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Electron Beam Physical Vapor Deposition (EB-PVD)

Penn State has a world-class electron beam evaporation facility. The first unit is an industrial prototype electron beam-evaporation unit that has six electron beam guns.  Each gun has an average power capacity of 45 kW, for a total of 270 kW.  This unit has the additional flexibility in having the capability to continuously feed three ingots individually or simultaneously for the synthesis of complex compounds via co-evaporation. Typical ingot diameters range from 2 to 7 cm with lengths up to 50 cm. Overall, the chamber is approximately 90 cm in length, 90 cm in width, and 90 cm in height. Evaporation rates range from 0.5 nm to 100 mm per minute depending on the material to be evaporated and desired microstructure. The second unit has one EB gun (8kw), four 25cc hearths, and a cold cathode ionization source with chamber size of 66 cm x 60 cm x 100 cm. Multilayered coatings, direct evaporation, reactive evaporation, IBAD processes can easily be performed in this unit. The EB-evaporation process offers extensive flexibility in controlling composition and microstructure of the deposited coatings. 

In the EB-PVD process, focused high-energy electron beams generated from electron guns are directed to melt and evaporate ingots as well as to preheat the substrate inside the vacuum chamber as shown in Figure 1. Due to the change in pressure, the vapor rises and traverses the vacuum chamber where it condenses on the substrate forming the coating. To obtain more uniform coatings, the sample is often rotated during the coating process.  In addition, nonuniform evaporation is reduced by scanning the electron beam over the surface of the melt pool.

The depth of the melt and degree of vaporization can be controlled by restricting the kinetic energy of the electron beam.  This restriction allows the deposition rates to range from a few angstroms per second to as high as 100 µm/minute, depending on the material. 

EB-PVD Illustration
a.		 EB-PVD Facility
b.		 EB-PVD Chamber
c.
Figure 1.  (a) schematic representation, and (b-c) digital images of an industrial proto-type electron beam‑physical vapor deposition (EB‑PVD) research facility with 6 EB guns and three continuous ingot feeding system.

Methods of Evaporation
When discussing electron beam (EB) evaporation techniques for compounds, there are three main methods used to obtain the proper stoichiometry or phase of the compound. These are direct evaporation, reactive ion beam evaporation, and co-evaporation. There are also derivatives of the EB-PVD process such as ion plating and activated reactive evaporation. 

Direct Evaporation
Direct evaporation of compounds (ZrO2) using an electron beam source generally results in products of the parent phase as well as the individual species within the vapor cloud that condense on the substrate and subsequently recombine to form the compound; the technical term is called fractionation. The stoichiometry depends on such factors as the ratios of the molecular fragments, deposition rates, degree of surface mobility, impingement of the gaseous atoms present in the environment, impurities present on the substrate surface, the amount of time these fragments stay on the substrate’s surface, and the reaction rate of the fragments reconstituting the compounds at or near the substrate’s surface.

The average energy of the condensing species for materials directly evaporated is approximately 0.1 eV with a narrow energy distribution.  In contrast, the average energy of the condensing species from sputtering processes is 2–10 eV, with typical values being 5 eV.  In addition, the energy distribution of the condensing species from sputtering techniques is broader (2-5 eV) than that of evaporation (0.1-0.2 eV).  The energy of the condensing species is very important in film growth in order to produce atom mobility.  At lower energies (<10 eV), very little modification to the microstructure occurs as it takes approximately four times the bonding energy (~3-5 eV) to break atomic bonds and result in surface mobility of the atoms.  The energy is only high enough to cause local (surface) bond breaking and molecular dissociation.  Between energies of 10 eV – 40 eV, not only is there enough energy present to break surface bonds, but also enough to result in an appreciable amount of surface mobility, which affects film growth and density. 

Short range or near-surface displacement (forward sputtering) can occur within this energy range, as well as a small amount of sputtering at the surface (~0.1 %).  Between 20 eV – 100 eV, a higher degree of energy transfer occurs which results in sub-surface damage and a greater degree of sputtering.  In addition, increased film modification occurs due to the higher amount of energy present to break bonds and increase surface mobility.  Energies greater than 100 eV can cause significant amounts of re-sputtering, which is heavily material dependent.  However, since there is a “trade off” between the deposition rate and the energy of the condensing species, higher energies are often needed to obtain similar microstructure effects as those with low deposition rates and energies.  In general, increasing the condensation rate at a constant energy (of the condensing species) decreases the effects of the energy as a result of kinetic limitations.

Reactive Evaporation
Reactive evaporation is the process whereby the metal (i.e., pure titanium) is evaporated in the presence of a reactive gas (argon/nitrogen) to form a compound (TiN).  If the thermodynamic conditions are met, the reactive gas dissociates into its component parts and reacts with the metal vapor to form a stable chemical compound.  The energy of the ionized gas can range from a few eV to greater than 1000 eV depending on the processing parameters and desired microstructure. Reactive ion beam assisted, electron beam-physical vapor deposition where the reactive gas is ionized prior to reacting with the evaporated material (titanium). 

Advantages and Disadvantages of EB-PVD
As with any process, there are certain advantages and disadvantages of the EB-PVD process.  Depending on the material to be evaporated and the microstructure desired for a particular application, small modifications of the deposition process can be made to exploit the advantages or limit the disadvantages.  These will be discussed below.

Advantages of the EB-PVD process
The EB-PVD process offers many desirable characteristics such as flexible deposition rates (1 nm/min up to 100 mm/minute, depending on material, desired microstructure, and property performance), dense coatings, strong metallurgical bonding, tailored composition, columnar and polycrystalline microstructure, and high thermal efficiency.  It should be noted that the desired microstructure is heavily dependent on the processing parameters.  For example, good adhesion is obtained at higher substrate temperatures due to diffusion bonding.  At lower temperatures, the bonding becomes more of a mechanical bond.  As a result, the surface finish of the substrate becomes increasingly important at low deposition temperatures for adhesion.
 
Coating density can be changed by controlling the deposition rate and method of rotation.  Samples rotated out of the vapor cloud tend to be less dense than those that are not rotated or rotated with the coating surface perpendicular to the source material at all times.  In addition, the angle of coating incidence will affect the density and microstructure, and thus the coating properties. In general, coatings with high incidence angles are less desirable due to increased amounts of porosity. Elements with low vapor pressure such as molybdenum, tungsten, and carbon are readily evaporated by this process. In addition, EB-PVD is capable of producing multilayered and nano-laminated metallic/ceramic coatings at relatively low temperatures, with distinct or diffuse interfaces, on large components by changing the processing conditions such as ingot composition, part manipulation, and EB energy. Lastly, in combination with IBAD, ceramic coatings can be deposited at lower substrate temperatures with increased adhesion as additional energy (from the ionized gas) is provided for diffusion and intermixing near the substrate / coating interface. 

Disadvantages of the EB-PVD process
The main disadvantages of the EB-PVD process are high capital equipment cost and that it is a line-of-sight process, which makes coating complex geometries more difficult. Other disadvantages of the EB-PVD process include filament degradation, which often results in nonuniform power densities and thus evaporation rates. In addition, if the proper accelerating voltage and current of the electron beam are not used, fractionation may occur.

Ion Beam Assisted Deposition (IBAD) Process
Ion beam assisted deposition is often used in conjunction with one of the conventional evaporation or PVD techniques to change the properties and microstructure of the depositing coating.  The three main applications that ion beams are used for are etching, deposition, and property / microstructure enhancement, each of which will be discussed briefly below.

Ion Beam Etching and Pre-cleaning
In ion beam etching, an ionized beam of gas is directed towards the substrate’s surface, prior to deposition, to remove material. This removal of surface material is the result of physical sputtering of the material from the surface, due to the momentum transfer between the energetic beam atoms and the substrate surface atoms.  Generally, for physical sputtering, an inert gas, such as argon, is used.

During surface etching or pre-cleaning, damage can occur to the substrate’s surface from heat generation.  The maximum removal of material generally occurs in the energy range of 300-400 eV, but is heavily dependent on the material and type of ion source. In addition, heavier ions can displace atoms from their lattice position resulting in strain and thus stress. If their energies are high enough, the ion beam gases can be entrapped or embedded beneath the surface, altering the substrate composition and microstructure.  It should also be noted that energies of 100 eV have caused considerable damage in some materials, but by increasing the energy to ~ 500 eV has shown little to no damage to the same material due to re-sputtering. This clearly shows how sensitive or insensitive certain materials are to various energy levels. In compound materials, fractionation or preferential etching can also occur depending on the material system and degree of bonding.  For example, material A (low atomic number and weakly bonded) may sputter faster than material B (high atomic number and strongly bonded) depending on the energy level and current density. This is explained based on the degree of bonding and the atomic number of the material. Since material A is weakly bonded, it takes less energy to break the weak bonds, as compared to material B. In addition, it takes less energy to move a lighter atom or molecule, than a heavier one, which results in greater surface mobility, and thus a higher probability of desorption.

Bombardment of the substrate surface prior to deposition (i.e., sputter cleaning) promotes better adhesion.  The two major effects occurring during this pre-cleaning step are: (1) removal of adsorbed hydrocarbons and water molecules and (2) increasing the density of nucleation sites for condensation.

Energetic ions bombarding the surface remove hydrocarbons and water molecules which are weakly adhered to the surface.  Not removing these materials /molecules prior to deposition results in poor adhesion as they serve as weak links for bonding.  In addition, energy and current density of the ion beam during sputter cleaning can have a significant impact on the number of nucleation sites. For example, high-energy atoms can cause localized defects which serve as nucleation sites. The increased number of nucleation sites is believed to result in a higher bond density between the substrate and deposited layer. In addition, when bombardment is introduced during deposition, it enhances the surface mobility of the atoms which increases the interactions with the incoming molecules causing intermixing of materials at the interfaces which results in better bonding, and thus increased adhesion (similar to the cathodic arc process).

Microstructure and Property Enhancement
In the last several years, ion beams have gained increased importance during the deposition process to enhance the properties of the depositing film.  Ion bombardment of the substrate occurs while the source material is evaporated by either resistance or EB.  In general, most property enhancement processes, a large number (5,000) of low energy (100 eV) ions can produce a similar microstructure of that produced by a small number (1,000) of high-energy (500 eV) ions, as long as the total energy (500,000 eV) input is the same. 

The state of the internal stresses developed in the coating can be changed from tensile to compressive stress by the forcible injection of high-energy atoms (i.e., ion implantation).  Thus the ability to control the stress level is an additional feature of the IBAD.  Chemical vapor deposited coatings generally form with tensile stresses due to the thermal expansion mismatch with the substrate, which often limit the coating thickness before spallation occurs.  Ion bombardment during deposition has a tendency to reduce the tensile stress and often changes the intrinsic stress from tensile to compressive.  Depending on the energy of the ion beam, texturing or preferred crystal growth orientation can be controlled. 

In addition, numerous authors have reported increases in the average hardness of coatings deposited with IBAD.  The increase in hardness is obtained by increasing the density, decreasing grain size, changing stress state, and controlling the crystallographic texture of the coating.  Ion implantation (higher degree of energy than IBAD) of Ni and Ti into the surface of high-speed tool steels increases the surface hardness (improved wear resistance), but also introduces large amounts of compressive stress which often leads to premature failure.  Improved step coverage (i.e., high surface roughness or complex geometries) has also been reported when using  IBAD.  This is most likely the result of increased atom mobility under bombardment.