Development of aerial image based aberration measurement technique

To maintain the best imaging performance of current high NA DUV scanners, in-situ aberration measurement is becoming more important than ever. In this paper, we present an aerial image based aberration measurement technique that can measure the aberrations up to 37(th) Zernike polynomial term. Our aberration measurement technique uses aerial image sensor (AIS) on DUV scanners. AIS is a slit scanning type aerial image sensor that can capture the one-dimensional intensity distribution of aerial images. Unlike previous photo resist image based aberration measurement technique, presented technique does not require the three-beam interference condition or the two-beam interference condition because it utilizes the image intensity information. This can eliminates the geometrical restriction in determination of the pupil sampling points. Thus, we made optimization of pupil sampling so that it can minimize the random error propagation in each Zernike coefficients. This optimization was done on a trial and error basis and we observed that the random error propagation significantly depended on pupil. sampling plan. The measured aberration was correlated to the programmed aberration induced by lens element displacement. Also the measurement repeatability was evaluated and confirmed. The overall performance of this aberration measurement technique is found to be appropriate for in-situ aberration monitor of current high NA scanners.