Progress Towards Traceable Nanoscale Optical Critical Dimension Metrology for Semiconductors

Non-imaging optical critical dimension (OCD) techniques have rapidly become a preferred method for measuring nanoscale features in semiconductors. OCD relies upon the measurement of an optical reflectance signature from a grating target as a function of angle, wavelength and/or polarization. By comparing the signature with theoretical simulations, parameters of the grating lines such as critical dimension (CD) linewidth, sidewall angle, and line height can be obtained. Although the method is sensitive and highly repeatable, there are many issues to be addressed before OCD can be considered a traceable metrology. We report on progress towards accurate, traceable measurement, modeling, and analysis of OCD signatures collected on the NIST goniometric optical scatter instrument (GOSI), focusing on recent results from grating targets fabricated using the single-crystal critical dimension reference materials (SCCDRM) process. While we demonstrate good correlation between linewidth extracted from OCD and that measured by scanning electron microscopy (SEM), we also find systematic deviations between the experimentally obtained optical signatures and best fit theoretical signatures that limit our ability to determine uncertainty in OCD linewidth. We then use the SCCDRM line profile model and a χ goodness-of-fit analysis on simulated signatures to demonstrate the theoretical confidence limits for the grating line parameters in the case of normally distributed noise. This analysis shows that for the current SCCDRM implementation, line height and oxide layer undercut are highly correlated parameters, and that the 3-σ confidence limits in extracted linewidth depend on the target pitch. Prospects for traceable OCD metrology will be discussed.