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.     

A single-layer PDMS-on-silicon hybrid microactuator with multi-axis out-of-plane motion capabilities-part II: fabrication and characterization

This paper reports on fabrication and characterization of a new electrostatic microactuator that achieves out-of-plane multi-axis motion with a single silicon device layer. The multi-axis motion with the simple actuator design is possible by incorporating a three-dimensional (3-D) polydimethylsiloxane (PDMS) microstructure. This paper develops a new device processing method named "Soft-Lithographic Lift-Off and Grafting (SLLOG)" to fabricate the previously designed PDMS-on-silicon hybrid actuator structure. SLLOG is a low-temperature (less than 150/spl deg/C) process that allows replica molded PDMS microstructures to be integrated in silicon micromachined device patterns. The fabricated actuator is characterized using laser vibrometry. The experimental results demonstrate actuation motions achieved in three independent axes with fast dynamic response reaching a bandwidth of about 5 kHz. The fabricated PDMS-on-silicon actuator yields a vertical displacement up to 5 /spl mu/m and rotational motions with a 0.6-/spl deg/ tilting angle at a 40-V peak-to-peak ac actuation voltage.     

基于硅塑性变形的蛇形梁垂直梳齿驱动器

设计了基于硅塑性变形的垂直梳齿驱动器,中央可动微镜由四组蛇形曲折梁支撑。驱动器的制作采用硅—硅键合技术,首先利用DRIE干法刻蚀技术释放可动梳齿与固定梳齿,然后通过各向异性湿法腐蚀制作的施压凸台实现可动梳齿和固定梳齿的精确自对准,最后利用硅塑性变形使可动梳齿和固定梳齿在垂直方向上产生位错,成功制作出在Z方向依靠位错梳齿实现垂直驱动的蛇形梁静电梳齿驱动器。     

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.     

Integration of displacement sensor into bulk PZT thick film actuator for MEMS deformable mirror

Bulk PZT thick film actuator integrated with displacement sensor, the so-called self-sensing actuator, is presented in this paper. The PZT film is used as not only an actuating layer but also a displacement sensor, which is achieved by dividing the electrode on the top surface of the PZT film into two parts: central top electrode for actuating and outer annular sensor electrode for piezoelectric displacement detection. When the actuator moves, the piezoelectric charge is induced in the outer annular PZT due to the piezoelectric effect. The total amount of accumulated charge is proportional to the stress acting on the PZT, which is in turn proportional to the actuator displacement. By collecting the piezoelectric charge, the actuator displacement can be detected. A theoretical model is proposed to determine the structure parameters of the sensor and predict the sensor sensitivity. Experiments were performed on the micro-fabricated sensor integrated PZT thick film actuator, and the measured piezoelectric charge is close to the theoretical predictions. The integrated piezoelectric sensor has a displacement sensitivity of approximately 4 pC/nm. In addition, the integration of displacement sensor into the actuator needs no additional fabrication process and has no influence on the actuator performances.     

Research on electrostatic actuator polymer thin-film for controlling light scattering phenomena

The purpose of this paper is based on micro fabrication technology, while integrating planar waveguide technology and the scattering phenomenon generated by electro-statically actuator thin film, to develop a 2-dimensional display technology capable of being cleared and re-displayed. For thin film displacement, the restoration of inward elasticity needs to be overcome. During thin film displacement, attraction due to suction occurs when coming into contact with light waveguide; electrostatic force and elastic force are restored and mutually balanced, causing display to light up. On the other hand, when input voltage is released, electrostatic force stops and thin film is restored to original position, causing display to darken. The design structure uses SU-8 as supporting posts, and PDMS as the electrostatic thin film suspended above the glass substrate (light waveguide). The experimental results show that a waveguide with an electrode length of 250 μm (sub-pixel length), a micro-post height of 27 μm, and a PDMS film thickness of 16 μm requires an actuator voltage of 314 V; and a micro-post height of 27 μm, and a PDMS film thickness of 8 μm requires an actuator voltage of 189 V. Thus, with an arrayed micro-electrode design, electronic paper and panels with large color display area could be manufactured.     

Tolerance analysis for comb-drive actuator using DRIE fabrication

Deep reactive ion etching (DRIE) process is specially invented for bulk micromachining fabrication with the objective of realizing high aspect ratio microstructures. However, various tolerances, such as slanted etched profile, uneven deep beams and undercut, cannot be avoided during the fabrication process. In this paper, the origins of various fabrication tolerances together with its effects on the performances of lateral comb-drive actuator, in terms of electrostatic force, mechanical stiffness, stability and displacement, are discussed. It shows that comb finger with positive slope generates larger electrostatic force. The mechanical stiffness along lateral direction increases when the folded beam slants negatively. The displacement is 4.832 times larger if the comb finger and folded beam are tapered to +1° and −1°, respectively. The uneven deep fingers generate an abrupt force and displacement when the motion distance reaches the initial overlap length. The undercut reduces both the driving force and the mechanical stiffness of the lateral comb-drive actuator. The fabricated comb-drive actuator, with comb finger of +1° profile and 0.025 μm undercut, and folded beam of −1° slope and 0.075 μm undercut, is measured and compared with the models where both show consistent results. These analytical results can be used to compensate the fabrication tolerances at design stage and allow the actuators to provide more predictable performance.     

A novel self-aligned electrostatic vertical actuator using plastic deformation technology

A novel method is presented to fabricate a self-aligned electrostatic vertical actuator using plastic deformation technology. The model of the vertical actuator is proposed and simulated using finite element analysis method. The fixed combs and movable combs, and spring beams are patterned using one mask and fabricated by DRIE technology simultaneously. The fixed combs and movable combs have staggered distance of 21.8 μm in the vertical direction on the condition of self-alignment using plastic deformation technology. The electrostatic vertical actuator has achieved large vertical drive displacement at low voltage. The measurement results show that the maximum drive displacement runs up to 17 μm at 17 V voltage between the fixed combs and movable combs. Experimental results verify the usefulness of the guidelines obtained from simulation and calculation.     

A self-aligned fabrication method of dual comb drive using multilayers SOI process for optical MEMS applications

A novel method for fabricating a self-aligned electrostatic dual comb drive using a multi-layer SOI process is developed. The present method utilizes four aligned masks, greatly simplify the existing SOI-MEMS fabrication methods in manufacturing optical MEMS devices. Here, the actuating structure consists of fixed combs and moving combs that are composed of single crystal silicon, nitride and polysilicon. One mask is used to provide a deep etching to etch polysilicon, nitride and single crystal silicon respectively. The nitride separates polysilicon and single crystal silicon and provides an additional dielectric for the purpose of producing bi- directional motion upon applying electrostatic forces. A dual comb drive actuator with optical structures was fabricated with the developed process. The actuator is capable of motion 250 nm downward and 480 nm upward with 30 V applied voltage at 4 kHz frequency. The dynamic characteristics of the first and the second resonant frequency of the dual comb-drive actuator are 10.5 kHz and 23 kHz respectively. Experimental results indicated that the measured data agreed well with simulation results using the ANSOFT Maxwell^® 2D field simulator, ANSYS^® and Coventor Ware^®.     

Development of copper based miniature electrostatic actuator using WEDM with low actuation voltage

Fabricating electrostatic micro actuator, such as comb-drive actuator, is one of the demanding areas of the MEMS technology because of the promising applications in modern engineering, such as, micro-switches, attenuators, filters, micro-lenses, optical waveguide couplers, modulation, interferometer, dynamic focus mirror, and chopper. For the fabrication, most of the cases silicon monocrystalline wafers are used through complex process. To etch the silicon substrates, researchers often use deep reactive-ion etching or anisotropic wet etching procedure which are time consuming and unsuitable for batch fabrication process. Again, resent research shows that comb-drive actuators need comparatively high voltage for actuation. In solving these problems, the study presents a copper based electrostatic micro actuator with low actuation voltage. Using wire electrical discharge machine (WEDM), the actuator is fabricated where a light weight flexible spring model is introduced. Capacitor design model is applied to present a voltage controlling electronic circuit using Arduino micro controller unit. The experimental result shows that the actuator is able to produce 1.38 mN force for 15 V DC. The experiment also proves that coper based actuator design using WEDM technology is much easier for batch processing and could provide the advantages in rapid prototyping.     

High fill-factor two-axis gimbaled tip-tilt-piston micromirror array actuated by self-Aligned vertical electrostatic combdrives

In this paper, we present a high fill-factor micromirror array actuated by self-aligned vertical electrostatic combdrives. To meet the requirements of applications in free-space communication and imaging, each micromirror has three degrees of freedom of motion: rotation around two axes in the mirror plane and linear translation perpendicular to the mirror plane. Our approach is to integrate the high fill-factor reflectors into the fabrication process of the actuators on the wafer-scale. Multilevel silicon-on-insulator (SOI) bonding is utilized to form the high optical quality reflectors and high aspect-ratio vertical combdrive actuators. The wiring for electrical access to the multielectrode per pixel array is fabricated on separate wafers by thin film processing, and flip-chip bonded to the reflector/actuator chip. This architecture overcomes the fill-factor limitation of top-side accessed electrical addressing of mirrors made on SOI. Our 360/spl mu/m pixel size mirror array achieves a 99% fill-factor with optically flat reflectors.     

Indirectly heated micro-electrothermal actuator with a monolithically integrated displacement sensor

In this article, an indirectly heated micro-electrothermal actuator is presented. The actuator has a capacitive sensor monolithically integrated to provide in vivo sensing of the actuator’s displacement. The actuator and sensor are fabricated using the PolyMUMPs process. Electrothermal and thermal-mechanical models of the actuator have been developed. Simulated displacement versus input current reasonably agrees with tested results. The integrated capacitive sensor readout is an indicator for the displacement.     

Electroplated compliant metal microactuators with small feature sizes using a removable SU-8 mould

In the work presented in this article a fabrication process for high aspect ratio metal microstructures has been developed using photoresist EPON SU-8. A smallest feature size of 2 μm with near vertical sidewalls has been achieved on a routine basis. The SU-8 photoresist has been used as mould for electroplating high aspect ratio metal micro actuators. A removal process has also been developed. The removal of the SU-8 mould after plating is done in a plasma asher using an oxygen or oxygen/fluorine plasma with plasma power as low as 200 W. To demonstrate the fabrication process a 10 μm thick nickel, electro-thermal micro actuator has been fabricated.     

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.     

A single-layer PDMS-on-silicon hybrid microactuator with multi-axis out-of-plane motion capabilities-Part i: design and analysis

This paper proposes a new single-layer electrostatic microactuator design that generates three-axis motion resulting in vertical translation and out-of-plane tilting. The new actuator design combines a micrometer-scale three-dimensional (3-D) polydimethylsiloxime (PDMS) structure fabricated using soft lithography with comb drives processed using a single mask on a silicon-on-insulator (SOI) wafer. The multi-axis actuation capability of the proposed actuator is enabled by coupling the in-plane actuation motion of the comb drives with the elastic bending of PDMS flexural microjoints. To predict the static and dynamic performance of the actuator, this paper develops a four-bar-linkage model and applies Lagrangian dynamics theory. The developed analytical model is validated using finite element analysis (FEA) and allows us to perform parametric design of the actuator. The analysis indicates that the proposed PDMS-on-silicon hybrid actuator can yield the desired multi-axis actuation capability with a dynamic bandwidth as large as 5 kHz.     

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 distributed electrostatic micro actuator (DEMA)

A distributed electrostatic micro actuator (DEMA) has been proposed. The actuator has many small driving units which consist of two wave-like insulated electrodes. Both ends of insulated electrodes are connected to each other, and the driving unit has narrow gap for deformation caused by electrostatic forces. The driving units have large area of electrodes and are distributed in series and in parallel. So, a strong electrostatic force can be obtained, and the deformation and the generated force of the actuator would be large. Macro model of the DEMA was fabricated with polyimide films, and the deformation of the actuator was measured. When the applied voltage was 200 V, the deformation ratio was 36%. A micro actuator was fabricated by use of photolithography and electroplating. The displacement of 28 μm was observed when applied voltage was 160 V. Experimental results of the micro actuator were compared with the results simulated by finite element method (FEM) analysis     

MEMS milliactuator for hard-disk-drive tracking servo

This paper describes the design, fabrication, and operational characteristics of a MEMS milliactuator designed for servo tracking in a hard-disk drive (HDD). The actuator is designed to increase the bandwidth of an HDD tracking servo and pack more recording tracks on a disk. An Invar (low thermal expansion metal) electrode position process was developed to meet the thermal stability requirement. The electroplated Invar's thermal coefficient of expansion is as low as 6.3×10^-6/K, which is almost half of that of pure nickel. For the plating mold pattern definition, a high-aspect-ratio polymer etching technique was developed. A high-aspect-ratio structure line-and-gap definition is required to achieve both a high directional stiffness ratio and electrode efficiency for the actuator. The etching technique described can etch through a thick (<40 μm) polymer layer with an aspect ratio of 16:1 at an etch rate of <2 μm/min. Low-cost/high-volume manufacturing is achievable by this batch fabrication technique. A milliactuator was fabricated and assembled with a suspension and a slider weighted at around 2 mg. The slider was successfully driven by the milliactuator while the slider was flying on a spinning disk. The operational characteristics (frequency response) of the in-flight milliactuator were measured, and the results indicate that the actuator is suitable for high-bandwidth HDD servo-tracking applications     

Self-aligned vertical electrostatic combdrives for micromirror actuation

In this paper, we analyze the effect of misalignment in electrostatic combdrives, and describe a fabrication technology that minimizes misalignment in vertical electrostatic combdrives by creating self-aligned, vertically staggered electrodes. Self-alignment of the interdigitated electrodes simplifies fabrication and minimizes failures due to electrostatic instability, thus enabling fabrication of narrow-gap, high-force actuators with high yield. The process is based on deep-reactive ion etching (DRIE) of buried-patterned silicon-on-insulator (SOI) wafers. Measurements on fabricated combdrives show relative misalignment of less than 0.05 /spl mu/m. This corresponds to less than 0.1% misalignment, which, according to our analysis, results in a travel range of 98% of that for perfectly aligned drives. The validity of the process is demonstrated by fabrication of scanning micromirrors measuring 300 /spl mu/m by 100 /spl mu/m. Optical angular deflections from 4/spl deg/ at low frequency to 40/spl deg/ at resonance were measured for an applied voltage of 75 Vpp. Resonant frequencies ranged from 5 kHz to 15 kHz for these devices, making them suitable for high-speed, high-resolution optical scanning and switching.     

Design and fabrication of twisted monolithic dielectric elastomer actuator

This paper presents a novel design of dielectric elastomer actuator (DEA), called single body dielectric elastomer actuator (SDEA), to improve the performance of existing DEAs. The DEA is typically configured with stacking multiple dielectric elastomer film and linearly contracted according to the applied voltage. SDEA is fabricated monolithically without external frame and has the advantages of flexibility and light weight. Thus, it is applicable to various configurations of actuators such as twisting or bending, etc. By exploiting the advantages of SDEA, we propose a new 2-ply design of SDEA. The design is configured with plying couples of monolithically fabricated SDEA. We explain how it can amplify the stroke with its basic principles of operations. In addition, its fabrication method is addressed. Finally, the results of performance evaluations are included with respect to stroke, force, speed, etc.