Deposition Technology: Electron Beam Evaporation

Electron beam (e-beam) evaporation involves heating the material by an electron beam as opposed to thermal evaporation which uses resistive heating (discussed in previous Blog). A typical e-beam evaporation system is shown schematically in Figure 1. E-beam heated sources have two major benefits:

  • Very high power density and as a result, a wide range of control over evaporation rates from very low to very high
  • Evaporant is contained in a water cooled Cu hearth, thus eliminating the problem of crucible contamination

While it will not be discussed in any detail, the two major types of e-beam sources (guns) are thermionic and plasma types (details of these guns can be found in reference 1). Deposition rate in this process can be as low as 1 nm/min to as high as a few μm/min. Material utilization efficiency is high relative compared to other PVD methods and the process offers structural and morphological control of films (discussed shortly). Due to the very high deposition rate, this process has been used in industrial applications for wear resistant and thermal barrier coatings in aerospace industries, hard coatings for cutting and tool industries, and electronic and optical films for semiconductor industries and thin film solar applications.

Note that one of the major problems with these processes is the low energy of the evaporated atoms, which can cause problems in the thin films such as poor adhesion to the substrate, reduced density, porous and columnar microstructure, increased moisture pick up and poor mechanical properties. This will be addressed in the next Blog.

Deposition of single elements by evaporation is straight forward. Deposition of alloys with two or more components, however, can be a challenge due to different vapor pressures and different evaporation rates. The solution to this problem is to use multiple sources, one for each constituent of the alloy. Additionally, the material evaporated from each source can be a metal, alloy or compound. Challenges of this process are calibrating deposition rates for each source, ensuring that the adatom beam from each source uniformly coincides with the beams from the other sources and that deposited atoms are sufficiently blended and obtaining uniform density of each material over the substrate surface.

Difficulties of direct evaporation to form oxides, nitrides, fluorides, carbides, etc. are due to fragmentation of vaporized compounds. Reactive evaporation can overcome many of these problems. Here metals are evaporated in the presence of reactive gas. A schematic of a typical reactive evaporation system is also shown in Figure 1. The problem here is that most oxides are substoichiometric due to the low energy of the evaporated adatoms and that deposition rates can suffer. In most cases stoichiometric films are deposited only as low deposition rates. Reaction kinetics (i.e., low energy of evaporated atoms/molecules) prohibit full oxidation or full reaction with the reactive species, which limits the potential applications of these films as optical coatings in particular. To this end, activated reactive evaporation (ARE) was developed [1,2]. This process generally involves evaporation of a metal or alloy in the presence of a plasma of a reactive gas, hence the term “activated”. For example, TiC and TiN coatings are deposited by evaporation Ti in a C2H2 or N2 plasma respectively. The plasma has the following roles:

  • Enhance reactions that are necessary for deposition of compound films
  • Modify growth kinetics and, as a result, structure/morphology of deposits

Applications for evaporated thin film coatings include:

  • Multilayer optical coatings
  • Metallization
  • Large area metal mirror coatings for telescopes
  • Metallization for roll coating
  • Decorative coatings
  • Hard coatings
  • Tribological coatings
  • Thin film solar cells
  • Diffusion barriers
  • Thermal barrier coatings
  • Organic thin films

Evaporation was one of the first processes used extensively for the deposition of thin films. This process enabled the use of thin films in a wide variety of applications, which lowered manufacturing costs and expanded functionality of bulk materials. Large area coverage was now possible. Evaporation and related processes are still used extensively to synthesize a wide variety of thin film coating materials.

A full range of evaporated optical coatings is being produced, including AR on glass and plastic, mirror coatings, beam splitters, filters, transparent conductive coatings, dichroic coatings, attenuation coatings and fiber optic coatings [3]. Coating of large areas, as well as high speed web coating, are possible with e-beam systems. Figure 2 shows the evaporated 8m Al Gemini telescope mirror coating. These have come a long way from the evaporated quartz/MgF2/cryolite/Al coatings in 1936 [4].

Reference:

  1. S Ismat et al., Chapter 4, Handbook of Deposition Technologies for Films and Coatings, Science, Applications and Technology, 3rd Ed., Peter M Martin Ed., Elsevier (2010).
  2. R F Buhshah & C Deshpandey, in Physics of Thin Films, J L Vossen and M H Francombe (Eds.), Academic Press (1987).
  3. See http://www.evaporatedcoatings.com/index.php?WTX=gaw.
    J Strong, J Opt Soc Am 26 (1936) 73.

Deposition Technology: Thermal Evaporation

This article sponsored by Maynards

 

Deposition technology is the backbone of thin film technology. It is based essentially on the history of improved adatom energetics, higher deposition rates, thickness uniformity, large area coverage, and of course, cost. This technology can be tracked from thermal evaporation and electrochemical deposition in the 1930’s (starting as early as the 1870’s [1]) to advanced processes such as atomic layer deposition (ALD) and high power impulse magnetron sputtering (HIPMS). Due to the low energy of evaporated atoms, quality, environmental stability and durability of thin films was initially a major problem. Additionally, since these are nonequilibrium processes, virtually and material composition is possible. Each process has made significant contributions to possible compositions, mechanical properties, electrical properties, optical properties, durability, environmental stability, manufacturability, and advanced applications. And evaporation technology is no exception. Additionally, a wide range of complex multilayer structures, substrate sizes and shapes, complex compositions, functionality, decorative coatings, microstructures and nanostructures is now possible.

Thermal evaporation is a physical vapor deposition (PVD) process that goes back as far as the 1850’s and was the first technology to facilitate numerous thin film applications [1,2]. The first evaporated thin films were probably prepared by Faraday in 1857 when he exploded metal wires in a vacuum [3]. Deposition of thin metal films in vacuum by thermal heating was initiated in 1887 by Nahrwold and employed by Kundt in 1888 to measure refractive indices of these films [4,5]. As early as the 1920’s it was initially used to metallize auto parts, metallize mirrors (including large telescope mirrors), deposit antireflection coatings, and is still used today for a number of advanced applications [3]. Production of fully dense coatings and self-supported shapes by high rate PVD processes started around 1961.

PVD processes, including thermal and electron beam evaporation involve atom-by-atom transfer of material from a source to a substrate. The three general steps in the formation of a deposit can be summarized as:

  • Synthesis of the material to be deposited:
    • Transition from a condensed phased (solid or liquid) to a vapor phase
    • For compounds, a reaction between the components of the compound, some of which may be introduced into the chamber as a gas or vapor
  • Transport of the vapors between source and substrate
  • Condensation of vapors (and gases) followed by film nucleation and growth

In evaporation and other PVD processes, steps 1 and 3 can be independently controlled, giving the process a high degree of flexibility in controlling structure, properties and deposition rate.

Thermal evaporation (TE) involves resistive heating a solid material inside a high vacuum chamber, taking it to a temperature which vaporizes the material, while electron beam (e beam) evaporation uses an electron beam to heat the source (next Blog). In TE, metal material (in the form of wire, pellets, shot) is fed onto heated semimetal (ceramic) evaporation boats. A pool of melted metal forms in the boat cavity and evaporates into a cloud above the source. Alternatively the source material is placed in a crucible which is radiatively (radio frequency-RF) heated by a resistive filament or the source material may be hung from the filament itself (filament evaporation). MBE is an advanced form of thermal evaporation. Figure 1 shows a schematic of a thermal evaporation system and Figure 2 shows a thermal evaporation source.

This process requires a good vacuum (generally < 10-4 torr), however, even a relatively low vapor pressure is sufficient to raise a vapor cloud inside the chamber. This evaporated material now constitutes a vapor stream, which traverses the chamber and condenses on the substrate, forming a coating or film.

In most processes, source material is heated to its melting point and is liquid, and is usually located in the bottom of the chamber, often in some sort of upright crucible. The metal vapor then rises above this bottom source, and substrates are held inverted in appropriate fixtures at the top of the chamber. Multiple sources are used to deposit alloys, and care must be taken since each material evaporates at a different rate. Although many elements evaporate, a number of materials such as Cr, Cd, Mg, As and C sublime and elements such as Sb, Se and Ti are borderline between evaporation and sublimation [2]. Sn, Al, Ga and Pb have low vapor pressures, just above their melting point while materials such as Cr have high vapor pressures. Although most elements vaporize as atoms, Sb, Sn, C and Se can vaporize in clusters of atoms [2].

Deposition rates for evaporated materials are generally very high, which minimizes incorporation of impurities into the film and can result in a very fine grain microstructure. Microstructure, durability and environmental stability of thin films to a large extent depends on the energy of the atoms arriving at the substrate. While deposition rates can be very high, thermally evaporated atoms arrive at the substrate with an energy of ~ tenths of eV. Adatom energetics are thus not impressive, which can result in a porous microstructure and poor film adhesion. As a result, high substrate temperatures or ion assist are often required to achieve fully dense and adherent films. Ion assist will be addressed in a future Blog.

Reference:

  1. Historical Timeline of Vacuum Coating and Vacuum/Plasma Technology, Society of Vacuum Coaters: svc.org/history of vacuum coating/History-of-Vacuum-Coating.cfm*.
  2. Donald Mattox, Handbook of Physical Vapor Deposition, Noyes (1998).
  3. S Ismat et al., Chapter 4, Handbook of Deposition Technologies for Films and Coatings, Science, Applications and Technology, 3rd Ed., Peter M Martin Ed., Elsevier (2010).
  4. F Soddy, Proc R Soc Lond 78 (1967) 429.
  5. I Langmeir, J Am Chem Soc 35 (1913) 931.