A transmissive mask is placed in contact with the resist, or a small distance above it for proximity printing. Chromium-patterned glass or quartz is a typical mask. Illumination is normally collimated ultraviolet light, with refractive illumination optics used to prepare the beam.
There is no projection objective imaging a reduced mask pattern onto the resist: the mask shadows the resist approximately one-to-one. The finite gap in proximity printing introduces diffraction and limits edge sharpness; contact printing reduces that gap at the cost of mask wear and contamination risk.
Use multilayer reflective optics and a reflective patterned mask. Near the usual extreme ultraviolet wavelength of 13.5 nm, ordinary lenses and thick transmissive masks absorb too strongly. Multilayer mirrors focus and project the image, while an absorber pattern on a multilayer-coated mask defines the exposed regions.
The optical path operates in vacuum to avoid absorption by air. The key distinction is reflective mask projection, rather than a transparent chromium-on-glass mask or a scanned electron probe.
Use electron lenses and a digitally scanned, blanked electron beam; no physical mask is required for direct writing. Electromagnetic lenses focus the charged-particle beam, deflectors place it, and the pattern generator controls position and dose. Electrostatic elements can also form part of electron optics, but these are not refractive glass lenses.
The extreme ultraviolet mask mentioned in the problem is the object being manufactured by this maskless write process, rather than a mask placed in the electron path.
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