Pattern placement and shape distortion control using contour-based metrology

Despite of the fact that thousands of CD-SEM (critical dimension scanning electron microscope) images are acquired in a daily basis in a fab, limited metrology is performed. Usually these images will not serve other purposes after they are collected and measured, but as they are stored, post-process analysis can be applied. Initially, most of these images are used to perform CD metrology, even though many other types of metrics could be extracted from the same images, especially when using contour metrology. In this paper two use cases will be explored, where contour-based image processing is performed on typical in-line metrology targets. In both cases, initial intended metric was CD but thanks to contour based image computing, complementary information can be extracted. In the first use case, CD and overlay metrics can be extracted, while in the second CD, etch slanting and asymmetry analysis is performed across the wafer. In the first use case, retro-processing of about 6 months of in-line data will also show the interest to retrieve, a posteriori, overlay information on stored CD metrology images. Also, thanks to the possibility of combining the contours with multiple sources of information, such as layout, it is possible to obtain metrics even from the hidden areas, created by the overlay phenomenon. This can be seen in the Figure 1, where it is shown a via-to-line overlay case. In the second use case, the CD information has been “augmented” by sidewall angle local and global variability and asymmetry information leading to the revelation of a within-wafer etch slanting signature which causes a shape distortion. Depending on the stack configuration, this effect can be also plotted as an overlay or more specifically as a pattern placement shift map. The metrology was possible due to the combination of information coming from the contours and height, which allows also a local analysis of the 3D geometry of the features. This can be better visualized in Figure 2. Contour-based metrology offers new capabilities to dissociate several layers (e.g. via and line) or elements (e.g. top and bottom) in the image so that interlayer and intralayer metrics, other than width dimensions, can be computed. Besides, a solution not integrated in the tool provides excellent versatility to re-process images, thus allowing the obtention of new metrics, which can be very helpful also for retro-analysis.