Sven Holmstrom

MEMS Laser Scanners: A Review

Laser scanners have been an integral part of MEMS research for more than three decades. During the last decade, miniaturized projection displays and various medical-imaging applications became the main driver for progress in MEMS laser scanners. Portable and truly miniaturized projectors became possible with the availability of red, green, and blue diode lasers during the past few years. Inherent traits of the laser scanning technology, such as the very large color gamut, scalability to higher resolutions within the same footprint, and capability of producing an always-in-focus image render it a very viable competitor in mobile projection. Here, we review the requirements on MEMS laser scanners for the demanding display applications, performance levels of the best scanners in the published literature, and the advantages and disadvantages of electrostatic, electromagnetic, piezoelectric, and mechanically coupled actuation principles. Resonant high-frequency scanners, low-frequency linear scanners, and 2-D scanners are included in this review.

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.

Electromagnetically Actuated FR4 Scanners

A torsional micromechanical scanner is fabricated from thin fire resistant 4 substrates using standard printed circuit board technology. The top and the bottom copper layers are connected with vias and shaped as a single coil to enable one- and two-dimensional electromagnetic actuation with an external magnet. Using 5 mm times 5 mm mirrors, the following scan angle and resonant frequency combinations are achieved: 17deg at 1.8 kHz and 140deg at 417 Hz. Another 10times improvement in magnetic actuation torque seems feasible. The technology offers a unique advantage by allowing a high degree of integration with microoptics and electronics directly on the mechanical platform and offers a low-cost alternative to silicon microelectromechanical systems devices particularly when large or low-frequency devices are required.

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.

NiFe Plated Biaxial MEMS Scanner for 2-D Imaging

A two-axis microelectromechanical systems micromirror actuator is developed for retinal scanning display and imaging applications. The device operation makes use of magnetostatic torque produced by the combination of a flux generating custom-made high-frequency electrocoil and the NiFe layer deposited on the movable part. Modeling of the actuation in the magnetic domain, as well as the experimental characterization of the mechanical part is described. The device is capable of full optical scan angles of 88deg (at 100-mA root-mean-square coil current) and 1.8deg for slow and fast-scan directions, respectively. In combination with a mirror size of 1.5 mm, resulting thetasoptmiddotD products are 132degmiddotmm and 2.7degmiddotmm for slow and fast axis, respectively. Atmospheric operation of the device is enabled due to high mechanical quality factors of the order of 3000

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.

Mechanically coupled comb drive MEMS stages

An electrostatic large clear-aperture in-plane scanner with a novel actuation principle is presented for fast and large stroke scanning applications 9 mum resonant deflection at 11.51 KHz with 100 Vpp excitation is observed.

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.

Two-Dimensional MEMS Stage Integrated With Microlens Arrays for Laser Beam Steering

A novel microelectromechanical stage with one uniaxial set of combs capable of 2-D actuation is presented. A polymer microlens array (MLA) is mounted vertically onto the stage. Driven at resonance, the stage deflects 124 μm out of plane and 34 μm in plane. Finally, laser beam steering is demonstrated using two cascaded MLAs.

FR-4 as a New MOEMS Platform

FR4 is a well-engineered material and widely used in the PCB industry and lend itself to high degree of integration of optoelectronic, micro-optic, and electronic devices. FR4 is used for the first time as an actuated mechanical device that integrates several functions on the same device. Two different approaches to 2D laser scanning using a single electromagnetic actuation coil and application to Fourier Transform spectroscopy are presented; many other applications can be envisioned.