High-Resolution EDS Mapping of Insulators with Zero Sample Preparation

Scanning Electron Microscope (SEM) imaging quality depends on the electrical conductivity of the material under investigation. Higher surface conductivity produces better electron images. SEM imaging and analytical Energy Dispersive X-ray Spectroscopy (EDS) investigation of low-surface conductivity materials is commonly required in various fields, including engineering, life sciences, materials science, forensics, etc. Low-vacuum SEM imaging mode is commonly used for such insulating materials to improve surface charge dissipation of insulating materials. The latter is sometimes avoided due to: (a) absence of low-vacuum imaging mode in certain SEMs, (b) technical limitations, (c) problems with beam distortion and skirting, and (d) higher noise in data along with lower X-ray fluorescence generation. Conversely, high-vacuum SEM imaging conditions are essential for sub-micron chemical mapping of insulating materials.

Cross-section of a cable specimen was investigated in this study. The SEM image (Fig. 1a) shows an inner core of multiple twisted copper (Cu) wires at the bottom left corner. An insulating layer covers the inner core Cu wires, providing electrical isolation from the second layer of Cu wires. The outermost sheath or jacket layer completely encapsulates the inner three layers, providing mechanical strength, chemical stability, and flame retardance.

Fig. 1b shows the chemical composition of the different layers. The Cu wires in the map are shown in magenta. The inner insulating polymeric layer mainly contains carbon (C) and oxygen (O), isolating the inner core Cu wires from the second layer of Cu wires. The insulating layers are non-conductive and appear brighter due to surface charge accumulation. The outermost layer, shown in blue, has a high concentration of chlorine (Cl), indicating that the material could be chlorosulfonated polyethylene, which is commonly used in cable sheaths. Sub-micron to few-micron-sized MgO particles in the sheath layer improve high-temperature and fire-resistance properties (fig. 2).

The presence of a single Aluminum (Al) wire in the second layer could serve multiple purposes, including cost reduction, signal differentiation, or acting as a sacrificial galvanic anode to monitor sheath integrity.

The annular four-segment design of the XFlash^® FlatQUAD EDS detector enables measurements in deep trenches and sides of vertical features. A thin layer of tin (Sn) coating on the vertical walls of copper wires was mapped with FlatQUAD. Sn coating improves electrical conductivity and provides long term chemical stability against sulfidation and oxidation of Cu.

To minimize strong surface charging of insulating layers, the SEM was operated at a low probe current of 250 pA and an accelerating voltage of 5 kV. The proximity of the XFlash^® FlatQUAD detector to the sample enables the detection of very weak X-ray signals emitted under gentle beam conditions, allowing high-quality EDS mapping at low probe doses.