The role of secondary electron emission in the charging of thin‐film phase plates
Authors: Dries Manuel , Janzen Roland , Schulze Tina , Schundelmeier Jonas , Hettler Simon , Golla‐Schindler Ute , Jaud Bianca , Kaiser Ute , Gerthsen Dagmar 
Published: December 2016

In the past few years, physical phase plates (PP) emerged as an interesting tool to achieve phase contrast of weak‐phase objects in transmission electron microscopy (TEM). Research has focused on thin‐film PPs, which are typically fabricated from amorphous carbon (aC)‐films [1]. However, the illumination with high‐energy electrons initiates an irreversible degeneration of the aC‐film, which causes electrostatic charging and affects the phase‐shifting properties.

Electrostatic charging is a limiting factor in the application of thin‐film PPs. However, the mechanisms of charging are not well‐understood. This work shows that charging predominantly occurs due to secondary electron (SE) emission. For this purpose, Hilbert PPs were fabricated from different materials to study their charging behavior under electron beam illumination. Besides aC, thin films of the metallic glass alloy Pd77.5Cu6.0Si16.5 (PCS) were used for PP fabrication. The amorphous PCS‐alloy is characterized by a high electrical conductivity and a strong resistance towards oxidation [2], which at first sight suggests minor charging of PCS‐films. For use as a Hilbert PP, the film thickness was adjusted to a phase shift of π. At 200 keV electron energy, this corresponds to a film thickness of 49 nm for aC and 19 nm for the PCS‐alloy.

Fig. 1 shows phase‐contrast TEM images and corresponding power spectra of an aC test‐object, which were acquired using a Hilbert PP fabricated from an aC‐ (Fig. 1a) or a PCS‐film (Fig. 1b). Despite its excellent electrical properties, charging occurs for the PCS Hilbert PP as can be deduced from the strong distortion of the Thon‐ring system in Fig. 1b. However, charging is significantly reduced if the PCS Hilbert PP is coated on both sides with a thin aC‐layer of 6 nm thickness as shown in Fig. 1c. It is noted, that for smaller cut‐on frequencies charging also occurs for the aC (Fig. 1a) and the aC/PCS (Fig. 1c) Hilbert PP. In contrast to pure aC‐films, the carbon content is drastically reduced for the aC/PCS Hilbert PP, which improves its stability under electron beam illumination.

The reduced amount of charging in Figs. 1a,c is attributed to the low SE emission of aC compared to the PCS‐alloy. The emission of secondary and backscattered electrons was investigated in a Zeiss NVision 40 scanning electron microscope. The Hilbert PPs were mounted on a specific device, where the absorbed and transmitted current can be measured separately. The absorbed current to ground I_S was measured for different primary electron energies E from 0.5 keV to 20 keV. Fig. 2 shows the absorbed current normalized with respect to the current of the incident electron beam I_PE for aC and the PCS‐alloy. Between 1 keV and 3 keV aC is negatively charged. Above 3 keV the aC‐film only shows a small tendency for charging. Although the blue curve is close to zero, I_S/I_PE takes finite values below the measurement accuracy. The PCS‐alloy has a negative charge below 5 keV, but is positively charged for electron energies up to 20 keV. The low tendency for charging of aC and the positive charging of the PCS‐alloy are consistent with the observations made in Fig. 1. It is noted, that electron energies are much higher in TEM. However, the curves in Fig. 2 show trends, which might continue towards higher energies.

Electrostatic charging implies poor electrical conductivity. Otherwise, any positive (negative) charge balance would be immediately compensated by an electrical current from (to) ground. We assume that our Hilbert PPs are properly grounded and that aC as well as the PCS‐alloy have a sufficient electrical conductivity for the small beam currents used in TEM. A possible reason for poor electrical conductivity is beam‐induced contamination. Low energy SEs, which get trapped in the highly insulating contamination layer, cause the formation of an electrical dipole layer at the interface between the contamination and the PP film.