Structuring of Low Firing Temperature PZT Thick Films by Photolithography
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0928-GG09-18
Structuring of Low Firing Temperature PZT Thick Films by Photolithography Stefan Schimpf, and Bertram Schmidt IMOS, Otto-von-Guericke University, PO Box 4120, Magdeburg, Sachsen-Anhalt, 39016, Germany
ABSTRACT Structured piezoelectric PZT thick films were fabricated on silicon wafers. Different supplements, like lead oxide and glasses, were added to the PZT to enhance the sintering. The films were structured by doctor blading on an epoxy based photoresist mask. This allows very accurate micro patterns of high quality piezoelectric material. A firing process was developed to gain PZT structures with 200 µm in lateral size and a 200 µm pitch. The structures were electrically contacted, poled and characterized. INTRODUCTION PZT (Lead Zirconate Titanate) is a cost-efficient and readily available piezoelectric material. Generally it is used by sintering powder into macro-sized electroceramics. PZT thick films are a flexible and cost effective way to generate forces or torques that are needed to drive MEMS. The conventional process to deposit the thick film is screen printing [1]. The screen is virtually impossible to align, so the process proves unsuitable due to its low accuracy, low reproducibility and large feature sizes. Standard MEMS structures are usually defined by photolithography. To overcome the disadvantages of thick film technology in terms of alignment accuracy and reproducibility compared to photolithographic processes, a photo resist is used as a mask in the thick film process. Thus, the structure size and the alignment accuracy are limited by the particle size and the lithographic restrictions, only. This greatly helps to apply thick films in MEMS devices of all kinds. PZT has to be fired at temperatures higher than 1100°C to achieve sufficient sintering. Temperatures of more than 800°C lead to a degradation of the electrodes [2] and harm other structures or reduce their lifetime. To reduce the thermal load on the system sintering aids are used. Standard supplement is lead oxide [1]. Apart from being poisonous and harmful to the environment lead oxide leaks out of the PZT and interferes with other structures on the substrate[3]. To avoid lead oxide as a sintering promoting agent, glass powder is used. Glass properties can be adapted in a wide range. Low melting glasses are at 800°C, or at even lower temperature, soft enough to form the matrix for the PZT. Regardless the glass melt is too viscous to leak out of the thick film structure. Thereby, other structures on the substrate are not affected by the sintering of the thick film and the process is compatible to other MEMS processes.
EXPERIMENT
Fabrication process For the thick film manufacturing different PZT slurries were prepared. They consisted of 80% PZT (PIC 151 by PI Ceramic) and 20% sintering agent. Table I shows the used sintering aids. 15% terpineol (C10H18O) was added as vehicle to the powders. Table I. Sintering Aids Paste Nr.
Sintering Aid
Particle Size
Chemical Composition
Manufacturer
1
Lead Oxide
10 µ m
PbO
Chemos
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