Thin Film Deposition Technologies: Introduction to Thin Films and Processes

Thin films are deposited by a wide variety of processes. However, we begin this blog first with the advantages of using thin films and deposition processes applications. Thin film is the general term used for coatings that are used to modify and increase the functionality of a bulk surface or substrate. Typical thicknesses range from 50 nm to 10 μm. They are used to protect surfaces from wear, improve lubricity, improve corrosion and chemical resistance, modify optical and electrical properties and provide a barrier to gas penetration. In many cases thin films do not affect the bulk properties of the substrate material. They can, however, totally change the optical, electrical transport, and thermal properties of a surface or substrate, in addition to providing an enhanced degree of surface protection. Thin film deposition technology and the science have progressed rapidly in the direction of engineered thin film coatings and surface engineering [1]. Plasmas are used more extensively. Accordingly, advanced thin film deposition processes have been developed and new technologies have been adapted to conventional deposition processes. The market and applications for thin film coatings have also increased astronomically, particularly in the biomedical, display and energy fields.

Thin films have distinct advantages over bulk materials. Because most processes used to deposit thin films are nonequilibrium in nature, the composition of thin films is not constrained by metallurgical phase diagrams. Crystalline phase composition can also be varied to certain extent by deposition conditions and plasma enhancement. Virtually every property of the thin film depends on and can be modified by the deposition process and not all processes produce materials with the same properties. Microstructure, surface morphology, tribological, electrical, and optical properties of the thin film are all controlled by the deposition process. A single material can be used in several different applications and technologies, and the optimum properties for each application may depend on the deposition process used. Since not all deposit technologies yield the same properties or microstructures, the deposition process must be chosen to fit the required properties and application. For example, diamond-like carbon (DLC) films are used to reduce the coefficient of friction of a surface and improve wear resistance, but they are also used in infrared optical and electronic devices. Titanium dioxide (TiO2) is probably the most important and widely used thin film optical material and is also used in photocatalytic devices and self-cleaning windows, and may have important applications in hydrogen production. Zinc oxide (ZnO) has an excellent piezoelectric properties but is also used as a transparent conductive coating and spintronics applications. Silicon nitride (Si3N4) is a widely used hard optical material but also has excellent piezoelectric response. Aluminum oxide (Al2O3) is a widely used optical material and is also used in gas barriers and tribology applications. The list goes on…..

Thin films thus offer enormous potential due to the following:

  • Creation of entirely new and revolutionary products
  • Solution of previously unsolved engineering problems
  • Improved functionality of existing products; engineering, medical and decorative
  • Production of nano-structured coatings and nanocomposites
  • Conservation of scarce materials
  • Ecological considerations – reduction of effluent output and power consumption

Engineered materials are the future of thin film technology. Engineered structures such as superlattices, nanolaminates, nanotubes, nanocomposites, smart materials, photonic bandgap materials, molecularly doped polymeres and structured materials all have the capacity to expand and increase the functionality of thin films and coatings used in a variety of applications and provide new applications. New advanced deposition processes and hybrid processes are being used and developed to deposit advanced thin film materials and structures not possible with conventional techniques a decade ago. For example, until recently it was important to deposit fully dense films for all applications, but now films with engineered porosity are finding a wide range of new applications. Hybrid processes, combining unbalanced magnetron sputtering and filtered cathodic arc deposition for example, are achieving thin film materials with record hardness.

Organic materials are also playing a much more important role in many types of coating structures and applications, including organic electronics and organic light emitting devices (OLED). These materials have several advantages compared to inorganic materials, including low cost, high deposition rates, large area coverage, and unique physical and optical properties. It is also possible to molecularly dope and form nanocomposites with organic materials. Hybrid organic/inorganic deposition processes increase their versatility, and applications that combine organic and inorganic films are increasing.

In addition to traditional metalizing and glass coating, large area deposition, decorative coating and vacuum web coating have become important industrial processes. Vacuum web coating processes employ a number of deposition technologies and hybrid processes, most recently vacuum polymer deposition (VPD) and have new exciting applications in thin film photovoltaics, flexible displays, large area detectors, electrochromic windows, and energy efficiency.

Thus we see that each thin film deposition process can be used for a range of applications and that some are more conducive than others with respect to certain applications and materials. The following processes with be reviewed:

  • Thermal and electron beam deposition
  • Magnetron sputtering and its cousins
  • Ion assisted deposition
  • Pulsed laser deposition
  • Chemical vapor deposition and its cousins
  • Atomic layer deposition
  • Ion plating
  • Cathodic arc deposition
  • Vacuum polymer deposition
  • Vacuum web coating

This series of Blogs will describe deposition processes with respect to the following criteria:

  • Adatom energetics varies significantly with deposition process and must be taken into account. Some processes require additional components to increase energy.
  • Substrate temperature is critical.
  • Thickness uniformity of the film on the substrate
  • Microstructure
  • Materials usage
  • Deposition rate.
  • Process scale up
  • Materials, Some processes are work better with certain substrate materials that others

 

Reference:

  1. Peter M Martin, Introduction to Surface Engineering and Functionally Engineered Materials, Wiley/Scrivener (2011).

How vacuum affects quality and microstructure of thin films

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 [1].

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 [3].

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.

 

Reference:

  1. Peter M Martin, Introduction to Surface Engineering and Functionally Engineered Materials, Wiley/Scrivener (2011).
  2. J Greene, in Multicomponent and Multilayered Films for Advanced Microtechnologies: Techniques, Fundamentals and Devices, O Auciello and J Engemann, eds., Kluwer (1993).
  3. John A. Thornton, Ann. Rev. Mater. Sci, 7 (1977) 239.
  4. R. Messier et al., J. Vac. Sci. Technol., A2(2) (1984) 500.
  5. Russell Messier, J. Vac. Sci. Technol., A4(3) (1986) 490.
  6. B. A. Movchan and A. V. Demchishin, Phys. Met. Metallogr., 28 (1969) 83.
  7. J. V. Sanders, in Chemisorption and Reactions on Metal Films, J R Anderson, ed., Academic Press (1971).