In Situ AFM-SEM-EDS, a Correlative Microscopy Platform for Surface Characterization and Elemental Analysis

Here, we present a highly integrated correlative microscopy platform (see Fig. 1) which incorporates complementary techniques such as Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray Spectroscopy (EDS/EDX). In combination with a unified coordinate system, each of these techniques reveals different properties/characteristics of the sample from the same region of interest easily, in contrast to the challenges of ex-situ correlative techniques [1]. This would be very advantageous in fields seeking correlative information, such as the life sciences [2, 3], semiconductor industry [4, 5], earth sciences [6], and pharmaceutical industry [7]. And until now, to our knowledge, no single platform has offered a turnkey in-situ correlative solution of AFM-SEM-EDS with a unified coordinate system for a seamless correlation experience. Recently, we have addressed the challenges of AFM and SEM correlation with the introduction of FusionScope, a highly integrated microscopy platform incorporating a state-of -the-art AFM based on self-sensing cantilever technology and an SEM with a Schottky electron source [8, 9]. SEM provides high-resolution lateral information but lacks the ability to provide high-resolution information of real topographical surface features [10]. Scanning probe techniques, on the other hand, can provide atomic resolution of topography, as well as magnetic, electrostatic, and conductive information. In FusionScope, the surface topography is mapped both laterally and vertically thanks to SEM and AFM, while the unified coordinates sustain the correlation accuracy (see Fig. 2), eliminating a major shortcoming of ex-situ correlation approaches. We have now integrated an EDS system into the FusionScope platform. The addition of EDS brings new functionality, because – although some modes of SEM (e.g. backscattered electron images) and AFM (e.g. phase mode, see Fig. 3) can be used to deduce compositional differences in material – neither technique is able to give chemical sample information. EDS, in contrast, can identify and localize elements in high-resolution [11]. Integration of an EDS system into FusionScope allows the platform to correlate elemental maps with high-resolution scanning electron and scanning probe microscope micrographs. The EDS system has an X-ray detector based on a silicon drift detector (SDD), that provides high energy resolution for recording single spectra and elemental maps [12]. Detecting light elements (down to C) is possible thanks to the thin graphene window (1 µm +/-150 nm). An electron trap with optimized geometry maximizes the X-ray collection (solid angle) and minimizes the stray magnetic fields. Integrated detector electronics and the SEM scan boards also allow X-ray maps, see Fig. 4. The NIST DTSA-II database [13] and a background subtraction algorithm have been incorporated for the elemental analysis. The unified coordinate system allows the correlation of high-resolution AFM data with X-ray data. Thanks to the recent integration of the EDS system, FusionScope can now perform in-situ AFM-SEM-EDS measurements, where the elemental composition can be identified in the region of interest while a high-resolution topographic image (in 3D) is created. This offers meaningful potential for users to overcome challenges of ex-situ correlation, drastically minimizing time and errors while providing high-resolution information in multiple modalities. Schematic of FusionScope™ chamber, combining AFM-SEM-EDS with a Trunnion Stage capable of up to 80° tilt. The Profile View provides a unique angle to inspect the AFM tip and its interaction. SEM-AFM correlative image of a test sample made of brass. SEM measurement shows the high-resolution view of the surface in the lateral dimension, and AFM maps the height variation of the surface in high resolution. SE image (-a) of the region of interest at 7.5 keV with 2000 x 2000 pixels. Image-b is the correlated AFM-SEM image with added transparency to the AFM image. Image-c is the correlated AFM-SEM image with zero transparency. See Fig. 3 for the details of the AFM image. AFM measurements of the region of interest. Image-b is the topography measurement of the 20 µm x 20 um region in dynamic mode with 1000x1000 pixels; the color values correspond to the height of the surface. Image-b is the phase image of the same measurement; the colorbar represents the phase of the cantilever in dynamic mode. The SEM-EDS technique reveals the elemental composition of the region of interest. The images -a, -b, and -c are individual elemental maps for Cu, Zn, and Fe, respectively. Image-d is SE measurement at 15 keV of this region; the image-e is the superimposed map of three elements with added transparency. The images are 200 x 200 pixels. The field of view is 50 µm. Image-f is the correlative image of AFM scan (Fig. 3a) and the EDS-Fe map (Fig. 4c).