The development of physical vapour deposition (PVD) represented a truly remarkable scientific advancement in the scientific industry as it enabled the deposition of thin films onto a range of materials, giving way to essential products ranging from medical implants to LCD screens.

Co-evaporation of materials in PVD goes a step beyond this technology, applying more than one coating material to a substrate at a time. It opens the door to a wide range of new compositions with impressive advanced qualities not previously possible within PVD processes. Learn how co-evaporation could benefit your manufacturing processes and how Korvus Technology makes co-evaporation of materials in PVD simple and accessible. 

What Is Co-Evaporation?

Co-evaporation is the process of heating and evaporating materials from several sources with the idea of depositing multiple materials onto a single substrate. In contrast, typical physical vapour deposition involves depositing just one material at a time. By combining the vapour states of multiple materials, new compositions can be achieved that provide more specialised qualities and features for thin film applications. 

Co-evaporation is one of three main evaporation techniques for producing compounds, with the other two being direct evaporation and reactive evaporation. It’s a type of co-deposition within thermal evaporation and has a range of use cases. Co-evaporation works like this:

• Each material is heated separately in a high vacuum environment.
• The materials melt and evaporate.
• The vapour phase of each material reacts with the other.
• The vapour is deposited onto the substrate.
• The sample can be heated or rotated depending on the desired uniformity of the thin film.

Advantages Over Single Material Evaporation

Single and co-evaporation of materials in PVD both have their place within materials manufacturing. However, co-evaporation is often the preferred choice for achieving uniform film deposition with multiple depositing materials. 

Conducting co-evaporation with radiative heating methods, such as E-beam evaporation, allows manufacturers to achieve a high degree of uniformity within thin films. Single material evaporation poses challenges for achieving complete uniformity, especially with large or complex substrates. Many developers prefer sputtering over thermal evaporation when uniformity is necessary. However, co-evaporation overcomes these challenges, producing a more even and consistent coating. 

The Co-Evaporation Process in PVD

Co-evaporation of materials in PVD follows a relatively straightforward process that your company can achieve with the right tools and technology. Several key components ensure the uniformity and tactile strength of the deposited coating. 

The co-evaporation process consists of three main stages. The best PVD technology handles each of these stages for you with little intervention. 

Heating Materials Separately

The first step in co-evaporating and depositing materials onto a substrate is to heat the materials separately within a high vacuum chamber. The vacuum environment is key, as it allows manufacturers to contain the vapour phase of the source materials and enable the materials to react with each other to create the unique composition for coating. 

PVD devices typically use resistive filament boats or wire baskets to contain the source material, though some use an electron beam to evaporate the material from a crucible. 

Vapour Phase Reactions

After heating and evaporating the source material separately, the vapour phase of each material reacts with the others, creating the compound that will make up the coating. The two or more vapours physically combine, while they chemically react to create a new composite. It is then this composite that will condense onto the substrate and form your thin film.

Substrate Preparation and Handling

Preparing a substrate for physical vapour deposition usually involves pre-cleaning it to remove native organics. Sometimes, etching the surface is necessary to create better adhesion. 

To ensure complete uniformity in the PVD coating, many co-evaporation devices include features to heat and rotate the substrate material. Technicians can also manually rotate the substrate on one or two axes within the deposition chamber to promote optimal consistency. 

Equipment and Technology for Co-Evaporation

Using the right equipment and technology for the co-evaporation of materials in PVD ensures successful co-evaporation. These tools play critical roles in the process: 

High Vacuum Environment

Maintaining a high vacuum environment — i.e., one that maintains a pressure between 10-7 and 10-3 millibars — allows for the vapour stage of the source materials to be contained within the environment after evaporation. PVD technology creates vacuum chambers by removing air molecules from the space using a vacuum pump or by reducing the pressure with a fast flow of fluid. 

Heating Sources: Boats and Baskets

Co-evaporation requires reliable heating sources that effectively heat the source materials to evaporation. Two common sources include boats and baskets. 

Filament boats consist of refractory metals like tungsten, molybdenum, and tantalum. They come in several designs that serve as vessels to contain the evaporated solid source materials. 

Wire baskets, commonly made from tungsten, can be heated to a high temperature and allow for the even distribution of the deposited material. 

Both of these vessels consist of metals with high melting points and low vapour pressure to maintain their shape and conduct heat within a vacuum environment. 

Enhancing Uniformity: Heating and Rotation 

Manufacturers use several techniques within the co-evaporation process to enhance uniformity. Heating the sample could allow for better adhesion of the evaporated, heated source materials. Additionally, rotating the sample on one or two axes in the deposition chamber facilitates an even thickness across the film. Achieving a uniform finish ensures that the thin film produces even conductivity across the entire surface. 

Features of E-Beam Evaporation

E-beam evaporation presents an ideal heat source within the co-evaporation of materials in PVD. It allows for greater precision and control than other heat sources.

This technology uses a high-power electron beam to evaporate the source material under a high vacuum. With E-beam evaporation, developers can evaporate materials that are challenging to process with standard resistive thermal evaporation, like gold or ceramics. Using this technology is just another way to ensure the uniformity of the thin film.

TAU E-Beam Evaporation Source

Traditionally, e-beam technology uses magnets to focus electrons and form a beam. But the inclusion of magnets sometimes disrupts metal heat sources. 

Korvus Technology’s TAU E-beam evaporation system uses a “mini” source without beam-bending magnets. It utilises an enclosed head that allows for low substrate temperatures, reducing the thermal energy required in the coating process. This technique allows for precise, direct heating and evaporation ideal for the co-evaporation of multiple source materials into a single thin film.

References

[1] Wolfe, Douglas E, Jogender Singh, &  Krishnan Narasimhan. (2002). Synthesis of titanium carbide/chromium carbide multilayers by the co-evaporation of multiple ingots by electron beam physical vapor deposition. Surface and Coatings Technology, 160:2-3, pp. 206-218. https://doi.org/10.1016/S0257-8972(02)00404-8

[2] Hurtado Morales, Mikel Fernando. (2013). 

[3] Singh, J. and D. E. Wolf. (2005). Review Nano and macro-structured component fabrication by electron beam-physical vapor deposition (EB-PVD). Journal of Materials Science, 40, pp. 1-26.  https://doi.org/10.1007/s10853-005-5682-5

[4] Tesfamichael, T, Nunzio Motta, Thor Bostrom, & J.M. Bell. Development of porous metal oxide thin films by co-evaporation. Applied Surface Science, 253:11, pp. 4853-4859. https://doi.org/10.1016/j.apsusc.2006.10.065

[5] Momblona, Cristina, et al. Co-evaporation as an optimal technique towards compact methylammonium bismuth iodide layers. Scientific Reports, 10.