Physics jobs

The exposure itself will not only create the wanted photochemical reactions in the photosensitive material, but will also warm up the silicon wafer. This is an unwanted effect because this heat load will locally expand the material on an exposed layer, and if the next layer has different thermal conditions during exposure then these two layers on top of each other ( stack of these layers eventually form a microchip) would not precisely align as desired without any corrective action. The physical model describing such expansion will depend on the wavelengths of the exposure light (e.g. EUV photons carry more energy than DUV photons), the transmission of the reticle, the number of photons exposed, the material properties and the corresponding heat transfer in dependency of time and - last not least also on the environment around the wafer. We have to deal with the vacuum environment in EUV machine while our DUV machines have either air or the immersion fluid (water) on top the wafer – all creating differences in the physical models needed for the different systems.

Once we have a good physical model established and calibrated, we are able to forecast the corrections needed caused by such thermal expansion for every single exposure and will actuate these coordinate corrections (in all directions/rotations) with the wafer stage, reticle stage, and lens elements movements – this we call a feedforward correction. Needless to say that all calculations required for such feedforward corrections have to be executed in synchronization and within allowed time budgets in order meet with wafer per hour productivity target of our scanner.