Resonant Micromirrors Research Articles

Single-drive 2-DOF wideband micromirror via mode interaction.

Micromirror technology is one of the current research hotspots. In this work, what we believe to be a novel electrostatic 2-DOF micromirror structure with double-biased torsional axes is proposed. By introducing internal resonance, synchronous motions of the two axes with a locked frequency ratio under a single driving force were achieved within a wide frequency range. The mechanical structure can thus be greatly simplified, as well as the operating frequency band is broadened. Also, two driving methods were proposed to realize the density spot acquisition with a 1:2 frequency ratio. The macroscopic experiment is further carried out to verify the validity of the theoretical model, which successfully realized a 1:2 internal resonance. The structural optimization design of internal resonance micromirrors is discussed, and a band expansion of at least 135.58% can be achieved in the simulation results. Compared with the traditional resonant micromirrors, the proposed one greatly increases the operating band at a very small sacrifice of vibration amplitude, and the resonant state can be guaranteed in complex environments. This novel micromirror provides a new chip solution for portable devices in complex environments and greatly simplifies the structure of dual-axis resonant micromirrors, reduces processing costs, and improves processing reliability.

One-to-one internal resonance in a symmetric MEMS micromirror

Resonant modal interaction is a nonlinear dynamic phenomenon observed in structures excited at large vibration amplitudes. In the present work, we report the experimental evidence of a 1:1 internal resonance in a symmetric resonant micromirror. The experiments are complemented with a reduced model obtained from the 3D finite element discretization of the device by parametrizing the system motion along a low dimensional invariant set of the phase space. The presence of coupling monomials in the governing equations makes the resulting dynamics non-linearizable. Both model and experimental data show the existence of a complex pattern of multiple stable solutions for a given value of the excitation frequency.

Evaluation of robustness against external vibrations for long-range MEMS lidar with one-dimensional resonant micromirror

We describe the verification of a long-range one-dimensional (1D) scanning micro-electro-mechanical systems (MEMS) lidar specifically considering the robustness against external vibration influences. The 1D scanning MEMS lidar exploits a multichannel horizontal line laser to scan the scene vertically for a 10 deg × 11 deg horizontal and vertical field of view at a frame rate of up to 29 Hz. To evaluate the robustness against vibrations, a vibration evaluation setup is developed to apply a wideband vibration based on the automotive standard LV124. The vibration tests are performed in three conditions open loop without control and two phase-locked loops (PLLs) with default and high gain settings. The test results demonstrate that vibration can cause wobbly distortion along the scan angle in the open loop case and the PLLs can suppress effectively this influence in the mean and standard deviation of the standard point to surface error up to 69.3% and 90.0%, respectively. This verifies the benefits of the MEMS mirror control, ensuring stable point cloud measurements under vibrations in harsh automotive environments.

Resonating AlN-thin film MEMS mirror with digital control

Piezoelectrically actuated resonant micromirrors were designed to meet the light detection and ranging (LiDAR) system requirements. Key features were a 3-mm mirror aperture, a 40-deg field of view, and a 50-Hz refresh rate. The presented micromirror provides biaxial symmetrical beam steering with ±12.7 deg mechanical tilt angle, resulting in a 50-deg field of view with an adjustable Lissajous XY-scanning pattern for a forward-looking LiDAR system. The mirrors were fabricated using silicon on insulator wafers, and actuation was based on piezoelectric aluminium nitride thin film. The mirrors were vacuum packaged for high-quality factor resonator operation. The device design contained eight separate piezoelectric aluminium nitride elements arranged as differential pairs for each axis, where each actuator was equipped with a sensing element providing a mechanically coupled electrical feedback signal. The piezoelectric elements connected as actuators required only minimal power and were directly compatible with CMOS low-voltage logic, which eases integration to driving digital systems. The sense elements are used to monitor phase, amplitude, and frequency. A digital control system connected to each of these elements provides accurate frequency and phase control of independent orthogonal resonators, permitting control of the X and Y amplitudes and the refresh rate of the Lissajous pattern.

Nonlinear Response of PZT-Actuated Resonant Micromirrors

In this work, we address the modelling of piezo-micromirrors with large opening angles. We focus on various sources of nonlinearites induced by the interaction with the surrounding fluid and by occurrence of geometric large transformations. Specific attention is also devoted to the piezoelectric coupling coefficient. Piezoelectric thin films have gained attention as key materials for the actuation of micro devices since they provide high drive forces, enabling devices with higher performances compared to electrostatic actuation. However they also induce material nonlinearities that cannot be neglected. The model is validated using experimental data obtained for various excitation levels.

Erratum to “Design and Evaluation of a Fast, High-Resolution Sensor Evaluation Platform Applied to MEMS Position Sensing” [May 18 1014-1027

We present the design and implementation of an adaptable field-programmable gate array (FPGA)-based sensor evaluation platform. This platform is developed to benchmark a capacitive position sensor for a resonant micromirror system. The sensor is developed in a smart packaging solution as a multilayer inkjet-printed electrode structure on a 3-D-printed metal housing. Very high required resolutions of respos $r_{m} = {1000}~\mu \text{m}$ at an offset of $d_{0} ={1000}~\mu \text{m}$ , motivate the development of such a platform. Yet, it is fully adaptable to other sensing principles (e.g., inductive). The suggested platform provides high sampling rates (up to ${\approx} {10}$ ns) and enables generation of trigger signals, i.e., the mirror control signal, without time lag (as could result from high-order filters). The online configurable FPGA block structure in combination with host software blocks enables flexible and individual design. The sensor read-out circuitry is designed as a carrier frequency system. Such a carrier frequency system enables flexible choices of bandwidth and measurement signal frequency. It thus allows for separation in frequency from coupling parasitics, i.e., other frequencies present in the device under test (e.g., actuation frequency in case of the micromirror system).

Design optimization of a dynamically flat resonating micro-mirror for pico-projection applications

One of the main design limitations of resonant micro-mirrors, intended for visual projection display applications, is inertia-driven dynamic deformation. In order to achieve high scan angles, micro-mirrors used for high frequency (20–30 kHz) laser beam scanning are typically operated at resonance and in the region of their torsional modal frequency. Although the optical resolution of the projected image is defined by the micro-mirror dimensions, the scanning frequency and the maximum scan angle, significant dynamic deformation (> 1/10 of the incident wavelength) can develop and give rise to a loss in contrast between adjacent projected spots. A design optimization scheme for a one directional resonant micro-mirror intended for laser projection with XGA optical resolution is performed and presented in this study. The minimization of dynamic deformation is considered as one of the main objectives but other parameters related to micro-mirror optical performance, structural reliability and gas damping characteristics are also investigated. The optimization scheme is performed using Design of Experiments and response surface methodologies. The design process demonstrates the technical feasibility of including features, such as a gimbal structure, that improve the dynamic mirror flatness without compromising the target scanning frequency, mode separation, maximum shear stress and power consumption.

PZT-Actuated and -Sensed Resonant Micromirrors with Large Scan Angles Applying Mechanical Leverage Amplification for Biaxial Scanning.

This article presents design, fabrication and characterization of lead zirconate titanate (PZT)-actuated micromirrors, which enable extremely large scan angle of up to 106° and high frequency of 45 kHz simultaneously. Besides the high driving torque delivered by PZT actuators, mechanical leverage amplification has been applied for the micromirrors in this work to reach large displacements consuming low power. Additionally, fracture strength and failure behavior of poly-Si, which is the basic material of the micromirrors, have been studied to optimize the designs and prevent the device from breaking due to high mechanical stress. Since comparing to using biaxial micromirror, realization of biaxial scanning using two independent single-axial micromirrors shows considerable advantages, a setup combining two single-axial micromirrors for biaxial scanning and the results will also be presented in this work. Moreover, integrated piezoelectric position sensors are implemented within the micromirrors, based on which closed-loop control has been developed and studied.

Design of a tunable resonant micromirror

This paper presents the concept, theoretical analysis, microfabrication processes and experimental characterization of a micromirror with tunable resonant frequency. For scanning applications where a large scanning angle is required, comb-drive based micromirrors are typically operated at resonance due to the limited force/energy density of electrostatic actuators. However, errors in microfabrication processes often cause natural frequencies to offset from the design value. This issue may be resolved by a frequency-tunable micromirror. In our design, the shifting of the resonant frequency is achieved by adjusting the axial stress of the micromirror’s torsion hinge via integrated chevron thermomechanical actuators. The scanning motion of the micromirror is generated by double silicon-on-insulator (SOI) wafer-based electrostatic comb-drive actuators. The experimental results show that the micromirror has achieved a frequency tuning range of approximately 3%, which is close to the analytic model. With precise frequency control, the micromirror may compensate the undesired frequency shift due to fabrication errors, which is useful for applications requiring precise scanning synchronization such as the laser scanning endomicroscope systems.

Design and Fabrication of Bulk Micro-machined, High Resilience, High-Q, High Tilt Angle, Low Driving Voltage, Flexure Beam Micro-mirrors on Mono-crystalline Silicon

We report the design, fabrication and characterization of electrically tunable, bulk micro machined, doubly clamped, circular cross-section flexure based square-plate Metal-On-Oxide, DC bias tension applied, resonant micro-mirrors for different frequencies starting from 1 kilohertz to 172 kilohertz on silicon {100} oriented wafers with high Q factors in the air, that have high resilience. Though silicon shows inherent reflective behavior, the surface quality of the top-deposited metal layer is treatment specific. The sputtered chrome-gold films have good light scattering properties, for precise measurements within 6 ×6 ROI matrixes for vibrometry and AFM. We introduce novel circular cross-section flexure hinges with the advantages of compact design, 5 fold better precision of rotation compared to similar constant cross-section flexure, lesser stiffness, and beam length and beam width that are less decisive for the micro-manufacture. We have fabricated the device, with a double mask, replacing torsion beams with flexure beams. These suspended-in-air-in-trapezoidal-cavity square plate micro-mirrors fabricated on the silicon {100} oriented wafers, are electrically tunable at ±30 Volts, with onset of motion at as low as 3 Volts electrical potential, with the largest achieved tilt angle as high as 11 degrees and parabolic capacitance-voltage characteristics, that confirm the motion of the device in a cavity. The motion of these devices was recorded as a Doppler frequency shift. The result shows a slight improvement in mirror surface quality when resonant micro-mirrors are applied with bias tension, that deviates from the results presented in prior art. The micro-mirrors initially show inactivity in motion, which was stabilizes later. The exact mechanism responsible for this initial inactivity is not known, we attribute it to the sluggish behavior of joined mechanical elements with large time constants, which slow down the vibrancy.

Modeling and Simulation of a Parametrically Resonant Micromirror With Duty-Cycled Excitation.

High frequency large scanning angle electrostatically actuated microelectromechanical systems (MEMS) mirrors are used in a variety of applications involving fast optical scanning. A 1-D parametrically resonant torsional micromirror for use in biomedical imaging is analyzed here with respect to operation by duty-cycled square waves. Duty-cycled square wave excitation can have significant advantages for practical mirror regulation and/or control. The mirror's nonlinear dynamics under such excitation is analyzed in a Hill's equation form. This form is used to predict stability regions (the voltage-frequency relationship) of parametric resonance behavior over large scanning angles using iterative approximations for nonlinear capacitance behavior of the mirror. Numerical simulations are also performed to obtain the mirror's frequency response over several voltages for various duty cycles. Frequency sweeps, stability results, and duty cycle trends from both analytical and simulation methods are compared with experimental results. Both analytical models and simulations show good agreement with experimental results over the range of duty cycled excitations tested. This paper discusses the implications of changing amplitude and phase with duty cycle for robust open-loop operation and future closed-loop operating strategies.

6PM3-PMN-046 周波数補償のための可変ばね付き共振マイクロミラースキャナ(微細加工とMEMS,ポスターセッション)

6PM3-PMN-046 周波数補償のための可変ばね付き共振マイクロミラースキャナ(微細加工とMEMS,ポスターセッション)

Optical position encoding and phase control of an electrostatically driven two-dimensional MOEMS scanner at two resonant modes

We have developed compact devices to control electrostatically driven resonant micromirrors with one and two axes. For stable oscillation with large amplitude, operation close to resonance must be ensured under varying environmental conditions. Our devices feature optical position sensing and driver electronics with closed loop control. In this contribution, we present in much detail the novel two-dimensional device and highlight specific aspects of this system.

Compact Low-Voltage Operation Micromirror Based on High-Vacuum Seal Technology Using Metal Can

In this paper, we present the design, fabrication, and vacuum package of an electrostatic comb-drive resonant micromirror. The micromirror having a high resonant frequency suitable for a laser scanning display was fabricated using a silicon-on-insulator (SOI) wafer. An individual die which was an array of four micromirrors with a total size of 5.5 × 5.5 mm2 was diced from the SOI wafer by dry etching. The die was packaged in a transistor-outline-8 metal can with a window in a vacuum-packaging machine at a pressure of 10-4 Pa. To evacuate the residual gases generated after the package process, a nonevaporable getter was used in the metal can. The resonant frequencies of the fabricated micromirrors were 13 and 25 kHz, respectively. An optical rotation angle of about 10° was achieved at a low driving voltage of 5 V. Due to the decrease of air-friction loss, the operation voltage decreased by a factor of 32 compared with the voltage operated at atmospheric pressure. The operation pressure in the vacuum package was evaluated to be about 0.7 Pa from the amplitude and quality factor of the mirror oscillation. Moreover, several properties, such as the surface profile of the micromirror, were evaluated before and after the packaging. The durability of the packaged mirror was tested at temperatures of up to 75°C. The theoretical explanations about air friction were also described.

Fabrication and Testing of a Micromirror Device Actuated by PZT Thin Films

The paper describes the fabrication and characterization of two-dimensional resonant micromirror device actuated by sol-gel deposited PZT thin films. The actuation principle is based on the bimorph beam structure, which consists of an oxide layer and a piezoelectric PZT layer. The two-dimensional scanning performance can be achieved by applying AC voltages with phase shifts at resonance to the actuating beams. The devices are fabricated through thin film depositions, lithography, dry plasma etching and the ICP release process. For a micromirror structure with a 300μm ×300μm mirror plate, the first four resonance frequencies are measured to be in the range of 10–30 kHz. To investigate the vibration modes, the deflections on different locations of the mirror plate are measured. The two dimensional scanning angle is determined to be in one direction and 11° at 23.4 kHz in the perpendicular direction.

Necessary load for room temperature vacuum sealing

This paper presents the effects of bonding load and surface roughness on room temperature vacuum sealing carried out using the surface-activated bonding (SAB) method. Vacuum sealing is necessary to achieve highly reliable packaging and for the functioning of devices such as infrared sensors, resonance sensors and resonant micromirrors. If the vacuum seal bonding is carried out at room temperature, then it can be applied to diverse combinations of materials. In the previous experiments, we found that room temperature bonding of Si/Si and Si/Cu using SAB could be successfully applied to vacuum sealing of microcavities. In order to design sealing structures and estimate suitable bonding conditions, simple models for vacuum sealing are considered. Based on the percolation theory, we have concluded that for vacuum sealing, the degree of contact between seal surfaces is greater than 0.6. Further, we introduce a model to provide a criterion for vacuum sealing applied to smooth metal surfaces with plastic deformation using average clearance. We then discuss the necessary load and surface roughness for room temperature vacuum sealing applied to metal film surfaces.

Resonant micro-mirror excited by a thin-film piezoelectric actuator for fast optical beam scanning

A silicon resonant torsional micro-mirror excited by piezoelectric bimorph actuators is presented. The bimorph actuators consist of a single-crystal silicon flexible beam or membrane and a PZT thin film. The mechanical elements are etched in the 20 μm thick superficial layer of a SOI substrate, thereby ensuring the flatness of the 500 μm-diameter gold-coated mirror. This device achieves optical beam scanning with large angular deflection at high frequency. In air, we have measured scanning up to 19.8° at 1.8 kHz, 99.1° at 10.6 kHz and 40.8° at 25.4 kHz. The applied driving voltage is sinusoidal with an amplitude under 42 V PP and with a 6 V DC bias voltage. At large oscillation amplitude, a non-linear hard-spring effect has been observed. In vacuum, a 78° optical scanning has been obtained with less than 1 V PP.