This paper presents the feasibility of implementing various scan patterns, ranging from quasi-static to resonant driving methods, using newly developed 3D-constructed Al(Sc)N piezoelectric MEMS mirrors. A description of the assembled device and how each driving method was tried and characterized are also provided. According to the quasi-static driving test result, tilting angle of ±10° was achieved by both AlN and AlScN based-MEMS mirrors. For 1D resonant scanning, total optical scan angle (TOSA) of 20° was obtained under the low applied voltage of 1.5 Vpp. Further investigations were conducted on various 2D scan patterns, including Lissajous and circular scans. The characterization results show that Lissajous scan patterns with various field-of-view (FOV) sizes can be realized by adjusting the applied voltage and driving frequencies. In the case of circular scan, a TOSA of 10° was achieved, demonstrating the potential for 360° of omnidirectional scanning using the presented mirror devices. In addition, assessments of the electrical stability of fabricated piezoelectric material under high voltage and the mechanical robustness based on long-term cycling tests were conducted to ensure the reliability of the device. The presented low-power compact Al(Sc)N-based piezoelectric MEMS mirror device possesses a wide range of specifications, affording it the capability for application and customization to meet various purposes, while also holding significant potential for further advancements in its utility.
Future industrial production will be characterized by the collaborative work of humans and robots, sharing the same factory area (almost) simultaneously. To realize this cooperation in an efficient and safe way, powerful LiDAR systems with a large field of view are essential in order to permanently supervise the human-robot interaction and give instructions for the robotic motion based on the current human behavior. Piezoelectrically driven, resonant MEMS mirrors are often at the heart of such LiDAR systems, due to their high speed, electric power efficiency, and compactness. However, not only the achievable optical field of view, but also the resonant frequencies of the mirror eigenmodes are key parameters that need to be suitable for the specific application scenario and the system components, such as the laser. In this study, we present the FEM-simulation-based development of biaxial MEMS mirrors with 3 mm aperture, specifically optimized for the use in a LiDAR system to monitor and control the human-robot collaboration. The gimbal-less mirrors of Design 1 (and Design 2) exhibit a diminishing coupling between the two resonant modes at 2.4 kHz (2.3 kHz) and 5.6 kHz (5.0 kHz). This enables the individual control of the mirror movement along the two orthogonal axes and a very good light density. The two presented mirrors realize scanned field of views of 60° x 32° and 42° x 42° rectangles, respectively, showing almost no pincushion distortions. Due to the reduced mechanical coupling and mutual influence, the sensing signals possess a high signal-to-noise ratio, enabling the precise determination of the mirror position.
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.
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.
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