Focus characterization using end of line metrology

Abstract

A new method is introduced to measure relative focus using conventional optical overlay instruments. Optical end of line metrology (OELM), is based on patterning a wide frame in which adjacent sides are constructed of submicron sized lines that run perpendicular to the center opening. Because truncation is size dependent, line and space features exhibit significantly more line shortening effects than the solid sections. When measured with a conventional optical overlay tool, the difference in line shortening between the solid and line and space sections are measured as an alignment offset, which is a relative measure of actual line shortening. Reproducibility for measuring the apparent misalignment of these modified box-in-box structures was 3 nm (3/spl sigma//sup est/), which is similar to the repeatability for measuring conventional resist box in box alignment boxes. Truncation is sensitive to focus and the utility for using OELM toward characterizing focus-dependent lens parameters was investigated. Estimated 3/spl sigma/ for calculating best focus and astigmatism were 0.04 /spl mu/m and 0.01 /spl mu/m, respectively. Focus corrections were accurate to 0.05 /spl mu/m within /spl plusmn/0.3 /spl mu/m of best focus. Additionally, a general method is presented to estimate the error in the calculated best focus.