Aerial image based lens metrology for wafer steppers

Abstract

Phase Measurement Interferometers (PMI) are widely used during the manufacturing process of high quality lenses. Although they have an excellent reproducibility and sensitivity, the set-up is expensive and the accuracy of the measurement needs to be checked frequently.This paper discusses an alternative lens metrology method that is based on an aerial image measurement. We discuss the Extended Nijboer-Zernike (ENZ) method and its application to aberration measurement of a high-NA optical system of a wafer stepper. ENZ is based on the observation of the through-focus intensity point-spread function of the projection lens. The advantage of ENZ is a simple set-up that is easy to run and maintain and provides good accuracy. Therefore the method is useful during lens assembly in the factory.The mathematical framework of ENZ is shown and the experimental procedure to extract aberrations for a high-NA lens is demonstrated on a high-NA DUV lithographic lens. PMI data is given as reference data. It is shown that ENZ provides an attractive alternative to the interferometer.

General imaging of advanced 3D mask objects based on the fully-vectorial extended Nijboer-Zernike (ENZ) theory

“…The development of the ENZ-theory was intended to provide a method to characterize optical systems by means of intensity measurements in the focal region. 6 In order to achieve this, it was fundamental to have both an accurate and fast algorithm to compute the point-spread function for a general aberrated system. In terms of computations, this comes down to computing the point-spread function from the otherwise uniform exitpupil, for an exit-pupil that is influenced by a general aberration.…”