Fabrication of PZT composite thick films for high frequency membrane resonators

High frequency, thickness mode resonators were fabricated using a 7 μm PZT thick film which was produced using a modified composite ceramic sol-gel process. Initial studies dealt with the integration of the PZT thick film onto the substrate. Two different diffusion barrier layers were tested, titanium oxide and zirconium oxide, in conjunction with the use of 2 types of silicon substrate (differing in the etch stop layer employed, either silicon nitride or silicon oxide). Zirconium oxide gave good results in conjunction with silicon oxide. Using these conditions, devices were produced and the acoustic properties measured for different electrode sizes ranging from 45*45 to 250*250 μm². The best electrode size, which maximised the acoustic response and minimised the insertion loss, was found to have an area of 110*110 μm². This device showed a resonant frequency of about 200 MHz, an effective electro-mechanical coupling coefficient of 0.29 and a Q factor of 22.