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Modem package designs generate a large amount of stress on the die which can be controlled using a thick film of polyimide over the passivation layer. Polyimide film thicknesses in excess of twenty microns at exposure are becoming common for very thin packages. The standard polyimide lithographic process frequently utilizes a trilayer film consisting of an adhesion layer, a polyimide film, and photoresist. A major advance in polyimide technology occurred with the introduction of photosensitive polyimide materials. These materials reduce the total number of process steps in the polyimide process. They also offer the opportunity to combine the passivation and polyimide lithography steps into one process level resulting in significant process simplification and manufacturing cost reduction. Consequently, there is a rapid increase in the use of photosensitive polyimides in the semiconductor industry. There are a number of important issues associated with photosensitive polyimide processing. Because most photosensitive polyimides are negative tone, residual film formation has a major impact on resolution and the usable process window. The high exposure doses required for thicker polyimide films exacerbates the residual film problem. Also, resolving small features such as fuse windows in DRAMs is frequently required in thick photosensitive polyimide layers. These small features result in polyimide height-to-linewidth aspect ratios that are comparable to many photoresist applications. Because of these requirements, photosensitive polyimide applications could benefit from detailed process characterization to enhance resolution and increase process latitude. Unfortunately, there is scant literature pertaining to lithographic performance and lithographic process modeling for photosensitive polyimide films. An extension of basic photoresist characterization techniques for thin films can be applied to thick photosensitive polyimide processes. The develop rate characteristics and lithographic performance for several commercial photosensitive polyimide products were studied at a thickness of 12 microns. Cross sectional SEM analysis, Bossung plots, and film retention plots are used to establish relative lithographic capabilities. These experimental results are used to study the effects of polyimide physical and chemical properties on lithographic performance.
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