By Donald M. Mattox
Management Plus, Inc.
Albuquerque, NM

In physical vapor deposition (PVD) processes, atoms or molecules are vaporized from a solid or liquid source, transported in the form of a vapor through a vacuum or low-pressure gaseous environment and condensed on a substrate. PVD processes can be used to deposit films of elemental, alloy and compound materials as well as some polymeric materials. Typically, PVD processes are used to deposit films with a thickness range of a few angstroms (Å)1 to thousands of angstroms, and deposition rates vary from 10-100 Å/sec. The deposits can be of single materials, layers with a graded composition, multilayer coatings or very thick deposits.

PVD processes can be categorized as vacuum evaporation, sputter deposition, arc vapor deposition and ion plating.

Vacuum Evaporation
Vacuum evaporation is a PVD process where material from a thermal vaporization source reaches the substrate without colliding with gas molecules in the space between the source and substrate. The trajectory of the vaporized material is line-of-sight. The vacuum environment also provides the ability to reduce gaseous contamination in the deposition system to any desired level. Typically, vacuum evaporation takes place in the pressure range of l0-5-10-9 Torr2, depending on the level of gaseous contamination that can be tolerated in the deposit.

Substrate fixturing is an important deposition component. Fixturing allows the substrate(s) to be held in a facedown position so that particulates do not settle on the surface causing pinholes in the film. It also provides a way to heat the substrate and allows movement during deposition to obtain uniform deposition. The strong dependence of deposition rate on geometry and time often requires that fixturing and tooling be used to randomize the substrate(s) position during deposition in order to increase the film thickness uniformity. This fixturing also randomizes the angle of incidence of the depositing vapor flux, increasing the uniformity of the film properties over the substrate(s) surface.

The shutter is an important component because it allows the vaporization source to be heated without exposing the substrate to volatile contaminant material that may vaporize from the source first. It also minimizes radiant heating from the vaporization source. Opening and closing the shutter can accurately control the deposition time. The "glow bar" allows the formation of a plasma in the system for in situ cleaning of the substrate surface in the deposition chamber.

The thermally vaporized materials equilibrium vapor pressure is an important property. In a closed container at equilibrium, as many atoms return to the surface as leave, and the pressure above the surface is the equilibrium vapor pressure. Vapor pressures are strongly dependent on the temperature. The vaporization rate from a hot surface into a vacuum (free surface vaporization rate) depends on the temperature and the equilibrium vapor pressure of the material at that temperature. Materials with a high vapor pressure at low temperatures are typically vaporized from resistively heated sources. Refractory materials, which require a high temperature to vaporize, require the use of focused high-energy electron beam heating for vaporization.

For vacuum evaporation, a reasonable deposition rate can be obtained only if the free surface vaporization rate is fairly high. A vapor pressure of 10-2 Torr is typically necessary to provide a useful deposition rate. Materials with that vapor pressure above the solid are described as subliming materials (e.g., chromium, carbon); materials with that vapor pressure above the liquid are described as evaporating materials. Many materials, such as titanium, can be deposited either by sublimation or evaporation, depending on the temperature of the source. For alloys, the vaporization rate of each constituent is proportional to the relative vapor pressures (Raoults Law). Therefore, during vaporization, the higher vapor pressure material vaporizes more rapidly, and the source is progressively enriched in the lower vapor pressure material as evaporation progresses.

Most elements vaporize as atoms, but some, such as antimony, carbon and selenium, have a portion of their vapor as clusters of atoms. For these materials, special vaporization sources called baffle sources can be used to ensure that the depositing vapor is in the form of atoms.

Some compounds, such as silicon monoxide, silicon nitride, hafnium carbide, tin dioxide, boron nitride, lead sulfide and vanadium dioxide, sublime. The molecules of many compound materials partially dissociate on vaporization; however, some may vaporize primarily as molecules. Notable among the materials that vaporize without much molecular dissociation are silicon monoxide and magnesium fluoride, which are widely used in optical coating technology. The degree of dissociation of a compound depends strongly on the vaporization temperature. When depositing a compound that dissociates, the depositing film is generally deficient in the gaseous constituent. This loss of gaseous constituents during vaporization can be partially compensated for by using reactive evaporation or activated reactive evaporation where there is a low-pressure reactive gas or plasma of reactive gas in the deposition environment, or by continuous reactive ion bombardment of the depositing material from an "ion gun." This type of deposition is also called quasi-reactive deposition.

For low vaporization rates, the material from a point vaporization source deposits on a substrate with a distance and substrate orientation dependence given by the cosine deposition distribution.

In actuality, the flux distribution from the source may not be cosine but can be modified by source geometry, collisions in the vapor above the vaporizing surface when there is a high vaporization rate, level of evaporant in the source, changes of vaporization source geometry with time, etc. Evaporation rates are typically monitored in situ and in real time by collecting the vapor on the surface of a quartz crystal oscillator, causing the oscillation frequency to change. Calibration allows the change in frequency to be related to deposited film mass and thickness.

Vacuum evaporation is used to form optical interference coatings, mirror coatings, decorative coatings, permeation barrier films on flexible packaging materials, electrically conducting films, and corrosion-protective coatings.

Sputter Deposition
Sputter deposition deposits particles vaporized from a surface by the physical sputtering process. Physical sputtering is a non-thermal vaporization process where surface atoms are physically ejected by momentum transfer from an atomic-sized energetic bombarding particle, which is usually a gaseous ion accelerated from a plasma or ion gun. If the surface atom that is struck attains enough energy, it shakes other atoms in the near-surface region and a "collision cascade" develops. Some momentum from the collisions is directed back toward the surface and, if sufficient, can physically eject atoms from the surface, i.e., sputtered. Most of the energy that is transferred by the bombarding particle appears as heat in the near-surface region.

At all but the lowest bombarding energies, the flux of atoms that are sputtered from the surface leave the surface with a cosine distribution. They typically have average kinetic energies higher than that of thermally vaporized atoms and have a high energy "tail" in the energy distribution that can be several tens of eV3.

The sputtering yield is the number of surface atoms that are sputtered for each incident energetic bombarding particle. The sputtering yield depends on the bombarding particle energy, relative masses of the bombarding and target species, the angle of incidence of the bombarding species and the chemical bond strength of the surface atoms. The most common inert gas used for sputtering is argon. As the angle of incidence of the bombarding particles becomes off normal, the sputtering, yield can increase 2-3 times, up to a point where the bombarding particles transfer little momentum because of the high collision angle. After this, the sputtering yield drops off rapidly since most of the bombarding species are reflected from the surface. The apparent sputtering yield can be affected by the surface topography, since in sputtering a rough surface some of the sputtered particles are "forward sputtered" and redeposited on the surface.

Since the sputtering process removes each solid surface atomic layer consecutively, if there is no diffusion, the composition of the vapor flux leaving the surface is the same as the composition of the bulk of the material sputtered. This allows the sputter-vaporization of alloy compositions that cannot be thermally evaporated because of the greatly differing vapor pressures of the constituents.

Often, sputtered surfaces have a surface layer composed of a reacted material such as an oxide or nitride. Since the chemical bonding of the compound materials is stronger than that of the elemental material, the sputtering yield is initially low until the surface layer is removed. Also, if reactive gases are present, they can continuously "poison" the target surface by forming compounds on the surface, giving low sputtering yields.

Particles that are sputtered or reflected from the sputtering target surface at low gas pressures will travel in a line-of-sight path. If the gas pressure is higher, gas-phase collisions can take place, reducing the energy of the particles and scattering them from a line-of-sight path. If there are enough collisions, the energetic particles are "thermalized" to the energy (temperature) of the ambient gas. Energetic gaseous particles bombarding the surface of the growing film can affect the film formation process and the properties of the deposited film material. Thus, it makes a difference in film properties whether the sputter deposition is done at a low gas pressure or a higher gas pressure.

The most simple plasma configuration is DC diode sputtering where a high negative DC voltage is applied to a conductive surface in a gas. A plasma is formed that fills the container and positive ions are accelerated to the surface.

Magnetron sputtering uses a magnetic field, usually from permanent magnets near the target (cathode) surface, to confine the electrons near the surface. When an electron ejects from the target surface, it accelerates away from the surface by the electric field, but is forced to spiral around the magnetic field lines.

Wright first reported film formation by sputter deposition in 1877, and Edison patented a sputter deposition process for depositing silver on wax phonograph cylinders in 1904. Sputter deposition is widely used to deposit thin-film metallization on semiconductor material, energy-controlling coatings on architectural glass, transparent conductive coatings on glass, reflective coatings on compact disks, magnetic films, dry film lubricants, wear-resistant coatings and decorative coatings.

Arc Vapor Deposition
In arc vapor deposition the deposit is formed from the anode or cathode of a low-voltage, high-current DC arc, in a low-pressure gaseous atmosphere. In cathodic arc vaporization, which is the most common PVD arc vaporization process, the high current density arc moves over a solid cathodic electrode causing local heating and vaporization. The arc movement may be random or "steered" using a magnetic field. In many cathodic arc vapor deposition systems multiple cathodic arc sources are used to give deposition over large areas.

In anodic arc vapor deposition, electrons melt and vaporize the anodic electrode. The electrons for the anodic arc can come from a number of sources such as a hot hollow cathode, a hot thermoelectron-emitting filament or from ion bombardment. The configuration of an anodic arc vapor deposition system is similar to that of an e-beam thermal evaporation system.
An arc can also be ignited in a vacuum forming a vacuum arc. In the vacuum arc, the electrodes must be closely spaced and a positive space charge is generated in the space between the electrodes. This space charge repels positively charged ions to high energies.

In arc vaporization, the vaporized atoms pass through a high-density electron cloud and many, if not most, are ionized. These "film-ions" can then be accelerated to high energies by an electric field in the low-pressure gas. The electrons in the arc also ionize the gaseous material, and those ions can also be accelerated to high energies in an electric field. If some of the gaseous ions are of a reactive gas, then a compound material such as a nitride can be deposited.

In cathodic arc vaporization, some of the material is ejected as molten globules called "macros." These macros deposit on the film material and create bumps that create pinholes when dislodged. Some materials are more prone to form macros than are others. The macros can be eliminated using a "plasma duct." In the plasma duct, a magnetic field bends the electrons and the film-ions follow the electrons to keep volumetric charge neutrality in the plasma. The macros are deposited on the duct walls rather than on the substrate.

Cathodic arc vapor deposition is used in depositing wear-resistant decorative/functional coatings such as TiN (gold-color) and ZrN (brass-color) on a variety of substrates such as plumbing fixtures and door hardware. Anodic arc vapor deposition is used to deposit adherent elemental materials such as chromium and diamond-like carbon for wear resistance and compound materials for optical coating applications.

Ion Plating
In the early 1960s, it was shown that controlled concurrent energetic bombardment of the depositing film material by particles of atomic or molecular dimensions could be used to modify and tailor the properties of the deposited film material. Thus, concurrent or periodic bombardment during deposition can be a major PVD processing variable.

Ion plating uses concurrent or periodic energetic particle bombardment of the depositing film material to modify and control the composition and properties of the depositing film. In ion plating, the deposited material source can be evaporation, sputtering, arc erosion, laser ablation or other vaporization source. The energetic particles used for bombardment are usually ions of an inert or reactive gas; however, when using an arc erosion source, a high percentage of the vaporized material is ionized, and ions of the film material can be used to bombard the growing film. Ion plating can be done in a plasma environment where ions for bombardment are extracted from the plasma, or it may be done in a vacuum environment where ions for bombardment are formed in a separate ion gun. The latter ion plating configuration is often called ion beam assisted deposition (IBAD).

The most common form of ion plating is the plasma-based process where the substrate and/or its fixture is an electrode used to generate a DC or Rf plasma in contact with the surface being coated. If an elemental or alloy material is deposited, the plasma can be of an inert gas, usually argon. In reactive ion plating, the plasma provides ions of reactive species such as nitrogen, oxygen or carbon (from a hydrocarbon gas) that are accelerated to the surface to form compounds such as oxides, nitrides, carbides or carbonitrides. Typically, in plasma-based ion plating, the substrate fixture is the cathode of the DC circuit; however, the plasma can also be formed independently of the substrate and ions are accelerated from the plasma to the surface of the growing film.

Concurrent or periodic bombardment during film growth modifies the film properties by affecting the nucleation of the depositing atoms, densifying the film by compaction or "atomic peening," introducing significant thermal energy directly into the substrate surface region and by sputtering and redeposition of the film material. Energetic particle bombardment can also introduce compressive film stress by recoil implantation of surface atoms into the lattice structure and, in the case of reactive deposition, enhancing chemical reactions on the surface by bombardment-enhanced chemical reactions and sputtering/desorbing untreated species. In the case of reactive deposition in plasma-based ion plating or ion sources using a plasma of reactive species, the plasma also activates the reactive species, which enhances the kinetics of chemical reactions at the surface.

It has been determined that for argon ion bombardment, the energy of the bombarding ions should be greater than ~50 eV and less than ~300 eV to effectively modify the film properties. For lower ion energies, momentum transfer is not sufficient to displace the film atoms, and for higher energies, the bombarding species will be incorporated into the film unless the substrate temperature is high. This gas incorporation can result in void formation and microporosity in the film. To completely disrupt the columnar growth morphology in deposited films of refractory materials requires that ~20 eV per depositing atom be added by the concurrent bombardment. This means that a bombardment ratio of about one energetic ion (200 eV) per 10 depositing film atoms is used. For example, at a 30 Å/sec deposition rate, the ion flux of 200 eV ions should be at least 1015 ions/cm2/sec or an ion current (singly charged ions) of ~0.1 mA/cm2. At these ion energies and fluxes, an appreciable portion (10-30%) of the depositing atoms are sputtered from the growing film surface giving a growth rate less than the deposition rate.

Generally, in ion plating, the high-energy bombarding particles are positively charged ions that are extracted from a plasma and accelerated to the growing film surface, which is at a negative potential with respect to the plasma or the ion, and are extracted and accelerated from an ion gun. In plasma-based ion plating, the negative potential on the substrate surface can be formed by applying a continuous DC potential to an electrically conductive surface, applying a pulsed DC or Rf potential to an insulating surface, by applying a combination of DC and Rf bias, or by inducing a "self bias" on an electrically insulating or electrically "floating" surface. When using an ion gun, the high-energy ions can be injected into a field-free region so that a negative potential does not have to be applied to the substrate to achieve high-energy bombardment of the surface.

Concurrent bombardment during film growth affects nearly all film properties, such as film-substrate adhesion, density, surface area, porosity, surface coverage, residual film stress, index of refraction and electrical resistivity. In reactive ion plating, the use of concurrent bombardment allows the deposition of stoichiometric, high-density films of compounds such as titanium nitride and zirconium nitride at low substrates.

Ion plating is used to deposit hard coatings of compound materials, adherent metal coatings, optical coatings with high densities and conformal coatings on complex surfaces.

Each of the PVD processes has its advantages and disadvantages. Each requires different process monitoring and controlling techniques. Generally, the most simple technique and configuration that will give the desired film properties and most economical product throughput should be used. There are no "handbook values" for the properties of deposited thin films, and the properties depend on the details of the deposition process. In order to have a reproducible process and product it is important to have good process controls.

NOTES:
1 1 angstrom (Å) = 10-10 meters = 0.1 manometers (nm) = 10-4 microns (mm) = 0.004 microinches.

2 Standard atmospheric pressure = 760 mm mercury (or Torr) or 1.01 × 105 Newtons per meter square (or Pascals). 1 Torr=103 milliTorr (mTorr). 1 mTorr = 7.5 Pascals (pa)=1 (mm of mercury).

3 An electron volt (eV) of energy is the amount of energy attained by a singly charged particle accelerated through a potential of 1 V. A kinetic energy of one eV is equivalent to a thermal temperature of ~11,000C

.

Serving the Finishing Industries. Since 1936.
PFONLINE and all contents are properties of Gardner Publications, Inc.
All Rights Reserved.