It has been a long time since I posted my thoughts on lithography.
Recently, my group has been investigating thin e-beam resist for better resolution. It turns out the thickness is not independent.
In fact, if you assume the fraction of non-absorbed radiation goes as exp(-az), where a is the absorption coefficient, and z is the depth thickness, you find that your dose must increase for thinner resist, i.e. 1/[1-exp(-az)]. At the same time, thinner resist lets more radiation reach underlying layers and substrate, leading to more damage. The damage increase factor with compensated dose is exp(-az)/[1-exp(-az)].
It works out to roughly inverse proportion to resist thickness, i.e., if you have 70% the resist thickness, you have to add nearly 40% your dose, which increases the damage more than 40%.
The added damage is bad for the product, the extra power consumed for extra dose is bad for the environment.
There should be less green for technology that's becoming less green.