A new mask evaluation tool, the microlithography simulation microscope aerial image measurement system

Abstract

In a previous paper we have discussed a prototype aerial image measurement system (AIMS1). This prototype has evolved into a new product, the Zeiss MSM1002microlithography simulation microscope, which can evaluate phase shift and conventional photolithographic masks. In this paper we will discuss the MSM design, its operation, image capture and analysis methods, and typical applications. The toolts unique optical system captures Through focus' images of a mask for a selected NA, sigma, and wavelength, thus paralleling the characteristics of a particular optical stepper. The MSM operates at I-line (365mu) and DUV (248 urn) wavelengths, and handles commonly used 5 or6 inch reticles. The images obtained are optically equivalent to that incident on resist, but are highly magnified so that they may be recorded using a DUV CCD camera. Typically, features of interest are recorded as a through focus series; image intensity is digitized and may be analyzed in a variety of ways using newly developed aerial image measurement system (AIMS ) software. Combined with simple models for predicting resist behavior, a great deal of information may be obtained on the expected printing perfonnance of a given reticle as a function of intensity and depth of focus, prior to actual resist tests. A number of applications have been explored. Phase and transmission errors for alternating line structures have be readily quantified as an aid to optimizing mask fabrication techniques. Application to the assessment of defect printability, both before and after repair, is presented. Masks have been analyzed to predicted CD values which are in good agreement with subsequent resist work. Unconventional illumination schemes have been studied.