STEM Imaging with Beam-Induced Hole and Secondary Electron Currents

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 (SEs), which 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 SEs 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 SE 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 with the current produced in standard EBIC imaging, which highlights only the regions in a sample that contain electric fields, the EBIC produced by SE emission, or SEEBIC, is small (picoampere 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.

Figure 1

STEM SEEBIC imaging of a metal electrode. (top) A cartoon representation of the basic setup shows the electron beam rastering over a bare $\mathrm{Al}$ electrode. Scattered electrons produce the usual ADF image (middle). SEs ejected from the electrode leave holes behind, producing a positive current and corresponding bright contrast in the EBIC image (bottom). The imaged electrode is maintained at virtual ground by the TIA.

Figure 2

Conventional STEM and double STEM SEEBIC imaging of $\mathrm{Ti}$/$\mathrm{Pt}$ electrodes. The simultaneously acquired (a) BF, (b) ADF, and (c),(d) STEM EBIC images show $\mathrm{Ti}$(5 nm)/$\mathrm{Pt}$(25 nm) electrodes on a ${\mathrm{Si}}_3{\mathrm{N}}_4$ membrane. TIAs are connected to the electrodes as indicated by the blue and red symbols in the STEM EBIC images, with the symbol shown as opaque in the image generated by that TIA. Bright (dark) contrast corresponds to positive (negative) current, as indicated by the current scale for (c) and (d) that is to the left of (c). The gray, blue, and red plots in (e) show line profiles from (b)–(d) taken in the region indicated by the gray rectangle in (b). The gray plot is in arbitrary units and the blue and red plots use the scale on the $y$ axis. The images in (a)–(d) are rotated ${90}^{\circ }$ counterclockwise, so that the fast scan direction is bottom to top. EBIC values in (c),(d) are given relative to their respective averages from the region outlined in green in (c).

Figure 3

EBIC vs beam current and SE yield vs inverse beam energy. In the plot (a), the EBIC current from $\mathrm{Ti}$(5 nm)/$\mathrm{Pt}$(25 nm) and 100-nm $\mathrm{Al}$ electrodes is shown (with fits to $I_{\mathrm{SE}}=\delta \times I_B$) for five different beam currents $I_B$ at each accelerating voltage of 80, 200, and 300 kV. The EBIC uncertainty is taken to be 0.1 pA. In the plot (b), the SE yield $\delta$ for each material, determined from the six fits in the plot (a), is linear in $E_B^{-1}$, as predicted by Eq. (2).

Figure 4

ADF and EBIC images of $\mathrm{Ti}$/$\mathrm{Pt}$ and $\mathrm{Al}$ electrodes and average SEEBIC from each material vs electrode bias. The images show four different material combinations: the bare ${\mathrm{Si}}_3{\mathrm{N}}_4$ membrane, $\mathrm{Ti}$(5 nm)/$\mathrm{Pt}$(25 nm), 100-nm $\mathrm{Al}$, and $\mathrm{Ti}$(5 nm)/$\mathrm{Pt}$(25 nm)/$\mathrm{Al}$(100 nm). The plot shows the average signal in the SEEBIC image measured on $\mathrm{Al}$ (blue box) and $\mathrm{Ti}$/$\mathrm{Pt}$ (red box), with the average current measured on ${\mathrm{Si}}_3{\mathrm{N}}_4$ (green box) subtracted, for each bias value. The solid lines are fits to the SE-emission-current equation [Eq. (2)] for $V>0$, with the work functions and zero-bias currents as free parameters.

Figure 5

Conventional STEM and STEM SEEBIC images of metallic grains in a $\mathrm{Ti}$/$\mathrm{Pt}$ film. Simultaneously acquired BF (a), ADF (b), and EBIC (c) images of a $\mathrm{Ti}$(5 nm)/$\mathrm{Pt}$(25 nm) electrode on a ${\mathrm{Si}}_3{\mathrm{N}}_4$ membrane.