Finite element modeling of PAG leaching and water uptake in immersion lithography resist materials

The fundamental understanding of photo acid generator (PAG) leaching and water uptake is important for the design of robust immersion imaging processes; including resist, scanner and track hardware design. Experimental studies show that PAG leaching occurs over a very short time scale (< 10 seconds). Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis also reveals that PAG leaching occurs at the top surface of a resist film. The time scale and depth of PAG leaching is important to understanding the fundamental impact of immersion process steps on imaging performance. Finite element modeling is used to study the diffusion of water into a resist and the diffusion of PAG out of a resist into flowing water. Experimental mass uptake of water in a 150 nm resist film was collected experimentally using a quartz crystal microbalance (QCM). The diffusion coefficient of water in the thin resist is calculated to be 1.5e-11 cm²/s. PAG leaching data was collected from an experimental apparatus that can flow water over a resist coated 200 mm wafer (dynamic WEXA2). The PAG diffusion model shows that the diffusion coefficient transitions from 1.4 e^-14 cm²/s in the surface of the film to 1.0 e^-16 cm²/s in the bulk of the film. The finite element simulations show an excellent physical correlation to the experimental PAG leaching data. The extraction of resist component diffusion coefficients enables the modeling of component depth profiles in thin resist materials. The component depth profile information is then used to model the impact of resist design and immersion resist processing on 32 nm node imaging performance.