Introduction

SPUTTERING is a nonthermal vaporization process in which surface atoms are physically ejected from a surface by momentum transfer from an energetic bombarding species of atomic/molecular size. Typically, sputtering uses a glow discharge or an ion beam (Ref 1) to generate a flux of ions incident on the target surface. These ions cause atoms, and occasionally clusters of atoms, to be knocked free from the target surface by impact transfer, or sputtering. Sputtering is used in two principal applications: sputter etching, in which the primary objective is removal of material from the target surface (Ref 2); and sputter deposition, in which redeposition of these sputtered atoms onto another surface, or substrate, is the primary goal. The latter application is discussed in this article. The fundamentals of plasma formation and the interactions on the target surface are discussed first, followed by the differences between reactive and nonreactive sputtering, and several methods of process control. In the third section, the basic principles and relative advantages and disadvantages of the most common sputtering techniques are examined, specifically direct-current (dc) diode, radio-frequency (rf) diode, triode, magnetron sputtering, and a relatively new technique known as "unbalanced" magnetron sputtering.

Compared to other thin-film deposition methods, sputter deposition techniques have several distinct advantages:

• Use of an unlimited range of source and film materials (i.e., metals, semiconductors, insulators, alloys, and compounds)

• Small sputtering-yield variations from one material to another as compared to the relative variation in the evaporation rates at a given temperature

• Ease of low-temperature deposition of refractory materials

• Elimination of droplet emission from the source that can occur in thermal evaporation

• Absence of droplets, which are common in arc-deposited films

• Ease of forming multicomponent films

• Uniformity of film thickness over large areas

• High degree of film adhesion

• Environmentally friendly processing

Sputter deposition processes have several limitations as well:

• Target (source) materials must ordinarily be in sheet or tube form.

• Deposition rates are typically less than 300 nm/min (3000 A/min).

• Setup costs are high because of the required vacuum environment.

• Line-of-sight process may not be suitable for three-dimensional components.

• Energy efficiency is low (70% or more of the input energy is expended in target heating).

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