The coming of age of tilt CD-SEM

Abstract

The need for 3D metrology is becoming more urgent to address critical gaps in metrology for both lithographic and etch processes. Current generation lithographic processing (ArF source, where λ=193 nm) sometimes results in photoresist lines with re-entrant profiles or T-topping, as do many etch processes. A re-entrant profile misleads top-down metrology into reading the critical dimension (CD) as too large. Recent advances in gate process technology also raise challenges to traditional top-down metrology. One such example is the FinFET, which is truly a 3D device with 3D metrology needs. The ability to measure the bottom width of a profile is crucial for process control. Recently, tilt-beam critical dimension-scanning electron microscopy (CD-SEM) applications have been developed to measure the bottom CD of such features, using the tilted-view to see the bottom, avoiding the feature's larger top. This is an important achievement, as the bottom of a profile is the main feature of interest in many processes. Estimation of sidewall angle (SWA) is also important. For several years, tilt-beam CD-SEM has been an available technique for this measurement, with limited adoption by the litho-metrology community. However, in this paper we will explore another method to use the tilt feature to measure average sidewall angle, based on edgewidth measurement and the assumption of basic trapezoidal profile and known height and combined with the ability to sample multiple-features. While it will not provide exact profile shape, this technique can be quite useful in providing average profile information and will definitely exhibit good throughput. Samples used will be photoresist and etched FinFET structures to measure sidewall angles. Correlations of the results to a traceable CD-atomic force microscopy (AFM) reference measurement system are provided. Conclusions will show preliminary findings of the readiness of tilt-beam CD-SEM for measuring profile and, by extension, the status of measuring 3D structures such as FinFETs, and using CD-SEM as a direct control of lithographic tooling for T-topped photoresist profiles.