Monte Carlo softwares are widely used to understand the capabilities of electron microscopes. To study more realistic applications with complex samples, 3D Monte Carlo softwares are needed. In this paper, the development of the 3D version of CASINO is presented. The software feature a graphical user interface, an efficient (in relation to simulation time and memory use) 3D simulation model, accurate physic models for electron microscopy applications, and it is available freely to the scientific community at this website: www.gel.usherbrooke.ca/casino/index.html. It can be used to model backscattered, secondary, and transmitted electron signals as well as absorbed energy. The software features like scan points and shot noise allow the simulation and study of realistic experimental conditions. This software has an improved energy range for scanning electron microscopy and scanning transmission electron microscopy applications.

Scanning transmission electron microscopy (STEM) was used to image gold nanoparticles on top of and below saline water layers of several micrometers thickness. The smallest gold nanoparticles studied had diameters of 1.4 nm and were visible for a liquid thickness of up to 3.3 µm. The imaging of gold nanoparticles below several micrometers of liquid was limited by broadening of the electron probe caused by scattering of the electron beam in the liquid. The experimental data corresponded to analytical models of the resolution and of the electron probe broadening, as function of the liquid thickness. The results were also compared with Monte Carlo simulations of the STEM imaging on modeled specimens of similar geometry and composition as used for the experiments. Applications of STEM imaging in liquid can be found in cell biology, e.g., to study tagged proteins in whole eukaryotic cells in liquid, and in materials science to study the interaction of solid:liquid interfaces at the nanoscale.

We present a three-dimensional (3D) version of the CASINO Monte Carlo software; the current 2D version is widely used in the microscopy community. CASINO is used for the simulation of images and linescans of electron beam instruments. The software has an easy-to-use graphical user interface (GUI). The creation of the sample, setting of the simulation parameters and the viewing of the results are done through this GUI. The software now implements a full 3D sample, allowing users to create realistic geometries for their simulations. Other new features of the software include models for: 1) fast secondary and secondary electrons, 2) annular dark field scanning transmission electron microscopy (ADF STEM) [1], 3) absorbed energy, and 4) elastic cross sections based on the software ELSEPA [2] allowing modeling of the electron scattering in the range up to 500 keV. CASINO is available in 32 and 64-bit version (the latter allowing larger simulations) and uses multi-CPU and multi-core hardware to reduce simulation time [3]. We will present the features of CASINO and examples of its applications.The electron trajectory calculation is based on to the previous version of CASINO [4]. The fast secondary electrons (FSE) are generated using the Möller equation [5] while the slow secondary electrons (SE) are generated from the plasmon theory [6]. Fig. 1 shows backscattered electrons (BSE) and SE images generated with CASINO of tin balls on a carbon substrate sample. These images are used to understand the impact of microscope parameters on image resolution. The difference in contrast between the BSE and SE signal for 1 and 10 keV incident energy is analyzed from these simulations. The largest contrast (2.3) is obtained with the SE signal at 1 keV and is four time larger than the contrast obtained with BSE signal for the same energy.The 3D version of the Monte Carlo software CASINO includes features to analyze the absorbed energy within the sample. These features are the simulation of complex beam scanning pattern and the calculation of the absorbed energy inside a 3D matrix unit volume. Absorbed energy modeling can assist the user in the determination of the exposure parameters and resist thickness when fabricating nanometer-scale semiconductor devices using electron beam lithography (EBL) technique. Fig. 2 shows an example of the impact of incident energy on PMMA resist lines by EBL. Fig. 2B and 2C show a cross section view of the energy summed over 300 nm along the line axis in the PMMA layer. The side view at 3 keV shows that the absorbed energy between the lines is more important at the bottom due to the larger interaction volume. From the first line at the left, we observed that the absorbed energy can occur as far as 50 nm (at the resist/SiO 2 interface) away from the line pattern at 3 keV. At 20 keV, no absorption was observed outside the line pattern, except for a barely visible enlargement at the bottom, which should not cause any problem during the resist development step. From this example, it is clear that such low e...

Scanning transmission electron microscope (STEM) images of three-dimensional (3D) samples were simulated. The samples consisted of a micrometer(s)-thick substrate, and gold nanoparticles at various vertical positions. The atomic number (Z) contrast as obtained via the annular dark field detector was generated. The simulations were carried out using the Monte Carlo metihod in the Casino software (freeware). The software was adapted to include the STEM imaging modality, including the noise characteristics of the electron source, the conical shape of the beam, and 3D scanning. Simulated STEM images of nanoparticles on a carbon substrate revealed the influence of the electron dose on the visibility of the nanoparticles. The 3D datasets obtained by simulating focal-series showed the effect of beam broadening on the spatial resolution, and on the signal-to-noise-ratio. Monte Carlo simulations of STEM imaging of nanoparticles on a thick water layer were compared with experimental data by programming the exact sample geometry. The simulated image corresponded to the experimental image, and the signal-to-noise levels were similar. The Monte Carlo simulation strategy described here can be used to calculate STEM images of objects of an arbitrary geometry and amorphous sample composition. This information can then be used, for example, to optimize the microscope settings for imaging sessions where a low electron dose is crucial, for the design of equipment, or for the analysis of the composition of a certain specimen.

The Monte Carlo software CASINO has been expanded with new modules for the simulation of complex beam scanning patterns, for the simulation of cathodoluminescence (CL), and for the calculation of electron energy deposition in subregions of a three-dimensional (3D) volume. Two examples are presented of the application of these new capabilities of CASINO. First, the CL emission near threading dislocations in gallium nitride (GaN) was modeled. The CL emission simulation of threading dislocations in GaN demonstrated that a better signal-to-noise ratio was obtained with lower incident electron energy than with higher energy. Second, the capability to simulate the distribution of the deposited energy in 3D was used to determine exposure parameters for polymethylmethacrylate resist using electron-beam lithography (EBL). The energy deposition dose in the resist was compared for two different multibeam EBL schemes by changing the incident electron energy.