In our previous blog post, Introduction to Vacuum Coating Technology, we shared various PVD (Physical Vapor Deposition) methods and showed typical products being coated using vacuum technologies. Thin films can be adhered to metal, glass, plastics, ceramics, or paper. In this blog post we will focus on Thermal Evaporation techniques.
What is thermal evaporation?
Thermal evaporation is the oldest vacuum coating technology. A material is melted and evaporated at high temperatures, and the vapour is deposited on the substrate. Image 1 illustrates the temperatures required for commonly used materials.
Image 1: Saturation vapour pressure of different metals (source: Fundamentals of Vacuum Technology, Leybold print 0.200.02)
Two methods of vacuum coating by Thermal Evaporation
The material must be heated to a temperature that results in substantial high vapour pressure, which limits the use of high-melting material and the choice of the container. Evaporation can be achieved by heating wires electrically or depositing them in electrically conductive crucibles made of a material with a significantly higher melting point. Oxides can be evaporated out of boat-shaped evaporators. Image 2 shows a selection of various thermal evaporators.
Image 2: Examples of evaporators (source: Fundamentals of Vacuum Technology, Leybold print 0.200.02)
The required base pressure in the coating device is 10-07–10-05 mbar depending on the required quality of the layer. This is to:
- Ensure a mean free path of evaporated atoms much longer than the distance from source to substrate. This ensures atoms arrive unscattered by residual gas molecules.
- Provide a clean surface, otherwise the evaporated atoms won’t stick well, forming an unstable layer.
The material can also be melted by an electron beam. The materials then can be evaporated at higher temperatures, enabling higher evaporation rates and the ability to melt oxide materials. Water-cooled crucibles ensure that evaporated crucible material won’t contaminate the films. The evaporation rate can be controlled by varying the electron beam power. The electron beam is deflected by 270 degrees via a magnet, as seen in Image 3. By wobbling the beam, the molten mass can be kept at homogenous temperature and used to full capacity.
Keep in mind that the start-up investment in this type of device is slightly higher, since a high-voltage supply and cooling water feedthroughs are required.
Image 3: The path of a deflected electron beam
Image 4: Electron Beam Evaporators 10 KW (left) and 6 KW (right) by Beamtec
Vacuum systems for Thermal Evaporation
Generally, vacuum systems for thermal evaporators need an hour or less of evacuation time from atmosphere to base pressure 10-06 mbar.
Most units today use turbomolecular pumps in the 300–1000 l/s range, backed by either double-stage rotary vane pumps, dry, scroll or multistage roots pumps. Ideally, these turbomolecular pumps are mounted horizontally. This prevents debris (flitters from coatings, filaments, tiny screws etc.) from falling into the pump. Most systems don’t use valves in front of the turbomolecular pumps.
A shutter — manually or electrically/pneumatically actuated — is located above the evaporator. This avoids coating the complete chamber continuously while the evaporated material is hot, warming up or cooling down. This also ensures reproducible layers can be produced by fixing the time. Most units also use a thin film monitor to measure (and control) the thickness of the coating layer.
A typical laboratory coating device for film research is shown in Image 5.
Image 5: A laboratory coating unit with evaporator, and a chamber size of 250 mm (courtesy of Leybold, Cologne)
Common applications of Thermal Evaporation
Thermal evaporation is often used to coat optics and ophthalmic lenses. Multiple layers are evaporated to improve the properties of the lenses. Among these are anti-reflective layers, hard coatings, protection against infrared or ultraviolet light, sun protection and mirror coatings. The vacuum chambers have diameters up to 1500 mm — each holding up to several hundred lenses, depending on their diameter. The lenses are fixed in specially designed rotable calottes to ensure uniform thin films on all the products in one batch. The vacuum system therefore consists of a larger turbomolecular pump in the 2000 /s class or a cryopump combined with a small roots blower fore vacuum system. Advanced literature can be found on the websites of the system manufacturers such as the Bühler Group, or lens suppliers such as Zeiss.
Larger coating machines typically create web coating for packaging foils. Films like aluminium are applied to plastic foils in “roll-to-roll” web coaters. These thin films create a protective barrier against air and moisture which prolongs the freshness and shelf life of consumer goods.
Image 6: Examples of vacuum-coated foils (source: A brochure on 'Roll-to-Roll Web Coating, by Applied Materials, Alzenau, Germany)
In these production machines with high foil throughput (several metres per second!) the gas flow into the vacuum system is immense! The large foil surfaces to be coated generate large amounts of degassing. Generally, the vacuum systems consist of large oil diffusion pumps for pumping the air supported by cold panels to condense the water vapour. These cold panels are cooled by cryogenic refrigerators, or cryochillers, sometimes called “Polycold machines”.
Special cryochillers are designed to cool to temperatures of 110 K. This provides up to 200,000 l/s of pumping speed for water vapour inside the chamber. Fore-vacuum is generated by a roots blower systems (see left on Image 7).
Image 7: Web coater (courtesy of Applied Materials, Alzenau, Germany)
Image 8 shows an example of costume jewellery. The optical effects are generated by special coatings adhered via thermal evaporation.
Image 8: Examples of coated jewellery (see more examples at www.swarovski.com)
As you can imagine, the list of such applications can go on. This blog post series is just an introduction to get you started on this engaging field and vacuum systems application. In our next blog post we’ll continue our journey and introduce sputter technology and some of its typical applications.
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