Mastering the resist-leaching and aqueous-contact-angle challenges

A unique aspect of 193nm immersion (193i) lithography is the water between the front lens and the wafer, which forms a meniscus. This moves with the exposure head across the wafer. The resist stack on the wafer is exposed under the water in a dynamic way with the step and scan process. As 193i lithography moves into manufacturing, two new technical challenges must be mastered to fully realize its benefits without degrading performance: the resist stack used in this immersion process must offer both low leaching behavior and a hydrophobic surface. The first of these requirements is caused by the fact that the photo-acid generator (PAG), quencher, or other small molecular components in the resist leach into the water, they contaminate it and may also degrade the resist’s performance. The contaminatedwater can further contaminate the lens and the wafer stage of the scanner. Thus, a resist stack with minimal leaching is required for the immersion exposure. The second requirement is related to the fact that the water meniscus moves fast: it follows the exposure head as it moves across the wafer at a peak speed of ∼500mm/s. As a result, water droplets may be left behind forming circular defects. A hydrophobic surface reduces the number of water droplets and allows the meniscus to move easily. Consequently, static and dynamic contact-angle measurements are being introduced to evaluate the surface hydrophobicity of resist stacks.