Fabrication of Ultraplanar Aluminum Mirror Array by Novel Encapsulation CMP for Microoptics and MEMS Applications

Abstract

For some recent microoptic and microelectromechanical systems (MEMS) applications, metal structures are used as micromirror arrays, e.g., liquid-crystal-on-silicon (LCoS) microdisplays. The present paper reports a new approach to aluminum patterning to produce ultraplanar mirror arrays for microdisplay applications. A thin film of (interlayer dielectric or ILD) is deposited on a silicon wafer. The mirror pattern is etched onto the ILD using reactive ion etching. Aluminum is deposited, followed by a deposition, which caps the aluminum thin film such that the total thickness of the aluminum and cap-oxide layer is equal to the ILD etch depth. This structure is planarized using a two-step chemical mechanical polishing (CMP) process. First, a short oxide CMP is performed to remove the cap-oxide from the “high areas.” Finally, aluminum removal in the high areas is completed, using an unoptimized Al CMP process. The inlaid aluminum pattern is protected by the oxide cap. The experimental results show about 5 nm of dishing in the cap oxide for 25 μm pitch and 84% pattern factor, while the encapsulated aluminum is dish-free. Additionally, the process compares favorably in terms of insensitivity to slurry chemistry and large end-point detection window to conventional mirror array fabrication techniques. © 2004 The Electrochemical Society. All rights reserved.