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

Abstract

For some recent microoptic and microelectromechanical systems ͑MEMS͒ applications, metal structures are used as micromirror arrays, e.g., liquid-crystal-on-silicon ͑LCoS͒ microdisplays. The present paper reports a new approach to aluminum patterning to produce ultraplanar mirror arrays for microdisplay applications. A thin film of SiO 2 ͑interlayer dielectric or ILD͒ is deposited on a silicon wafer. The mirror pattern is etched onto the ILD using reactive ion etching. Aluminum is deposited, followed by a SiO 2 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 ͑CMP͒ 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 m 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.Microoptics and microelectromechanical systems ͑MEMS͒ have become increasingly important technologies in the microfabrication industry. Some of these devices require mirror-array structures to interact with light, unlike the ubiquitous complimentary metal oxide semiconductor ͑CMOS͒ devices, e.g., liquid crystal on silicon ͑LCoS͒ microdisplays. The novel functionality imposes stringent structural constraints. For instance, the top metal layer, used as a mirror array in LCoS microdisplays, has to be locally planar and smooth for specular reflection of light. 1,2 The top dielectric, that forms a passivation layer between the liquid crystal ͑LC͒ alignment layer ͑e.g., polyimide with microgrooves͒ and the metal mirrors, has to be locally planar because the liquid crystal flow and alignment is sensitive to surface topography. 3,4 Hence, a locally planar passivation dielectric layer would avoid any structural perturbation of the nanometer-scale deep grooves that align LC material for proper optical retardation during microdisplay operation in LCoS microdisplays. From the global planarity point of view, the nonplanar film thickness causes stress-related die deformation, which adversely affects the LC cell gap which is maintained at 10% of the mean value of about 3 m. 5 The fabrication process of such devices requires innovation from the standard techniques used in CMOS microfabrication. Aluminum is the metal of choice for mirror fabrication for two reasons. First, it exhibits good reflectivity in the visible spectrum. 6 Second, it is compatible with CMOS fabrication. However, the two fabrication approaches to mirror patterning, (i) etching aluminum and (ii) inlaid metal patterning, both produce unsatisfactory results. Et...

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