Deposition of ZnO thin films by magnetron sputtering for a film bulk acoustic resonator

Abstract To fabricate lateral-field excitation (LFE)-mode solid mounted resonator (SMR)-type film bulk acoustic resonators (FBARs), piezoelectric ZnO layers were deposited in an RF magnetron sputtering system. Control of the crystallinity, microstructure and electric properties of the piezoelectric layers was essential for fabricating high-quality LFE-mode SMR-type FBARs. In the appropriate deposition condition for FBAR devices, ZnO thin films with highly c-axis-preferred orientation (XRD rocking curve, σ=2.17°), high resistivity of 106 Ω cm and surface roughness of 10.6 A were deposited. Optimal substrate rotation was especially important for improvement of the c-axis-preferred orientation of ZnO films. Plasma properties such as the electron temperature, plasma density and saturated ion current were also analyzed for optimal ZnO deposition conditions using a Langmuir double-probe system. The resonator, for which the active piezoelectric area was 200×200 μm2, consisted of 1.25-μm-thick ZnO film and a 110-nm Au electrode. Its series and parallel resonance frequencies appeared at 1.68 and 1.71 GHz, respectively, and the quality factor was 201.4±7.4.