Broadband Microwave Dielectric Properties of BST Thin Films on Quartz Substrates

Abstract

Thin Films of (Ba _0.5 , Sr _0.5 ) TiO ₃ have been deposited on quartz substrates by RF magnetron sputtering. The percentage of oxygen in sputtering atmosphere was varied between 25 to 100%. The as-deposited films were x-ray amorphous and crystallized on post deposition annealing at 900°C for 1 min. Microstructure as determined by Atomic Force Microscope (AFM) showed a strong dependence on oxygen mixing percentage (OMP) in the sputtering gas. The microwave dielectric properties of these thin films are determined up to 20 GHz using a calibration comparison technique with patterned coplanar waveguide (CPW) transmission lines deposited on the surface of the film. Two CPW test structures of 100-micron gap and 200 micron width are deposited simultaneously one on the test film and the other on the bare substrate by directly sputtering silver through a shadow mask. The scattering parameters of these two CPW transmission lines were measured using a Vector Network Analyzer and a probe station mounted with the GSG probes of 250-micron pitch. The dielectric constant of the BST films crystallized on quartz vary between 150–220 and the loss tangent is found to be varying from 0.018 to 0.021 depending upon the processing conditions. The relaxation currents present in the films due to the structural defects associated with the processing conditions may be responsible for the observed dielectric loss as well as the frequency dependent permittivity.

Keywords:

• Ferroelectric thin films
• microwave permittivity
• BST

Acknowledgments

The authors acknowledge the financial support from “National Programme on Smart Materials” India. Facilities provided by the ISRO, DLRL, UGC under the plan grant, UPE and SAP and DST in the FIST and ITPAR programmes are gratefully acknowledged. KVS acknowledges a project associateship from NPSM. KS acknowledges a project associateship from DLRL. The technical support extended by Dr. K. Natarajan, Dr. P. Nagasayanu of BEL, Bangalore and Dr. Guru Subrahmanyam of university of Dayton, Ohio is gratefully acknowledged.