Coated Grids and Custom Coating Service from SPI Supplies
Measuring the thickness of the coating
One of the most often asked questions about the SPI Supplies custom coated
grids has to do with the thickness. Some researchers believe they have a
need to accurately know the thickness of their support films while others
seem to just be curious about it.
One can take a Formvar® coated (copper) TEM grid, embed it, diamond knife
thin section it, and arrive at a coating thickness. Since most embedding
resins can potentially interact with the Formvar to yield a thickness result
that is larger than reality, we recommend metallizing both sides of the
Formvar. Until not too long ago, this was usually done with gold, but now
the recommended method is to metallize both sides with osmium metal, as
would be done in the OPC line of osmium plasma coaters.
The layer of osmium metal,
unlike a gold layer, has absolutely no grain size and can be deposited far
thinner, roughly 1 nm. And the metal layer does protect the Formvar film
from interacting with the embedding resin.
Unfortunately, when the grids are being made, a grid with a larger hole
(e.g. smaller mesh size), needs a more robust Formvar layer then a smaller
hole mesh (one with a higher mesh size). Therefore since the Formvar films
are made to be just thick and strong enough to withstand the insult of the
electron beam, by necessity then, grids with higher mesh sizes have thinner
Formvar layers. And grids with smaller mesh sizes and larger holes to span
have thicker Formvar layers.
The thickness can also depend on environmental factors such as relative
humidity. And while we do try to control these factors, there are some
seasonal variations. So these are the reasons why we can not quote a firm
Formvar thickness. Generally speaking, we believe the thickness to be on
the order of 20 nm but it can be as thin as 12-15 nm and might even be at
times as thick as 30 nm.
One can also take a carbon coated grid and measure its thickness the same
way. The osmium metal layers are, this time, very important because there
are clear indications that the embedding resin can have a swelling effect on
the carbon film. Over all we believe the carbon thicknesses are on the
order of 10-20 nm, again, the thicker layers being used for the lower mesh
sizes and the thinner value for the higher mesh sizes.
We do know that workers cite absolute values for both Formvar and also
carbon films being used on their grids. We do not think their thickness
values are what they say they are. But if one really wanted to check and
see, they would have to practice the kind of approach described above. We
would of course welcome any comment one might have pertaining to the
thickness either of our own SPI Supplies produced grids, or grids that they
have made themselves and measured, or even the thickness of grids made by
our competitors.
But if you are one of those workers who are really concerned about knowing
the exact thickness of your support film and also, one of those persons who
would have some concern over the amount of "sag" of any support film over
the grid squares (because of the uncertainties "sag" can cause to the
magnification calculation), you might want to consider the
SPI Supplies silicon nitride membrane window grids.
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Friday July 25, 2008
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