Stepper Scanner Step and Repeat Projection Optics Throughput Wafer Table

Exposure is the step where the scanner actually fires light through the reticle and lens to print one chip's pattern onto one location on the wafer — repeated field by field until the whole wafer is covered. How that light is delivered (all at once vs. swept through a slit) and shaped (which angles illuminate the mask) determines both the image quality and how many wafers per hour the tool can produce.

Nearly every leading-edge tool today is a scanner, not a stepper. An older stepper exposes an entire chip field in one static flash, then mechanically steps the wafer to the next position and flashes again — simple, but it means the lens has to deliver uniform, low-aberration imaging across the *entire* field simultaneously, which gets exponentially harder as fields get larger. A scanner instead exposes through a narrow rectangular slit while the reticle stage and wafer stage move in a precisely synchronized scan — the reticle moves 4x faster than the wafer, matching the lens's reduction ratio, so the pattern sweeps across the full field one thin strip at a time. The lens only ever needs to be excellent across that narrow slit, not the whole field — a dramatically easier optical design problem that also runs faster, because synchronized scanning sustains higher sustained throughput than flash-and-step motion.

Illumination shape matters as much as exposure timing. The light isn't simply aimed straight down at the mask — its angular distribution (how much comes in straight-on vs. at an angle, controlled by the partial coherence factor, σ) changes which spatial frequencies of the pattern get captured by the lens at all. Off-axis illumination schemes — annular (a ring of angles), dipole, and quadrupole — deliberately shape that angular spread to boost contrast for specific pattern orientations and pitches, at the cost of making other orientations worse. Choosing illumination shape is one of the first, highest-leverage decisions in setting up a process window for a given layer.

Throughput is the business metric this entire step optimizes for. A single EUV scanner represents well over $150M of capital; its value is measured in wafers-per-hour, because every additional second per exposure multiplies across the hundreds of layers and thousands of wafers a fab runs through it per week. Scan speed, illumination intensity, and resist sensitivity are tuned together specifically to maximize that number without pushing dose variation or overlay error outside tolerance.