Soft bake effect in 193-nm chemically amplified resist

We investigated the thickness and optical constants, n and k, changes of the 193 nm chemically amplified resist for different thicknesses and soft bake conditions with in-situ measurements. During soft bake, the thickness, n and k change abruptly up to 90 s, then they settled down to certain values. It has been found that the optical properties of the resist after soft bake depend on the final resist thickness. The relationships between the optical constants and the resist thickness after soft bake were extracted from the experimental results and applied to our simulation. A series of simulations were carried out for various resist thicknesses. The simulation results showed considerable changes in line width when the changes of n and k after soft bake were considered. The results indicate that the changes of the optical constants by soft bake are not negligible and they can affect the lithography process significantly. Especially for the thin resist with a smaller critical dimension, the line width variation due to n and k change by soft bake becomes more significant and should be considered in simulation.