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

 

Lead Zirconate Titanate (PZT) is a high dielectric constant erS, high coupling kt2 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 acoustical attenuation of PZT tends to preclude its usage 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 FBAR resonators 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 in this study 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 with an Agilent 8722C swept frequency network analyzer and a Cascade MicrotechR probe station. Using the Mason equivalent circuit model, the DPE coefficients were extracted by varying parameters in the Mason model until the predicted S11 vs frequency fit the measured data. The devices exhibited erS in the range of 300 to 500, coupling constants kt2 between 5% and 35%, and acoustic attenuation from 400 to 2100 dB/cm at 1 GHz.

 
citation

Larson III, J. D.; Gilbert, S. R.; Xu, B. PZT material properties at UHF and microwave frequencies derived from FBAR measurements. IEEE International Ultrasonics, Ferroelectrics, and Frequency Control 50th Anniversary Joint Conference; 2004 August 24-27; Montreal; Canada.