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Title: The development of electrical plasma diagnostics for HiPIMS discharges
Author: Poolcharuansin, Phitsanu
ISNI:       0000 0004 2729 0546
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2012
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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 1E19 per cubic metre, 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 microseconds 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 microseconds 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 ExB drift in the magnetised region, are able to azimuthally escape from the discharge with a mean velocity of 8E3 metre per second, 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 1E15 per cubic metre to assist the HiPIMS build-up at the low-pressure range. Using an energy-resolved mass spectrometer, it was demonstrated that the average energy and charge state of ions can be enhanced in the low-pressure operation.
Supervisor: Bradley, James Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: TK Electrical engineering. Electronics Nuclear engineering