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Projection lithography images a mask through reduction optics; its resolution scales approximately as $k_1\lambda /\mathrm{NA}$. It offers parallel exposure, high throughput and accurate repeat patterning, but small features demand short wavelengths, large numerical aperture, costly optics and high-quality masks, with a shrinking depth of focus.

Electron beam lithography directly writes a programmable pattern using a focused electron beam. Its much shorter wavelength and small probe permit nanometre-scale features without an optical mask, making it attractive for research prototypes, low-volume devices and mask fabrication. Its limitations are serial writing time, charging of insulating substrates, beam drift, field stitching and forward/backscattering proximity effects in electron-beam lithography. Electron wavelength alone does not set the resist resolution. E-beam is often preferable for a few very small custom structures; projection remains preferable for high-volume repeated manufacture.

A photomask is a patterned optical component used to define the illumination reaching a photosensitive layer in photolithography. Its pattern controls transmission, reflection or optical phase, depending on the imaging system. In projection lithography, optics form a reduced image of the mask on the substrate.^[ref]

The printed boundary depends on diffraction, illumination and photoresist response as well as the drawn mask edge. Optical proximity correction deliberately changes mask geometry to compensate for those effects. A mask feature is therefore part of an imaging-and-material process, rather than a guarantee that an identical outline will appear on the wafer.