New critical dimension uniformity measurement concept based reticle inspection tool

The Critical Dimension Uniformity (CDU) specification on photo-mask is getting increasingly tighter which each successive node. The ITRS roadmap for optical masks indicates that, the CDU (3 sigma) for dense lines for binary or attenuated phase shift mask is 3.4nm for 45nm half-pitch (45HP) node and will go down to 2.4nm for 32HP node. The current variability in mask shop processes results in CDU variation across the photo-mask of ~2-3nm. Hence, we are entering in a phase where the mask CDU specification is approaching the limit of the capability of the current POR (process on record). Hence, mask shops have started exploring more active mechanisms to improve or compensate for the CDU of the masks. A typical application is in feeding back the CDU data to adjust the mask writer dose and compensate for non-uniformity in the CDs, resulting in improved quality of subsequent masks. Another option is to feed the CD uniformity information forward into the wafer FAB and adjust the scanner dose to correct for reticle non-uniformity. For these purposes mask makers prefer a dense measurement of CDs across the reticle in a short time. Mask makers are currently using the CD-SEM tool for data collection. While the resolution of SEM data ensures its position as the industry standard, an output map of CDU from a reticle inspection tool has the advantage of denser sampling over larger areas on the mask. High NA reticle inspection systems scan the entire reticle at high throughput, and are ideally suited for collecting CDU data on a dense grid. In this paper, we describe the basic theory of a new, reticle inspection-based CDU tool, and results on advanced memory masks. We discuss possible applications of CDU maps for optimizing the mask manufacturing and wafer production processes.