Electrostatic Comb-drive Microactuator Research Articles

Challenges in fabrication of high aspect ratio electrostatic comb-drive microactuator using one-step X-ray lithography

A High Aspect Ratio (HAR) electrostatic comb-drive microactuator of polymethyl-methacrylate (PMMA) is designed to deliver nearly 40 µm uniaxial displacement at 25 V DC. The HAR comb-drive microstructures are fabricated via one-step X-ray lithography (OXL). Polyimide-Au X-ray mask is fabricated and used to pattern 200 μm, 500 μm and 800 μm thick PMMA by deep X-ray lithography (DXRL), followed by drying/release and selective metallization for the development of HAR electrostatic microactuator. The release was optimized with the help of various low-surface-tension liquids. 200 μm and 500 μm thick microstructures were successfully released whereas, 800 μm structures could not be released due to the higher depth and associated capillary force. In addition, non-uniform distribution of Au on the side walls of overlapped region of comb fingers, during metallization, resulted in uneven distribution of electrostatic force followed by short-circuiting of HAR (i.e. 40) microactuator. These are the potential issues in the fabrication of HAR microstructures and devices by OXL and discussed in details in this paper.

Development of polymer MEMS process technology as an approach to a sustainable production system

Polymethyl methacrylate (PMMA) has been proposed as a material for micro-electromechanical systems (MEMS) to initiate the research on environmentally friendly micro-nano machining technology using polymer materials. A polymer MEMS process has been developed using hot embossing and precision machining. MEMS structures less than 2 μm were successfully embossed. The PMMA layer that remained after hot embossing was removed by a polishing process to release the movable parts. A PMMA electrostatic comb-drive microactuator was fabricated. Both finger width and gap between fingers were 5 μm, and thickness was larger than 70 μm. An operated displacement of 11 μm at a drive voltage of 100 V was obtained. It was 20 times larger than that of an identical silicon device. A torsional micro mirror device driving with vertical comb actuator was fabricated. The size of the mirror was 1×1 mm 2. The maximum tilt angle of 5.6 was obtained with driving voltage of 100 V and frequency up to 100 Hz. A chevron-shaped PMMA thermal actuator with a thickness of about 50 μm has been fabricated and tested successfully. The displacement was about 5 times larger than that of a Si counterpart at the same power consumption.

Ultra-small silicon waveguide coupler switch using gap-variable mechanism

Submicron-wide silicon waveguide coupler with gap variable mechanism is proposed for a compact optical waveguide switch. Two freestanding silicon waveguides are placed parallel with a submicron gap. The gap is changed by electrostatic comb-drive micro-actuators to control the coupling coefficient of the coupler. The fabricated device consisted of the silicon waveguides of 400 nm in width and 260 nm in thickness. The total size of the switch was 100 μm wide and 150 μm long. Decreasing the gap between the waveguides to 110 nm, the output intensity at drop port became a maximum while the output intensity at through port became a minimum. The extension ratio of the switch output was 17 dB for the waveguide displacement of 300 nm.

Development of polymer electrostatic comb-drive actuator using hot embossing and ultraprecision cutting technology

We report the fabrication and evaluation of a PMMA electrostatic comb-drive microactuator using hot embossing and ultra-precision cutting technology. First, a two-step silicon mold is fabricated by bulk micromachining technology. Next, comb-drive microactuator structures are formed on a PMMA plate by hot embossing. Both finger width and gap between fingers are 5 µm, and finger thickness is larger than 70 µm. Then, the PMMA layer that remained after hot embossing is removed by ultra-precision cutting to release the movable parts. Last, the device is coated with a gold layer for the electrode. The PMMA comb-drive microactuator has been tested successfully.

Submicrometer Comb-Drive Actuators Fabricated on Thin Single Crystalline Silicon Layer

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Electrostatic comb-drive microactuators were fabricated by electron beam lithography on a 260-nm-thick silicon layer of a silicon-on-insulator wafer. The actuators consisted of comb electrodes, springs, and a frame. Two kinds of microactuators with doubly clamped and double-folded springs were designed and fabricated. The comb electrode was as small as 2.5 <formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula> wide and 8 <formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula> long and was composed of 250-nm-wide, 260-nm-thick, and 2-<formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula>-long fingers. The air gap between the fingers was 350 nm. The spring was 250 nm wide, 260 nm thick, and 17.5 <formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex> </formula> long, and the spring constant was 0.11 N/m. The force and displacement generated by the microactuator were <formula formulatype="inline"><tex Notation="TeX">$2.3 \times 10^{-7} \ \hbox{N}$</tex></formula> and 1.0 <formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula>, respectively. Applying an ac voltage, the oscillation amplitude became maximum at a frequency of 132 kHz. The mechanical and electrical characteristics of the fabricated actuators were investigated quantitatively. </para>

Pitch-variable blazed grating consisting of freestanding silicon beams

Theoretical analysis is presented for a pitch-variable blazed grating which consists of freestanding silicon beams. The pitch-variable blazed grating is implemented by combining silicon-on-insulator (SOI) technology with microelectronicmechanical system (MEMS) technology. The whole device is fabricated on a 10 microm silicon device layer to guarantee sufficient stiffness. The 4-level blazed surface profile is realized by combining a two-mask process with fast atom beam etching. Electrostatic combdrive microactuators with double folded springs are proposed to stretch the freestanding grating beams. In association with reactive ion etching and vapor HF release, the freestanding grating beams and the microactuators are obtained, and a Cr/Au film is deposited onto the blazed grating surfaces by a protective mask process to improve the diffracted power. Mechanical response and diffraction efficiency of fabricated devices are characterized and the experimental results indicate that the fabricated 4- level blazed gratings extend both the tuning range and the diffraction efficiency of the 1st diffraction order of present MEMS diffraction gratings.

A millimeter-sized nanomanipulator with sub-nanometer positioning resolution and largeforce output

Nanomanipulation in space-limited environments (e.g., inside a SEM (scanningelectron microscope), and particularly in a TEM (transmission electronmicroscope)) requires small-sized nanomanipulators that are capable of producingsub-nanometer positioning resolutions and large output forces. This paperreports on a millimeter-sized MEMS (microelectromechanical systems) basednanomanipulator with a positioning resolution of 0.15 nm and a motion range of ± 2.55 µm. An amplification mechanism is employed to convert micrometer input displacements,generated by a conventional electrostatic comb-drive microactuator, into sub-nanometeroutput displacements. The device has a high load driving capability, driving a load as high as98 µN without sacrificing positioning performance. Based on the pseudo-rigid-body approach,closed-form analytical models of the minification ratio and stiffness of the amplification mechanismare developed. Finite element simulation and testing results verify that the theoretical modelsare valid with an error smaller than 6.2% and that the mechanism has a high linearity(± 2.4%). The amplification mechanism and analytical models have general applicability to otherMEMS transducer designs. A capacitive displacement sensor is integrated fordetecting input displacements that are converted into output displacements via theminimization ratio, allowing closed-loop controlled nanomanipulation. The MEMS-basednanomanipulators are applicable to the characterization/manipulation of nanomaterialsand construction of nanodevices. Corrections were made to this article on 29 August 2007 (in the and sections 3.2.3 and 5.2). The corrected electronic version is identical to the print version.

Electrostatic Microactuators for Precise Positioning of Neural Microelectrodes

Microelectrode arrays used for monitoring single and multineuronal action potentials often fail to record from the same population of neurons over a period of time likely due to micromotion of neurons away from the microelectrode, gliosis around the recording site and also brain movement due to behavior. We report here novel electrostatic microactuated microelectrodes that will enable precise repositioning of the microelectrodes within the brain tissue. Electrostatic comb-drive microactuators and associated microelectrodes are fabricated using the SUMMiT V (Sandia's Ultraplanar Multilevel MEMS Technology) process, a five-layer polysilicon micromachining technology of the Sandia National labs, NM. The microfabricated microactuators enable precise bidirectional positioning of the microelectrodes in the brain with accuracy in the order of 1 microm. The microactuators allow for a linear translation of the microelectrodes of up to 5 mm in either direction making it suitable for positioning microelectrodes in deep structures of a rodent brain. The overall translation was reduced to approximately 2 mm after insulation of the microelectrodes with epoxy for monitoring multiunit activity. The microactuators are capable of driving the microelectrodes in the brain tissue with forces in the order of several micro-Newtons. Single unit recordings were obtained from the somatosensory cortex of adult rats in acute experiments demonstrating the feasibility of this technology. Further optimization of the insulation, packaging and interconnect issues will be necessary before this technology can be validated in long-term experiments.

Surface Oxide Effects on Static Fatigue of Polysilicon MEMS

ABSTRACTStatic fatigue – crack growth causing delayed failure under constant stress and involving stress corrosion cracking – was investigated in polysilicon MEMS using two different surface-micromachined devices. One exploited residual tensile stresses to create stress concentrations at micromachined notches, and the other involved a single-edge notched beam specimen integrated with an electrostatic comb-drive microactuator. Tests with both devices revealed that polysilicon is not susceptible to static fatigue in humid environments. However, when a relatively thick (45 to 140 nm) surface oxide was thermally grown on the microactuator devices, the specimens demonstrated delayed fracture in a humid ambient, presumably due to static fatigue of the surface SiO2. The stress intensities at the resulting cracks in the SiO2were then sufficient to cause catastrophic crack propagation through the polysilicon specimens. The implications of our data on the issue of fatigue in polysilicon is discussed.

Focused ion beam tuning of in-plane vibrating micromechanicalresonators

Strategies for permanently tuning the resonant frequency of in-plane micromechanical resonators are compared. It is shown that tuning of the suspension stiffness may have advantages compared with tuning of the mass. Iterative frequency tuning of laterally resonant electrostatic comb-drive microactuators by focused ion beam machining is demonstrated.