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Resolution enhancement techniques (RETs) have become indispensable for the sub-wavelength optical lithography. Inverse lithography technology (ILT) is one kind of RETs, which attempts to consider the mask synthesis as an inverse problem and compute the optimum mask by using the entire area of the design pattern with a rigorous mathematical approach. Doubledipole lithography (DDL) uses two orthogonal dipole illuminations and one or two masks to print the desired wafer pattern. The main challenge of using such an IC-manufacturing technique remains how to properly synthesize the proper mask pattern for the arbitrarily given target pattern. This paper presents a gradient-based ILT approach addressing the problem above, called DDL-ILT. This method has been demonstrated by synthesizing various kinds of masks for printing 45-nm critical dimension (CD) features. Images with good fidelity have been achieved.