Photomask linewidth measurement comparison including an improved model-based through-focus measurement approach

Abstract We introduce a new method for structure width (critical dimension (CD)) measurements, the ultraviolet through-focus optical microscopy (UV-TFOM). It is based on through-focus measurements, providing additional information about the phase change introduced by the sample and the measurement system for the sophisticated data analysis, which is based on rigorous simulations of the underlying diffraction problem and Bayesian target vector optimization. In addition to the dimensional structure parameters of interest, such as the CD, the method allows to determine and incorporate the most significant aberrations of the imaging system. To test the performance of this new method, line structures on a chromium-on-glass photomask down to nominal widths of 100 nm were measured. The results and the derived measurement uncertainties are compared with the results of a previous comparison using standard CD calibration tools used at the Physikalisch-Technische Bundesanstalt (PTB), including CD atomic force measurement (CD-AFM), CD scanning electron microscopy (CD-SEM) and transmission ultraviolet optical microscopy (UV-OM). The measured values are CD at 50% of the height of the structures. Here, the edge angle of the structures is also taken into account. While some of the results of the old comparison have already been published previously, all the results obtained at PTB are presented here for the first time. The comparison confirms the excellent agreement between PTB’s CD-AFM, and SEM measurements and the good agreement also with the UV-OM results, most of which are well within the combined expanded uncertainties. However, in contrast to UV-OM, the measurement results obtained with UV-TFOM show no significant systematic deviations from the non-optical methods, has an extended range down to 100 nm, and also offers a reduced measurement uncertainty. We present the new method including the setup used, provide all measurement results including the related uncertainty budget and the expanded uncertainties.