Fabrication of a Micromirror with Sidewall Electrodes

We designed and fabricated a micromirror with sidewall electrodes in the Cornell NanoScale Science & Technology Facility (CNF) for endoscopic application. The micromirror can perform 2-dimensional scanning. The static and dynamic performances of the micromirror are improved over that of a micromirror without sidewall electrodes. In addition, undesired spring-softening effect that is commonly found in electrostatic actuation is significantly reduced. Based on the fabrication process of a single micromirror, the 2 × 2 micromirror array with sidewall electrodes and 4 × 4 micromirror array with sidewall electrodes were fabricated in CNF as well. Summary of Research: Micromirrors are important micro-electric-mechanical system (MEMS) devices adopted in many application fields, such as barcode readers, digital light processors (DLP), optical switches, optical coherence tomography (OCT) and confocal microscopes. The performance and limitation of these devices are highly depended on the micromirror. Our goal is to design and fabricate a micromirror for an endoscope used in OCT system. The application of a biaxial micromirror used in OCT system to acquire 3-D images has already been demonstrated [1]. One of the major challenges is to design and build the actuators. The micromirror actuating mechanisms such as electrostatic, thermal, piezoelectric, electromagnetic, electrowetting of liquid metals or mixed actuators are the subjects of intense research. Among these actuators, electrostatic actuator is widely used in different micromirror designs [2-4]. Electrostatic actuators are easy to fabricate by traditional integrated circuit (IC) process. Based on the electrostatic force, F = 1/2 e0V 2/d2, (e0dielectric constant of vacuum, dthe distance between electrode and mirror plate), if the same drive voltage is used, an increase in d will decrease the electrostatic force rapidly. Meanwhile, increasing the effect area of electrodes is effective to raise the electrostatic force. As a result, a micromirror with sidewall electrodes is proposed in Pu et al [3], however, its fabrication process is so complicated. Therefore, we created a model for sidewall electrodes and designed a novel micromirror with sidewall electrodes [5]. The micromirror with sidewall electrodes consists of a rigid mirror plate, spring hinges, gimbal frame, supporting frames, and actuating electrodes. The micromirror plate is suspended by the double-gimbal structure, which consists of two pairs of torsion bars for 2-degree-of-freedom (DOF) scans: a-scan and b-scan, rotating about the x-axis and y-axis, respectively. The micromirror plate is electrically grounded and the quadrant electrodes (sidewall and bottom electrodes) offer V1, V2, V3, and V4 voltages. The parameters of the micromirror are listed: micromirror plate: 1 mm × 1 mm, torsion bar width: 2 μm, the gap between micromirror plate and bottom electrodes: 290 μm. Figure 1 presents a scanning electron micrograph (SEM) image of the asfabricated micromirror with sidewall electrodes. The fabrication process is divided into three steps; (i) micromirror plate fabrication, (ii) sidewall electrode fabrication, and (iii) assembling of micromirror plate and sidewall electrodes. The micromirror plate was built on the silicon-on-insulator (SOI) wafer. First, we fabricate the micromirror plate feature on the device layer and then pattern the back side of the micromirror on the handle layer. Reactive ion etching (RIE) and deep RIE (DRIE) were adopted during the process. After obtaining all the micromirror features on SOI wafer, we used wet etching to release the micromirror plate. Figure