The temperature of a substrate, while the covering is drawing on it’s surface, страница 28

Figure 8.2 Focusing action of magnetic(а) and electric(b) fields on electrons and occurrence of drift electron movements (c): 1 - a cathode - target, 2 - a field line of the magnetic field, 3 – an equipotential surface on border of the dark cathode space.

As width of DCS is inversely proportional to the density of an ionic current (look at the formula (8.10)), borders of this area are deformed, that entails occurrence of heterogeneity of the electric field (fig. 8.2, b). An equipotential surface on border of DCS is concave, and electrons, accelerated in the DCS area, obtain speeds in the direction to the center of the dispersion zone under action of the electric field.

The further analysis of electron movement in the discharge of magnetron sputtering systems shows, that at the considered geometry of a magnetic field they also make gradient and centrifugal drifts [39], and both transverse (), and longitudinal() gradients of an induction exist. At represented in figure 8.3,c directionsЕ,B and  electrons will drift upwards perpendicularly to the plane of figure. Due to a longitudinal gradient of the induction, electrons, which have a speed component in its direction, will be braked and reflected by the magnetic field to the center of a plasma ring before they achieve the cathode surface, because every electron aspires to preserve its magnetic moment .

8.1.4 Formation of spatial charges in plasma

Researches of the discharge in the crossed electric and magnetic fields, carried out in magnetron system with coaxial cylindrical electrodes, have shown, that the magnetic field renders very strong influence on parameters and structure of the discharge [25, 29, 38, 39]. As it has been shown above, movement of ions in plasma poorly depends on geometry of magnetic fields used in magnetron sputtering systems, electron mobility across the magnetic field becomes much lower, than ion mobility, and electrons leaving on the anode at big magnetic fields is difficult. It results to the formation of the negative volumetric charge and the anode layer in which the anode potential drop takes place. At big enough magnetic field almost all enclosed voltage can drop in the anode layer. Electrons, being accelerated in this area, ionize atoms of gas therefore the anode area becomes a dominating area of ionization, and thickness of the anode layer is the function of a magnetic field. Thus, in case of the crossed electromagnetic fields three types of discharges depending on value of the magnetic field can be observed: the discharge with area of cathode drop, the discharge with area of anode falling and the discharge with simultaneous existence of both areas (figure 8.3) [39].

Figure 8.4 Distribution of potential in the discharge gap of magnetron sputtering systems with cylindrical electrodes: 1 - area with positive spatial charge at the cathode; 4 - area with negative spatial charge at the anode; 2, 3 - simultaneous existence of both areas (the distance is postponed from the cathode) [39].

The first type of discharge is observed in usual sputtering systems and in magnetron systems with weak magnetic fields (induction up to 0,01 … 0,03 Tl) and characterized by presence of area with a positive spatial charge at the cathode. At enough strong magnetic fields the negative spatial charge is formed at the anode, the area of the anode potential drop starts to be formed, and there are obviously expressed areas of cathode, and anode potential drop at induction of the magnetic field about 0,1 Tl in the discharge. At the further increasing of field the anode potential drop grows and the discharge with a negative spatial charge and anode area of ionization is formed. The value of the induction of the magnetic field at which this type of discharge is formed, depends on a design of the discharge system, working pressure and some other factors. The most detailed researches have been carried out in cylindrical discharge system. These researches have shown, that for the purposes of dispersion of materials the discharge with a positive spatial charge and cathode potential drop is most effective. However the discharge with simultaneous existence of both cathode, and significant anode potentials also appeared rather effective for dispersion of materials because it provides a uniform distribution of density of a current on a sprayed surface of a target [25, 29, 38, 39].