Electrostatically operated micromirrors for Hadamard transformspectrometer

The paper presents the development of a linear micromirror array which can be used as a switchable entrance mask for a double-array Hadamard transform spectrometer. In addition to the detector array, the double-array spectrometer has a linear multislit array realized by independently switchable micromirrors at the entrance side. Two different switch positions of the electrostatically operated mirrors allow the reflection of light into or away from the spectrometer. With this arrangement (mirror array, concave grating, and array detector) and the use of the Hadamard transform principle it is possible to increase the signal-to-noise ratio and the resolution of the system compared to conventional spectrometers of the same size. In order to use a micromirror array as an entrance mask the dimensions of the switching elements must be similar to those of entrance slits and the mirror surface must have a high reflectivity in the desired spectral range. To obtain a high resolution a given pitch between the mirrors must be realized. Using thin-film technology we developed a linear array with 18 tiltable mirrors. The mirrors consist of a multilayer system of aluminum and silicon nitride deposited on a structured sacrificial layer of polyimide. The aluminum has a comparatively high reflectivity in the UV/VIS region and the silicon nitride yields good mechanical properties for realizing stress reduced mirrors and bending structures. After etching of the sacrificial layer, the mirrors can be switched electrostatically. The angle between on and off position is about 5°. Low switching voltages in the range of 15-45 V can be realized     

A Large-Area High-Reflectivity Broadband Monolithic Single-Crystal-Silicon Photonic Crystal Mirror MEMS Scanner With Low Dependence on Incident Angle and Polarization

In this paper, we introduce a single-axis resonant combdrive microelectromechanical systems (MEMS) scanner with a large-area highly reflective broadband monolithic single-crystal-silicon (SCS) photonic crystal (PC) mirror. PC mirrors can be made from a single monolithic piece of silicon through alternate steps of etching and oxidation. This process allows the fabrication of a stress-free PC reflector in SCS with better optical flatness than deposited films such as polysilicon slabs on low-index oxide. PC mirrors can be made in IR transparent dielectric material and can achieve high reflectivity over a broad wavelength range. PC reflectors have several advantages over other mirror technologies. They can tolerate much higher processing temperatures and higher incident optical powers as well as operate in more corrosive environments than metals. Compared to multilayer dielectric stacks, PC mirrors allow for simpler process integration, thus making them highly compatible with CMOS and MEMS processing. In this paper, we fabricate a PC mirror MEMS scanner in SCS without any deposited films. Our PC mirrors show broadband high reflectivity in the wavelength range from 1550 to 1600 nm, and very low angular and polarization dependence over this same range. The single-axis MEMS scanners are fabricated on silicon-on-insulator (SOI) wafers with the PC mirrors also fabricated in the SOI device layer. The scanners are actuated by electrostatic comb drives on resonance. Dynamic deflection measurements show that the scanners achieve 22deg total scan angle with an input square wave of 67 V and have a resonance frequency of 2.13 kHz.     

Single-Crystal-Silicon Continuous Membrane Deformable Mirror Array for Adaptive Optics in Space-Based Telescopes

In this paper, we present a single-crystal-silicon (SCS) continuous membrane deformable mirror (DM) as a corrective adaptive-optics (AO) element for space-based telescopes. In order to correct the polishing errors in large aperture (~8 m) primary mirrors, a separate high-quality surface DM array must be used. Up to 400000 elements and a mirror stroke of ~100 nm are required for the correction of these polishing errors. A continuous membrane mirror formed by the the SCS device layer of a silicon-on-insulator (SOI) wafer is used to achieve a high-quality optical surface and to minimize the additional diffractive effects in the optical system. To achieve substantial local deformation needed to correct high-order errors, we use a highly deformable silicon membrane of 300-nm thickness. This thin membrane is able to deform locally by 125 nm at an operating voltage of 100 V with a pixel pitch of 200 mum. The resonance frequency of a pixel is 25 kHz with a low Q-factor of 1.7 due to squeeze-film damping. The device is fabricated by processing the microelectromechanical system (MEMS) and electronic chips separately and then combining them by flip-chip bonding. This allows optimization of the MEMS and electronics separately and also allows the use of an SOI layer for the mirror by building the MEMS bottom up. A small prototype array of 5times5 pixels with 200-mum pitch is fabricated, and we demonstrate single pixel and multiple pixel actuation     

Mathematical Modeling and Analysis of MEMS Deformable Mirror Actuated by Electrostatic Piston Array

In this report, we constructed a simple mathematical analysis model for a new MEMS deformable mirror using an electrostatic piston array. The deformable mirror has been developed as a wavefront compensation device in adaptive optics for retinal observation. The device realizes a large convex deformation with a low operation voltage because of moving bottom electrodes on the pistons. The constructed model is based on the plate theory and simple superposition of the actuation components. The calculated deformation analyzed using this model agrees well with that analyzed using the finite element method not only in deformation shape but the peak‐valley deformations. In addition, the calculation time is much shorter, so the model can be used for design optimization of the device.     

Electrostatic actuation of three-dimensional MEMS mirrors using sidewall electrodes

We propose and demonstrate electrostatic sidewall-electrodes actuation of three-dimensional (3-D) microelectromechanical systems (MEMS) gimbal mirrors. The linearity of the mirror angle dependence on actuation voltage is improved with the sidewall-electrodes actuation. In addition, the undesired spring-softening effect commonly found in electrostatic actuation, where the mirror resonance frequency decreases with increased tilt angle, is significantly reduced. Sidewall actuation enables superior performance of 3-D MEMS mirrors including large pull-in angles, reduced actuation voltages, improved device reliability, and fast switching times.     

Development of Flexible Piezoelectric Strain Sensor Array

In this paper, we present a novel flexible sensor array manufacturing process that involves transfer printing methods using a chip mounter with a vacuum collet. We successfully transfer‐printed continuously very fragile microelectromechanical systems (MEMS)‐based 5‐mm‐long, 1‐mm‐wide, 5‐μm‐thick high‐aspect‐ratio ultrathin PZT (1.9 μm)/Si (3 μm) strain sensors onto a polyimide based flexible printed‐circuit (FPC) substrate with etched Cu wiring. Then, we connected the sensors to the Cu wiring by printing insulating and conductive pastes using a screen printer. The output voltage based on the deformation behavior of the test plate was generated from the flexible piezoelectric strain sensor array attached to the plate. Therefore, the developed piezoelectric sensor array is capable of easily performing the distribution measurement of the strain leading to damage such as cracks.     

用于激光雷达的双层梳齿驱动MEMS扫描镜

提出了一种新型双层梳齿驱动的大尺寸、大偏转角度、低驱动电压微机电系统(MEMS)扫描镜的设计方法。该扫描镜设计嵌套式内外双层垂直梳齿结构,梳齿间采用静电排斥力驱动,改进后的S型扭转梁有效地降低敏感轴刚度。基于垂直梳齿驱动理论分析,建立了该扫描镜的理论模型,并利用MAXWELL和ANSYS仿真软件进行了其静态、动态分析与验证,研究了该扫描镜的体硅制备工艺。仿真实验结果表明,在110 V的驱动电压扫描下,该双层梳齿驱动下的MEMS扫描镜可以实现最大偏转角±13.46°;此外,结构的谐振频率为1.79 kHz,远低于其他高阶模态,有效地抑制了其他非工作模态的交叉干扰运动,获得良好的工作带宽。     

Effects of electrical leakage currents on MEMS reliability and performance

Electrostatically driven MEMS devices commonly operate with electric fields as high at 10/sup 8/ V/m applied across the dielectric between electrodes. Even with the best mechanical design, the electrical design of these devices has a large impact both on performance (e.g., speed and stability) and on reliability (e.g., corrosion and dielectric or gas breakdown). In this paper, we discuss the reliability and performance implications of leakage currents in the bulk and on the surface of the dielectric insulating the drive (or sense) electrodes from one another. Anodic oxidation of poly-silicon electrodes can occur very rapidly in samples that are not hermetically packaged. The accelerating factors are presented along with an efficient early-warning scheme. The relationship between leakage currents and the accumulation of quasistatic charge in dielectrics are discussed, along with several techniques to mitigate charging and the associated drift in electrostatically actuated or sensed MEMS devices. Two key parameters are shown to be the electrode geometry and the conductivity of the dielectric. Electrical breakdown in submicron gaps is presented as a function of packaging gas and electrode spacing. We discuss the tradeoffs involved in choosing gap geometries and dielectric properties that balance performance and reliability.     

Microfabrication of ionic polymer actuators by selective plasma irradiation

A novel method for fabricating a miniaturized ionic polymer–metal composite (IPMC) with plated gold electrodes is developed by combining selective plasma irradiation and electroless plating. The membrane is irradiated by SF₆ or CF₄ plasmas through a stencil mask prior to plating to prevent deposition of a gold electrode in the plasma‐irradiated area. Miniaturized IPMCs are fabricated with a 400‐µm‐wide line‐and‐space pattern. The proposed method has the potential to realize miniaturized IPMCs combined with microelectromechanical system (MEMS) devices because this provides a direct and simple fabrication process. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

MTL‐based implementation of current‐mode CMOS RMS‐to‐DC converters

In this paper, systematic implementation of current‐mode RMS‐to‐DC converters based upon MOS translinear (MTL) principle, utilizing symmetric cascoded MTL cell (SCMC) is proposed. Theory of operation and mathematical analysis of both explicit (direct) and implicit (indirect) techniques for realization of SCMC‐based RMS‐to‐DC converters are discussed. The SCMC includes a folded MTL loop and realizes an MTL equation. MTL principle utilizes the square law characteristics of saturated MOS transistors to realize square‐root domain (SRD) functions. The SCMC is constructed by two connected cascoded current mirrors and has a compact, symmetric, and multi‐purpose structure, with capability of implementing the circuits into the programmable and configurable structures. The proposed RMS‐to‐DC converters utilize the SCMC along with a configurable current mirror array. The required squaring and square‐rooting functions are realized using the SCMC, after proper configuration of the current mirror array. The proposed circuits have been implemented using a reconfigurable architecture fabricated in a 0.5 µm CMOS technology. Copyright © 2014 John Wiley & Sons, Ltd.     

Surface micromachined segmented mirrors for adaptive optics

This paper presents recent results for aberration correction and beam steering experiments using polysilicon surface micromachined piston micromirror arrays. Microfabricated deformable mirrors offer a substantial cost reduction for adaptive optic systems. In addition to the reduced mirror cost, microfabricated mirrors typically require low control voltages (less than 30 V), thus eliminating high-voltage amplifiers. The greatly reduced cost per channel of adaptive optic systems employing microfabricated deformable mirrors promise high-order aberration correction at low cost. Arrays of piston micromirrors with 128 active elements were tested. Mirror elements are on a 203-μm 12×12 square grid (with 16 inactive elements, four in each corner of the array). The overall array size is 2.4 mm square. The arrays were fabricated in a commercially available surface micromachining process. The cost per mirror array in this prototyping process is less than $200. Experimental results are presented for a hybrid correcting element comprised of a lenslet array and piston micromirror array, and for a piston micromirror array only. Also presented is a novel digital deflection micromirror that requires no digital to analog converters, further reducing the cost of adaptive optics systems     

A behavioral model for optically powered OCT endoscope with a micro electrostatic vertical‐comb optical scanner

This paper presents a novel device architecture for optically actuated microelectromechanical systems (MEMS) endoscopes for optical coherence tomography (OCT) measurement. A 10‐mW infrared light beam at a wavelength of 1.5 µm is transferred through the single‐mode fiber to provide a scanning MEMS mirror with the drive voltage (maximum 11 V) by exciting a photovoltaic cell, while also providing with a secondary light beam at a wavelength of 1.3 µm for the OCT measurement. An electrostatic vertical comb‐drive optical scanner (1.5 mm × 2.0 mm × 0.5 mm) has been developed by using the deep reactive ion etching (DRIE) of a silicon‐on‐insulator (SOI) wafer. The design of the scanner module is discussed, along with the experimental results of electrostatic operation. An equivalent circuit model for the optical scanner is developed to explain the behavior of the optically powered actuation mechanism, including the hysteresis loop in the frequency response and the voltage dependence of oscillating angle (mechanical peak ±3.2°/7 V around the resonance frequency of 250 Hz). OCT measurement of a tissue is demonstrated to reconstruct the cross‐sectional image of a fingerprint at a resolution of lateral 40 µm × vertical 8 µm and penetration depth of 2.5 mm. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Theory of double-chirped mirrors

A theory of double-chirped mirrors (DCMs) for dispersion compensation in ultrashort pulse laser sources is presented. We describe the multilayer interference coating by exact coupled-mode equations. They show that the analysis and synthesis of a coating with a slowly varying chirp in the layer thicknesses can be mapped onto a weakly inhomogeneous transmission line problem. Solutions of the transmission line equations are given using the WKB-method. Analytic expressions for reflectivity and group delay are derived. The solutions show that the main problem in chirped mirror design is the avoidance of spurious reflections, that lead to Gires-Tournois-like interference effects responsible for the oscillations in the group delay. These oscillations are due to an impedance matching problem of the equivalent transmission line. The impedance matching can be achieved by simultaneously chirping the strength of the coupling coefficient and the Bragg wavenumber of the mirror. An adiabatic increase in the coupling coefficient removes the typical oscillations in the group delay and results in broad-band mirrors with a controlled dispersion. Finally, the mirror is matched to air with a broadband antireflection coating. We discuss a complete design of a laser mirror with a reflectivity larger than 99.8% and a controlled dispersion over 300-nm bandwidth. Using such mirrors in a Ti:sapphire laser, we have demonstrated ≈30-fs pulses, tunable over 300 nm, as well as 8-fs pulses from the same setup. A different design resulted in 6.5-fs pulses     

Movable vertical mirror arrays for optical microswitch matrixes andtheir electromagnetic actuation

Movable vertical mirror arrays optical microswitch matrixes and their electromagnetic actuation with microcoils are presented as an extension of previous results on a first prototype of microswitch (Helin et al., 2000) a new mechanical design is developed to further increase the density of integration of the optical bypass. Deep reactive ion etching and KOH etching are successively used to realize the whole structure getting the advantages of both (self-alignment between vertical smooth mirrors and fiber guides). Microcoils are fabricated in order to actuate the microswitches electromagnetically. Efforts are done to increase the electromagnetic force generated by the microcoils in order to allow down scaling the sizes and large displacements of the microswitch movable parts in the same dimensions as the optical fibers. A high aspect ratio SU8 pattern and the use of ferromagnetic materials should achieve these requirements     

Arrays of High Tilt-Angle Micromirrors for Multiobject Spectroscopy

Micromirror arrays are promising components for generating reflective slit masks in future multiobject spectrographs. The micromirrors, 100 mum times200 mum in size, are etched in bulk single crystal silicon, whereas a hidden suspension is realized by surface micromachining. The micromirrors are actuated electrostatically by electrodes located on a second chip. The use of silicon on insulator (SOI) wafers for both mirror and electrode chip ensures thermal compatibility for cryogenic operation. A system of multiple landing beams has been developed, which latches the mirror at a well-defined tilt angle when actuated. Arrays of 5times5 micromirrors have been realized. The tilt angle obtained is 20deg at a pull-in voltage of 90 V. Measurements with an optical profiler showed that the tilt angle of the actuated and locked mirror is stable with a precision of 1 arcmin over a range of 15 V. This locking system makes the tilt angle independent from process variations across the wafer and, thus, provides uniform tilt angle over the whole array. The surface quality of the mirrors in actuated state is better than 10-nm peak to valley and the local roughness is about 1-nm root mean square     

Microelectromechanical deformable mirrors

A new class of silicon-based deformable mirrors is described. These devices are capable of correcting time-varying aberrations in imaging or beam forming applications. Each mirror is composed of a flexible silicon membrane supported by an underlying array of electrostatic parallel plate actuators. All structural and electronic elements were fabricated through conventional surface micromachining using polycrystalline silicon thin films. A layout and fabrication design strategy for reducing nonplanar topography in multilayer micromachining was developed and used to achieve nearly flat membrane surfaces. Several deformable mirrors were characterized for their electromechanical performance. Real-time correction of optical aberrations was demonstrated using a single mirror segment connected to a closed-loop feedback control system. Undesirable mirror contours caused by residual stress gradients in the membrane were observed     

1.55-μm InP-lattice-matched VCSELs with AlGaAsSb-AlAsSb DBRs

We review the design, fabrication, and characterization of 1.55-μm lattice-matched vertical-cavity surface-emitting lasers, operating continuous wave up to 88°C. For one embodiment, the threshold current is 800 μA, the differential quantum efficiency is 23%, and the maximum output power is more than 1 mW at 20°C and 110 μW at 80°C. The basic structure consists of AlAsSb-AlGaAsSb mirrors, which provide both high reflectivity and an InP-lattice-matched structure. The quaternary mirrors have poor electrical and thermal conductivities, which can raise the device temperature. However, a double-intracavity-contacted structure along with thick n-type InP cladding layers circumvents these drawbacks and finally leads to an excellent performance. The measured voltage and thermal impedances are much lower for the intracavity-contacted device than an air-post structure in which current is injected through the Sb-based quaternary mirror. The structure utilizes an undercut aperture for current and optical confinement. The aperture reduces scattering loss at the etched mirror and contributes to high differential efficiency and low threshold current density     

A compact modelling of double-walled gate wrap around carbon nanotube array field effect transistors

The effects of multi-walled CNTs and array of channels are combined to form Double-walled Gate Wrap Around Carbon Nano Tube array Field Effect Transistor (DWGWA CNTFET). Numerical model is proposed for the device to study its performance. Screening and imaging effects of adjacent and inter walls in array of channels are included for calculating the drive capacitance, subsequently the drive current. This model suits for a wide range of chiralities and diameters. The change in drive capacitance of double-walled and single-walled device with respect to various drain and gate voltage for different values of number of channels, diameters are studied. The number of channels, CNTs diameters, chiralities of the tubes, source/drain length are varied and the corresponding responses of drive current, cut off frequency, signal delay time for both double and single walled devices are compared. In all cases, DWGWA CNTFET excels in its performance over Single-walled Gate Wrap Around Carbon Nano Tube array Field Effect Transistor (SWGWA CNTFET).The model of the proposed device can be utilized for designing the Nano devices with high power and high speed capability.     

Operation of a dual excitation multiphase electrostatic drive by amplitude‐modulated ac voltage

Dual Excitation Multiphase Electrostatic Drive (DEMED) is a synchronous motor that is driven by electrostatic force. Because of its light weight, thinness, and large power per weight ratio, it has promise as a small‐size, high‐power motor in the future. Of the three types of driving method that had been formerly developed for the motor, the single frequency method, which drives the motor by applying a three‐phase ac voltage to the electrodes of the motor, is the most useful since it needs the least number of phases of power supply. However, this method has problems with operation at very low speed such as 10 μm/s. In order to solve such problems, this paper proposes a novel add‐on operation method. In the new method, called modulation drive, the excitation voltage to each phase of the electrodes is amplitude‐modulated by a carrier signal with higher frequency before being applied to the electrodes. In tests regarding the single frequency method, the modulation drive successfully drove a motor with typical power operational amplifiers that were boosted by transformers with ferrite cores, and eliminated the force degradation at low drive speed. Additionally, the force generation of modulation was analyzed, and the results agreed very well with the experimental results. © 2000 Scripta Technica, Electr Eng Jpn, 131(4): 78–84, 2000