Short- and long-term plasma phenomena in a HiPIMS discharge

Abstract

High power impulse magnetron sputtering (HiPIMS) is a plasma-based thin film deposition technique in which extremely high power pulses are applied to a conventional magnetron sputtering source. As a result, the plasma density in HiPIMS discharges is considerably increased up to 10 19 m −3 , about three orders of magnitude higher than that in conventional direct current magnetron sputtering (DCMS) discharges. Hence the vapour of the sputtered species becomes highly ionised, leading to remarkable improvement in the microstructure and the properties of depositing films. To better control the deposition process, it is important to gain insights into the properties and the dynamics of the HiPIMS plasmas. This thesis is associated with the investigations on HiPIMS plasmas using a number of electrical diagnostic tools including a Langmuir probe, a retarding field energy analyser and a gridded quartz crystal microbalance. It was shown, using a Langmuir probe analysis, that there are three distinct groups of electrons generated during first the 4 µs of the HiPIMS pulse. These electrons are super-thermal or beam-like electrons with effective temperatures of up to 100 eV, hot electrons with temperatures up to 7 eV and cold electrons with temperatures < 1 eV. As time progresses, however, these electrons develop into single-temperature Maxwellian electrons. Using the retarding field energy analyser located at typical substrate positions, it was found that ions travel to a grounded substrate with an average energy of up to 10 eV during 20-40 µs into the HiPIMS pulse, and with an energy of 3 − 5 eV for the rest of the pulse. Ions escaping to the side of the discharge axis are also investigated using the movable and rotatable retarding field analyser. It was found that ions, circulating with a similar direction as the electron E×B drift in the magnetised region, are able to azimuthally escape from the discharge with a mean velocity of 8 × 10 3 m s −1 , unless there are collisions with residual gases. Together with the knowledge of radial electric field, determined from plasma potential, the equations of circular motion of an ion fluid element have been solved numerically. Using a biased quartz crystal microbalance in combination with a gridded electrode, the ionised metal flux fraction in a HiPIMS discharge has been investigated. The average discharge power was varied from 0.3 to 1.3 kW and, irrespective of the power control method used, an associated decrease in the flux fraction (from 50% to 30%) was observed. The mechanisms responsible for this decrease in the time-averaged flux fraction of metal ions are associated with the probability of ionisation of the sputtered species and the effect of the ions returning to the target. Finally, a technique of the superposition of a dc pre-ioniser and a HiPIMS power supply is proposed to operate a HiPIMS discharge at a pressure down to 0.08 Pa. The pre-ioniser provides a background plasma with a density of 10 15 m −3 to assist the HiPIMS build-up at the low-pressure range. Using an ener...

An introduction to thin film processing using high-power impulse magnetron sputtering

“…4, reported deposition rates are given for various target materials based on the data from Helmersson et al 62 and Samuelsson et al 8 The most common explanation to the reduction in deposition rate stems from a work by Christie, 63 where back-attraction of metal ions to the target followed by selfsputtering causes a reduction in the amount of sputtered particles reaching the substrate. Controlling and optimizing the potential profile in the cathode region will, therefore, greatly affect the number of metal ions incident on the target surface, 64 which so far has not been fully explored. Gas rarefaction, which was discussed in the previous section, is also likely to affect the deposition rate in HiPIMS processes.…”

Argon metastables in HiPIMS: time-resolved tunable diode-laser diagnostics

“…Very little is known about the afterglow regime of the HiPIMS discharge, but from Langmuir probe measurements [33] it is found that a weak plasma can be sustained for several microseconds (> 10 µs) after pulse-off. It is therefore possible that the increased amplitude of the first peak of m Ar n as the pulse width increases is related to the fact that the plasma decays slower and thus the bulk plasma will contain more Ar m at the beginning of the next HiPIMS pulse.…”

Plasma potential mapping of high power impulse magnetron sputtering discharges

“…Not surprisingly, HiPIMS plasma has been extensively studied using Langmuir probes. [19][20][21][22][23] Various levels of time resolution have been demonstrated for selected positions of the probe, mostly focusing on the interesting region above the racetrack. However, measurements generally lack the survey character that may allow us to gain greater insights.…”