Aerial image sensor for self-calibration of wafer steppers

The requirement for higher resolution is pushing up the NA of the projection lens. As a result, DOF becomes shallower, and the focus budget becomes tight. Precise measurement of best focus is becoming more and more important. A new aerial image sensor is presented that is suitable for use on leading edge wafer steppers. This sensor detects the intensity distribution of aerial images down to 0.15 micrometer isolated lines, and is currently used as a best focus calibration sensor for wafer steppers. This sensor can measure best focus using both dense and isolated patterns with a precision of < 20 nm (3(sigma) ). In actual operation, determination of best focus on a wafer stepper requires only a few minutes. The functionality of this sensor is being expanded to include additional self- calibration tasks, such as magnification, illumination telecentricity, distortion, and other aberrations.