Wavefront Performance Measurement on Small NA Lens

ASML is a global leader in lithography for mass producing semiconductor chips. The company uses optical techniques to measure wafer topology in its commercial lithography systems. ASML’s current method for qualifying the lenses used in level sensors relies on a double-pass interferometer. While this method is effective, it also has serious limitations: It is complex, requires a large facility and is time-consuming. To address these problems, the team explored alternative single-pass metrology
techniques that can improve efficiency while maintaining accuracy and repeatability.

The team evaluated three approaches and then selected the Shack-Hartmann Wavefront Sensor for its simplicity and availability. After confirming this sensor’s potential with extensive theoretical analysis and modeling, the team designed and built a test setup around the sensor to validate the theoretical findings through experimental testing. This testing confirmed that the system successfully measures and displays wavefront aberrations introduced by the test lens. Repeated measurements demonstrate the setup’s accuracy and repeatability, confirming its potential as a viable alternative to the more complex double-pass interferometer that ASML currently uses.