Abstract:
PZT films up to 35 μm thick were fabricated, using a composite sol gel route combining a PZT
powder and a PZT sol. The maximum temperature for the process was 710°C. A
demonstration of single layer and multilayer structures was given to show the flexibility of this
technology. With Stoney’s Equation, studies of the in-situ film stress development as a
function of the film thickness and density was effectuated. It helped to understand that the
internal forces increase considerably with the film thickness and density. This study yields to
set up experimental conditions in which a crack free surface finish of a 28μm thick film
revealed the adaptability of the spin coating technique to fabricate thick films.
The wet etching technology revealed the possibility of a great adaptability to pattern and
shape innovative devices such as bars 10 μm wide of 21μm PZT thick film. The results open
the way to a wide range of new industrial application requiring small features and/or multilayer
PZT thick film with embedded electrodes.
The single element and annular array devices have been shown to resonate at approximately
60MHz in air and 50 MHz in water. Three types of the composite thick film – 2C+4S, 2C+5S
and 2C+6S – were used to fabricate the Tm-pMUT devices. In each case the most effective
poling was obtained by maintaining the poling field of 8.4V/μm during cooling from the poling
temperature (200ºC) to ‘freeze’ poled domains in place. This ‘freezing’ was required to
prevent the tensile stresses within the film from reorienting the domains at high temperatures
when the poling field is removed.
Increasing values of thickness mode coupling coefficient (kt) were observed with increasing
levels of sol infiltration (decreasing density). Such behaviour is thought to be due to non linear
effects on the piezoelectric coefficient (e33) at high levels of porosity. For very dense thick film
material a kt of 0.47 was observed which is comparable to that observed for the bulk material.
