Russell Farrugia

Capacitance measurement techniques in MOEMS angular vertical comb-drive actuators

This paper presents three novel techniques for the measurement of capacitance variation in high-frequency operation of angular comb-drive structures, with applications including optical projection micro-mirrors. The first technique uses a lock-in amplifier in order to measure the impedance of the comb structure by superimposing a test signal having a frequency that is at least one order of magnitude higher than the micro-mirror resonant frequency. The second measurement setup uses a digitizing oscilloscope to measure the capacitance from the instantaneous voltage and current in the comb structure. The third setup consists of a micro probe controlled by a high-resolution positioner in order to tilt the mirror to a desired angle. This allows for static capacitance measurement at different mirror deflection using a high precision LCR meter. The measurement results from the different setups were found to be in agreement with each other. The results were further verified against analytical and numerical modelling.

Design optimization of a high frequency resonating micro-mirror with low dynamic deformation

One of the main design limitations of resonant micro-mirrors intended for visual projection display applications is inertia-driven dynamic deformation. Micro-mirrors used for high frequency (20–30 kHz) laser beam scanning are typically operated at resonance in the region of their torsional modal frequency in order to achieve high scan angles. Although the optical resolution of the projected image is defined by the micro-mirror frequency, maximum scan angle and dimensions, significant dynamic deformation (> 1/10 of the incident wavelength) results in a loss in contrast between adjacent projected spots. This paper presents a structural design optimization scheme for a one directional resonant micro-mirror intended for laser projection with XGA optical resolution. The minimization of dynamic deformation is considered as one of the partial objectives together with other micro-mirror performance and reliability characteristics. The optimization scheme is performed using the response surface method and multi-objective genetic algorithms. This design process demonstrates the technical feasibility of including features, such as a gimbal structure, that improve the dynamic mirror flatness without compromising on the target scanning frequency, mode separation and maximum shear stress.

Analysis of dynamic deformation in 1-D resonating micromirrors

Dynamic deformation is an important characteristic in high frequency resonating microscanners intended for high definition raster scanning display applications. Out-of-plane deformation resulting from high acceleration loads causes beam divergence which will in turn reduce the optical resolution. This paper presents a detailed analysis on the mechanical design aspects contributing to dynamic deformation such as the micromirror layout and the micromirror-spring linkage design. Applicability of the one dimensional plate theory in evaluating micromirror bending due to inertial loads is investigated using finite element analysis. A comparison among a number of layout designs was carried out with the aim of increasing micromirror bending stiffness in a direction parallel to the axis of rotation. Moreover spring-linkage effects were also addressed and significant improvement in the dynamic deformation was achieved by the inclusion of a gimbal structure between the micromirror and the torsional springs.

Analytical modeling of an electrostatically actuated torsional micromirror

This paper presents an analytical model for an electrostatically actuated torsional micromirror having both a yoke layer, and a mirror layer. In particular, a relationship between the actuation voltage and the angle of rotation of the micromirror was derived from first principles. This model was then verified by means of finite element analysis and it was noted that there is a good agreement between the analytical and the simulated results. This model was used to study the behaviour of a micromirror designed using a semi-custom fabrication process featuring dimensions of 10 μm× 10 μm and an angle of rotation of 20°. In addition, a mathematical model representing the complete second-order system including damping as well as electrostatic actuation is also presented and verified on Matlab.

Performance analysis of an RF MEMS TPoS resonator using FE modelling

This paper presents an analysis, using finite element modelling, of a 40 MHz contour-mode RF MEMS resonator based on the SINTEF thin-film piezoelectric-onsubstrate (TPoS) fabrication technology. Novel simulation techniques are proposed which enable an exhaustive analysis of the relevant resonator characteristics including temperature drift and quality factor. This model also enables the analysis of possible tuning techniques which can be used to compensate for temperature and process-induced variations in the resonant frequency. The effect of the thickness ratio of the PZT film to the substrate on the resonant frequency and frequency shift is also discussed.

Investigation of warpage in wafer-level Molding: Measurements and FE analysis

Novel 3D packaging technologies which require large area mold embedding are being developed in order to achieve further minimization and cost reductions. Compression molding using epoxy molding compounds is one technique being considered for wafer-level encapsulation. However significant warpage in molded wafers is a critical issue which may hinder successive processes from being carried out. Cases of both symmetric (spherical) and asymmetric (cylindrical)-shaped warpage have been reported in wafer-level compression molding trials on blank wafers. This paper presents finite element models of the molded wafer, with and without embedded dies, which take into account the observed complex multi-state warpage characteristics. Molded wafer warpage measurements were carried out in order verify the applicability of the small and large deformation theories for layered plates, to deduce the cure shrinkage molding compound properties and to validate the finite element model of the molded blank wafer. The latter was used to analyze possible factors (nonplanar mold layer thickness, anisotropic wafer elastic properties) leading to asymmetric warpage. The numerical model will thus enable the prediction of the optimum process and material conditions for the warpage to be minimized together with the expected deformation of the molded wafer model with embedded dies.

Warpage issues in large area mould embedding technologies

The need for higher communications speed, heterogeneous integration and further miniaturisation have increased demand in developing new 3D integrated packaging technologies which include wafer-level moulding and chip-to-wafer interconnections. Wafer-level moulding refers to the embedding of multiple chips or heterogeneous systems on the wafer scale. This can be achieved through a relatively new technology consisting of thermal compression moulding of granular or liquid epoxy moulding compounds. Experimental measurements from compression moulding on 8” blank wafers have shown an unexpected tendency to warp into a cylindrical-shape following cooling from the moulding temperature to room temperature. Wafer warpage occurs primarily as a result of a mismatch between the coefficient of thermal expansion of the resin compound and the Si wafer. This paper will delve into possible causes of such asymmetric warpage related to mould, dimensional and material characteristics using finite element (FE) software (ANSYS Mechanical). The FE model of the resin on wafer deposition will be validated against the measurement results and will be used to deduce appropriate guidelines for low warpage wafer encapsulation.