Growth and Growth-Related Properties of Films Formed by Physical Vapor Deposition
THE PROPERTIES of atomistically deposited films depend strongly on the material being deposited, the substrate
surface chemistry and morphology, the surface preparation process, and the details of the deposition process and the
deposition parameters. The origin of the unique properties of physical vapor deposition (PVD) film can be understood by
understanding the film formation process.
The formation of a useful and commercially attractive engineered surface using any PVD process (vacuum deposition,
sputter deposition, or ion plating) involves several stages:
1. Choice of the substrate ("real surface") and development of an appropriate surface preparation process
2. Selection of the film material(s) to produce the surface properties required
3. Choice of the PVD process to provide reproducible properties, compatibility with subsequent
processing, and long-term stability
4. Development of the fabrication process parameters, parameter limits, and the monitoring/control
techniques
5. Development of appropriate characterization techniques to determine the film properties and stability of
the product
6. Creation of written specifications and manufacturing processing instructions to cover the substrate
material, surface preparation, deposition process, and characterization procedures
The properties of a film of a material formed by any PVD process depends on four factors:
· Substrate surface condition--e.g., surface morphology (roughness, inclusions, particulate
contamination), surface chemistry (surface composition, contaminants), mechanical properties, surface
flaws, outgassing, preferential nucleation sites, and the stability of the surface
· Details of the deposition process and system geometry--e.g., angle-of-incidence distribution of the
depositing adatom flux, substrate temperature, deposition rate, gaseous contamination, and concurrent
energetic particle bombardment (flux, particle mass, energy)
· Details of film growth on the substrate surface--e.g., substrate temperature, nucleation, interface formation, interfacial flaw generation, energy input to the growing film, surface mobility of the
depositing adatoms, growth morphology of the film, gas entrapment, reaction with deposition ambient
(including reactive deposition processes), and changes in the film properties during deposition
· Postdeposition processing and reactions--e.g., reaction of film surface with the ambient, thermal or
mechanical cycling, corrosion, interfacial degradation, burnishing of soft surfaces, shot peening, and
overcoating ("topcoat")
In order for the film to have reproducible properties, each of these factors must be reproducible.
Technological (Real) Surfaces
Technological surfaces or engineering surfaces are terms that are used to describe the real surfaces of engineering
materials. These layers, along with the underlying bulk material, are the real substrate that must be altered to produce the
desired surface properties. Invariably the real surface differs chemically from the bulk material by having surface layers
of reacted and adsorbed material such as oxides and hydrocarbons. The surface chemistry, morphology, and mechanical
properties of the real surface can be very important to the adhesion and film formation process. The underlying bulk
material can be important to the performance of the surface. For example, a wear coating on a soft substrate will not
function well if, under load, it is fractured by the deformation of the underlying substrate. Also, good film adhesion
cannot be obtained when the substrate surface is mechanically weak, because failure can occur in the near-surface
material. The bulk material can influence the surface preparation and the deposition process by continual outgassing and
outdiffusion of internal constituents.
Some of the surface properties that affect the formation and properties of the deposited film are:
· Surface chemistry--affects the adatom-surface reaction and nucleation density. Chemistry can affect the
stability of the interface formed by the deposition.
· Contamination (particulate and film, local or uniform)--affects surface chemistry and nucleation of the
adatoms on the surface. Particulate contamination generates pinholes in the deposited film.
· Surface morphology--affects the angle-of-incidence of the depositing atoms and thus the film growth.
Geometrical shadowing of the surface from the depositing adatom flux reduces surface coverage.
Surface morphology can affect the film properties and stability.
· Mechanical properties--affects film adhesion and deformation under load
· Outgassing and outdiffusion--affects nucleation and film contamination
· Homogeneity of the surface--affects uniformity of film properties over the surface
In particular, the surface morphology can have an important effect on the film properties. Figure 1 shows the effect of
surface morphology and particulate contamination on surface coverage and pinhole formation. Also, the surface
morphology can affect the average angle-of-incidence of the adatom flux, which has a large effect on the development of
the columnar morphology in atomistically deposited films.


The nature of the real surface depends on its formation, handling, and storage history. In order to have reproducible film
properties, the substrate surface must be reproducible. This reproducibility is attained by careful specification of the
substrate material, careful incoming inspection procedures, careful surface preparation, and appropriate handling and
storage of the material.
Surface preparation is the process of preparing a surface for the film/coating deposition process (Ref 1). Surface
preparation may mean cleaning (removal of contaminants), but it can also include surface treatments to change the
properties of the surface in a desirable way, such as roughening or smoothing the surface, making a harder surface by
plasma treatment (i.e., plasma nitriding) or shot peening, or "activating" the surface, such as the oxygen plasma treatment
of a polymer surface. Often surface preparation consists of two distinct stages. The first is "external cleaning," which
takes place outside the deposition system in a controlled environment. This processing environment is designed to control
recontamination after cleaning. For example, to control recontamination by particulates, a filtered air "cleanroom" is used.
External cleaning can consist of both "gross cleaning," which removes a portion of the substrate surface material, and
"specific cleaning," which removes specific contaminants such as hydrocarbons or salts. The second stage of surface
preparation is "in situ cleaning," which is performed in the deposition system. For example, hydrocarbon contamination
can be removed from some surfaces by exposing them to an oxygen plasma in the deposition system.
Care must be taken to ensure that the surface preparation process does not change the surface in an undesirable or
uncontrolled manner, such as selective leaching of one phase of a two-phase surface. One objective of any surface
preparation procedure is to produce as homogeneous a surface as possible. Reproducible surface preparation, as well as
associated handling and storage techniques, are obtained by having appropriate specifications for the process, handling,
and storage procedures used. In addition, recontamination of the prepared surface in the deposition chamber and by the
deposition process is a major consideration.

Atomistic Film Growth
Atomistic film growth occurs as a result of the condensation of atoms ("adatoms") on a surface. The stages of film
formation are:
1. Vaporization of the material (adatoms) to be deposited
2. Transport of the material to the substrate
3. Condensation and nucleation of the adatoms
4. Nuclei growth
5. Interface formation
6. Film growth--nucleation and reaction with previously deposited material
7. Changes in structure during the deposition process--interface and film
8. Postdeposition changes due to postdeposition treatments, exposure to the ambient, subsequent
processing steps, in-storage changes, or in-service changes
All of these stages are important in determining the properties of the deposited film material (Ref 2, 3, 4, 5).
Vaporization
In physical vapor deposition, vapors can be formed by thermal and nonthermal techniques. Thermal techniques require
heating, such as vacuum evaporation and sublimation (see the article "Vacuum Deposition, Reactive Evaporation, and
Gas Evaporation" in this Volume). Nonthermal vaporization includes sputtering (see the article "Sputter Deposition" in
this Volume), arc vaporization, laser ablation, and others.
Transport
The vaporized material can be transported through a vacuum, gas, or plasma. The vacuum environment allows control of
the contamination in the ambient environment to any desired level. The gaseous environment may thermalize energetic
particles and cause vapor phase nucleation, depending on the gas density (see the article "Vacuum Deposition, Reactive
Evaporation, and Gas Evaporation" in this Volume). The plasma environment "activates" reactive species, making them
more chemically reactive.
Condensation and Nucleation
Atoms that impinge on a surface in a vacuum environment either are reflected immediately, reevaporate after a residence
time, or condense on the surface. The ratio of the condensing atoms to the impinging atoms is called the sticking
coefficient. If the atoms do not immediately react with the surface, they will have some degree of surface mobility over
the surface before they condense. Re-evaporation is a function of the bonding energy between the adatom and the surface,
the surface temperature, and the flux of mobile adatoms. For example, the deposition of cadmium on a steel surface
having a temperature greater than about 200 °C (390 °F) will result in total re-evaporation of the cadmium.
Surface Mobility. The mobility of an atom on a surface will depend on the energy of the atom, atom-surface
interactions (chemical bonding), and the temperature of the surface. The mobility on a surface can vary due to changes in
chemistry or crystallography. The different crystallographic planes of a surface have different surface free energies that
affect the surface diffusion. For example, for face-centered cubic metals the surface free energy of the (111) surface is
less than that of the (100) surface, and the surface mobility of an adatom is generally higher on the (111) surface than on
the (100) surface. This means that different crystallographic planes will grow at different rates during adatom
condensation. Various techniques have been developed to study surface mobility and the surface diffusion rate of adatoms
on a surface (Ref 6, 7).
The surface mobility of adatoms can be an important factor in surface coverage. For example, the surface coverage of a
silicon device is improved by depositing an Al/Cu metallization on a TiN barrier layer at 500 °C (930 °F). The Al/Cu has
a higher surface mobility on the TiN surface than on the silicon surface and is able to completely fill a 0.5 m diameter

Additional comments :