Design and Fabrication of MEMS Piezoelectric Rotational Actuators
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Design and Fabrication of MEMS Piezoelectric Rotational Actuators Danny Gee, Wayne A. Churaman, Luke J. Currano, and Eugene Zakar U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783, U.S.A. ABSTRACT As Microelectromechanical Systems (MEMS) continue to mature and increase in design complexity, the need to exploit rotation in MEMS devices has become more apparent. An inplane piezoelectric rotational actuator is proposed that provides free deflections on the order of 1.5° with applied biases of less than 25V and nanoampere currents. Moments up to 6x10-8 N·m, corresponding to forces of 125 µN, were measured using MEMS cantilever springs. The actuator utilizes the low-power, high-force characteristics of lead zirconate titanate and a coupled, dual offset-beam design to provide effective rotational displacement. The resulting power consumption is three orders of magnitude less than current electrothermal rotational designs. INTRODUCTION The vast majority of MEMS actuators generate translational motion. A few rotational actuators have been documented in the development of micro-robotics, locomotives, and other biologically-inspired devices [1-3]. While piezoelectric materials are scrutinized for limited strain and fabrication complexities, they exhibit excellent characteristics that are attractive for MEMS actuators. Piezoelectric actuators have demonstrated wide bandwidths, high sensitivity, and large stroke forces [4]. Also with high power densities, piezoelectric actuators generally exhibit the most efficient transduction mode [5]. Lead zirconate titanate (PZT) is a well-studied piezoelectric material that is used in MEMS for its attractive thin film properties. Previous research in piezoelectric rotational actuation has been largely limited to optical microstages that operate with non-planar behavior [6]. As the longitudinal piezoelectric coefficient for PZT is nearly double the value of the transverse coefficient, it is convenient to develop out-of-plane actuators. This presents a unique challenge for applications that require inplane actuation. In this report, an in-plane, piezoelectric MEMS rotational actuator is proposed. The underlying operation of the rotational actuator is presented and the process sequence of the device fabrication is detailed. Preliminary results of the actuator are given and the defects associated with device fabrication are examined within this paper. THEORY Operational Concept The premise behind the rotational actuator is similar to a previously designed electrothermal actuator [7]. The actuator is composed of two parallel, yet offset beams that are connected to a free perpendicular beam, which is shown in Figure 1. A released cantilever is also integrated into the device to serve as a resisting spring to the actuator movement.
Figure 1. Labeled image of a fabricated rotational actuator.
The two offset beams are the actuation beams where deformation occurs. A piezoelectric stack, consisting of a PZT film sandwiched between a top and bottom metal electrod
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