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In this paper, we present the design, fabrication, and characterization of a long-arm comb-drive actuator with an externally mounted large mirror for optical applications. The provision for an externally mounted mirror can be useful for many optical applications, which require large and high-quality reflecting surfaces. The angular stretch of the comb fingers at the far end is kept smaller than those near the anchor point. This structural feature allows the design of actuator arm lengths of over 5 mm. The truss of the actuator is designed carefully to support the large movable arm. An analytical model for the electrostatic forces in the actuator was developed, and detailed analyses for side instability and resonance frequency were performed. The complete fabrication process involving bulk micromachining of silicon-on-insulator (SOI) wafer using a two-mask process is described. A procedure for efficient mounting of the mirror utilizing die bonder, vacuum tip, and silver epoxy is demonstrated. Static characterization measurements show that the actuator can achieve a rotational angle of 3° with an applied voltage of 130 V. A resonance frequency of $\sim 246$ Hz for a mirror size of $2~\textrm {mm} \times 100~\mu \textrm {m} \times 1$ mm was determined by optical characterization; this closely matches with the simulation results and the analytical model. [2014-0318]

An approach for controlling sidewall smoothness through deep reactive ion etch (DRIE) was developed for silicon micro-optical applications. The process relies on carefully controlled over-passivation of the etched sidewall during the etch/passivation cycles of the advanced Si etch (ASE) process. Several parameters of the ASE process such as etch/passivation cycle time, coil power, platen power, and etching gas flow were systematically varied to achieve an optimized process recipe. The rms roughness of the sidewall after gold deposition was found to be around 3.9 nm. Our structure represents the lowest reported sidewall roughness achieved using only a DRIE process.

Rotary comb-drive electrostatic actuators with virtual pivotal point of rotation were designed and fabricated for their application in external cavity tunable lasers in Littman configuration. Silicon on insulator (SOI) wafers with device layer thickness of 50 μm and buried oxide layer of 2 μm were used. The structures were fabricated through bulk micromachining of the device layer followed by release of movable arms through wet etching. Six pairs of comb arms were used to symmetrically distribute electrostatic forces on an interconnected movable arm structure. The movable structure consisting of stiff truss connectors in arc shapes on inner and outer periphery was anchored through three beams. Actuation of the movable arms on a circular profile with a virtual pivotal point was achieved for the designed range of operation. A maximum rotation of ± 1.5 degrees at 190 V was achieved. The dimensions of the movable arm are on the order of 1.2-1.5 mm. The actuators have a physical clearance of up to 2.8 mm from the device structure to the virtual pivot point. The sidewalls of the movable arms are accessible for optical applications. The geometrical configuration of the fabricated structures supports the stringent requirements of optical path alignments in external cavity lasers. The fabricated structures can be reliably operated in the kilohertz range

Rotary comb-drive electrostatic actuators with smooth vertical sidewalls were designed and fabricated for photonic applications. Silicon on insulator (SOI) wafers with device layer thicknesses of up to 100 μm and oxide layers of 1-2 μm were used. Structures were fabricated through bulk micromachining of the handle layer followed by removal of the buried oxide. Deep reactive ion etching (DRIE) of the device layer defined the comb structure with simultaneous release of the moving arm. Thick photoresist and oxide masking layers were used for the high aspect ratio deep etches. Notching effects or depth variations of device layers were not observed. Etch/ passivation cycles in DRIE were optimized to obtain smooth sidewalls. Traces of etch scallops were confined only to the first few microns of the top and bottom edges of the reflecting sidewall. A movable arm length of 2 mm with a maximum rotation of 2.8 degrees was achieved. The edge deflection was over 100 μm at 100 volts. The fabricated structure provides a long reflecting sidewall that is accessible for hybrid integration in three dimensional space. The fabricated structures can be reliably operated in the kilohertz range.

Rotary comb-drive electrostatic actuators with smooth vertical sidewalls were designed and fabricated for photonic applications. Silicon on insulator wafers with device layer thicknesses of up to 100 μm and oxide layers of 1 to 2 μm were used. Structures were fabricated through bulk micromachining of the handle layer, followed by removal of the buried oxide. Deep reactive ion etching (DRIE) of the device layer defined the comb structure with simultaneous release of the movable arm. Thick photoresist and oxide masking layers were used for the high aspect ratio deep etches. Notching effects or depth variations of device layers were not observed. Etch/passivation cycles in DRIE were optimized to obtain smooth sidewalls of less than 5 nm roughness. Traces of etch scallops were confined only to the first few microns of the top and bottom edges of the reflecting sidewall. A movable arm length of 2 mm with a maximum rotation of 2.8 deg was achieved. The edge deflection was over 100 μm at 100 V. The fabricated structure provides a long reflecting sidewall, that is accessible for hybrid integration, in three dimensional space. The fabricated structures can be reliably operated in the kilohertz range.