Nanoscale Metrology of Line Patterns on Semiconductor by Continuous Wave Terahertz Multispectral Reconstructive 3-D Imaging Overcoming the Abbe Diffraction Limit

Abstract

This paper demonstrates overcoming of the Abbe diffraction limit (ADL) on image resolution. Here, terahertz multispectral reconstructive imaging has been described and used for analyzing nanometer size metal lines fabricated on a silicon wafer. It has also been demonstrated that while overcoming the ADL is a required condition, it is not sufficient to achieve sub-nanometer image resolution with longer wavelengths. A nanoscanning technology has been developed that exploits the modified Beer-Lambert’s law for creating a measured reflectance data matrix and utilizes the “inverse distance to power equation” algorithm for achieving 3-D, sub-nanometer image resolution. The nano-lines images reported herein, were compared to scanning electron microscope (SEM) images. The terahertz images of 70 nm lines agreed well with the transmission electron microscope images. The 14 nm lines by SEM were determined to be ~15 nm. Thus, the wavelength dependent Abbe diffraction limit on image resolution has been overcome. Layer-by-layer analysis has been demonstrated where 3-D images are analyzed on any of the three orthogonal planes. Images of grains on the metal lines have also been analyzed. Unlike electron microscopes, where the samples must be in the vacuum chamber and must be thin enough for electron beam transparency, terahertz imaging is non-destructive, non-contact technique without laborious sample preparation.