EUV Lithography
# EUV Wafer Stage Interferometer Positioning Metrology
The wafer stage inside an EUV scanner has to know its own position to within a fraction of a nanometer at every instant during exposure, and the way it achieves that knowledge is a laser interferometer system that measures position by counting light wavefronts rather than relying on any mechanical encoder. The stage carrying the wafer moves continuously during a scan exposure, and every feature printed on the wafer depends on the scanner knowing exactly where that stage is at the moment each point gets exposed. A mechanical position sensor simply can't resolve position changes at the scale EUV lithography requires; laser interferometry instead measures position as a count of optical wavelengths, a measurement technique whose resolution is tied to the wavelength of light used, not to any moving mechanical part's manufacturing tolerance.
Interferometer-based stage metrology works by splitting a laser beam, bouncing part of it off a mirror mounted on the moving wafer stage, and recombining it with a reference beam — the resulting interference pattern shifts in a way that directly and precisely tracks how far the stage has moved. As the stage moves, the path length the reflected beam travels changes, and that change shows up as a measurable shift in the interference pattern between the two recombined beams. Counting those pattern shifts gives a position measurement accurate to a small fraction of the laser's wavelength — far finer than any mechanical measurement system could achieve, and exactly the resolution needed to keep successive exposures correctly registered to each other, which is what overlay accuracy between lithography layers ultimately depends on.
Modern EUV scanners increasingly supplement or replace pure interferometer systems with encoder-based position metrology for certain stage-position measurements, because interferometer accuracy can be affected by air turbulence and pressure fluctuations along the beam path, whereas an encoder reads position directly off a physical scale mounted near the stage. This hybrid approach exists because no single position-sensing technology is free of every error source at the tolerances EUV demands — interferometers excel at absolute long-range precision but are sensitive to the medium the beam travels through, while encoders are less sensitive to air disturbance but have their own calibration and scale-accuracy considerations. Combining both technologies, cross-checking one against the other, is how scanners push stage positioning accuracy beyond what either method alone could reliably deliver.
Wavefront aberration metrology verifies the optics themselves are shaped correctly, and vibration isolation keeps the whole optical frame from moving unexpectedly; wafer stage interferometry is the complementary piece that tells the scanner exactly where the wafer is at every instant during a deliberate, controlled scan — together, all three position-and-shape measurements are what let a lithography system claim nanometer-scale accuracy across an entire production wafer.