Resistivity is very important, if you are trying to make good devices or grow uniform Thermal Oxides. The more uniform the resistivity across the wafer and across the cassette of 25, the more uniform the Thermal Oxide you can grow. Understand that how uniform the resistivity of your start material is directly related to how your Oxide will grow. Most engineers know that Thermal Oxide grows faster in areas of low resistivity (more Dopant) and slower in areas of High resistivity (less dopant) To simplify things think of dopant as fertilizer on your lawn, wherever there is more fertilizer, the grass always grows faster... Right? Thermal Oxide growth works the same way! If the dopant uniformity of the start wafer is good you can expect a good Thermal Oxide. Of course that transforms into terrific gate Oxides and more stable and reliable devices. Ingots can have resistivity problems as well as some wafers. We have seen resistivity changes across a finished wafer of 2-4% and on Epitaxial wafers we have seen radial resistivity vary across a epitaxial wafer with a median resistance of 3 -20 Ohm/cm2 up to 15% across the wafer! As you can see epitaxial wafers are not always a good choice for uniform thermal Oxides.