Strategies for Nanoscale Contour Metrology using Critical Dimension Atomic Force Microscopy

Contour metrology is one of the techniques used to verify optical proximity correction (OPC) in lithography models. These methods, known as resolution enhancement techniques (RET), are necessary to continue the decrease in integrated circuit feature sizes. Broadly speaking, RET are used to compensate for lithography errors to ensure better image transfer from the mask to the wafer and subsequence processing. Contours extracted from the printed features are used to verify the OPC models. Currently, the scanning electron microscope (SEM) is used to generate and verify the contours. The critical dimension atomic force microscope (CD-AFM), which is being used as a reference instrument in lithography metrology, has been proposed as a supplemental instrument for contour verification. This is mostly due to the relative insensitivity of the CD-AFM to material properties, the three-dimensional data, and the ability to make the instrument traceable to the SI unit of length. However, although the data from the CD-AFM is inherently three dimensional, the planar two-dimensional data required for contour metrology is not easily compared with the top-down AFM data. This is mostly due to the effect of the CD-AFM tip and the scanning strategy. In this paper we outline some of the methods for acquiring contour data using the CD-AFM. Specifically, we look at different scanning strategies, tip types, contour extraction methods, and imaging modes. We compare contours extracted using our method to those acquired using the SEM.