ADVANCING MATERIALS SERIES: WHAT IS PVD?
PVD PROCESSES AND COATING MATERIALS
The deposition of thin film layers from the vapor phase is accomplished through several techniques. We review the physical vapor deposition (PVD) techniques and equipment that are in common use in the high-volume production of coatings that find application in the optical, display, decorative, tribological and energy-generating /saving industries. Numerous PVD processes and coating materials have been developed and optimized for the specific applications. Coating materials are classified as dielectric compounds, metals, alloys, or mixtures. The same material can exhibit different optical, electrical, and mechanical properties depending on the deposition process.
Titanium oxide is a unique example of a metal oxide compound that, depending on deposition process parameters, can be made into film layers that are: transparent, electrically conductive, chemically reactive to light and bio-agents, chemically inert or exhibit spectrally selectively absorption. The dependent parameters are starting composition, oxidation state, crystalline structure and packing density. There are basically two PVD techniques used in production:
• Thermal evaporation by resistively heating or by using an electron-beam heating
• Sputtering, a non-thermal process
Variations and additions are made to the basic PVD techniques to permit different coating materials and substrate types to be accommodated. Process additions designed to alter the growth, nano-structure or composition of the film through control of the dependent variables listed above include bombardment of the growing film by high energy inert- or / and reactive ions, substrate heating, atmosphere composition and partial pressure, rate and vapor incidence angle. A further important variable contribution to the nucleation and self-assembling growth structure of the condensing adatoms, that we have discussed frequently, is the condition — both chemical and physical — of the substrate surface.
Resistance-heated (RH) sources are constructed of metal containers that can be open and boat- shaped or closed, as with a baffle box and exit opening. The type of source used, and its metal (or surface lining) depends on whether the material melts when heated or sublimes. Refractory metals (Ta, Mo, W) and ceramic or graphite crucibles are used to form sources. Since most fluoride compounds melt, an open container is often used, and evaporation proceeds from a large melted area. If the material sublimes, as do sulfide and selenide compounds and some oxide compounds, a baffled box source is used that emits the vapor.
Materials that require high temperature (>~1000° C) to vaporize, such as refractory oxide compounds and refractory metals, require the higher temperature of a focused electron beam source (E-B). Nearly any material that can be evaporated by RH can be evaporated by E-B; however, the power (high voltage) must be lowered for fluoride compounds for example, to prevent dissociation. Metals such as aluminum, gold and copper have lower evaporation temperatures than dielectrics and RH is generally used. Oxide and nitride compounds generally require the presence of a reactive atmosphere to recompose the compound or to establish the correct composition of the film. A partial pressure of the appropriate gas, deposition at the appropriate rate and substrate temperature all influence film composition.
Physical Vapor Deposition (PVD) Process
PVD is a line-of-sight process, where the vapor stream profile is approximately a cosine distribution, provided that the mean free path (MFP) of the evaporant molecules is larger than the scattering depth of the residual atmosphere. At a pressure of 1 e-05 Torr (0.01 Pa), the MFP is 1 m. The distance between the substrate and source must be less than the MFP to prevent loss of rate due to excessive scatter by the resident gas background. To achieve uniform thickness deposition over a large substrate area requires special geometrical considerations. The substrates are typically in motion through the plume distribution to provide random time and area sampling. Substrate tooling can be rotated in planetary motion to improve film density. Various shapes of occluding masking might be added to fine tune the thickness uniformity. Monitoring of thickness can be done indirectly using a quartz crystal oscillator or directly with an optical monitor. Coating systems are now available that can automatically execute a multi-layer coating design and control the individual film thicknesses according to a programmed design.
IAD Source and Additional PVD Forms
Ionization of a partial pressure oxygen or of the evaporant species produced in an energetic plasma or with the use of an ion source for ion-assisted deposition (IAD) supplies higher energies. The IAD source ionizes and accelerates argon and oxygen ions toward the substrates. Reactive oxygen completes the oxide compound, and energetic Ar+ ions compact the growing film to increase its packing density. Ion energies of hundreds of eV are possible with IAD sources. In some processes, the substrate or its holder can have a bias with respect to the source that accomplished the same purpose, but at lower energies.
Other forms of PVD for optical applications include pulsed laser deposition (PLD) and atomic layer deposition (ALD). PLD has the advantages of preserving the composition of the starting material for a large variety of compositions. It has not found use in high volume production due to small area coverage and expensive laser accessories. ALD is not based on evaporation, but on chemically reacting precursor gases under temperatures of 200° C or hotter. It has the advantages of producing dense films and consistent repeatable composition and predictable thickness without the need for real-time monitoring.
Evaporation processes are based on vaporizing a material by heating it beyond its melting or subliming temperature. In sputter deposition, atoms of materials are dislodged by the impact of ions, atoms or other particles that are created in an energetic plasma when the kinetic energy of these particles exceeds the binding energy of the target surface. The basic sputter technique is configured as a diode with a plasma discharge between the anode and cathode. Figure 1 shows the basic configurations for sputtering techniques. The cathode target material can be of nearly any composition, for example, insulators, metals and alloys and can be sputtered to deposit solid thin films of predetermined composition. Oxide and nitride compounds can be reactively sputtered from metal targets typically using a DC plasma. Targets composed of dielectrics, ceramics and targets with low electrical conductance are sputtered using plasma-based variations including RF sputtering. Argon is generally the working gas and a reactive component gas is added to the final composition of the sputtered film.
Figure 1. Sputtering configuration of basic technique.
The addition of concentrated magnetic fields near the target increases the deposition rate and distribution (magnetron sputtering) by increasing the density of the plasma and power density on the target surface. Energies are 1-10 eV for classical sputter deposition, about a factor of 10 greater than R-H or E-B energies. A beneficial consequence is that sputtered films are denser than evaporated films; a negative is higher compressive stress that for some applications needs to be reduced through process optimization. Alloys of materials can be sputtered with preservation of the starting composition, unless the sputter yields of the two materials differ significantly from each other. Despite such differences, the target can be conditioned to control the composition of the deposited film.
Recently, high-and mid-frequency pulsed power is used to generate high sputtering rates, and dual magnetrons have been configured to mix two different materials to create films with entirely new properties. Alternatively, in the ion beam sputtering technique (IBS), a remote source producing ions of energies of hundred’s is directed toward the target. The energy and mass of the impacting particles determines how many target atoms are sputtered. Because the sputtering process is very repeatable, thickness control is done by controlling power and deposition time. When sputtering from targets that react with oxygen or nitrogen, greater stability of rate and composition is achieved using an optical emission controller that maintains a chosen oxidation state as indicated by its line emission intensity.
The pressure in the sputtering chamber is three orders of magnitude higher than that for evaporation, consequently, scatter due to collisions is very high and spacing between target and substrate is reduced to a few centimeters. A beneficial consequence of the scatter at the high pressures is that the distribution can more easily be made to conform to non-planar shapes. Discussions containing more detail on the many aspects of sputter deposition processes have appeared in previous Coating Materials News issues. Reach out to the Materion team for additional information on deposition materials and sputtering targets by emailing us at OrderChemicals@Materion.com.