Vertical Comb-drive Actuators Research Articles

Driving Principle and Stability Analysis of Vertical Comb-Drive Actuator for Scanning Micromirrors.

We have developed a manufacturing process for micromirrors based on microelectromechanical systems (MEMS) technology. The process involves designing an electrostatic vertically comb-driven actuator and utilizing a self-alignment process to produce a height difference between the movable comb structure and the fixed comb structure of the micromirror. To improve the stability of the micromirror, we propose four instability models in micromirror operation with the quasi-static driving principle and structure of the micromirror considered, which can provide a basic guarantee for the performance of vertical comb actuators. This analysis pinpoints factors leading to instability, including the left and right gap of the movable comb, the torsion beams of the micromirror, and the comb-to-beams distance. Ultimately, the voltages at which device failure occurs can be determined. We successfully fabricated a one-dimensional micromirror featuring a 0.8 mm mirror diameter and a 30 μm device layer thickness. The height difference between the movable and fixed comb structures was 10 μm. The micromirror was able to achieve a static mechanical angle of 2.25° with 60 V@DC. Stable operation was observed at voltages below 60 V, in close agreement with the theoretical calculations and simulations. At the driving voltage of 80 V, we observed the longitudinal displacement movement of the comb fingers. Furthermore, at a voltage of 129 V, comb adhesion occurred, resulting in device failure. This failure voltage corresponds to the lateral torsional failure voltage.

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.

SOIMUMPs micromirror scanner and its application in laser line generator

A SOIMUMPs 1-D rotation micromirror is presented. The micromirror is driven by electrostatic vertical comb-drive actuators to work at resonant mode to scan a laser beam. The residual stress in the metal film coated on the SOI device layer is used to generate vertical offset in the comb-drive actuators with the combs located far from the rotation axis to increase the torque. A concave lens is designed to put after the micromirror to amplify the laser beam scanning angle, as well as to compensate for the curvature of the micromirror. A micromirror-based scanning system is used to build a laser line generator with a continuously adjustable fan angle, which solves the limitation of a fixed fan angle in conventional laser line generators. Prototypes of the micromirror and the laser line generator are fabricated and measured. A driving circuit that can generate a high-voltage square wave driving signal with adjustable amplitude and frequency is designed. All the parts are integrated in a 44 mm×88 mm×44 mm box and powered with a single 5-V power supply. The optical scanning angle under 100 V with or without the concave lens is 27 deg and 12 deg, respectively, at a resonant frequency of 900 Hz.

SU-8 as a Torsional Spring: Viscoelastic Creeping and Closed Loop Control

The torsional viscoelastic creeping and recovery of an SU-8 spring is reported by studying the deflection angle variation of a vertical comb-drive actuator suspended by an SU-8 spring at three different stress levels. A maximum creep of 0.43° (corresponding to a relative change of strain of 5.7%) is observed under a constant stress of 8.3 MPa in 1 h ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V =22$ </tex-math></inline-formula> .7 V). A capacitive readout circuit is designed to sense the torsional deflection of the comb-drive actuator. The circuit is capable of actuating and sensing the capacitance change by the same comb-drives with a resolution of 0.03° (corresponding to a capacitance resolution of 0.05 pF). The readout capacitance is then used as the input variable for a proportional-integral (PI) controller in order to eliminate the time and load dependent drift by creeping of the SU-8 spring. The PI controller is tuned by the Ziegler–Nichols method and has a settling time of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 2.3$ </tex-math></inline-formula> s for the output. Compared with the open loop deflection of 0.43°, the closed-loop deflection is reduced by a factor of 7 down to 0.06°. This controller error is prominent at high actuation voltages, and is attributed to the actuator’s lateral and vertical cross-sensitivity. [2014-0124]

Hybrid micro electrostatic actuator

A hybrid micro-electrostatic actuator is presented. The actuator integrates a vertical comb driving (VCD) unit and a parallel-plate driving (PPD) unit. The hybrid actuator is fabricated using a one structural layer microfabrication process, i.e., MetalMUMPs instead of a two-layer microfabrication process needed for traditional vertical comb-drive actuators by taking advantage of the residual stress gradient in the MetalMUMPs nickel layer, which raises the moving parts of the actuator above the substrate after release. The hybrid actuator significantly simplifies the fabrication process for vertical comb-drive actuators, i.e., turning a process requiring two structural layers into a process requiring only one structural layer and thus avoids any misalignment between the two layers. The hybrid actuator can generate larger force and then a larger displacement than the actuator having only the VCD with the same area since no extra space is needed for the PPD unit which uses the moving electrode existing in the VCD unit and a fixed electrode under the VCD unit. The VCD and PPD units in the hybrid actuator are subject to the same driving voltage and work together to pull the moving parts of the actuator downward. A model is established for the hybrid actuator to analyze its displacement. The analytical results show that displacement of the moving part of the hybrid actuator is about half of the gap between the electrodes of the PPD unit. Prototypes are fabricated and tested. With a driving voltage of 150 V, the hybrid actuator achieved a measured displacement of 6.48 µm.

A novel vertical comb-drive electrostatic actuator using a one layer process

This paper presents the design, fabrication and testing of a new residual stress gradient based vertical comb-drive actuator. Conventional vertical comb-drive actuators need two structural layers, i.e. one for the moving fingers and a second for the fixed fingers. A vertical comb-drive actuator based on a single structural layer micromachining process, using the residual stress gradient along the thickness of the nickel of the MetalMUMPs (Metal Multi-User MEMS process) fabrication process, is developed. The MetalMUMPs provides a 20 μm thick nickel film and is subject to residual stress gradients along its thickness. Two curve-up beams are devised to curve out of plane after release. The curve-up beams raise a plate with comb fingers above the substrate to form the moving fingers. The fixed comb fingers are connected to the substrate via anchors. When a voltage is applied across the moving and the fixed fingers, the moving fingers move down towards the fixed fingers. Experimental measurements on prototypes have verified the design principle. A vertical displacement of 4.81 µm at 150 V was measured.

Miniature Flipping Disk Device for Size Measurement of Objects Through Endoscope

Finding the size and distance of objects viewed through a single optical path-a typical endoscopic condition-is a challenge. Stereo imaging, which would provide the size and distance information, typically requires multiple optical channels or other elaborate techniques, increasing the endoscope diameter and system complexity. This paper reports the development of a miniature flipping disk device that enables stereo measurement, and demonstrates the operation of the device installed in an endoscope. The fabricated Pyrex disk (1.2 mm in diameter and 485 μm in thickness), anodically bonded to a 50-μm-thick silicon vertical comb-drive actuator, is flipped to ±10.7° at a resonant frequency of 414 Hz by 10 VAC of electrostatic actuation. The image shift made by such glass flipping provides a reference dimension, against which the real size and distance of objects seen through a standard single-channel endoscope can be estimated.

Novel fabrication process for a monolithic PMMA torsion mirror and vertical comb actuator

This paper presents a novel fabrication process for monolithic PMMA (polymethylmethacrylate) vertical comb-drive actuators utilizing hot embossing, surface-activated direct bonding and elliptical vibration cutting. The robustness and capability of the method are demonstrated through the fabrication of sophisticated PMMA freestanding microstructures, such as a rotation mirror device integrated with vertical comb actuators. The fabrication process consists of silicon mold fabrication by deep-RIE, PMMA microstructure formation by hot embossing, PMMA–PMMA direct bonding by surface activation, releasing or removal of a PMMA layer remaining after hot embossing by elliptical vibration cutting, and finally, metallization by sputtering to form electrodes. An efficient technique to protect the PMMA microstructures during the release process using reinforcement material is applied. A monolithic PMMA torsion mirror driven by a vertical comb actuator has been fabricated successfully.

A photovoltaic pn junction micro vertical comb-drive actuator

This study reports a novel micro electrostatic vertical comb-drive actuator (VCA) driven by the photovoltaic effect that results from incident light. The vertical comb electrodes have a pn junction structure (named pn-combs). The VCA with pn-combs can be driven by the photovoltaic effect. The combination of the photovoltaic effect and reverse bias can further increase the driving amplitude of the VCA. To demonstrate the feasibility of the proposed concept, the VCA with pn-combs was fabricated on an epitaxial silicon wafer (epi-wafer) and successfully driven resonantly by intensity-modulated laser light. The first torsional mode of a typical fabricated VCA is at 1.46 kHz. Moreover, VCA driven using (1) optical driving, and (2) optical driving with dc reverse bias, were also demonstrated. The angular displacement of the VCA was ±3.8 m deg by optical driving. The angular displacement of the VCA significantly increased to ±41 m deg after applying an additional dc reverse bias of 5 V.

Angular vertical comb actuators assembled on-chip using in-plane electrothermal actuators and latching mechanisms

We have designed, fabricated and tested self-aligned angular vertical comb-drive (AVC) actuators by on-chip assembly using in-plane electrothermal actuators and latching mechanisms. The on-chip assembly process is carried out by engaging latching mechanism connected to the torsion bars through the off-centered thinned down silicon beams. When the latching mechanism is fully engaged, the assembled AVC actuator forms permanent initial tilt angle by the retraction force of electrothermal actuators. The AVC actuators and latching mechanisms are fabricated on a silicon-on-insulator (SOI) wafer using three photomasks and three times of deep etch steps. The maximum optical scan angle of 30.7° is achieved at 4.56 kHz under the sinusoidal driving voltage of 0–80 V applied to the AVC actuator. After the reliability test performed by operating the actuator for 1.6 × 10 8 cycles at its resonance, the measured optical scan angle variation and resonant frequency change were within 1.1% and 8 Hz, respectively, and the robustness of the latched mechanism was ensured.

Experimental characterization of two-axis MEMS scanners with hidden radial vertical combdrive actuators and cross-bar spring structures

In this paper, we perform the experimental characterization of two-axis MEMS scanners driven by radial vertical combdrive actuators. The dc scan ranges are limited by the pull-in effect. Each scanner utilizes a cross-bar spring structure to achieve two rotational degrees of freedom (DOFs) without employing any gimbal. Both the actuators and torsion springs are hidden underneath the mirror to obtain a small form factor. The devices are fabricated by a five-layer polysilicon surface micromachining process (SUMMiT-V). Devices with different combinations of parameter values are experimentally characterized and compared.

Two-axis MEMS scanners with radial vertical combdrive actuators—design, theoretical analysis, and fabrication

We propose the employment of radial vertical combdrive actuators to implement two-axis micro-electro-mechanical systems (MEMS) scanners. The devices are designed based on a five-layer polysilicon surface micromachining process. A cross-bar spring structure consisting of lower and upper torsion springs is incorporated to achieve two rotational degrees of freedom, enabling the dual-axis rotation. Both the vertical combdrive actuators and the torsion springs are hidden underneath the mirror to achieve a small form factor. Theoretical analysis is performed for comparison of various designs. Preliminary experimental results are also obtained.

Application of Vertical Comb-Drive MEMS Mirror for a Phase Shift Device

We have proposed a vertical comb-drive MEMS mirror which can be applied for a spectrometer with a phase-shifting method, and fabricated the mirror device using the micro fabrication technology. The configuration of the MEMS mirror was designed with three vertical electrostatic comb-drive actuators and hinges at the periphery of the mirror. In the proposed spectrometer, the phase of the light which is reflected at the movable mirror is changed, and the interferogram is obtained by the movable mirror which is driven precisely. We evaluated the moving characteristics of the fabricated MEMS mirror and confirmed that the MEMS mirror could be driven vertically to about 23μm and also could be tilted. Using this MEMS mirror, we configured the spectrometer with a source of monochromatic light (λ= 405 nm), and evaluated the spectroscopic characteristic in principle. Then, we have confirmed that the fabricated MEMS mirror can be applied to the spectrometer with the phase-shifting method.

Linearization of a two-axis MEMS scanner driven by vertical comb-drive actuators

A driving scheme using a pair of differential voltages (Vx, Vy) over a bias voltage is proposed to linearize the dc characteristic (angle versus voltage) of a two-axis MEMS scanner. The micromirror has a gimbal-less structure and is driven by vertical comb-drive actuators in conjunction with a leverage mechanism. At an optimal bias voltage of 53 V, a linear optical scan range of ±3.2° is achieved experimentally in both the x and y directions with the differential voltages ranging from −10 V to + 10 V.

Design of Novel Sequential Engagement Vertical Comb Electrodes for Analog Micromirror

The vertical comb-drive actuator (VCA) is a promising component to drive analog as well as scanning micromirrors. This work demonstrates the concept of using ldquosequential engagement of vertical comb electrodesrdquo to improve the linearity and to maximize the deflection for analog micromirror. This concept is achieved by varying the in-plane distribution of comb electrodes. The simulation and experiment results of vertical comb-drive actuator VCA with four different electrode-distribution designs demonstrated the feasibility of this study. In comparison, a modified (curved-profile) VCA was remarkably improved the performance of conventional (straight-profile) VCA. Experiments show that the curved-profile VCA improves the nonlinearity by 34% and increases the maximum angular motion for 2.3-fold.

Development of Vertical Electrostatic Comb-Drive Actuator Using Magnified Cascade Configuration

In this study, a vertical electrostatic comb-drive actuator with a novel cascade configuration called cascade vertical comb-drive actuator (CVCA) is developed to magnify the actuation stroke. The CVCA combines vertical comb-drive actuators and multi-stage structures to form a stacked actuation configuration that can be used to overcome the stroke constraint of the traditional vertical comb-drive actuator. To implement the CVCA, the Taiwan Semiconductor Manufacturing Company (TSMC) 0.35 µm two polysilicon layers and four metal layers (2p4m) complementary metal–oxide–semiconductor (CMOS) fabrication processes and post-CMOS micromachining steps are employed to fabricate the device. Due to the multilayer characteristic of the microstructure, the proposed CVCA can achieve the vertical motion and cascade structures. The experimental results indicated that the stroke of the proposed CVCA can be magnified three times as compared with that of the standard vertical comb drive with the same chip size.

Dynamic Response Modeling of MEMS Micromirror Corner Cube Reflectors With Angular Vertical Combdrives

A model for the dynamic response of microelectromechanical-systems micromirrors with angular vertical combdrive electrostatic actuators has been proposed based on a numerical solution of the nonlinear dynamic equations for torsional oscillation. The torque generated by the electrostatic drive is assumed to depend at least on the second order of the turn angle, and the model shows a shift in the resonant frequency with drive voltage, in agreement with the experimental results obtained from components fabricated by self-assembly of bonded silicon parts. The dynamic response of a corner cube reflector (CCR) based on a pair of scanning mirrors and a fixed mirror is predicted using the model, and the theoretical response agrees well with the experimental results. From the model, a small initial misalignment of the CCR mirrors can be determined

A Fourier transform spectrometer using resonant vertical comb actuators

The design, fabrication and characterization of a novel out-of-plane vertical comb-drive actuator based lamellar grating interferometer (LGI) is reported. The interferometer utilizes resonant mode vertical comb actuators, where comb fingers are simultaneously used for actuation and as a movable diffraction grating, making the device very compact. The Fourier transform of the zeroth order intensity pattern as a function of the optical path difference gives the spectrum of light. The main advantages offered by the proposed device are a long travel range (i.e. good spectral resolution), a large clear aperture (i.e. high light efficiency), and a very simple, robust and compact spectrometer structure. Peak-to-peak 106 µm out-of-plane deflection is observed in ambient pressure and at 28 V, corresponding to a theoretical spectral resolution of about 0.4 nm in the visible band and 3.6 nm at 1.5 µm. A simple CMOS compatible process based on bulk micromachining of a silicon-on-insulator wafer is used for the device fabrication.

A Poly-Si-Based Vertical Comb-Drive Two-Axis Gimbaled Scanner for Optical Applications

This work presents a novel two-axis gimbaled mirror for optical scanning applications. This scanner has been implemented using the molded surface-micromachining and bulk etching release (MOSBE II) process. Thus, the vertical comb-drive actuators are employed to realize the large stroke. The thin-film electrical routings are employed to drive the actuators individually. The rib-reinforced structures are exploited to increase the stiffness of mirror plate and supporting frame. In addition, the moving space created by backside deep reactive ion etching enables the mirror to perform large out-of-plane angular motion. The measured radius of curvature of reinforced mirror plate is 0.43 m. The maximum scanning angle of two driving axis are plusmn3.8deg at 55 V and plusmn2.4deg at 100 V, respectively. The resonant frequencies associated with the scanning modes are 5.8 and 7.8m kHz, respectively. In addition, the two-dimensional scanning images such as Lissajous patterns are also demonstrated

Design, Fabrication, and Characterization of a High Fill-Factor, Large Scan-Angle, Two-Axis Scanner Array Driven by a Leverage Mechanism

We report on the design, fabrication, and characterization of a high fill-factor, large scan-angle, two-axis scanner array. The two-axis microelectromechanical-systems (MEMS) mirror is driven by electrostatic vertical comb-drive actuators through four motion amplifying levers. The maximum mechanical rotation angles are <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$pm 6.7^circ$</tex> at 75 V for both axes, leading to total optical scan angle of 26.8 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> . The resonant frequency is 5.9 kHz before metallization. A linear fill factor of 98% is achieved for the one-dimensional (1-D) micromirror array. This 1D array of two-axis micromirrors was designed for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1times N ^2$</tex> wavelength-selective switches (WSSs). In addition to two-axis rotation, piston motion with a stroke of 11.7 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$mu m$</tex> is also attained.1731