With decreasing Critical Dimensions (CD), the negative influence of line edge roughness (LER) and line-width
roughness (LWR) on CD uniformity and mean-to-target CD becomes more pronounced, since there is no corresponding
reduction of roughness with dimension reduction. This applies to wafer metrology as well as to mask metrology. In
order to better understand the types of roughness as well as the impact of the CD-SEM roughness measurement
capabilities on the control of the mask process, the sensitivity and accuracy of the roughness analysis were qualified by
comparing the measured mask roughness to the design for a dedicated LER test mask. This comparison is done for
different LER amplitude and periodicity values and for reference structures without nominal LER using the built-in CD-SEM
algorithms for LER characterization.
Mask Manufacturers are continuously asked to supply reticles with reduced CD (Critical Dimension) specification, such as CD Uniformity and Mean to target. To meet this on-going trend the industry is in a quest for higher resolution metrology tools, which in-turn drives the use of SEM metrology into standard mask manufacturing process. As dimensions of integrated circuit features reduce, the negative effects of roughness of the features, and/or of components such as photo-resist and ancillary structures used to produce the features, become more pronounced since there is not necessarily a corresponding reduction of roughness with dimension reduction. As a result of the increased problems, metrics that quantify roughness of specific sections of an integrated circuit have been developed; for example, line edge roughness (LER) measures the roughness of a linear edge.
This paper concentrates on one specific area of the Mask Metrology, being measurement of the different Roughness metrics of the reticle features such as lines and contacts, using a new SEM metrology tool, the Applied Materials RETicleSEM. We describe the comprehensive Roughness Analysis Algorithm package that performs precise measurements of the different Roughness metrics including Fourier analysis, auto-correlation function and correlation length. This package can be used to isolate and characterize the roughness of specific wavelength ranges that may be of interest for mask manufacturing process and/or mask quality control considerations. We conclude with sample results of Roughness Analysis on real SEM images of Reticle lines. The influence of CD roughness on the precision of measurements is considered. The proof that long-wave roughness can be one from the sources of flyers during CD measurements is presented.