A Multi-Step Sraf to Improve Process Windows in Metal Layer

In the metal layer of the integrated circuit, the layout has dense lines with high density and isolated lines with low density. The T-opening is in an isolated position in the layout. However, dense lines have mismatched process windows with isolated lines in the metal layer, lowering the process window with T-openings and analogous structures. Sub-Resolution Assist Feature (SRAF) is often used to improve the contrast, depth of field, and process redundancy of the lithography process to improve the process window of the layout in integrated circuits. Due to the different density features between the upper and lower (longitudinal) distribution of the horizontal pattern in the T-shaped opening, the diffraction spectrum distributions around the horizontal main pattern generate high discrepancy. As a result, the diffraction spectrum difference will lead to the mismatch of the process windows and reduce the process window of the layout. Here, we introduce a multi-step SRAF addition method evenly to reduce the contrast of graphics with different sizes, then improve the difficulty of the photolithography process.