Vertical Combdrives Research Articles

Computational analysis of vertical comb-drive microactuator with extended mirror for manipulation of light

Squeeze film damping is a dominant mechanism to examine the losses in the micro-opto-electro-mechanical systems. It plays an important role in understanding the dynamic performance of vertical-comb drive (VCD) microactuator. In the present paper, finite element method is used to estimate the Q factor and damping ratio at varying ambient pressures ranging from atmospheric (105 Pa) to vacuum (0.1 Pa) in COMSOL Multiphysics. The effect of ambient pressure on dynamic performance of the VCD is discussed in detail. Based on the applications, two different VCDs (VCD-L and VCD-SR) with extended micromirrors have been proposed in the current study for the manipulation of light. VCD-L having lower eigen frequency of 670 Hz can be used in laser and biomedical applications. VCD-SR with higher eigen frequency nearly 90 kHz has the potential use in a synchrotron radiation source. In addition, a parametric study is done to understand the performance of VCD for varying process parameters such as variable offset length among the comb fingers, spring dimensions, and orientation of single crystal silicon. For spring design, the von-Mises stress theory is preferred over maximum shear stress theory.

Study of metal-assisted chemical etching of silicon as an alternative to dry etching for the development of vertical comb-drives

Metal-assisted chemical etching (MaCEtch) has recently emerged as a promising technique to etch anisotropic nano- and microstructures in silicon by metal catalysts. It is an economical wet chemical etching method, which can be a good alternative to deep-reactive ion etching (DRIE) process in terms of verticality and etch depth. In the present study, gold is used as a metal catalyst and deposited using physical vapour deposition. It has already been demonstrated that (100) p-type Si wafer can be etched with vertical and smooth side walls. Effects of varying concentrations of etchant constituents and various other parameters, that is, porosity of deposited Au, surface contaminants, oxide formation, metal catalyst, etching time, role of surface tension of additives on the etch depth and surface defects are studied and discussed in detail. By increasing the hydrofluoric acid (HF) concentration from 7.5 M to 10 M, lateral etching is reduced and the microstructure’s width is increased from 17 µm to 18 µm. Porous defects are suppressed by decreasing the hydrogen peroxide (H2O2) concentration from 1.5 M to 1 M. On increasing the etching time from 30 min to 60 min, the microstructures are over-etched laterally. Smoother side walls are fabricated by using the low-surface-tension additive ethanol. The maximum etch depth of 2.6 µm is achieved for Au catalyst in 30 min. The results are encouraging and useful for the development of vertical comb-drives and Micro-Electro-Mechanical Systems (MEMS).

Interactive Modelling and Simulation of Micromirror MEMS Devices

This paper presents an interactive software package for the modelling and simulation of microelectromechanical system (MEMS) devices utilizing the MATLAB high-level programming language and its interactive environment. The package provides a dynamic analysis and frequency responses of a 1-D torsional micromirror electrostatically actuated with staggered vertical comb-drives. Applying a frequency sweep to the micromirror equation, the torsional mode natural frequency may be estimated. The developed package consists of several graphical user interfaces used for modeling MEMS devices. The software package is useful for computing the forces and torques for different micromirror geometries.

High quality factor RF MEMS tunable capacitor

This paper presents the design of a tunable capacitor that integrates two vertical comb-drive tunable capacitors and a parallel plate tunable capacitor. The parallel plate tunable capacitor is in the middle of the comb-drive tunable capacitors. The vertical comb-drive part has sets of fixed and moving fingers while the parallel plate has fixed and moving plates. The capacitor is fabricated using the MetalMUMPs microfabrication process, which has only one thick structural layer i.e. metal (20 μm nickel and 0.5 μm gold). The nickel of the metal layer is subject to residual stress gradients along its thickness. After release of the metal layer, the stress gradients bend the curve-up beams to raise the moving fingers and the moving plate of the capacitor above the substrate and the fixed plate. Hence, one structural layer (i.e. the metal) is used to form the moving and the fixed comb fingers without requiring two structural layers. Vertical comb-driven tunable capacitors offer high capacitance density when a large number of fingers with narrow gap are used. Therefore, high capacitance ratio and high quality factors can be achieved by using vertical comb-drives with in a relatively small device area compared to lateral comb-drives. The parallel plate driving part increases the displacement and capacitance ratio while reducing the actuation voltage required for driving the capacitor. The quality factor and tuning ratio of the fabricated tunable capacitor are 118.5 and 101% at 0.8 GHz; respectively at a driving voltage of 100 V. Based on the experimental results of the fabricated tunable capacitor, an optimized design is presented for both high tuning ratio and high quality factor. The tuning ratio of the optimized capacitor is found to be 143.1% at a “pull-in” displacement of 9.5 µm and a “pull-in” voltage of 90 V. The quality factors of the optimized capacitor are 520 at 0 V and 363.5 at 90 V, respectively at 0.8 GHz. The tunable capacitor achieves a displacement of more than one-half of the gap between the parallel plates without “pull-in” effect.

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.

Large Stroke Staggered Vertical Comb-Drive Actuator for the Application of a Millimeter-Wave Tunable Phase Shifter

This paper presents the design, fabrication, and characterization of a MEMS actuator with large static deflection as a waveguide-mounted variable millimeter-wave phase shifter. The actuator is composed of a pair of interdigitated microplates actuated by vertical comb-drives and suspended by SU-8 torsional springs. The SU-8 spring possesses a thin metallization top layer and a reverse-T-shaped cross-section enabling low torsional stiffness and high in-plane stability. A maximum mechanical deflection of 10.3 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$^{\circ}$</tex></formula> is obtained under a dc actuation voltage of 35 V. The dynamic characterization of the device shows that the resonance frequency of the torsional mode is well separated from the other three bending modes, confirming the designed low torsional stiffness and high in-plane stability. The torsional viscoelastic creeping is measured as a function of time at different loads and shows a maximum of 0.5 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$^{\circ}$</tex></formula> for an applied voltage of 27.5 V. A high operation cycle test is conducted and the metalized SU-8 spring withstands 800 million cycles without showing fatigue. RF measurements show that a variable mechanical deflection angle between 0 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}$</tex></formula> and 8.2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}$</tex></formula> results in a variable transmission phase shift up to 58.0 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}$</tex></formula> . The measured insertion loss is always below 5.1 dB at 98 GHz, corresponding to a figure of merit of 11.5 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}/{\rm dB}$</tex> </formula> . [2012-0338]

Pull-In Analysis of Torsional Scanners Actuated by Electrostatic Vertical Combdrives

This paper presents pull-in analysis of torsional MEMS scanners actuated by electrostatic vertical combdrives with general comb gap arrangements and cross sections. The analysis is based on a 2-DOF actuator with a single voltage control. Three failure modes of the scanners are identified as in-plane twist, transversal motion, and out-of-plane twist. For each failure mode, analytical expressions of pull-in deflection are obtained by applying 2D analytical capacitance models to the derived pull-in equations. From these, the dominant pull-in mechanism is shown to be in-plane twist for scanners with high-aspect-ratio torsional springs. The analytical calculations for both symmetric and asymmetric capacitances are shown to be in good agreement with simulation results. The optimum scanner design is achieved when the pull-in deflection matches the capacitance maximum angle. The condition can be expressed in terms of the ratio of the comb thickness to the comb gap, which is smaller than the typical aspect ratio of deep reactive ion etching. The optimum tradeoff between the maximum deflection angle and the number of movable combs is achieved by adjusting the overlap of the movable and fixed combs and the distance of the comb sets from the axis of the rotation.

Two-axis MEMS scanners with radial vertical combdrive actuators—design, theoretical analysis, and fabrication

We propose the employment of radial vertical combdrive actuators to implement two-axis micro-electro-mechanical systems (MEMS) scanners. The devices are designed based on a five-layer polysilicon surface micromachining process. A cross-bar spring structure consisting of lower and upper torsion springs is incorporated to achieve two rotational degrees of freedom, enabling the dual-axis rotation. Both the vertical combdrive actuators and the torsion springs are hidden underneath the mirror to achieve a small form factor. Theoretical analysis is performed for comparison of various designs. Preliminary experimental results are also obtained.

Fabrication of a polymer-based torsional vertical comb drive using a double-side partial exposure method

A novel approach which uses a double-side partial exposure method to fabricate a polymer-based torsional vertical comb drive (VCD) with thick photoresist AZ9260® as the structural material is proposed in this paper. Front-side partial exposure defines the height of the fixed lower fingers, and back-side partial exposure creates the suspending space of the upper fingers, where the overlap and self-alignment between fingers can easily be achieved in this way. It does not need any sacrificial layer and etching process. A metal layer is finally deposited on the structural surface by a sputtering system for suitable electric conductivity to activate the polymer torsional VCD. The finite element method is used here to simulate the capacitance at different finger positions, and twelfth-order polynomial curve fitting is performed to obtain capacitance as a function of finger displacement. Then the rotation angle can be calculated analytically with the capacitance function. Also, the static deflection and dynamic response of polymer torsional VCDs are characterized experimentally, where the dimensions of the torsion plate are 300 µm wide and 360 µm long with movable fingers of length 100 µm; the torsion spring is 60 µm long and 4 µm wide. Both have a thickness of 31 µm, and the initial overlap is 11 µm in depth and 80 µm in length between the lower and upper fingers. By comparing the simulated and experimental results, the feasibility of the proposed fabrication method of polymer torsional VCDs is verified with a measured rotation angle of 2.31° under a driving voltage of 158.3 V.

A novel self-aligned vertical electrostatic combdrives actuator for scanning micromirrors

A novel self-aligned vertical electrostatic combdrives actuator has been developed and demonstrated, which enhances the capabilities and applications of high aspect ratio silicon-on-insulator microelectromechanical systems (SOI-MEMS) by enabling additional independent degrees of freedom of operation: both upward and downward vertical pistoning motion as well as bi-directional rotation. The present method utilizes four aligned masks greatly simplifying the existing SOI-MEMS fabrication methods for manufacturing high performance scanning micromirrors. Results from the micromirror device for the novel vertical combdrives show that a mechanical tilt angle of ±1° at 100 Vdc was achieved for a 450 µm diameter micromirror. The scanning micromirror can scan a large angle 62° at the resonance frequency of 10.46 kHz with a sinusoidal voltage input of 60 V in amplitude.

Vertical Combdrive Based 2-D Gimbaled Micromirrors With Large Static Rotation by Backside Island Isolation

We introduce a backside island isolation method for silicon-on-insulator (SOI)-based microelectromechanical systems technology and demonstrate vertical comb drive-based two-dimensional gimbaled micromirrors with large static rotation using the isolation method. The proposed isolation method provides electrical isolation and mechanical coupling of SOI structures without additional dielectric backfill and planarization by utilizing timed etched backside handle wafer structures. The backside island is a hidden layer beneath the gimbal and allows independent application of actuation potentials to the gimbal and inner mirror. We developed the fabrication process that accommodates the backside island isolation structures into an established vertical comb drive process, thereby allowing implementation of two-axis gimbaled structures. The maximum static optical deflections of the gimbal and mirror are 46/spl deg/ and 15/spl deg/, respectively.

Multilevel Beam SOI-MEMS Fabrication and Applications

A microfabrication technology has been developed and demonstrated, which enhances the capabilities and applications of high aspect ratio silicon-on-insulator microelectromechanical systems (SOI-MEMS) by enabling additional independent degrees of freedom of operation: both upward and downward vertical pistoning motion as well as bi-directional rotation. This is accomplished by applying multiple-mask high aspect ratio etches from both the front- and back-side of the SOI device layer, forming beams at different levels. The processes utilize four masks, two for front-side and two for back-side etching. As a result, single-crystal silicon beams with four different cross-sections are fabricated, and can be combined to form many additional beam cross-sections. This provides a wide variety of possible mechanical designs that can be optimized for optical and other applications. By this methodology, unique high aspect ratio micromirror devices were demonstrated with fully isolated and accurately self-aligned vertical combdrives in the SOI device layer, with initial combfinger overlap. Examples of fabricated devices are shown with performance summaries.