBL resolution, it is nearly substrate independent with higher throughput compared to EBL. For the IDT fabrication, we have deliberately chosen a rather standard Si substrate to demonstrate that our method can result in extremely high-frequency SAW devices, which can be monolithically integrated with common electronic circuitry. Integration of high-frequency SAW devices is normally restricted by the incompatibility of piezoelectric films with CMOS processing. We showed that SAW delay line devices with 125nm down to 65 nm metal lines can reach resonance frequencies up to 16 GHz in ZnO/SiO2/Si multilayer system, which is the highest frequency for ZnO based transducers on Si reported so far. The finite element analysis confirmed the results and showed good agreement with the experiment.