A UV inspection tool has been used to image and inspect Next Generation Lithography (NGL) reticles. Inspection images and simulations have been used to provide feedback to mask makers so that inspectability of NGL masks can be optimized. SCALPEL masks have high optical contrast and look much the same in reflection as conventional chrome on glass masks do in transmission. EPL stencil masks can be imaged well in reflection, but defects below the top surface, in the cutouts, may not be detectable optically. EUV masks that have been made to date tend to have relatively low contrast, with line edge profiles that are complex due to interference effects. Simulation results show that improved EUV inspection images can be obtained with a low reflectivity absorbing layer and proper choice of buffer layer thickness.
KLA-Tencor and industry partners are collaborating on a project for developing early capabilities of inspecting NGL masks. The project, partially funded by NIST as part of the ATP program, is focusing on building a research tool that will provide experimental data for development of a production capable tool. Some of the key technical issues include contrast in transmission and reflection, defect sources and types, and maintaining mask cleanliness in the absence of pellicles. The masks need to be inspected at multiple process stages, starting with unpatterned substrates, and ending with the pattern inspection. System issues include defect sensitivity and inspection time, which need to be balanced.
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