Oblique sputtered and evaporated magnetic thin films for tape recording


Bijker, Martin D. (1998) Oblique sputtered and evaporated magnetic thin films for tape recording. thesis.

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Abstract:After having introduced some aspects of tape recording in chapter one, chapter two deals with the magnetron sputtering process itself. The kinetic energy of the electrons which are bound to the groove region by the combined action of the B and E field, was calculated. It was found that for a magnetic target, the calculated kinetic energies are clearly too low to be able to ionise the argon gas. Since this is in contradiction with reality, it was inferred on a collective movement of the electrons, instead of the particle behaviour which was assumed in the calculation of the drift speed. This should then be accompanied by an increase in drift speed. After having established the current that is associated with the groove electrons, also the local argon gas temperature just in front of the cathode was measured to be around 330 K. Furthermore it was concluded that the collision probability of an sputtered atom with the neutral argon gas is mainly dependent upon the argon pressure. Collision with either argon ions or electrons in the plasma are much less frequent or have little effect. Finally, from a TRIM simulation, it turned out that atoms which are ejected under a larger angle with respect to the normal on the target, generally carry more energy but'are less frequent in number compared to atoms which are ejected under low angles of incidence. Furthermore it was found that the spread in energy was also larger in the case of high scattering angles. In chapter 3, the oblique growth of sputter deposited films was briefly- discussed. In contrast to evaporation, with sputtering there are even more complicating factors affecting the film growth. On average, the argon atoms are impinging on the growing film a factor of 10.000 times more than the atoms which contribute to the growing film do. Furthermore the presence of high energy neutrals is also affecting film growth. Moreover the energy and angular distributions in the case of sputtering are not as readily calculated compared to the case of evaporation. In order to find some relief, a commercial simulation tool was purchased. Although some trends can be found, the complex interaction of all the quantities affecting the oblique growth and thus the columnar inclination angle is still far from understood. After having introduced the measurement and analysis equipment in chapter 4, chapter 5 deals with obliquely evaporated tape. Here several experimental tapes are compared with respect to microstructure, magnetic and recording properties to a commercial Hi8ME tape. It was concluded that although experimental ME tapes can be deposited with varying parameters, it does not seem to be possible to deposit an experimental tape which incorporates all the macroscopic properties like Hi8ME tape. It is however not a priori clear that such a tape could not be deposited with the mini roll coater set-up, but it seems to be highly dependent on the way in which the oxygen is supplied. Although a similar morphology is obtained, the crystal sizes in the experimental tapes are larger compared to the Hi8ME one. In all tapes it was found that the non- ferromagnetic oxide grains are anti-ferromagnetically coupled to the ferromagnetic Co-Ni grains. From the microstructural analysis two models could be deduced where the model with the oxide shells around the grains seems to fit best to the other measurements (see section discussion of chapter 5). Furthermore if the oxidation process is diffusion limited, then a much more plausible explanation is found as to why the oxygen incorporation process seems so inefficient with reference to sections and From pressure and impingement considerations it is concluded that the initial columnar inclination angle can indeed be affected by the oxygen. Furthermore, the Co atoms seem to scatter a lot by the oxygen on the inlet side (600in our case). Furthermore it was concluded that a large difference in oxygen exposure and lower drum temperature might account for the more uniform and thinner oxide shells around the ferromagnetic grains in the case of the Hi8ME sample. Finally in chapter 6, the oblique sputtering process is discussed. The first part of the chapter deals with sputtering under fixed angles of incidence and the influence of some principle deposition parameters is studied. The influence of the effective angle of the substrate with the target, target ,materials, argon pressure and input power is analysed with respect to the microstructure. It was found that the geometry in combination with the argon pressure play a vital role in the subsequent microstructure. The magnetic properties of samples prepared using a different geometry and argon pressure, could to some extend be explained from the observed microstructures. The dependence upon the input power is less clear .Furthermore it was shown that none of the analysed samples showed a pronounced texturing and in most cases the direction of the oblique magnetic anisotropy was found to coincide with the columnar inclination angle. The one exception was found for the case when high argon pressures were employed. Furthermore it seems that the main anisotropy source is found in the shape anisotropy of the columns. In tbe second part of the cbapter, some preliminary results on sputtered tape were presented. It was sbown tbat a tape witb reasonable recording properties could be deposited in a reproducible way. Tbe output versus frequency cbaracteristic was found equal to a commercial MP-pro tape. It was found tbat the wear properties of the sputtered tape were mucb better compared to the experimental evaporated tapes, since even noise aDd overwrite measurements could be performed. Moreover aD oblique morpbology was determined by cross sectional TEM, whicb could already be deduced from tbe good aDd bad recording direction. Finally from the preliminary microstructural cbaracterisation of tbe sputtered tape, some similarities witb a sample deposited under a fixed aDgle were observed. This bas also been compared witb the commercial Hi8ME tape of cbapter 5.
Item Type:Thesis
Electrical Engineering, Mathematics and Computer Science (EEMCS)
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