High performance microwave air-bridge resonators

We have used pulsed laser deposition and a dry release process to produce freestanding zinc oxide films for bulk mode microwave acoustic resonators. Laser assisted deposition is used to produce ZnO films with very low (slightly tensile) stress at temperatures below 200°C. They have a high degree of orientation, C axis X-ray ½ width ~0.24°, resistivity greater than 10¹⁰ ohm cm and loss tangent below 0.01 at 10 MHz. Resonators with Qs of over 350 with K_em of 0.22 at 1.5 GHz have been produced. An air-bridge process has been used to produce undistorted freestanding membranes up to 400 μm sq., with single mode resonance characteristics. The use of an inorganic sacrificial layer and a dry release etch avoids the problem of surface tension that occurs in wet chemical etching, resulting in high yield. The process also allows for on-wafer frequency trimming. Multipole band pass filters have been designed based on the discrete resonator results, and have been fabricated using this process. A rejection of 28 dB and an insertion loss of -7 dB have been achieved. The high yield and good uniformity make this a practical, low-cost process for producing complex multipole filter structures suitable for 1-3 GHz mobile communications