= Solution
<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. \b[E-beam is often preferable for a few very small custom structures; projection remains preferable for high-volume repeated manufacture.]
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