Piezoelectric thin film actuation of RF MEMS devices
This thesis investigates the piezoelectric thin film actuation of RF MEMS devices. lt is demonstrated that piezoelectric actuation using silicon structures with integrated PZT thin film is suitable for RF MEMS switches and mechanical filters. Both these devices are studied, fabricated and tested. To assist the mechanical design of a piezoelectric microswitch an electrical model is developed, from which design parameters can be derived to meet the requirement of an insertion loss of -1dB and an isolation loss of -40dB at 4GHz. The model shows how the switching gap and the overlapping dimension are two key parameters and the ratio between the two should be larger than 0.15. Switching gaps are chosen as 5m and 7.5m from the designed range of 3 ~ 15m. An equivalent circuit network incorporating a transmission line model of the coupling beam is developed to demonstrate a design method for the mechanical filter. The model reveals that the coupling beam stiffness, coupling position, electromechanical coupling factor and the quality factor of the piezoelectric cantilever have substantial effects on filter characteristics. A narrower/wider bandwidth with higher/lower filter Q can result if the coupling position is near/further to a cantilever's anchor position. Extreme situations of the above two coupling cases result in filter bandwidth's disappearing and filter centre frequency's shifting. For a desired filter insertion loss source and load resistances need to be determined by quality factors of terminal circuit and coupled resonators. A bulk micromachining process compatible with the integration of PZT thin film onto silicon is developed. ln general, for PZT pattern size above 100m wet etching could give satisfactory pattern definition, however for pattern size below 50m dry etching is needed. To etch a thickness 2 1m PZT film long dry etching lime demands for the survivability of a masking material. ln this study a technique combined of dry and wet etching is developed for a 1m PZT film. lt is found that a low pressure promotes a higher etch rate and the best etching conditions are a gas mixture with a composition of 1/4 for CHF3/Ar at a total gas flow of 25sccm under a RF power of 150W. Dry etching is also developed to release cantilevers from the buried S102 layer with conditions of a gas mixture of 13sccm CHF3 and 3sccm 02 at 80mTorr and a RF power of 100W in 30mins for 1m Si02. To be compatible with PZT elements silicon cantilevers are processed using deep reactive ion etching. PZT thin film actuation has been demonstrated successfully with PZI' thin film integrated silicon cantilevers. A static deflection of 2.89m is measured at an actuating voltage of 20V for a 100m wide and 450m long cantilever. A displacement of 556nm at 12.98 kHz resonance is measured for a 200m wide and 850m long cantilever under a 10mV AC plus a 10V DC. A piezoelectric strain constant d31 of 30.15pC/N has been obtained for the PZT thin film used in this study. Filter resonant modes and impedance responses are measured. The filter concept design is justified by the obtained results. Using the impedance data the electromechanical coupling factor, filter centre frequency, filter bandwidth and filter Q are determined. There is a good agreement between the measured and calculated filter centre frequencies. At the first filter resonant mode a maximum piezoelectric coupling factor of 0.19 and a maximum resonator-Q of 235 are recorded. At the second filter resonant mode a maximum of 0.12 and a maximum resonator-Q of 360 are obtained. Results show smaller coupling factors than required at the 13' and 2" centre frequencies for all measured filters. This implies that pass bands of designed filters will be missing from their responses at the 18' and 2" centre frequencies. Design improvements are given in the relevant results discussions. Residual stresses are studied for deflected cantilevers after release. The tensile stress of PZT, the converted tensile stress of Pt and the compressive stress of remaining buried SiO2 layer could result in the upwards-deflected cantilevers. The compressive stress of P could compensate the tensile stress of PZT and flatten the cantilever beam. lf the buried SiO2 layer is removed incompletely or if the tensile stress of PZT layer is very high, then upwards-deflected cantilevers would result. Measurements of the most switch cantilevers show larger initial deflections than the designed switching gaps. Finally, conclusions and suggestions of future work are given.