The fusion of metrology and inspection: challenges and solutions

Abstract

The extension of optical projection lithography beyond the 22-nm node requires strong Resolution Enhancement Techniques (RETs) such as aggressive Optical Proximity Correction (OPC), computational lithography, double patterning and others. These strong RETs make photomask patterns more complex, photomask specifications tighter, and the metrology demands greater. Currently Photomask Critical Dimension (CD) and image placement are characterized by measuring a few hundred points sampled across the entire mask area; however more sampling points in active patterns or even full area inspection will be required to support strong RETs. Inspection tools are developing capabilities in CD and Image placement mapping. Although measurement precision, resolution, stage position accuracy and other metrology functions would be inferior to a metrology tool, the sampling is intrinsically larger, including the entire active area. The inspection tool could satisfy the large sampling requirements while a subset of patterns could be measured with traditional CD and Image placement metrology tools with the required precision. Therefore the fusion of metrology and inspection tools would satisfy both the full area CD and Image placement measurement: coarse inspection in CD and Image placement with the inspection tool and verification with metrology tools measuring the CD and/or Image placement errors identified by the inspection tool. Next generation metrology tool and inspection tools also leverage the aerial plane image for inspection and metrology. This is important since the strong RETs generate very complex patterns that are difficult to measure reliably with a CDSEM. The aerial image retains a simple shape though photomask patterns are extremely complex. Using aerial image metrology techniques simplifies CD measurement on complex patterns and enables measurement of lithographically critical features. Measuring what matters most of wafer could lead to relaxed mask CD specifications and permit more sophisticated CD correction methods.