Resist Exposure Photon Absorption Acid Generation Latent Image Formation
The latent image is the invisible chemical copy of the aerial image that exists inside the resist film for the brief window between exposure and development — a spatial map of photoacid concentration (in a chemically amplified resist) that mirrors the light intensity pattern the lens projected, but isn't yet a physical, developable pattern. Nothing has dissolved or crosslinked at this point; the chip's geometry exists only as a gradient of chemistry waiting to be acted on.
The latent image is where "light" formally becomes "chemistry." In a chemically amplified resist, each absorbed photon triggers a photoacid generator (PAG) molecule to release a single hydrogen ion at that location. Where the aerial image was bright, more PAG molecules activate, producing a higher local acid concentration; where it was dark, almost none do. The result is a spatial map of acid concentration that tracks the optical intensity pattern — the latent image — but it's purely chemical. Nothing is soluble or insoluble yet; that transformation doesn't happen until the post-exposure bake lets the acid diffuse and catalyze deprotection.
Dose — not peak intensity alone — is the quantity that actually matters. A resist is specified by the minimum exposure dose (measured in mJ/cm²) needed to fully convert it from unexposed to exposed state. Dose is the product of intensity and exposure time, which is why scanner throughput and resist sensitivity trade directly against each other: a more sensitive resist needs less dose, so the same light source can expose wafers faster.
Photon energy sets a second, independent constraint. Photon energy rises as E = hc/λ, so EUV's 13.5nm photons each carry roughly 15x the energy of a 193nm ArF photon. To deliver the same total dose, an EUV exposure therefore uses far fewer individual photons landing on any given pixel than a DUV exposure would. Fewer discrete events means more relative statistical scatter in exactly how many photons (and how much acid) land in any one feature — the latent image itself becomes noisier purely from photon-counting statistics, before chemistry or optics contribute anything at all. That photon-starvation problem is a direct, unavoidable consequence of using a shorter wavelength, and it's the root cause of the stochastic defects that make EUV resist design so much harder than its DUV predecessors.