Forbidden pitch or duty-free: revealing the causes of across-pitch imaging differences

As resist image responses vary with duty ratio, the identification of a particularly challenging instance leads to its classification as a “forbidden pitch.” The increased application of various RET methods has often resulted in the misuse of this label for anything unexplained by linear effects. This paper attempts to dispel the myths regarding the imaging variations that occur with pitch. Furthermore, by describing the basis of these behaviors, insight is provided for the appropriate design of mask, illumination, OPC, and exposure parameters to best accommodate a broad range of duty ratio values.