Biaxial resonant MEMS-scanners are considered as promising core-device in state-of-the-art imaging and projection systems due to their compactness, the large field-of-view, high speed, and comparably low power consumption. However, the usage in three-dimensional LIDAR modules or projectors for industrial applications is often limited by non-optimal Lissajous-scanning patterns. To achieve dense and spatially uniform Lissajous-trajectories, a suitable frequency ratio of the two oscillation modes is essential. In previous works, the frequency ratio was either maximized or minimized, which often led either to mechanical fragility or undesirable coupling of the two normal modes. For solving the abovementioned problems, a piezoelectrically-driven biaxial MEMS-scanner exhibiting large design flexibility, enabling the individual tailoring of the two orthogonal rotational oscillation-modes and Lissajous-patterns with large fill factor, was developed. This design freedom and decoupling of two axes motions are achieved by a gimbal-less design with individual actuator systems for the two oscillatory axes. Driven by the CMOS-compatible piezoelectric Al(Sc)N, the Q-factor of the resonant mirror with large optical aperture of 5 mm is enhanced by hermetic wafer-level glass-encapsulation. A projection module, which combines the biaxial MEMS-scanner, an RGB-laser-beam combiner, and the electronics for both read-out and control, was developed in the frame of a funded research project (”MEMS-scanner-based laser projection system for maritime augmented reality”). The target of the project was the development of a smart window, in the sense of a MEMS-scanner-based laser projection system for maritime augmented reality, which offers the possibility to fade in safety-relevant information of navigation and ship sensors into the field-of-view of the bridge personnel on the ship’s bridge. Such projector is promising also for further applications in industry, for instance in 3D cameras.
Despite the high linearity of Al(Sc)N as piezoelectric actuator material, quasi-static MEMS mirrors show exemplary differences due to intrinsic stress. To control the static and dynamic behavior of the mirror, an electronic control system may be used. Rapid control prototyping (RCP) can be a helpful tool for developing generic or application-specific control schemes. This paper provides a practical introduction to the RCP approach and demonstrates it in practice with a gimbal-less bi-axial micro mirror. The application example is a long-range LIDAR system with optical positioning tolerance <0.1 degree and <400 µs point-to-point transition rate. An open loop control is implemented with a digital filter (finite-duration impulse response, FIR), using standard functions from MATLAB®/Simulink® to generate a random signal, a model of the mirror and a Gaussian filter. The response to the filtered input signal is simulated before running the control scheme on the RCP system. The modeling process relies on automatic code generation to program the RCP target system or other supported platforms.
Presented is a high-resolution AR micro display based on Laser Beam Scanning (LBS) applying a two-dimensional resonantly operated vacuum packaged MEMS mirror with large mirror diameter, high scan frequencies, high Q-factor and large field-of-view (FoV). The image is projected to the retina using a diffractive waveguide leading to a comfortably large eyebox. Advanced control algorithms and image processing methods are implemented to accurately drive, sense and control the biaxial resonant MEMS mirror as well as to optimize image projection quality. Due to a sufficiently large mirror diameter this micro display does not need any beam expansion optics between MEMS mirror and waveguide enabling an ultra-compact projection unit. Resonant operation of the MEMS mirror in both axes and exploiting the significant advantage of a hermetic vacuum package effectively reduces energy loss by damping and thus minimizes drive voltage and power consumption. The display setup demonstrates the successful realization of a small form factor high resolution micro projector that meets important requirements for enabling fashionable AR smartglasses.
In this work, 2D MEMS quasi-static mirrors based on piezoelectric, non-ferroelectric AlScN/AlN actuators with three different mirror plates (diameters of 2 mm, 5 mm and 10 mm) using a design and manufacturing platform will be reported. While the AlN/AlScN driving actuators ensure high linearity and large tilting angles, the multiple-waferbonding technique via glass fritting enables 3D construction of the MEMS mirrors and hermetic sealing. Even though there is no request on vacuum package for quasi-static driving, hermitic sealing on wafer level with appropriate interior pressure level within the sealing improves the mechanical robustness of the MEMS components and protects them from the particles and humidity from the environment. Since the main design concept was adopted from the previous work and adapted for different aperture sizes, this paper will focus on reporting further simulation results on mechanical behaviors, especially shock survivability under very harsh environment, the technology efforts and results of utilizing such a design and manufacturing platform for AlScN/AlN driven MEMS mirrors.
In this work, 2D piezoelectrically driven MEMS circular scanners have been designed, fabricated and tested. These mirrors own large optical apertures of 7 mm, 10 mm and 20 mm for good beam shapes. Also HR-coating layers for 515 nm and 1050 nm reaching up to 99.99% reflexion and 0.1% transmission were applied onto the mirror surface for the suitability of high power laser, where the wavelengths were specified according to the laser source development demands. Based on piezoelectric position sensing elements integrated on the MEMS mirrors a closed-loop control was developed. In this paper the design efforts, realizing circular-scanning and eliminating non-linearity during mode superposition, and fabrication efforts will be reported. Characterization results focusing on mechanical behaviors, position sensing signal, HR-coating will be also important parts of this work.
Presented here is the world’s first resonant 1D MEMS mirror achieving mechanical scanning angles exceeding ±45° and thus providing a field of view of up to 180°. The MEMS scanner features a 2 mm x 4 mm ellipsoid mirror plate and oscillates at a scan frequency of about 1.5 kHz. Integrated sensors and closed-loop control allow for an accurate position detection below 0.1°. To achieve the scan angles as well as to guarantee long lifetime and reliability, the MEMS mirror is hermetically sealed on wafer level by a dedicated glass cover and operated in vacuum.
This paper presents a 2D MEMS mirror for smart headlights, combing high-Q vacuum package with AR (Anti Reflecting)-coating, piezoelectric driving and Lissajous scanning. While the vacuum package protects the MEMS device and the AR-coating suppresses parasite reflections from the glass lid, the AlN-based piezoelectric actuators are robust against shock and vibration in harsh environment, comparing to fragile capacitive finger structures. This gimbal-less MEMS mirror with a large circular aperture (diameter = 5.5 mm) utilizes Lissajous scanning possessing two perpendicular torsion modes with frequencies of fx = 2.26 kHz, fy = 2.30 kHz fulfilling high light density and large total optical scanning angles of 55°, 30° at ± 40 VAC. A 2D projection of 50° x 20° was realized, where the angle loss comparing to the 1D testing arose from pincushion distortion, whose effect was severely reduced by the redesign run. Due to the great long-term stability of AlN and protection of vacuum packages, the MEMS mirror also shows a good reliability. This paper will describe and discuss the design, fabrication and characterization results of this MEMS mirror.
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