Quantification of Gas-Based Charge Compensation by Off-Axis Electron Holography in Open-Cell Environmental TEM

Samples with low electrical conductivity when imaged in transmission electron microscopy (TEM) can undergo complex charging processes which cause various distortions to measurements. These distortions can include apparent movement of the sample, changes to the effective optical parameters [1], distortions of objects at different heights (Fig. 1a), and additional phase ramps (Fig. 1b). Several methods exist to compensate for or reduce charge buildup on samples. These include coating samples with thin films of high electrical conductivity or imaging a sample in close proximity to a material with high electrical conductivity [2]. The coating method is often irreversible and is not possible for all samples. The proximity method can require specific apertures or sample preparations and is not always consistent. Another method that has been established in scanning electron microscopy (SEM) but has not been widely applied or studied in TEM is the use of gas flow over a sample [3]. This technique is applicable to almost any sample and is dynamically reversible. Off-axis electron holography is often used to quantitatively measure long-range fields. The presence of uncontrolled charging can mask the fields of interest. Being able to modulate the charging process in situ may also allow for deeper studies of how beam-induced charges are distributed on different samples and interfaces. While the effects of gasses on TEM imaging [4] and off-axis holography [5] have been studied before, the effects on the charging process have not. In the present study, we quantify the gas-mediated charge reduction process on thin dielectric films through off-axis electron holography in an open-cell type environmental TEM. First, we show that the large phase ramp present in a low-conductivity silicon nitride film (Fig. 1b) can be greatly reduced by the introduction of a gas (Fig. 2a). This demonstrates the charge compensation effect. The degree of charging is a complicated interplay of many parameters and we study the effects of gas pressure and dose rate as well as specimen material, thickness, and gas species. We show that although the additional scattering of the electron beam by the gas reduces the beam’s coherence and intensity at the sample plane, the charge compensation effects still improve the fringe contrast on the film (Fig. 2b). Finally, we discuss how this in situ reversible charge-modulation method may be utilized to investigate other processes such as beam-induced specimen vibrations. Lorentz TEM image of an unbiased electron biprism below the interface of a 20nm thick SiN film and vacuum. b) Reconstructed Lorentz-mode electron hologram of the interface of a vacuum region and 20nm thick SiN film. Both images are obtained in standard high-vacuum conditions with no gas flow. a) Reconstructed electron hologram of the interface of the same area shown in Fig. 1b but with 1950 Pa N2 gas pressure around the sample area. b) Trends in the hologram fringe contrast with N2 gas pressure when both arms of the hologram pass through a 20nm thick SiN film or through an empty gas filled region far from the membrane. Average and standard deviation over 10 measurements.

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