Wafer-scale nondestructive metrology on subwavelength diffraction gratings by means of Wood's anomaly

Abstract

Anomalistic behavior in diffraction responses of grating can be easily detected and can indirectly provide information about the grating parameters such as the grating period, height, duty-cycle and profile. More precisely, the absorption resonance (Wood's anomaly) which arises from the excitation of a surface plasmon polariton (SPP) in reflective sub-wavelength diffraction gratings are of interest as well as Rayleigh's anomaly which takes the form of a discontinuity in the diffraction response and which is the consequence of the excitation of a new propagating mode. In this paper we describe how these anomalies can be used as a non-destructive metrology tool to estimate the grating parameters by an IR spectral scatterometry measurement. We briefly describe the theoretical conditions for which SPP are excited. We investigate the wavelength sensitivity of Wood's anomaly in the zeroth order diffraction response to individual grating parameter variations at CO2 laser wavelengths. A numerical electromagnetic grating solver software package "Gsolver" was used for the theoretical modeling. We show that this non-destructive IR spectral scatterometry measurement based on feature extraction allows us to measure grating parameter variations with nanometer resolution. The measurement time needed to scan a 4" wafer has been shown to be of the order of a few minutes. This is much faster as compared to traditional techniques as (deconstructive) SEM inspection or white light interferometry. Furthermore, the extension of this technique to larger wafers does not impose any difficulties.