A new algorithm for optical photomask CD metrology for the 100-nm node

Manufacturing devices at the 100nm node presents new problems for the photomask metrologist. The metrologist is required to measure dense features with arbitrary line and space widths. While existing optical metrology tools are very successful at measuring isolated features with very high precision and repeatability, the conventional threshold algorithms exhibit Optical Proximity Effects (OPE) that affect the accuracy of CD measurements of both isolated and dense features. This paper presents a new CD metrology algorithm that is highly linear and largely insensitive to the influence of OPE while maintaining high precision and repeatability. The algorithm has been implemented on the new 244nm DUV optical metrology tool, the KMS-100. Demonstrated performance for the new algorithm on the DUV tool on binary masks shows better than 1.5nm, 3 sigma static repeatability down to 0.25um. Linearity, without multipoint calibration, is better than 5nm down to 0.25um for isolated lines. The OPE sensitivity (difference between measurements of isolated, dense and half isolated lines) for mask features down to 0.4um has been demonstrated to be better than 5nm over a wide range of dense lines and spaces widths.