Characterization of PZT thick films fired on LTCC substrates

Abstract

Ferroelectric ceramic materials based on solid solutions of Pb(Zr,Ti)O3 (PZT) are used in the electronics industry for sensors and actuators and for electromechanical transducers, to name just a few examples. Thick-film technology, i.e., the deposition of thick-film pastes by screen printing, primarily on alumina substrates, is a relatively simple and convenient method to produce layers with a thickness up to 100 μm. The characteristics of thick-film ferroelectrics are similar to those of bulk materials [1–4]. Low-temperature co-fired ceramics (LTCC) materials, which are sintered at the low temperatures typically used for thick-film processing, i.e., around 850 ◦C, are based either on crystallizable glass [5, 6] or a mixture of glass and ceramics, for example, alumina, silica or cordierite (Mg2Al4Si5O18) [7, 8]. Jones et al. have presented a comparison of the mechanical and chemical characteristics of both green and fired LTCC tapes from different suppliers in [9]. Ceramic multi-chip modules (MCM-C) are multilayer substrates with buried conductor lines. An additional contribution to the smaller size and the higher density of MCM-C is the ability to integrate screenprinted resistors, or sometimes capacitors and inductors. These screen-printed components can be placed either beneath the discrete components on the surface of the multilayer dielectric or buried within the multilayer structure. For an overview of passive integrated components in MCM see, for example [10]. For some applications, for example integrated sensors or micro-actuators, PZT thick-films on LTCC that are sintered at relatively low temperatures (around 850 ◦C) comparable with LTCC’s firing temperatures, would be of interest [11, 12]. The aim of this work was to study the compatibility between LTCC and screenprinted PZT as well as the electrical characteristics of the PZT layer. PZT 53/47 powder (PbZr0.53Ti0.47O3) with an excess 6 mol% of PbO was prepared by mixed-oxide synthesis at 900 ◦C for 1 h from high-purity PbO (litharge) 99.9% (Fluka), ZrO2 99% (Tosoh), and TiO2 99% (Fluka). To this was added 2 wt% of lead germanate, with the composition Pb5Ge3O11 (melting point 738 ◦C) as a sintering aid. Lead germanate (PGO) was also prepared by mixed-oxide synthesis from PbO and GeO2 99% (Ventron) at 700 ◦C. After synthesis, both compositions were ball milled in acetone for 1 h and dried. A thick-film paste was prepared from the PZT (2% PGO) and an organic vehicle (ethyl cellulose, alpha-terpineol and butil carbitol acetate) by mixing on a three roll mill. The green LTCC 951 tape (Du Pont) and alumina ceramics were used for substrates. The thick-film structure was prepared by first printing gold film (Remex 3243) and then the PZT film. The PZT film was printed 6 times with intermediate drying. The gold and PZT layers were cofired at 850 ◦C for 8 h in a closed alumina crucible. The thickness of the PZT films after the thermal treatment was around 50 μm. The green and fired Du Pont LTCC 951 tapes were analyzed by X-ray diffraction (XRD) analysis with a Philips PW 1710 X-ray diffractometer using Cu Kα radiation. X-ray spectra were measured from 2 = 20 ◦ to 2 = 70 ◦ in steps of 0.04 ◦. X-ray spectra are shown in Fig. 1. The unfired material is a mixture of alumina and glass. After firing at 850 ◦C peaks of anorthite ((Na,Ca)(Al,Si)4O8) phase appear. The peaks of alumina and anorthite are denoted by “A” and asterisk, respectively. For the electrical measurements gold electrodes were sputtered onto the PZT films. The values of the remanent polarization and the coercive field were determined from ferroelectric hysteresis curves measured with an Aixact TF Analyzer 2000 at 50 Hz. The real and imaginary parts of the complex dielectric constant were measured with an HP 4284 A Precision LCR Meter at 1 kHz. In Table I the electrical parameters, i.e., remanent polarization Pr, coercive field Ec, dielectric constant e′ and dielectric loss tan δ, of the co-fired LTCC/Au/PZT structure are presented. The electrical characteristics of this structure are compared to the characteristics of a similar structure printed on alumina substrates [13]. Hysteresis loops of the PZT films on the alumina and LTCC substrates are shown in Fig. 2.