Introduction to Vacuum Coating or Thin Film Technology
November 9, 2020
Thin Films are layers of material on surfaces with a thickness well below a nanometer and up to a micrometer. There are multiple reasons to coat a device with a thin film. Just a few examples include:
Protective films to prevent corrosion
Decorative layers on jewelry or bathroom fittings
Wear protection on tools
Multiple layers to improve optical properties of ophthalmic lenses
Semiconductor or solar cell production
Head-up displays in automotive industry
Mirrors of reflector lamps
Packaging foils for freshness preservation
Architectural glass for thermal insulation
This list is not exhaustive, and new applications are continually emerging
Today we differentiate the various techniques used to deposit a thin film layer on a substrate into Physical Vapour Deposition (PVD) or Chemical Layer Deposition (CVD). Vacuum plays an essential role in PVD which requires high vacuum. Vacuum also forms part of most CVD applications.
The most mature technology is thermal evaporation. A material is melted and evaporated at high temperatures and the vapour is deposited on the target. The temperatures required can be taken from the graph pictured below.
Evaporation can be achieved by heating wires electrically or depositing it in crucibles of material with a significantly higher melting point. Another way is to melt it by using an electron beam.
In both cases a high vacuum of 10-07 to 10-05 mbar is required during the coating process, depending on the size of the vacuum chamber and the required quality of the layer. The reasons are:
To ensure a mean free path of evaporated atoms that’s much longer than the distance from source to target. This ensures that the atoms arrive unscattered by residual gas molecules.
To provide a clean surfaces. Otherwise the evaporated atoms would not stick well and would form an unstable layer.
Another way to coat samples is by sputtering. Sputter deposition uses a target material which is bombarded by ions accelerated out of a plasma. The most commonly used plasma gas is Argon. The Argon ions sputter atoms of target material which coat the substrate. Due to the higher energy of the sputtered atoms they stick better than if applied through thermal evaporation. However, sputter deposition requires a more extensive system engineering operating under vacuum conditions. While a sputtering process that uses Argon plasma is running at pressures above 5 x 10-04 (and up to 1 x 10-02) mbar, an ultimate pressure in the 10-06 mbar range is required for cleaning and to ensure the purity of each layer.