subscript

The passage around this formula

where $\lambda$ is the exposure wavelength, $\mathrm{NA}$ is the numerical aperture of the projection optics, and $k_1$ is a dimensionless process factor absorbing illumination scheme, mask engineering and resist behaviour. Two-beam interference sets a hard floor: for a periodic line-and-space pattern the smallest printable half-pitch is $\lambda$ / (2\,$\mathrm{NA}$) , which is $k_1$ = 0.25 in the convention where R is a half-pitch. Everything the industry calls resolution enhancement — off-axis illumination, phase shifting, optical proximity correction, inverse lithography — is an attempt to drive $k_1$ down toward that floor without printing garbage.

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