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Optical Proximity Correction (OPC)

A way to improve wafer printability by modifying mask patterns.
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Description

Optical proximity correction (OPC) is often associated with a photomask. The photomask is produced in a photomask facility. In the flow, a chipmaker designs an IC, which is then translated into a file format. Then, a photomask is developed based on that format.

A photomask is a master template for a given IC design. After the mask is developed, it is shipped to the fab and placed in a lithography scanner.

A resolution enhancement technique (RET), OPC makes use of tiny sub-resolution assist features (SRAFs), or decoration-like shapes, on the photomask. OPC modifies the mask patterns to improve the printability on the wafer.

For years, photomask makers used little or no OPC on the mask. Mask complexity began to escalate starting at 22nm/20nm. The OPC-based features began to pile up and overlap on the mask, making it difficult to print some of the features on the wafer. As a solution, the industry turned to multiple patterning. With this approach, the OPC-based features are split into two or more masks, which provide more room to manipulate the objects. At advanced nodes, though, the OPC features are becoming smaller and more complex.

More advanced RETs have been introduced, such as curvilinear assist features. One version of curvilinear assist features is called inverse lithography technology (ILT). ILT makes use of curvilinear shapes on the mask that improve the latitude of a process and the depth of focus for a lithography system. However, the increased complexity of mask shapes and finer geometries means it will take a longer time to write or pattern the mask.

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