Trench geometry and resist profiles from modeling of polarized optical spectra

Profiles of trenches in silicon were calculated by fitting measured polarized reflectance spectra with theoretically calculated RCWA (rigorous coupled wave analysis) models. The agreement of trench geometry from optical measurements with SEM cross-sections was good. The contact-free optical measurement on productive wafers constitutes a serious alternative to the time consuming and destructive cross-section analyses. Critical dimensions (CD) of gratings of resist lines were obtained from RCWA modeling of optical spectra. An i-line resist with lines of larger spacing and a UV resist with lines of smaller pitch were investigated. In both cases the top and bottom CDs varied over a wide range due to the change in exposure dose across the wafer. The good agreement between optically obtained CD values, in-line CD SEM data and cross-section SEM data makes the optical method an ideal tool for process control.