A method of forming a mask. The method includes providing design information of a design layer. The design layer includes M original design features and N original dummy features. The method further includes (i) creating a cluster of P representative dummy features, P being a positive integer less than N, (ii) performing OPC for the cluster of the P representative dummy features but not for the N original dummy features, resulting in P OPC-applied representative dummy features, and (iii) forming the mask including N mask dummy features. The N mask dummy features are identical. 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. The N mask dummy features have the same relative positions as the N original dummy features.