DESIGN AND ANALYSIS OF MICROMECHANICAL SYSTEMS FOR MICRO ELECTROSTATIC ACTUATORS

Abstract

In order to meet the requirements of much faster response speed and more accurate positioning in hard disk system, micro electrostatic actuators (MESA) with interdigitated fingers (comb) structures and beam suspensions are focused on in this thesis. The conceptual design of MESA is given pertaining to the coupling of mechanical and electrostatic, design limitations and temperature variation effects. The present investigation on MESA is further sub-divided into three parts, namely, static analysis, dynamic analysis and residual stress analysis. Firstly, MESAs with straight beam and folded-beam suspensions are addressed in the static analysis. The straight-beam suspensions are not applicable for the hard disk system because of its non-linear character. The fold-beam suspensions are stressed in this thesis because of its advantage in linearity. Comparison of the results of the static beam model with those of Finite Element Method (FEM) indicates that the former is very simple and yet accurate enough. It is noted that the normal stress in the shin dominates in the members of suspensions. Secondly, the dynamics of micro electrostatic actuators is analyzed using the single-degree dynamic model. To get the shortest possible settle time, the ideal damping factor is around 0.8 which may be obtained by adjusting the feedback voltage. To shorten the time to reach the desired position, decreasing the value of the total mass of the actuator is an effective way, because its total mass greatly affects the time needed for the actuator to get to the desired position. However, its resistance to impact is reduced as a result of this. If the maximum amplitude of the displacement response is limited to within a small range, the ability of the system to withstand an impulse will be limited. An appreciable reduction in the amplitude of response is achieved by increasing the value of the damping factor. Finally, the numerical study of the redistribution or residual stresses in micro-actuators is confined to folded-beam suspensions during etching. The normal residual stress components are of much higher values than the shear components. The residual stress distribution along the folded-beam suspension is nearly constant except at the ends of the shin and the thigh. At the initial stage of etching, the normal residual stress components can actually surge to a high value, as if an additional load has been applied; this may be detrimental to the structural integrity of the folded-beam suspension. As etching progresses, the residual stresses diminish rapidly to a low value attributed to the release of constraint in the lateral deformation of the folded-beam suspension. Upon complete removal of the sacrificial layer, the values of the final residual stresses in the folded-beam suspension are grossly reduced.