Reactive Deposition – Enabling Enhanced Thin Film Performance
There are aspects of thin film deposition where the main compound not only dominates the growing film but is also engineered to optimize the difference between the evaporation charge and the thin film itself. In comparison to metal thin films, dielectric compounds degrade during evaporation. Or, they can seriously challenge sputtering processes with variable conductivity, mechanical toughness, particles and damaging arcs. Whether to compensate for decomposing evaporation material or to create the highest quality full compound thin films, the reactive deposition process enables high quality performance coatings.
Figure 1 - Classical E-beam Deposition Diagram
In the first and perhaps lowest energy and lowest film density case, the material of interest is an oxide compound. In the classical e-beam deposition (Figure 1), the high-energy beam is directed onto the evaporation material held inside a water-cooled crucible. While often used for easier clean up, liners also moderate the heat flow from the starter charge or granules. This can contribute to rather exotic competing cooling challenges during the iterative heating and cooling of a typical high and low index optical design. For example, the most prolific high index material in the Visible and Near Infrared (VIS-NIR) region is Titanium Dioxide (TIO2). Figure 2 shows the complexity presented by TIO2 competing for dominance during evaporation and cool-down in the pocket.
Figure 2 - Ti - TiO2 Binary Phase Diagram (Huilian Cao)
The diagram shows that during and after typical evaporation, the process must compensate for an increasingly complex stew of sub-oxides based on proximity to evaporation and heat sink. Figure 3 shows an example of characteristics of the most stable sub-oxides which form and compete in the pocket. They are also examples of individually engineered coating materials which are meant to enable a stable and repeatable deposition process with variable success.
Figure 3 - Table of Basic Titanium Oxides
During evaporation, temperatures exceed 2500 C – the boiling point of the most volatile stable sub-phase. Because any missing oxygen site in the growing film will contribute to absorption, a background pressure of oxygen is added at some point between the substrate and the source. This is optimized with some external energy, such as substrate heaters and/or some type of ion assistance. The latter can also densify the film and maximize the index of refraction of the growing film.
As one can imagine, the process becomes exceedingly challenging for very long and complex optical designs. Changes in density, conductivity and shifting melting points all contribute to “spit” and require strong process engineering. While TiXOY is the most challenging example, similar practices are necessary for the other oxides HfO2, Nb2O5, Ta2O5, Al2O3 and even SiO2. While the materials industry has made strides in creating stable sub-phases over the years that certainly help, such as c-Ti3O5 and fused SiO2, there remain gaps in stability, rate and performance that only higher order reactive processes can address.
There is a more sophisticated reactive process that is completely dependent on ion assistance. It uses chargers, pellets, shavings or granules of the pure metal in the pocket of the e-beam gun. In this case, heat transfer (to crucible and to substrate), carbon from principle melting and gas injection remain key process considerations. Engineers can maximize film properties, realize higher rates and offer repeatability through a fully reactive non-sputtering approach.
For UV optical thin films, any unreacted metal centers or missing oxygen sites are breakdown points for high performance protected mirrors, antireflection coatings and filters. Similar to TiO2, - but to a lesser extent - HfO2 reduces to a similarly complex sub-oxide state. Various studies (like that in Figure 4) show that HfO2 films improved with increased substrate temperature and post-deposition recrystallization. This implies that for different applications, different degrees of reactive deposition can balance or optimize mix of substrate temperature and post deposition annealing. It can also have a direct influence on long-term stability of the coating.
Figure 4 - HfO2 Thin Films at Increasing Substrate Temperature with Annealing (M. Ramzan)
Coupled with UV-grade fused SiO2 with makeup oxygen, the metal reactive HfO2 process currently challenges some sputtered reactive processes. It reflects a cost point less than that of using the full compound due to its high level of required pocket maintenance. Perhaps more importantly, for semiconductor gate dielectrics than for optics, Hafnium Oxide has four standard crystalline phases (monoclinic, tetragonal, cubic and orthorhombic), each having different dielectric constants. For optical applications, the ability to control the amorphous-to-crystalline character can lead to increasingly sophisticated choices for specific coating platform applications.