Stress analysis, dielectric, piezoelectric, and ferroelectric properties of PZT thick films. Fabrication of a 50MHz Tm-pMUT annular array

摘要

PZT films up to 35 μm thick were fabricated, using a composite sol gel route combining a PZTpowder and a PZT sol. The maximum temperature for the process was 710°C. Ademonstration of single layer and multilayer structures was given to show the flexibility of thistechnology. With Stoney’s Equation, studies of the in-situ film stress development as afunction of the film thickness and density was effectuated. It helped to understand that theinternal forces increase considerably with the film thickness and density. This study yields toset up experimental conditions in which a crack free surface finish of a 28μm thick filmrevealed the adaptability of the spin coating technique to fabricate thick films.The wet etching technology revealed the possibility of a great adaptability to pattern andshape innovative devices such as bars 10 μm wide of 21μm PZT thick film. The results openthe way to a wide range of new industrial application requiring small features and/or multilayerPZT thick film with embedded electrodes.The single element and annular array devices have been shown to resonate at approximately60MHz in air and 50 MHz in water. Three types of the composite thick film – 2C+4S, 2C+5Sand 2C+6S – were used to fabricate the Tm-pMUT devices. In each case the most effectivepoling was obtained by maintaining the poling field of 8.4V/μm during cooling from the polingtemperature (200ºC) to ‘freeze’ poled domains in place. This ‘freezing’ was required toprevent the tensile stresses within the film from reorienting the domains at high temperatureswhen the poling field is removed.Increasing values of thickness mode coupling coefficient (kt) were observed with increasinglevels of sol infiltration (decreasing density). Such behaviour is thought to be due to non lineareffects on the piezoelectric coefficient (e33) at high levels of porosity. For very dense thick filmmaterial a kt of 0.47 was observed which is comparable to that observed for the bulk material.