Bow & Warp are two important specifications in determining the quality of a wafer at final inspection. First let's discuss Bow, this is the measure of how concave or convex the deformation of the median surface of the wafer at the center point, independent of any thickness variations.In this measurement the wafer is in an unclamped state on the chuck of the measurement system a normal or good Bow number on an 8" Prime wafer would be < 30 um. The easy way to visualize Bow is if you think of the wafer as a bowl turned either upside down or rightside up on a flat surface (convex or concave). Next let's talk about Warp, this measument is also made in an unclamped state. Warp is the difference between the maximum and minimum deviations of the median surface relative to the backside reference plane. Now thats a mouthfull, to simplify things think of warp as the deformation of a wafer kind of like a potato chip, thats the easiest way to explain it. A good Warp number on an 8" wafer would be <20 um.