Parameters determining the optical and mechanical properties of thin films by PVD coatings

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The fundamental requirements for optical thin films are as follows:

  • Stable optical properties such as refractive index, transmittance range, and reflectance
  • Good adhesion to the substrate
  • Low optical loss through, for example, low absorption and scattering
  • Low stress or the capability to control such stress
  • Low likelihood for spectral shifts
  • Heat-resistance, inertness, and sufficient durability and wear-resistance

In short, good optical, chemical, and mechanical properties are required for practical thin films. And there are certain specific parameters that influence these properties. The schematic model above illustrates one such example.

Substrate temperature for physical vapor deposition coating

This parameter has a considerable influence on the adhesion and microstructure, and therefore, the final optical properties of the thin film. Most thin films tend to form amorphous surfaces, but high substrate temperatures often crystallize the thin film and some substrates may even tend crystallize the film because of their intrinsic characteristics.

In practice, a substrate temperature of approximately 300KC (depending on the material used) is often used during deposition. However, substrates such as PC and PMMA cannot endure such temperatures, and so, several other techniques have been developed for depositing at low substrate temperatures, for example, IAD, which uses ions to bombard the evaporation materials.

Substrate condition for physical vapor deposition coating

The condition of the substrate surface on which the thin film must be deposited is also a critical issue: no matter how good the deposition conditions and materials are, if the surface of the substrate is contaminated or does not have sufficient roughness, it would be quite difficult to obtain good optical properties.

Moreover, some substrates tend to expel moisture during or after deposition; such substrates should be properly out-gassed before coating.

Evaporation material for physical vapor deposition coating

The coating material used should be specifically designed for thin film deposition. This includes not just the purity of the material but its structure and behavior during deposition as well-all of which affect the final thin film quality.

The so-called "out-gassing" phenomenon may stem from various sources: remnant air inside the evaporation material, oxides and fluorides that dissociate during deposition, remnant moisture inside the chamber, and even gasses released by equipment inside the chamber. The tendency for out-gassing depends not just on the physical properties of the evaporation material but also on its structure.

"Spitting" is a critical problem often encountered during evaporation, and is difficult to prevent. Spitting may be invisible to the naked eye, or may appear as pinholes or other surface defects on the thin film surface. The tendency for spitting depends on the intrinsic properties of the evaporation material, itself, but can be minimized to acceptable levels by using suitable materials and appropriate deposition conditions. Spitting is related to not only to the physical properties of the material but also its form, grain size, density, purity, and the various deposition conditions.

"Hole-drilling" is another problem that can decrease the production efficiency and also sometimes causes defects. When irradiating the material with an e-beam, unexpected deep holes may be drilled through the melted surface of the pellet or tablet. The point of evaporation will change suddenly and this will affect the distribution of the thin film. Unexpected hole-drilling is also one cause for spitting.

When using tablet- or pellet-type evaporation materials, the material itself may crack under the irradiation. This phenomenon often spoils the tablet or pellet, halting the production process.

Deposition rate for physical vapor deposition coating

The evaporation rate significantly determines the microstructure of the deposited thin film, and therefore, its optical properties. When severe spitting occurs during preconditioning, reducing the deposition rate is a common option to regain control. The most suitable rate then depends on the material, substrate, and coater.

Electron beam

Materials with high melting points can be evaporated by an e-beam, but others are compatible with both e-beams and resistive heating. The method used for sweeping the e-beam spot, as well as the spot size, accelerating voltage, and emission of the e-beam, must be optimized for producing high-quality thin films.

Hearth and crucible

The crucible and hearth used must not only be heat-resistant but also inert to the material you are going to heat. Moreover, recent research has shown that the shape and material of the crucible also influence the thin film production process.

Vacuum condition

This parameter is not only determined by the power of the vacuum pump but also by the amount of remnant moisture inside the chamber and the various gases emitted during deposition. However, though the degree of vacuum can be measured, there is another parameter that cannot: for example, each time the coating chamber is vented, the condition of the exhaust system is also changed, along with the resulting contamination level inside the coating chamber. This "quality of vacuum" also influences the thin film quality.