Total test repeatability: a new figure of merit for CD metrology tools

Evaluations of CD-metrology tools usually focus on resolution, repeatability and accuracy. These are traditional metrics which relate to the capability to measure a local line width. These metrics do not cover the capability to map the CD fingerprint (uniformity map) of the wafer and scanner field, which are essential for sub-100nm lithography process control. In this study, CD-uniformity wafers of state-of-the-art step-and-scan systems were measured with different metrology tools. Analysis of the results revealed a random contribution that could not be attributed to the exposure tool or to the repeatability of the metrology tool. A test and analysis method was developed to separate out this random contribution from the test results. The level of this random CD variation, called the Total Test Repeatability (TTR), is proposed here as a new metric to compare CD-metrology tools in their capability to generate CD uniformity maps. The method was applied to study CD-SEM, Electrical Line width Measurements and CD-scatterometry. In general, the TTR appears to be much larger than the metrology tool repeatability. As such it is an important figure of merit for CD metrology tools used to reveal fingerprints of reticles, exposure tools or processing tools. The TTR is dependent on the metrology tool, measurement algorithm, but also on materials and processing flow and conditions. Some root causes have been identified, such as the wafer resistivity properties for ELM or line width roughness that appears as CD variation in CD-SEM tools. Modifications can be made in the metrology strategy to suppress the TTR and reveal more reliable CD-uniformity fingerprints.