Elsevier

Micron

Volume 96, May 2017, Pages 38-47
Micron

Carbon contamination in scanning transmission electron microscopy and its impact on phase-plate applications

https://doi.org/10.1016/j.micron.2017.02.002Get rights and content

Highlights

  • Systematic investigation of focused electron-beam induced contamination in TEM.

  • Contamination is identified as graphitic amorphous carbon with high sp2-fraction.

  • Contamination layer is not prone to electrostatic charging.

  • Deposited contamination can be used as a phase plate for phase-contrast TEM.

  • Evaluation of different methods to inhibit contamination.

Abstract

We analyze electron-beam induced carbon contamination in a transmission electron microscope. The study is performed on thin films potentially suitable as phase plates for phase-contrast transmission electron microscopy. Electron energy-loss spectroscopy and phase-plate imaging is utilized to analyze the contamination. The deposited contamination layer is identified as a graphitic carbon layer which is not prone to electrostatic charging whereas a non-conductive underlying substrate charges. Several methods that inhibit contamination are evaluated and the impact of carbon contamination on phase-plate imaging is discussed. The findings are in general interesting for scanning transmission electron microscopy applications.

Introduction

The prevention of undesirable hydrocarbon contamination on the sample has been a challenge since the early days of electron microscopy. While residual oil molecules in the vacuum have been a considerable contribution to the contamination in microscopes equipped with oil-pumps (Ennos, 1954, Fourie, 1978), this problem has been eliminated in modern transmission electron microscopy (TEM) by using dry pumping systems leaving the sample as the main source of contamination (Hren, 1979). Molecules adsorbed on the sample surface are decomposed and deposited under electron-beam illumination mainly by primary (PE) and secondary (SE) electrons with surface diffusion playing a major role in transport of the mobile adsorbates along the sample surface. Common strategies to avoid sample contamination are sample cooling (Egerton and Rossouw, 1976) or heating, beam showering (Egerton et al., 2004) and plasma (Isabell et al., 1999) or UV cleaning (Hoyle et al., 2011). The strategies are summarized and evaluated for scanning transmission electron microscopy (STEM) by Mitchell (2015).

Alternatively, hydrocarbon contamination can be exploited to deposit nanometer sized carbon-based structures with defined shape and interesting properties both in STEM and TEM (Broers et al., 1976, Ueda and Yoshimura, 2004, Shimojo et al., 2005, Malac et al., 2005, Rykaczewski et al., 2008). Comprehensive simulations have been carried out to predict size and shape of structures formed by electron beam induced deposition (EBID) (Rykaczewski et al., 2007).

Contamination also affects the performance of physical phase plates (PPs) for phase-contrast TEM (Danev and Nagayama, 2001, Schultheiss et al., 2006, Glaeser, 2013). PPs usually consist of structured thin films or electrostatic devices which are typically implemented in the back focal plane (BFP) of the objective lens. PPs then allow to induce a relative phase shift between the scattered and unscattered part of the electron wave resulting in a phase-contrast enhancement. Meanwhile, similar PPs are also applied in different planes of the electron microscope, such as in the selected area plane for phase-contrast TEM (Minoda et al., 2011) or in the condenser lens system to shape the illuminating electron beam (Verbeeck et al., 2014). An additional phase shift caused by contamination or electrostatic charging of the PP limits the applicability of PPs. An alternative PP approach utilizes a uniform film placed in the BFP of the objective lens that is locally modified by the incident beam itself. This approach is referred to as a hole-free (HF)PP (Malac et al., 2012a). Both, positive and negative phase shift, corresponding to a positively or negatively charged area induced on the film by the incident electron beam, were reported for HFPPs and their various implementations (Malac et al., 2012a, Danev et al., 2014). While the underlying physical origin of the phase shift in HFPPs is not fully understood, the hydrocarbon contamination, its electrical properties and rate of buildup are expected to have a significant effect on PP imaging and on PP methods that utilize uniform thin films in particular.

In this work we aim at understanding both contamination and charging effects on thin films responsible for the functionality of HFPPs. We systematically studied thin films made of materials intended for PP application such as amorphous carbon (aC), metallic glass alloys (ZAC: Zr65Cu7.5Al27.5 and PCS: Pd77.5Cu6Si16.5) and amorphous Si in a transmission electron microscope. We present the experimental results on the properties of carbon contamination obtained by electron energy-loss spectroscopy (EELS) and HFPP imaging. The results confirm previously published findings on the behavior of contamination in (S)TEM (Hren, 1979, Egerton et al., 2004, Mitchell, 2015). Additionally, it is shown that contamination deposited by a high-current density electron-beam illumination can be well described by graphitic carbon with a high content of sp2-hybridized bonds. The deposited material does not carry additional charges whereas an underlying non-conducting substrate may still charge. The impact of our findings on PP TEM is discussed.

Section snippets

Materials and methods

Most of the experiments were carried out using a dry-pumped Hitachi HF 3300 transmission electron microscope (Hitachi High Technologies, Naka, Japan) equipped with a cold field emission gun operated at 300 kV. The microscope is operated in free lens mode allowing us to customize the electron optics or adjust the accelerating voltage between 35 kV and 300 kV for a particular experiment (Malac et al., 2010). The electron optics were adjusted in order that the BFP of an additional test specimen

Results and discussion

In this section we first present a model for the principle of contamination buildup based on previous studies (Hren, 1979, Mitchell, 2015, Rykaczewski et al., 2008) which is confirmed by our experimental results. A description of the physical properties of the deposited contamination layer obtained from EELS analysis follows. Subsequently, the charging of the contamination layer is addressed. Finally, we present the findings on the prevention of hydrocarbon contamination by different measures

Conclusions

We studied the properties of carbon contamination on amorphous thin films of different composition. These thin films are potentially suitable as PPs for phase-contrast TEM. Two different electron-optical setups allowed the determination of the relative thickness, the phase shift induced by the deposited contamination layer and the evaluation of the carbon bonds. The main findings can be summarized as follows and are valid for both HFPP imaging and conventional STEM where a thin film is

Acknowledgements

S. Hettler acknowledges funding from the Karlsruhe House of Young Scientists (KHYS). The work performed at KIT was supported by the German Research Foundation (DFG) [Project GE 841/16]. The work at NINT was supported by the National Research Council of Canada. The ongoing support of Hitachi High Technologies, Naka, Japan especially Dr. Y. Taniguchi and Hitachi High Technologies Canada was crucial in developing a custom experimental setup of the Hitachi HF-3300. Collaboration with JEOL Ltd. and

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