Design and optimisation of high performance resonating micro-scanners through a multiphysics investigation

Abstract

Resonating micro-scanners based on MEMS fabrication technologies have been extensively implemented in laser beam scanning systems for micro-display and imaging applications. The optical resolution obtained from laser beam scanning micro-mirrors for micro-display applications is dependent on the mirror size, the scanning frequency and scan angle amplitude. However, other factors need to be considered in the design of such devices, namely, the structural strength, dynamic deformation and the scanning efficiency. Understanding the structural, fluidic and electrostatic characteristics of resonant micro-scanners is crucial in order to maximize the optical performance of such devices. In this dissertation, the multi-physical domains governing the micro-scanner operation will be evaluated using experimentally-validated computational fluid dynamics and finite element models. The numerical simulation models presented will be utilised (i) to investigate the validity of a number of analytical equations found in literature, (ii) to propose analytical equations with improved accuracy and range of validity for microscanner performance predictions and (iii) to develop and optimize novel micro-scanner designs. While micro-scanner designs with angular vertical comb actuation are the focus in this work, a number of research outcomes are also applicable to high frequency resonating micro-scanners employing other actuation methods. A detailed investigation is performed on the two principal limiting factors in micro-scanners for high performance applications: air damping and dynamic deformation. Three-dimensional transient Navier–Stokes simulations are performed to analyse the complex air flow interactions of a high frequency scanning micro-mirror. The damping effects due to device thickness and the depth of the mirror cavity are also evaluated. It is shown that analytical damping models which are applicable to resonant MEMS devices, are not valid within the operating range of high performance micro-scanners. On the other hand good agreement in the overall quality factor is achieved between Navier-Stokes simulation and measurement results. The inertia loading acting on the mirror plate, as the micro-scanner oscillates in out-ofplane rotation, results in the dynamic deformation of the mirror surface. A detailed analysis is presented on the micro-mirror design aspects contributing to dynamic deformation. An improved analytical equation for dynamic deformation predictions is proposed, which takes into consideration the two-dimensional mirror plate twist. A comparison among a number of layout designs was carried out with the aim of improving the dynamic deformation distribution on the mirror surface. A significant improvement in dynamic deformation was achieved with the inclusion of a gimbal-type structure between the mirror plate and the torsional beams. A design optimization scheme based on numerical simulations and meta-modelling methodologies is introduced. The minimization of dynamic deformation is considered as one of the design objectives, which include output parameters related to micro-mirror optical performance, structural reliability and gas damping characteristics. The design optimization scheme was implemented in order to develop three high performance resonating micro-scanner prototypes using the SOIMUMPs MPW process. The simulations and measurements presented in this dissertation demonstrate that the optical performance of resonating micro-scanners fabricated from a single device layer can be maximized by incorporating the indirect-drive configuration and a gimbal-type mirror support structure.