Characterization of ultrathick photoresists for MEMS applications using a 1X stepper

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Abstract
There is a growing interest in using optical steppers for Micromachining and Microfabrication (MEMS) applications due to the tighter overlay and improved critical dimension (CD) control possible with these lithography tools versus a contact printer or full wafer scanner. MEMS applications frequently require the use of ultra-thick photoresists which can easily exceed fifty microns. Extremely large structure heights and high aspect ratios are often required for micro- electrodeposition of mechanical components such as coils, cantilevers and valves. A stepper has an additional advantage with these structures since the focus can be adjusted at various levels into a thick photoresist, which will result in improved wall angles and enhanced aspect ratios. The patterning of high aspect ratio structures in these ultra- thick photoresist films is extremely challenging. The aspect ratios easily exceed those encountered in submicron lithography for standard integrated circuit (IC) manufacturing. In addition, the specific photoresist optical properties and develop characteristics degrade the CD control for these ultra-thick films. The bulk absorption effect of the photoresist reduces the effective dose at the bottom of the film. This effect is exacerbated by the isotropic wet development process, which produces sloped profiles. Unlike thin photoresist for IC manufacturing, lithography modeling and characterization tools are not available for ultra-thick photoresist films. For this study the performance of several commercially available positive and negative ultra-thick photoresists is examined at a thickness of fifty microns using both high throughput i-line and gh-line lithography systems optimized for thick photoresist processing. The photoresists used in this study are selected to represent the full range of available chemistries available from different suppliers. Basic photoresist characterization techniques created for thin films are applied to the ultra-thick photoresist films. Cross sectional SEM analysis, process linearity and Bossung plots are used to establish relative lithographic capabilities of each photoresist. The trade-offs between the various photoresist chemistries are reviewed and compared with the process requirements for high aspect ratio applications.

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