High power impulse magnetron sputtering (HIPIMS) : Fundamental plasma studies and material synthesis.
Physical vapour deposition (PVD) technology is widely used for deposition of coatings with applications in various industries; coatings in semiconductor industry, optical coatings or hard coatings mostly used in car and aerospace industries. In 1999 novel physical vapour deposition technology entitled high power impulse magnetron sputtering (HIPIMS) was suggested. The principle of HIPIMS is to utilize impulses (short pulses) of high power delivered to the target in order to generate high amount of metal ions with degree of metal ionization up to 90% and ions with energies up to 100 eV. Given that HIPIMS technology has been developed only 10 years ago, extensive amount of fundamental properties of the discharge has not yet been investigated. This thesis describes fundamental research of the plasma properties, mainly temporal and spatial evolution of ions and electrons in dependence on the external parameters such as applied power and working gas pressure. The initial period of the PhD project consisted of learning to use and interpret a data from plasma diagnostic techniques. These diagnostic techniques are Optical emission spectroscopy, Optical absorption spectroscopy, Plasma sampling mass spectrometry and Langmuir probe. The dependence of ion energy distribution function (IEDF) of metal and gas ions on applied power and working gas pressure was investigated. The results show increase in the amount and both average and maximum energy of metal and gas ions with increase in the power. Increase in the pressure, on the other hand, resulted in reduction of the amount of detected ions and lower average energies. The IEDF of both the target and gas ions was found to comprise of two Maxwellian distributions. First cold Maxwellian distribution was fitted to the main peak of the IEDF that had the lower average energy which is attributed to collisions with thermalised gas atoms and ions. The higher energy distribution is assumed to originate from a Thompson distribution of sputtered metal atoms which due to collisions are thermalised and appear as a Maxwell distribution. The time resolved measurements of the IEDF during HIPIMS of Cr target show that hot Maxwellian distribution originates from the pulse-on phase of the pulse while first cold Maxwellian distribution originates from period after the end of the pulse when metal ions are thermalised in collisions with gas particles. Additional measurements using Optical absorption spectroscopy showed increase in the ion to neutral ratio with increasing power.Time resolved measurements of the IEDF at different distances from the target showed that at low pressure the metal ions promptly diffuse through the chamber and at 1.2 ms metal ion density was equally distributed in the chamber. At high pressure metal ions diffuse slower due to short mean free path, nevertheless after 1.2 ms significant density of metal ions could be found at distance of 10 cm reducing equally in space closer to the target and further away from the target. The HIPIMS plasma discharge of most frequently used elements; C, Al, Ti, Cr, Cu and Nb was measured to investigate the temporal evolution and life-span of each element. The life-span of elements measured at low pressure showed dependence on the mass of the element with only Nb ions being detected at the start of the consequent pulse. At high pressure metal ions of all elements except C were detected at the beginning of the consequent pulse. Calculated metal to gas ion ratio showed increase with sputter yield equally at high and low pressure.In the final stage of the PhD project CrAISiN nanocomposite coatings were deposited. One coating with DC power applied on both Cr and AlSi target and three coatings with DC power applied on AlSi target and HIPIMS power applied on Cr target were deposited. Increase in metal to gas ratio was observed, using Mass spectrometer, with increase of the discharge power, particularly increase of Cr ions. Surface of the deposited coatings, recorded with SEM, was considerably smoother when deposited with HIPIMS compared to DC magnetron sputtering. Column size, calculated from micrographs recorded with AFM, was compared to metal to gas ion ratio and it showed decrease in columnar size with increase in metal to gas ion ratio.
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Sheffield Hallam University (United Kingdom).