Method of detecting aberrations of an optical imaging system

Abstract

Aberrations of an imaging system (PL) can be detected in an accurate and reliable way by imaging, by means of the imaging system, a circular phase structure ( ) on a photoresist (PR), developing the resist and scanning it with a scanning detection device (SEM) which is coupled to an image processor (IP). The circular phase structure is imaged in a ring structure ( ) and each type of aberration, like coma, astigmatism, three-point aberration, etc. causes a specific change in the shape of the inner contour (CI) and the outer contour (CE) of the ring and/or a change in the distance between these contours, so that the aberrations can be detected independently of each other. Each type of aberration is represented by a specific Fourier harmonic (Z-), which is composed of Zernike coefficients (Z-), each representing a specific lower or higher order sub-aberration. The new method enables to determine these sub-aberrations The new method may be used for measuring a projection system for a lithographic projection apparatus.