Robust determination of asymmetric side wall angles by means of coherent scanning Fourier scatterometry

Abstract

Optical metrology of grating parameters with small scattering volumes, such as side wall angles (SWAs), is an indispensable prerequisite for accurate process control in modern semiconductor lithography. However, current scatterometric technologies suffer from low sensitivity towards SWA and hence, large measurement uncertainties. In order to overcome this deficit, we propose an interferometric sensor design which enables the precise determination of asymmetric SWAs with values that deviate by less than 1° from the ideal 90°. Our measurement technique is based on coherent scanning Fourier scatterometry, extended by a reference arm in Mach-Zehnder/Linnik configuration, a spatially-structured aperture stop in the object arm, and a self-referencing shearing element in front of the detector. We demonstrate the validity and advantages of our approach by presenting rigorous simulations of an exemplary silicon line grating with a grating period of 800 nm. Each grating line consists of a fine sub-grating with 40 nm pitch and 20 nm critical dimension. A variation of the major grating parameters height and critical dimension highlights the robustness of the method. Although our simulation study focuses on the determination of asymmetric SWAs, it should be noted that the presented technique features high sensitivity towards all kinds of structural asymmetries, such as floor tilt or asymmetric bottom roundings.