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Chapter 3:
Resist Leaching and Water Uptake
One unique aspect of 193i lithography is the use of water situated between the final lens element and the resist. The resist stack (with or without topcoat) on the wafer is dynamically exposed through this water with the step-and-scan process. The photoacid generator (PAG), quencher, and other small molecular components of the resist may leach into the water. These leached components contaminate the water and may degrade resist performance. The contaminated water can additionally contaminate the lens and wafer stage of the scanner. To master these leaching problems, we must understand the dynamics of resist leaching and the transportation of leached contaminates in the immersion water, as well as the impact of these contaminants on the lens during exposure. Additionally, water can penetrate the topcoat and diffuse into the resist film, causing the topcoat or the resist to swell, which, in turn, affects their lithographic performance. This chapter specifically addresses the following issues: (1) leaching test methods, (2) leaching dynamics, (3) leaching with 193-nm exposure, (4) pre-rinse to partially remove leached contaminants, (5) lens contamination caused by resist leaching, and (6) water uptake in resist film. 3.1 Leaching Test Methods A general approach to evaluating the leaching characteristics of a resist involves several steps. First, a puddle of DI water is formed on the surface of the resist stack. After a specific amount of time, the water in the puddle is sent for analysis. The leaching test measures the amount of resist components that have leached into the water over time. Various methods of immersing the resist film and extracting the water sample have been developed and reported; however, the results have been inconsistent with variability as high as 2−3x. So far, no standard test method or specifications for leaching have been accepted by the entire 193i community. However, these are worthwhile goals and more reliable methods continue to be sought.
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