Leaky SAW Devices with Beryllium Electrodes

The reduction of losses in SAW devices continues to be a relevant topic. Leaky SAW devices on rotated Y-cut of LiTaO3 suffer from losses due to “leakage”, i.e. radiation of slow shear bulk-acoustic waves weakly coupled to the surface wave. It is well known that the leakage on LiTaO3 can be reduced by optimizing the thickness of aluminum electrodes and the crystal cut angle. It is also known that heavy metal electrodes (Pt, Mo,W) can slow down the leaky wave in LiNbO3 so strongly that the leakage can be suppressed. But what about other metals? In this work we show that the amount of leakage on rotated Y-cut LiTaO3 depends mainly on two parameters: the crystal cut angle and mass of the electrodes. With suitably chosen combination of the two, the leakage can be essentially suppressed, at least at one frequency point. This leakage cancellation can be achieved with many different metals. However, many heavy materials are not feasible in practice - the demanded thickness is so small that the high resistivity of electrodes is not acceptable. An excellent candidate could be Beryllium (Be) - being a very light material with low internal acoustic losses and relatively low resistivity. For a synchronous resonator on LiTaO3 we can minimize the leakage at a single frequency within the resonance-antiresonance region. For the leaky losses all studied metals seem fairly interchangeable. The electrode structure was optimized to give approximately the same leaky loss at resonance and at the anti-resonance, with these losses being minimal at around the center between these frequencies (around 2p*f = 4020 m/s). For thicker Be electrodes of h/λ = 13.5% we have resonance at 2p*fr = 3951.3 m/s and antiResonance at 4114.0 m/s (Q-factors due to the leaky loss >4000).