Question: Recently we had a student ask us why he was unable to maintain a plasma when trying to sputter a new boron nitride target with an rf generator power supply.
Answer: When utilizing an rf generator and associated tuning network to strike a plasma for the sputtering of certain materials it is sometimes necessary to raise the initial partial pressure of the working gas above the normal operating range momentarily. Typically it is advisable to operate in the 2-5 millitorr range for rf deposition with zero reflected power but to ignite the plasma initially, especially for a new unconditioned target, it may require increasing the flow rate of the working gas (or reducing the pumping speed by reducing the opening of the variable orifice plate) to allow the partial pressure within the chamber to increase until the plasma ignites. This may be in the 25 to 150 or even up to the 200 millitorr range. Once the plasma ignites it should be possible to return the pumping parameters back to the normal operating range and then maintain the plasma in the 2-5 millitorr range.
When the plasma initially ignites at the higher partial pressure the reflected power may be quite high. This is normal and of no consequence. It is not necessary to adjust the impedance in the tuning network to try and null out the reflected power at this point. However, once the plasma is sustained at normal operating pressures it is imperative that the reflected power be as close to absolute zero as possible. When utilizing an auto tune system make certain that the tuning network does not settle in a saddle point. If the reflected power remains above a few watts try to manually adjust the impedance upwards or downwards to see if the reflected power drops to a lower point after a small increase is observed in either direction. Finally, if it is impossible to maintain a low reflected power at these pressure levels check to make certain that the system is directly connected to a good sustainable earth ground.