Comb Actuator 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.

Investigating the effect of interatomic distance on friction force through MEMS-AFM based experiment

Investigations on the influence of the interatomic distance (IAD) on friction are crucial for gaining a better understanding of friction and comprehending the essential nature of graphene friction. We obtained friction force microscopy (FFM) images of strained surfaces to examine how the IAD affects the friction force. First, using microelectromechanical system (MEMS) fabrication techniques, we fabricated a strain-generating microdevice that can generate surface strain on its Si surface by an electrostatic comb actuator. The microdevice was installed in a high vacuum atomic force microscope. Next, FFM measurements were performed on the stress concentration section of the microdevice using a Si cantilever. The driving voltage on the comb actuator was switched on/off for each scan line during a single raster scan. This procedure allowed us to obtain FFM images representing the surface strain (i.e., the IAD variation) by excluding friction/wear-induced disturbances. The FFM images agreed well with the strain distribution calculated by the elastic finite element method analysis. The relative reduction of the friction was approximately 9% against the calculated strain of 1.2%.

Modelling and control design of the electrostatic linear comb actuator

Control design plays a major part in electrostatic linear comb actuator (ELCA) systems. Most existing control methods of ECLA mainly focus on traditional PID method. However, proportional–integral–derivative (PID) controller is only appropriate for the reference to be known as constant. In this article, the problem of establishing the dynamic model and designing the controller is investigated for ECLA working in the time-varying reference. The tracking effectiveness is constructed for ECLA systems based on the consideration of roots of the equivalent differential equation in tracking error model. Moreover, the convergence velocity is also tuned by changing the parameters in the presented control scheme. Simulation results guarantee the performance of the nonlinear control scheme as well as dynamic model to be established.

A new wide tunability MEMS based variable capacitor using two separate electro-static vertical comb drive actuators

This paper presents a new MEMS variable capacitor to achieve high stable region and extremely wide tunability. The idea is based on increasing the stable region in the gap between the plates of the capacitor. It is done by combining the functionality of two different vertical comb drive actuator sets to takeover a unity air gap variation. The design of the structure is carried out so that the performance of these two actuator sets is fully independent of one another and without any destructive or damaging effect on each other. The advantage of this scheme is that adding the second mechanism of actuation does not change the overall structure thickness compared with when the structure uses a single actuation mechanism. Therefore, the tunability increases sharply. Aluminum is the structural material used for the designs. Comb actuators are widely used as MEMS motors due to their long range of linear motion, low power consumption and ease of fabrication. A full review of electrostatic actuator portion is done. The structure is calculated using MATLAB software. To verify, the calculated results are compared with simulated results using Intellisuite software. The natural frequency is 1.173 KHz. According to calculation and simulation results the achieved minimum tuning range is 2300%.

MEMS-SOA Spectrum-Sliced Auto-Equalized Source Enabling Uniformly Tunable Microwave Photonic Filter

In this work, we report a Micro-Electro-Mechanical System (MEMS)-Semiconductor Optical Amplifier (SOA) active interferometer as a low cost and miniaturized tunable broadband sliced source and optical loss auto-equalizer for single pass band microwave photonic filter (MPF). A low finesse Fabry-Perot MEMS interferometer is formed from a movable deeply etched micro-mirror in front of a cleaved fiber facet. The interferometer slices the amplified spontaneous emission (ASE) output of the SOA, while the output is feedback to the SOA again for amplification and optical loss auto-equalization through the SOA gain saturation effect. This configuration improved the uniformity of the MPF passband attenuation along the tuning range down to 3.72 dB only. The filter center frequency can be finely tuned electrically for ±241.6 MHz with a MEMS electrostatic comb actuator driving the mirror. The center frequency is controlled by the distance between fiber facet and micro-mirror. The operation of the proposed MEMS-SOA based MPF is experimentally verified as a proof of concept from 1.2 to 4.5 GHz and can be easily extended to higher frequencies. A tuning resolution of 6.04 MHz/ $\mu \text{m}$ is achieved with stopband attenuation larger than 20 dB and 3-dB bandwidth mean value of 141 MHz.

Design and Fabrication of a MEMS-Based Break Junction Device for Mechanical Strain-Correlated Optical Characterization of a Single-Molecule

Here, we propose a robust lab-on-a-chip system for multi-dimensional (electrical, optical, and mechanical) characterization of a single-molecule utilizing a Si micromachined micro-electromechanical-system (MEMS) chip. Our MEMS-based break junction (MEMS-BJ) utilizes conventional MEMS chip fabrication techniques to create an electrostatic comb actuator capable of trapping a single-molecule between two nanoscale metal electrodes with sub-nanometer displacement resolution. In order to achieve this functionality, components of the chip (e.g flexural spring, comb drive fingers) are delicately designed based on an analytical model and basic performance parameters are verified using a finite element analysis simulation and capacitive sensor measurements. Using this system, we are able to perform combined single-molecule Raman spectroscopy and molecular conductance measurements in real-time. Finally, we present a representative case study of the multi-dimensional characterization of a single-molecule junction by analyzing correlated electrical, optical, and mechanical information while mechanically modulating the strain on the junction which allows for direct observation of the interplay between molecular structure and electron transport. We believe that our MEMS-BJ system will enable a deeper multi-dimensional study at a single-molecule level promoting the development of new methods for chemical and biological sensor applications.

Tunable terahertz band-pass filter based on MEMS reconfigurable metamaterials

We present a tunable terahertz (THz) band-pass filter based on electrostatically actuated microelectromechanical systems reconfigurable metamaterials. The unit cell of the filter consists of a U-shaped structure and a fat-T shaped structure. The fat-T shaped structure connects with a movable frame which is actuated by two columns of electrostatic comb actuators. Initially, the filter has two passbands whose central resonance frequencies locate at 0.66 THz and 1.31 THz, respectively. When the actuators are biased with direct currents (DC) voltages, the fat-T shaped structures will move to the U-shaped structures. Thus, the transmition of the peak at 0.66 THz will decrease, and the modulation depth can reach 84.4%. Meanwhile, the resonance frequency of the high-frequency passband shows distinct blueshift, and the largest frequency shift arrives to 0.195 THz. The simulation results obtained from the finite-intergration-technology show that these two resonance peaks are produced by two LC resonators on the two arms and one resonator on the bottom of the U-shaped structure. A flinite element analysis method is introduced to study the electromechanical performance of the actuator, and a displacement of 4 µm is achieved at DC 120 V. The tunable THz band-pass filter can be applied in THz communication, THz frequency selection and THz sensing.

A New Analysis on Reduction of Undesired Beam Bending in Electrostatic Comb Drive MEMS Actuator

In electrostatic comb drive-based sensors and actuators, often there is a possibility of nonparallel motion or bending in individual comb which considerably affects the device performance including pull-in characteristic. In this paper, we have presented a theoretical and experimental investigation to reduce such undesired beam bending in gap closing-type comb drive actuator. It has been envisaged that three key parameters, responsible for beam bending, are spring stiffness, beam stiffness, and number of actuation beams. Subsequently, a design guideline has been proposed to reduce the undesired bending by appropriate selection of these parameters. A specific comb drive actuator having multiple actuation regions with varying number of beams is designed, simulated, and analyzed using a semianalytical approach and FEM tool. Further, the structure is fabricated by establishing a silicon-on-insulator (SOI)-based single mask fabrication process and tested with an in-house experimental setup. The theoretical predictions are validated with the experimental results to demonstrate how the undesired beam bending can be reduced significantly by suitable selection of number of actuation combs for a given ratio of spring and beam stiffness. The proposed guidelines enable the micro-electromechanical system (MEMS) designers to cautiously choose the key parameters while designing in order to avoid the undesired comb bending and operate the device safely.

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.

Design of tunable terahertz bandstop filter based on electrostatically actuated reconfigurable metamaterials

A tunable terahertz (THz) bandstop filter based on electrostatically actuated microelectromechanical systems (MEMS) reconfigurable metamaterials is presented. The central part of the filter is a periodic array structure, and each unit cell consists of a movable bar and a fixed inverted trapezoidal ring. When direct current (DC) voltage is applied to actuate the electrostatic comb actuators symmetrically positioned on both sides, the movable frame and all movable bars can move in-plane. This will reconfigure the unit cell of the filter, and thus change its resonance frequency. Finite integration time domain (FITD) method is used to study the working mechanism and the influence of structure parameters. The results show that the performance of the filter strongly depends on the distance between the trapezoidal ring and the cross bar, the width of inverted trapezoidal ring and the basic angle of trapezoidal ring. A finite element analysis (FEA) method is introduced to study the electromechanical performance of the actuator, and a displacement of 3µm is achieved at DC 30V. The corresponding modulation of central frequency is about 24%. The potential applications of this tunable THz bandstop filter are THz communication, THz beam control and frequency selective detection.

A 50–100 GHz ohmic contact SPDT RF MEMS silicon switch with dual axis movement

We firstly show the prototype of an ohmic contact Single-Pole Double-Throw Radio Frequency Micro-Electro-Mechanical Systems (SPDT RF MEMS) switch operating at 50–100GHz. The fabricated ohmic contact SPDT RF MEMS silicon switch moves both laterally and vertically, to improve the isolation at high frequencies by initially misaligning the contact part of the switch over a Coplanar Waveguide (CPW) transmission line. The lateral and vertical movement of the switch is operated by using comb and parallel plate actuators, respectively. The proposed switch was fabricated using Silicon-On-Glass (SiOG) bonding process. The insertion loss of the fabricated switch is measured according to the different operation states of the switch, in the range from 50 to 100GHz. The fabricated length of the transmission line is 4.6mm and the measured insertion loss and isolation are 9.13dB and 24.37dB at 70GHz, respectively.

In-plane monolithic microscanner with two synchronized, self-aligned flat mirrors and compliant springs

This work proposes an architecture for a wide-angle, self-aligned, in-plane monolithic scanner fabricated using the deep reactive ion etching technology. The fabricated microscanner provides an optical scanning in-plane angle of about 86 deg and operates at the speed of 2.73 kHz. The scanning is achieved using two synchronized, flat mirrors coupled mechanically to allow for wide-angle scanning and connected through a compliant structure to allow the use of a linear comb actuator. This wide-angle, in-plane scanning opens the door for many applications, especially for handheld optical displays.

A Microgripper with a Post-Assembly Self-Locking Mechanism.

In this work, we report a new design for an electrostatically actuated microgripper with a post-assembly self-locking mechanism. The microgripper arms are driven by rotary comb actuators, enabling the microgripper to grip objects of any size from 0 to 100 μm. The post-assembly mechanism is driven by elastic deformation energy and static electricity to produce self-locking and releasing actions. The mechanism enables the microgripper arms to grip for long periods without continuously applying the external driving signal, which significantly reduces the effects and damage to the gripped objects caused by these external driving signals. The microgripper was fabricated using a Silicon-On-Insulator (SOI) wafer with a 30 μm structural layer. Test results show that this gripper achieves a displacement of 100 μm with a driving voltage of 33 V, and a metal wire with a diameter of about 1.6 mil is successfully gripped to demonstrate the feasibility of this post-assembly self-locking mechanism.

A High-Bandwidth MEMS Nanopositioner for On-Chip AFM: Design, Characterization, and Control

We report the design, characterization, and control of a high-bandwidth microelectromechanical systems (MEMS) nanopositioner for on-chip atomic force microscopy (AFM). For the fabrication, a commercially available process based on silicon-on-insulator is used. The device consists of a scan table, moved in the $x$ – $y$ plane by two sets of electrostatic comb actuators, capable of generating strokes in excess of $\pm 5~ {\mu }$ m. The first resonance frequencies of the nanopositioner are approximately 4.4 and 5.3 kHz in lateral directions. Electrothermal sensors are used to measure the displacement of the scan table. To enable fast scans, a dynamic model of the system is identified and used to design a feedback controller that damps the oscillatory behavior of the device. The nanopositioner is tested as the scanning stage of an AFM to perform high-speed scans.

High-stroke silicon-on-insulator MEMS nanopositioner: control design for non-raster scan atomic force microscopy.

A 2-degree of freedom microelectromechanical systems nanopositioner designed for on-chip atomic force microscopy (AFM) is presented. The device is fabricated using a silicon-on-insulator-based process and is designed as a parallel kinematic mechanism. It contains a central scan table and two sets of electrostatic comb actuators along each orthogonal axis, which provides displacement ranges greater than ±10 μm. The first in-plane resonance modes are located at 1274 Hz and 1286 Hz for the X and Y axes, respectively. To measure lateral displacements of the stage, electrothermal position sensors are incorporated in the design. To facilitate high-speed scans, the highly resonant dynamics of the system are controlled using damping loops in conjunction with internal model controllers that enable accurate tracking of fast sinusoidal set-points. To cancel the effect of sensor drift on controlled displacements, washout controllers are used in the damping loops. The feedback controlled nanopositioner is successfully used to perform several AFM scans in contact mode via a Lissajous scan method with a large scan area of 20 μm × 20 μm. The maximum scan rate demonstrated is 1 kHz.

Controlling Levitation and Enhancing Displacement in Electrostatic Comb Drives of MEMS Actuators

Capacitive comb actuators are widely used as MEMS motors due to their long range of linear motion, low power consumption, and ease of fabrication. Here, we present data from a thin comb capacitive actuator where fringe fields contribute significantly to the device performance. We characterize the observed levitation effect and discuss two methods to control the out-of-plane forces: 1) by means of alternating the comb polarity; and 2) by using an additional electrode below the comb. Considering two alternative designs, it is shown how the levitation force can be mitigated. One design decreases the out-of-plane motion by a factor of two, but also reduces the lateral range. An alternative design proved successful in decreasing out-of-plane motion by 75%, while enhancing the in-plane displacement of the linear comb actuator by over 35%.

Characterization of an In-Plane Single-Sided Lateral Comb Actuator Fabricated by a 3D-Printer

Characterization of an In-Plane Single-Sided Lateral Comb Actuator Fabricated by a 3D-Printer

Time Response of a Microelectromechanical Silicon Photonic Waveguide Coupler Switch

Time response of a silicon photonic nanowire-waveguide microelectromechanical switch is studied by measuring optical output and analyzing mechanical and electrical dynamics. The switching mechanism is based on the gap variation of a directional waveguide coupler consisting of 400-nm-wide and 260-nm-thick waveguides. One of the coupler waveguides is moved by an ultrasmall comb actuator. A theoretical model is proposed by combining the mechanical motion and the electrical circuit response of the switch. By analyzing the model, an optimum condition for switching rise time is obtained to be ~10 μs, where the mechanical oscillation is suppressed by the driving electrical circuit with a large resistance. The rise time measured from the optical output intensity was 18 μs.

Two-dimensional optical scanner with monolithically integrated glass microlens

A miniaturized two-dimensional forward optical scanner with a monolithically integrated glass microlens was developed for microendoscopic imaging applications. The fabricated device measures 2.26 × 1.97 × 0.62 mm3 in size and a through-silicon microlens with a diameter of 400 µm and numerical aperture of 0.37 has been successfully integrated within the silicon layer. An XY stage structure with lens shuttle and comb actuators was designed, and proprietary glass isolation blocks were utilized in mechanical and electric isolation of X- and Y-axis actuators. Resonant frequencies of the stage in X and Y directions were 3.238 and 2.198 kHz and quality factors were 168 and 69.1, respectively, at atmospheric pressure. Optical scanning test has been performed and scan angles of ±4.7° and ±4.9° were achieved for X and Y directions, respectively.

An Electromechanical Tunable Grating on Silicon-on-Insulator Platform

We report here the design and fabrication of an electromechanical tunable grating on silicon-on-insulator (SOI) wafer. The tunable grating consists of a submicron electrostatic comb actuator and an expandable freestanding grating. Rigorous coupled-wave analysis (RCWA) method is utilized to analyze the optical responses of freestanding grating with different periods and filling factors. Obvious shift of the resonant peaks is obtained by changing the grating period and the grating filling factor. The electromechanical tunable grating is realized on the silicon device layer by a combination of electron beam (EB) lithography, deep reactive ion etching (DRIE) and wet etching. Scanning electron microscope (SEM) micrographs indicate that the grating is well fabricated. Via applying biased voltage, the force generated by the electrostatic comb actuator can modulate periods and filling factors of the freestanding grating. The electromechanical tunable grating with simple fabrication process shows bright prospects for optical telecoms and miniaturized spectrometers.