Influence of line edge roughness (LER) on angular resolved and on spectroscopic scatterometry

Abstract

Scatterometry or optical CD metrology (OCD) has become one of the most common techniques in quantitative wafer metrology within the recent years. Different tool configurations are either available in commercial inspection tools or subject of recent and present research activities. Among these are normal incidence reflectometry, 2-θ scatterometry, spectroscopic ellipsometry and angle resolved Fourier scatterometry. The two latter techniques appear to be promising for future use in semiconductor fabs. Spectroscopic ellipsometry is well established, and Fourier scatterometry has become of increasing interest within the recent time. Line edge roughness, i.e. an edge position variation of printed lines in lithography, has been of less importance up to now, as its amplitude could largely be neglected with respect to the feature dimensions. This will, however, not be the case for future nodes, as on the one hand CDs are getting smaller and smaller, and on the other hand, even the absolute amplitude is expected to increase due to the higher complexity of lithography and etch processes. In this paper a comparison of scatterometric reconstructions in both spectroscopic and angle resolved techniques considering LER afflicted samples is presented. The validity and benefit of a simple effective medium model is investigated.

Analytical modeling and three-dimensional finite element simulation of line edge roughness in scatterometry

“…stitching errors) may be larger than the wavelength and roughness induced scatter as described by the formalism presented here is observable. For small structures and small spatial wavelength perturbations, however, no other diffraction orders than the zeroth order exist and other descriptions like the effective layer model [15,16] must be used to account for roughness.…”

“…However, this recent treatment was for a 1D structure, which inherently only describes in-plane scatter between the diffraction orders. In realistic investigations of roughness in scatterometry [5,6,15] also out-of-plane scatter has to be included. It will be shown that in the Fraunhofer far-field approximation, the impact of the line roughness is given by a multiplicative term also for 2D structures.…”

First steps towards a scatterometry reference standard

“…As one example, recent investigations [12][13][14][15] have shown, that even small feature details like line edge roughness (LER) or line widths roughness (LWR) do significantly affect the analysis of scatterometric measurements. To quantify this effect we performed numerical investigations based on finite element (FEM) based rigorous modelling [16].…”

Simulations of Scatterometry Down to 22 nm Structure Sizes and Beyond with Special Emphasis on LER

“…Restricting to resist structures the computation times keep bearable, since the RCWA algorithms converge much faster for dielectric gratings than for conductive ones. The authors of this work presented a first approach to consider true 3D roughness [25]. In that work strongly bearable computation times could be achieved due to a purely periodic modeling of the LER instead of a truly random model.…”