Carbon Coating Archives - Quorum Technologies Ltd

Electron microscopes use electrons rather than visible light to investigate and image samples, thus providing orders-of-magnitude enhancements on the magnification and resolution of traditional optical microscopes.

However, electron microscopes are much more complicated than optical microscopes, and the conditions that samples are subjected to are very difficult to work with.

SEM and TEM both depend on the transfer of electrons between a sample and the microscope itself, and thus imaging is performed under high vacuum. Moreover, samples must be conductive to enable electron transfer. This poses a difficulty for some sample types.

Although it is comparatively easy to use SEM or TEM to image solid metallic samples, for instance, imaging soft, hydrated and/or non-conductive samples — for example, biological tissues — can be challenging.

When left untreated, such samples lead to several issues. Moisture and gas from the sample easily evaporate inside the vacuum environment of an electron microscope sample chamber, thereby contaminating the microscope and damaging the sample.

Moreover, upon subjecting non-conductive samples to the incident electron beam within an electron microscope, the electrons do not conduct through the sample, making them accumulate in one place. This “charging” results in several imaging artifacts and can even make it impossible to image samples.

It is possible to solve these issues by the careful preparation of samples, specifically by sample coating. Using a layer of conductive material (carbon or metal) to coat samples has several purposes. There are two key reasons for coating: to make the sample conductive, which avoids “charging” effects, and to encapsulate samples to eliminate off-gassing or evaporation.¹

Specifically, metallic coatings can facilitate better thermal conductivity, safeguarding the sample from heat damage from the incident electron beam. They can even localize the signal to the actual surface of the sample, thus enhancing the signal-to-noise ratio and secondary electron emission.

Carbon coatings offer certain distinct advantages. Carbon coatings for electron microscopy are amorphous, conductive layers transparent to electrons. This implies that carbon coatings are particularly valuable for making non-conductive samples amenable to energy-dispersive x-ray spectroscopy (EDS).²

Achieving High-Quality Carbon Coatings

For SEM and TEM applications, metallic coatings such as tungsten and gold can be done through sputtering. However, the same is not true of carbon. Although carbon can be sputter-coated, the resulting coatings exhibit high hydrogen concentrations, which makes carbon sputtering unsuitable for electron microscopy applications.

Alternatively, high-quality carbon coatings can be performed through thermal evaporation of carbon in vacuum. Two similar techniques can be used to achieve this — using carbon fiber or using a carbon rod.

In the carbon rod coating method, two carbon rods with a sharpened contact point between them are used. This is also called the Brandley method.

The process involves passing current between the two rods, ensuring very high current density at the sharpened contact point, which leads to very high levels of resistive heating. This causes the evaporation of carbon from the surface. This can be achieved with a ramped current or a pulsed current.

In the carbon fiber technique, a carbon fiber is mounted between two clamps, and a pulsed current is passed along it. This leads the carbon to evaporate from the surface of the fiber.

Both techniques exhibit unique differences in quality. The carbon fiber technique facilitates certain control over coating thickness by tweaking the number of current pulses and a pulse length. This makes it appropriate for TEM grid applications and analytical SEM applications like EDS and electron backscatter diffraction (EBSD). Yet, pulsed carbon fiber coatings essentially contain higher levels of debris.

Carbon rod coatings are “cleaner” and of better quality. Carbon rod coatings made in high vacuum with ramped current offer the maximum quality coatings, suitable for high-resolution TEM applications and crucial SEM applications.

In its pulsed version, the technique can be employed to achieve thicker coatings for SEM, particularly for wavelength-dispersive X-ray spectroscopy (WDS) and EBSD. In such applications, it is important to select carbon rods with the maximum purity to achieve the maximum possible coating quality.

Carbon Coating Solutions from Quorum Technologies

Quorum Technologies has come up with the new Q Plus Series that offers an all-in-one solution to attain high-quality carbon coatings for all electron microscopy applications.

With the ability to coat both carbon fiber/cord and carbon rod, the carbon evaporators employ easy-change inserts to allow users to easily switch between the two modes.

The new Q150V Plus from Quorum Technologies offers the highest vacuum of 1 x 10⁻⁶ bar for superior results. The lower background pressure implies oxygen, nitrogen and water vapor are eliminated from the coating chamber, restricting chemical reactions while executing the coating process. This leads to impurities or defects. Lower scattering also implies amorphous carbon films of higher purity and high density.

References

Goldstein, J. I. et al. Coating Techniques for SEM and Microanalysis. in Scanning Electron Microscopy and X-Ray Microanalysis: A Text for Biologist, Materials Scientist, and Geologists (eds. Goldstein, J. I. et al.) 461–494 (Springer US, 1981). doi:10.1007/978-1-4613-3273-2_10.
Heu, R., Shahbazmohamadi, S., Yorston, J. & Capeder, P. Target Material Selection for Sputter Coating of SEM Samples. Microscopy Today 27, 32–36 (2019).
Image shown – Fungi spores on a TEM grid (Cu, 300 mesh with 5nm carbon film produced with Q150VES Plus coater). Image Credit: Quorum Technologies

Electron microscopy techniques rely on the transfer of electrons between sample and microscope. For conductive samples, this is easily achieved – however, non-conductive, or poorly conducting samples must be coated with an electrically conductive coating to produce usable images. A high-quality coating is essential to obtain high-quality images. Quorum Technologies developed the Q Plus Series to provide researchers with a versatile and high-performance coating to rival major manufacturers, without the associated price tag.

The Role of Coatings in Electron Microscopy

Scanning Electron Microscopes (SEM) and Transmission Electron Microscopes (TEM) work similarly to Optical Microscopes but rather than probing materials using light, they use electrons. Optical microscopes are diffraction-limited (to a maximum resolution of about 200 nm) whereas electron microscopes can produce beams of electrons with much smaller wavelenght¹ and surpass the resolving power of optical microscopes by several orders of magnitude. As a result, they are the most powerful microscopy techniques in the world.

Using electrons instead of light, however, introduces other complications. Both techniques (SEM and TEM) rely on the transfer of electrons between sample and microscope; therefore, it can be difficult (or near impossible) to obtain a usable image signal from samples with poor or no conductivity. This is especially true of SEM, where samples are bombarded with an electron beam: poorly conductive or non-conductive samples will rapidly accumulate charge under these conditions, leading to image distortion as well as thermal and radiation damage to the sample. In extreme cases, the sample may accumulate sufficient charge to decelerate the primary beam, acting as an “electron mirror” and preventing an image altogether.²

To overcome this, poorly conducting samples are coated with a thin layer of metal or carbon. This makes the surface conductive, eliminating charge accumulation and enabling a better signal to be obtained by the microscope. Coating techniques are widely used for imaging biological or organic samples since these are typically non-conductive and easily damaged by the electron beam.

While the primary role of coating in SEM is to increase electrical conductivity and prevent “charging”, it also has several other useful effects:

Coating a sample with a thermally conductive material such as gold, silver, copper, aluminium can reduce thermal damage from the primary electron beam.
Particulate matter and fragile organic samples can be mechanically stabilized and held in place by a thin layer of carbon.
Coating organic samples that contain trapped gas or moisture protects both sample and microscope from being contaminated by off-gassing.
Metallic coatings can be used to minimize the volume of penetration of the electron beam, localizing scanning to the very surface of a sample. This can also dramatically increase the emission of secondary and backscattered electrons.

Download our Guide to Coating for Electron Microscopy Here

The Impact of Coating Quality

When working with a coated sample in an electron microscope, it is the coating itself that gets directly imaged. The quality of the coating, therefore, places a hard limit on the quality of the images that can be obtained.

When imaging very small structures (such as electrospinning fibres doped with copper nanocrystals), depositing a coating that is too thick can easily bury meaningful information. It is vital that coating thickness can be precisely controlled and tailored to the features that are being interrogated.³

In the worst cases, poor quality coating equipment introduces contamination and can irreparably damage samples. Researchers often opt for cheap coaters to save money, only to find that their costs increase due to additional microscope time and ruined samples.

However, thanks to the Q Plus Series from Quorum Technologies, it is no longer necessary to pay a premium to obtain state-of-the-art coatings.

The Q Plus Series: Affordable and High-Quality Coating

The Q Plus Series is the latest iteration of Quorum’s world-leading range of coaters; offering cutting-edge sputter and evaporation coating in a single easy-to-use platform. Quorum’s turbomolecular-pumped coaters are suitable for both oxidizing and non-oxidizing metals, while our low-cost rotary-pumped sputter coaters are suitable for non-oxidizing metals. The Q Plus Series is suitable for sputter coating and evaporating carbon coating for SEM, FE-SEM and TEM applications.

This new range of coaters is designed to enable researchers to exercise precise control over coating thickness, whatever their application requirements. For the highest level of performance, the Q150V Plus provides an ultimate vacuum of 10^-6 mbar; removing oxygen, nitrogen and water vapour from the chamber and eliminating chemical reactions during the sputtering process. The Q150V Plus also enables the production of finer grain size and thinner coating for ultra-high-resolution applications (beyond 200,000x magnification). Low scattering enables the formation of high-purity amorphous carbon films of high density.

All models in the Q Plus Series feature a touch-screen interface as well as status LEDs and audio notifications for straightforward and intuitive control. Integrated 16 GB memory allows the storage of over 1000 recipes to be stored, and a USB port enables upgrades and downloads of log files.

To find out more about the Q Plus Series of coaters, view our brochure or get in touch with us today.

To view our latest webinars on coating technologies, we invite you to view our series here:

References and Further Reading

The Diffraction Barrier in Optical Microscopy. Nikon’s MicroscopyU https://www.microscopyu.com/techniques/super-resolution/the-diffraction-barrier-in-optical-microscopy.
Goldstein, J. I. et al. Coating Techniques for SEM and Microanalysis. in Scanning Electron Microscopy and X-Ray Microanalysis: A Text for Biologist, Materials Scientist, and Geologists (eds. Goldstein, J. I. et al.) 461–494 (Springer US, 1981). doi:10.1007/978-1-4613-3273-2_10.
Ahire, J. J., Neveling, D. P. & Dicks, L. M. T. Polyacrylonitrile (PAN) nanofibres spun with copper nanoparticles: an anti-Escherichia coli membrane for water treatment. Appl Microbiol Biotechnol 102, 7171–7181 (2018).

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