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Plasma activated growth of reactively sputtered optical thin films

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posted on 27.06.2017, 14:26 by Kazuki Oka
The effect of the configuration of the confining magnetic field on the operating characteristics of a planar magnetron has been observed. In particular, the heat load and charged particle bombardment of the substrate were measured. A circular planar magnetron was constructed with a magnetic field which could be controlled by the adjustment of the current flowing through coils placed around the circumference and with a separate supply feeding a coil around the centre pole. This allowed the magnetic field configuration to be unbalanced and caused the negative glow plasma to be incident on the substrate. It was found that the "unbalanced" electrically-controlled model could give six times the heat load to a substrate in comparison with a balanced permanent magnet structure. Such a load resulted from the bombardment of the substrate with the plasma (revealed by the measurement of the I-V characteristics), and gave a 30 volt negative bias as well as saturated electron and ion currents of around 90 and 1 mA respectively (a magnetron operating current: 0.8A). The heat load was lkWm⁻². This compared to a 1 volt positive bias with current of 1.4 and 0.5 mA for the balanced case. This bombardment was used as a neutralised ion beam to enhance film growth during sputtering. The effect of the magnetic field configuration has been observed on the operating characteristics of a planar magnetron. In particular, the deposition rate, the currents to a probe and heat load, as a function of the outer pole solenoid current, were measured. The deposition rate/input power was constant at 19 mÅ/s/W, independent of the outer pole solenoid current. In the region from 20 to 40 A of the outer pole solenoid current, the configuration of the magnetic field seemed to be concentric and the electrons were confined by it. But from 40 to 80 A, the magnetic field became dispersed outwards along the direction to the magnetron and the electrons followed it. Consequently, following bombardment of the surface by electrons and neutralising ions, the currents to a probe and the heat load, gave a behaviour. The effect of the pressure has been observed on probe current, floating potential and heat load. With the pressure increasing, they decreased. It seems that the mean free path of particles becomes shorter and then scattering of them increases. The influence of the magnetic field strength on the plasma, created at the rear of a substrate, has been observed. In particular, the floating potential and the charged particle bombardment of the substrate were measured. Permanent magnets at the rear of the substrate, outside of the chamber, could make the configuration of the magnetic field change and control the floating potential from -3 to -59 V. At the same time, they influenced the current-voltage characteristics of a probe and made it change following the direction of the field. Ti0₂ is a very important thin film material in optics because the refractive index is high and it can also satisfy the requirements of a hard, dense and chemically stable coating. In our study, the effect of the floating potential, and the resulting ion beam bombardment on the properties of Ti0₂ thin films deposited by reactive planar magnetron sputtering, has been observed. In reactive magnetron sputtering of Ti0₂, as the partial pressure of oxygen is increased, reaction products form on the target (it is poisoned) which lead to an unstable situation with the cathode switching from metal to oxide uncontrollably. More sophisticated control techniques are required in order to get a stoichiometric Ti0₂ film. The oxygen flow was controlled by observation of the light emission of the Ti spectral line. This was measured using a band pass filter and a photomultiplier. It maintained the oxygen flow by regulating a piezovalve. The optical emission controller gave a complete range of Ti to oxygen ratios. As a result, we obtained samples with a high refractive index of 2.52 at 633 nm and a high deposition rate of 4 Å/s. The properties measured were the refractive index, the surface composition and morphology, and the crystallinity. The relationship between the refractive index and the proportions of 'anatase' and 'rutile' crystal structures of the Ti0₂ films was especially considered and the dependence on the floating potential was assessed. Also, Al₂0₃, Cuo, ZnO have been sputtered and the effect of pressure instability assessed. Their properties were measured.



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© K. Oka

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A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.



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