My introduction in the November MRS BULLETIN to this two-part series on deposition processes discussed the extensive use of thin films in science and technology. That it takes two issues and nine articles to cover this topic — and by no means exhaustively — is testimony to the manifold ways thin films are prepared.

If all deposition processes resulted in the same product, then such extensive coverage would be redundant and unnecessary. Thin films, however, cover a virtual infinity of free energy states — and related crystal structures, micro-structures, defects, defect densities, impurities, compositions, composition modulations, etc. — that are sensitive to the particular deposition process and its conditions. It is this richness of choice that makes thin film science and technology both exciting and, at times, frustrating.

Along with the freedom to extensively vary thin film characteristics, resulting properties and applications comes the difficulty in understanding preparation-characterization-property relations in enough detail to control and reproduce deposition processes.

The November articles covered molecular dynamics computer modeling of nucleation and growth processes, molecular beam epitaxy, organometallic vapor phase epitaxy, and chemical vapor deposition. This month's articles continue the sequence of ways to deposit films, the general direction being toward lower substrate temperatures. Plasmas, which offer both increased flexibility and complexity, are primarily considered. The last article covers thermal plasmas, not to control the vapor deposition but to melt powders which result in a multiple splat-quenched array of particles that form coatings important to industry.

Physical vapor deposition (PVD) technology consists of the basic techniques of evaporation deposition and sputter deposition. PVD is used to deposit films and coatings or self-supported shapes such as sheet, foil, tubing, etc. The thickness of the deposits can vary from angstroms to millimeters.

Applications range widely, from decorative to utilitarian and over significant segments of the engineering, chemical, nuclear, microelectronics, and related industries. They have been increasing rapidly because modern high technology demands multiple and often conflicting sets of properties from engineering materials, e.g., combination of two or more of the following: high temperature strength, impact strength, specific optical, electrical or magnetic properties, wear resistance, fabricability into complex shapes, biocompatibility, cost, etc. A single or monolithic material cannot meet such demands. The solution is a composite material, a core material and a coating each having the requisite properties to meet the specifications.

This article will review evaporation-based deposition technologies, theory and mechanisms, processes, deposition of various types of materials, and also the evolution of the microstructure and its relationship to the properties of the deposits.

The first evaporated thin films were probably prepared by Faraday in 1857 when he exploded metal wires in a vacuum. The deposition of thin metal films in vacuum by Joule heating was discovered in 1887 by Nahrwold and was used by Kundt in 1888 to measure refractive indices of such films. In the ensuing period, the work was primarily of academic interest, concerned with optical phenomena associated with thin layer of metals, research into kinetics and diffusion of gases, and gas-metal reactions.

Ion beam processing for thin film deposition is rapidly overtaking some of the more conventional plasma-based thin film processing techniques. This is due to strong improvements in the types and reliabilities of the sources available as well as a growing understanding of the advantages and capabilities of using ion beams.

An ion beam process can be differentiated from a plasma-based process in that the plasma in an ion beam is generated away from the sample and a beam of ions is directed at the sample. In a plasma-based process, the sample is usually immersed in the plasma. This highlights the fundamental advantage of ion beam processing—control of the flux and energy of the ions incident on either a sample or a target (for sputter deposition). It is this control which is missing in plasma-based processing, where the ion flux (current), ion energy, chamber pressure, and gas species are all hopelessly intertwined. In addition, certain aspects of the ion bombardment—angle of incidence, complications of gas scattering, etc. —are essentially fixed in plasma-based processing, leaving no room to vary parameters, and in conjunction, film properties.

A wealth of different types of ion sources cover a broad range of beam currents and energies. At the high energy end (0.1 – 20 MeV) are the implantation sources, typically used for doping semiconductors and treating surfaces (hardening, for example) and for various types of nuclear chemical analysis. These sources, however, tend to be very low current (μA). At slightly lower energies (tens of kilo-electron volts), but significantly higher currents (50 A), are the ion sources used for heating fusion plasmas.

Deposition of films by sputtering was observed first in 1852 by Grove. The technique was in general use through the 1920s for preparing reflective coatings and other thin film samples. Western Electric deposited gold on wax masters for phonograph recordings. The improvement in diffusion pump technology at that time caused thermal evaporation deposition to replace sputtering.

Not till the 1950s did sputter deposition reappear… Bell Laboratories developed tantalum hybrid circuit technology using sputter deposition. Besides depositing Ta, they created a new material, Ta2N, by reactively sputtering tantalum in gas mixtures of argon and N2. Since then, these two methods, sputtering of metals and alloys and reactive sputtering of compounds, have been investigated for many applications of thin film materials.

Although the general aspects of the methods have changed little in the past 30 years, the implementations have changed significantly, particularly since the introduction of magnetron systems in the 1970s. This review will concentrate mainly on these flexible, high rate magnetron deposition systems.

The term sputtering actually applies to the physical processes by which atoms are removed from a material. Momentum is transferred from an incident, energetic particle, usually in the form of an ion, to atoms of the target material. A large number of these atoms are displaced from their normal sites in the crystal lattice, producing a disordered structure that also contains some of the incident particles, which are implanted. Some of the target atoms are displaced from the surface; if they have enough energy, they escape from the target as sputtered atoms.

Over the past two decades a vast number of publications have emerged from laboratories all over the world, describing the application of plasmas for preparing and processing materials. MRS symposia, scientific journals and books, and complete conference series are solely devoted to this specific topic.

Modern VLSI integrated circuits, for instance, would simply not exist without sophisticated plasma etching techniques. But highly reactive, partly ionized and dissociated, quasi-neutral gases—plasmas—are not only useful for etching purposes, i.e., the removal of materials. They are also very valuable tools for the deposition of materials with unique structures and compositions at lower temperatures than for conventional thermally induced chemical vapor deposition processes. Backed by intensive research activities and more than a decade of practical experiences, plasma deposition technologies are now penetrating a number of industrial manufacturing processes.

Plasmas can be classified into two basic categories — non-isothermal, and isothermal or thermal plasmas.

Within the high electric fields applied for non-isothermal plasma generation at reduced pressure, free electrons are accelerated to energies that correspond to several thousand degrees in the case of thermal activation. The neutral species in the gas phase and the heavy ions are either not influenced by the fields or cannot follow changing fields fast enough. Their temperature stays low, resulting in a difference between electron and gas temperature. In these nonequilibrium plasmas, the collisions of high energy electrons and gas molecules result in dissociation processes that would only occur at very high temperatures of more than 5,000 K in the case of thermal equilibrium. Therefore, non-isothermal plasmas allow the preparation of materials and compositions that are difficult to obtain using thermally activated, conventional CVD. Due to the initiation of chemical reaction by collisions with “hot” electrons rather than hot gas molecules, the processing temperature can, in many cases, be kept lower than in conventional deposition processes.

The concept of plasma is central to many scientific and engineering disciplines—from the design of neon advertisement lights to fusion physics. Plasmas vary from low density, slight states of ionization (outer space) to dense, thermal plasmas (for extractive metallurgy). And plasmas are prominent in a wide range of deposition processes — from nonthermal plasma-activated processes to thermal plasmas, which have features of flames and which can spray-deposit an enormous variety of materials. The latter technique, arc plasma spraying (or simply, plasma spraying) is evolving rapidly as a way to deposit thick films (>30 μm) and also freestanding forms.

This article will review the technology of plasma spraying and how various scientific disciplines are contributing to both an understanding and improvement of this complex process.

The plasma gun dates back to the 1950s, when it was introduced for the deposition of alloys and ceramics. Due to its high temperature flame it was quickly discovered that plasmas could be used for depositing refractory oxides as rocket nozzle liners or to fabricate missile nose cones. In the latter technique, the oxide (e.g., zirconia-based ceramics, spinel) was sprayed onto a mandrel and the deposited material was later removed as a free-standing form.

The technique's versatility has attracted considerable industrial attention. Modern high performance machinery is commonly subjected to extremes of temperature and mechanical stress, to levels beyond the capabilities of present-day materials. It is becoming increasingly common to form coatings on such material surfaces to protect against high temperature corrosive media and to enhance mechanical wear and erosion resistance. Several thousand parts within an aircraft gas turbine engine have protective coatings, many of them plasma sprayed. In fact, plasma spraying has emerged as a major means to apply a wide range of materials on diverse substrates. The process can be readily carried out in air or in environmental chambers and requires very little substrate surface preparation. The rate of deposit buildup is rapid and the costs are sufficiently low to enable widening applications for an ever increasing variety of industries.