Ph. D oral exam: Daesung Lee (Monday, Feb. 14, 2 pm)

Daesung Lee daesung at stanford.edu
Thu Feb 10 12:34:40 PST 2005


University Ph.D. Oral Examination

 

DESIGN AND FABRICATION OF SOI-BASED MICROMIRRORS FOR OPTICAL APPLICATIONS

 

Daesung Lee, Department of Electrical Engineering

Stanford University

Monday, February 14, 2005, 2:00pm - 3:00pm

Packard 101 (Refreshments served at 1:45pm)

 

Abstract

 

In this talk, we present the fabrication processes and designs of two types of Silicon-on-insulator (SOI)-based micromirrors for optical applications: Single-axis, two-axis scanning mirrors actuated by self-aligned vertical combdrives in double SOI layers and MEMS actuated vertical mirrors for optical bench technology

 

MEMS scanning mirrors enable a variety of optical applications including displays, confocal microscopy, and optical fiber switches. In many of these applications, mirrors require optically flat surface, large actuating force, and large deflection angle. High aspect ratio vertical combdrive actuators in SOI layers offer these desired features. Electrostatic actuators exhibit an important behavior called pull-in which limits the maximum stable deflection. Analytical derivation shows that the alignment of two comb sets in the vertical combdrive is critical because the misalignment reduces the maximum stable deflection. In the first part of the talk, we present the fabrication process and design of vertical combdrives with self-aligned comb sets in double SOI layers. The two oxide layers between the device layers provide electrode isolation and etch stops for thickness control. With three masks from the front-side of the wafers, we demonstrated mirrors capable of a single-axis, bi-directional rotation and piston. Furthermore by adding a non-critical backside etch, we demonstrated two-axis, bi-directional gimbaled scanning mirrors. Double-stacked SOI layers are used to provide both electrical isolation and mechanical connections for gimbaled structures. 

 

In the second part of the talk, we present the fabrication process of MEMS actuated vertical mirrors with optically very flat surface for optical bench technology. The optical bench technology enables passive alignments of optical components including vertical mirrors, fiber U-grooves, and lens holders, thus reducing the packaging cost significantly. Previously demonstrated applications of this technology include optical switches, variable optical attenuators, and optical phase shifters. DRIE etch in SOI layer is, due to its simplicity, the most widely used technique to fabricate this type of mirror. However, obtaining good optical-quality surfaces with DRIE remains a challenging task. Wet anisotropic etch with KOH of (110) silicon on the other hand, provides close to atomically smooth vertical surfaces, but anisotropic etching only supports fabrication of very simple structures defined by the crystalline orientation of silicon. In this talk, we present the fabrication process of vertical mirrors that uses a combination of KOH and DRIE etches to take advantage of the merits of each process: Vertical mirror surfaces are fabricated by KOH, while the rest of the structures are fabricated by DRIE. The vertical mirrors are fabricated in a (110) silicon device layer of SOI wafers.
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