The processes discussed so far have focussed on the deposition of purely metallic coatings; the sputtered target material deposits onto the substrate, without reacting with other species. This is ideal when the intended coating is the same composition as the target material. However, if the desired coating has a different composition to the target, additional processes are required.
One such process is reactive sputter deposition, also known as reactive mode – in this mode, reactive gas species are introduced into the deposition chamber. During deposition, the evaporated target material undergoes
reactions with present reactive gas species, to produce a compound coating.
Common reactive gasses include oxygen and nitrogen, to produce metal oxide coatings and metal nitride coatings, respectively. While the reactive mode enables deposition of compound coatings, it is not without additional complex processes to consider. Introduction of reactive gasses can have profound effects on the structure of the coating; equally, the stoichiometry of compound coatings must be considered. Additionally, attention must be given to the rate of deposition.
The chief factor in these considerations is the partial pressure of reactive gas in the chamber. At low partial pressures, reactive gas species may react with deposition material (from the target) at the substrate; this is known as the metallic mode. In the metallic mode, deposition rates can be relatively high and are similar to non-reactive modes. However, there are usually too few reactive gas species present to create a stoichiometric coating. This leads to the production of a sub-stoichiometric coating, which is not desirable.
In order to produce coatings of stoichiometric composition, the partial
pressure of the reactive species must be increased. This is usually achieved by increasing the flow rate of the species into the chamber. However, the relationship between coating properties and flow rate of the gas is non-linear (Musil et al., 2006). As the reactive gas flow is increased, only small changes in the partial pressure of the gas are observed. This is because there is sufficient deposition material within the chamber to react with the gas, preventing increases in pressure. However, if the flow rate is increased
58 | P a g e further and exceeds the absorption/reaction capacity of the deposition
material, the reactive gas will begin to react with the target surface. This creates a more resistive oxide layer, which reduces the target voltage. The lower target voltage reduces the acceleration of argon ions towards the target, lowering the amount of kinetic energy transferred on impingement.
This makes sputtering less efficient and drastically reduces deposition rate;
this mode of action is the ‘oxide mode’ and the target is said to be ‘poisoned’.
When in oxide mode, any further increase of the flow rate of reactive gas will result in linear increase in partial pressure of that gas. Thus, despite lower deposition rates, stoichiometric coatings may be produced.
Once the oxide layer has been formed, it may only be removed by reverting to the metallic mode. This is achieved by reducing the partial pressure of the reactive gas species within the chamber past a secondary critical value and allowing the targets to ‘sputter off’ the oxide layer – this is known as ‘sputter cleaning’. Interestingly, this secondary critical value is lower than the first critical value, where the target enters the poisoned mode. This behaviour is called hysteresis and is represented graphically in Figure 12.
Figure 12: Hysteresis characteristics with target voltage as a function of reactive gas flow. The critical value for the change from metallic mode (A-B) to poisoned mode (C-D) is higher than the critical value (E) for reverting back to metallic mode. Adapted from Banyamin (2014).
59 | P a g e The lower critical value for reverting back to metallic mode is caused by the reduction in target voltage. The oxide sputters off at a much lower rate than the metal and takes a long time to be removed.
In reactive mode systems, it is desirable to increase the partial pressure to produce stoichiometric coatings, but the partial pressure must not become too high, or coating structure may be negatively affected (discussed in
section 2.3). Additionally, exceedingly high pressures may reduce deposition rates further. Therefore, control of the partial pressure of reactive gas within the chamber is vital. The flow rate and thus partial pressure, of the reactive gas can be controlled automatically using an optical emission monitoring (OEM) system. Such systems consist of a fibre optic cable adjacent to a window of the chamber connected to a monochromator and detector, which can then control the flow rate of the reactive gas into the chamber. When the plasma is ignited, a glow discharge effect occurs; it emits a spectrum of photons characteristic to its constituent particles, i.e. the target material. A prominent, characteristic wavelength within this spectrum can be selected using the monochromator and measured using the detector; as deposition rate remains constant, intensity of the wavelength remains constant. When in metallic mode, the maximum is known as the ‘full metal signal’.
When the reactive gas is added to the chamber, the resulting reactions cause the colour of the plasma to change and the intensity of the characteristic wavelength to decrease. In this way, the intensity of the characteristic wavelength can act as an analogue for the partial pressure of the reactive species. Therefore, a specified percentage of the full metal signal can be set, such that the OEM system increases the flow rate of the reactive gas until the detected emission is at the set percentage of the full metal signal. The flow rate is then tightly throttled by a rapid response valve to ensure constant emission percentage is maintained.
As discussed, the formation of an oxide layer on the target drastically
reduces target voltage and sputtering rate. However, being a semiconducting material (and so more resistive than the pure metal target), charging effects must also be considered. When a potential difference is applied across an
60 | P a g e anode and cathode separated by an insulating material, the material will charge until a dielectric barrier discharge occurs, also known as an arc.
These arcs can result in droplets of material being ejected from the target and condensing on the substrate, causing damage and stress to the growing coating and negatively impacting the structure. In addition, the arc may provide a nucleation point on the target for further arcs to form, resulting in a feedback loop that ultimately reduces the effectiveness of the coatings and damages the magnetron. Finally, the arc can cause rapid and unpredictable fluctuations in voltage and plasma density, which may affect the
stoichiometry of the growing coating. It is clear that control of arcing behaviour is critical.
One such manner of control is the use of a pulsing DC power supply – the use of a pulsing magnetron system is termed pulsed magnetron sputtering process. When the target voltage is pulsed, the build-up of charge is
reduced, minimising the probability of an arc forming. The effect of pulsing is on coating structure illustrated in Figure 13. In Figure 13a, no pulsing was used during the deposition process; it can be seen that the coating is
heterogeneous, porous and has a rough surface. This contrasts with Figure 13b, where pulsing was used – the coating is dense, homogenous, with a smoother surface.
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Figure 13: SEM micrograph of fracture sections of alumina deposited by a) DC reactive sputtering processes, without pulsing and b) pulsed DC reactive sputtering coatings (Kelly and Arnell, 2000). The coating deposited via a pulsed system is much more homogenous with fewer stress features than the coating deposited in the DC mode.
Pulsed DC deposition is a combination of the frequency of the cycle and the ratio between voltage on and off times, i.e. negative and positive voltages phases. Pulsing was only found to be effective at reducing arcing when the frequency was above 20 kHz. Above this, suppression of arcing was most prominent when the voltage off-time was approaching the same as the voltage on-time (Kelly and Arnell, 2000).
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