STEM Imaging with Beam-Induced Hole and Secondary Electron Currents

Abstract

In standard electron beam-induced current (EBIC) imaging, the scanning electron beam creates electron-hole pairs that are separated by an in-sample electric field, producing a current in the sample. In standard scanning electron microscopy (SEM), the scanning electron beam ejects secondary electrons (SE) that are detected away from the sample. While a beam electron in a scanning transmission electron microscope (STEM) can produce many electron-hole pairs, the yield of SE is only a few percent for beam energies in the range 60-300 keV, making the latter signal much more difficult to detect on-sample as an EBIC. Here we show that the on-sample EBIC in a STEM registers both SE emission and capture as holes and electrons, respectively. Detecting both charge carriers produces differential image contrast not accessible with standard, off-sample SE imaging. In a double-EBIC imaging configuration incorporating two current amplifiers, both charge carriers can even be captured simultaneously. Compared to the currents produced in standard EBIC imaging, which only highlights the regions in a sample that contain electric fields, the EBIC produced by SE, or SEEBIC, are small (pA-scale). But SEEBIC imaging can produce contrast anywhere in a sample, exposing the texture of buried interfaces, connectivity, and other electronic properties of interest in nanoelectronic devices, even in metals and other structures without internal electric fields.

Adjusting the STEM Sample Holder Potential for Improved EBIC Contrast

“…Of primary electrons with energies typical for TEM (80-300 keV), only a few percent yield secondaries [1]. Usually an SE detector, such as an ETD [3][4][5], is used to detect a portion the SEs emitted from the top (i.e.…”

“…Adjusting potentials near the sample can significantly change the SE signal contrast [1,4,5]; SEs have only a few eV of kinetic energy and are therefore strongly influenced by local electric and magnetic fields. For example, SEs emitted into an electric field directed away from the emission point can be recaptured, thereby diminishing the detected SEEBIC.…”

Secondary-Electron Electron-Beam-Induced Current Measurements at Lattice Resolution

“…Aberration-corrected SE imaging has been previously demonstrated using an off-sample detector [2]. Aside from the aforementioned advantages of SEEBIC over remote SE detection, SEEBIC has the additional advantage of providing differential contrast depending on where current is measured [1]. While imaging a device, for example, SEEBIC can be collected from individual electrodes, providing information about electronic structure, such as conductance changes, which are invisible to both STEM and SE imaging.…”

Mapping Electronic State Changes with STEM EBIC

“…In nanoelectronic devices, conductivity is a determiner of device function. To investigate such functioncritical properties with the high-resolution of TEM, we are developing scanning TEM electron beam induced current (STEM EBIC) imaging [1,2]. In STEM EBIC imaging, as the focused electron beam is rastered over the sample, the current induced in the sample is measured and associated with the beam position to produce an image.…”

“…In the low resistance state (LRS) at 1.8V, finite lead resistance and sizable bias currents combine to cause much of the potential drop to occur outside the field of view. Linear combinations of maps [2] acquired with opposite bias polarities separate, for example, the signals due to internal device fields from those due to conductivity or temperature changes, revealing further details relevant to actual device function that cannot be detected with standard TEM [5]. Matrix of STEM images acquired under various device bias conditions.…”