Fabrication and Characterization of Micromachined Piezoelectric T-Beam Actuators

This paper presents a monolithically fabricated microelectromechanical piezoelectric cantilever beam with a T-shaped cross section capable of in-plane and out-of-plane displacements and sensing. High-aspect-ratio T-beams are achieved by direct micromachining of bulk lead zirconate titanate (PZT-4) via reactive ion etching of 65- μm-deep features. Electrodes deposited on the top and bottom web and flange regions of the T-shaped structure allow in-plane and out-of-plane motion actuation and sensing. The T-beam structures were tested for in-plane and out-of-plane tip displacements, out-of-plane blocking force, and impedance response. These results are explained using analytical models that predict static deflection, blocking force, and resonance frequency. Nine prototype micromachined T-beams are fabricated that achieve up to 129 μm of out-of-plane displacement, 11.6 μm of in-plane displacement, and 700 μN of out-of-plane blocking force.

I. Introduction

Micromachined piezoelectric unimorph and bimorph structures have been used in many applications such as out-of-plane actuators [1], deformable mirrors [2], RF switches [3], accelerometers and gyroscopes [4], [5], microrobotic legs [6], and microair vehicle wing actuators [7]. Lead zirconate titanate (PZT) has been the most popular piezoelectric material in microelectromechanical systems (MEMS) applications because of its large piezoelectric and electromechanical coupling coefficient, as well as compatibility with micromachining processes [8]–[12]. RF sputtering [13], metal—organic chemical vapor deposition [14], and sol—gel techniques [3] have been used to deposit PZT thin films, usually less than 3 thick, whereas screen printing [15], [16] and spray coating [17] techniques have been used to deposit PZT thick films from 20 to 200 thick. Ideally, thin and thick PZT films should exhibit the same piezoelectric properties as bulk PZT. However, in practice, the properties of thin films are often degraded due to the following: 1) high sensitivity to stoichiometry, grain size, thickness, and orientation [18]; and 2) the clamping effect of the underlying substrate, which reduces the strain produced by an electric field. In general, deposited PZT film properties are inferior to bulk PZT properties, except for the very few cases in which thin PZT films have been realized via templated growth [19]. Further challenges in successfully designing and fabricating MEMS structures using PZT films arise from unwanted curvature due to residual film stress and/or thermal expansion coefficient mismatch [16], [20]. For certain applications, such as RF switches and optical devices, obtaining cantilevers with tight flatness tolerance is critical. In principle, it is possible to account for the expected curvature during the design process; however, such compensation is premised on precise knowledge of material parameters [21], which are difficult to reproduce in practice. A more robust method of creating flat cantilevers is by increasing the substrate (passive layer) thickness to withstand the residual stress at the expense of reduced tip displacement [17]. This, however, leads to suboptimal sensor and actuator designs for the sake of a flat structure.