This is a massive stepper lens assembly in a very heavy steel enclosure. The external jacket is either stainless steel or INVAR and weighs close to 50 pounds. The tube assembly is 16 inches long and 165mm in diameter. The front element (mask side) measures approximately 75 mm in clear aperture. The front element bezel is engraved GCA / Tropel, 5-2135-KRF, with the serial number 212011.
The KRF designation denotes a Krypton Fluoride excimer laser. Therefore the lens would be corrected for a wavelength of 248nm, meaning the elements would be fused silica or quartz glass.
The output side of the lens (substrate side) has an annular attachment that appears to have several functions. Most obviously the surrounding metal flat serves to protect the output element. Built into the annular ring is some sort of proximity sensor system. There is a photomultiplier tube (the pins of which are visible in one of the photos) and an elaborate lens system built into the side of the collar. There is a forward-facing lens element with externally adjustable deviating mirrors which are probably used to regulate the focus of the lens.
These photolithography lenses are exceptionally highly corrected for distortion. Typical construction in a lens of this vintage (early-1990's) might consist of 25 or more elements. We do not have any of the electronics used to drive the PMT system. We only have what is shown in the accompanying photos. The lens appears to be in perfect condition, but we have no means to rigorously test it.
Photo set 1725