1. A method of forming a mask, comprising: providing design information of a design layer, wherein the design layer includes M original design features and N original dummy features, M and N being positive integers; creating a design cluster of P representative dummy features, P being a positive integer and less than N; performing OPC (Optical Proximity Correction) for the design cluster of the P representative dummy features but not for the N original dummy features, resulting in P OPC-applied representative dummy features such that if the P OPC-applied representative dummy features were printed on the mask and if a lithographic process was performed on a photoresist layer through the printed mask, then the P representative dummy features would be created in the photoresist layer; forming the mask including N mask dummy features, wherein the N mask dummy features are identical, wherein each mask dummy feature of the N mask dummy features of the mask has an area which is a function of at least an area of an OPC-applied representative dummy feature of the P OPC-applied representative dummy features, and wherein the N mask dummy features have the same relative positions as the N original dummy features.