Thin-Film Vacuum Deposition Sources

Introduction & Bibliography

We examine methods of manufactruing thin film and deposition sources in Vacuum Systems. Here, we give a brief introduction to the mechanics and physics of sputtering and thermal evaporation sources.

For those interested in greater detail, the book list below gives a good crosssection of the information. The subject matter is so huge there is little content overlap between these books. Also, despite their age, the first two remain frequently quoted references. (Given the short publication lifespan of technical books, do not expect to find many “in print”).

  • Handbook of Thin Film Technology, Maissel & Glang, McGraw Hill 1970, Lib. of Congress Card No. 73-79497
  • Thin Film Processes, Vossen & Kern, Academic Press, Inc. 1978, ISBN 0-12-728250-5
  • Thin Film Processes 2, Vossen & Kern, Academic Press, Inc. 1991, ISBN 0-12-728251-3
  • Handbook of Sputter Deposition Technology, Wasa & Hayakawa, Noyes Publications 1992, ISBN 0-8155-1280-5
  • The Materials Science of Thin Films, Ohring, Academic Press 1992, ISBN 0-12-524990-X
  • Thin-Film Deposition—Principles & Practice, Smith, McGraw Hill 1995, ISBN 0-07-058502-4
  • Handbook of Thin Film Process Technology, ed. Glocker et. al., Institute of Physics Publishing 1995 ISBN 0-7507-0311-5
  • Physical Vapor Deposition of Thin Films, Mahan, Wiley Interscience 2000, ISBN 0-471-33001-9
  • Sputter Deposition, Westwood, AVS 2003

Sputtering

What Is Sputtering?

Sputter deposition, commonly called sputtering, removes atoms/molecules from a solid target’s surface, projecting them into the gas phase from which they condense on another surface. In the simplest arrangement, a high voltage is applied between two circular, plane-parallel disks: a target (cathode) and a substrate (anode) mounted a few inches apart. An inert process gas at 1 to 100 mTorr flows between the electrodes. Initial electrons from the target’s surface cause cascade ionization in the gas, forming a plasma—loosely defined as a confined region with equal concentrations of electrons and positive ions.

Because the plasma is both electrically neutral and highly conductive, there is little voltage drop across it. The drop occurs across thin "dark space" regions between the plasma and each electrode. The target’s negative potential attracts positive ions from the plasma’s edge. They hit the target with sufficient kinetic energies to eject neutral target atoms/molecules by energy transfer.

While traveling from target to substrate, each ejected atom hits numerous gas atoms/molecules that deflect them and cause energy loss. By optimizing the target-substrate distance, the atoms approach the substrate’s surface from partially randomized directions, producing a reasonably uniform film thickness across a textured substrate’s surface.

For circular sources, the optimum throw distance between target and substrate is larger than the target’s diameter to "smooth out" the source’s ring-like deposition pattern. By contrast, a linear production source used to coat large area substrates moving across it, has a short optimum throw distance. Here the linear source’s end-effects disturb the uniformity.

Sputter Sources

The differences between diode, magnetron, balanced/unbalanced, RF/DC power options, etc. for various sputter sources are discussed in Vacuum Systems.

A source’s vacuum compatibility depends on its specific design, but all are suitable for 10-6 Torr to 10-7 Torr range. Some true UHV versions are compatible with 10-10 Torr and bakeable to 400° C (with magnets removed from the air side). The latter group is used where the process chamber must reach UHV conditions (for cleanliness) before backfilling with process gas.

In R & D applications, sources are often mated to existing chambers and the relative positions of the source’s mounting port and the substrate holder determine which source mounting is most appropriate: (i) The axial source has its target’s surface perpendicular to its utilities tube; (ii) The right angle has its target’s surface parallel to the utilities tube; and (iii) The flex has the source and tube connected by a flex metal hose, enabing the angle between their centerlines to vary from 0° to 45°.

An additional consideration for R & D applications is varying the film composition. For simultaneous or sequential deposition of different materials and quick changes in film compositions, a cluster of 3 or 4 sputter sources mounted on one flange with integral cross-contamination shields, shutters, and gas feeds, may be an ideal arrangement.

That implies thicker targets which, in turn, demand correctly shaped magnetic fields to penetrate those targets. Another common production need is depositing films on large rectangular substrates. This is addressed with linear sputter source (longer than the substrate is wide) and then moving the substrate across the source during sputtering.

 

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