Step and Scan

Step-and-repeat and step-and-scan are the two fundamentally different ways a lithography tool images a reticle's pattern across a wafer, and the difference between them is exactly what separates an older "stepper" from a modern "scanner." A step-and-repeat tool flashes the entire reticle field onto the wafer in one static exposure, then steps the wafer to the next field position and flashes again; a step-and-scan tool instead sweeps a narrow slit of illumination across the reticle while the wafer stage moves in perfect synchrony beneath it, building up a full field one thin strip at a time before stepping to the next position — same end goal, completely different physics inside the tool.

The reason scanning won out is a lens-design problem, not a speed problem. A projection lens that stays perfectly corrected for aberrations across a full, large static field is extraordinarily hard and expensive to build, but a lens only has to be well-corrected across a narrow slit at any given instant if the wafer and reticle stages are scanning that slit across the field together — this is what let step-and-scan tools image a field roughly 26 by 33 millimeters, noticeably larger than step-and-repeat's practical limit around 22 by 22 millimeters, while using a comparably sized, comparably correctable lens.

Scanning also forces a mechanical engineering problem that flashing never had: two stages moving in lockstep. The wafer stage has to track the reticle stage's scan motion at precisely the reduction ratio the optics use — a 4x reduction system means the wafer moves at exactly one-quarter the reticle's velocity, continuously, with nanometer-level synchronization error or the pattern smears — so a scanner's wafer and reticle stages are themselves a precision instrument in their own right, not just a conveyor that stops and starts between flashes the way a step-and-repeat tool's stage can get away with being.

ASML built its business on winning exactly this transition. GCA shipped the first commercial wafer stepper, the Mark II, in 1978 as a pure step-and-repeat tool, and the stepper market that followed was dominated by step-and-repeat machines through the 1980s; ASML's step-and-scan architecture, introduced commercially in the 1990s, is what let it catch up to and eventually overtake established players like Nikon and Canon, and essentially every leading-edge immersion and EUV lithography tool shipped today, regardless of maker, is a scanner rather than a stepper for exactly the lens-correction reason that made scanning win in the first place.

Read step-and-scan through a *correct-a-slit-not-a-field* lens rather than a *faster-exposure* lens — the scanning motion looks like it exists for throughput, but the real reason it exists is that it let lens designers solve a much easier optical problem and still cover a bigger field, which is the trade that actually decided how every modern lithography tool is built.