Density and microstructure of thin films depends on a number of factors, but primarily on the energy of the species (adatoms) incident on the substrate. Microstructure of the thin film includes the crystalline structure, morphology, density, defects and inclusions, voids and grain structure. Microstructure can strongly influence physical properties such as strength, toughness, ductility, hardness, corrosion resistance, high/low temperature behavior or wear resistance. Electrical properties also depend on microstructure. As we described in an earlier Blog, adatom energies range from a few tenths of an eV for thermal evaporation to tenths to hundreds of eV for sputtering. Adhesion of the thin film to the substrate also critically depends on energy of the species. Columns and articles on how vacuum affects thin film performance, as well as vacuum equipment, can be found in virtually every issue of VT&C.
Because they are nonequilibrium processes, each atomistic deposition process has the potential to deposit materials that vary significantly from the source material in composition, microstructure, mechanical properties, tribological properties, and physical properties (conductivity, optical properties, etc.), depending on deposition conditions. The resulting films may have high intrinsic stress, high concentration of point defects, extremely fine grain size, highly oriented microstructures, metastable phases, incorporated impurities, and micro to macro porosity. These properties have significant influence on the physical, corrosion resistance, and mechanical and tribological properties of the deposited film .
Microstructure can be varied over wide ranges depending on deposition process and conditions. To fully understand thin film microstructure it will be instructive to first elucidate film growth processes: how the film evolves during growth and the major factors that affect nucleation, growth, and microstructure. Defects are also an integral aspect that must be understood since they can severely degrade film performance. Microstructure of a thin film evolves as it nucleates and grows on the substrate surface. How a thin film grows is not a trivial process, and depends on a number of factors, including deposition process, substrate surface quality, temperature, energy of incident particles, angle of incidence, etc. These factors are all interconnected, and as a result, it is hard to segregate the dependence on vacuum. Additionally, film growth mechanisms can be complicated by the fact that not only is the species from the source deposited, but additional processes such as resputtering, reflected neutrals, shadowing and ion implantation can occur.
The focus here will be on role of energy of the adatoms and impurities resulting from the vacuum during deposition (note however that vacuum is only one of many factors that determine film quality). As discussed earlier, atoms and molecules ejected from the source collide with and scatter off gas atoms (inert and reactive) in the region between source and substrate. How the film grows on the substrate depends critically on the energy is has when it reaches the substrate and how this energy is utilized on the surface of the substrate. We see for sputtering that inert gas (Ar, Kr, etc.) pressure influences film structure through indirect mechanisms such as thermalization of deposited atoms with increased pressure and increased component of atom flux due to gas scattering [2-7]. Reduction of gas pressure results in more energetic particle bombardment which densifies the film, while the opposite is true for increased pressure. The structure zone model (SZM) attempts to quantify this relationship, and differs for each deposition process. Basic SZMs used to predict film microstructure depend on chamber pressure, substrate temperature, deposition rate and substrate placement .
While beyond the scope of this Blog, SZMs have been revised and refined over the last two decades, and incorporate ion bombardment, grain structure, film thickness effects, substrate roughness, angle of incidence, and external substrate bias. Ion bombardment effects are important both in sputtering and evaporation. However, ion bombardment is an integral part of sputtering (DC and RF) processes but must be introduced externally into evaporation processes. Here the floating potential replaces chamber pressure. This makes sense since particle energy can be related to both pressure and substrate bias (expect more energetic bombardment at low pressures and higher bias: VS ~ 1/P). Zone T is widened compared to zone 1, which can be interpreted as increased ion bombardment enhancing adatom mobility similar to higher substrate temperatures.
How does film microstructure affect mechanical properties of thin films?
- It is generally desirable for the film to have high density, low porosity, and tightly bound grain structure
- Columnar structure is a strong function of deposition temperature
- A highly columnar or porous film may not be as hard or corrosion resistant as a dense, small grained and defect free film
- Stable optical properties are observed for dense, small grained, or amorphous films
- Structural defects tend to increase with increased thickness
- Columnar structure is required for a number of applications, such as piezoelectric and ferroelectric and magnetic thin films
- Compressive stress generally increases microhardness and microfracture toughness
- Interfacial phases and associated microporosity and contaminants all degrade coating adhesion
Unless there is a highly specialized application that requires porous films, special film microstructure or films with large columnar structure, highest quality films are dense, tightly packed and smooth. Thus we conclude that virtually all properties of PVD films depend on vacuum (pressure) at some level.
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