Electron microscopes and analytical methods using X-rays are powerful tools that provide valuable information at the nanoscale of various samples. Some samples need to undergo a preparation step before being analyzed. The first group of samples that need to be prepared are radiation-sensitive samples. This group of samples comprises most biological samples, but other sensitive materials, such as plastics, are also included. The second groups of samples that need to be prepared are non-conductive materials or materials with poor electrical conductivity. Due to the non-conductivity properties of these materials, their surface acts as a trap for electrons. The accumulation of electrons on the surface of these materials causes the surface to become so-called “charging”.
The charged portions of the surface are white areas in the image taken by electron microscopy which actually reduces the information obtained from the sample. To prevent this phenomenon, it is necessary to create a nanometer-sized layer of conductive material on the sample surface before conducting electron processes. This conductive layer, like a channel, results in the electrical discharge of the sample surface.
Despite the advantages of deposition of the specimens prior to analysis or electron microscopy, there are disadvantages too. The greatest disadvantage though is the surface of the sample is cloaked with the coating, meaning information on the sample’s atomic number information is lost. In addition, in some cases, the topography of the surface can be adjusted, or incorrect atomic information can be collected. Considering the stated cases, the choice of conducting material for deposition is very important considering the elements present in the sample.
Gold is the material most commonly used for the preparation of electron microscope specimens because of its high electrical conductivity and fine grain size resulting in high-resolution images.
Preparation of the samples for analysis by Energy-dispersive X-ray spectroscopy (EDX) and Backscattered Electron Imaging (BEI) are usually coated with carbon. This is because carbon has a low atomic number and the peak of the X-ray graph of carbon doesn’t interfere with other peaks of other elements.
Carbon film with evaporated carbon fiber or carbon rod is deposited on the sample. Carbon evaporates from the fiber or rod as a result of electric current passing through them and deposited on the sample. The thickness of the deposited film depends on the thickness of the thread used and the time it passes through it. Suitable thicknesses for sample preparation for electron microscopy or X-ray analysis are between 5 and 20 nm.
EDX is an analytical method used for structural analysis or chemical properties of a sample. This method generally works on the principle that each element has a unique atomic structure and as a result, we will have a unique set of peaks in its X-ray spectrum. To emit characteristic X-rays from a sample, a high-energy beam of charged particles such as an electron or proton, and an X-ray beam is concentrated into the under study sample. The number and energy of X-rays emitted from a sample are measured using an energy-dispersive spectrometer. Since the energy of the X-rays reflects the energy difference between the two layers as well as the atomic structure of the element from which they are emitted, it is possible to measure the composition of the sample elements. Figure 2 shows the EDX mechanism of operation.
Since the elements of the first period, (hydrogen and helium) cannot produce X-rays, so these elements cannot be measured in EDX. Secondary period elements (such as carbon, nitrogen, oxygen, etc.) can produce X-rays but the energy of the X-rays produced by these elements is low and their energy is close together, Therefore, the measurement of these elements by EDX method is associated with a significant error. This error varies depending on the type and condition of the detector and the sample type. Figure 3 is an example of an EDX spectrum.
Carbon rods are normally used in a Bradley type evaporation source with shaped carbon rods. Using carbon rod as carbon source in the coating process, due to the small surface area of the rod, it will have less contamination than the carbon fiber coating. Thickness control is also enhanced by the use of carbon rods and by a thick carbon rod can be controlled on average of between 5 and 50 nm. However, systems that use carbon rods as carbon sources require higher power (around 200 amps).
Carbon fibers are used to make strands which are then braided into threads. The finished braided threads are thermally threated to reduce the amounts of impurities. High surface areas of carbon fibers increase chance of contamination. The thickness of the deposited layer represented by the carbon thread depends on the thickness of the fiber, and as the fiber thickens, the thicker layer will deposited.
Flash mode is usually used to evaporate the fiber because in pulse method by repeating each pulse due to the disappearance of the fiber strands different thicknesses will be deposited. Systems that use carbon fiber as a carbon source require less power (about 50 amps).
Considering the above mentioned differences, it is clear that carbon rod evaporation not only gives better control over the thickness, but also delivers a higher quality carbon film without contamination and debris.
However, the application of the pre-heat process to systems that are equipped with a shutter greatly reduces the contamination of the thin film deposited from the carbon fiber source.
Some systems using carbon fiber have a more precise control over the power supply and use thicker fiber threads; they have better control over the thickness. However, contamination and debris are still a contingency.
Carbon evaporated in low vacuum (<10-3 mbar using a rotary
pump) tends to have a more crystalline structure which becomes visible at
magnifications of 25,000 – 35,000x. This type of coating is sufficient
for most SEM/EDX applications.
Carbon evaporated in high vacuum (>10-5 mbar using a turbo pump and a backing pump) appears to be virtually amorphous and has a conductivity which is about 10x better than when evaporated in low vacuum.
The high vacuum carbon coater model DCT, equipped to turbomolecular pump and the Desk carbon coater model DCR, equipped to rotary pump are produced by Vaccoat Co., and suitable for deposition of the high quality carbon film with controllable thickness. They also have the capability to be fitted with a carbon rod head in order to carbon coating from the rod. For more information about these devices, visit the Vaccoat Company website.
- https:// blog.phenom-world.com / sputter-coating-sem
- https:// www.microtonano.com / TIN-Carbon-rods-versus-carbon-fibers.php
- https:// www.microtonano.com / TIN-Target-material-selection-for-coating-SEM-samples-using-an-SEM-sputter-coater.php
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