Reactive and Non-Reactive Deposition:
Comparison of PVD Processes
Physical vapor deposition (PVD) processes are the most commonly used deposition techniques for the widest variety of materials and coating applications. The process for transforming solid materials into vapors of their atoms requires the addition of energy in order to release atoms from their bonds. The energy source can be thermal, such as that supplied by electron-beam or resistance-heated sources. Or alternatively, the source can be non-thermal vaporization, such as that achieved by high kinetic energy impacting species in sputtering techniques. Depending on the material used, (i.e. metals, metal compounds of oxides or fluorides, or alloys and mixtures), and the desired layer composition, vaporization energy will range from < 1eV to 10’s of eV. Vaporization energy is a key parameter in PVD as it relates to reactive vs non-reactive processes.
In the case of the deposition of pure metals, a non-reactive vaporization process is required. The vaporization of compounds results in partial decomposition of the starting compound. When depositing optical thin-film layers, incomplete stoichiometric composition such as sub-oxidation produces absorption and refractive index differences. The presence of absorption is more severe in the UV (ultra violet) than in the IR (infrared) and for coatings that are intended for high laser damage threshold applications. When compounds are to be deposited, reactive processes must be involved. A few common exceptions exist: SiO2, SiO, and Al2O3 can be deposited in a non-reactive environment with acceptable optical and mechanical properties.
With the introduction of reactive processes that function to completely oxidize metal-oxide compound layers, the quality of coatings produced by PVD processes is currently state-of-the-art. The resulting improved coating technology incorporates ions in an energetic plasma environment. Also, uses accelerated ions to deliver energy and to influence film layer growth morphology. Previous Coating Materials News issues have discussed these processes in relation to specific materials; the following is an overview of the technology.
Reactive Thermal Deposition
High substrate temperature promotes oxidation or nitridation and densification for many compounds. An alternate to the high-temperature process is ion-assisted deposition (IAD). IAD employs an ion gun to bombard the growing metal-oxide film with a mixture of Ar+ and reactive O+ or N+ ions, thereby supplying energy to complete the reaction as well as compact the layer and increase adhesion. Typical ion energies are 50 to 70eV. In plasma ion-assisted deposition (PIAD), an energetic plasma consisting of electrons, Ar+ and reactive O+ or N+ ions, fills the coating chamber and promotes complete reactivity. Highly reactive ionized species condense on the substrate. PIAD is also used with resistance-heated and E-beam processes to produce dense, transparent, adhesive coatings for severe environments including high laser damage threshold coatings.
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