29 Nov 2018

Quality from Finland: excellence to surfaces from thin films

What exactly are thin films and what are they good for?

Thin film technology got a lot of publicity in the spring as Tuomo Suntola, father of the ALD (Atomic Layer Deposition) technique was awarded the one million Euro Millennium technology price. This was the first time the Millenium price was awarded to a Finn as a whole. Go Finland!

What exactly are thin films and what are they good for? Materials interact with the environment by their surface layers. By adding a, smartly selected, layer of another material to the surface it is possible to tailor the interactions, e.g. making the surface immune to rust.

In Industrial applications, thin films typically aim to improved wear and/or corrosion resistance, lower friction or to reduced adhesion of contaminants and dirt. In medical applications, additional requirements are the ability to prevent bacterial and viral adhesion and e.g. the formation of blood clots. With implant coatings, it is necessary to tailor biological response of the coating to suit the application.  Furthermore, essential requirements for implant materials are biocompatibility and blood compatibility.

Thin or even thinner films?

The thickness of the required coating depends on the application. As moisture barrier for flexible electronics, some tens of nanometers may be sufficient. However, at the thicker end of thin e.g. in high stress situations, coatings thickness of tens of micrometers may be required. Of course, it is good to remember that we are still talking about thickness thinner than the human hair. Such coatings are prepared in high vacuum. The growth of the film is such, that atmospheric gas molecules and dust sticking to the growing surfaces have a negative effect on the coating properties and more importantly on the interfacial adhesion.

Applied research at the Universities of Jyväskylä, funded by EU and with industrial support

The ongoing Industrially Functional Surfaces (IFS) -project at Jamk University of Applied Sciences focuses on finding ways of utilizing existing coating technologies in new application areas. The project is turning on its third year and its funding comes from the European Regional Development Fund and eight companies operating in Central Finland. The Academic partner in the project is the Faculty of the Information Technology of the University of Jyväskylä. Their research concentrates on developing fast methods for measuring and simulating the properties of thin films utilizing hyperspectral imaging.

Physics rules

An essential part of the project is to deposit coatings for fast experiments in new applications. The coatings are prepared with a vacuum evaporator. Vacuum evaporation is a physical vapor deposition (PVD) method. In PVD methods, some form of energy vaporizes a solid target material to gas phase. The “vapor” produced flies through the vacuum and hitting the sample forms a coating. Evaporation methods include the cathode arc method, where high voltage is used use to vaporize the target material forming plasma, and laser ablation. In sputtering methods, mechanical energy in the form of energetic ions, is used bombard the target.

Gunning  for a surface

In vacuum evaporators, typically either an electron beam gun or electric current is used to warm up and evaporate the target material. Electron beam is the far more effective way of the two.   With an electron beam, even materials with very high melting points, such as metal oxides (~ 3000°C) can be vaporized. The simpler method, thermal evaporation with electric current, is used with metals and compounds having lower melting points (~1800°C). The vacuum coater at Jamk University of Applied Sciences includes both an electron beam gun and a thermal evaporator. They can be operated simultaneously to produce hybrid and nanocomposite coatings (read= materials mixes with novel properties).

… and additional gadgets

The evaporator is equipped with a rotating crucible (target) carousel. This device enables the preparation of multilayer coatings without breaking the vacuum. If samples were to be introduced to ambient air between depositions, dust particles and air molecules would contaminate the surfaces endangering the interfacial adhesion.

Surfaces get contaminated also in vacuum, with residual gas molecules. To remove these contaminants and to introduce adhesion enhancing functional groups to surfaces the sample is treated with oxygen plasma immediately prior to layer deposition. An essential parameter affecting the structure of the growing film is the substrate temperature. For this reason, the system includes a substrate heating system. The structure of the film, its grain structure, grain size, crystalline defects and impurity concentrations, determines its properties and ultimately its appropriate performance in an application.

Limitations and other ways of doing things

Although the system is versatile and suits well for fast experiments, it has its limitations. In its earlier life, before coming to the University of Applied Sciences, it was used for the coating of optical components. Optical components typically do not have complex surface geometries and, sure enough, the coating method and system are best suited for planar samples. If complex surface geometries are to be deposited with a conformal coating, the first and foremost technique is the millennium price winning ALD.

Another downside of thermal evaporation techniques is that by simply adding thermal energy to the target it is impossible obtain energetic vapor particles.  This limits the use of thermally evaporated coatings in application were the adhesion of thick coating is essential e.g. in high load-bearing tools and artificial hip or knee implants. A way to produce coating suitable for such applications is the cathodic arc method. With this technique it is possible obtain such energies e.g.  to carbon plasmas that upon collision with a sample diamond is produced. The conditions, temperature and pressure, in the collision are locally comparable to conditions in which natural diamond is former under earth’s crust.

Towards a closer co-operation in the field of thin films in Jyväskylä

The Nano-science center and the Accelerator Laboratory of the University of Jyväskylä have a comprehensive battery of devices for the deposition and analysis of thin films. This battery includes a modern ALD device and a helium ion microscope. Professor Timo Sajavaaras group, is one the leading research groups in the world, on ion beam analysis of materials. The groups  1,7 MeV Pelletron accelerator is an excellent tool for the analysis of thin films. The aforementioned infrastructure provides a basis for the continuation of the excellent research on nanotechnology and thin films conducted at University of Jyväskylä during the last decades.

One of the aims of IFS project is to improve and intensify the co-operation between Jamk University of Applied Sciences, University of Jyväskylä and the private sector in the field of surface science and technology. The project has provided a good forum for the establishment of this co-operation. The co-operation utilizes the different strengths of the partners. The University of Jyväskylä provides its research expertise, experience and infrastructure.  The role of the University of Applied Sciences is to bring the research closer to industrial interface via its large contact network.

The co-operation is rising to a new level.  During last spring several ALD –devices have found their new home in Jyväskylä. These devices are being set up with the joint efforts of the universities and planning of research projects utilizing them is ongoing. A special focus of this research co-operation is the roll-to-roll coating of fiber-based materials and the scaling up of this technology to industrial level.

Jamk University of Applied Sciences is focusing its R&D resources on materials science and engineering through the newly established Center of Applied Materials Science (CAMS). Development projects, education and services concentrating on state-of-the-art thin film technology are going to be an essential part of operation of this center.

More information: Esa Alakoski, +358 40 547 0520, firstname.lastname@jamk.fi, JAMK University of Applied Sciences