Pushing the limits of environmental scanning electron microscopy
Authors: Rattenberger Johannes , Fitzek Harald , Schroettner Hartmuth , Wagner Julian , Hofer Ferdinand 
Published: December 2016

In environmental scanning electron microscopy (ESEM) electrical insulating, wet and biological samples can be investigated without additional sample preparation. The imaging gas inside the chamber suppresses charging and outgassing of the sample but it also decreases the signal to noise ratio (SNR) [1]. Especially applications in the kPa regime are limited by poor image quality (e.g. wetting experiments).

Recent publications on high pressure capabilities of state of the art microscopes have shown that they are working far away from physical limits and that there is plenty of room for improvements [2].

The key to high image quality at high pressures is to reduce scattering of the primary beam electrons inside the imaging gas as far as possible while maintaining ideal operation conditions for the SE‐detector [3].

In the FEI Quanta 600 ESEM the gaseous environment in the sample chamber is separated by a differential pumping system and two pressure limiting apertures (PLA) from the high vacuum inside the electron column. Nevertheless, a lot of gas streams through the PLA upwards and a significant amount of scattering takes place even before the electron beam is entering the sample chamber [2].

Based on the insights of Monte Carlo and finite element simulations a new aperture holder was designed that significantly reduces this gas flow and therefore also the primary beam scattering. The PLAs are exchangeable and smaller diameters further increase the SNR at the expense of a smaller field of view.

In a conventional ESEM the secondary electron detector is a positively biased electrode which attracts and accelerates the secondary electrons. On their way to the detector the secondary electrons undergo collision ionization which amplifies the signal and generates positively biased gas ions. With increasing chamber pressure this SE signal amplification strongly decreases because the electron mean free path decreases and the SEs do not gain enough energy between collisions to ionize the imaging gas anymore.

By replacing the position and modifying the shape of the detector it can be optimized for high pressure applications. Nearby a needle detector with very small tip radius (R < 10 µm) the electric field is strong enough for SE amplification and by positioning the needle on the sample table it operates at ideal conditions regardless of pressure and working distance. The distance sample to PLA and sample to detector is no longer coupled. A by‐product of this design is that the conventional position of the backscatter electron detector (BSE) at the end of the column is no longer blocked by the SE detector.

With this outstanding signal to noise ratio at high chamber pressures the limits of conventional ESEM technology can be crossed. Wetting experiments at low acceleration voltages and low dwell times are possible as well as imaging liquid samples without cooling (see figure 1,2). In figure 3 a BSE image of gold nanoparticle in oil at 10 kPa chamber pressure can be seen and the overall improvements are shown in figure 4.