Energy of sputtered atoms
In order to enhance the density of the coating in PVD, energy should be provided to the growing film.
In the case of magnetron sputtering, the atoms emanating from the target as a result of sputtering, gain a kinetic energy as a result of the momentum transfer from the incoming working gas ions. The working gas ions get their energy from the target bias voltage. The target atoms, when traveling towards the substrate thanks to this kinetic energy, undergo collisions with the working gas atoms and with themselves. Therefore, at the arrival at the substrate their energy is lower as compared to the initial state near the target.
Parameters to increase the energy provided to the coating
Decreasing the target to substrate distance will therefore increase the deposition rate and the probability of having a high energetic target atoms at the substrate which will result in coatings with better density.
The substrate bias voltage is another parameter that can be used to increase the energy of the incoming flux to the substrate. However, for that to be effective, the incoming flux should be ionized. In conventional direct current magnetron sputtering (dcMS) the ionized flux is low and is made from argon ions. As the ionized flux is low, a higher bias voltage is needed to achieve the desired energy to the coating. The energy to the adatoms is provided by particle momentum transfer.
However, a high bias voltage for a flux made mostly of argon ions can lead to argon implantation increasing thereby residual stress and lowering the adhesion.
HiPIMS
So, it is important that the flux of particles towards the substrate is made of ions of the sputtered species. Increasing the magnetic field unbalancing degree can increase the flux of ions at the substrate vicinity. But this flux is made from argon ions since the electron density is still not high to ionize also the target atoms.
In order to increase the electron density so that the probability of electron impact ionization is enhanced, the current to the target is increased. However, a very high power to the target will damage the target. For that the high power is applied in pulses of low duty cycle while the average power over time is maintained similar to that of dcMS.
As result of this method, the sputtered species in addition to argon species are also ionized. This increases the flux of ions to the substrate and as result decreases the substrate bias voltage needed.
This is known as high flux low energy bombardment of the substrate that increases the density without generating two much residual stress and avoid reduction of the deposition rate because of the high substrate bias voltage.
Deposition rate in HiPIMS
Since the sputtered species are now ionized, the bias voltage at the target can attract them back after being sputtered. This will decrease the deposition rate.
In order to increase the deposition rate, one need to push the ionized sputtered species towards the substrate. To do that, different methods are used such as increasing the unbalancing degree and reducing the magnetic field strength, using a pulse train with short pulses, or using a positive pulse after the negative pulse.
Using these procedures, the deposition rate can be equal to that of conventional magnetron sputtering and can be even higher in the case of reactive magnetron sputtering.
In fact, reactive magnetron sputtering induces poisoning of the target when high reactive gas flow is needed. The poisoning of the target is the deposition of the compound layer on the target.
Using HiPIMS the high current at the pulse increases the sputtering rate and therefore the sputtering of compound layer formed at the target is favored keeping the target metallic. Moreover, because of the high electron density, the dissociation rate of the reactive gas molecule is high and therefore the reactive gas flow needed is low which reduces the poisoning as well.
