Spatial averaging technique for ellipsometry and reflectometry

Abstract

This invention relates to ellipsometry and reflectometry optical metrology tools that are used to evaluate semiconductor wafers and is directed to reducing errors associated with material surrounding a desired measurement area or pad, either by minimizing the uncertainties in positioning the measurement beam or by taking into account the effects of the surrounding material in analyzing the measured data. One aspect the present invention utilizes a technique where initially one purposefully aims to place the optical spot of the measurement beam a few microns away from the center of the target pad. Then a series of measurements are made with each measurement separated by a small stage jog as the optical spot is scanned over the measurement pad. Provided the surrounding material is the same on both sides of the pad, one finds that the data invariably has either a cup or inverted “U” shape or an inverted cup or “U” shape when viewed as a function of position. The minimum or maximum of the curve is then used to identify the center of the pad. Another aspect the present invention makes use of a novel method of data analysis that allows for the correction of the effects of the surrounding material in analyzing the data. In essence, the data collected at the center of the pad is treated as being created by a superposition of light coming from the pad material itself and light coming from the surrounding material. The influence of the two materials is weighted by the proportion of the light that reflects off the pad as compared with the light that reflects off of the surrounding material. Given knowledge of both the dimensions of the pad and the size and profile of the beam spot, the resulting signal may be mathematically modeled according to the present invention to account for both the contribution of the light reflected from the pad and the light reflected from the surrounding material.

Dependencies are inferred from the claim text.