Introduction

Electron microscopy provides structural information about materials such as solids, crystals, and electronic devices on a nanometer scale. ^[1][2] Scanning Electron Microscopy (SEM) is a method for imaging and analyzing specimens to provide surface information about the sample.^[1]

 It involves the generation and evaluation of secondary electrons (and a smaller amount of back-scattered electrons) utilizing a finely focused electron beam to obtain high resolution and high depth of field imaging.^[3] The digital images obtained are also called SEM micrographs. According to Stokes, Knoll was the first to demonstrate scanned electron images in 1935, and the first SEM was built in 1942.^[4]

Theory of SEM

Primary electrons are generated from a source by using electrical potential and high voltage applied to a filament, along an evacuated column (the filament may be a thermionic emitter or a field emission source). This way, a finely focused electron beam is formed and the surface of the sample can be systematically scanned with it. Electron optics, such as electromagnetic lenses, are used to shape and focus the electron beam. The primary electrons cause various interactions on the sample surface such as the generation of particles and photons (Figure 1). Emitted (or transmitted, through a surface thin enough) particles can be collected and used to form an image, diffraction pattern or chemical spectrum. The most important signals used in SEM are back-scattered electrons and secondary electrons. Formed images are a result from variations in electron beam signal intensity at each point (pixel) within the scanned sample area.^[4]

Figure 1. Different electron interactions with the sample (Figure: Riia Vidgren).

SEM in operation

When operating the SEM, the primary electron beam is scanned (rastered) over a square area of the specimen. The intensity of each screen pixel is determined by the signal level (intensity) arriving at the detector at each point. The beam diameter (or spot size) is determined relative to the size of features of interest. In terms of quantitative data, the image can be shown as a 8-bit grey scale image where pixels are assigned to 256 different grey level values between 0-255, where 0=black and 255=white. If high magnification is required, the beam diameter must be smaller, simultaneously lowering the beam current.^[4]

The operation principle of SEM is shown in Figure 2. The voltage difference between anode and cathode is used to accelerate the electrons from the Schottky or field-emission cathode. The smallest beam cross-section is demagnified by an electron lens system so that a desirable electron probe is formed at the specimen surface. Electron-probe current, aperture and probe diameter can all be varied to, e.g., increase the depth of focus. In front of the final lens, a deflection coil system is used to focus the electron probe and scan across the specimen. This is synchronized with the electron beam from a separate cathode-ray tube (CRT) to adjust the focus and other probe parameters.  The image is formed by modulating the intensity of the CRT, and one simple way to increase magnification of the sample is to decrease the current between the final scanning beam and the coil current, which helps to keep the image size constant.

 
Figure 2. Schematic image of the SEM equipment (Figure: Riia Vidgren).

Some general requirements and applications for SEM imaging are listed below.

• high-vacuum conditions inside the column (at least 10^-3 Pa)
• analyzed specimens must be vacuum-friendly (non-volatile) or the volatile substances (e.g. water) must be removed before analysis
• SEM analysis in liquid or gas states is only possible by employing preparation methods e.g. cryogenic procedures
• with less conductive samples, negative charge builds up (as a result of the bombardment of sample by high-energy electrons) which leads to deterioration in image quality. Metallic coating (sputtering) can be used to prevent this. Metals and electrical conductors are also easier to image.^[4]
• the higher the Z of the sample material, the lower the beam energy should be for better resolution
  
• unlike Transmission electron microscopy (TEM), SEM can be used for bulk specimen analysis
• major advantage of SEM is that besides the back-scattered and secondary electrons a large variety of electron-specimen interactions can be used to form an image and to obtain qualitative and quantitative information^[2]

SEM Example: Characterization of CuO

Nasser et al.^[5] prepared thin films of copper oxide CuO by chemical bath deposition (CBD) method. The films were prepared on glass substrates at room temperature for 20 seconds using heated liquid of sodium hydroxide up to 70 °C and a copper thiosulfate complex. The substrates were annealed at different temperatures (200 °C, 300 °C and 400 °C) in the air and the crystalline structure of the prepared samples was studied by using XRD and SEM. The SEM images (Figure 4) showed a small and spherical morphology which was believed to arise from the different chemistry of precursor solutions, the changing of the crystalline structure of Cu₂O (Cuprite) to CuO (Tenorite) and the temperature of annealing.

Figure 3. SEM images of the CuO thin film (License: CC BY-NC-ND 4.0).^[5]

References