Defects in Silicon are one of the principle causes for rejection of Silicon wafers both by the large Silicon manufacturers and the IC manufacturers. A lot of the time if you buy wafers from a Silicon Vendor and not a manufacturer they might be called "Prime wafers", but chances are they have some faults. Most likely they have been rejected by some Fab or the Wafer manufacture for one thing or another, and if you can find out what for, you are way ahead of the game. Remember though this is not a bad thing, most of these wafers are perfectly fine for most tool R & D uses and they are able to be sold for reasonable prices. True prime wafers that meet all the specs and are suitable for device manufacture are very expensive! So smile when you get a good price on Prime and you get a good wafer with very few defects. There are many defects associated with Silicon wafers and most are caught at the Silicon Manufacturer, some are not. Wafer defects range from pits in the Silicon surface and tiny scratches, to things that are Hidden from view, buried below the Silicon surface like the picture at the RIGHT of Copper Precipitates (a common defect). Found just below the Silicon surface by means of a SIRTL etch for 60 sec. This defect exhibits a crows foot like appearance and would cause a bad Gate Oxide over this region, causing a failed device(s). If this defect was under a few points on your 49 point Thermal Oxide Measurement map, you would definitely see non-uniform Oxides in these areas (Thermal Oxide growth can accelerate or decelerate over defects in the Silicon). There are many Hidden defects in Silicon wafers and to date no one has made a perfect one. Perhaps someday when they grow ingots on the Space station we will get Silicon with zero defects but until then, the major Silicon manufacturers back on Earth are turning out some incredible Silicon Wafers