The temperature of a substrate, while the covering is drawing on it’s surface, страница 29

The analysis of various variants of magnetron sputtering system design shows, that all of them are characterized by presence both positive at the cathode, and negative at the anode spatial charges in a range of magnetic fields used for the discharge (0,03 … 0,1 Tl), and anode potential drop grows with growth of an induction of a magnetic field. The given factor is necessary for taking into account when determining the energy of bombarding ions, necessary operating voltage at which process of dispersion will be maximum effective, and technological process of generation of a coating will be optimal.

8.1.5 Model of the discharge and definition of current-voltage characteristic

Development of the physical model of processes occuring in the discharge of magnetron sputtering systems and their analytical description enable to explain the observable phenomena and to choose design values and operating characters of these systems.

Proceeding from the considered features of the discharge [20, 25, 29, 38, 39], the following model of processes occuring in ithas been offered:

-     Secondary electrons play the basic role in formation and maintenance of plasma in magnetron sputtering system. Electrons under action of the crossed electric and magnetic fields make the directed movement along the dispersion zone at the target along the trajectory close to a cycloid;

-          Secondary electron makes cycloidal movement until the length of the way gone by it will not exceed average length of free path of electron in gas at the given pressure. Then ionizing collision of electron and atom of working gas occurs with the certain probability, then moves away from the target on distance, approximate equal to cyclotron radius;

-          There is rather small voltage drop caused by necessity of secondary electron dispersal from the target up to energies at which effective ionization of working gas begins in the field of dark cathode space, other voltage drops in anode area (near to the conditional anode);

-         The accumulation of negative charge in anode area occurs due to presence of enough strong transverse magnetic field, therefore the strong electric field arises both in cathode, and in anode areas where processes of ionization of atoms of working gas basically take place;

-          As the magnetic field does not render essential influence on character of movement of ions the last do not participate in process of ionization and after formation they move at once from the zone of ionization to the target, forming at this surface positive spatial charge that allows not to take into account concentration of ions in the anode area. Energy of an ion bombarding a target depends on the area of plasma, where it was formed. Average energy of ions in magnetron sputtering systems as researches have implied, makes approximately Wi=0.7eUp.

Formulas (8.10) - (8.13) for calculation of position of the conditional anode are deduced on the basis of the suggested model. Having limited to the analysis of the anode area of plasma which plays a determining role in maintenance of the discharge in a strong transverse magnetic field, it is possible to consider a stationary problem for a flat layer to a first approximation (fig. 8.5).

Figure 8.5 Distribution of potential in the discharge gap of magnetron sputtering system, assumed when taking out its current-voltage characteristics.

Axisox we shall direct across a layer, an axisоу - lengthways, and a magnetic field – along oz axis. Considering, that the prevailing mechanism of electron carrying to the anode on an axisox  is the electric drift determined by classical transverse electron mobility in a magnetic field, and, neglecting an ionic current in a layer, because concentration of ions is small, it is possible to write down initial system of the equations: