During sputtering, a vacuum is first produced in a sputtering chamber, and then an atmosphere of a sputtering gas having a defined pressure is created. A gas discharge of the sputtering gas is ignited in the vicinity of a sputtering target that is attached to a sputtering head and generally kept at a negative potential. A sputtering plasma comprising positively charged ions and free electrons is created from the electrically neutral atoms or molecules of the sputtering gas. The positively charged ions are accelerated by the negative potential of the target on the surface thereof and there, by way of momentum transfer, knock out material, some of which travels in the direction of the substrate to be coated as a result of rebound and is deposited there. At the same time, these ions release electrons from the target as a result of this bombardment, which are accelerated by the electric field in the direction of the sputtering plasma and there ionize further atoms or molecules of the sputtering gas by way of collisions. The sputtering plasma in this way is self-sustaining.
So as to be able to coat larger substrates in one operation, the use of larger sputtering targets is being pursued. However, as the target size increases, the plasma becomes increasingly unstable. During magnetron sputtering, this effect is counteracted by the field lines of a permanent-magnetic field extending through the plasma. In the case of round sputtering targets, this field generally runs between a permanent-magnetic ring that is provided at the edge of the receptacle for the sputtering target and an additional permanent magnet that is provided in the center of this target receptacle. A portion of the stray magnetic field runs in curved field lines through the space in which the sputtering plasma is located. This stray magnetic field forces free electrons onto long cycloid tracks transversely relative to the electric and magnetic fields through the sputtering plasma, where these electrons ionize atoms of the sputtering gas by way of a large number of collisions and thereby contribute to sustaining the plasma.
The disadvantage is that this method works only at a comparatively low pressure. At higher pressure, the mean free path becomes too short for the electrons, so that these can only accumulate where the magnetic field is the strongest. Where this field is weaker, the plasma is also weaker. The intensity of the sputtering plasma consequently becomes inhomogeneous over the surface of the sputtering target. In the extreme case, the plasma breaks down into several separate parts, which are usually localized next to the magnetic poles of the permanent magnet.
However, higher pressure is specifically required for sputtering oxidic layers in an oxygen atmosphere. The shorter mean free path causes the undesirable effect that fewer negative oxygen ions are thrown against the substrate due to repulsion from the target and damage the layer already deposited on the substrate, or ablate the same nonstoichiometrically (back sputtering effect). In addition, a high pressure is advantageous so as to transfer an oxidic layer from the target to the substrate with the proper stoichiometry during the deposition process. Some materials can only form a stable stoichiometric phase in the deposited layer when the partial pressure of oxygen is relatively high.
Thus, it is the object of the invention to provide a sputtering head that generates a stable plasma over the entire surface of the sputtering target at high pressure. It is another object of the invention to provide a method by which target material can be deposited on a substrate in a homogeneous layer thickness at high pressure.
These objects are achieved according to the invention by a sputtering head according to the main and additional independent claims, and by a method according to another independent claim. Further advantageous embodiments will be apparent from the respective dependent claims.