SPI Replacement Sputter Coater Cathodes

A few comments and frequently asked questions (FAQ's)

1. How long could I expect my sputter cathode to last?
This is like asking how long your automobile tires will last. The answer then is a function of how often you are coating samples (e.g. once an hour, once a day, once a month, etc.), how much thickness is being applied for each run, and the starting thickness of the original cathode (thicker cathodes obviously last longer than thin ones).
2. What are the arguments for a thicker vs. thinner cathode?
Most important, there is no difference in the quality of the coating itself. However, it is simply a matter of economics. When cathodes are produced, the total cost of made up of two parts: a) "fabrication" and b) value of the metal itself. Cost elements under "fabrication" include the cost to create the highest purity and also the cost of rolling the metal into a foil of the required thickness and then cutting out the metal discs in the desired diameter. The interesting part of this, and the one that is least understood by end users, is that the fabrication costs are independent of foil thickness! The cost to cut discs out of a thicker foil are the same as the costs to do the cutting from a thicker foil. When it comes to the cost of rolling the metal into a foil, the cost is more or less the same, whether the foil it thicker or thinner. Hence, when one purchases foils that are thinner (e. g. 3 mils), per dollar spent, less is for the precious metal than when the foil is thicker (e.g. 10 mils).

3. Why not foils even more thick than 10 mils?
There is a point where there is diminishing returns in the thickness argument and security and concerns about theft become more of a consideration. After all, we won't want to have that much value that one would feel compelled to lock up the cathode into a safe every night. There is yet another reason and that is the generally uneven wear that occurs across the diameter of a cathode. In most sputter coaters, the sputtering rate is not the same everywhere on the cathode surface and that is why very often, some areas of a cathode "burn through" in some regions, yet some other areas, most notably around the circumference, retain almost the original thickness. So again there is a trade off in over all costs, because as the cathode gets thicker, at the end of its useful life and it has to be replaced, the percentage of gold remaining (relative to when new) is larger.

4. My gold cathode is taking on a decidedly non-gold appearance and it won't sputter any more. What is the reason for this?
This is not that uncommon of a question. The first rule of thumb is that if it does not look like gold, then it probably is not gold. There is one exception however, and that is when the sputtering process creates some kind of surface texture that leads to an optical effect, a non-gold looking gold color (more like gray). But that typically does not adversely effect the sputtering rate.

The other cases then would be either a) contamination from external sources (e.g. finger prints) or b) build up of contaminants from the use of gold with insufficient purity. If (a), then that "problem" is solved by a good solvent washing and scrubbing with something like acetone. If (b), then it will not rub off with solvent and it would be something building up in the way of impurities from within the gold foil itself. If this is the reason for the apparent "failure" of a gold cathode, one should try polishing off that surface of impurity concentration on a standard metallographic polishing table. If the cathode worked at least for a while, this should rejuvenate it to its original state and it should sputter as it did when new . But if this is indeed the problem, our advice is to not be using such low purity cathodes, this is not the route for quality work. Consider purchasing high purity precious metal cathodes from SPI Supplies and enjoy the benefits of trouble free and problem free sputtering, including high sputtering rates which leads to reduced radiant heat exposure to fragile and possibly heat sensitive samples.

5. What would be the "recommended purity" for SEM applications?
This would probably be one of those kinds of questions for which there is no universally accepted answer. We believe that for most if not all SEM applications, 0.9999 purity is quite fine and using 0.99999 could very well be overkill. In any case, there is a noticeable difference in the cost when comparing 0.9999 vs. 0.99999 purity.

But there is a cost associated with purification. For example, taking gold from 0.98 to 0.99 involves a higher cost, and there is even more of a cost when going to 0.999 purity. And as one goes from 0.9999 to 0.99995 and then to 0.99999 is still more cost. In other words, while one really does want the highest purity gold in their cathodes, there is a cost consideration.

But on the down side of purity, there clearly is a point where the impurity elements that don't sputter begin to build up on the surface, resulting in a mostly non-gold layer. When when this happens the sputtering rate essentially "stops". Now we know this would never ever happen with a cathode purchased from SPI Supplies, and we would report, it would be highly likely that is would happen with cathodes purchased from any of the original equipment manufacturers, or even our primary competitors in the microscopy supply and consumables business. But if you have a gold cathode that appears to have "stopped sputtering", and if the color is no longer "gold" in appearance, then there is a very real possibility that someone in the laboratory, in a mistaken decision to try to save money made their own cathode from sheeting rolled perhaps at a jewelry shop, working under conditions of unknown and uncontrolled cleanliness. And as the impurities build up on the surface, and the surface gold is depleted, the sputtering rate drops and finally stops sputtering at all.

Now is this scenario sounds like it could be applying to you, we offer this advice: Try polishing off this layer in a metallographic polishing table, in order to renew the original composition that apparently did work for some time. This should return the surface of the gold to its original composition and while the sputtering rate will again drop down as it did the first time, at least you will be able to get some emergency use out of the cathode, at least until such a time that you can order in from SPI Supplies a high purity cathode.

6. What is the value of my "spent" cathode?
An individual cathode, by itself, as virtually no economic value. This comes as somewhat of a surprise to some, but in order for a spent cathode to have any value, it has to be refined to high purity again and put back into the metal stream. But for that to happen, a chemical analysis must be done by the refiner and by the time one factors in not only the cost of the analysis, but also the set up to melt and purify, these costs end up being far in excess of the intrinsic value of any gold in a spent gold cathode.

However, a large number of cathodes, if melting together, and one analysis done of the batch, then not only does the reclamation of this gold become economically viable, but we are all, both individually and also, institutionally, doing our part to keep this non-renewable resource part of the gold metal stream. Therefore, be sure you consider participation in the SPI Supplies sputter coater cathode recycling program.

7. Are there ways to extend the useful lifetime of my cathode?
Actually, in some cases, there are! You might think it analogous to rotating the tires on your car to gain longer tire life. Many sputter coater head designs, including the SPI Module Sputter Coaters operate with magnetic fields that are not uniform around the cathode. So although there is a tendency on some units to uniformly sputter around a 360° "ring", in other units there is a tendency for higher sputtering rates from specific points on that ring. So we recommend the sputter coater equivalent of a tire rotation: Rotate the cathode some angle, for example, 30°, so that these points now will start their faster sputtering rates from a less sputtered area of the cathode. If this is done conscientously, we have seen people extend the useful lifetimes of their cathodes from anywhere from three to six times relative to the unrotated life times!

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