||In this study, 1/2 λ mode SMR filters on Si substrates by reactive RF magnetron sputtering method were fabricated. In addition, the thermal annealing process was adopted to improve the insertion loss of SMR filter.|
The Bragg reflector in SMR is alternately mounted by high and low acoustic impedance materials, with low acoustic impedance material of SiO2 and high acoustic impedance material of W. We could obtained three kinds of crystal structures of W, α - phase W、β - phase W and α & β - mixed phase W, respectively, it could be obtained by modulating the sputtering recipe. α - phase W possesses higher acoustic impedance and is suitable for high acoustic impedance material in bragg reflector.
The piezoelectric layer of ZnO is sputtered by a 2-step deposition method on Si substrates with different temperature. The ZnO film with stronger C-axis (002) orientation and lower surface roughness value could be obtained at substrate temperature of 200 ℃, which is suitable for fabricating SMR device.
After the SMR filter had completed, the device is thermal annealed with CTA、RTA and RTA in O2 ambient. After thermal treatment, the properties of filters are improved. The properties could be optimized with RTA in O2 ambient condition. The insertion loss was improved from -12.03 dB to -6.96 dB. The film characteristics of ZnO changes after the SMR processed thermal treatment. The strongest C-axis (002) intensity with the lowest surface roughness value at 400 ℃ annealing temperature could be obtained, in that, approximate equal Zn:O ratio could be achieved by XPS examination.
The central frequency of SMR filter drifted to higher value as the temperature of thermal treatment increased, which is attributed to the changes of the ZnO acoustic velocity(υ) after thermal treatment.