Lowering the Cross Correlation between Different Shape Parameters of the Inverse Grating Problem in Coherent Fourier Scatterometry

Angular Fourier Scatterometry techniques to solve inverse-grating problems have been widely used in semiconductor metrology since the initial work by den Boef et al. [1] because it is simple, fast and robust and yet yield accurate measurements. These systems use incoherent light as a source and are commonly referred to as Fourier Scatterometry (FS). Coherent Fourier Scatterometry (CFS) was introduced a few years ago [2] as an alternative technique to the existing tools. This new method differs from Incoherent Fourier Scatterometry by the aspect of using coherent light as a source to illuminate the grating to be characterized. It is shown [3] that the use of the information about the phase difference of successive orders generates significant gain in sensitivity. This phase difference is implicitly obtained by laterally scanning the grating with respect to the optical axis of the set-up within one grating period and obtaining a few intensity measurements. This lateral scan affects phases of only orders higher orders than zeroth. Using this property, besides common parameters of grating reconstruction without making any additional measurement, the parameter bias [3] (lateral misalignment of the grating in experiment and its numerical model) could also available in CFS enabling one to align the target with respect to optical axis of the system to an accuracy of few nanometers. However, this works only when higher orders are available.