An Introduction to Thin Films and the Deposition Process

Thin films are an inseparable part of modern technology. When deposited on the surface of an object, thin film coatings can alter its qualities, for example, increasing durability, changing electric conductivity, or improving optical properties.

But what is thin film in physics? Thin films are layers of material ranging in thickness from sub-nanometers to microns. Thin film deposition techniques fall under the broad categories of physical vapour deposition and chemical vapour deposition.

The history of thin solid films begins in antiquity [1], with metallic films (usually gold platings) used on various artefacts for decorative and protective purposes. Today, many industries rely on precise atomic layer deposition to produce high-purity thin films.

Industrial applications include thin film solar cells, optical lenses with a high refractive index, anti-reflective optical coatings, semiconductor devices, light crystal displays, and more.

In this article, you’ll find out more about thin films and the thin film deposition process.

Thin Films and What They’re Used For

Thin films are coatings deposited upon a substrate material (like a silicon wafer, electronic part, or optical lens). Depending on the material and purpose, the film thickness can be anything from one atomic layer to several micrometres.

Different applications will call for distinct types of thin films. These include:

• AR/HR (antireflective or high-reflectivity) coatings alter the optical properties of a material, for example by filtering visible light or deflecting a light beam. Common uses for these thin films include electronic displays, low optical thickness lenses, and output mirrors.
•  TCO (transparent conductive oxide) coatings are electrically conductive, transparent coatings with many uses in touchscreens, LCDs, and photovoltaics. One example is hydrogenated microcrystalline silicon-germanium thin films in solar cells.
• DLC (diamond-like carbon) thin films increase the coated object’s hardness and scratch resistance. These electrically resistive protective coatings improve the lifespan and durability of microelectronics, medical devices, sensors, and more.
• Biocompatible hard coatings protect implanted devices, biosensors, and artificial limbs to improve the devices’ longevity and prevent contamination. Biocompatible thin films typically use compounds like titanium nitride and chrome nitride.

In this article, we discuss two main deposition techniques: physical vapour deposition (PVD) and chemical vapour deposition (CVD). These methods have shown significant advantages over other techniques, like dip or spin coating, in most applications.

Physical vapour deposition involves vaporising materials in a low-pressure chamber to deposit them upon a substrate. Films deposited by PVD can create highly durable, corrosion-resistant coatings that tolerate exposure to high temperatures.

Chemical vapour deposition typically involves chemical reactions between precursors, which combine to create a thin film coating on the substrate surface. CVD may involve several advanced techniques, such as reactive ion etching.

What is Thin Film Deposition?

The thin film deposition process may vary depending on the techniques used, but all the methods we discuss involve placing a thin layer of deposited film on the substrate surface within a vacuum chamber.

The first creation of metal films by chemical vapour deposition took place in the mid-17th century. Experiments in oxide deposition began circa 1760, while sputter deposition took its first steps in the 1850s [2].

By the 1930s, manufacturers already used early-stage thin films for high-reflectivity mirrors. The 1960s technologies of ultra-high vacuum and in situ electron microscopy enabled the creation of more advanced, pure, and uniform thin films. In 1970, Peter J. Clarke launched the first sputter gun that created atomic-scale films by using ion and electron collisions.

Advanced technologies, such as atomic resolution surface imaging, allowed the progress of the thin film industry as we know it today. Methods for depositing thin films, such as sputtering-based methods, continue to grow and develop into new applications.

Physical vapour deposition (PVD), which involves the vaporisation and depositing of solid material on a substrate, encompasses a range of methods from a basic evaporation process to magnetron sputtering and pulsed laser deposition [3].

Many advanced thin-film devices, like the HEX Series by Korvus Technology, use physical deposition methods. PVD, in particular sputtering, allows manufacturers to create highly precise and uniform thin films.

In contrast, chemical vapour deposition relies on a reaction between precursors in the deposition chamber. CVD has several common uses, like creating Si thin films. One drawback of CVD is the need for extra-high temperatures to incite the process.

How Does Thin Film Deposition Work?

At this point, you still might be wondering what is thin-film technology. The production process of thin films depends on the technique used, e.g. evaporation, sputtering, ion beam deposition, or chemical vapour deposition. Let’s summarize its several methods below to answer that question.

Thermal Evaporation

Thermal evaporation deposition occurs in a chamber under the pressure of 10^(-6) to 10^(-5) mbar. A crucible that connects to a high-current source holds the target material.

Here are the basic steps for thermal evaporation deposition:

1. As the target material heats, it releases vapour particles that create vapour pressure.
2. The vapour stream crosses the chamber and coating particles attach to the substrate.
3. Throughout the process, the vacuum pump keeps working to maintain a high-vacuum environment and ensure a free path for the film particles.

Chemical Vapour Deposition

Under conditions of heat and low atmospheric pressure, CVD creates thin films using the following steps:

1. Precursors to the desired coating undergo heat vaporisation and flow into the reaction chamber through a flow-controlled inlet.
2. A chemical reaction occurs between the precursors, which form the coating molecules.
3. The gas molecules condense on the much cooler substrate, just like water vapour on a cold drinking glass.

Magnetron Sputtering

Sputtering is a physical phenomenon leveraged by thin film deposition to expel microscopic particles of solid materials from their surface. This ejection is achieved by attacking the solid material with energetic particles. The particles are then deposited in the target materials forming the thin film layer.

Like other deposition methods, sputtering takes place in a chamber with a vacuum pump constantly working to remove air. The vacuum is a crucial condition for this process, which occurs naturally in outer space.

Magnetron sputtering involves the following steps:

1. An inert gas, usually argon, flows continuously into the chamber. Magnet arrays within the cylindrical rotating target generate a magnetic field.
2. High voltage creates a gaseous plasma near the target’s magnetic field. The plasma contains argon gas atoms, argon ions, and free electrons. Electrons that hit argon atoms continuously create positively charged ions.
3. The negatively charged sputtering target attracts the positively charged ions. As the ions hit the target, they eject atoms.
4. The ejected atoms settle on the substrate’s surface and create a film.

Molecular Beam Epitaxy (MBE)

MBE, a technique for growing epitaxial films, requires ultra-high vacuum conditions and is more complex compared to other thin film deposition processes [4].

This method strictly controls deposition parameters and can create highly pure polycrystalline thin films, a fundamental requirement of many nanotechnologies. In some cases, the epitaxial film consists of the same material as the substrate but is purer and with a lower defect rate.

The creation of epitaxial thin films by MBE includes the following stages:

1. The material to be deposited absorbs heat in the effusion chamber.
2. The target material atoms or molecules leave the effusion cell in a beam and move towards the substrate. The particles move in a highly controlled beam, which ensures a high level of control over film formation.
3. The particles settle on the substrate in precise monolayers that preserve the crystalline structure.

The recommended thin-film creation method in each case will depend on the application, the target and substrate materials, and the desired film uniformity and physical and chemical properties, such as high thermal conductivity or corrosion resistance.

For example, chemical vapour deposition works well for thin-film polycrystalline silicon often used in integrated circuits, while the production of coatings to improve optical properties often works best with sputtering.

The Benefits of Thin Film Deposition for Scaling Production and Technological Progress

Thin films grown by both PVD and CVD methods form the cornerstone of many industries today. Thin film technology has countless applications, including semiconductors, medical devices, fibre lasers, LED displays and other consumer electronics, and more.

Here are just a few examples of advanced thin-film applications:

• Solar cells. A thin film solar cell owes its energy generation properties to the layer of Silicium thin film (also known as “Si Thin Films”) deposited on a glass or metal substrate. Second-generation Si thin film solar cells are flexible and lighter than crystalline silicon cells, which enables applications like photovoltaic glazing [5]. Amorphous silicon, a non-crystalline form of silicon, is an affordable substrate that helps reduce the cost and scale the production of thin film panels.
• Thin film transistors. Thin film transistors are a vital component of liquid crystal displays. This type of transistor is inexpensive, energy-efficient, and offers better response time.
• Thin film batteries. A thin film battery is more efficient, charges faster, and lasts longer compared to a conventional Li-ion battery. This type of battery can improve the performance of medical products, implants, smart cards, and green energy storage banks.

The thin-film industry keeps evolving. Researchers never stop looking for ways to improve the purity, accuracy, optical properties, and other qualities of thin films. Meanwhile, industry experts work towards reducing costs and overcoming scaling obstacles in thin film production.

In order to implement thin film deposition in your production chain, you’ll need to deploy a thin film deposition system.

Korvus Technology is a reliable provider of modular deposition systems and other thin-film accessories and components for laboratories and manufacturers.

Thin Film Technologies: Final Words

The world as we know it today relies on thin films. Thin films are everywhere, from a bag of chips to the device you are using to read this article.

Often invisible, thin films are the quiet superheroes of modern industry, with a range of uses from prolonging shelf life to improving optical properties of lenses. Thin films may consist of metals, oxides, and organic materials.

The technologies for making thin films vary depending on the application and industry demands. If you are interested in learning more about thin films, we invite you to browse other articles at Korvus Technology.

References

[1] Greene, J.E. Review article: Tracing the 5000-year recorded history of inorganic thin films from ∼3000 BC to the early 1900s AD. Applied Physics Reviews, Volume 1, Issue 4, 22 July 2014.

[2] Greene, J. E. Review Article: Tracing the recorded history of thin-film sputter deposition: From the 1800s to 2017. Journal of Vacuum Science & Technology, Volume 35, Issue 5, 10 May 2017.

[3] Baptista, A., et al. On the Physical Vapour Deposition (PVD): Evolution of Magnetron Sputtering Processes for Industrial Applications. Procedia Manufacturing, Volume 17, 2018, Pages 746-757.

[4] Nurzal, N. The Growth of Hexagonal Boron Nitride Quantum Dots on Polycrystalline Nickel Films by Plasma-Assisted Molecular Beam Epitaxy. Crystals, Volume 12, Issue 3, 3 March 2022.

[5] Luceño-Sánchez, J.A., et al. Materials for Photovoltaics: State of Art and Recent Developments. International Journal of Molecular Sciences. Volume 20, Issue 4, 23 February 2019.