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MRC for curvilinear mask shapes

Image showing examples of MRC violations based on External Distance limits. The left violated the first rule, so it is flagged independently of its run-length. The right shows two situations that violate the external distance of 28 nm.
Generating curvilinear mask shapes in OPC instead of pure rectangular shapes is becoming more and more realistic as a method for improving wafer lithography performance. The main benefit of using curvilinear shapes is an improved process window, meaning that the wafer image is less sensitive to dose and focus conditions during the exposure. With the increased compute power of the latest high-performance clusters and the availability of multi-beam mask writers (MBMW), those wafer lithography benefits can be realized at technology nodes currently being developed.

A very practical challenge for putting masks with curvilinear shapes in production is the availability of reliable mask rule checks (MRC). The OPC engine not only needs to generate shapes which are manufacturable, the mask shop also needs a method of verifying that incoming mask data is manufacturable. For curvilinear mask shapes this is more challenging than for rectangular mask shapes, since simple width and space checks as used for rectangular masks are no longer sufficient.

In this paper, we discuss a comprehensive set of MRC limits and demonstrate the effectiveness of an MRC engine operating on curvilinear input data. Rules used here include minimum width of exposed features, minimum space between exposed features, minimum curvature of convex and concave shapes, as well as minimum area of exposed features.

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