Characterization Study of an Aqueous Developable Photosensitive Polyimide on 300 mm Wafers

摘要

The advent of 300 mm -wafer processing for semiconductor manufacturing has had a great impact on the development of photolithographic materials, equipment and associated processes. At the same time advanced packaging techniques for these semiconductor devices are making strides for smaller, faster and lower cost parts with improved reliability. Photosensitive polyimides are used for passivation stress buffer relief and soft error protection on almost all memory devices such as DRAM as well as final passivation layers for subsequent interconnect bumping operations on most of today's advanced microprocessors. For processing simplicity and total cost of ownership, it is desirable to use an aqueous developable polyimide to maintain compatibility with standard photoresist processes. This study will investigate the feasibility of processing photosensitive polyimides on 300 mm wafers. The performance of a commercially available, positive acting, aqueous developable polyimide is examined at a thickness appropriate for logic devices. A broadband stepper is utilized since polyimides are highly aromatic polymers that strongly absorb UV light below 350 nm. This stepper exposes photosensitive films using mercury vapor spectrum output from 390 nm to 450 nm (g and h-line) and allows rapid exposure of both broadband as well as narrow spectral sensitive films. The system has been optimized for thick photoresists and polyimides and uses a combination of low numerical aperture with maximum wafer level intensity to achieve well formed images in thick films. Process capability for 300 mm wafers is determined by analyzing polyimide film thickness uniformity and critical dimension (CD) control across the wafer. Basic photoresist characterization techniques such as cross sectional SEM analysis, process linearity and process windows are also used to establish lithographic capabilities. The trade-offs for various process capability windows are reviewed to determine the optimum process conditions for different polyimide applications.