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The requirements for highly specialized photosensitive materials for nanotechnology and Micro-Electro-Mechanical Systems (MEMS) applications are being driven by the rapid growth of consumer products incorporating these devices. These high volume consumer devices including accelerometers for air-bag sensors, biomedical sensors, optical switches and ink jet print heads. These applications all require ultra-thick photosensitive materials with highly controllable lithographic properties. For ink-jet print head applications, the lithography requirements include the formation of high aspect ratio structures with a negative (re-entrant) profile for nozzle formation. In order to form the required nozzle geometry for high resolution ink-jet printers, photosensitive materials need to be capable of providing up to 10 degree negative profiles at a film thickness of up to 25 microns. For consistent print dot size it is necessary to maintain excellent control and repeatability of the sidewall angle of the nozzle. Since this material remains on the substrate as a permanent part of the ink-jet print head, the mechanical and adhesive properties of the material are as important as the lithographic properties. This paper investigates modifications to an existing MicroChem epoxy-based SU8-4000 thick photoresist to generate highly re-entrant sidewall angles for next-generation high resolution ink-jet nozzle formation. Multiple versions of SU8-4000 with different levels of dye tuned for the exposure wavelength are exposed using a 1X lithography system optimized for thick photoresist processing. This stepper uses a combination of low numerical aperture, broadband exposure and large focus offsets for optimal processing of thick photosensitive materials. Basic photoresist characterization techniques in conjunction with cross sectional SEM analysis are used to establish lithographic capabilities for nozzle formation.
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