PZT thick films for pressure sensors: Characterisation of materials and devices

Different piezoelectric materials can be used in micro-electro-mechanical systems (MEMS) for transducers i.e. actuators and sensors of mechanical quantities. Lead zirconate titanates (PZTs) are the most common ceramic materials used as piezoelectric transducers. The ceramic MEMS are made by LTCC (low temperature cofired ceramic) and thick-film technology. They have been extensively used due to their superior piezoelectric properties. The use of thick-film PZT materials on LTCC substrate is not a trivial task due to the interaction between the printed PZT layers and the LTCC substrates during firing. The microstructural, electrical and piezoelectric characteristics of the thick PZT films on relatively inert alumina substrates and on LTCC tapes were studied. To minimise the possible interactions between glassy LTCC substrates and active PZT films for some samples the intermediate barrier layers were used. To characterise the thick PZT films on ceramic substrate the dielectric permittivities (epsiv), dielectric losses (tgdelta), and piezoelectric coefficients (d₃₃) were measured. The thick-film piezoelectric resonant pressure sensor on LTCC structure was designed and samples were fabricated (Figure 2). The thick-film PZT actuator on the ceramic diaphragm induces the vibration of the diaphragm at its resonant frequency. The applied pressure bends the diaphragm and generates additional stress, which shifts the resonant frequency. This is used as the output signal of the piezoelectric resonant pressure sensor. The resonant frequencies are shifted around 26 kHz. The calculated sensitivities are 2.5 Hz/kPa.