Low pressure behaviour of the sputter magnetron discharge

Summary form only given. For successful simulation of the complete magnetron sputter deposition process, the modelling of the plasma is crucial. This is because the position and extension of the plasma, more specific its ionisation distribution, determines the erosion profile, i.e. the area on the target (cathode) from where the atoms are removed due to argon ion bombardment. The pressure dependence of the width of the erosion profile was experimentally studied: it is nearly constant above a certain pressure (0.51 Pa), but below this pressure, the width increases strongly with decreasing pressure. The width of the ionisation distribution, and thus also of the plasma, along the direction parallel with the target surface has the same pressure dependence. As the ionisation of the argon gas is primarily due to the secondary electrons (SE), which are released from the target by impinging ions, we developed a model for simulating the orbits of these SE. This is done by solving the Lorentz equation of motion for charged particles. The magnetic field is calculated analytically, and the electric field is assumed to vary linearly over a known distance. Our simulations show that the arch shaped ionisation region of a single SE, emitted at a certain position at the cathode surface, does not change with pressure. Thus, the change in the plasma must be due to a change of emission profile of the SE. For explaining this change we investigated the SE movement: due to its arch shaped orbit, a SE is brought back towards the surface after one cycloidal bounce. If the initial energy of the SE is set to zero, as is common practice, it is reflected by the combined influence of the electric and magnetic field. However, if the initial energy is given a realistic value (typical 4 eV), the SE can interact with the cathode which can lead to recapture of the SE. This recapture is only possible before a SE undergoes any interaction with the discharge gas. As a result, the effect only appears at- low pressures and affects the SE emitted near the centre of the erosion profile more than the ones close to the edge because the first have a shorter cycloidal bounce. Consequently, lowering the operating pressure favours the SE emitted near the edge and they will relatively be more present. This effect causes the change in SE emission profile and the resulting increase in plasma and erosion profile width. Hence, for modelling the magnetically confined plasma of a magnetron discharge at low pressures the small initial energy of the SE has to be taken into account, and should not be set to zero as is usually done in such simulations.