Analysis of SAW propagation in gratings on ZnO/diamond substrates
The space harmonic method is used to analyze surface acoustic wave (SAW) propagation under an infinite periodic metal grating on ZnO/diamond composite layered substrates. Dispersion properties for shorted and open gratings are derived as a function of the thickness of the grating electrodes. From these dispersion relations, the coupling of modes (COM) parameters are derived. Energy profiles inside ZnO/diamond show that the energies contained in each of the ZnO and diamond layers are of the same order when the thickness of the ZnO layer is P//spl pi/r (P=grating period) and that the energy is contained within two wavelengths below the ZnO/diamond interface.
Wireless identification and sensing using surface acoustic wave devices
Wireless Surface Acoustic Wave (SAW) devices were fabricated and tested using planar Lithium Niobate (LiNbO3) as substrate. The working frequencies were in the 180 MHz and 360 MHz range. Using a network analyser, the devices were interrogated with a wireless range of more than 2 metres. Trials with Electron Beam Lithography (EBL) to fabricate SAW devices working in the 2450 MHz with a calculated feature size of 350 nm are discussed. Charging problems became evident as LiNbO3 is a strong piezoelectric and pyroelectric material. Various attempts were undertaken to neutralise the charging problems. Further investigation revealed that sputtered Zinc Oxide (ZnO) is a suitable material for attaching SAW devices on irregularly shaped material. DC sputtering was used and several parameters have been optimised to achieve the desired piezoelectric effect. ZnO was sputtered using a magnetron sputtering system with a 75 mm Zn target and a DC sputter power of 250 Watts. Several trials were performed and an optimised material has been prepared under the following conditions: 9 sccm of Oxygen and 6 sccm of Argon were introduced during the process which resulted in a process pressure of 1.2×10 mbar. The coatings have been characterised using Rutherford Backscattering, X-ray diffraction, SEM imaging, and Atomic force microscopy. SAW devices were fabricated and tested on 600 nm thick sputtered ZnO on a Si substrate with a working frequency of 430 MHz. The phase velocity has been calculated as 4300m/s. Non-planar samples have been coated with 500 nm of sputtered ZnO and SAW structures have been fabricated on using EBL. The design frequency is 2450 MHz, with a calculated feature size of 1 μm. The surface roughness however prevented a successful lift-off. AFM imaging confirmed a surface roughness in the order of 20 nm. Ways to improve manufacturability on these samples have been identified.
High velocity pseudosurface waves (HVPSAW)
Current interest in low-loss UHF filters for use in mobile and personal communication systems has led to a number of pseudo-SAW (PSAW) and SH mode based SAW devices. These filters can operate at higher frequencies than SAW-based ones for a given line-width because PSAW and SH mode velocities can be significantly higher than corresponding SAW velocities. Furthermore for certain orientations the attenuation of the PSAW is acceptably small. In this paper the existence of an independent high-velocity pseudo-SAW (HVPSAW) mode is discussed. The HVPSAW is shown to have a phase velocity close to the longitudinal bulk wave velocity and to be quasi-longitudinally polarized. The nature of this HVPSAW mode is described for both electrically open-circuited and layered metallic short-circuited conditions for several piezoelectric materials. Numerical and experimental data are presented which discuss the existence and properties of these high-velocity pseudosurface waves.<<ETX>>