Curves ahead! IC manufacturing prepares for curvilinear masks

When manufacturing an integrated circuit, we have always made masks with Manhattan and 45-deg edges. Contact holes, for example, are designed square, even though the shape that is actually printed on wafer is a circle. It turns out that the ideal mask pattern to print such a circle is in fact a circle; the ultimate process-window-maximizing curvilinear mask shape. Curvilinear masks are now possible using multi-beam mask writers and EDA tools that handle curvilinear data are coming online.

This paper discusses the benefits of curvilinear (CL) masks and how electronic design automation (EDA) tools including inverse lithography technology enable this new generation of IC manufacturing.

Why curvilinear masks?

IC Design and manufacturing has historically represented shapes as Manhattan. Rectilinear polygons are a very efficient way to use available space. Circles and curves waste space, but in the real world, corners are always rounded to some extent. Rectilinear designs exhibit rounded corners on wafer due mostly to the low-pass nature of projection optics. Rectilinear designs exhibit rounded corners on mask due to finite corner resolution by optical/e-beam writers.

Patterning requirements of next-generation lithographic processes have pushed lithographers to explore the advantages of curvilinear masks. The arrival of multi-beam mask writers (MBMW) addresses the mask write runtime penalty associated with high vertex counts and brings curvilinear (CL) masks closer to reality.

The industry is moving full speed ahead to enable CL mask tools, data handling, and flows.