Electrostatic Comb Actuators Research Articles

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.

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.

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 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.

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.

A micromirror with CNTs hinge fabricated by the integration of CNTs film into a MEMS actuator

This paper reports a robust fabrication process to integrate carbon nanotubes (CNTs) film into a micro electromechanical systems actuator by the design and fabrication of a silicon micromirror supported by CNTs hinges. Vertically aligned single wall carbon nanotubes forest film was synthesized by water-assisted chemical vapor deposition. CNTs film was then condensed, manually maneuvered and patterned by EB lithography to form a flexible hinge of a mirror. The mirror is actuated by a electrostatic angular vertical comb actuator and the performance had been characterized. The mirror could be driven by a low voltage with a rotate angle of 1.5° and a response frequency of 500 Hz.

Design of curved photonic cavities for a narrow-band widely tunable resonance ranging 200 nm

We propose a type of photonic resonator with a tunable curved cavity that enables efficient tuning of the optical length of a resonant cavity made of a solid material; we call this a "tunable curved resonator" (TCR). Its integration with a "tunable curved waveguide" (TCWG) and their actuation by a MEMS (micro electromechanical systems) electrostatic comb actuator are also designed for integrated photonic circuits. With this kind of structure, a widely and continuously tunable narrow-band resonance ranging up to 200 nm is achieved with a MEMS actuation voltage less than 70 V. Its applications in widely tunable photonic filters and lasers are promising.

Silicon-on-insulator out-of-plane electrostatic actuator with <italic>in situ</italic> capacitive position sensing

An out-of-plane electrostatic comb actuator with in situ capacitive position sensing is presented. Asymmetrical vertical combs are used as the comb drives and in differential out-of-plane position sensing capacitors. A spring which can realize out-of-plane translational displacement of high-aspect-ratio structures was designed and analyzed both analytically and numerically. A robust silicon-on-insulator (SOI) process was proposed and verified to successfully fabricate the actuator with asymmetrical combs. The in situ capacitive position sensing capability was realized by the integrated out-of-plane sensing capacitors, and calibrated with the gravity as an acceleration reference. Treated as an accelerometer, the scale factor and nonlinearity of the actuator are evaluated to be 93.4 mV/g and 0.03%, respectively, with the input range of ±1 g. The tested static out-of-plane displacement ranges from −0.88 to 1.39 μm when the actuator is placed horizontally, and −1.16 to 1.25 μm when placed vertically, both with the supply voltage of 12 volts.

Design of narrow band photonic filter with compact MEMS for tunable resonant wavelength ranging 100 nm

A prototype of planar silicon photonic structure is designed and simulated to provide narrow resonant line-width (∼2 nm) in a wide photonic band gap (∼210 nm) with broad tunable resonant wavelength range (∼100 nm) around the optical communication wavelength 1550 nm. This prototype is based on the combination of two modified basic photonic structures, i.e. a split tapered photonic crystal micro-cavity embedded in a photonic wire waveguide, and a slot waveguide with narrowed slabs. This prototype is then further integrated with a MEMS (microelectromechanical systems) based electrostatic comb actuator to achieve “coarse tune” and “fine tune” at the same time for wide range and narrow-band filtering and modulating. It also provides a wide range tunability to achieve the designed resonance even fabrication imperfection occurs.

A novel micro transportation system with fast movement of a micro container based on electrostatic actuation and a ratchet mechanism

This paper presents a novel micro transportation system (MTS), which can drive micro containers in both straight and curved paths based on an electrostatic comb actuator and a ratchet mechanism. The micro container, which has four driving wings and four anti-reverse wings attached to its central ‘backbone’, is driven to move forward only by an electrostatic actuator. While the driving wings act as the legs of a water strider to push the container forward, the anti-reverse wings work as a ratchet mechanism to prevent the container from moving backward. The container with a length, width and thickness of 500 µm, 250 µm and 30 µm, respectively, moves unidirectionally with a desirable velocity up to 1000 µm s−1 in straight and curved paths. The velocity can be changed by varying the frequency and/or amplitude of the driving voltage. The MTS has been fabricated from SOI (silicon on insulator) wafer utilizing silicon micromachining technology with only one mask.

Design and Simulation of a MEMS Control Moment Gyroscope for the Sub-Kilogram Spacecraft

A novel design of a microelectromechanical systems (MEMS) control moment gyroscope (MCMG) was proposed in this paper in order to generate a torque output with a magnitude of 10−6 N·m. The MCMG consists of two orthogonal angular vibration systems, i.e., the rotor and gimbal; the coupling between which is based on the Coriolis effect and will cause a torque output in the direction perpendicular to the two vibrations. The angular rotor vibration was excited by the in-plane electrostatic rotary comb actuators, while the angular gimbal vibration was driven by an out-of-plane electrostatic parallel plate actuator. A possible process flow to fabricate the structure was proposed and discussed step by step. Furthermore, an array configuration using four MCMGs as an effective element, in which the torque was generated with a phase difference of 90 degrees between every two MCMGs, was proposed to smooth the inherent fluctuation of the torque output for a vibrational MCMG. The parasitic torque was cancelled by two opposite MCMGs with a phase difference of 180 degrees. The designed MCMG was about 1.1 cm × 1.1 cm × 0.04 cm in size and 0.1 g in weight. The simulation results showed that the maximum torque output of a MCMG, the resonant frequency of which was approximately 1,000 Hz, was about 2.5 × 10−8 N·m. The element with four MCMGs could generate a torque of 5 × 10−8 N·m. The torque output could reach a magnitude of 10−6 N·m when the frequency was improved from 1,000 Hz to 10,000 Hz. Using arrays of 4 × 4 effective elements on a 1 kg spacecraft with a standard form factor of 10 cm × 10 cm × 10 cm, a 10 degrees attitude change could be achieved in 26.96 s.

An ultrasmall wavelength-selective channel drop switch using a nanomechanical photonic crystal nanocavity

We propose a nanomechanical channel drop switch by controlling the submicron distance between a nanocavity and photonic crystal (PC) waveguide with an ultrasmall electrostatic comb actuator. Light propagating in the PC waveguide is coupled with a nanocavity and emitted as a drop signal under a resonant condition. The proposed switch has been fabricated by silicon micromachining. The drop efficiency has been controlled by 12.5 dB and gap change of 600 nm.

Submicron silicon waveguide optical switch driven by microelectromechanical actuator

A submicron silicon waveguide optical switch driven by a microelectromechanical actuator is fabricated. The switch is composed of single-mode silicon input and output waveguides and a movable waveguide suspended by silicon springs. Switching occurs when the air gap between input and output waveguides is closed by displacing the movable waveguide 540±20nm by an electrostatic comb actuator. The switch, fabricated on a silicon-on-insulator wafer, is monolithic, and active switching region is as small as about 40×60μm2. An extinction ratio of 15±2dB is experimentally obtained between switch off and on states at 1.55μm wavelength.

Theoretical Analysis of Mechanical-Contact-Based Submicron-Si-Waveguide Optical Microswitch at Telecommunication Wavelengths

We present the theoretical analysis of a novel mechanical-contact-based submicron-Si-waveguide optical switch. The switch is composed of an identical input and output waveguides, and a movable waveguide driven by a miniature electrostatic comb actuator. The movable waveguide closes the gap between input and output waveguides. Due to mechanical-contact of submicron-Si-waveguides, input light propagates from input waveguide to output waveguide through the movable waveguide. The switch uses a transverse-electric-like polarized single-mode light at 1.55 μm telecommunication wavelength as the incident light. Theoretical calculations suggest that the optical switch with contact-tip waveguides at 45° tip angle can realize very sharp output signal change. As a result, 97.0 % transmission at the on state and 0.2 % transmission at the off state, corresponding to approximately 96.8 % optical output signal change between on and off states, are theoretically achieved by a displacement of 400 nm. Possible fabrication imperfections are estimated and their effects on the optical performance are calculated. Enlargement of all contact-tip surfaces is understood to be a promising approach for the switch performance enhancement against possible fabrication imperfections.

Light-Intensity-Feedback-Waveform Generator Based on MEMS Variable Optical Attenuator

Increasing demands on the dynamical behavior of microelectromechanical systems (MEMS) devices are reaching a point where mechanical design by itself cannot provide further improvements. Alternative approaches based on control theory, such as the open-loop or the closed-loop driving strategies, must be used instead to provide further enhancements in device performance. In this paper, the design of a light-intensity control system for an optical waveform generator is presented. The optical waveform generator is based on the light-modulation ability of MEMS variable optical attenuators (VOA). The VOA is of shutter-insertion type driven by an electrostatic comb drive. The control system consists of an inner position-control loop and an outer light-intensity-control loop. The inner control loop improves the dynamic response of the position of the MEMS electrostatic comb actuator by using position feedback, whereas the outer feedback loop handles both light-intensity regulation and tracking. An experimental setup and a practical system characterization are given. Based on this, the feedback control system is implemented on an actual MEMS VOA. The results verify that the control system proposed in this paper does significantly improve both the accuracy and the dynamical behavior of the existing device.