Improved evaluation of dose-dependent atom contrast based on image simulation for LVEM

October 12, 2014 - As part of the special issue "Low-Voltage Electron Microscopy" in Ultramicroscopy 2014, researchers from Ulm University have in the frame of the SALVE-project explored the dependence of signal-to-noise ratio, atom contrast, and specimen resolution on electron dose and sampling [1]. By using first experimental results of the C_S/C_C-corrected SALVE II microscope, image simulation under low-dose and infinite dose conditions is carried out, see for example Fig. 1, and the sampling for optimum image resolution is determined.

Introduction and parameters for image simulation

The instrumental resolution of transmission electron microscopes (TEMs) has dramatically improved over the last years, mainly due to the introduction and practical realization of hardware aberration correction [2 - 4]. Since low-voltage aberration corrected TEM is developed, the aim has mainly been to reduce radiation damage. Materials can be damaged via the knock-on damage mechanism, where atoms are displaced by direct impacts of the imaging electrons [5 - 8]. Knock-on can be reduced by decreasing the electron energy. The first low-voltage aberration-corrected TEMs used an accelerating voltage of 80 kV. Currently TEMs with voltages down to 20 kV [9], 30 kV [10] and 40 kV [11] are under development.

Especially for lower accelerating voltages, fully corrected transmission electron microscopy is very important, because spherical correction alone does not provide sufficient information transfer to unambiguously resolve the atomic positions in graphene [12], and other sensitve materials. Fully-corrected TEM means corrected for higher-order geometrical aberrations, as well as chromatic aberrations of the imaging lenses [13]. Another advantage of a C_C-corrected microscope is that contrast delocalization caused by inelastic scattering in a single layer of graphene at voltages as low as 20 kV is negligible. In this case, the influence of inelastic scattering on the image is mainly the decrease of the intensity contributed by the elastic scattered electrons [14].

While knock-on damage decreases, the electron–electron (inelastic) scattering cross section increases at lower electron energies and, depending on the material, ionization can become the dominating damage mechanism [8]. An effective way of reducing ionization damage may, e. g., be conductive coating [15]. As extreme examples of the latter, samples have been enclosed within carbon nanotubes [16], or between graphene layers [17, 18], greatly reducing radiation damage during imaging. Such approaches are not always feasible, however, and to reduce ionization images need to be acquired with limited electron doses.

Additionally, the stability of the microscope is another factor limiting the electron dose in a single image. The microscope tends to drift away from the corrected state, and as a result images can be acquired only within a small time window before resolution is deteriorated [19]. Also all kinds of instabilities including electrostatic and magnetic field noise [20] and instabilities caused by the sample stage can lead to blurring of the images, if long exposure times are used. The combined effect of these disturbances on the image contrast must be taken into account for the image simulation and is given in Ref. 13.

Improvement of image simulation and interpretation

In order to study the dependence of signal-to-noise ratio, atom contrast and specimen resolution on electron dose and sampling, Lee et al. 2014 [1] have simulated images of graphene obtained with the C_S/C_C-corrected SALVE II microscope operated at an accelerating voltage of 80 kV (Fig. 2). For the first time, the noise for low dose conditions is treated as stochastic fluctuations around the ideal electron count at each image pixel, instead of the previously used additive noise, e.g. Ref. 21. Also introduced for the first time is a modified definition for the image contrast, which takes the electron dose into account. A former formulation by Rose [7] is adapted to include the improved dose-dependent contrast in the calculation of the specimen resolution. Subsequently, the authors have determined the optimum sampling for 20 kV, 40 kV, 60 kV and 80 kV.

The image simulations are based on the experimental data of all necessary parameters of the SALVE II microscope [1]. As a result of this study, it could be shown that large signal-to-noise ratio, high contrast and atomic resolution can be achieved under high-dose conditions. However, it is still possible to obtain fine resolution and good image contrast under low-dose conditions (~5x10⁶ e^-/nm²), as long as the optimum sampling is employed [18].