With the ever increasing demand for higher transistor density and improved functionality, the nanostructures on modern semiconductor chips become more and more complex and their modeling requires a multitude of parameters. As a result, the performance of scatterometry as state-of-the-art optical inspection tool is limited by insufficient sensitivities towards certain parameters and high cross-correlations between them. In order to improve the model-based reconstruction, it is essential to generate as many uncorrelated datasets as possible. In this paper, we propose to combine conventional Fourier scatterometry with Mueller polarimetry and white-light interferometry to measure both angle- and wavelength-resolved Mueller matrices. This approach takes advantage simultaneously of the most relevant information channels of the light field: intensity, wavelength, phase, propagation angle, and polarization. We validate the performance improvement in case of multi-parameter problems by means of a comprehensive simulation study. In general, both the measurement uncertainties and the cross-correlations are reduced in comparison to other scatterometric configurations. Furthermore, our approach facilitatesthe reconstruction of target asymmetries, such as asymmetric sidewall angles, or the analysis of isolated line gratings at low technology nodes. Aiming at an experimental validation as well, we finally show results from first proof-of-principle measurements performed during the ongoing setup implementation.
The characterization of nanoscale grating asymmetries is an indispensable prerequisite for improving the accuracy of process control in modern semiconductor lithography. Model-based scatterometry is the state-of-the-art optical waferinspection technique. We suggest to extend it by coherent phase-structured illumination. The resulting intensity and phase profiles are evaluated in the far-field image plane. By means of rigorous simulations, it has already been demonstrated that the use of phase-structured illumination increases the sensitivity towards any kind of grating asymmetry including asymmetric sidewall angles (SWAs), floor tilts, asymmetric top and bottom roundings, and even overlay errors. Furthermore, it is possible to determine both the absolute value and the sign (magnitude and direction) of an asymmetry. In this paper, we will recapitulate the evaluation strategy and summarize the most important simulation results. Subsequently, we will present first proof-of-principle measurements obtained by digital off-axis holography. A direct comparison between simulation and measurement demonstrates the validity of the suggested approach.
Phase-structured illumination is investigated as a possible extension of scatterometric measurement methods for silicon line gratings. This is done by means of rigorous simulations. Special emphasis is put on the capability of this approach to detect nanoscale fabrication asymmetries such as sidewall angles, bottom rounding, and floor tilt. The studied setup features a focused spot (numerical aperture=0.7), i.e., scanned over the sample, while analyzing the phase distribution in the image plane. This phase distribution can be accessed via holographic imaging. The results are compared to conventional nonstructured illumination. It is shown that by employing phase structuring, the resulting phase changes are larger, even if only symmetric deviations are considered. For asymmetric deviations, phase-structured illumination provides much higher sensitivity and better capability to detect the sign of the asymmetry.
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.
We propose a measurement technique which enables the precise determination of side wall angles (SWAs) with absolute values below 1°. Our simulations show that a differentiation between asymmetric SWAs is also possible. The grating structure under investigation has a grating period on the order of a few micrometers. Each grating line consists of a fine sub-grating with 40 nm period and 20 nm critical dimension. Our approach is based on coherent high-NA Fourier scatterometry, extended by a lateral scan over the sample. Additionally, a 180°-shearing element allows for coherent superposition of the higher diffraction orders.
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