However, in discharge conditions in magnetron sputtering system to operate a hade is not obviously possible, at the same time, as some researchers suppose [7, 8, 30, 32, 47] the hade practically of all ions on the cathode - target is equal to zero. Therefore influence of the hade on factor of dispersion of the cathode - target in MSS can be neglected, and one can accept for calculations value of the dispersion coefficient at normal falling of particles.
Influence of the surface condition on the dispersion coefficient. The concept « a surface condition is meant as a roughness and a kind of machining, chemical cleanliness and hardness, a microstructure and density of dispositions. Many of the named questions still are not investigated or there is fractional information about them [8, 32, 47, 78].
Roughness and kind of machining. It is possible to believe, that these factors do not render essential influence on the value of the dispersion coefficient, espessially at long dispersion and if energy of ions is not too small. In the beginning of process of dispersion figures of etching are formed on the surface because of weak connection of atoms on borders of grains. In result the smooth surface becomes rough. Thus there is a capture of a part of emitted atoms and on a surface grow a cone [78]. However it results in increase in hade, so also numbers of emitted atoms. In result both effects can compensate mutually each other.
Results of experiments show [78], that polishing of surfaces of metals brings a mistake in definition of the dispersion coefficient, not exceeding 10 % when energy of ions makes about 500 eV.
Let's compare results of experiences to the polished copper plate [78] and a target on which it was continuously made sputtering of copper. At the ion energies of copper 6 keV values of the dispersion coefficient appeared accordingly 8,7 and 8,0. If energy of ions is small (about 100 eV), angular distribution of emitted atoms goes below a cosine curve, that is there is an output of atoms mainly along the tangent to a surface. In this case the number of emitted atoms should depend on a roughness of a surface. The direct experiment confirms it. Comparison of the dispersion coefficient of a smooth nickel core and a core having carving cutting (fig. 5.12) has been carried out.
The surface of many metals (Аl, Ta, Тl, etc.) is usually covered by oxides which are sprayed much worse, than metals. It was considered long time, that aluminium is one of the most proof materials against dispersion. However after removal of oxide layers speed of dispersion of aluminium became in 2,5 times less, than speed of dispersion of copper [78].
With reduction of energy of ions the roughness affects more and more. At 100 eV the dispersion coefficient of a core with carving cutting is in 2,5 times less, than at 600 eV where they are approximately equal. Besides the "hysteresis" of a curve of dispersion is observed. The last is connected to increasing of microroughnesses caused by ionic etching.
Figure 5.12 Ratio of the dispersion coefficients of a smooth nickel core and a core having a groove, depending on energy of ions of argon [78] (the scheme of experience is shown on the right).
So, at small energies of ions the roughness of a surface influences the dispersion coefficient, with growth of energy of ions this influence becomes insignificant. In a range of energies, interesting to us 1 … 2 keV influence of roughness on the dispersion coefficient is insignificant.
The number of the atoms which have left a sample, depends on pressure of the gas surrounding a target because, first, the quantity of molecules of residual gases which pollute a surface, is proportional to pressure, second, with increase in pressure the number of atoms which come back to a sample owing to collision with molecules of residual gas grows. When there is an evaporation from one infinite plate to another, parallel with the first and distant from it on the certain distance, owing to the outdiffusion only the part of evaporated atoms will reach the collector plate. It is possible to believe, that the similar ratio is carried out for emitted atoms [47, 78].
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