Lithography tutor

So how does this optical path difference affect the formation of an image? For light, the path length traveled is equivalent to a change in phase. Thus, the OPD can be expressed as a phase error,

Our interpretation of defocus is that it causes a phase error as a function of radial position within the aperture. Light in the center of the aperture has no error, light at the edge of the aperture has the greatest phase error

the effect of defocus is to add a phase error to the higher order diffracted light relative to the zero order. When the lens recombines these orders to form an image, this phase error will result in a degraded image.

how depth-of-focus changes with feature size， the phase error can be expressed as a function of the mask feature:

maximum tolerable defocus (called the depth-offocus,DOF)

using an aqueous base as the developer, the exposed PAC（photoactive compound） is soluble but the unexposed PAC is not, creating a solubility differential with exposure that forms the basis of resist image formation.