In vacuum evaporation and low-mean free-path sputter deposition, where high-energy neutrals reflected from the sputtering target are thermalized, the most important processing variables that influence the properties of PVD-deposited films are substrate temperature and the angular distribution of the vapor flux impinging on the substrate. In the early 1960s, it was shown that controlled, concurrent energetic particle bombardment of the depositing film material by massive particles of atomic or molecular dimensions could be used to modify and tailor the properties of deposited film material. Concurrent bombardment during deposition then became a major processing variable. In the case of long-mean-free path sputter deposition, such as low pressure magnetron sputter deposition, the high-energy reflected neutrals are often an important, uncontrolled processing variable.

Techniques that use concurrent energetic particle bombardment to control film properties are termed Don Plating," or, on-Assisted Deposition" (IAD). The most common form of ion plating uses the substrate and its fixturing as an electrode to generate a DC or rf plasma in contact with the surface being coated, as shown in Fig.1. The plasma can be of an inert gas, such as argon, or a reactive gas(es) that provides energetic ions and such reactive species as nitrogen, carbon or oxygen to deposit compound films, oxides, nitrides, carbides or carbonitrides (reactive ion plating). The plasma can also be formed independently of the substrate. If the process takes place in a vacuum with the inert or reactive ions produced by an "ion gun," it is often called "Vacuum ion Plating," or Don Beam-Assisted Deposition" (IBAD), shown in Fig.2. In ion plating, the source of the condensable film material can be from thermal vaporization, sputtering (sputter ion plating) or a chemical vapor precursor gas, like that used in chemical vapor deposition (chemical ion plating).

Concurrent or periodic bombardment during film growth modifies the film properties by affecting the nucleation of the depositing adatoms, densifying the film by compaction, or "atomic peening." This introduces significant thermal energy directly into the substrate surface region by depositing the kinetic energy of the particle and releasing the ionization energy by recombination. Compressive film stress is also initiated, by recoil implantation of atoms into the atomic structure and, in the case of reactive deposition, enhancing chemical reactions on the surface. During reactive deposition, the plasma also "activates" the reactive species, contributing to the reactive deposition process.

It has been determined that, for argon ion bombardment, the energy of the bombarding ions should be greater than about 50 eV and less than about 250 eV, to effectively modify the film properties. For lower 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 there is a high substrate temperature. 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 (see this column, January and February 1994), it is necessary that about 20 eV per depositing atom be added by the concurrent bombardment. This means a bombardment ratio of about one energetic ion (200 eV) per 10 depositing film atoms. For example, at a 30 Å/sec deposition rate, the ion flux (200 eV ions) should be at least 105 ions/cm2, or an ion (singly charged) current of about 0.1 ma/cm2. At these ion energies and fluxes, an appreciable portion (10-30 percent) of the depositing atoms is sputtered from the growing film surface. Energetic particles of atomic size can be formed by:

1. acceleration of positive ions from a plasma to a negatively charged (biased) surface;

2. reflection of high-energy neutrals from a sputtering target;

3. acceleration of positive ions from a positive space charge, such as that found in vacuum arcs;

4. acceleration of negative ions such as O- from an oxide sputtering cathode; and

5. formation and extraction of ions from an "ion gun."

Generally in ion plating, the high-energy bombarding particles are positively charged ions that are extracted from a nearby plasma and accelerated to the growing film surface. The negative potential on the substrate surface can be formed by applying a DC potential to an electrically conductive surface, applying an rf potential to an electrically insulated surface, by applying a combination of DC and rf bias, or by inducing a "self bias" on an electrically insulating or electrically "floating" surface.

All large-area surfaces in contact with a plasma will have a negative potential with respect to the plasma. The negative potential (self-bias) on the surface is generated because the highly mobile electrons from the plasma strike the surface at a greater rate than the less-mobile ions. This high flux of electrons to the surface generates a "sheath potential" between the plasma and the surface that is capable of accelerating the less-mobile, positive ions from the plasma and the surface at a rate sufficient to give an equilibrium of electron and ion flux to the surface. For low-temperature plasmas, this sheath potential is on the order of a few volts, and the accelerated ions are not capable of modifying the film properties by momentum transfer, but can be used to "ion scrub" or" reactive plasma etch" a surface. To obtain high sheath potentials, electrons can be generated from a hot filament or a hollow cathode, accelerated away from the electron source, and magnetically confined so as to strike the substrate surface. With this technique, sheath potentials of 50-100 volts can be generated and the ions accelerated to this surface are capable of modifying the film properties.

All film properties are affected by concurrent energetic particle bombardment. Of particular note are enhanced adhesion between the film and substrate, densification of the deposited film material, ability to control residual film stress, influence on grain size and orientation, and, in the case of reactive deposition, the ability to enhance chemical reactions on the surface without raising the substrate temperature and the selective desorption/sputtering of unreacted species. One aspect of ion plating that is often important is the improved surface coverage achieved by the sputtering and redeposition of a portion of the depositing atoms. O

Bibliography

D.M. Mattox, Ion Plating," (Chapter 6), Deposition Technologies for Films and Coatings (revised), edited by Rointan F. Bunshah etal, Noyes Publications (1994).