SPIE 2000 #3999-47Process Characterization of an Ultra-Thick Strippable Photoresist Using a Broadband Stepper

The number of lithographic applications that require the use of ultra-thick photoresists is rapidly increasing. Extremely large structure heights and high aspect ratios are often required for micro-electrodeposition of mechanical components such as coils, cantilevers and valves. These ultra-thick photoresists can also be used as a mold in micromachining (MEMS) applications. Ultra-thick photoresists are also used in bump bond applications to define the size and location of the bonds for advanced packaging. The process optimization required to obtain high aspect ratio structures in ultra-thick photoresist films is extremely challenging. The aspect ratios far exceed those encountered in advanced submicron lithography for integrated circuit (IC) manufacturing. MicroChem’s epoxy-based SU-8 thick photoresist, while yielding high performance in the thickness range greater than 100µm, uses organic solvent development and can not be removed using standard stripper chemistries. This process issue limits the use of SU-8 to applications where photoresist removal is not necessary. For this study an experimental chemically-amplified, aqueous-developable, strippable, negative photoresist designated STFN v1 was examined at a thickness of 50µm using a broad band lithography system optimized for thick photoresist processing. This stepper uses a combination of low numerical aperture, high wafer plane irradiance and broadband exposure from 350 to 450nm for optimal processing of thick photosensitive films. Basic photoresist characterization techniques established for thin films in IC manufacturing are applied to STFN v1 photoresist using a ghi-line lithography system. Cross sectional SEM analysis, process linearity and process windows are used to establish the lithographic capabilities of the photoresist. The performance results for the strippable STFN v1 photoresist are then compared with the non-strippable material SU-8. Key Words: thick photoresist, micromachining, MEMS, photoresist characterization, broadband stepper 1.