Éger Ferenc schreef op vr 11-12-2020 om 22:34 [+0100]:

To counter this, illumination systems in steppers contain a narrow-band filter (Bragg-filter or similar) to filter out exactly one narrow spectral line (hence I-line and G-line), and additional optics to eliminate off-axis components and increase coherence (therefore, what reaches the reticle is more or less "laser-like", highly coherent and monochromatic radiation with very narrow FWHM). Also, the projection optics in the stepper is usually a "narrow-band" system in terms of wavelength due to dispersion, and fine-tuned to work only at the wavelength of the line it is specified for.

It is true that old steppers contain spectral line filters; more recent steppers use monochromatic lasers.

But what in lithography is actually done is to use off-axis illumination for resolution enhancement. In the extreme one can reach double the resolution over coherent illumination (0.25*λ/NA versus 0.5*λ/NA). More info on off-axis illumination: http://www.lithoguru.com/scientist/litho_tutor/TUTOR42%20(Aug%2003).pdf

Off-axis illumination can be implemented by a second lens system before the light reaches the reticle where a pupil plane is present where one can insert the shape of the off-axis illumination one wants. Actual scanners from ASML (which I am familiar with) use more complex systems with so-called diffractive optical elements to optimize for other effects like lens aberrations, light intensity uniformity, minize effect of temperature expansion of materials, etc.