An Introduction to Thin Films and the Thermal Evaporation Deposition Process

Do you want to learn about the fascinating processes of thin film manufacturing and thermal vapour deposition? Thin film deposition is a critical component of virtually every modern technology we rely on today. For example, look at your mobile phone. The manufacturer likely used thin films to enhance the screen’s optical properties, like reducing visible light glare. However, electronic displays are just one of the numerous applications for thin film technology.

This article will cover the basics regarding thin films, including the thermal evaporation technique, thermal deposition systems, material selection and the many industries and applications of thin films.

What Is Thin Film Technology? A Quick Primer on Thin Films

The thin film deposition process involves depositing thin film coatings onto a substrate material. It allows for enhancement of the physical and chemical properties of the source material to fit the application, such as improving optical properties or providing scratch resistance. [1, 4, 6]

Thin film deposition techniques fall under two categories: thermal chemical vapour deposition, or CVD (i.e., atomic layer deposition, atmospheric pressure CVD or spin coating, where a chemical reaction occurs) and physical vapour deposition, or PVD (i.e., sputtering or pulsed laser deposition, where a physical reaction occurs). [4] Common types of thin films include amorphous silicon thin films, thin film polycrystalline silicon, and epitaxial thin films. [1] We’ll explore thin film technologies right now, starting with thermal vapour deposition.

What Is Thermal Vapour Deposition?

The oldest PVD technique is thermal evaporation deposition or thermal deposition. The thermal evaporation process uses vacuum technology to deposit thin films onto the substrate surface in one or more layers, forming protective coatings. [6] Standard methods include resistance-heated evaporation, electron-beam evaporation and molecular beam epitaxy.

Why is it called thermal evaporation? The process involves heating thin solid films in a vacuum to their melting point, producing vapour pressure. Then, the vapour stream moves across the chamber, condensing and forming a thin layer on the target material. [1] Typical uses include creating metallic films on thin film transistors or solar cells.

Thermal evaporation has many advantages compared to other thin film deposition processes. [1]

●   You can change deposition parameters during the production process to customise film properties, e.g., depositing one atomic layer at a time, maintaining target deposition accuracy or ensuring film uniformity.

●   You can use a vast range of target materials to suit various applications.

●   Thermal evaporation works with virtually any material (like those with a low or high refractive index)

●   If the source material is of high purity, the deposited film will also have a high purity level.

●   You can easily control film thickness and the thermal evaporation deposition rate.

●   It’s the least expensive and most straightforward PVD process.

Breakdown of the Thermal Vapour Deposition Process

Would you like to know more about how thermal evaporation can create thin films? The thermal evaporation technique can deposit thin films using various methods, but this section explores a general overview of the process.

Thermal Evaporation Technique: Thin Film Deposition

1) Evaporation

Before starting thin-film deposition by thermal evaporation, you’ll need the deposition and target material, plus the necessary equipment. The first step involves feeding the source material inside a vacuum chamber, typically with the target material (the object upon which you want the film deposited) placed up top and facing downward.

Next, apply sufficient heat to the deposition material, causing evaporation through sublimation or vaporisation. Standard heating methods include a high electric current or electron beam. [1] You can feed the source material onto a hot ceramic evaporator, called a boat, or use a crucible and electric filament. The boat/crucible material should have high thermal conductivity. [2]

2) Conveyance

Post-evaporation, the vapour particles in the vacuum deposition chamber are in flux. The vapour then travels through the vacuum (usually in a straight path) to the target substrate with little interference. [1,6]

3) Condensation and Formation

Next, the vapour condenses and solidifies on the surface of the substrate, resulting in film formation. [1] By repeating the thermal deposition cycle with the same substrate, you can apply layers of thin films grown on the surface. It’s a valuable technique for creating thin film devices and has numerous applications. [6]

Thermal Vapour Deposition Systems and How They Work

Thin films deposited via thermal evaporation require specific equipment and conditions to achieve success. Laboratory work requires a deposition system, a vacuum pump and chamber and other equipment related to the current technique. You’ll also need accessories for film thermal evaporation, like a mechanical shutter, a quartz crystal resonator and vacuum load docks.

The HEX Series deposition system for thin films is suitable for many techniques, including sputtering, thermal evaporation, e-beam evaporation, surface analysis and more. In fact, the platform is entirely modular, allowing for precise customisation and easy upgrades.

Depositing thin films in a lab takes more than just the correct equipment; it also requires optimal conditions. Conditions too similar to normal atmospheric pressure can create inconsistent and poor-quality thin films. As such, a vacuum is necessary to eliminate all other substances except what you want to evaporate. [6]

In a high vacuum environment featuring a long mean free path, the vapour particles move directly to the target with little interference from gaseous impurities. [2] In addition, the absence of a vacuum could cause unintended chemical reactions if other particles are present and collide with the vapour.

What Kind of Materials Are Used in the Thermal Evaporation Process?

A primary advantage of creating thin films by thermal evaporation is the vast selection of suitable source materials, which you’ll select based on the physical, chemical and optical properties you want to enhance. You can use any material, ranging from ceramics and metals to alloys and even organic materials. [6] However, alloys are challenging to evaporate. An alternative method for alloys involves evaporating two sources of the same material simultaneously in different crucibles.

Common materials for depositing thin films via thermal evaporation include the following:

Ceramics

●   Boron nitride

●   Alumina

●   Graphitic carbon

Metals

●   Molybdenum

●   Tantalum

●   Tungsten

●   Gold

●   Silver

●   Nickel

●   Germanium

●   Aluminium

Thermal evaporation also allows you to target a wide variety of substrate materials. You can deposit thin films onto metal, plastics, ceramics, glass, and even paper. [5] With the opportunity to enhance a material’s optical properties, thermal evaporation is often a part of manufacturing ophthalmic lenses and optic equipment. For example, it can increase the optical thickness or create an anti-reflective coating.

Another application for thin films using aluminium as source material is metallised plastic for consumer food packaging. Aluminium thin films are ideal because they form a protective layer to prevent moisture and air infiltration, preserving food freshness. These films are also cheaper and lighter than aluminium foil. [5]

Applications of Thermal Vapour Deposition and Thin Film Technologies

We already discussed enhancing the source material’s optical properties and learned about thin films enhancing optical properties to create optical coatings (i.e., anti-glare) and improve functionality. Thermal evaporation is vital in many more industry applications, like manufacturing thin film batteries, electrical contacts, thin film transistors and liquid crystal displays.

The creation of photovoltaic solar cells and panels also involves thin films and thermal evaporation. [4] Developing renewable energy generation techniques is finally becoming a global priority, and thin film solar cells are a vital piece of the puzzle.

Common Applications for Thin Films by Thermal Evaporation: PV Solar Cells

Standard deposition materials that most manufacturers use to create a thin film solar cell are silicon (Si thin film) and hydrogenated microcrystalline silicon. However, solar cell manufacturers that traditionally deposit Si thin films now use amorphous silicon instead of polycrystalline thin films, primarily because they’re cheaper to fabricate. [4]

Additional benefits of using thin films in the photovoltaic solar cell sector include increased efficiency, reduction in the total cost of deposition materials and a higher level of the absorption coefficient to reduce material thickness. [1]

The scientists who developed thermal evaporation deposition technology first used thin films for decorative coatings, such as those used in costume jewellery creation. However, thermal evaporation now has countless uses, including data storage devices, mirror coatings and electrically conductive films; even NASA uses it to create spacesuits!

Manufacturers Considering Thin Films by Thermal Evaporation

For manufacturers contemplating an investment in thin film deposition by thermal evaporation for their production line, it’s crucial to know the advantages and disadvantages. Remember, thermal evaporation is the least expensive PVD process and the easiest to monitor and control the deposition rate.

Unfortunately, certain alloys are challenging to deposit. Plus, this process requires purchasing a vacuum chamber with a large volume, as well as the entire deposition system. However, there’s no denying the many benefits and practical uses of this technology.

Final Thoughts

Hopefully, we’ve helped answer your question, “What is thin film in physics?” Thermal vapour deposition for thin films is integral for virtually every modern industry, from manufacturing LCDs and making mobile phone screens to enhancing the optical properties of glasses and creating thin film panels for photovoltaic solar cells.

At Korvus Technology, we’re proud to have over 20 years of experience in the thin film sector, developing, fabricating, and constructing deposition systems. As a result, our HEX platform is unparalleled in its customisation, control and flexibility, making it ideal for students, researchers, manufacturers and more.

When you’re ready to incorporate thin film deposition into your organisation, we have all the equipment and accessories you need to get started. Contact Korvus Technology today on +44 1628 201 329 for a quote, or read through our other informative blog posts about thin films and how they can benefit your business.

References

[1] Acosta, E., & Ares, A. E. (2021). Thin Films/Properties and Applications (Revised ed.). https://doi.org/10.5772/intechopen.87838

[2] Cui, B. (2021). Deposition [Slides]. University of Waterloo Electrical and Computer Engineering. https://www.inrf.uci.edu/wordpress/wp-content/uploads/day4-deposition.pdf

[3] Darling, R. B. (2013, December). Physical Vapor Deposition [Slides]. University of Washington Electrical & Computer Engineering College. https://labs.ece.uw.edu/cam/tutorials/PhysicalVaporDeposition.pdf

[4] Garcia-Barrientos, A., Bernal-Ponce, J. L., Plaza-Castillo, J., Cuevas-Salgado, A., Medina-Flores, A., Garcia-Monterrosas, M. S., & Torres-Jacome, A. (2021). Analysis, Synthesis and Characterization of Thin Films of a-Si:H (n-type and p-type) Deposited by PECVD for Solar Cell Applications. Materials, 14(21), 6349. https://doi.org/10.3390/ma14216349

[5] Müller, D. (2021, January 7). Introduction to Vacuum Coating by Thermal Evaporation. Vacuum Science World. Retrieved March 2, 2022, from https://www.vacuumscienceworld.com/blog/vacuum-coating-thermal-evaporation

[6] Oluwatosin Abegunde, O., Titilayo Akinlabi, E., Philip Oladijo, O., Akinlabi, S., & Uchenna Ude, A. (2019). Overview of Thin Film Deposition Techniques. AIMS Materials Science, 6(2), 174–199. https://doi.org/10.3934/matersci.2019.2.174

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