THE LITHOGRAPHY EXPERT: Diffusion and resolution

摘要

The diffusion of chemical species during post-exposure bake is a necessary evil. Diffusion is often used to smooth out standing waves when printing on partially reflecting substrates. For a chemically amplified resist, acid must diffuse in order to find reactive sites on the polymer to deblock. It is the deblocking that changes the resist solubility, and the greater the amount of diffusion, the greater the amount of deblocking. But diffusion will also reduce resolution. Consider a diffusion length (the average distance that a particle will diffuse) that is greater than the feature size-all of the information of the feature will disappear as it smears itself out due to this diffusion. Of course, a good resist used properly will have a diffusion length much smaller than the desired feature size. But how much diffusion is too much? Since reducing diffusion reduces the amount of deblocking (and thus the sensitivity of the resist), we don't want to drive diffusion too low. In this column we'll derive some simple rules to quantitatively explain the impact of diffusion on resolution.