SPI-Module™ Sputter/Carbon Coater System

VIII. Comments About Carbon Coating in General and the SPI-Module Carbon Coater in Particular

Carbon coating is one of the oldest electron microscope sample preparation procedures in use and, in some respects, it could also be the least understood.

Until roughly 1975, virtually all carbon coating was done in vacuum evaporators (diffusion pumped systems). Whether it was for "backing" TEM specimens, making carbon support films for TEM or the coating of samples for electron microprobe (WDS) or energy dispersive spectroscopy (EDS), the vacuum evaporator was the only tool available.

With the advent of sputter coaters in the early to mid-1970's, the temptation was just too great to not "make" a carbon cathode that would be substituted for a gold cathode to "see what happens". And indeed, quite remarkably, one could, after about thirty minutes of "deposition" cause a white piece of paper to turn grey, at least to a level that one would have thought there would have been sufficient surface conductivity. For several years, one of the more interesting demonstrations at electron microscopy oriented trade shows, both in the USA and Europe were firms "showing off" how one could deposit carbon from a sputter coater. Unfortunately seeing is not always believing, and while the coaters surely did turn a white piece of paper grey, the bottom line was that the carbon coating was just not conductive. And if the sample was heat sensitive, such as a membrane filter , the long term exposure to the heating from the cathode tended to destroy many of the surfaces one would have wanted to coat in the first place. So that is why one does not just screw in or other wise attach a carbon disk and expect to obtain the kind of carbon coating one would need for EDS or only SEM examination.

And there were some really wild attempts to coat carbon and these attempts were to limited to simple notions for example of the type for using a screw in carbon cathode. One contraption was introduced using using acetylene. These must have been before the days when one worried about safety considerations! But the important point was that none of these approaches was successful in producing a carbon coating that would be acceptable for the intended use.

SPI was the first manufacturer to introduce a carbon coater module, as an add-on to the original SPI Sputter Coater in 1976. And since that time, the carbon module concept has been modified and improved to the point that today it is recognized as being one of the easiest to use systems applying the best possible coatings, and for a price that is typically significantly below what others claim to be doing the same thing!

The current version of the device, now in the form of the SPI-Module Carbon Coater comes "standard" with a carbon filament head for use with high purity SPI carbon fiber. There is also available an optional carbon rod head which can be ordered at the time of instrument purchase or at any later date.

The carbon fiber head is clearly the easiest to use, results in a far higher sample throughput and is the method of choice by most users. The carbon filament is also a lower cost consumable than carbon rods. And for heat sensitive samples, the significantly shorter exposure time to the hot filament the carbon fiber represents yet another advantage.

However, the carbon rod head (SPI#11439) results in a slightly more controllable coating, and the coating itself seems to be more homogeneous with a lower population of small submicron sized particulates which could be confused with particulates in the sample to be examined.

Some users believe that the carbon rod head applies a coating of slightly higher conductivity than the carbon fiber, and we believe that probably is correct, however if there is a difference, we think that the difference is not very great.

But since the carbon fiber head does seem to result in a slightly elevated particulation of the coating, for users studying, for example, fine particles on a membrane filter, the carbon rod approach would be recommended.

Also, for carbon coating, unlike sputter coating, the very best vacuum is required and no external gas is bled into the system. Hence, the needle valve is completely closed to obtain the best vacuum with the vacuum pump. So while one might be able to get away with the use of a used and not so perfect vacuum pump might be perfectly acceptable for sputter coating, for the best carbon coating, one really should be using the best possible mechanical pump.

But both carbon fiber as well as carbon rods, having high surface area, tend to adsorb moisture, and if the evaporation is done without some type of off-gassing cycle, the evaporation will be done in a rather poor vacuum environment resulting in a less than acceptable coating. That is why the SPI-MODULE Carbon Coater module also has a test mode. One can actually watch the moisture burn off and, once this is done, the sample coated. We have found this test mode and means of validating that all adsorbed moisture is literally burned off during the test cycle demonstrably does result in a higher degree of reproducibility results for most users.

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