Low Voltage - Scanning Transmission Electron Microscopy (LV-STEM)

Introduction

Scanning Transmission Electron Microscopy (STEM) was first applied in a TEM. The idea is to navigate a beam over an area with a very fine probe similar as in SEM, but to collect transmitted electrons through a very thin sample (50-250 nm) to form images. In this mode, by varying the collection angle of the electron signals underneath the sample, Z-contrast imaging is possible of which the intensities render back directly to the composition or concentration. It also enables EDS mapping but with a much higher spatial resolution since the interaction volume is much reduced in the case of thin section samples. 

Low-voltage STEM is a dedicated term for STEM mode performed in an SEM. The highest possible acceleration voltage of the electron beam in SEM is 30 kV, which is much lower than 80-300 kV in modern TEMs. Thus to avoid conflicts, the STEM imaging in SEM is referred to as LV-STEM, or Transmission SEM (TSEM). 

Imaging modes in STEM

According to the angular range of transmitted electron collection, the imaging modes can be classified as bright field (BF), annular dark field (ADF) and high angle annular dark field (HAADF). Each mode collects a different group of electron signals and offers a very different contrast. 

The following images are acquired from a coccolith section with a thickness of about 60-70 nm, showing examples from different imaging modes as well as a typical secondary electron imaging by InLens detector from Zeiss Crossbeam 550. The structure in the middle has a size of 9.3 ┬Ám in diameter.     

<p>bf imaging</p>
BF
<p>adf imaging</p>
ADF
<p>haadf&nbsp;imaging</p>
HAADF
<p>inlens&nbsp;se&nbsp;imaging</p>
SE

Energy Dispersive Spectroscopy (EDS) in STEM

EDS analysis can be performed in STEM mode. The advantage compared to EDS in SEM mode is that it offers much higher resolution and less interference due to the highly reduced interaction volume because of the limited thickness of the sample sections. To be mentioned, since the sample needs to be electron transparent and has a limited thickness, the working distance must be small to increase the signal/noise ration; or if it's possible, the sample sections are made thicker to generate more x-ray signals.

On the side, the colour images are example results of carbon, oxygen, lead and aluminium element maps obtained from the same area of the coccolith sample as shown in the above images.


Sample preparation

As described above, to be able to use the STEM mode in a SEM, similar as in TEM, the specimen needs to be thin for the electrons to penetrate through. Therefore, specific sample preparations are necessary. Conventionally, we use microtomy machine to cut big piece samples, please check our supporting equipment page for more information. And if the sample is soft, such as biological samples, it needs to be embedded in resin beforehand. However, this is not possible for certain crystalline samples, minerals for instance, such samples need special preparation method, like thin section lift-out in our FIB-SEM system.