Improvement of focus accuracy on processed wafer

ABSTRACT As feature size shr inkage in semiconductor device progress, process fluctuation, especially focus strongly a ffects device performance. Because focus control is an ongoing challenge in optical lithography , various studies have sought for improving focus monitor ing and control . Focus errors are due to wafer s, exposure tools, reticles, QCs, and so on. F ew studies a re performed to minimize the measurement error s of auto focus (AF) sensor s of exposure tool, especially when processed wafers are exposed. With current focus measurement techniques, the phase shift grating (PSG) focus monitor 1) ha s been already proposed and its basic principle is that the intensity of the diffraction light of the mask pattern is made asymmetric by arranging a ŒSKDVHVKLIWDUHDRQ a reticle. The resist pattern exposed at the defocus position is shifted on the wafer and shifted pattern can be easily measured using an overlay inspection tool. However, it is difficult to measure shifted pattern for the pattern on the processed wafer because of interruption s caused by other patterns in the underlayer. In this paper, we ther HIRUHSURSRVHµµ6(0 -36*¶¶ technique , where the shift of the P SG resist mark is measured by employing critical dimension -scanning electron microscope (CD -SEM) to measure the focus error on the processed wafer. First, we evaluate the accuracy of SEM -PSG tech nique . Second , by applying the SEM -PSG technique and feeding the results back to the exposure, we evaluate the focus accuracy on processed wafers. By applying SEM -PSG feedback, the focus accuracy on the processed wafer was improved from 40 to 29 nm in 3 1 . Keywords : processed wafer, auto focus sensor , SEM -PSG, feedback correction