An Experimental Examination of Mems Microactuator Material Issues

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W. P. ROBBINS *, AND L. F. FRANCIS*

*Department of Chemical Engineering and Materials Science "**Deparment of Electrical Engineering University of Minnesota, Minneapolis, MN 55455-0132

ABSTRACT

Thermally induced interactions between materials in complex microactuator structures were investigated. The device structure contained a combination of a piezoelectric layer (lead zirconate titanate - PZT) an electrode with adhesion layer (Pt/Ti), buffer layer (Si0 2 or TiO2), structural material (polysilicon and/or silicon nitride), and sacrificial oxide (SiO 2). The presence of a SiO2 sacrificial layer did not affect either the bottom electrode or PZT layer. XRD results showed significant platinum and titanium silicide formation in the Pt/Ti electrode at 700 °C (PZT crystallization temperature) on both polysilicon and silicon nitride structural materials when no buffer layer was used. Auger analysis shows that the Ti adhesion layer oxidizes, that measured levels of silicon increase in the electrode zone, and that electrode elements diffuse into the structural material. Buffer layers of SiO 2 (0, 0.1, 0.73, 1.3, 1.5 tin) and amorphous TiO2 (0.065 pm) were inserted between the electrode and the structural material. XRD and sheet resistance measurements demonstrated that SiO 2 thicknesses greater than 0.73 pma reduced pyrochlore formation in the PZT and reduced the degradation of the electrode. However, this thickness was incompatible with overall surface micromachining processes. The TiO 2 layer effectively prevented pyrochlore formation and electrode degradation, while being compatible with overall actuator processing.

INTRODUCTION

Materials characterization is an important component in the effort to develop surfacemicromachined microactuators and PZT-based systems in general. A wealth of information has been presented detailing the effect of Si on Pt/Ti electrodes [1,2], the interaction of Ti with Sibased materials [3,4], and the effect of substrate and electrode materials on the microstructural development of PZT [5,6]. This information has been mainly oriented towards the characterization of material interactions and the development of ferroelectric random access memories (FRAM). Microactuator processing imposes limits on candidate structural materials, and the required response output of force or displacement limits the overall thicknesses.[7,8] The structural materials, silicon nitride (Si 3N4) and polysilicon, are fairly common materials in IC fabrication [9], but have not been studied as substrates for the deposition of sol-gel derived PZT. In this paper, the effects of silicon nitride and polysilicon structural materials, sacrificial oxide wells, and barrier layers for silicon diffusion on PZT and electrode quality are examined.

EXPERIMENTAL PROCEDURE

The multilayer substrates fabricated in this study were designed to mimic materials interactions expected in microactuators, as shown in Figure 1. Multilayer substrates were prepared with a range of thermally grown oxide layers (0, 0.1, 0.73, 1.3, 1.5 pm). The s