“Arcing” is quite often an issue with any sputtering process and can lead to disastrous results if not properly maintained or, more preferably, eliminated. When we say “arcing” we are referring to the electrical potential applied to the sputtering target surface shorting over to ground potential through the plasma during the deposition process causing a short circuit. This high energy displacement can cause severe damage to the equipment being employed (power supply, cathode assembly, magnetic enhancement, substrate, etc.) and more than likely will cause damage to the target itself, depending on the amount energy being dissipated.
There are a number of potential causes for arcing in the plasma, but we are only going to deal with one possible area of concern here. NOTE: Another possible cause was briefly discussed in our January 2015 blog involving the proper preparation or “Break In” procedure for a new and reused targets. What we would like to address here is the function of the dark space shield in magnetically enhanced cathode designs. By design, and based on the geometrical configurations within a magnetically enhanced cathode, a “Dark Space” is created within the plasma around the outer perimeter of the target. This is created by directing the impinging ions toward the target surface with the electrical and magnetic component created by the cathode assembly in such a way as to direct the charged particles (gaseous ions) with enough energy to break the atomic bonds within the target material and propel them into space, preferably toward the substrate. To facilitate the directionality and placement of this momentum transfer a dark space shield, which is at ground potential, is configured around the perimeter of the cathode to impede the plasma. The exact position or placement of this ground potential is critical in contributing to the efficiency of creating a dark space (an area where no plasma is present) and avoiding arcing over to ground. To facilitate the efficiency of creating this dark space, most dark space shields are adjustable. The mechanical adjustment is generally capable of being altered in only the “Z” or vertical height direction; with the “X” and “Y” (horizontal) directions being fixed by design, usually between 3-6mm or so. The position of the dark space shield can generally be adjusted in the “Z” direction to accommodate different thicknesses of target and/or backing plate configurations. The dark space shield should always be adjusted such that it extends BEYOND the target surface by about 3-6 mm. This will establish the ground potential in the plasma to create a dark space away from the outer perimeter of the target and thus reduce the probability of the sputtering target to arc over to ground through the plasma.
So when targets are changed within the system and targets of varying thicknesses are replaced within a given cathode, check to see that the dark space shield extends beyond the target surface to that critical 3-6mm distance. Also, during the target change, make certain that no debris or flakes have accumulated or fallen between the dark space shield and the perimeter of the target surface which may cause a short circuit. Finally, make certain that the dark space shield is equally positioned around the outer perimeter of the target. Again, this spacing should be approximately 3-6mm in all directions.
