Simple and Low Cost Patterning Process for Sputtered Pb(Zr,Ti)O₃ Thin Films and Electrodes for Membrane-Based Microsystems Applications

Abstract

Micromachined silicon membranes actuated by a piezoelectric thin film are of great interest for microfluidic or ultrasonic applications. In this paper, we present a simple and low cost patterning process for sputtered Ti and Pt electrodes and Pb(Zr,Ti)O 3 thin films using lift-off and a wet HCl etching methods respectively. We have showed by finite element analysis the incentive to pattern electrodes in order to get higher actuation performances. Moreover, in order to reduce the stress on the diaphragm, the PZT film has been also patterned. We propose an optimized flow chart taking into account the different material properties in the multi-layer stack constituting the actuator and the need of a PZT crystallization annealing. To show the influence of the actuator layout, finite element models of a 20 µm thick, 3 x 3 mm 2 silicon membrane with variable electrode design, actuated by a 1 µm PZT (10 V bias) thin film have been developed. We found a maximum deflection for a 1.7 mm side length square electrode located at the membrane center. Sputtering was achieved on a non-heated silicon substrate, in pure Argon, using a stoichiometric PZT target. The metallic electrodes have been structured by standard liftoff technique. Different concentrations of HCl [4 % -35 %] with temperature up to 90°C have been tested for PZT patterning. Well defined PZT microstructures were better achieved by wet etching of unannealed PZT film on SiO 2 or Pt (HCl 35%, room temperature). The etching rate was 40 nm.s -1 . The presence of the perovskyte phase in the PZT films after a 700°C, 30 s RTA in air, has been checked by X-ray measurements. Finally, the silicon membranes have been realized by anisotropic chemical etching in an aqueous KOH (40 %, 60°C) solution. Membrane deflection under low bias have been successfully tested.