I needed to find the sputter-deposition rate for a few materials. This sounded a simple task but it was not as easy as I thought. First, there are many different ‘rates’ quoted. Some are a just a thickness per unit time others include the power applied and expressed as a dynamic deposition rate. Again this sounds to be fairly simple. However it became apparent that there was a very wide variation in the values quoted for each material. There are several factors that cause this variation. The maximum power that can be put into the magnetron source very much depends on the design of the source, in particular the water cooling circuit, and the target construction and quality.
If the water cooling is limited there will be a point reached where there is more heat generated by the surface bombardment and sputtering process than is being removed by the water cooling. Once this point is reached any further power will further increase the target temperature. This can change the sputtering performance significantly and I have seen a source where increasing the power reduced the deposition rate. As the source temperature increased it affected the ferrite magnets and the magnetic field strength reduced thus the plasma no longer could retain as many electrons and so the current carrying capacity declined and in turn the bombardment reduced and so the sputtering rate reduced. Thus the water cooling critically affects the amount of power that can be applied and so different manufacturers may quote different deposition rates for the same material.
The rate variation can further be widened by differences in the target thickness as well as being dependent on if the water cooling is direct onto the target material or onto a backing plate. If a backing plate is used the target may or may not be bonded onto the backing plate. If the target is bonded there can still be variations in the thickness and material of the backing plate as well as the bonding technique.
Added to this is the variation in magnetic design and variation in magnetic materials that may be used and hence the effectiveness of the electron capture. This will affect the sputtering racetrack size and shape.
Another source of variation in the values quoted comes from how the power is used to produce the values. Some take the whole target area and simply divide the power by the area. Others take only the area of the racetrack and thus these power density values would appear greater as the area is reduced. Others limit the racetrack area further by taking the half width of the racetrack.
The greatest difficulty is that in many of the papers that quote values they do not disclose enough information about how the values are derived. It becomes impossible to compare the deposition rates without knowing these details.
Another aspect to be careful of is that many values are only calculated and not verified in practice. Years ago experimenters such as Sigmund produced sputtering yield tables for different gases and different specific ion energies for many materials. These tables can give a sputtering yield expressed in a ratio relative another material. The relevant ratio can then be used to convert the dynamic deposition rate of one material into a rate for any other material. It is thus open to question how accurate this calculated deposition rate will be in reality.
Thus for any of you out there also wanting to find out the deposition rate available from a magnetron sputtering source I hope this will help you understand the variations of values that you might come across.