High-end characterization techniques have long been sought after by researchers. Among these techniques, the scanning electron microscope (SEM) stands out for its ability to produce high-quality images at high magnification. Despite its wide usability and availability in the field of research and development, there is a lack of comprehensive knowledge about SEM among young researchers and students. Specifically, understanding the effects of controlling parameters on image quality is not common among researchers, which hinders their productivity and the quality of their research.

The purpose of this article is to address this knowledge gap and bridge the divide between researchers, scientists, and the SEM. By providing valuable insights into the impact of controlling parameters on image quality, we aim to equip researchers with the necessary understanding to utilize SEM effectively in their work. This will not only enhance their productivity but also improve the overall quality of their research.

​

What is Scanning Electron Microscope (SEM)?

A Scanning Electron Microscope (SEM) utilizes a focused electron beam to magnify samples and create images. By systematically scanning the electron beam across the sample's surface, the emitted electrons are collected to generate the image. It can be compared to a person using a narrow flashlight to scan a wall in a dark room, gradually building up an image in their memory. Instead of a flashlight, the SEM uses an electron beam, an electron detector instead of eyes, and a viewing screen or camera for memory storage. Electrons, which are negatively charged particles within an atom, are focused using electromagnets in an electron microscope, whereas glass lenses are used to focus light photons in a light microscope.

The interaction between the electron beam and the sample's surface influences the resulting image. SEMs are powerful tools for visualizing the otherwise invisible worlds of microspace (10^-6 m) and nanospace (10^-6 m). They can magnify objects from around 10x to 300,000x. SEM images typically lack color but may be artificially colored, appear three-dimensional due to depth of field, and only represent the surface of the sample due to minimal penetration of the electron beam.

Detectors in SEMs can capture two main types of images: secondary electron images and backscattered electron images. In a secondary electron image, the shades of gray correspond to the topography of the sample. In a backscattered electron image, the shades of gray relate to the atomic weight of the constituent elements in the sample, providing valuable compositional information.

​

How it works?

Electron microscopes have similarities to light microscopes, despite their more complex technology. While light microscopes use simple components like a light bulb, glass lenses, the eye as a detector, and a specimen, electron microscopes employ advanced electronics, vacuum equipment, voltage supplies, and electron optics systems. This comparison serves as a starting point for understanding scanning electron microscopes (SEMs).

​

​

​

​

​

​

​

​

​

​

​

​

Within a scanning electron microscope (SEM), an electron gun produces a high-energy electron beam that is directed and focused onto the surface of the specimen using magnetic lenses. The beam then systematically scans (rasters) across the specimen. Unlike in a light microscope, the electrons in an SEM do not form a real image of the sample. Instead, the SEM image is created by illuminating the sample point by point in a rectangular scanning pattern, with each point's signal reflecting differences in the sample, such as its topography or composition. The viewing screen synchronously scans with the beam, ensuring a one-to-one relationship between specimen points and image points, leading to a point-by-point translation. Higher magnification is achieved by reducing the scanned area size on the sample.

To create contrast in the SEM image, the signal intensity resulting from the interaction between the beam and the specimen must be measured across the specimen surface. Signals emitted by the specimen are collected by an electron detector, transformed into photons through a scintillator, amplified by a photomultiplier, and converted back into electrical signals to modulate the image intensity on the viewing screen.

​

​

​

​

​

​

​

​

​

​

​

​

​

​

When an electron beam interacts with a sample in a scanning electron microscope (SEM), various outcomes occur due to interactions with the atoms in the sample. These interactions result in different phenomena. For instance, some electrons may be reflected back out of the sample, known as backscattered electrons. Other electrons may collide with atoms and displace electrons, which then emerge from the sample as secondary electrons. Additionally, these interactions can lead to the generation of X-rays, light, or heat within the sample. Generally, most of the energy is dissipated as heat.

In order to produce traditional SEM images, known as micrographs, we collect the electrons emitted from the sample during these interactions. To better understand the electron-matter interaction, it is helpful to consider two modes of scattering: elastic scattering and inelastic scattering. During elastic scattering, the trajectory of the electron within the specimen changes, but its kinetic energy and velocity remain relatively constant. This process gives rise to the generation of backscattered electrons (BSE).

On the other hand, secondary electrons emerge from the surface layers of the sample due to inelastic scattering. In this case, the incident electron trajectory is slightly perturbed, resulting in the transfer of energy to the specimen. Secondary electrons provide information about the surface features of the sample. Features such as edges, peaks, and fine structures like crests tend to appear particularly bright in the secondary electron image because these electrons can easily escape from these areas on the sample. It's important to note that the brightness of features in the secondary electron image can be influenced by various factors beyond the ones mentioned above.

​

​

​

​

​

​

​

​

​

Key parameters of SEM

​

Accelerating Voltage: Accelerating voltage, also known as the voltage difference between the filament and the anode in a scanning electron microscope (SEM), is responsible for accelerating the electron beam towards the anode. The typical range of accelerating voltage in an SEM is 0 to 30 kV, and a higher accelerating voltage generally allows for greater penetration of the electron beam into the sample. It is commonly believed that increasing the accelerating voltage leads to a higher signal and lower noise in the final image, but the situation is more complex due to several disadvantages.

One of the disadvantages of using a higher accelerating voltage is a reduction in the resolution of structural details on the specimen surface, particularly in the secondary electron (SE) imaging mode. As the accelerating voltage increases, the electron beam penetrates deeper into the sample, resulting in a larger interaction volume. Consequently, the spatial resolution of surface features in the sample created from those signals is reduced. Another issue is the increased electron build-up in insulating samples, which can cause charging artifacts. Insulating samples are less conductive, and at higher accelerating voltages, the accumulation of electrons on the sample's surface becomes more pronounced, leading to distortions and interference with image quality. Additionally, higher accelerating voltages can lead to increased heating and the possibility of specimen damage. The greater energy carried by the accelerated electrons can cause heating effects on the sample, which can be detrimental, especially when imaging sensitive or delicate specimens.

​

​

​

​

​

​

​

​

​

​

​

To select an appropriate accelerating voltage, it is essential to consider the specific requirements of the imaging task and the characteristics of the sample. Factors such as the desired resolution, sample conductivity, and potential heat sensitivity should be taken into account. Optimal accelerating voltage selection requires a balance between achieving sufficient signal intensity and minimizing the negative effects mentioned above.

​

​

 

​

Spot Size: The spot size refers to the size of the electron beam focused onto the specimen surface. It determines the spatial resolution and the level of detail that can be observed in the resulting image. The spot size is typically measured as the diameter of the beam at a specified intensity, such as the full-width at half-maximum (FWHM) of the beam intensity.

​

​

​

​

​

​

​

​

​

When capturing images in a scanning electron microscope (SEM) at the same magnification, accelerating voltage (kV), and working distance, but with different spot sizes, the variation in resolution becomes apparent. The level of blurriness or sharpness in the images is influenced by the spot size, which may be reported differently depending on the specific SEM instrument being used.

​

​

​

​

​

​

​

​

​

​

​

Working Distance: In addition to the accelerating voltage and spot size, the selection of working distance and spot size significantly impacts the quality of images obtained in SEM. The working distance refers to the distance between the objective lens of the SEM and the specimen surface. It plays a crucial role in determining the depth of field and resolution of the images. In general, a working distance of around 10 mm is often recommended as it provides a good balance between depth of field and resolution. This distance allows for a reasonable range of focus, ensuring that a significant portion of the specimen remains in focus while maintaining acceptable resolution. However, there are cases where reducing the working distance can lead to improved resolution, particularly when using lower accelerating voltages. Decreasing the working distance brings the objective lens closer to the specimen, allowing for a narrower depth of field. This narrower depth of field can enhance the sharpness and details of the in-focus areas, resulting in higher resolution. However, it's important to note that reducing the working distance excessively may limit the depth of field too much, causing other parts of the specimen to become out of focus.

​

​

​

​

​

​

​

​

​

​

Cheat code for SEM

1. Low depth of field?

• Increase working distance

• Decrease spot size

2. Surface Charging?

• Lower accelerating voltage

• Use correct detector

• Increase the conductivity of sample (sputter coat or connect the sample surface and stage mount by carbon tape)

3. Low resolution?

• Reduce scan speed

• Shorten working distance

• Reduce spot size

4. Grainy or noisy image?

• Reduce scan speed

• Enlarge spot size

5. Out of focus?

• Adjust working distance

• Adjust the focus and stigmation at higher magnification than the working magnification

• First focus the image, then fix astigmatism, repeat the process until image become crisp and clear.

 

 

In summary, while SEMs are generally user-friendly, optimizing the various parameters during image capture can be challenging and requires experience and familiarity with the equipment. It involves understanding the impact of each parameter on image quality and adapting the settings to achieve the desired results. Practical experience and knowledge of the subject matter being studied are essential for efficient and effective SEM operation.