One essential precondition for a successful characterization of material by using electron microscopy is a sufficient electronic conductivity. Insulating or non sufficient conductive specimens becomes negatively or positively charged beside the balance points E_1,2,3 where the same amount of backscattered and secondary electrons leave the sample than primary electrons remain in the sample [1]. This charging effects are additionally an important factor for microscopy accessories like phase, monochromator plates etc [2]. We have implemented a modified transmission unit [3,4] in a SEM/FIB NVISION 40 SEM, which allows to measure the adsorbed and transmitted current simultaneously with the BioLogic SP-200 Potentiostat and the specimen current monitor of the SEM respectively. Four different free supporting films were investigated: formvar (C₅H₈O₂), leaf gold, amorphous carbon and amorphous PCS (Pd_77.5Cu_6.0Si_16.5) alloy. Fig. 1 shows the studied formvar film at a) 3 kV and b) 20 kV without and with charging effects. The measurement of the transmitted current delivers the electron range and the application of the formula of Fitting [5], the film thickness of the investigated samples. Fig 1 c) show the normalized transmitted currents I_t/I_p (I_t= transmitted current, I_p =probe current) in dependence on the accelerating voltage for two objective aperture sizes of 30 and 120 µm. The electron range is given as the point where the first time transmitted electrons can be detected. The results show the expected independence of the electron range on the aperture size and therefore on the beam current, except for formvar where the differences are caused by charging effects. The obtained thicknesses for gold (157±16 nm) and , Formvar (209±21nm ) agree well with Monte Carlo simulations and those obtained for  amorphous carbon (56±6 nm ) and   amorphous PCS (20±2 nm ) with the expected film thicknesses of 50 nm and 20 nm respectively [2]. With the knowledge of sample thickness, scan speed and adsorbed current, the implemented charge per volume N can be calculated. Fig 1 d) present N in dependence on the energy of the incident electrons exemplary for formvar and gold with the 30 µm aperture. The obtained E₃ points are 2,0±0,3 keV and 4,3±0.3 keV. In a second step the adsorbed and transmitted current was measured with and without an additional potential of 50 eV on the sample for two different working distances. With the experimentally obtained currents,this enables to calculate the secondary electron yield δ and the backscattered coefficient η in dependence on the energy of the incident electrons. The results are shown for the amorphous carbon film in Fig.1 e) and f).

[1] Reimer L., Golla U., Böngeler R., Kässens M., Schindler B., Senkel R. Charging of bulk specimens, insulating layers and free-supporting films in scanning electron microscopy. (1992) Optik 92 14-22

[2] Dries, M., Janzen, R., Schulze, T.,  Schundelmeier, J.,  Hettler, S., Golla-Schindler,U.,  Jaud, B.,  Kaiser, U., Gerthsen, D.: The Role of Secondary Electron Emission in the Charging of Thin-Film Phase Plates (2016) Conference Proceeding Microscopy and Microanalysis Ohio; submitted

 [3] Inventor of the Patent of Carl Zeiss SMT GmbH, Entwicklung und Prototypenbau eines STEM Detektors für ein Rasterelektronenmikroskop: Patentnr.: DE       10211977, EP 1347490,US 2003/0230713 A1

[4] Golla U., Schindler B. and Reimer L. Contrast in the transmission mode of a low-voltage scanning electron microscope. (1994)  Journal of Microscopy 173 219-    225

 [5] Fitting, H.-J.: Transmission, energy distribution, and SE excitation of fast electrons in thin solid films. In: physica status solidi (a) 26 (1974), Nr. 2, 525–535.

[6] We kindly acknowledge M. Dries and R. Janzen of the KIT Karlsruhe for the amorphous carbon and PCS sample, the BMBF “Li-EcoSafe” joint research project  (FKZ: 03X4636A) and the Ministry of Science, Research and the Arts (MWK) of Baden-Wuerttemberg in the frame of the SALVE (Sub Angstrom Low-Voltage Electron microscopy and spectroscopy project) for financial support.

Figures:

Figure 1. Free supporting films investigated in transmission mode in the SEM: a), b) formvar film at 3 and 20 keV without and with charging effects, respectively c) normalized transmitted current of the electrons for different films as indicated d) the charge per volume N for gold and formvar e) the secondary electron yield and f) the backscattering coefficient in dependence on the energy of the incident electrons for amorphous carbon films.

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