Universal Pressure Scanning Electron Microscopy

Pushing the Limits of Environmental Scanning Electron Microscopy

  • Fig. 1: Schematic representation of a conventional ESEM (left) and the USPEM (right).Fig. 1: Schematic representation of a conventional ESEM (left) and the USPEM (right).
  • Fig. 1: Schematic representation of a conventional ESEM (left) and the USPEM (right).
  • Fig. 2: Yeast cells on membrane (3 keV, 4°C, 800 Pa water vapor).
  • Fig. 3: Wetting experiment on human hair (5 keV, 2°C, 990 Pa water vapor).

Recent publications on high pressure capabilities of state of the art environmental scanning electron microscopes have shown that they are working far away from physical limits and that there is plenty of room for improvements. In this publication an optimized pressure limiting aperture holder and secondary electron detection system is presented which pushes the limits. Imaging at higher chamber pressures up to one atmosphere investigations of living organisms and much more are possible.

Introduction

Environmental Scanning Electron Microscopy (ESEM) is a well-established technique in microcopy to investigate electrically insulating, biological or wet sample. In comparison to the conventional Scanning Electron Microscope (SEM) the imaging gas inside the sample chamber (up to a few hundred Pascal pressure) suppresses the outgassing of wet samples and the negative charging of insulating samples. The gaseous environment inside the ESEM sample chamber is separated from the electron column by a pressure limiting system; a positively biased electrode (a few hundred volts) works as Secondary Electron (SE) detector. The SE signal is amplifying inside the imaging gas by collision ionization and the generated positively charged gas ions suppress negative charging of the sample [1].

The negative side effect of this gaseous environment is a relatively poor image quality at high chamber pressure (p > 600 Pa) caused by scattering effects and the insufficient secondary electron detection system. As a result high pressure experiments must be done at lowest possible pressure, high electron energies, long dwell times and large electron beam currents. The hereby introduced Universal Pressure Scanning Electron Microscope (UPSEM, patent pending) is putting an end to the above mentioned limitations.

Pressure Limiting System

The pressure limiting system has two major advantages: (1) the protection of  the electron source from too high pressure and (2) minimization of the scattering of beam electrons inside the gaseous environment. The pressure limiting system is usually realized by one or two Pressure Limiting Apertures (PLA) at the end of the pole piece and an additional pumping system.

Caused by the steady gas flow from the sample chamber into the electron column scattering events start before the electron beam is entering the sample chamber. In consequence the Stagnation Gas Thickness (SGT - effective distance, the electron beam is inside the gaseous environment) is significantly larger than the working distance (pole piece to focused point) or the environmental distance (distance final PLA to focused point) [1].

Secondary Electron Detection System

In state-of-the-art ESEMs the secondary electron detector is a flat positively biased electrode positioned sideways or directly at the end of the pole piece. Secondary electrons are attracted and accelerated by the electric field of the detector, on their way through the gas collision ionization amplifies the signal. With increasing chamber pressure the mean free path of SEs decreases and the electrons do not gain enough energy from the electric field in between collisions to ionize the gas any longer. As a result, after a maximum of several hundred Pascal the amplification efficiency strongly decreases [1].
Since the first commercially available ESEM in 1988, it became a widely used microscopy technique and is well established in life and material sciences. However, recent publications have shown that commercially state-of-the-art ESEMs do not reach the physical limits and there is plenty of room for improvement [2,3].

Optimizing the System

One of the most widespread types of ESEMs are the FEI (meanwhile Thermo Fisher Scientific) Quanta Series microscopes, where the gaseous environment in the sample chamber is separated by two PLAs and a differential pumping system from the high or ultra high vacuum inside the electron column. The gas quantity flowing from the sample chamber into the electron gun is determined by the diameter of the first PLA and by the pressure difference between the regions above and below the aperture [1]. By approaching the PLA from below, the pressure gradually decreases from a distance of about one diameter below PLA1 followed by a sharp decrease immediately above the PLA.

A trivial approach to minimize the gas inside the electron column would be a smaller aperture diameter. However, the available field of view and the probe current strongly decrease. A problem that might be solved by optimizing the shape of the apertures and the shape of the aperture holder which redirect the gas flow into less disturbing directions and minimize the interaction of the primary beam electrons with the imaging gas.

With the help of Monte Carlo simulations we identified a design error of the aperture holder [2]. The conical form focuses the gas flow into the direction of the electron gun, the gas flow is chocked and a supersonic gas jet is formed. That is the reason why a cylindrical construction was chosen for the optimized aperture holder. In the original design the final PLA is relatively thick (750 µm) and has a 90 degree lip angle. By decreasing the thickness and increasing the lip angle the direction of the gas flow is also positively influenced. Additionally, the gas quantity may be limited by the use of different PLA diameters. With this approach, the scattering of primary beam electrons is significantly reduced even when operated at the same field of view as the original design.

In the FEI Quanta series ESEMs the final PLA is part of the gaseous secondary electron detector which is positioned at the end of the pole piece. At high chamber pressure the homogeneous electric field is not strong enough to amplify the secondary electron signal. In contrast, the new designed SE detector is shaped like a needle and positioned sideways. Nearby the tip of the needle (tip radius < 10 µm), the inhomogeneous electric field  amplifies the signal, whereas the electric field in the remaining detector sample distance is weak enough to avoid arc discharge and influence on primary beam electrons.

Furthermore the presented aperture holder and secondary electron detector eliminate two big disadvantages which origin only in detector positions. With the new design, the environmental distance is not coupled with the gap between detector and sample and the SE detector needle no longer blocks the position of the backscatter detector. The simultaneous use of both detectors is extremely practical and decoupling the both distances improves the high-pressure performance significantly. In its original condition, there is always a trade-off between minimizing the environmental distance (respectively the stagnation gas thickness) and an ideal amplification distance for the SE detector (fig. 1).

This new designed system enables us to minimize the environmental distance while still maintaining ideal operation conditions for a dedicated high-pressure secondary electron detection system. The improved performance of the system can be seen in figure 2 and 3 (fig. 2: yeast cells without any sample preparation; fig. 3 wetting experiment on human hair).

Summary
The presented high-pressure optimized ESEM system allows the user to investigate samples in the range from the high vacuum to the ESEM regime up to one atmosphere chamber pressure within the same microscope. Thanks to these unique capabilities, this system is called Universal Pressure Scanning Electron Microscope (UPSEM).  It has to be noted that the presented high pressure performance of the system was already demonstrated years ago by G. Danilatos in different publications [4,5,6,7].  However, it is new that existing commercially available systems can be easily upgraded.
The significant improvement in signal to noise ratio enables a number of new possibilities: extending the maximum available chamber pressure, good time resolution for in situ investigations of dynamic processes, small beam currents for radiation sensitive samples and low acceleration voltages for surface sensitive imaging. By increasing the chamber pressure experiments can be performed closer to reality and new pressure regimes are made accessible for experiments.

Acknowledgment
The author wants to thank G.D. Danilatos (ESEM Research Laboratory, Sydney) for helpful discussions and the Austrian Research Promotion Agency (FFG - PN 839958), the “Bundesministerium für Digitalisierung und Wirtschaftsstandort” (bmdw) & Austrian Cooperative Research (ACR) (SP2016-002-006) and the Graz Centre for Electron Microscopy (ZFE) for financial support.

Authors
Johannes Rattenberger1, Harald Fitzek2, Hartmuth Schroettner1,2

Affiliation
1 Graz Centre for Electron Microscopy (ZFE), Graz, Austria.
2 Institute of Electron Microscopy and Nanoanalysis (FELMI), Graz University of Technology, Graz, Austria

Contact
Dr. Johannes Rattenberger

Graz Centre for Electron Microscopy (ZFE)
Graz, Austria
johannes.rattenberger@felmi-zfe.at
www.felmi-zfe.at/
 

References
[1] Gerasimos D. Danilatos: Foundations of environmental scanning electron microscopy, Advances in Electronics and Electron Physics Vol. 71, 109–250, (1988) doi: 10.1016/S0065-2539(08)60902-6
[2] Gerasimos D. Danilatos, Johannes Rattenberger and Vassilios Dracopoulos: Beam transfer characteristics of a commercial environmental SEM and a low vacuum SEM, Journal of Microscopy Vol. 242, 166–180 (2011) doi: 10.1111/j.1365-2818.2010.03455.x
[3] Harald Fitzek, Hartmuth Schroettner, Julian Wagner, Ferdinand Hofer and Johannes Rattenberger: High-quality imaging in environmental scanning electron microscopy – optimizing the pressure limiting system and the secondary electron detection of a commercially available ESEM, Journal of Microscopy, 85–91 (2016) doi: 10.1111/jmi.12347
[4] Gerasimos D. Danilatos, Design and Construction of an Atmospheric or Environmental SEM (Part 1), Scanning, Vol. 4, 9–20 (1981) doi: 10.1002/sca.4950040102
[5] Gerasimos D. Danilatos, and R. Postle, Design and Construction of an Atmospheric or Environmental SEM (Part 2), Micron, Vol. 14, 41–52 (1983) doi: 10.1016/0047-7206(83)90030-4
[6] Gerasimos D. Danilatos: Design and Construction of an Atmospheric or Environmental SEM (Part 3), Scanning, Vol. 7, 26–42 (1985) doi: 10.1002/sca.4950070102
[7] Gerasimos D. Danilatos: Design and Construction of an Atmospheric or Environmental SEM (Part 4), Scanning, Vol. 12, 23–27 (1990) doi: 10.1002/sca.4950120105

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