[Libre-silicon-devel] OK. That happened (Funding)

[Staf Verhaegen]

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.

greets,
Staf.

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