Dissolved solids, like chlorides, iron, manganese and nitrates work in two ways to reduce the performance of a sputter gun. These foreign elements can substantially increase the electrical conductivity of water. Coolant water running through the magnetron sputtering gun can, if the water is too conductive, bleed energy to ground because the coolant goes from cathode potential to ground (at the supply side of the water). If the coolant water is too electrically conductive it could also cause arcs between the cathode and the anode through the cooling tubes. Any time a conductive coolant flows between high potential and ground bad things can happen, if the coolant is not highly resistive.
The resistance through the power supplies themselves really determines the minimum resistivity of the coolant. The electrical resistance through a DC power supply to ground may only be 2KΩ. But this resistance is good enough to run the gun correctly. So the idea with the coolant is that it should never have the lowest resistance value in the power circuit. Some cathode manufacturers, while they state that tap water is an acceptable coolant, also specify it have a resistivity of 1MΩ – that is pretty great tap water!
Also, highly conductive water, i.e., containing high PPMs of dissolved solids, tends to be “dirty.” These solids will eventually deposit on the cathode and cooling system components, eventually leaving a sedimentary buildup sufficient to restrict the cooling circuit and overheat the sputtering gun. Processed forms of water, such as de-ionized (DI) and distilled water do have very high resistivity. Fresh DI water, before it is exposed to ambient air, is considered ‘perfect’ water with resistivity at the limit of what is possible, nominally 18.2MΩ. On exposure to air it soon drops to an equilibrium resistivity of about 8MΩ. Distilled water is also about 8MΩ. However, both of these water versions are highly reactive and will subsequently corrode the cathode and are not recommended for use in closed loop chillers.
The clear recommendation is that the resistivity of the water be “as high as practical.” “Practical” is usually defined by how much money you have to spend on things like closed loop chillers and synthetic fluids.
Category: Deposition Equipment