**Piezoelectric Thick Films: Preparation and Characterization**

J. Pérez de la Cruz *INESC Porto - Institute for Systems and Computer Engineering of Porto, Porto, Portugal* 

### **1. Introduction**

Sol-gel technology allows the deposition of 1m thick oxide films onto a variety of substrates at temperatures well below those conventionally used for bulk ceramic processing. Thin film processing temperatures as low as 500-700oC are typically used allowing ceramic materials to be incorporated into the silicon processing stages (Ohno et al., 2000; Wu et al., 1999). In addition, low processing temperatures reduce the inter-diffusion of atomic species between the different thin film layers and ionic vaporization, such as: lead in lead zirconate titanate oxide (PZT) films (Wu et al., 1999; Jeon et al., 2000). PZT thin films have been largely deposited in order to produce several types of devices, such as: membrane sensors, accelerometers and micromotors (Barrow et al., 1997). However, devices requiring larger actuation forces (i.e. high frequency transducers, vibration control devices) require thicker piezoelectric films (Tsurumi et al., 2000). In these cases, it is not practical to produce thick PZT films using standard sol-gel techniques, because of the increased cracking risk due to shrinkage nor is it desirable to produce thick films by a repetitive single layer deposition process due to the time required (Barrow et al., 1995; Zhou et al., 2000).

The interest in ferroelectric lead zirconate titanate thick films for device applications, including high-frequency ferroelectric sonar transducers (Bernstein et al., 1997), microelectromechanical system devices (Polla & Schiller, 1995; Myers et al., 2003; (Akasheh et al., 2004), elastic surface wave devices (Cicco et al., 1996), hydrophones (Chan et al., 1999) and sensors (Xia et al., 2001), has increased in the last decades because PZT ferroelectric thick films possess the merits of both bulk and thin film materials (Barrow et al., 1997; Ledermann et al., 2003). PZT thick films devices not only work at low voltage and high frequency, as they are compatible with semiconductor integrated circuit, but also possess superior electric properties approaching near-bulk values. Naturally, processing of PZT thick films has also become an increasingly popular research field. Some approaches that have recently been studied to process thick films include electrophoretic deposition (Corni et al., 2008), pulsed laser deposition (Yang et al., 2003), screen printing (Walter et al., 2002) and sol-gel (Xia et al., 2001; Wang et al., 2003; He et al., 2003). Among them, the hybrid sol-gel technique has a special interest due to its low preparation cost and excellent stoichiometric control. Sol-gel also offers the capability to lay down thick layers anywhere between 0.1m and 100m, a thickness range that is difficult to achieve by other deposition techniques.

Piezoelectric Thick Films: Preparation and Characterization 353

state oxide methods (dry method or DM) shows a mean grain size of 2.2 m, while PZT powder prepared by a coprecipitation process (web method or WM) shows a mean grain size of 1.9 m. Some readers could assume that a progressive decrease in powder size could contribute to a smaller powder agglomeration inside the sol-gel matrix, improving the films densification. However, this is not completely true, as when the power size decreases, the effective surface of the powder increases. This increase in the powder effective surface results in an increase of the surface tension force, which strongly contribute to agglomerate formation. An evident example can be observed in Fig. 1, the coprecipitation process produces smaller grain size PZT powders; however, during the sol-gel PZT power mixture a

Fig. 1. Grain size distribution of PZT and PT powders prepared by different synthesis routes, and dispersed using -terpineol solvent, BEEA dispersant, PVB binder and PEG

which cannot be filled by any post-deposition process, such as: infiltration.

Similar agglomeration behavior is also observed in high molecular weight free sol gel matrixes. In this case authors have used an ultrasonic dispersion process combined with a relatively high viscous sol-gel matrix in order to eliminate the PZT powder agglomeration (Pérez et al., 2007). As the ultrasonic dispersion time increases, agglomerate formation decreases, as shown in Fig.2. It is clear that the ultrasonic process disperses the agglomerates resulting in an increase of low diameter differential volume. The reduction of the PZT powder agglomeration and the increase in the low diameter differential volume improve the densification of the film during the preparation and reduce the formation of *closed pores*,

It is notable that the ultrasonic process eliminates the agglomerate formation; moreover, it also negates the use of the high molecular weight solvents, dispersants, binders and plasticizer, resulting in shorter drying and pre-annealing processes during the thick film preparation process. The elimination of high molecular weight compounds from the sol-gel matrix reduces in large scale pore formation, improving the PZT thick films densification.

plasticizer (Simon et al., 2001). (Copyright Elsevier)

~10 m agglomerated is observed.

In this chapter an exhaustive review of the preparation of PZT thick films by infiltration method will be presented. Solution powder agglomeration, film densification, phase formation temperature, among others, will be some of the topics that will be analyzed. Finally, the structural and electrical properties of the PZT thick films as a function of the number of solution infiltrations will be highlighted.
