Osmium Plasma Coaters

The ultimate coating device for coating SEM samples, especially for FESEM!

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SPI Supplies is the exclusive distributor for the complete line of OPC Osmium Plasma Coaters originally manufactured by Nippon Laser and Electronics Laboratory, and then Laser Techno, but as a result of a corporate restructuring, the manufacturing unit is now being done by Filgen, Inc., an important and respected scientific instrument manufacturer in Japan. This is a new and innovative concept for the deposition of fine structureless and featureless thin film coatings on surfaces for applications in electron microscopy, especially for those who are using a FESEM. We are honored to have been selected first by Nippon Laser, and then by Laser Techno, and more recently by Filgen to be the exclusive worldwide distributor (outside of Japan) for this exciting new family of instruments. Most of the worldwide network of agents and distributors of SPI Supplies have service departments set up to service electron microscopes and vacuum evaporator and sputter coaters, and servicing an OPC osmium plasma coater is actually easy in comparison to that of the typical electron microscope.

What is Osmium Plasma Coating?
While "gold coating" in a conventional sputter coater used in SEM laboratories is quite acceptable for users with conventional SEM's, it seems clear that for those fortunate enough to be owners of a new (actually any) FESEM, which of course has far higher resolving abilities than non-field emission SEMs, in order to actually achieve the resolution of their instrument on nonconductive samples, they have to do something about the grain size of the gold. After all, it is not possible to achieve resolution finer than the size of the grain of the gold metallization. And that is the main driving force for the popularity of coating systems offering the promise of a smaller grain size. For example, in North America, leading researchers found a number of years ago that a chromium coating exhibited substantially smaller grain size than what would have been possible for sputtered gold. Ion beam sputtering (IBS) offered another route to a smaller grain size. Unfortunately, even with these reduced grain sizes, the grain is still resolved in a typical FESEM and in the case of chromium, the metallization oxidizes so fast, it is a challenge to get the sample into the SEM before it starts to oxidize and become nonconductive. And of course, for a chromium coated sample, there is no such thing as a "second look".

The advantage of a thinner coating thickness:
With conventional sputtering processes, the thickness must reach some thickness before there is conductivity. Depending on whether one is using gold or any of the other mentioned methods, the coating thickness still tends to be minimum ~ 20 nm or higher. And with a thickness of that level, there are numerous important features that are just being covered up. But with the OPC osmium coaters, one achieves conductivity at a thickness of slightly under 1 nm. One ends up being able to "see" all kinds of features that would otherwise be getting covered up by a sputtered coating. See our video ease use and over view of operating.

Shortcomings of chromium coating:
Chromium, as a coating, suffers from two major drawbacks for the FESEM application, and while not normally found in the literature of those who manufacture such coating devices we would like to list them below:

Virtually no shelf life for a chromium coated sample chromium since chromium metal starts to oxidize almost immediately to a very nonconductive oxide. While one might contemplate storage of important specimens in dry oxygen free environments, the fact is that chromium metal, in this form, seems to want to oxidize no matter what is done to it. So the rule of thumb for chromium coated samples pretty much is to "coat and look" almost as one operation. And there is no such thing as going back to take a "second look" since that would require a second coating resulting in a much thicker overall coating, and running the real risk of covering up important structure that would otherwise be seen.
The grain size of chromium is certainly greater than zero simply meaning that while the grain size of sputtered chromium is far smaller than that of gold in a conventional gold coater, the fact is that there still is a measurable grain size. Of course the degree to which this grain size is a problem for any particular user would depend on what kind of resolution they are trying to achieve.
The minimum thickness of an electrically conductive sputtered chromium coated surface is greater than 10 nm and some report even larger numbers. This compares with the minimum electrically conductive coating of osmium when deposited in the OPC coaters of slightly less than 1 nm.

As if this was not enough reason to seriously consider the osmium plasma coating alternative, we do encounter at trade shows, on a regular basis, FESEM users who turn out to be using conventional gold sputter coating systems. But upon further questioning, they are using the gold for low magnification work that without the gold coating, lacks the kind of contrast one has come to expect in an SEM image. So if you are one of those FESEM users who finds their work does not require the ultra-high resolution that is normally associated with of a FESEM, then the osmium coater might not be for you.

But if you are one who does push the limits of resolution of your FESEM, and are working on nonconductive samples, or even low atomic number conductive samples, and are disappointed with the lack of meaningful contrast (which happens more often than not), then you would definitely be a good candidate for an osmium plasma coater.

There are also certain specific applications, not even necessarily at high magnification, such as immunogold tagging of cells and BSE imaging, or the examination of certain difficult to coat (with gold) samples, such as polytetrafluoroethylene (e.g. PTFE) or surfaces with thin lubricants (e.g. storage media, hypodermic syringe needles, catheters), then osmium plasma coating often times does what gold can not do, namely, one can coat with osmium with out the artifacts normally associated with gold deposited layers

There are also reports that the virtually zero heat contribution due to the operation of the plasma makes possible the coating of otherwise unstable explosive particles that absolutely could not be coated any other way.

So if you are experiencing any of the above mentioned problems, or are just unhappy with your ability to get artifact-free coatings on the samples you are studying, even if with a conventional SEM and at low magnifications, then one of the OPC systems is almost certainly going to be worthy of your serious consideration.

Osmium source:
Standard osmium tetroxide ampoules, specially sized to fit into the detachable reservoir on the front of the unit. From a 0.1g ampoule, the number of actual runs one could get would depend on the thickness being applied, but typically, one could expect to realize 10-15 runs. Note: Some of the earlier units manufactured by Nippon Laser and now, Laser Techno were produced to take a slightly non-standard size osmium ampoule. All units sold by SPI Supplies have been manufactured specifically to accept normal sizes with respect to the starting ampoules.

But isn't osmium tetroxide dangerous?
Well of course it is! But then again, so is gasoline and we don't give a second thought about getting into an automobile. The innovative design of all the OPC models permits one to install a normal glass ampoule of osmium tetroxide, into a special holding device which is then tightly capped. The analogy with the gasoline in an auto fuel tank is almost exact. When the system is ready to operate, the vapors sublime into the now evacuated and therefore sealed chamber, the molecule dissociates in the field of the DC glow discharge into its constituent elements, and osmium is reduced to the metal which literally plates out as a completely amorphous film on the sample to be coated. Unless there is a really good vacuum in the chamber, they system absolutely can not be operated.

We would recommend that the system be vented to the outside or else operated inside a fume hood. However, this might be over kill since many of these systems operate in Japanese laboratories using only a regular oil mist filter. Our own belief is that any osmium in the tetroxide form surely would be reduced (to the relatively harmless dioxide) instantly when in contact with the hot pump oil. But until more is known about the safety issue, we recommend the system be used only as indicated above.

One further comment: An OPC-40 and more recently, OPC-60A system has operated out in the open during the running of the Lehigh University "Short Course" on SEM for the past three years. It has received the most stringent scrutiny of some of the best known users of FESEMs in the world and no one has expressed concerns about the operation of the system that way. The newer models OPC-80N and the latest OPC-80T systems have the same identical safety features and interlocks and our comments for the earlier units would also apply. The OPC-80T coats osmium in the automatic mode but does other depositions including carbon in a manual mode of operation. The OPC-80N has only an automatic mode and will coat both osmium and carbon in the automatic mode only.

Examples of the superiority of osmium coatings:
Since a "picture is worth 10,000 words", we would prefer to have our prospective customers see for themselves some examples of the superior results that can be obtained relative to sputter approaches. And for those who would like to see results on a more controlled basis, we can present carefully obtained experimental results comparing different popular coating methods on clean glass microscope slides.

Publications citing osmium plasma coating
We are starting a list of publications that include results using osmium plasma coating. If you encounter publications not on this list, please let us know so that we can add them to our list.

Is this system "accepted"?
We will admit that we ourselves went into shock when first hearing about a coater that "coated with osmium tetroxide". But we have become believers. Some of the technical staff of SPI had extensive demonstrations of the first model, the OPC-40 (predecessor to the OPC-60 and OPC-80's) at the ICEM in Cancun in August 1998. We were impressed with the fast turnaround time between samples (relative to chromium coaters that require long pump down times with a turbo pump). The OPC units use an ordinary rotary vane (e.g. "mechanical") pump. We were further impressed when the marketing manager of a major FESEM manufacturer said to me that when they do demos to prospective customers, they prefer to use the osmium coater because, well, they do a "better demonstration" and have a better shot at making a sale.

Now that might not be sufficiently convincing, however in Japan, where the technology was developed, more than 100 units of the OPC series have been sold both to FESEM and conventional SEM users. Indeed, in Japan, there is an overwhelming preference for osmium coating instead of chromium coating. And at the recently held APEM meeting in Kanazawa, several customers in Japan expressed the thought "we can't understand why so many users in North America are still using chromium."

Plasma coating with carbon
The concept of plasma coating when the starting material is a subliming solid is novel and innovative. The Nippon Laser and Electronics and Laser Techno researchers have found that naphthalene, that also sublimes, just like osmium, can be introduced into the chamber, and decomposed, again, much the same way as osmium tetroxide, with the result being an ultrathin carbon coating that is completely amorphous, and therefore also, structureless and featureless. Where as arc evaporated carbon (e.g. in vacuum evaporator) is obviously "black", an evaporated film of carbon that has been done in the OPC system is actually, visually transparent! Yes, we know that this flies against conventional wisdom, but if the surface being coated is smooth, the deposited film is transparent. We believe that this unique coating capability for carbon has a number of obvious and also not-so-obvious advantages over vacuum evaporation deposited coatings. In addition to the potential to make the world's finest carbon support films for TEM, such coatings are also completely free of both nanotube and other nanoparticle artifacts that almost always are a bi-product of arc evaporation in a vacuum evaporator. Unfortunately, with the standard rotary vane mechanical pump, the carbon coater accessory does not produce carbon films that seem to be "clean" enough for use as TEM support films.

But when the carbon is applied to a sample destined for EDS analysis, the deposited carbon film does exhibit a smaller grain size small than we have seen with vacuum evaporation. For certain samples, such as catalysts, nano particles or other nano-features, this kind of property is invaluable.

To do carbon coating, one needs the Model OPC-80T Osmium Plasma Coater. Whereas the OPC-60N is an all-automatic coater designed for the automatic coating only of osmium, the OPC-80T can be operated in the automatic mode for both osmium and carbon deposition. It can not be operated in the manual mode. All coating is automatic and the only variable is the coating time, and therefore the thickness. In the earlier units, there was a manual mode but the newest models don't have that option.

If this all sounds very complicated, think of it this way: If you want only an osmium coater and don't care about carbon, consider the OPC-60A or even the OPC-40 which is still available. But if you want to do carbon as well, then you really do need the OPC-80T.

How does the price of the OPC-80T compare with that of chromium coater?
Well it is a bit more expensive, but not all that much more expensive. The system does not need a turbo pump and longer term, the operational and maintenance costs are far lower. And don't forget that you are getting a superior carbon coater as part of the "package". From our perspective, many who are purchasing a chromium coater, because of their relatively high cost, more often than not, since other elements can be deposited in a typical Magnetron head chromium coater, the costs can be split between the FESEM lab and other groups doing thin film coatings research. However, a very big trade-off is given up for the SEM lab application and those who have made this kind of compromise, in the end, never are able to realize the full capabilities of their FESEM.

But in the case of the OPC-60A, there is only this one application, that is, the SEM laboratory application and we don't see any comparable opportunity for the sharing of the funding for the purchase. However, with the new carbon coating capability of the OPC-80T, one can use the same unit both for coating SEM samples as well as samples to be analyzed by EDS. We want to stress that in our opinion, the carbon coating capability is not able to produce films suitable for support films for TEM applications.

Substantiation of the claims being made:
We have provided some typical examples of the high resolution benefits possible with the OPC family of osmium coaters. Some will be recognized as extremely high magnification and resolution SEM micrographs whereas some others offer direct comparisons between osmium plasma coating in one of the OPC systems vs. conventional sputtering, ion beam sputtering or even chromium coating. We believe some of these examples and comparisons are quite remarkable because it demonstrates that a conductive layer is formed at far thinner thickness in comparison to sputtered films, not only of gold but also chromium.

Model identification terminology:
The "60" in OPC-60A means that the chamber is 60 mm in diameter; the "80" means that the chamber is 80 mm in diameter. OPC systems for osmium have been delivered as research instruments with chambers as large as 180 mm. However, for all of the OPC units, the coating thickness has the kind of uniformity one generally wants in the inner half of the radius. In other words, for the OPC-80T, the area of uniform coating is confined to a circular area with diameter of ~ 40 mm. The "N" in the model designation denotes an enhanced model relative to the original model designations, OPC-60 and OPC-80.

The unique technology embodied in the OPC family of osmium plasma Coater is covered by United States Patent #5855682: Plasma thin-film forming apparatus which issued on January 5, 1999. It is also covered under a patent recently issued January 27, 1997 in the European Community (EC), #78952.

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