EUV Lithography
# EUV Illuminator Optics and Facet-Mirror Design
Before EUV light ever reaches the mask, it has to pass through the illuminator — a subsystem of mirrors whose job is to take the raw, non-uniform light emerging from the tin-plasma source and turn it into a beam that's uniform in intensity and shaped correctly in angle across the entire exposure field. Light coming off the collector mirror isn't uniform or well-shaped for lithography use — it carries the plasma's own intensity variations and spatial profile. If that raw beam were sent straight to the mask, different parts of the exposure field would receive different amounts and angles of light, directly causing uneven exposure and pattern distortion across the field. The illuminator's entire purpose is correcting that before the light ever reaches the mask.
Facet mirrors — arrays of many small individual mirror segments — are the specific optical trick that lets the illuminator homogenize the beam, because each facet redirects a small slice of the incoming light along a slightly different path, and recombining thousands of these redirected slices averages out the source's original non-uniformity. A single large mirror can redirect light but can't independently reshape different parts of the beam; a facet mirror array, by contrast, treats the beam as many small pieces that each get individually steered, so that the output beam is effectively a superposition of thousands of differently-angled contributions. Averaged together, source hot spots and cold spots wash out, and the resulting beam arrives at the mask far more uniform than the raw source output ever was.
The illuminator also sets the angular distribution of light striking the mask — the pupil fill shape — which is tuned separately from uniformity and chosen specifically to match the diffraction characteristics of the mask pattern being exposed, since different pattern geometries benefit from different illumination angles. Shaping pupil fill is a distinct optimization from beam uniformity: a given illumination angle distribution can enhance contrast for certain feature orientations and pitches while degrading it for others, so the illuminator's facet arrangement has to deliver both an evenly intense beam and a deliberately shaped angular profile simultaneously, two requirements that a simpler single-mirror design could never satisfy at once.
Wavefront aberration metrology verifies the projection optics delivering the mask pattern to the wafer stay correctly shaped; the illuminator is the stage before that, making sure the light hitting the mask in the first place is uniform and correctly angled — together they bracket the mask itself with correction on both sides of the optical path.