Ion plating — past, present and future
Introduction
Vacuum coating processes can be thought of as occurring in three stages. The first is the generation of the material to be deposited by a source. In PVD this is vaporization from a solid or liquid surface by thermal evaporation, sputtering, arc vaporization or laser ablation. In CVD it is the chemical vapor precursor such as C2H2, TiCl4, WCl6, SiH4, etc.
The second stage is transport through the vacuum to the substrate. This may be without collision with other particles, collision with other particles without nucleation or gas phase nucleation and ‘nanoparticle’ formation. In CVD processing, the initial decomposition or reduction of the chemical vapor precursor may begin in the vapor phase depending on the gaseous conditions such as temperature or the presence of a plasma. Plasmas also ‘activate’ the reactive gas in reactive deposition processing making them more chemically reactive.
The third stage is condensation on the substrate or growing film. In PVD processing this condensation involves nucleation and interface formation followed by build-up of the film material (film growth). The development of the interface may continue during the film growth. If the process is a reactive deposition process, the condensation also involves reaction of the elemental deposited species with a gaseous species, an adsorbed species or a co-deposited species to form a compound. If the depositing species is a non-stoichiometric molecule, such as an oxide that has lost some of its oxygen, the depositing species may react with the gaseous ambient in a ‘quasi-reactive’ deposition process. In CVD processing, the condensation may involve the final decomposition or reduction of the chemical vapor precursor species on the surface possibly with reaction with a gaseous or co-deposited species.
Ion plating is an atomistic vacuum coating process in which the substrate surface is sputter-cleaned and maintained clean until the depositing film material covers the surface. The depositing film is then continuously or periodically bombarded by energetic atomic-sized inert or reactive particles during film growth. The bombardment affects the nucleation, growth and properties of the coating. The source of depositing atoms can be from vacuum evaporation, sputtering, arc vaporization, a chemical vapor precursor or from a combination of sources, for example sputtered titanium and carbon from the decomposition of acetylene (low pressure-CVD) that condense and react to form TiC. Perhaps it should be noted that in some cases different species may condense without chemical reaction and thereby form a mixture.
Bombardment can occur by the impact of inert or reactive ions (and high-energy neutrals formed by charge exchange collisions between thermal neutrals and high energy ions) accelerated from a plasma in the deposition chamber (‘plasma-based’ ion plating) or from a separate ion source (‘ion gun’) (‘vacuum-based’ ion plating). The use of ion beams for bombardment became known as ion beam assisted deposition (IBAD). Fig. 1 depicts the two variations. Energetic bombardment can also be from accelerated ions of vaporized source material or from energetic atoms formed by neutralization and reflection of high-energy ions that are bombarding a sputtering target in a low gas pressure where there is little ‘thermalization’ of the reflected high-energy neutrals.
Ion plating was first described in the early 1960s [1], [2], [3], [4], [5], [6] and was initially used to enhance film adhesion and improve surface coverage. The initial description (and patent) covered using thermal evaporation and CVD precursors as deposition sources since sputter deposition, at that time, was a very slow deposition process and vacuum arc vaporization sources had not been developed. The development work was done at Sandia National Laboratories, a US Government facility, and the patent on ion plating has been in the public domain from the beginning.
Later it was shown that controlled bombardment during deposition could be used to modify film properties such as density, morphology, electrical conductivity, index of refraction, and residual film stress. More recently bombardment has been shown to enhance chemical reactions in reactive and quasi-reactive deposition processes. Presently ionization and acceleration of the depositing film atoms (‘film ions’) is being used for ‘directed deposition’ to improve filling of surface features in semiconductor processing.
The energetic particle bombardment associated with ion plating provides a powerful processing variable that can be used to modify and control the properties of the deposited material. The stages of ion plating can be differentiated into surface preparation, nucleation and interface formation, and film growth. The energetic bombardment can affect each stage.
Section snippets
Surface preparation
The condition of the substrate surface can be an important factor in the nucleation and interface formation stage of film deposition. The presence of a surface layer, such as an oxide or a contaminant such as a hydrocarbon, can affect nucleation density and prevent reaction and diffusion. This can result in poor adhesion, high electrical contact resistance, lack of long-term stability, etc. Sputter cleaning [7] has been used since the 1950s to clean surfaces for surface science studies. By
Nucleation, surface coverage and interface formation
Numerous studies have shown that the nucleation density on surfaces being ion bombarded is generally greater than that on non-bombarded surfaces. This can be due to a number of factors including: having a clean surface, defects in the surface, surface heating and/or recoil implantation.
In the ion plating process surface coverage is improved by sputtering and redeposition of the depositing material as well as some degree of backscattering of sputtered material. Gas scattering of the depositing
Film growth-elements, alloys, mixtures and compounds
With the advent of the scanning electron microscope (SEM) in 1965, morphology studies of deposited coatings became more refined. In the early 1970s many morphological studies of DC diode sputter-deposited elemental and r.f. sputter deposited films showed that the ion bombardment was densified by the deposited material [12], [13], [14] and influenced the residual stress in the films. Later the densification was modeled by a number of authors [15]. Important processing parameters are the mass,
Film growth-reactive and quasi-reactive deposition
In the mid-1970s, with the advent of high-rate DC magnetron sputtering, it was shown that bombardment densified the films and increased their hardness. In 1983 it was shown that bombardment using concurrent bombardment of inert gas ions increased the chemical reaction rate of reactive gases on surfaces [18]. At about the same time it was shown that bombardment with reactive gas ions beams from ion sources also increased the chemical reaction rate [19]. Bombardment effectively increased reaction
Applications of ion plating
The ion plating process is known by a number of other names. These include: ion vapor deposition (IVD) which is widely used in the aerospace industry, ion assisted deposition (IAD), bias sputtering, sputter ion plating (SIP), energy-assisted deposition, and ion beam assisted deposition (IBAD) which is widely used in the optical coating industry. Ion plating can also be considered a process variable for common deposition processes such as thermal evaporation, sputter deposition and arc vapor
Future directions
Ion plating has generated several new industrial markets and can be expected to create more in the future. In particular, applications that replace electrodeposition and its associated pollution, are to be expected. Deposition sources using ‘film ions’ (i-PVD) formed by postvaporization ionization in a plasma formed by an auxiliary plasma source [39]. An example is the ‘directed deposition’ processes for filling high-aspect-ratio surface features by using an r.f. plasma for postvaporization
Conclusions
The energetic particle bombardment used in the ion plating process provides a powerful processing variable that can be used to modify and control film properties in a controlled manner. By application of proper procedures its difficulties can be avoided. The use of periodic deposition and bombardment allows the process to be used on thermally sensitive substrates.
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Donald M. Mattox was awarded the Albert Nerkin Award (1995) by the AVS for his work in developing and understanding the ion plating process.