An electrostatic microactuator system for application in high-speedjets

The development of an electrostatic microactuator system for the study and control of high-speed jet flows is presented. The electrostatic actuator is 1.3 mm wide, 14 μm thick and has a head that overhangs a glass substrate, intruding into the flow by 200 μm. The actuator has been fabricated using a bulk-silicon dissolved-wafer process to increase device thickness for increased stiffness in the flow direction. Characterization of the new actuators demonstrated their ability to oscillate with amplitudes of up to 70 μm peak-to-peak at resonant frequencies of 5 and 14 kHz. This is a very large motion at such high frequencies when compared to existing macro or micro mechanical actuators. The full actuator system was mounted around the exit of a high-speed jet using several sector-shaped PC boards. This enabled detailed examination of the ability of the actuators to withstand the flow environment and generate substantial flow disturbances. The results showed that the microactuators functioned properly up to jet speeds of 300 m/s while generating disturbances in the shear layer surrounding the jet comparable to those produced by other macro-scale methodologies     

A micromachined impact microactuator driven by electrostatic force

This paper presents a novel micromachined actuator which is developed to produce precise and unlimited displacement. The actuator is driven by impact force between a silicon micro-mass and a stopper. The suspended silicon micro-mass is encapsulated between two glass plates and driven by electrostatic force. When the mass hits the stopper which is fixed on glass plates, impact force is generated to drive the whole actuator in a nano size step (/spl sim/10 nm). The overall dimension of the device is 3 mm /spl times/3 mm. The driving voltage is 100 V and average speed is 2.7 /spl mu/m/s. The total thickness is 600 /spl mu/m.     

Design and fabrication of a SU-8 based electrostatic microactuator

Comb-drive microactuator is widely used in MEMS devices and traditionally is made of silicon as structural material using silicon-based fabrication technology. Recent development in UV lithography of SU-8 has made it possible to fabricate the ultra high aspect ratio microstructures with excellent sidewall quality. In this paper, we report a low cost alternative to the silicon-based comb drive by using cured SU-8 polymer as structural material. The microactuator was designed to have a integrated structure without assembly or bonding. A unique integration fabrication process was successfully developed based on UV lithography of SU-8 and selectively metallizing SU-8 polymer structures. Preliminary experimental results have proved the feasibility of the microactuator and the fabrication technology.     

Integrated fabrication of electrostatic microactuator for HDD R/W head positioning

We designed an electrostatic actuator which can work with the voice coil motor in a dual-stage servo control system to accurately position the read/write (R/W) head in hard disk drives. An integrated fabrication process for this microactuator has been proposed, considering total fabrication of R/W head and alumina-titanium carbide (AlTiC) slider. Adhesive bonding technique using photoresist SU-8 was selected for wafer bonding between silicon and AlTiC. A microactuator prototype has been fabricated by the proposed process. The stroke of the microactuator has been measured during the static testing.     

Fabrication of an S-shaped microactuator

We have developed a new fabrication process for electrostatic actuators having an S-shaped film element, which we previously invented for such applications as gas valves. The developed process allows batch fabrication of the actuator whose S-shaped structure height, which is equal to the amount of vertical film displacement, is of the order of a few hundred micrometers. The microactuators are fabricated by stacking three wafers. The middle wafer contains the sputtered Ni film strip which is buckled into an S-shape during the stacking process. The length of film necessary for the S-bend profile has a folded structure which is stretched after stacking. The size of the fabricated chip was 5 mm×5 mm, and the vertical film displacement was 220 μm. The actuator was operated by electrostatic force when the applied voltage was more than 70 V     

An electrostatic microactuator using LIGA process for a magnetic head tracking system of hard disk drives

 We demonstrate an electrostatic micro actuator which is fabricated by LIGA process. The actuator is designed for a magnetic head tracking system of hard disk drives (HDDs). The actuator is essential to achieve very high track density of HDDs. We realize the aspect ratio of 125 by the LIGA process using a Si-Au mask. We propose to use PMMA molds both as the mechanical structure and as the insulator between electrodes. We believe there are great opportunity for the LIGA process in making micro actuators of HDDs. Received: 25 August 1997 / Accepted: 24 October 1997     

Fabrication of electrostatic MEMS microactuator based on X-ray lithography with Pb-based X-ray mask and dry-film-transfer-to-PCB process

Lithographie Galvanoformung Abformung (LIGA) is a promising approach for fabrication of high aspect ratio 3D microactuator for dual-stage slider in hard disk drive. However, this approach involves practically challenging X-ray lithography and structural transfer processes. In this work, electrostatic MEMS actuator is developed based on a LIGA approach with cost-effective X-ray lithography and dry-film-transfer-to-PCB process. X-ray lithography is performed with X-ray mask based on lift-off sputtered Pb film on mylar substrate and photoresist application using casting-polishing method. High quality and high aspect ratio SU8 microstructures with inverted microactuator pattern have been achieved with the interdigit spacing of ~5 μm, vertical sidewall and a high aspect ratio of 29 by X-ray lithography using the low-cost Pb based X-ray mask. A new dry-film-transfer-to-PCB is employed by using low-cost dry film photoresist to transfer electroplated nickel from surface-treated chromium-coated glass substrate to printed circuit board (PCB) substrate. The dry film is subsequently released everywhere except anchor contacts of the electrostatic actuator structure. The fabricated actuator exhibits good actuation performance with high displacement at moderate operating voltage and suitably high resonance frequency. Therefore, the proposed fabrication process is a promising alternative to realize low-cost MEMS microactuator for industrial applications.     

Optimization of an electro-thermally and laterally driven microactuator

 In previous research about thermally and laterally driven microactuators, Guckel et al. proposed a microactuator based on the variable cross sections but the same length of two adjacent beams. Pan and Hsu presented another type of microactuator based on different lengths but the same cross sections of two adjacent beams. Here, a microactuator that combines the traits of those two designs is proposed, optimized, and fabricated. Finite element analysis is performed to obtain the optimal dimensions of the structures when the maximum lateral displacement is achieved. When the air gap is 2 μm, the optimal performance of current design is similar to Guckel's design with optimal dimensions, but it has 65% larger displacement than Pan and Hsu's design at optimal dimensions. For the case of the large air gap beneath the structure (300 μm), the microactuator of current design has more than 25% tip displacement improvement compared to Guckel's or Pan and Hsu's designs. It is also found that the proposed structure is less sensitive to the beam length variations around optimal point no matter in small or large air gap case. Experimental results also verify that the dimension of the air gap affects the thermal boundary condition, so does the performance and optimal architecture of the microactuators. Received: 17 August 2001/Accepted: 3 May 2002 This project was supported by the National Science Council of the Republic of China, NSC89-2218-E009-005. The staffs at Semiconductor Research Center of NCTU are gratefully acknowledged for providing facilities and technical support. This paper was presented at the Conference of Micro System Technologies 2001 in March 2001.     

A design methodology for a bulk-micromachined two-dimensional electrostatic torsion micromirror

This paper presents a design methodology for a two-dimensional (2-D) electrostatic torsion micromirror fabricated with bulk-micromachining technology. The theoretical models in mechanical and electrostatic fields presented here provide insights into the influences of different design parameters on micromirror performance. Parametric numerical models built in ANSYS are used to more accurately predict its performance and further refine the design parameter values derived from the theoretical models. By use of the electrical analogy method, an equivalent electrical circuit is built in PSPICE to predict the static and dynamic performance of this micromirror, with the numerical simulation results as the input parameters. The equivalent electrical circuit has been demonstrated to be a simple and powerful approach to characterize the performance of this 2-D torsion micromirror. The test results for this micromirror reveal very good agreement between experimental and numerical results, taking into account fabrication tolerances and experimental accuracies. Incorporating the fabrication tolerances of bulk-micromachining technology, this design methodology can be readily applied to performance characterization and design optimization.     

Analytical behavior of rectangular electrostatic torsion actuators with nonlinear spring bending

In this paper, we study the pull-in effect for rectangular electrostatic torsion actuators by using analytical calculations that include the higher order effects of nonlinear spring bending. The calculation approach speeds the design of such systems. The method is found to be suitable for actuators with single long beam springs where the ratio of the resonant frequencies for the torsion and bending modes is up to at least 3.5, in the region where bending dominates torsion. After fitting the theory in this paper to Coventor simulation results with three nonphysical coefficients, the fractional differences between Coventor simulation and analytical calculation results are smaller than 6%. The method is also suitable for at least one class of folded spring designs, with greatly decreased bending mode displacement. The main results are also verified by comparing them with published experimental results.     

Design and fabrication of an angular microactuator for magneticdisk drives

Angular electrostatic microactuators suitable for use in a two-stage servo system for magnetic disk drives have been fabricated from molded chemical-vapor-deposited (CVD) polysilicon using the HexSil process. A 2.6-mm-diameter device has been shown to be capable of positioning the read/write elements of a 30% picoslider over a ±1-μm range, with a predicted bandwidth of 2 kHz. The structures are formed by depositing polysilicon via CVD into deep trenches etched into a silicon mold wafer. Upon release, the actuators are assembled onto a target wafer using a solder bond. The solder-bonding process will provide easy integration of mechanical structures with integrated circuits, allowing separate optimization of the circuit and structure fabrication processes. An advantage of HexSil is that once the mold wafer has undergone the initial plasma etching, it may be reused for subsequent polysilicon depositions, amortizing the cost of the deep-trench etching over many structural runs and thereby significantly reducing the cost of finished actuators. Furthermore, 100-μm-high structures may be made from a 3-μm deposition of polysilicon, increasing overall fabrication speed     

Performance improvement of an electrothermal microactuator fabricated using Ni-diamond nanocomposite

In this paper, a low-temperature stress-free electrolytic nickel (EL) deposition process with added dispersed diamond nanoparticles (diameter <0.5 /spl mu/m) is developed to synthesize Ni-diamond nanocomposite for fabricating electrothermal microactuators. Device characterization reveals dramatic performance improvements in the electrothermal microactuator that is made of the nanocomposite, including a reduction in the input power requirement and enhanced operation reliability. In comparison with the microactuator made of pure nickel, the nanocomposite one can save about 73% the power for a 3 /spl mu/m output displacement and have a longer reversible displacement range, which is prolonged from 1.8 /spl mu/m to more than 3 /spl mu/m. Furthermore, the nanocomposite device exhibits no performance degradation after more than 100 testing cycles in the reversible regime. The enhancements increase with the incorporation of the nanodiamond in a nickel matrix, so the Ni-diamond nanocomposite has potential for application in MEMS fabrication.     

Design and fabrication of an electrothermal microactuator for multi-level conveying

During the past years, a variety of microactuators developed for micro-conveyors have been presented. However, such micro-conveyors can only provide conveying motion in a single plane. Here an electrothermally driven microactuator with a capability of adjustable height is proposed, which may act as a basic unit for multi-level conveyors. This microactuator is based on the principle of thermal bimorph actuation with two long conveying fingers to exert out-of-plane bending motions in the transversal direction, which are connected and lifted by an initially curved height adjuster in the longitudinal direction. The devices can provide conveyance of micro-objects between two plane levels of different heights. The testing results show that the two fingers and a height adjuster can be actuated simultaneously and individually with little thermal crosstalk. The proposed device with a dimension of 900×100×4.5 μm³ can provide 5 μm vertical displacements by the height adjuster at 1 V and 18 μm lateral displacements by the conveying finger at 2 V. Simulations by finite-element program ANSYS 5.7 have been performed and widely match with testing results.     

A multiple electrostatic electrodes torsion micromirror device with linear stepping angle effect

Torsion micromirror devices that can achieve linear stepping angle effects play an important role in optical MEMS applications. However, traditional torsion micromirror devices driven by a single electrostatic electrode have difficulty meeting this requirement due to their nonlinear angle-voltage transfer characteristics. In this regard, the concept of a multiple-electrodecontrolled micromirror is proposed to eliminate this drawback. Through this novel design, linear stepping angles can be easily achieved by a set of linearly varied or constantly applied voltages. A simple mathematical model has been developed to predict the angle-voltage transfer characteristics of the proposed device and has been simulated with finite element simulations. The corresponding control strategies of this device, named the linear control strategy and the digital control strategy, are also proposed in this paper. The Cronos/MEMSCAP Multi-User MEMS Process (MUMPs) was used in conjunction with flip-chip bonding technology to fabricate the proposed torsion micromirror device. Experimental data indicates that the relative stepping angle error, between the fabricated device and the mathematical model, are within 5%.     

Investigation of the torsion and bending effects on static stability of electrostatic torsional micromirrors

In this paper the electromechanical behavior of a torsional micromirror was investigated using of a static model with considering torsion and bending characteristics of micro-beams. A set of nonlinear equations based on the parallel plate capacitor model was derived to represent the relationships between the applied voltage, torsion angle, and vertical displacement of the torsional micromirror. Step by step linearization method (Newton’s method) was used to calculate the rotation angle and vertical displacement of the micromirror due to the applied voltage. This method is fast and gave acceptable and accurate results which were in good agreement with the experimental data.     

Pull-in voltage study of electrostatically actuated fixed-fixed beams using a VLSI on-chip interconnect capacitance model

A highly accurate computationally efficient closed-form model has been developed to determine the pull-in voltage of an electrostatically actuated fixed-fixed beam. The approach includes the electrostatic spring softening effects due to the fringing field capacitances along with the nonlinear spring hardening effects associated with the load-deflection characteristics of a uniformly loaded fixed-fixed beam. Meijs and Fokkema's highly accurate empirical formula for the capacitance of a VLSI on-chip interconnect has been used to determine the spring softening effects due to the fringing field capacitances. The developed model has been verified by comparing the results with published experimentally verified three-dimensional (3-D) finite element analysis (FEA) results and with those from other published representative closed-form models. The developed model can determine the pull-in voltage with a maximum deviation of 1.27% from the FEA results for small deflections and for large deflections (airgap-beam thickness ratio =12), the deviation from the FEA results is 2.0%. A maximum deviation of 0.5% from the FEA results has been observed for extreme fringing field cases (beamwidth-airgap ratio /spl les/0.5). The model's accuracy range is better compared to the other published models.     

Gain-scheduled control of an electrostatic levitator

A simple gain-scheduling approach to the position control of electrostatically levitated materials is presented. An electrostatic levitator is currently being constructed at the German Aerospace Center (DLR). It can be used for containerless processing of metals and nonconducting materials. Time-varying sample charges and the fact that the plant is open-loop unstable make this a difficult and challenging control problem. The nonlinear parameter-dependent system is approximated by two local linear models. Based on these models, two local linear controllers are designed. LMI-based pole region assignment is used to reduce the design procedure to the tuning of three parameters with intuitive meaning. The sample charge is estimated with a discrete-time extended Kalman filter and used as scheduling parameter. Experimental results demonstrate that gain-scheduled control achieves a significantly better performance than fixed-gain, observer-based state feedback control.     

Optimal shape design of a rotary microactuator

Optimal shape designs of electrodes based on parallel-plate configurations are proposed for an electrostatic rotary microactuator to enhance actuating force generation capability. In most conventional electrostatic microactuators associating parallel-plate configurations with rotary mode, the shape of electrodes have not been rigorously evaluated in terms of optimality and straight shapes were simply adopted. As a result, two facing electrodes are spaced in the way of leading to relatively large clearance at the outer region from the center of rotation; hence force generation capability is inherently limited. To overcome the limitation of conventional rotary microactuators and enhance the capability, a tilting configuration is invoked and integrated into the optimal shape, which allows the gap size between two facing electrodes to be as small as the minimum gap size achievable by current fabrication technology. This study demonstrates that the proposed optimal shape integrating tilting configuration increases the force generation capability dramatically over conventional shapes     

静电吸合对力平衡式隧道加速度传感器的影响

隧尖与阳极之间的可控间距是关系到隧道加速度传感器能否正常工作的一个重要参数,静电吸合限制了该可控间距的范围,本文从理论上分析了静电吸合对隧道加速度传感器设计的影响。对于质量块作活塞式运动的平动型隧道加速度传感器,只有当隧尖与阳极之间的初始间距小于两个反馈电极之间的初始间距的三分之一加上发射间距的和时,力平衡才能实现。对于质量块由多根平行悬臂梁支承的隧道加速度传感器,首次提出并证明,增大隧尖与悬臂梁末端的水平距离可以扩大锥尖高度和锥尖与阳极之间的初始间距的取值范围,从而降低对传感器加工工艺的要求。