Definition of flatness

1. 1 : for wafer surfaces, the deviation of the front surface, expressed in total indicator reading (TIR) or maximum focal plane deviation (FPD), relative to a specified reference plane when the back surface of the wafer is ideally flat, as when pulled down by a vacuum onto an ideally clean, flat chuck. [SEMI M1-94 and ASTM F1241] Contrast bow and warp. Also see global flatness and site flatness. 2 : in a ceramic package or leadframe, the allowable deviation of a surface from a defined reference plane . The tolerance zone is defined by two parallel planes within which the surface must lie. [SEMI G61-94] Also see camber.