Contrast enhancement materials for thick photoresist applications

Abstract

The performance requirements for ultra-thick photoresists are rapidly increasing due to the dramatic growth of applications such as nanotechnology (MEMS) and advanced packaging. Commercial products such as accelerometers, ink jet print heads, biomedical sensors and optical switches are driving growth in the nanotechnology market. Advanced packaging techniques such as flip chip in package, flip chip in board and wafer level chip scale packaging have become widely adopted to address electrical device performance and chip form factor considerations. The common lithography requirement for these applications is formation of high aspect ratio structures with sufficient process latitude to allow devices to be manufactured in production volumes. The use of a contrast enhancement material (CEM) has been shown to be effective in improving lithographic performance and process latitude for thin photoresist applications. However, CEM technology can also be used for the thick photoresist materials in MEMS and advanced packaging applications. The lithographic performance of three representative thick photoresists was characterized with and without a top CEM. The first two materials are ultra-thick positive photoresists that are widely used for electroplated bump bonding structures. The third material is a thick negative photoresist widely used for electrical redistribution levels. All lithography was performed using a low numerical aperture, 1X stepper to control critical dimensions (CD), sidewall angles and aspect ratios. Cross sectional SEM analysis was used to establish the lithographic capabilities of the three photoresists with and without top CEM. The recommended process flow for each photoresist with top CEM is described. The advantages and disadvantages of using CEM for thick photoresist applications are also discussed.