Summary: | 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.
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