Wyatt O. Davis

MEMS-based pico projector display

A pico-projector based on scanned 3-color laser light has been developed. The scanning element is a dual-axis MEMS scanning mirror that produces WVGA display resolution. The laser light sources are red and blue laser diodes and a second harmonic green laser. Use of a MEMS and laser light sources leads to a small volume and a thickness of 7mm, enabling the use of the projector system as a component embedded into portable consumer electronics.

Comb-Actuated Resonant Torsional Microscanner With Mechanical Amplification

A comb-actuated torsional microscanner is developed for high-resolution laser-scanning display systems. Typical torsional comb-drive scanners have fingers placed around the perimeter of the scanning mirror. In contrast, the structure in this paper uses cascaded frames, where the comb fingers are placed on an outer drive frame, and the motion is transferred to the inner mirror frame with a mechanical gain. The structure works only in resonant mode without requiring any offset in the comb fingers, keeping the silicon-on-insulator-based process quite simple. The design intent is to improve actuator efficiency by removing the high-drag fingers from the high-velocity scanning mirror. Placing them on the lower velocity drive frame reduces their contribution to the damping torque. Furthermore, placement on the drive frame allows an increase of the number of fingers and their capacity to impart torque. The microscanner exhibits a parametric response, and as such, the maximum deflection is found when actuated at twice its natural frequency. Analytical formulas are given for the coupled-mode equations and frame deflections. A simple formula is derived for the mechanical-gain factor. For a 1-mm × 1.5-mm oblong scanning mirror, a 76° total optical scan angle is achieved at 21.8 kHz with 196-V peak-to-peak excitation voltages.

Vibration mode frequency formulae for micromechanical scanners

A torsional scan mirror suspended with two flexure beams can be used in various display, imaging and other scanning applications. Using various mirror shapes and flexure dimensions as parameters, a set of analytical formulae is presented to predict the natural frequency of the first five vibration modes, which are torsion, in-plane and out-of-plane sliding modes and in-plane and out-of-plane rocking modes. Mode frequencies are compared with the finite element model (FEM) predictions using ANSYS™ for a wide range of flexure beam dimensions. The formulae include the effective inertia of the flexure beams and orthotropic material anisotropy effects. The analytical formulae are verified for both isotropic (e.g. steel) and orthotropic (e.g. silicon) materials. These formulae work very well when the Euler–Bernoulli beam theory assumptions and the rigid mirror assumption are satisfied. The accuracy of analytical predictions is improved by introducing an empirical correction factor to the analytical predictions using non-dimensional flexure beam ratios. The correction factor reduces the error between analytical formulae and FEM predictions to within a few per cent for all five modes for a large range of flexure dimensions. FEM predictions and analytical formulae are partly verified by experimental results.

Resonant PZT MEMS Scanner for High-Resolution Displays

A resonant piezoelectric scanner is developed for high-resolution laser-scanning displays. A novel actuation scheme combines the principle of mechanical amplification with lead zirconate titanate (PZT) thin-film actuation. Sinusoidal actuation with 24 V at the mechanical resonance frequency of 40 kHz provides an optical scan angle of 38.5° for the 1.4-mm-wide mirror. This scanner is a significant step toward achieving full-high-definition resolution (1920 × 1080 pixels) in mobile laser projectors without the use of vacuum packaging. The reported piezoscanner requires no bulky components and consumes <; 30-mW power at maximum deflection, thus providing significant power and size advantages, compared with reported electromagnetic and electrostatic scanners. Interferometry measurements show that the dynamic deformation is at acceptable levels for a large fraction of the mirror and can be improved further for diffraction-limited performance at full resolution. A design variation with a segmented electrode pair illustrated that reliable angle sensing can be achieved with PZT for closed-loop control of the scanner.

Optical and mechanical performance of a novel magnetically actuated MEMS-based optical switch

A novel magnetically actuated 8/spl times/8-port MEMS-based fiber-optic switch is described. Fiber-to-fiber insertion loss measurements of six 8/spl times/8 switch units show average and worst-case insertion loss of 1.3 dB and 2 dB, respectively. Low insertion loss is achieved through a unique MEMS design that uses anisotropically etched single-crystal silicon sidewalls to provide a global mechanical alignment stop for an array of MEMS mirrors. This alignment surface produces a uniform and repeatable mirror angle across the mirror array. Mirror misalignment is attributed to the surface roughness of the silicon sidewalls. Repeated interferometric measurements of the mirrors of 24 8/spl times/8 switch units show repeatability of the mirror angle of 3/spl times/10/sup -3/ degrees, while the uniformity of the mirror angle across the MEMS array is 2/spl times/10/sup -2/ degrees, in agreement with the angular error predicted from measurements of sidewall surface roughness. In turn, the average repeatability and uniformity of the insertion loss are 0.01 dB and 1 dB, respectively, in agreement with predictions based on the interferometric measurements. Finally, the unique dynamics of the magnetic actuation and electrostatic addressing scheme are described. Measurements show that fast switching can be achieved by driving the mirrors with a magnetic pulse that is faster than the mechanical resonant frequency of the mirror, relying on an electrostatic clamping force to capture the mirror as it overshoots the magnetic field angle. This actuation scheme is shown to result in switching times of 8.5 ms to 13.5 ms, but requires accurate control of the kinetic energy of the mirror.

High frequency torsional MEMS scanner for displays

A high frequency resonant torsional microscanner actuated with thin film PZT is modeled, fabricated, and characterized. Sinusoidal actuation with 24 V at a mechanical resonance frequency of 39870 Hz provides a total optical scan angle of 38.5 deg. for the 1.4 mm wide mirror. It provides significant power and size advantages compared to electromagnetically and electrostatically actuated scanners. This scanner is a significant step towards achieving full HD resolution with mobile laser projectors.

Measuring Quality Factor From a Nonlinear Frequency Response With Jump Discontinuities

The convenient half-power bandwidth formula used for measurement of quality factor Q does not apply for nonlinear systems that have jump discontinuities in their frequency responses, since one of the half-power amplitudes is not observable. This paper shows alternatives to the half-power formula that do apply to such nonlinear systems, while preserving all of the convenience of the method. Their practical use is illustrated by experimental Q measurements for a microelectromechanical systems scanning mirror.

High-performance silicon scanning mirror for laser printing

This paper describes the design, fabrication, and characterization of the first MEMS scanning mirror with performance matching the polygon mirrors currently used for high-speed consumer laser printing. It has reflector dimensions of 8mm X 0.75mm, and achieves 80o total optical scan angle at an oscillation frequency of 5kHz. This performance enables the placement of approximately 14,000 individually resolvable dots per line at a rate of 10,000 lines per second, a record-setting speed and resolution combination for a MEMS scanner. The scanning mirror is formed in a simple microfabrication process by gold reflector deposition and patterning, and through-wafer deep reactive-ion etching. The scanner is actuated by off-the-shelf piezo-ceramic stacks mounted to the silicon structure in a steel package. Device characteristics predicted by a mathematical model are compared to measurements.

Bi-resonant scanning mirror with piezoresistive position sensor for WVGA laser projection systems

Fraunhofer IPMS developed a new type of small-sized scanning mirror for Laser projection systems in mobile applications. The device consists of a single crystal mirror plate of 1 mm diameter in a gimbal mounting enabling a bi-resonant oscillation of both axes at a resonance frequency of about 100 Hz and 27 kHz respectively. The mechanical scan angle (MSA) achieved is ± 7° for the slow and ± 12° for the fast axis. The mirror angle position and phase can be read out via two piezo-resistive sensors located at the torsion axes. In order to allow for a minimum device size of the resonantly driven slow axis the sensor of the inner fast axis was connected by a new kind of thin silicon conductors. Those are created by means of an etch stop in TMAH etch and kept as thin as possible in order to reduce their contribution to the mechanical stiffness of the mirror-supporting structures. This new system enables to lead six (or even more) independent electrical potentials onto the moving parts of the device, whereas the mechanical properties are mainly determined by only 2 torsion axes. The devices were subsequently characterized and tested. Technology details, simulation results, pictures of the device and the new conductor structures as well as measurement results are presented.

A high-frequency comb-actuated resonant MEMS scanner for microdisplays

A high-frequency novel torsional MEMS scanner is developed for high resolution microdisplays employing a multi-frame geometry. For the torsional mirror, 26.7° and 36.1° total-optical-scan-angle are achieved at resonance, at atmospheric pressure and vacuum respectively.