Lead zirconate titanate thin films on GaAs for microwave device applications

Summary form only given. Bulk Acoustic Wave (BAW) resonant devices based on quartz crystals are widely used in electronic systems at frequencies up to a few tens of MHz. However for operation at higher frequencies the crystal must be made much thinner and consequently-ceases to be mechanically self supporting. Similarly, Surface Acoustic Wave (SAW) device dimensions shrink at high frequencies requiring sub-micron electrode structures for microwave operation. In this paper we describe how the submicron critical dimensionality of acoustic wave devices can be achieved by depositing thin piezo-ceramic films on semiconductor substrates with the subsequent fabrication of BAW devices. The piezoelectric ceramic used in this work is sol-gel derived lead zirconate titanate (PZT) about 0.5 μm thick. Films having the composition Pb(Zr_0.53Ti_0.47)O₃, have been prepared on platinized silicon (Pt-Si) and platinized gallium arsenide (Pt-GaAs) substrates using a 1,3-propanediol and a novel 1,1,1-tris(hydroxymethyl)ethane based sol-gel technique. Crystallisation of the PZT films on the Pt-GaAs was achieved by firing the sol-gel coating at 650°C for a dwell time of 1 second using rapid thermal processing (RTP) techniques. Films having the required thickness of ~0.5 μm were produced from a single deposition of the precursor sol resulted. Average values of remnant polarisation (P_r) for the Films were 29 μC/cm² and 24 μC/cm² on Pt-Si and Pt-GaAs respectively, comparing very well with bulk values. Preliminary microwave characterisation performed on PZT/Pt-Si based BAW resonator structures indicates a fundamental parallel resonance at 0.1 GHz, having an unloaded Q of 1100. Higher frequency operation will be obtained when substrate thinning under the active layer has been optimised