Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 1649–1655 Improvement of composite sol–gel process for manufacturing 40 m piezoelectric thick films A. Bardaine a,∗ , P. Boy a , P. Belleville a , O. Acher a , F. Levassort b a Laboratoire Sol–Gel, CEA/Le Ripault, BP 16, 37260 Monts, France b Universit´ e Fran¸ cois Rabelais, LUSSI, 10 boulevard Tonnell´ e, BP 3223, 37032 Tours Cedex 1, France Received 28 May 2007; received in revised form 20 October 2007; accepted 26 October 2007 Available online 15 January 2008 Abstract In this study, high performance piezoelectric thick films were prepared by depositing only a few sol–gel infiltrated composite layers on Si/SiO 2 /Ti/Pt wafers. Lead zirconate titanate (PZT) powder, binder and infiltration sol were synthesised from the same-laboratory made time-stable polymeric PZT sol. Adding coarse powder (50 wt%) to the sol precursor, crack-free coatings were prepared by dip coating in the 30–50 m thickness range in only 3–6 layers. The results showed that good performance can be achieved by controlling sol concentration, powder charge, composite sol ageing, number of infiltrations and heat treatments. The initial results showed that only a limited number of infiltrations are needed in the final stack to obtain high piezoelectric performance. For instance, a 40 m thick film was prepared with 6 composite layers with a 0.7 M PZT sol charged with 50 wt% PZT powder after 2 months’ ageing. Poling was performed for 5 min under 15 V/m in an oil bath at 170 ◦ C after 5 infiltrations and final annealing at 700 ◦ C for 10min in RTA. Despite substrate clamping, this coating reached an effective thickness coupling factor up of to 39% at a resonance frequency of 30 MHz. This result was obtained with a non-doped PZT powder. © 2007 Elsevier Ltd. All rights reserved. Keywords: Sol–gel; Composite; Piezoelectric properties; PZT; Ultrasonic transducers 1. Introduction Ferroelectric lead zirconate titanate (PZT) is widely used for piezoelectric device applications such as MEMS, actuators and ultrasound transducers. In medical imaging, the exploration of superficial tissues in fields such as dermatology, ophthal- mology and microsurgery requires the development of high frequency (40–50 MHz) ultrasound probes. Piezoelectric poly- mers or copolymers such as PVDF and P(VDF-TrFE) have limited electromechanical performance (k t ∼25%) and lead- based ceramics (k t ∼50%) have also been widely investigated. However, for these materials, working at high frequency requires the preparation of films of few tens of microns thick (for exam- ple around 40 m for a resonance frequency of 30MHz). To deposit films with such high piezoelectric performance, sev- eral processes such as lapping, 1 tape casting, 2 screen printing, 3 sol–gel 4 and inkjet printing 5 have been studied. Sol–gel chem- ∗ Corresponding author. E-mail addresses: anthony.bardaine@cea.fr (A. Bardaine), philippe.boy@cea.fr (P. Boy). istry has certain advantages among these techniques such as low processing temperatures, low cost equipment, easy stoichiome- try and film thickness control. Furthermore, sol–gel chemistry is very appropriate for deposition onto complex shape substrates. PZT sol–gel films, first studied by Fukushima et al., 6 were devel- oped as thin films for various applications such as non-volatile memories, 7 capacitors 8 and MEMS. 9 To increase film thickness, a sol–gel route using a charged sol, also called a composite sol, was first developed by Barrow et al. 4,10 Using the spin-coating technique, 5–200 m thick films were prepared with good piezo- electric properties. The best reported results were obtained using 1–2 m thick single layers, since authors reported that increasing the single layer thickness leads to a degradation of film den- sity and piezoelectric properties. 11 Other studies showed that coating infiltrations with a PZT sol improved both densification and surface roughness and led to enhancement of the piezoelec- tric coefficient d 33 . 12,13 Infiltration is usually performed after deposition of each composite layer. 14 In the study reported here, the sol–gel composite process was used to prepare dip-coated PZT thick films deposited on Si/SiO 2 /Ti/Pt wafers. The piezoelectric and electromechani- cal properties were mainly studied by measuring the effective 0955-2219/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2007.10.014