Thermal evaporation is a common physical vapour deposition (PVD) method. This basic form of PVD involves the use of resistive heating at high temperatures in a high vacuum chamber to cause solid material evaporation and create high vapour pressure. Then, the stream of evaporated material coats the surface of the substrate present in the vacuum chamber.
As a thermal evaporation technique, thin-film deposition has many industrial applications, including the creation of metal bonding layers in solar cells, thin-film transistors, semiconductor wafers, and carbon-based OLEDs. A film can consist of a single component or form co-deposition layers of different materials.
How does thermal vapour deposition work? What does this type of thermal evaporation system involve, and what is the role of high-temperature PVD in the thin film manufacturing industry?
What Is Thermal Vapour Deposition?
As the term implies, thermal deposition requires heat. In a high vacuum chamber, a heat source acts on a solid material to produce vapour pressure. Most thermal evaporation systems provide an evaporation temperature ranging from 250 to 350 degrees Celsius to bring the source materials from a solid state to vapour.
The basic unit of a thermal evaporation system is a vacuum chamber that holds the substrate and the source material. The source material typically sits at the lower part of the chamber, while fixtures hold the substrate in an inverted position at the top.
Thanks to the vacuum environment, even comparatively low vapour pressure can produce a vapour cloud. Once this happens, the vapour stream consisting of evaporated particles can traverse the chamber and stick to the substrate surface as a thin film coating.
The resistive evaporation process is one of the most straightforward and efficient forms of physical vapour deposition (PVD). This common method provides cost-effective tools to create thin films of both metals and nonmetals like oxides and nitrides. Compared to the sputtering process, it produces higher deposition rates and thicker coatings, allows fast flash evaporation, and works particularly well for applications that use electric contacts.
Breakdown of the Thermal Deposition Process
In principle, thermal evaporation deposition consists of basic processes and is one of the oldest technologies in the thin film industry. Unlike thermal chemical vapour deposition, it does not require complex precursors or reactive gas. A simple schematic diagram can help illustrate the method.
For thermal evaporation to occur, the deposition chamber must supply proper conditions, namely a high-vacuum environment and a temperature that is high enough to turn the target atoms of the source material into vapour.
The Vacuum Chamber Environment
Effective thermal evaporation requires a high-vacuum environment to remove the gas particles that may interfere with deposition. A vacuum pump creates and maintains the vacuum in the thermal evaporator. In some systems, the vacuum chamber has low pressure around the source material and high pressure in the substrate area.
The bottom of the vacuum chamber contains the heater evaporator, which holds the source material and provides joule heating to bring it to the required temperature.
Physical Vapor Deposition
The heat dissipation of the source material creates vapour pressure. As the vapour stream rises, it meets the substrate. Then the evaporated materials condense on the target material surface to form a solid film. The substrate holder in the chamber rotates continuously to ensure that the deposited film creates an even layer.
PVD works on both metals and nonmetals and on many types of substrates, such as silicon wafers and various polymers.
The thin film quality depends on several factors, including the pressure in the vacuum chamber, the source material’s molecular weight and evaporation rate, and the substrate holder’s rotation speed.
A higher degree of vacuum will give the source material molecules an improved free path and reduce any impurities in the film. In contrast, a rough surface of the substrate may cause non-uniform deposition. The thermal evaporation deposition method may work less well for materials with a very high melting point.
Thermal Vapour Deposition Systems and How They Work
Industrial thermal evaporation deposition systems must operate under precise conditions to ensure film purity. The two main subdivisions of thermal evaporation processes are Filament Evaporation and Electron Beam (E-Beam) Evaporation.
The first method involves resistive evaporation filaments, also known as “boats.” Essentially, these are thin sheets of metal, often tungsten, that act as an evaporation source and hold the source material in appropriate-sized indentations. Filament evaporation is comparatively safe thanks to low voltage. However, this method requires very high currents.
Other thermal evaporation systems apply the more high power density method of E-Beam Evaporation. This method involves high voltages (typically upwards of 10,000 volts) and requires specialised safety features.
The energy source is an electron beam “gun” that accelerates electrons using high voltage and directs them as a targeted beam. This electron beam hits the crucible that contains the source material. Many E-Beam systems have several crucibles that hold multiple sources of material at once to enable seamless co-deposition and multi-layer coating.
To control evaporation speed and thermal evaporation deposition rate, most thermal evaporation systems use quartz crystal microbalances (QCMs) to measure and regulate deposited film thickness. Thermal evaporation systems may also use various software or hardware configurations to manage evaporation rate and film properties.
What Kind of Material Is Used in the Thermal Evaporation Process?
There are several examples of commonly used thermal evaporation sources. Metals, alloys, and ceramics can all work as source materials for evaporated films in thermal evaporation deposition. Thermal evaporation can deposit chrome, aluminium, silver, gold, and many other metals. Generally, resistive evaporation works best for elemental materials with a uniform melting point.
Since E-Beam evaporation debuted as a thermal evaporation technique over 50 years ago, it has enabled effective deposition in high-temperature source materials like transition metal oxides. These materials, which include SiO2, HfO2, Al2O3, and others, often function as UV coatings.
Thermal evaporation is a thermodynamic process with some inherent instability. To achieve more uniform processes and higher film purity, starting materials often undergo special processing like pre-melting, additive mixing, density control, and producing conductive sub-oxides. E-Beam evaporation may also use ion assist (IAD) for more stable, higher-density films.
However, even with the technological advancements of the past decades, E-Beam deposition may not offer enough accuracy for producing high-precision optical coatings in the astronomy, biotech, medical, and aerospace industries. These applications have mostly transitioned to other film deposition methods, like sputter deposition processes, that offer improved step coverage.
The Applications of Thermal Vapour Deposition
Thermal evaporation deposition has many industrial applications, including:
- Optics. Manufacturers use thermal evaporation to produce optic and ophthalmic lens coatings. Thermal evaporation can create hard coatings, anti-reflective layers, mirror coatings, and layers that protect against UV or infrared light.
- Electronics. Thermal evaporation works well for ultra-thin metal plating on devices such as OLEDs and solar cells.
- Consumer packaging. Packaging foils are among the largest-scale applications of thermal evaporation. Many foodstuffs benefit from improved shelf life and prolonged freshness thanks to a thin aluminium film applied to plastic packaging.
- Jewellery and accessories. Costume jewellery, accessories, buttons, and other apparel elements also often benefit from the aesthetic effects of a thin film coating deposited by thermal evaporation. In accessories, thin metal plating enables to keep production affordable and cost-effective.
Choosing a thermal evaporation method and investing in an industrial-scale system will influence a manufacturer’s entire line of production. When manufacturers plan to invest in the thermal evaporation technique, they should consider elements such as quality, consistency, film purity, and more.
Naturally, requirements will vary by line of production. Film quality that is acceptable in accessories or consumer packaging may not provide adequate optical or electrical properties for precision lenses or organic photovoltaics.
When commercial enterprises choose industrial-scale thermal evaporation systems, it’s important to make sure that, first, the system works well for the materials the company plans to apply as thin-film coatings, and second, that the system uses adequate quality controls like quartz crystal sensors or optical monitoring systems.
The base pressure in the vacuum chamber also plays a crucial role in the final product’s quality. Most systems should supply a partial pressure of 10(-07) to 10(-05) mbar to ensure that the evaporated material has a sufficiently long mean free path and avoid scattering of vapour particles by residual gases. An adequate base pressure level is also important for providing a clean substrate surface and a stable coating.
Thin-Film Deposition by Thermal Evaporation: Final Words
Thermal evaporation deposition, an early and common method of thin-film deposition, retains an essential place in various industries, including optics, electronics, and solar cells. Thanks to its high deposition rate and material utilisation efficiency, thermal evaporation has the edge over other methods in thin-film applications. Advanced technologies such as E-Beam deposition help thermal evaporation produce high-quality coatings with an excellent degree of accuracy.
Are you interested in further details about thin films and their applications? Browse other articles on thin film deposition by Korvus Technology.
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