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 M original design features and the design cluster of the P representative dummy features but not for the N original dummy features, resulting in M OPC-applied design features and P OPC-applied representative dummy features, respectively such that if the M OPC-applied design features and 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 M original design features and the design cluster of the P representative dummy features would be created in the photoresist layer; selecting an OPC-applied representative dummy feature of the P OPC-applied representative dummy features; and forming the mask including M mask design features and N mask dummy features, wherein the M mask design features are identical in terms of size and shape to the M OPC-applied design features, wherein the M mask design features have the same relative positions as the M original design features, wherein the N mask dummy features are identical in terms of size and shape to the selected 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.