Vacuum Web Coating

There have been a number of presentations that have focused on the magnetron sputtering design as a method of obtaining a high target utilisation. Ideally, we would all like to have high target utilisation, but often we will trade this off against improved coating uniformity or higher deposition rates. There has always been the option of then exchanging targets and obtaining a credit for the returned material. PlasmaQuest designed an alternative magnetron source where the plasma was generated away from the target and directed over the target and so enabling the whole target surface to be eroded at the same time and virtually the same rate. Thus for expensive targets this uniform whole target erosion gave an extremely high target utilisation. However, this is not an easy solution as the process does not easily transfer to large roll-to-roll coaters.

This is not a new problem and there have been other solutions used in the past.  One of the more interesting ones was to sputter from a molten target.  There is no rule that states that sputtering targets have to be solid. Solid targets do allow magnetron sources to be oriented in any direction which can be an advantage but in distributing the deposition around a deposition drum and so increasing the total deposition rate whilst minimising the substrate temperature rise. There can be advantages in sputtering from a molten target as the sputtering rate can increase and the power density applied will be much greater than conventionally as it is desired that the target is heated to become molten.  This does require some consideration to the magnetron source construction as the magnetic field ideally needs to remain constant even when the target temperature is increased.  So long as the magnets are sufficiently cooled that the field is not affected this should give very high sputtering rates compared to more conventional sources.

As mentioned this does limit the magnetron to being used the same as any evaporation source, at the base of the vacuum system pointing upwards.  What happens with a molten source is that you can still have the high deposition rate but instead of a deep erosion groove being formed the molten source will remain level throughout the target lifetime.  This then combines the best of high deposition rate and maximum target utilisation.  The limitation of this process is the material being sputtered. It is easier to work with a molten target if the melting point of the material is relatively low and it can be contained in a suitable backing plate/crucible easily. The work that was originally done with this type of source sputtered gold.  The products that were of interest were the heat mirror or solar rejecting coatings and so the standard materials were copper, silver and gold.  I do not recall this being extended to any other materials but there is no reason why the same concept cannot be used for other materials.

For those of you not quite so long in the tooth as I am, I hope this ancient, alternative way of getting high target utilisation is of interest.