Rect-SRAF method in inverse lithography technology

The effect of inserting curved and rectangular sub-resolution assist features (SRAFs) into the initial mask on inverse lithography technology (ILT) is studied. On the basis of level set based ILT, SRAFs are added when constructing the initial 3D surface, which will affect the output mask. Different initial masks can affect the imaging contour critical dimension (CD) of the output results. The method of using rectangular SRAFs as the initial mask for ILT is proposed and compared with the method of curved masks. The results indicate that both methods can obtain output masks with good imaging quality. However, after the Manhattan transformation, the output masks of the method with rectangular SRAFs have a significant advantage in maintaining imaging quality. The method with rectangular SRAFs reduces CD error under NC by more than 4% and increases the depth of focus (DOF) more than 35 nm after Manhattan transformation. The initial mask can control the type of SRAFs in the output mask by our method, for which the initial mask can also be selected according to the requirements of the output mask, reducing the difficulty and the impact on the imaging effect in the subsequent mask processing.