Production of a film of material often on a heated surface and in a vacuum. Physical vapor deposition technology is used in a variety of applications. Coatings are produced from a wide range of materials, including metals, alloys, compound, cermets, and composites. The two basic processes for physical vapor deposition are evaporation deposition and sputter deposition. In evaporation, thermal energy converts a solid or liquid target material to the vapor phase. In sputtering, the target is biased to a negative potential and bombarded by positive ions of the working gas from the plasma, which knock out the target atoms and convert them to vapor by momentum transfer. See also: Alloy; Cermet; Composite materials; Metal
Physical vapor deposition processes consist of three major steps: generation of the depositing species, transport of the species from source to substrate, and film growth on the substrate. The processes are versatile because the steps occur sequentially and can be controlled independently.
The first step involves generating the depositing species by evaporation using resistance, induction, electron-beam, or laser-beam heating (Fig. 1a), or by sputtering using direct-current or radio-frequency plasma generation (Fig. 1b).
In the second step, transport from source to substrate, various flow regimes are used. In molecular flow, the mean free path is larger than the source-to-substrate distance. Molecular flow occurs at low partial pressures of the depositing species and residual gas in the system, and is responsible for the line-of-sight feature typical of evaporation deposition processes and low-pressure magnetron-type sputter deposition processes. Viscous flow occurs at higher partial pressures (2.7–16 pascals or 20–120 millitorr) typical of diode sputter deposition; it also is intentionally added in the evaporation deposition process to cause gas scattering of the depositing species and thereby to increase the throwing power of the process.
An additional feature in the second step is the absence or presence of a plasma in the source-to-substrate region and the mode by which the plasma is excited, for example, direct current, radio frequency, or microwave. The mode of plasma excitation is an important aspect, since it controls the electron energy and distribution function and thereby the chemical reactions that take place in the plasma. The plot of ionization cross section versus electron energy peaks at low electron energies (50–100 eV). Processes that involve low electron energies, such as plasma-assisted evaporation, in which the electron energies generating the plasma can be controlled independently, offer a more versatile and a broader range of chemical reactions in the plasma than processes such as sputtering, in which the electron energies are dictated by other considerations such as target voltage (500–1000 eV), which controls the rate of sputtering. In sputter deposition, the electron energies cannot be controlled independently of other process parameters. The presence of a plasma is optional in the evaporation process but integral to the sputtering process.
The third step in physical vapor deposition is film growth on the substrate. The process depends on the energy of the incident species, which is typically 0.5 eV for evaporation and 10–100 eV for sputtering, and on the substrate temperature. The structure, composition, and residual stress in the film can be changed substantially by bombarding the growing film with energetic ions or neutral atoms. They can be generated by a separate ion source, or they can be attracted to the film from the plasma by electrical biasing of the substrate/film. Thus, the location of the substrate inside or outside the plasma can substantially change the nature and amount of ion bombardment. In magnetron sputtering, the plasma is confined to a narrow zone near the target. Therefore, if the film is to be bombarded, a second plasma has to be created near the substrate by a suitable method. See also: Magnetron
Single elemental species deposition
For single elemental species, the rate of evaporation is controlled by the temperature of the source, and this in turn governs the vapor pressure. At a given temperature, the rate of evaporation can vary over 10 orders of magnitude for the various elements in the periodic table. The sputtering rate for a given sputtering gas at a fixed pressure depends on the sputtering yield. The difference in sputtering yield for various elements is about 100 (ratio of the highest to the lowest sputtering yield), which is considerably smaller than the vapor pressure ratio for evaporation. In general, sputtering rates are 10–5000 times lower than evaporation rates.
Alloy deposition by evaporation can be accomplished in three ways: coevaporation, evaporation from a single source, and flash evaporation. Coevaporation involves the use of a number of sources corresponding to the number of elements in the alloy (Fig. 2a). Its advantage is the ability to control the evaporation rate from each source. Its disadvantage is that the alloy composition varies along the plane of the substrate.
Evaporation from a single source (Fig. 2b) operates under steady-state conditions where the composition and volume of the liquid pool on top of a solid rod are kept constant. For a thermodynamically ideal solution, the elemental composition of the vapor above the liquid alloy is proportional to the product of the mole fraction and the vapor pressure of the pure elements. For the example in Fig. 2b, if the alloy has a composition A1B1 and the vapor pressure of B is 10 times the vapor pressure of A, a pool of the composition A10B1 will generate a vapor species A1B1. The composition and volume of the molten pool is kept constant by raising the rod at a rate that allows the same amount of the solid alloy to be melted into the pool as is lost from the pool by evaporation. This technique is used commercially for coating a five-component alloy of nickel-cobalt-chromium-aluminum-yttrium (Ni-Co-Cr-Al-Y) onto turbine blades. Operationally, the maximum difference in vapor pressures between the elements in the alloy cannot be greater than 5000, due to the relationship between the rate of diffusion in the liquid and the rate of evaporation in maintaining the compositional homogeneity of the molten pool.
In the flash evaporation process, pellets of an alloy are dropped onto a very hot strip and are vaporized completely, thus maintaining the composition of the alloy in the deposit. This process works very well for elements with high vapor pressures. In a version of flash evaporation known as pulsed-laser or electron-beam ablation, a high-energy pulse vaporizes a small chunk of the alloy, thus retaining the composition of the alloy in the deposit. Cathodic arc evaporation, which evaporates small volumes of the target, may also be used. However, there is a problem of droplet ejection from the target that causes major flaws in the deposited film; this impedes its use in some applications of thin-film deposition. See also: Laser
Alloy deposition by sputtering is analogous to the evaporation method. In cosputtering, several sputter cathodes or sputter guns are used to generate the various vapor species. The same composition variation in the plane of the substrate occurs as in evaporation. Due to the low sputtering rates, there is not as much latitude to increase the source-to-substrate distance as with evaporation without making the deposition rates impractically low. A variation of cosputtering uses a segmented target. In this case, the sputtering target is built up of segments of the elements composing the alloy, the surface area of each element being inversely proportional to the sputtering yield. Increasing the number of segments increases the compositional homogeneity in the film.
An alloy can be sputtered from an alloy target if the modified surface composition is inversely proportional to the sputtering yield of the various elements. This is accomplished by a conditioning period of the target to allow progress from the alloy composition to the modified surface composition. See also: Sputtering
Deposition of compounds
Deposition of compounds can be performed in two ways. In direct evaporation/sputtering, the composition of the target is the same as that of the compound that is to be deposited; for example, both the target and film composition may be the oxide of yttrium (Y), barium (Ba), and copper (Cu), YBa2Cu3O7−x. In reactive evaporation/sputtering, the element or elements of the compound are evaporated/sputtered and react with the gas to form the compound. For example, Y, Ba, and Cu are evaporated/sputtered and react with oxygen to form YBa2Cu3O7–x . Plasma-assisted reactive processes are often used, since they activate the reactions leading to compound formation. The two plasma-assisted reactive processes for physical vapor deposition are activated reactive evaporation and reactive sputtering. These processes are of great importance for deposition of compound films in the manufacture of various devices.
Coatings provided by vapor deposition have many applications. For optical functions they are used as reflective or transmitting coatings for lenses, mirrors, and headlamps; for energy transmission and control they are used in glass, optical, or selective solar absorbers and in heat blankets; for electrical and magnetic functions they are used for resistors, conductors, capacitors, active solid-state devices, photovoltaic solar cells, and magnetic recording devices; for mechanical functions they are used for solid-state lubricants, tribological coatings for wear and erosion resistance in cutting and forming tools and other engineering surfaces; and for chemical functions they are used to provide resistance to chemical and galvanic corrosion in high-temperature oxidation, corrosion, and catalytic applications as well as to afford protection in the marine environment. Decorative applications include toys, costume jewelry, eyeglass frames, watchcases and bezels, and medallions. Coatings also act as moisture barriers for paper and polymers. They can be used for bulk or free-standing shapes such as sheet or foil tubing, and they can be formulated into submicrometer powders.
In some cases, a coating may serve multiple requirements. For example, a transparent conducting oxide may be a component of a pn junction as well as a current collector in a solar cell, or a gold-colored titanium nitride (TiN) coating may give a pleasing color as well as protect against wear. See also: Vacuum metallurgy