Site TIR or Total Indicator Reading is a site by site measurement of flatness of a Prime wafer. There are a few ways of taking this measurement Best fit, site Best fit, etc. If you are interested in great optical submicron Photolithography there is only one the Silicon guys will admit is the best way of measuring flatness. That way is Site flatness-Backside referenced measuring a 20x20mm or larger site and each one of these sites must meet <.3 um flatness across each site. Now thats a wafer you can resolve .25-.5 um geometries on, and if it meets this specification on 90-100% of the sites on the wafer you have got a really Great wafer! We are not even going to discuss Global flatness on this page because we think that parameter alone without Site TIR is next to worthless