When working with sputtering deposition processes, a complex not fully understood plasma process is used. In a vacuum chamber a low pressure inert gas is introduced at a pressure of appr. 10 mtorr. (0,01 mmHg). A voltage of minus 200-400 volt is applied between one electrod (cathode) and surronding body of vacuum chamber (anode).
The cathode is the sputter target. In our case DC is used but also RF discharge is possible and also other methods of excitation but we focus on DC discharges.
Due to the pressure a mean free path of 10-20cm for the electrones is possible. The pressure allows a plasma to conform. This plasma concists of electrones wich impacts Ar atoms and creates positively ionized argon plasma. To improve the probabiliy of impact a donut shaped magnet is placed just over the taget. This makes electrones take a spiral path and large probability of collision uccure. When an Ar ion has been created it will be accelerated towards the target, becouse the target are negatively charged and Ar ion is positively charged. Due to the heavy mass of the Ar atom it is little affected by the week magnetic field but the electrons are easily bend becouse of its little mass.
The plasma is conformed by the magnets. One large magnet
in the middle wich has Noth up and the other magnets surronding
the one in middle has South up.
When the Ar accelerates towards the target it will kick of one or several target atoms. In this case Aluminum atom, burt it can be ant type of metal. A dielectric compound can be used as a target but then RF-discharges must be used. Othervise charges will be build up on the target. In the DC case only electric conductive material can be used.
Sputtering in action. Plasma is concormed by the magnets behind the target
plate on top.
After the impact of Ar ion and when the aluminum atom has been liberated form the target it moves everywere in the chamber. Until it is deposited on the nearest cold surface. Due to the limited mean free path of atoms in the vacuum, the area wich will be coated should be quite close to the sputter target.
One big advantage in this process is that the coating material is evaporatet into gas phase directly form a solid source. This makes it possible to sputter material downwards. After a while the material will be eroded and even a 10 mm thick aluminum body will be eroded through its whole thickness.
plasma erotion of aluminum. The aluminum target is bonded by indium to a
copperbody, inside this copper body is the permanent magnets.
Reactive DC sputtering is also possible. In this case an reactive gas is mixed together with the inert sputtering gas. A small amount of nitrogen, flouride, oxide or other reactive gas is mixed with the Argon gas and oxide coating could be formed when the reactive gas is chemically bonded to the sputtering atom on the substrate surface.
Most of the energy from the impact of Ar ions arlost in the formof heat. Therefore extensive cooling is neccesery. This is done bye flushing pressurized water inside the copper backingplate. Inisde this copper body the magnets are placed so they are surrounded by flowing cooling water.
The backing plate are pressurized with cooling water, It is also electric isolated
becous of the negative voltage. inside this are the magnets.