There are a number of factors that can affect the shape of a wafer be it Silicon, GaAs or InP. While a wafer is at full thickness, it has the tensile strength to resist any external influences from changing its shape. However, as a wafer is thinned, external influences will cause a wafer to become concave or convex. Some of the more common influences is film type and thickness on the surface of wafer. Nitrides can cause a wafer to become concave while oxides can cause a wafer to become convex. Additionally, how heavily a wafer is doped can result in a potato shaped wafer. Aptek has extensive experience dealing with these issues. Sometimes a simple CMP polish can alleviate the strain from backgrind and allow the wafer to return to original form. Annealing is another method to relax the shape of a wafer through the use of heat and time.