All SEMs are supplied with a loading chamber or a sample holder where the sample is inserted. The use of aluminum stubs is recommended to load a sample in an SEM. These come in various standard sizes and are generously available on a commercial basis.
Image Credit: Thermo Fisher Scientific Phenom-World BV
It is essential that the sample is perfectly adhered to the stub’s surface before placing it in the sample stage or holder.
To adhere to the sample to the pin stub, the following can be used:
- Conductive paint
- Double-sided carbon sticker
- Special clamps
- Conductive tape
- A mixture of the above
It is also suggested that all loose particles are removed from the sample. To perform this, the following can be done:
- The aluminum stub can be held with tweezers, adjusted by 90°, and gently tapped on its side.
- Dry air can then be sprayed on the sample.
The following precautions should be taken:
- The sample should be held carefully to reduce damage.
- Tweezers must always be used to stop contamination.
- Ensure that there is a solid mounting procedure so that mechanical vibrations are avoided as a result of incorrect mounting.
- Dry air should not be sprayed in the direction of any scanning electron microscope or electronics as it may be flammable.
- Ensure that there is no condensed liquid in the spray air straw by initially spraying away from the sample.
These precautions will significantly decrease the contamination risk to the sample holder and system, and sample will ensure enhanced performance over time.
- Magnetic samples
- Non-conductive samples
- Powders and particles
- Beam sensitive samples
- Samples including outgassing or moist samples
SEM image of sugar cube charging. Image Credit: Thermo Fisher Scientific Phenom-World BV
SEM image of sugar cane in low vacuum. Image Credit: Thermo Fisher Scientific Phenom-World BV
When a non-conductive material is imaged, the electrons directed onto the surface of the sample do not have a path to the ground potential, which results in the accumulation of them on the surface. The image will become more and more bright or completely white until details can no longer be seen.
Mild movement can also be identified as a result of the mutual interaction between the electrons. This will create blurriness in the acquired image.
Multiple solutions are commonly used:
Conductive Tapes or Paints
By using a piece of conductive tape (such as copper tape) or a particular conductive paint to cover part of the sample, a bridge is created to the surface of the aluminum stub.
This will enable the sample to partially discharge and is sufficient to image mildly non-conductive samples when imaging regions near the edge of the tape.
Beam interaction with air molecules is enabled by introducing an atmosphere in the sample chamber. Positive ions are produced and attracted by the high number of electrons on the surface of the sample.
The ions will interact with the electrons more which discharges the sample. While this method introduces some noise to the last image, the sample can be analyzed more efficiently and cost-effectively with no additional processing.
SEM image of paper imaged with no precaution (charging.). Image Credit: Thermo Fisher Scientific Phenom-World BV
SEM image of paper imaged with gold coating. Image Credit: Thermo Fisher Scientific Phenom-World BV
SEM image of paper in low-vacuum model. Image Credit: Thermo Fisher Scientific Phenom-World BV
It is possible to produce a thin layer of a conductive material on the surface of the sample by utilizing a sputter coater. This makes a connection between the ground potential and the surface of the aluminum pin.
The selection of coating material heavily depends on the type of investigation to be performed on the sample. Platinum and gold are perfect materials for high-resolution images as both have very high conductivity.
Lighter elements, such as carbon, can be employed when Energy Dispersive Spectroscopy (EDS) analysis on non-organic samples is necessary.
An alloy of indium oxide and titanium oxide (ITO) can produce conductive, transparent layers, to be utilized on optical glasses to make them adequate for SEM. There are weaknesses when employing a sputter coater:
Further instrumentation is necessary, the investigation takes more time, and the samples must withstand more pumping cycles.
Any benefits to utilizing a backscatter electron detector (BSD) to image the sample is gone, because the contrast becomes highly homogeneous and there is no variation in gray intensity for alternate elements.
Samples that produce a magnetic field can detract from the precision of the electron beam, reshaping it and creating blurry, deformed images, normally lengthened along one axis. This issue is known as stigmation alteration and comprises an increase in the beam cross section’s eccentricity.
Every SEM provides the user with the possibility to tune the stigmation. Particular instruments need the user to correct stigmation values each time, while others can track standard values that are correct for the majority of samples.
The process changes the magnetic field of the lenses, which changes the shape of the beam. When the shape becomes circular again, the highest quality image can be captured. It may be required to fine-tune the focus again when changing the stigmation.
Image acquired with wrong stigmation settings. The image appears blurry and the particles (supposedly spheres) slightly deformed. Image Credit: Thermo Fisher Scientific Phenom-World BV
After correcting the stigmation, the image looks sharp and rich in details. Image Credit: Thermo Fisher Scientific Phenom-World BV
Delicate samples, such as biological samples or thin polymeric foils, can be harmed by the electron beam because of the heat emitted in the area of interaction or the rupture of chemical bonds.
This will create either a hole in the surface or an eventual deformation of the scanned area.
The most efficient way to decrease this influence is to use smaller values for current and voltage. In cases like these, the least possible values are suggested.
In the worst circumstances, the sample can have a thin coating layer applied to it to guard the delicate surface. Enhanced conduction will also optimize the resolution of the image.
Thermal influences can be decreased by employing a temperature-controlled device. If the heat produced by the beam is removed, it will shield the sample from any thermally-induced surface changes.
Spending a considerable amount of time on a particular area will result in damage to the sample over time. Efficiency during the investigation will stop significant alterations, but may not create the optimal results concerning image quality.
Zooming in suggests having an equal amount of electrons shot on a smaller area. The thermal drift is enhanced and the deformation characteristics will become more obvious. Low magnification is suggested where possible.
Powders and Particles
SEM image of powder samples using a spoon and SEM image of powder sample using a disperser. When using a disperser, the particles are clearly and evenly spread, and a software can be used to count them. Image Credit: Thermo Fisher Scientific Phenom-World BV
Information such as particle shape or size are essential in the design of the process flow when imaging particles.
The most efficient way to prepare particles or powder sample is to gather a small amount of sample using a spoon, let it fall on a double-sided carbon sticker, and then utilize spray air to take away the additional particles.
This technique will, unfortunately, result in many particles overlapping which will shield critical features, or may cause them to be blown off, promoting errors in particle counting schedules.
The best method of preparing a powder sample is to utilize a particle disperser unit. This will enable an even distribution of the sample on the sticker, decreasing the prevalence of overlapping particles and producing a pattern that can be employed to evaluate granulometry.
Operational parameters, for example, the amount of sample required and the vacuum level, mainly depend on the nature of the powder. The following are some factors to consider:
- Intricate samples may break as a result of a powerful pressure outburst.
- Hydrophilic samples may require a higher vacuum burst to be divided.
- Delicate powders need a smaller amount of sample.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific Phenom-World BV.
For more information on this source, please visit Thermo Fisher Scientific Phenom-World BV.