optical proximity correction

Optical proximity correction is the trick of deliberately distorting the shapes drawn on a photomask so that, after the blur of projection, what actually prints on the wafer comes out as the shape you wanted. Light used to pattern chips has a wavelength much larger than today's features, so it bends and smears at corners and line ends — squares print as blobs, and lines pull back from their neighbours. OPC pre-compensates: it adds little hammerhead tabs, serifs, and 'assist features' to the mask that, after the optics smudge everything, average back out to the crisp target.

It is essentially deconvolution baked into the mask: simulate how the optics will distort each shape, then warp the drawn polygon by the inverse so the printed result is correct. As features shrank far below the exposure wavelength, OPC grew from optional polish into an absolute necessity, and the corrected mask data can balloon to many times the size of the original layout — a major reason mask sets cost millions of dollars at leading nodes.