Micro-Mechanical Switch Array for Meso-Scale Actuation

Traditional MEMS actuators have limited stroke and force characteristics. This paper describes the development of a hybrid actuation solution, which utilizes a micro-machined actuator array to provide switching of mechanical motion of a larger mesoscale piezo-electric actuator. One motivating application of this technology is the development of a tactile display, where discrete mechanical actuators apply vibratory excitation at discrete locations on the skin. Specifically, this paper describes the development fabrication and characterization of a 4 x 5 micro-actuator array of individual vibrating pixels for fingertip tactile communication. The individual pixels are turned ON and OFF by pairs of microscopic thermal actuators, while the main vibration is generated by a vibrating piezo-electric plate. The fabrication sequence and the actuation performance of the array are also presented. INTRODUCTION Mechanical actuators are one of the defining features of micro-electromechanical systems (MEMS). Yet due to the increased role of surface forces most moveable structures are free standing attached to the substrate through an elastic hinge. This feature of MEMS presents difficulties in designing actuators with large displacements, since a large displacement would imply large elastic deformation of the hinge and correspondingly significant driving force. Contrary, macroscopic actuators utilize bearings of various kinds in which the frictional forces are negligible in comparison to the driving and inertial body forces. Thus in the field of mechanical micro-actuation large stroke and force is still a significant engineering challenge. The development of thermal micro-actuators for the first time presented a hope for a device with large stroke under reasonable driving voltages. Using poly-crystalline thermal actuators relatively large displacements were easily attainable [1,2]. While these devices had the advantage of ease of integration with on-chip electronics and relatively low power consumption (86mW, [3]), the polysilicon layers were only a few microns thick and brittle, thus limiting the usefulness of the actuator. These limitations have been partially addressed by the advent of actuators made from selectively doped single-crystal silicon [4-5] as well as entirely metallic actuators [6]. Further improvements of the efficiency and available force were achieved by designing V-shaped (also known as “bent-beam”) actuators [7], where the role of the cold arm is performed by the substrate itself. In comparison with the (poly)silicon devices, metallic micro thermal actuators have a larger thermal expansion coefficient and can undergo larger deformations without fracture. Yet due to their higher reactivity and lower melting point the maximum operating temperature is limited to 300-400°C thus reducing the benefit of larger thermal-expansion coefficient. Further in order to develop good quality lateral actuator a high out-of-place stiffness is needed. This can be achieved with a technique known as LIGA [8], where x-ray lithography is used to develop deep trenches in PMMA substrates, later to be filled with metal via electro deposition. The limited access and high cost of this process soon resulted in the development of alternative processes using thick photoresist films. In this paper we describe the development of thermal micro-actuators formed in photoresist mold, which are later released via sacrificial metal seed layer etch. The specific application of these devices is a tactile display, where the actuators perform mechanical switching function [9]. I. DESIGN OF THE THREMAL MICRO-ACTUATOR ARRAY Vibro-tactile displays are devices utilizing an array of vibrating points in order to stimulate the tactile sensors (Pacianian corpuscles) in the skin and create sensation of touch [10-12]. The required frequency, stroke and force for such stimulation was previously tested and the results are listed in Table 1. As evident from the table the required force and stroke of the actuators is significantly larger than the typical values for MEMS devices. To achieve these requirements we have developed a hybrid solution: (1) meso-scale vibrating plate driven by a set of piezo-actuators is coupled with (2) an array of MEMS mechanical switches (clutches), which re-direct the vibrations of the plate to the individual protruding pins (pixels). The switching of individual pixels is provided by the array of thermal actuators displacing laterally a lock mechanism as illustrated in Figure 1. The MEMS array is mounted on top of the vibrating plate assembly as illustrated in Figure 2. The pixels are turned ON and OFF by pairs of micro-actuators, while the main vibration is generated by a meso-scale vibrating plate as illustrated in Figure 2(left). A 3D close-up view of one such thermal actuator is shown in Figure 2 (right) indicating the need for a second layer of metal rising above the surface of the actuator (arrow). 1 Author to whom the correspondence should be addressed ([email protected]) Table 1. Settings on Solenoid Tactile Illusion-Producing Device