EUV Lithography
# EUV Scanner Dose Sensor Feedback Control
An EUV scanner has to deliver exactly the right amount of light energy to every point on the wafer, and because the tin-droplet plasma source's output fluctuates pulse to pulse, the scanner relies on in-situ dose sensors reading real exposure energy during the scan itself, not a pre-calibrated assumption about what the source should be putting out. Each EUV pulse from the plasma source varies slightly in intensity — droplet-to-droplet variation in size, position, and laser timing all introduce pulse energy noise that no source design fully eliminates. A wafer exposed to too little cumulative dose under-develops the resist pattern; too much dose over-exposes it. Because the source itself can't be made perfectly stable, the scanner instead measures the actual delivered dose in real time and adjusts exposure on the fly to compensate.
Dose sensors positioned at or near the wafer plane integrate pulse energy across the exposure slit as the wafer scans underneath it, feeding that measurement into a control loop that adjusts scan speed or source triggering to hold cumulative dose within a tight tolerance band — typically a fraction of a percent across the entire wafer. Because the scanner exposes the wafer by scanning it under a narrow illumination slit rather than flashing the whole field at once, dose uniformity depends on every point receiving the same integrated exposure as it passes through that slit, which in turn depends on pulse-to-pulse source stability being measured and corrected continuously rather than assumed constant. This real-time correction is what keeps resist exposure uniform despite a light source that is fundamentally noisy pulse to pulse.
Dose uniformity directly determines critical dimension uniformity on the finished wafer, since resist development sensitivity to exposure dose is one of the strongest levers on how accurately a printed feature matches its target size — making dose sensor accuracy and response speed a direct throughput-versus-yield tradeoff for scanner makers. A faster-responding, more accurate dose sensor allows tighter real-time correction and therefore better CD uniformity, but sensor response speed and scan speed interact: correcting dose fast enough to matter at production scan speeds is itself a nontrivial sensor-engineering problem, not a solved one.
The collector mirror and hydrogen-radical cleaning keep the source's average output usable over the scanner's operating life; the dose sensor feedback loop handles the source's instantaneous, pulse-to-pulse unsteadiness — together they're the two halves of the engineering effort needed to turn an inherently noisy plasma light source into exposures precise enough for leading-edge lithography.