Mapping Conductivity in the TEM with SEEBIC

The TEM is a central tool for studying nanoscale features in electronic devices. TEM’s scattering-based contrast excels at determining the physical structure of materials, and, with spectroscopic attachments like EDS and EELS, can precisely determine the composition and arrangement of atoms in devices. Combined with in situ capabilities, the TEM can precisely map physical structure changes in devices during operation and defect formation. However, in many situations the function or failure of devices is the result of small-scale electronic changes that do not present as detectable physics signals until changes become pathological. In order to detect these electronic changes in the TEM, complementary imaging must be employed with contrast that is directly related to electronic structure. One technique for obtaining electronic contrast in the TEM is electron beam induced current (EBIC) imaging, wherein current produced in the sample by the beam is mapped pixel-by-pixel in STEM. The “standard” mode of EBIC current generation, in use since the 1960’s [1], is the separation of electron-hole pairs (EHPs) in local electric fields. Recently, a new mode of EBIC was demonstrated in which current is produced by holes generated in the sample by beam-induced secondary electron (SE) emission [2]. This SE emission EBIC (SEEBIC) mode does not require the presence of a local electric field, is typically a much smaller current than standard EBIC, and is capable of much higher resolution imaging [3]. Unique among TEM-based techniques, SEEBIC is also capable of producing contrast directly related to local conductivity in samples [4], even in operating devices [5]. Here we discuss the STEM EBIC conductivity mapping technique and provide several examples of its use in both passive imaging and in situ experimentation. A simple demonstration of SEEBIC resistance mapping is shown in Fig. 1. The device consists of a GeSbTe (GST) strip spanning two TiN electrodes patterned on a thin SiN membrane. The STEM EBIC image in Fig. 1 contains both standard EBIC and SEEBIC contrast. As indicated in the diagram, when the beam is incident on the TiN/GST interfaces, the electric field at the Schottky barriers separate EHPs, with holes traveling towards the GST at each interface producing dark contrast on the right electrode, connected to the EBIC amplifier, and dark contrast on the left, which is grounded. Outside of these interfaces, SEEBIC contrast is proportional to the resistance to the left (grounded) electrode [4]. Holes produced by SE emission closer to the EBIC electrode (i.e. less resistance to the EBIC electrode than the grounded electrode) are more likely to reach ground via that electrode, producing a brighter (hole) current. SEEBIC is brightest on the right (EBIC) electrode, decreases steadily across the GST strip due to the uniform resistivity of the amorphous GST, and is darkest on the left electrode [6]. Annular Dark-Field (ADF) STEM and STEM EBIC images of a TiN/GST/TiN device, along with a diagram of the EBIC contrast. EBIC is measured on the right electrode with the left grounded. As indicated in the diagram, each TiN/GST interface has standard, EHP separation EBIC. The contrast outside of the interfaces is due to SEEBIC and is proportional to the resistance at a given pixel to the grounded left electrode.