Model sensitivity analysis of Monte-Carlo based SEM simulations

The sensitivity of simulated scanning electron microscopy (SEM) images to the various physical model ingredients is studied using an accurate, but slow simulator, to identify the most important ingredients to include in a reliable and fast SEM image simulator. The quantum mechanical transmission probability (QT) model and the electron-acoustical phonon scattering model are found to have the most significant effect on simulated 2D and 3D metrology results. The linewidth measurement error caused by not including these models in the simulation is less than 2 nm. Specifically, it was found from a comparison to experimental data that the QT model is essential in accurately predicting particular signal features in linescans such as “shadowing”. The simulator is compared with two other publicly available simulators, JMONSEL and CASINO, where the first one is also based on first-principle physics models and the latter one is using phenomenological models. CASINO is the fastest simulator on CPU, but Nebula on GPU is two orders of magnitude faster compared to a single threaded CPU simulation. Only up to 6% speed increase has been achieved by different model choices.ImPhys/Microscopy Instrumentation & Technique

Optimization of focused ion beam performance

The authors have analyzed how much current can be obtained in the probe of an optimized two-lens focused ion beam (FIB) system. This becomes relevant, as systems become available that have the potential to image and/or fabricate structures smaller than 10 nm. The probe current versus probe size curves were calculated for a commercial gallium-FIB, the nano-FIB system, and the helium microscope, using partly published, partly estimated system parameters. The current in sub-10 nm probes in the Ga systems turns out to be limited by the reduced brightness of the source and the chromatic aberration of the objective lens. In probes larger than 40 nm the current is limited by the angular current density and the spherical aberration of both lenses. The He system is limited at all probe sizes by the angular current density of the source and the chromatic aberration of both lenses in sub-5 nm probes and the spherical aberration of both lenses at probes larger than 10 nm. As the emission current of the He source is much smaller than that of the Ga source, the statistical Coulomb interactions in the gun lens region do not contribute to the total probe size, as is the case for the Ga systems.IST/Imaging Science and TechnologyApplied Science

Nebula: Monte Carlo simulator of electron–matter interaction

Monte Carlo simulations are frequently used to describe electron–matter interaction in the 0–50 keV energy range. It often takes hours to simulate electron microscope images using first-principle physical models. In an attempt to maintain a reasonable speed, empirical models are sometimes used. We present an open-source software package with first-principle physical models, which can run on GPUs for fast results. Typical electron microscope images can be obtained in minutes instead of hours on consumer-grade hardware, without any loss of accuracy.ImPhys/Microscopy Instrumentation & Technique

Model improvements to simulate charging in SEM

Charging of insulators is a complex phenomenon to simulate since the accuracy of the simulations is very sensitive to the interaction of electrons with matter and electric fields. In this study, we report model improvements for a previously developed Monte-Carlo simulator to more accurately simulate samples that charge. The improvements include both modelling of low energy electron scattering and charging of insulators. The new first-principle scattering models provide a more realistic charge distribution cloud in the material, and a better match between non-charging simulations and experimental results. Improvements on charging models mainly focus on redistribution of the charge carriers in the material with an induced conductivity (EBIC) and a breakdown model, leading to a smoother distribution of the charges. Combined with a more accurate tracing of low energy electrons in the electric field, we managed to reproduce the dynamically changing charging contrast due to an induced positive surface potential.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.ImPhys/Charged Particle Optic

Micro x-ray source

The invention relates 'to a micro X-ray source comprising a target acting as anode, and a cathode, which during operation interacts with the target and functions as electron source, wherein the target is embodied as a metal foil possessing a spot where the electrons from the electron source arrive, the metal foil being locally thinner at the spot.Applied Science

Model improvements to simulate charging in scanning electron microscope

Background: Charging of insulators is a complex phenomenon to simulate since the accuracy of the simulations is very sensitive to the interaction of electrons with matter and electric fields. Aim: In this study, we report model improvements for a previously developed Monte-Carlo simulator to more accurately simulate samples that charge. Approach: The improvements include both modeling of low energy electron scattering by first-principle approaches and charging of insulators by the redistribution of the charge carriers in the material with an electron beam-induced conductivity and a dielectric breakdown model. Results: The first-principle scattering models provide a more realistic charge distribution cloud in the material and a better match between noncharging simulations and experimental results. The improvements on the charging models, which mainly focus on the redistribution of the charge carriers, lead to a smoother distribution of the charges and better experimental agreement of charging simulations. Conclusions: Combined with a more accurate tracing of low energy electrons in the electric field, we managed to reproduce the dynamically changing charging contrast due to an induced positive surface potential.ImPhys/Charged Particle Optic

Brightness measurements of a gallium liquid metal ion source

The virtual source size of a liquid metal ion source is an order of magnitude larger than the size of the region from which the ions are emitted at the source. This source size has a direct effect on the reduced brightness and, hence, on the performance of these sources. The variation of the virtual source size of a gallium liquid metal ion source as a function of the angular current density at the source has been measured. This was done by measuring the source image size from images of a pencil lead sample taken with an FEI focused ion beam system. The measurements indicate that the virtual source size grows from about 50–80?nm when the emission current increases from 1?to?10??A. The experimental data on the virtual source size are compared with the theory on stochastic Coulomb interactions in the source region. On the basis of these measurements the authors show that the reduced brightness deteriorates with an increasing angular current density. The maximum reduced brightness measured was 1×106?A/(m2?sr?V).IST/Imaging Science and TechnologyApplied Science

Determination of line edge roughness in low-dose top-down scanning electron microscopy images

We investigated the off-line metrology for line edge roughness (LER) determination by using the discrete power spectral density (PSD). The study specifically addresses low-dose scanning electron microscopy (SEM) images in order to reduce the acquisition time and the risk of resist shrinkage. The first attempts are based on optimized elliptic filtering of noisy experimental SEM images, where we use threshold-based peak detection to determine the edge displacements. The effect of transversal and longitudinal filterings cannot be ignored, even when considering an optimized filter strength. We subsequently developed a method to detect the edge displacements without the use of a filter and thus avoiding biasing. This makes it possible to study how much image noise is acceptable and still determine the LER. The idea is to generate random images of line edges using the model of Palasantzas and the algorithm of Thorsos. We study the simulated PSDs as a function of the number of line edges and report on the convergence of the parameters (LER, correlation length, and roughness exponent) by fitting the Palasantzas model extended with a white noise term. This study demonstrates that a very noisy image with 12 line edges and about 2 electrons per pixel on average (charge density ?10???C) already produces an estimation for LER with a relative error (one-sigma) of about 10%. Furthermore, increasing the dose beyond 20 electrons per pixel does not significantly improve the LER determination.Imaging PhysicsApplied Science

Chemistry for electron-induced nanofabrication

ImPhys/Charged Particle Optic

Multibeam scanning electron microscope: Experimental results

The authors present the first results obtained with their multibeam scanning electron microscope. For the first time, they were able to image 196 (array of 14×14) focused beams of a multielectron beam source on a specimen using single beam scanning electron microscope (SEM) optics. The system consists of an FEI Novanano 200 SEM optics column equipped with a multielectron beam source module. The source module consists of the multibeam source and an accelerator lens. In the multibeam source, the wide angle beam of a high brightness Schottky source is divided into 196 beamlets and focused by an aperture lens array. The accelerator lens is positioned on the image plane of the multibeam source to direct the beams toward the SEM column. The array of source images is further imaged by the SEM magnetic lenses, and the beam opening angle is defined at the variable aperture of the SEM. The system is designed to deliver 14×14 arrays of beamlets with a minimum probe size of 1 nm. In this article, the performance of the system is examined for a fixed magnification case.IST/Imaging Science and TechnologyApplied Science