Scanning Micromirror Research Articles

MEMS Scanning Mirrors for Optical Coherence Tomography

This contribution presents an overview of advances in scanning micromirrors based on MEMS (Micro-electro-mechanical systems) technologies to achieve beam scanning for OCT (Optical Coherence Tomography). The use of MEMS scanners for miniaturized OCT probes requires appropriate optical architectures. Their design involves a suitable actuation mechanism and an adapted imaging scheme in terms of achievable scan range, scan speed, low power consumption, and acceptable size of the OCT probe. The electrostatic, electromagnetic, and electrothermal actuation techniques are discussed here as well as the requirements that drive the design and fabrication of functional OCT probes. Each actuation mechanism is illustrated by examples of miniature OCT probes demonstrating the effectiveness of in vivo bioimaging. Finally, the design issues are discussed to permit users to select an OCT scanner that is adapted to their specific imaging needs.

Temperature stability study of resonant angular scanning micromirrors with electrostatic comb-drive actuators

Resonant rotational scanning micromirrors with electrostatic comb-drive actuators are widely used in generating structured lights for 3D sensing, but their resonant scanning stability is affected considerably by the working temperature, which in turn compromises the sensing accuracy. The resonant scanning instability is mainly exhibited as the temperature drifts of the quality factor (Q) and the resonance frequency. In this paper, we focus on the Q variation due to the temperature dependence of air damping. The temperature dependence of the air damping in a comb-drive micromirror is investigated using both simplified analytical damping models and computational fluidic dynamic (CFD) models. The modeling results show that the temperature dependence of the viscous damping of the comb drive is opposite to that of the drag damping of the mirror plate, providing a means of minimizing the temperature dependence of Q by adjusting the structural parameters of the comb drive and the mirror plate. It has also been found from the modeling that the lengths of the comb-drive fingers and the mirror plate play the most significant role while the cavity height under the mirror plate has little effect if it is more than 10 % of the mirror plate length. Meanwhile, four micromirrors with different comb-finger or mirror-plate lengths are designed and fabricated. The testing results from these four fabricated mirrors have confirmed that the temperature coefficient of the Q value of an angular scanning comb-drive micromirror can be altered and ultimately compensated by changing the mirror-plate and/or comb-finger lengths.

Kirigami Engineering-Nanoscale Structures Exhibiting a Range of Controllable 3D Configurations.

Kirigami structures provide a promising approach to transform flat films into 3D complex structures that are difficult to achieve by conventional fabrication approaches. By designing the cutting geometry, it is shown that distinct buckling-induced out-of-plane configurations can be obtained, separated by a sharp transition characterized by a critical geometric dimension of the structures. In situ electron microscopy experiments reveal the effect of the ratio between the in-plane cut size and film thickness on out-of-plane configurations. Moreover, geometrically nonlinear finite element analyses (FEA) accurately predict the out-of-plane modes measured experimentally, their transition as a function of cut geometry, and provide the stress-strain response of the kirigami structures. The combined computational-experimental approach and results reported here represent a step forward in the characterization of thin films experiencing buckling-induced out-of-plane shape transformations and provide a path to control 3D configurations of micro- and nanoscale buckling-induced kirigami structures. The out-of-plane configurations promise great utility in the creation of micro- and nanoscale systems that can harness such structural behavior, such as optical scanning micromirrors, novel actuators, and nanorobotics. This work is of particular significance as the kirigami dimensions approach the sub-micrometer scale which is challenging to achieve with conventional micro-electromechanical system technologies.

Waferlevel Vacuum Packaged Microscanners: A High Yield Fabrication Process for Mobile Applications

Packaging of MEMS is an important expense factor within the production costs and, to ensure mass producibility, the packaging has to be performed on a waferlevel. While for inertial MEMS this is state of the art, it has not yet been reported for scanning micromirrors. Therefore, Fraunhofer ISIT has developed a process technology based on two 30 μm thick epitaxially deposited polysilicon layers for the manufacturing of waferlevel vacuum packaged MEMS scanning mirrors. It allows the fabrication of vertically stacked combdrives for out-of-plane mirror operation and a low damping environment for the reduction of needed driving signals. An anodically bonded structured glass wafer seals the devices at the front side, while an Au-Si eutectically bonded silicon wafer with an integrated 400 nm thick Ti getter layer is used to seal the devices from the backside. The measurement of the quality factor allows the estimation of the internal cavity pressure of sealed devices, which is in the range of 10-3 mbar.Waferlevel measurements showed that the fabrication process reaches a high mechanical yield of Ym = 95%. Vacuum packaged devices needed 6V driving voltage to reach a total optical scan angle of above 50°.

2D Scanning Micromirror with Large Scan Angle and Monolithically Integrated Angle Sensors Based on Piezoelectric Thin Film Aluminum Nitride.

A 2D scanning micromirror with piezoelectric thin film aluminum nitride (AlN), separately used as actuator and sensor material, is presented. For endoscopic applications, such as fluorescence microscopy, the devices have a mirror plate diameter of 0.7 mm with a 4 mm2 chip footprint. After an initial design optimization procedure, two micromirror designs were realized. Different spring parameters for x- and y-tilt were chosen to generate spiral (Design 1) or Lissajous (Design 2) scan patterns. An additional layout, with integrated tilt angle sensors, was introduced (Design 1-S) to enable a closed-loop control. The micromirror devices were monolithically fabricated in 150 mm silicon-on-insulator (SOI) technology. Si (111) was used as the device silicon layer to support a high C-axis oriented growth of AlN. The fabricated micromirror devices were characterized in terms of their scanning and sensor characteristics in air. A scan angle of 91.2° was reached for Design 1 at 13 834 Hz and 50 V. For Design 2 a scan angle of 92.4° at 12 060 Hz, and 123.9° at 13 145 Hz, was reached at 50 V for the x- and y-axis, respectively. The desired 2D scan patterns were successfully generated. A sensor angle sensitivity of 1.9 pC/° was achieved.

Design optimization of a 6.4 mm-diameter electromagnetic 2D scanning micromirror.

We present the design optimization, fabrication, and analysis of an electromagnetic biaxial scanning micromirror with 6.4 mm-diameter. The scanner is composed of a micromirror supported by two concentric gimbal structures with unique single turn coil. A cylindrical permanent magnet assembly is placed under the micromirror to provide a radial magnetic field for actuation. Lumped element model parameters and magnetic circuit have been optimized to maximize the driving torque. Fabricated micromirror has been actuated at 300 Hz and 1,010 Hz and maximum optical scan angle of 25.6° and 35.3° have been obtained for the vertical and horizontal scans, respectively. Crosstalk during the actuation has been analyzed, and improved models have been proposed to reduce the crosstalk.

Lissajous scanning structured illumination microscopy.

High-resolution fluorescent microscopic imaging techniques are in high demand to observe detailed structures or dynamic mechanisms of biological samples. Structured illumination microscopy (SIM) has grabbed much attention in super-resolution imaging due to simple configuration, high compatibility with common fluorescent molecules, and fast image acquisition. Here, we report Lissajous scanning SIM (LS-SIM) by using a high fill-factor Lissajous scanning micromirror and laser beam modulation. The LS-SIM was realized by a Lissajous scanned structured illumination module, relay optics, and a conventional fluorescent microscope. The micromirror comprises an inner mirror and an outer frame, which are scanned at pseudo-resonance with electrostatic actuation. The biaxial scanning frequencies are selected by the frequency selection rule for high fill-factor (> 80%) Lissajous scanning. Structured illumination (SI) was then realized by modulating the intensity of a laser beam at the least common multiple (LCM) of the scanning frequencies. A compact Lissajous scanned SI module containing a fiber-optic collimator and Lissajous micromirror has been fully packaged and coupled with relay optics and a fiber-based diode pumped solid state (DPSS) laser including acousto-optic-modulator (AOM). Various structured images were obtained by shifting the phase and orientation of the illumination patterns and finally mounted with a conventional fluorescent microscope. The LS-SIM has experimentally demonstrated high-resolution fluorescent microscopic imaging of reference targets and human lung cancer cell PC-9 cells. The LS-SIM exhibits the observable region in spatial frequency space over 2x, the line-edge sharpness over 1.5x, and the peak-to-valley (P-V) ratio over 2x, compared to widefield fluorescent microscopy. This method can provide a new route for advanced high-resolution fluorescent microscopic imaging.

Mirror Angle Tuning of Electromagnetic Micro-Mirrors With Oscillation Compensation

Underdamped feature and hysteresis existing in electromagnetic scanning micro-mirrors (EMSMs) often lead to oscillation and undesired positioning error of deflection angle. In this paper, a model-based robust scheme is proposed for the angle tuning of mirror plate of the EMSM. To build a proper model for describing the behavior of EMSM, which is also convenient for the design of tuning strategy, a data-driven model composed of linear dynamic submodel as well as hysteresis submodel is developed. In the data-driven model, the deflection mechanism is described by a linear dynamic submodel while the hysteresis in EMSM is depicted by a nonsmooth hysteresis basis function submodel. Then, in order to compensate for the internal oscillation caused by both underdamping factor and hysteresis, a pseudo-feedforward oscillation compensator (PFOC) not relying on the inverse model is proposed. Then, the design procedure of the angle tuning strategy with the PFOC is presented. Afterward, the robust design of the angle tuning scheme is also studied to overcome the effect of compensation residual. Subsequently, the proposed scheme is applied to the angle tuning of EMSM.

The stability of the MEMS 2D mirror's operating point in terms of eye tracking systems

This paper describes research on the stability of the MEMS mirror for use in eye tracking systems. MEMS mirrors are the main element in scanning methods (which is one of the methods of eye tracking). Due to changes in the mirror pitch, the system can scan the area of the eye with a laser and collect the signal reflected. However, this method works on the assumption that the inclinations are constant in each period. The instability of this causes errors. The aim of this work is to examine the error level caused by pitch instability at different points of work. Full Text: PDF ReferencesW. Fuhl, M. Tonsen, A. Bulling, and E. Kasneci, "Pupil detection for head-mounted eye tracking in the wild: an evaluation of the state of the art," Mach. Vis. Appl., vol. 27, no. 8, pp. 1275-1288, 2016, CrossRef X. Wang, S. Koch, K. Holmqvist, and M. Alexa, "Tracking the gaze on objects in 3D," ACM Trans. Graph., vol. 37, no. 6, pp. 1-18, Dec. 2018 CrossRef X. Xiong and H. Xie, "MEMS dual-mode electrostatically actuated micromirror," Proc. 2014 Zo. 1 Conf. Am. Soc. Eng. Educ. - "Engineering Educ. Ind. Involv. Interdiscip. Trends", ASEE Zo. 1 2014, no. Dmd, 2014 CrossRef E. Pengwang, K. Rabenorosoa, M. Rakotondrabe, and N. Andreff, "Scanning micromirror platform based on MEMS technology for medical application," Micromachines, vol. 7, no. 2, 2016 CrossRef J. P. Giannini, A. G. York, and H. Shroff, "Anticipating, measuring, and minimizing MEMS mirror scan error to improve laser scanning microscopy's speed and accuracy," PLoS One, vol. 12, no. 10, pp. 1-14, 2017 CrossRef C. Hennessey, B. Noureddin, and P. Lawrence, "A single camera eye-gaze tracking system with free head motion," Eye Track. Res. Appl. Symp., vol. 2005, no. March, pp. 87-94, 2005 CrossRef C. H. Morimoto and M. R. M. Mimica, "Eye gaze tracking techniques for interactive applications," Comput. Vis. Image Underst., vol. 98, no. 1, pp. 4-24, Apr. 2005 CrossRef S. T. S. Holmström, U. Baran, and H. Urey, "MEMS laser scanners: A review," J. Microelectromechanical Syst., vol. 23, no. 2, pp. 259-275, 2014 CrossRef C. W. Cho, "Gaze Detection by Wearable Eye-Tracking and NIR LED-Based Head-Tracking Device Based on SVR," ETRI J., vol. 34, no. 4, pp. 542-552, Aug. 2012 CrossRef T. Santini, W. Fuhl, and E. Kasneci, "PuRe: Robust pupil detection for real-time pervasive eye tracking," Comput. Vis. Image Underst., vol. 170, pp. 40-50, May 2018 CrossRef O. Solgaard, A. A. Godil, R. T. Howe, L. P. Lee, Y. A. Peter, and H. Zappe, "Optical MEMS: From micromirrors to complex systems," J. Microelectromechanical Syst., vol. 23, no. 3, pp. 517-538, 2014 CrossRef J. Wang, G. Zhang, and Z. You, "UKF-based MEMS micromirror angle estimation for LiDAR," J. Micromechanics Microengineering, vol. 29, no. 3, 201 CrossRef

Lissajous scanned variable structured illumination for dynamic stereo depth map.

Structured illumination plays an important role in advanced photographic and microscopic imaging applications. Here we report variable structured illumination (VSI) using Lissajous scanning techniques. The variable structured illumination module comprises Lissajous scanning micromirror and fiber-based diode pumped solid state (DPSS) laser with intensity modulation, combined with a stereo camera for dynamic stereo depth map. The micromirror projects static and discrete patterns by modulating the intensity of a laser beam at the least common multiple (LCM) of two scanning frequencies. The pattern density is increased by either decreasing the greatest common divisor (GCD) of scanning frequencies or decreasing the duty rate of the laser modulation. The scanning amplitude also controls the field-of-view (FOV) for the exact illumination of a target object for dynamic stereo depth map. The variable structured illumination module provides a new route for advanced imaging applications such as high-quality depth map, super-resolution, or motion recognition.

A Novel Indirect Quasi-Static Actuation Concept for 2D MEMS Micromirrors

We present a 2D MEMS raster scanning micromirror based on a novel actuation concept for the (quasi-)static axis. This is based on the distortion of the angular momentum generated by the primary resonant axis using a secondary oscillation. After introducing a model of the device dynamics using a rigid body approximation, several simulation studies are carried out to investigate the nonlinear dynamics of such a device. In the next step, a fully operational MEMS design is shown, that is actuated by piezoelectric as well as electromagnetic drive schemes. After manufacturing, we performed a series of experiments to obtain the characteristics of the resonant as well as the quasistatic axis, proving the viability of the proposed actuation concept. [2018-0250]

Air damping of high performance resonating micro-mirrors with angular vertical comb-drive actuators

The maximum scan angle amplitude of resonating micro-mirrors, intended for micro-projection display applications is limited by air damping. Three-dimensional transient Navier–Stokes (N–S) simulations are performed to analyse the fluid flow interactions with a high frequency scanning micro-mirror driven by angular vertical comb (AVC) actuators. The time-dependent damping moment contributions due to viscous shear and pressure drag are subsequently computed for both the mirror plate and comb-drive structures. A computational-efficient N–S model of the AVC structure, based on the sliding mesh technique available in ANSYS Fluent, is proposed. The effect of scan angle amplitude on the damping moment and the flow regime surrounding the oscillating micro-mirror plate is analysed in detail. It is shown that the simplified damping models applicable to resonant MEMS devices are not valid within the operating range of high performance micro-scanners. From dynamic mesh N–S simulations, the effect of the underlying mirror cavity wall on pressure drag damping is also evaluated. Good qualitative agreement in the overall quality factor is achieved between simulation and measurement results.

Vacuum packaged electromagnetic 2D scanning micromirror

We herein present the design, fabrication, and wafer-level packaging of an electromagnetic biaxial scanning micromirror. The vacuum packaging of the micromirror was implemented by anodically bonding the glass cap wafer to both sides of the silicon substrate with the micromirror. The measured quality factor of the vacuum packaged micromirror is 1923.2 for the horizontal scan, whereas that of the device in the air is 649.6. The maximum optical scan angle of the vacuum packaged micromirror is 24.19° at 155 mApp and 11.9 kHz for the horizontal scan, and 7.1° at 204.7 mApp and 60 Hz for the vertical scan. The estimated vacuum level of the package is 1.65 ± 0.85 Torr.

Spontaneous Parametric Down-Conversion Induced by Non-Degenerate Three-Wave Mixing in a Scanning MEMS Micro Mirror

Scanning micro-mirror actuators are silicon-based oscillatory micro-electro-mechanical systems (MEMS). They enable laser distance measurements for automotive LIDAR applications as well as projection modules for the consumer market. For MEMS applications, the geometric structure is typically designed to serve a number of functional requirements. Most importantly, the mode spectrum contains a single high-Q mode, the drive mode, which per design is expected to yield the only resonantly excited geometric motion during operation. Yet here, we report on the observation of a resonant three-mode excitation via a process known as spontaneous parametric down-conversion. We show that this phenomenon, most extensively studied in the field of nonlinear optics, originates from three-wave coupling induced by geometric nonlinearities. In combination with further Duffing-type nonlinearities, the micro mirror displays a variety of nonlinear dynamical behaviour ranging from stationary state bifurcations to dynamical instabilities observable via amplitude modulations. We are able to explain and emulate all experimental observations using a single fundamental model. In particular, our analysis allows us to understand the conditions for the onset of three-wave down-conversion which if not accounted for in the design of the MEMS structure, can have drastic impact on its functionality even leading to fracture.

Hybrid assembled MEMS scanner array with large aperture for fast scanning LIDAR systems

Abstract This article presents a large aperture micro scanning mirror (MSM) array especially developed for the panoramic 3D-ToF camera Fovea-3D. The Fovea-3D system uses a fiber amplified pulsed laser ToF technique at λ = 1550 nm \lambda =1550\hspace{0.1667em}\text{nm} with 1 MVoxel distance measuring rate. It targets for real time 3D imaging with a panoramic optical field of view (FOV) of 360 ° × 60 ° 360^\circ \times 60^\circ (horizontal × vertical) combined with a large distance measurement range of 100 m and a video-like frame rate of 10 Hz. For fast vertical scan axis a MEMS scanner module with large receiver aperture was especially developed. It increases the scanning rate to 3200 Hz which is four times faster in comparison to state-of-the-art fast macroscopic polygon scanning systems used for LIDAR systems. To guarantee at the same time a large reception aperture of D eff = 23 mm {D_{\mathit{eff}}}=23\hspace{0.1667em}\text{mm} , large FOV of 60 ° 60^\circ and high vertical scanning rate of 3200 Hz, a hybrid assembled MSM array was developed consisting of 22 reception mirrors and a separate emitting mirror for laser scanning of the target. For Fovea3D hybrid assembly of frequency selected scanner elements was chosen instead of a monolithic MEMS scanner array to guaranty a high yield of MEMS fabrication. All MSM are driven in parametric resonance to enable a fully synchronized operation of all individual MEMS scanner elements. Therefore, piezo-resistive position sensors are integrated on each MEMS chip for position feedback of driving control. The paper discusses details of the MEMS system integration including the synchronized operation of multiple MEMS scanning elements.

Low-power three-degree-of-freedom Lorentz force microelectromechanical system mirror for optical applications

A low-power three-degree-of-freedom scanning micromirror is presented. The 2- × 2-mm mirror is a gimbaless structure, directly supported by single-crystal microsprings. It is actuated by Lorentz force and is able to tilt about two axes and has linear motion in a third-axis. The transient and frequency responses of the micromirror are analyzed. The Lagrange’s equations of motions describing the dynamic behavior of the system are presented and show a good agreement with the experimental results. The fabricated microelectromechanical system mirror demonstrates a tilt angle of 22.8 deg at 247.5 Hz about y-axis, and 13.3 deg at 292.7 Hz about x-axis, in a 0.1 T magnetic field and 20-mA current on the mirror. Power consumption is 2.6 mW of power in tilting motions in resonant operation. With a total DC-drive current of 110 mA, 232-μm linear motion is achieved.

Electromagnetic 2D Scanning Micromirror for High Definition Laser Projection Displays

We present the design, fabrication, and analysis of an electromagnetic biaxial scanning micromirror for a high definition laser pico-projection display system. The device consists of a microfabricated double gimbaled structure for mechanical amplification of horizontal scan angle and a set of uniquely designed permanent magnets for high magnetic field generation in a small volume. With the fabricated 1.2-mm-diameter micromirror, the maximum scan angle of 52° at 28.9 kHz and 33° at 60 Hz have been obtained for horizontal and vertical scans, respectively. The effective volume of the packaged micromirror measures 0.34 cm3. A prototype full-color pico-projection display system has been fabricated with micromirror package and laser sources integrated into a 3.7 cm3 volume.

Investigation of Electromagnetic Angle Sensor Integrated in FR4-Based Scanning Micromirror

This paper performs a detailed investigation on the electromagnetic angle sensor integrated in the flame retardant 4 (FR4)-based scanning micromirror. An accurate theoretical model is presented, especially considering the coupling effect between the driving and sensing coils. Experimental results agree well with the theoretical results, and show a sensitivity of 55.0 mVp/° and a high signal-to-noise ratio (SNR) of 71.9 dB. Moreover, the linearity of the angle sensor can still reach 0.9995, though it is affected slightly by the coupling effect. Finally, the sensor’s good feasibility for feedback control has been further verified through a simple closed-loop control circuit. The micromirror operated with closed-loop control possesses better long-term stability and temperature stability than that operated without closed-loop control.

AlN film based piezoelectric large-aperture MEMS scanning micromirror integrated with angle sensors

This paper reports on the successful implementation of an aluminum nitride (AlN) piezoelectric film for the fabrication of a large-aperture MEMS scanning micromirror. To overcome the shortcoming of the relatively low piezoelectric coefficients of AlN film, a leverage amplification mechanism and resonant amplification effect are employed to amplify the scan angle of the mirror plate. Compared to conventional PZT piezoelectric micromirrors, the fabricated AlN film-based micromirror has demonstrated excellent compatibility with the current MEMS process. In particular, piezoelectric angle sensors are monolithically integrated without any additional process or material. The test results indicate the great linear actuation relationship and the good signal quality of the integrated angle sensors. Therefore, the proposed AlN micromirror will provide a new promising option for vast optical microsystem applications.

Speckle measuring instrument based on biological characteristics of the human eyes and speckle reduction with advanced electromagnetic micro-scanning mirror

Laser speckle is a major issue for laser projection displays. In various techniques of speckle reduction, speckle is quantified with a speckle contrast value. However, the measured speckle contrast is poorly suited for the subjective speckle perception of a human observer. Here, we investigate the characteristics of human eyes and propose a simplified optical transfer function of human eyes. Accordingly, two human-eye-modeled speckle measuring sets are configured. Based on the experimental set, an advanced electromagnetic micro-scanning mirror (EM-MSM) is exploited; which is of 6.5 mm in diameter and its half angle is 7.8° for a horizontal scan and 6.53° for a vertical scan. Finally, we quantitatively show that images generated with an EM-MSM exhibit superior quality. By providing human-eye-modeled speckle measuring instruments and an EM-MSM for speckle reduction, it has a promising promotion to laser projector development.