Atomically Resolved Secondary Electron Imaging for Bulk Materials

The development of electron microscopes has revolutionized our ability to study materials at high resolution, providing us with new insights into the relationship between a material's structure and its properties. Among these techniques, the scanning electron microscope is widely used in both academic and industrial research and has played a major role in the development and analysis of new materials and devices. SEMs primarily reveal the surface topography of a material by imaging secondary electrons (SEs) that are generated within a shallow depth of the surface. In general, the resolution of these images is as high as a few nanometers, but it has been shown that atomic level visualization can be achieved [1-3] using a spherical aberration-corrected scanning transmission electron microscopy (STEM). Notably, these achievements have been accomplished with samples that were thin enough to be electron-transparent, which raises the question of whether atomic-resolution SE imaging can be achieved with thick, bulk samples. Traditional (S)TEM imaging necessitates specimen thinning to mitigate signal degradation from multiple scattering. Nonetheless, the interest in thick samples persists due to their closer representation of real material properties. In this study, we evaluated the feasibility of atomically resolving SE imaging for bulk samples. As a model system, a silicon specimen, shaped as an isosceles right-angle triangle with a maximum thickness of 18 μm, was fabricated from a (100) Si wafer substrate utilizing a focused ion beam in situ lift-out technique. The distinctive geometry of the specimen (Figure 1A) enables precise measurement of sample thickness along the beam trajectory, as it mirrors the lateral distance from the thin edge of the sample. SE images were obtained using the Hitachi HD2700C dedicated STEM equipped with a spherical aberration corrector, operating at 200 kV with a convergence semi-angle of 22 mrad. Figure 1B showcases a series of SE images obtained from different sample thicknesses, illustrating the atomic structure with discernible "dumbbells" (pairs of Si atoms positioned around 0.14 nm apart) in general. It is noteworthy that the overall intensity of the averaged image increases with the thickness of the sample. We employ a Gaussian function to quantify and compare the signal and background intensities based on the atomic positions within the SE image intensity profile (Figure 2A): where IPk, x0, w and IBkg are the amplitude, position, width (full width at half maximum) of the peak and background intensity, respectively. Figure 2B shows the peak intensity remains generally constant while the background intensity increases across varying sample thicknesses. Signal-to-background ratio (IPk/IBkg) slightly decreases with sample thickness (Figure 2C), primarily attributed to the increase of the background intensity with increasing thickness. This study showcases the ability to acquire SE images of atomic columns from a thick (bulk) sample using a 200 kV aberration-corrected STEM. These findings unveil a new avenue for investigating the atomic-scale structure of bulk materials without the necessity of thin samples [4]. (A) a scanning electron microscopy image of Si wedge sample acquired from a 52° angled direction. (B) SE images of Si from different thicknesses. Each image displayed here is an averaged results from 5x5 unit cells to improve the signal-to-noise ratio and remove scanning distortion. The upper and lower intensity limits were manually adjusted using the same window settings to facilitate a comparative analysis of background and signal variations corresponding to the sample thicknesses. (A) Intensity profile along the red line in the inset, presenting the averaged SE image at a sample thickness of 1 μm. Experimental data points are depicted as dots, with a red curve indicating the Gaussian-fitting results. ISi and Iv indicate image intensities at Si column and at the valley between Si dumbbells, respectively. (B) IPk (= ISi-Iv) and IBkg (≈ Iv) at different sample thicknesses. (C) Signal-to-background ratio as a function of sample thickness.

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