Sputter Coating - Operating Characteristics

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5. Operating Characteristics

Table of Contents:

1. Introduction
2. Gaseous Conduction
3. Glow Discharge
4. Sputter Coating
5. Operating Characteristics
6. Specification

The glow discharge in sputtering is significantly dependent on the work function of the target material and pressure of the environmental gas. A range of target materials are used including Gold, Gold-Palladium, Platinum and Silver, although Gold is the most common having the most effective electrical conductibility characteristics. The sputter head and sputter power supply should be effective over a range of anticipated target materials. The deposition rate is current dependant, and if we operate in the correct glow region of the characteristic previously described, several fold changes in current should be available for a relatively small change in sputtering voltage. The deposition rate should not be sensitive to small changes in pressure, which may be experienced in the system.

If an efficient sputter head design, operating on low voltage and as a result low energy input, is achieve, then radiant heating from the target and high energy electrons, (potentially the most significant sources of damage to delicate specimens) should be considerably reduced. There is also evidence to suggest that such a sputter head system may also produce finer grain size for a given target material. The presence of an inert gas, which will not decompose in the glow discharge, is obviously desirable. Argon, having a relatively high atomic weight, provides a suitable source of ions for effective bombardment of the target material. The effectiveness is also dependent on the mean free path (m.f.p.) that is inversely proportional to pressure. If the m.f.p. is too short, insufficient energy will be gained for effective bombardment and will inhibit movement of sputtered material from the target. If the m.f.p. is too long, insufficient collisions occur and, in addition, the flow of sputtered material may change from diffusion in the gas to free molecular flow with a reduction in the effectiveness of omni-directional deposition.

If these characteristic of sputter heads are achieved it should not be necessary to cool the specimen stage for the majority of applications. If not, however, such cooling will only serve to reduce the baseline temperature, the thermal conductivity of most specimens we are considering being relatively poor. For sensitive specimens pre-cooling and subsequent reduction of the baseline may still be desirable and there is also evidence to suggest a reduction in grain size of the coating. It may be apparent that Scanning Electron Microscopy requires a versatile system without compromising performance. Specifically, fine grain size, uniform coating and low heat input. Consideration of these characteristics in design and development should enable a suitable coating system to be realised. It was indicated previously that while empirical design may be in evidence, it should now be apparent that efficient production of positive ions for glow discharge is required. The sputter head and its associated power supply should be a primary objective of design and development.

Figure 3.

Certain sputter heads can employ an annular magnet and shroud assembly, with disc target. The magnetic lines of force form enclosed loops at the target surface; deflection and retardation of electrons resulting in increased ion yield sputtering efficiency. The power supply employing solid stage switching for applied voltage control.

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