Self-aligned Vertical Comb Research Articles

A mems variable optical attenuator based on a vertical comb drive with self-elevated stators

Vertical comb drives are critical for electrostatic optical switches and variable optical attenuators (VOAs), but fabricating vertical comb drives requires the formation of two levels of comb fingers and thus suffers from the difficulty of dealing with the comb-finger alignment. This paper presents a vertical comb drive design and its micro-fabrication method that can realize self-aligned two-level comb fingers. The self-aligned vertical comb fingers are enabled by a novel vertically-elevated flat-end (VEFE) bimorph structure. Both the stator and rotor fingers of the vertical comb drive are formed by the same photomask and the same silicon etching step, which automatically ensures accurate alignment of the stator and rotor fingers. The vertical separation between the stator and rotor is created by the VEFE structure. A 1mm-aperture MEMS mirror with the proposed VEFE comb drive has been fabricated using SOI wafers with buried cavities. The mirror rotates 0.94° at 8 Vdc. The resonant frequency is 1.428kHz. The MEMS mirror has been assembled into a VOA module. Measurements show that the VOA achieved a dynamic range of 55dB and an insertion loss of less than 0.4dB and met the telecommunications standards on shock, vibration and long-term stability.

High fill-factor micromirror array using a self-aligned vertical comb drive actuator with two rotational axes

We present a two-axis micromirror array with high fill-factor, using a new fabrication procedure on the full wafer scale. The micromirror comprises a self-aligned vertical comb drive actuator with a mirror plate mounted on it and electrical lines on a bottom substrate. A high-aspect-ratio vertical comb drive was built using a bulk micromachining technique on a silicon-on-insulator (SOI) wafer. The thickness of the torsion spring was adjusted using multiple silicon etching steps to enhance the static angular deflection of the mirrors. To address the array, electrical lines were fabricated on a glass substrate and combined with the comb actuators using an anodic bonding process. The silicon mirror plate was fabricated together with the actuator using a wafer bonding process and segmented at the final release step. The actuator and addressing lines were hidden behind the mirror plate, resulting in a high fill-factor of 84% in an 8 × 8 array of micromirrors, each 340 µm × 340 µm. The fabricated mirror plate has a high-quality optical surface with an average surface roughness (Ra) of 4 nm and a curvature radius of 0.9 m. The static and dynamic responses of the micromirror were characterized by comparing the measured results with the calculated values. The maximum static optical deflection for the outer axis is 4.32° at 60 V, and the maximum inner axis tilting angle is 2.82° at 96 V bias. The torsion resonance frequencies along the outer and inner axes were 1.94 kHz and 0.95 kHz, respectively.

Electrostatic scanning micromirrors using localized plastic deformation of silicon

Electrostatic scanning micromirrors made of single-crystal silicon from a SOI (silicon-on-insulator) wafer using self-aligned and localized plastic deformation of silicon have been demonstrated. Localized plastic deformation is achieved by selective Joule heating of silicon structures while the whole substrate is kept at low temperature. Novel designs for torsional springs to prevent the buckling problem due to the non-uniform thermal expansion by localized heating are presented and the optimal range of electric power for successful Joule heating is discussed. The prototype micromirror is actuated by self-aligned vertical comb sets at a measured resonant frequency of 4.13 kHz with optical scanning angles up to 50.9° under driving voltages of 30 Vdc plus 14 Vac. After continuous operation of 20 billion cycles (56 days) at the maximum scanning angle, the device shows no observable signs of degradation or fatigue.

Design and fabrication of MEMS devices using the integration of MUMPs, trench-refilled molding, DRIE and bulk silicon etching processes

This work integrates multi-depth DRIE etching, trench-refilled molding, two poly-Si layers MUMPs and bulk releasing to improve the variety and performance of MEMS devices. In summary, the present fabrication process, named MOSBE II, has three merits. First, this process can monolithically fabricate and integrate poly-Si thin-film structures with different thicknesses and stiffnesses, such as the flexible spring and the stiff mirror plate. Second, multi-depth structures, such as vertical comb electrodes, are available from the DRIE processes. Third, a cavity under the micromachined device is provided by the bulk silicon etching process, so that a large out-of-plane motion is allowed. In application, an optical scanner driven by the self-aligned vertical comb actuator was demonstrated. The poly-Si micromachined components fabricated by MOSBE II can further integrate with the MUMPs devices to establish a more powerful MOEMS platform.

A novel microfabrication of a self-aligned vertical comb drive on a single SOI wafer for optical MEMS applications

A novel method for fabricating a self-aligned electrostatic vertical comb drive using a single layer of a SOI wafer is introduced. The fixed combs are anchored to bimorph cantilevers made of two materials with dissimilar thermal coefficients of expansion, i.e., silicon dioxide and single crystal silicon. The cantilever, which provides the vertical offset between the fixed comb and the movable comb, is deflected by residual stress during cooling down from oxidation temperature to room temperature. In piston motion, the vertical amplitude at the resonant frequency of 3.5 kHz is 30 µm. In torsional motion, the angle of optical deflection at the resonance of 830 Hz is changed by 6.5°. The measured resonant frequencies correspond to the results from a finite element analysis within 10%. This vertical comb drive is useful for optical and biophotonic MEMS requiring out-of-plane torsional or piston motion.