Sputtering is a term to describe the physical process of ions impinging on a target of desired material, ejecting atoms and ions of the target material.
These atoms and ions are then free to move within the deposition chamber, with the aim of adsorbing and reacting on a substrate. To accomplish this process, several steps must be taken. Firstly, the deposition chamber must be evacuated of air to create a vacuum, as described in section 2.2.1. A typical single-magnetron setup is shown in Figure 5.
Once the chamber is under vacuum, an inert gas such as argon must be introduced into the chamber (Figure 6). This is known as the process gas.
Target Substrate Holder
Magnets
Figure 5: Deposition chamber with single-magnetron setup. Powerful magnets are located on the exterior of the camber behind the target, which is the desired deposition material. In this example, a planar substrate holder is used. The square chamber is evacuated of air using pumps via a vent.
51 | P a g e A potential difference between the target (cathode) and chamber (anode) is introduced – this causes a build of charge to occur on the target. Seed electrons within the chamber, generated either by random ionising thermal collisions between argon atoms, or ionisation of argon caused by cosmic rays, are accelerated away from the cathode. Electrons that gain enough energy from this acceleration can then proceed to ionise more argon. The energy gained from this acceleration is impacted by the MFP within the chamber, which also highlights the importance of low and precisely
controlled pressures in this system. The potential difference then causes the positively-charged ionised argon to accelerate towards the target. When the argon ion strikes the target, kinetic energy is transferred and electrons and target atoms are ejected. Argon ions, electrons from the argon and electrons from the target collide with the process gas, causing further ionisation (Figure 7).
Figure 6: Deposition chamber with argon as a process gas. Argon has been added to the evacuated chamber at such a rate as to reach an equilibrium with the evacuation rate; in this manner, the amount of argon present is tunable to the specific parameters required.
52 | P a g e The resulting cascade generates a plasma. Argon atoms that received
energy from the electrons, but fail to ionise, return to the ground state by emission of a photon. This causes the plasma to have a characteristic colour emitted from it, called a glow discharge. Another effect of the argon ions impinging on the target, is the subsequent ejection of target material. When the argon ion impinges upon the target, depending on the energy of the collision, it may ‘burrow’ into the target and cause a cascade reaction; the collisions cause target atoms, ions and electrons to be ejected from the target (Figure 8). The ejected material may deposit and condense on the chamber walls, fixtures and substrate (Westwood, 2003). It is therefore the aim of sputter processes to eject target material into the chamber towards the substrate, with the goal of uniformly coating the substrate material to produce a homogenous, dense film.
The rate at which deposition occurs is of great importance in these processes, as it can influence coating properties as well as influencing
Figure 7: Schematic diagram of seed electron-argon ionisation mechanics, and subsequent electron ejection from target. Several concurrent mechanics occur here; cosmic waves and thermal collisions cause random ionisation of argon, and the applied potential difference between the target (cathode) and the chamber (anode) causes the ionised argon to accelerate towards the target and collide. This collision causes a cascade reaction which causes electrons to eject from the target, causing further ionisation of argon.
53 | P a g e commercial viability. The deposition rate is influenced by many factors, such as plasma locality and density, in addition to target species; heavier species require more kinetic energy to be imparted in order to cause a cascade and subsequent ejection of target material.
Figure 8: Burrowing argon ions lead to ejection of electrons and target atoms. The accelerated argon ions collide with (ejecting an electron from the target) and burrow into the target, causing the atoms of the target material to displace. A cascade reaction occurs, wherein the argon becomes more burrowed into the target and the atoms of the target displace further towards the boundary, eventually ejecting into the chamber.
The processes described so far concerns PVD sputtering techniques in general. What separates magnetron sputtering from other kinds of sputtering is the use of powerful magnets to manipulate the plasma; as the plasma contains most of the electrons and ions, manipulation of such can result in a more efficient and controlled process, with greater deposition rates. Magnets are mounted behind the target and so a magnetic field is applied parallel to the target. When an electron is ejected from the target, it is subject to the Lorentz force; this force changes the direction of the ejected electron, so that it forms a semi-circular orbit close to the target. If the electron does not make any collisions during its first orbit, it returns to the target. However, if the electron loses energy due to a collision with an argon atom, it cannot return to the target and so makes a series of semi-circular ‘hops’ around the target,
54 | P a g e dictated by the arrangement of the magnets (Figure 9) (Rossnagel, 1991;
Westwood, 2003).
Figure 9: Path of ejected electron around a target, dictated by an arrangement of the magnets in the magnetron. The solid line indicates the path of trapped electrons, while the dashed lines indicate magnetic field lines. Adapted from Westwood (2003).
Ejected electrons therefore become trapped in a spiral tunnel path close to the surface of the target, as opposed to escaping like the atoms and ions.
Magnetically trapping these electrons increases the probability of ion bombardment and so promotes further ionisation, vastly improving the sputter rate (Kelly and Arnell, 2000).
As mentioned, ejected target atoms and ions can deposit anywhere within the chamber. In order to increase the probability of deposition on the
substrate, an unbalanced magnetron system can be used. In conventional or balanced systems, the outer and inner regions of each magnetron have equivalent magnetic strengths. Contrastingly, in an unbalanced system the outer and inner regions of each magnetron have different magnetic strength (Parsons, 1991). There are two types of unbalanced system; inner regions stronger than outer regions, known as type-1 and outer regions stronger than inner regions, known as type-2. This leads to changes in the magnetic field
55 | P a g e lines – escaped electrons are guided by these field lines, either converging back towards the target (type-1), or out towards the substrate (type-2).
Maintaining plasma neutrality, ions are dragged along with these electrons (Kelly and Arnell, 2000). Thus, in type-1, the plasma is confined to the vicinity of the target, but in type-2, the plasma is extended into the chamber and to the substrate (Figure 10). Extending the plasma to the substrate has the direct effect of increasing ion density at the substrate surface, resulting in an increased ion bombardment of the growing film. This ion bombardment can be thought of as an atomic peening mechanism, imparting energy into the growing film, promoting diffusion of coating species and increasing density.
These effects are further explored in section 2.3.
Figure 10: Effect of balanced, type-1 and type-2 unbalanced magnetron configurations on plasma density. The size of the solid blue quadrangles correlates to the amount of magnetic material in each region. By adjusting the polarity and amount of magnetic material, at each magnet, the magnetic field lines can be manipulated. As plasma density follows magnetic field lines, this property can be used to manipulate the position and density of the plasma.
While the use of type-2 unbalanced magnetrons represents improvements in the quality of sputtered films over balanced magnetrons, the processes described so far have only discussed the use of a single magnetron. The plasma can be manipulated further by using multiple-magnetron systems. In these systems, the magnetic field lines of adjacent magnetrons can interact and so the orientation (and thus polarity) of secondary magnetrons must be considered.
56 | P a g e It follows that the secondary magnetron can have identical (‘mirrored’)
polarity, or opposite polarity (termed ‘closed field’). If a mirrored polarity is used, the magnetic field lines interact in such a way as to direct secondary electrons towards the chamber walls; this is sub-optimal, as electrons directed in this manner do not contribute to the plasma density at the substrate, reducing ion bombardment of the growing film, creating sub-optimal structures.
In the more effective polarity orientation, closed field, the field lines interact in order to direct electrons towards the substrate; this effectively extends the plasma into the chamber towards the substrate, increasing plasma density at the substrate, resulting in more efficient deposition (Figure 11) (Kelly and Arnell, 2000).
Figure 11: Effects of mirrored configuration (left) and closed-field configuration (right) on plasma (purple) around rotating substrate holder (centre). By manipulating the orientation to a closed-filed configuration, plasma density can be increased at the substrate holder. Adapted from Kelly and Arnell (2000).