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Ultra High Aspect Ratio Plating and 3-D Air Bridges with KMPR Photoresist
Keywords: Air Bridge, High Aspect Ratio, Plating
Ultra-high aspect ratio photoresists, designed for high density electro-plated structures are finding their way into new applications for MEMS devices. Radiometrically transparent, chemically amplified photoresists are required for such applications to produce vertical sidewall images in thick coatings. More important than the physical properties of the photoresist, which lends itself to the ultra-high aspect ratio images, is the ability of the resist to survive the harsh conditions of the chemical plating bath yet be readily removed after plating. Yet another requirement of the photoresist is for the stripping chemistry to pose no environmental hazards. Once these challenging and highly selective properties for specialty photoresists are met, optical photolithography is able to miniaturize many devices which have never been manufactured on this scale. Many of these new MEMS devices, such as 3-D induction coils, fuel cells and heat sinks; can now be fabricated on a wafer-scale. KMPR 1000 is a new, chemically amplified i-line photoresist based on a carboxylated epoxy cresol novolac resin. Unlike other epoxy-based photoresists, it is developable in metal-ion free aqueous developers, such as TMAH, and can be readily stripped. KMPR is an excellent plating resist and has excellent adhesion to metals. KMPR also has very high plasma resistance making it a versatile resist for MEMS, electrolytic plating and DRIE applications. KMPR 1000 photoresist, developed through a joint-venture with Nippon Kayaku Co., Ltd and MicroChem Corp. has demonstrated copper plated structures with an ultra-high aspect ratio of 10:1 in 50 um coatings (Figure 4). The high aspect ratio imaging properties of KMPR, together with its metal adhesion, etch resistance and ability to completely dissolve in wet strip chemistry allows the fabrication of plated structures, which are unprecedented in optical lithography and electrolytic plating. In addition to the excellent plating properties, the vertical sidewall imaging and plasma resistance of KMPR make straight sidewall DRIE etching possible with a selectivity to silicon greater than 250:1. These imaging and pattern transfer properties of KMPR make it a MEMS technology enabler that has never existed before under i-line illumination.
Harris R. Miller, Product Manager New Product Development
MicroChem Corp.
Newton, MA

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