Thin film deposition is an integral part of the manufacturing process for medical equipment, semiconductors, and many other consumer products. Electron beam evaporation of aluminium (Al) is one deposition method used when processing optical coatings, automotive technologies, and architectural structures — primarily due to the metal’s high thermal conductivity and low melting point.
E-Beam Evaporation Process
Electron beam evaporation is a physical vapour deposition method (PVD) [1] that administers a beam of high-energy electrons to melt metals like aluminium. After processing, a thin coating covers the target substrate, protecting it from rust and corrosion. Unlike other heating techniques, e-beam evaporation typically reduces the risk of radiation damage to substrates.
Evaporating aluminium material is relatively straightforward (click this link for an introductory video). First, you need a substrate, a laser, and a coaxial powder-feeding nozzle. Place the substrate and aluminium into a vacuum chamber, such as the HEX system, and point the e-beam laser at the metal.
Most people evaporate aluminium inside intermetallic titanium diboride-boron nitride crucible liners. However, due to its insulating material, an intermetallic boron nitride liner can delay the deposition process. The best way to avoid this problem is by soaking the metal at low power for one to five minutes to create heat in the crucible liner, causing an electrically conductive effect.
Al evaporation differs from melting many other metals. When aluminium evaporates, it forms a chemical reaction with oxygen, creating an aluminium oxide that can cover the entire melt pool in a robust skin. Without the proper attention, oxidised inclusions in deposited clad and molten bubbles can damage the substrate and surrounding walls of the vacuum-sealed box.
Applications of Aluminium Deposition
Evaporated aluminium films are present in many everyday consumer products. An Al liner can protect materials from ultraviolet radiation [2], water vapour, and microorganisms. Controlled Al deposition also allows users to adjust the conductivity of substrates by adding aluminium layers.
Semiconductor Industry
Thin film deposition is vital to the semiconductor industry. Evaporating aluminium into an atomised form helps manufacturers produce electronic materials and line substrates with thermal and chemical barrier coatings.
Many companies also use electron beam deposition to develop high-power fibre and semiconductor lasers. Since the mid-20th century [3], manufacturers have explored the use of modern Al deposition to reinforce substrates and high-density computer materials. Today, evaporating a target material through the use of an electron beam source and vacuum, such as the HEX system, requires less energy than required decades ago.
Optical Coatings
Evaporating aluminium with an e-beam is ideal for controlling the production of thin film optical coating. Some of the most popular products that rely on optical coatings include building glass, camera lenses, and eyeglasses.
Optical thin films, like cool filters, can shield the surface of a glass substrate from infrared radiation. High-reflector coatings can also give a mirroring effect to the glass. Heating aluminium can create a “silvering” effect that gives sunglasses their shiny appearance.
Alternatively, transparent conductive Al coatings can dissipate static charge in a target material. This solution can provide electrodes to a substrate where light is necessary to pass, such as a flat panel display.
You can create specific thin film characterisation by layering several aluminium coatings and heating them in the hearth at a determined temperature and vacuum pressure.
Thin Film Solar Cells
The surfaces of solar panels are vulnerable to a combination of heat from the sun and water damage from humidity. Thin Al films can significantly extend the life expectancy of these products, protecting them from natural deterioration.
A thin film solar panel typically costs less, requires less material, and creates less environmental waste than conventional cells. These lightweight, Al-coated arrays are also versatile, allowing technicians to install them on glass panes and rooftops.
Many large corporations are starting to feel the pressure of switching to alternative energy sources, like solar power. Hence, thin film panels are the practical long-term investment [4]. These durable products can provide power for satellites in the vacuum of space, industrial processes, and more.
Many solar-powered calculators, laptops, and mobile devices also have Al films.
Advantages and Limitations of the Electron Beam Source
E-beam Al evaporation has several advantages and limitations when compared to other vacuum-based deposition techniques, like sputtering. If you aren’t sure which method to use on your substrate, consider the following information:
Advantages of Electron Beam Evaporation of Aluminium
E-beam deposition is the best choice if you need to heat a material with a high melting point, like aluminium. This method will ensure that the entire surface of the substrate has the protection it needs.
It is easier to line the surface of glass with aluminium using electron beam devices. Better control over the pressure, vacuum density, and power of the thin film coating device allows you to set the deposition rate without much trouble.
Limitations of Electron Beam Evaporation of Aluminium
Controlling the evaporation rate of aluminium is a huge advantage in the deposition process. However, you can easily damage the crucible liner or substrate if you are new to this technology.
You’ll need to carefully monitor your vacuum chamber, melt pool, and other components regularly. You may also need to adjust the laser and heat settings or swap out the intermetallic boron nitride crucible liner often to minimise the risk of crucible cracking.
Electron beam aluminium evaporation will not adequately line the surface of a substrate with a complex geometric form. Evaporated aluminium could also create filament degradation that produces inconsistent results.
Sometimes, using a combination of electron beam and sputtering techniques may be the best way to form a protective layer around your target substrate.
Conclusion
The electron beam aluminium melting technique is an effective way for manufacturers to fill quotas and improve product quality. However, several factors — crucible liner strength, substrates used, etc. — can impact how effective this film deposition method is for you.
Fortunately, researchers continue to perfect this evaporation technique and apply it to other industries. In time, this thin film deposition method may become easier to control and less expensive to maintain.
Fill out our Korvus Technology Ltd contact form for more information about thin film deposition and which crucible liner to use.
References:
[1] Li, Zhong, Review article: Preparation and Properties of Coatings and Thin Films on Metal Implants. Encyclopedia of Biomedical Engineering, Pages 203-212, 2019.
[2] Cui, Yanyu, Research article: The Effect of Ultraviolet Wavelength on Corrosion Behavior of 7075 Aluminum Alloy in the Marine Atmospheric Environment. International Journal of Chemical Engineering, Volume 2021, 29 April 2021.
[3] Greene, J.E., Review article: Tracing the recorded history of thin-film sputter deposition: From the 1800s to 2017. Journal of Vacuum Science & Technology, 8 September 2017.
[4] Romeo, A., & Artegiani, E. CdTe-based thin film solar cells: Past, present and future. Laboratory for Photovoltaics and Solid-State Physics, 18 March 2021.
