PZT material properties at UHF and microwave frequencies derived from FBAR measurements

Lead zirconate titanate (PZT) is a high dielectric constant, ε_r^S, high coupling, k_t², piezoelectric material with possible application to film bulk acoustic resonators (FBAR) at low microwave frequencies. PZT is a widely used material at audio and low MHz range frequencies, but the high acoustic attenuation of PZT tends to preclude its use above UHF. Likewise, the bulk dielectric/piezoelectric/elastic (DPE) properties are well known at low frequencies, but the thin film properties have not been as well measured into the microwave range. To obtain better data, the material properties of thin film PZT deposited by several methods were measured at GHz frequencies from air- or silicon-backed FBARs fabricated on silicon wafers. PZT thin films, deposited by metal-organic chemical vapor deposition (MOCVD), sol-gel deposition, RF sputtering, or jet vapor deposition, were obtained from various sources. The films examined ranged from 0.4 to 2 μm in thickness, and were deposited on thin (∼0.1 μm) Pt or Ir bottom electrodes. The samples had compositions within the tetragonal phase (Zr/Ti = 30/70, 40/60) or near the morphotropic phase boundary (Zr/Ti = 50/50). Scattering parameter, S₁₁, vs frequency measurements were made. Using the Mason equivalent circuit model, the DPE coefficients were extracted by varying parameters in the Mason model until the predicted S₁₁ vs frequency fitted the measured data. The devices exhibited ε_r^S in the range of 300 to 500, k_t² between 5% and 35%, and acoustic attenuation from 400 to 2100 dB/cm at 1 GHz.