Best method for mounting thin polished mineral wafers

[AnneF]

Hello!

I have been analyzing a bunch of doubly polished quartz wafers for FTIR, and now I need to mount them for EPMA. In the past, I have used 1in rounds with holes drilled in them where I put samples and epoxy (see attached photos). The problem was that a lot of the time, the epoxy dried with a semi-concave surface, causing the samples to be tilted, which affected my probe data quality. Most of the time, I cure the epoxy overnight at low temperature in a drying oven before letting it sit for longer at room temperature. I used this method because I always do LA-ICP-MS after EPMA for trace elements, and 1 in rounds work well for both.

Does anyone have any suggestions for how to get epoxy to dry flat in this type of sample? Or do you have a better mounting procedure for such tiny, fragile samples that will allow me to do both EPMA and LAICPMS?

Thank you for your help!

[Beamsys]

Hi Anne,

There could be multiple ways for achieving flat mounting of thin fragile samples - all would depends on technical capabilities available at your disposal. For example:

You can try using 1" mount without any holes and attach samples with very small quantity of liquid epoxy, such as Allied 110 or M-Bond 610 - with thin layer of adhesive coplanarity would be improved;

To get nearly perfect planarity of mounting you can make custom 1" mount with tiny holes that are much smaller then size of your wafers. Use same liquid epoxy and gently pull vacuum through the holes during curing - excess of epoxy would be drawn into the holes and wafers would lay perfectly flat;

In both cases above you can try using Mikrostik instead of epoxy - it is very thin and will not induce much of tilt while drying

For mounting on 1" block without any holes you can try using mounting films - either push-n-stick variety, or better yet there are very thin thermally-curable epoxy sheets available for sample mounting. FL901S from Master Bond is only .003" or ~75um thick, providing very uniform flat mounting. Sometimes it could be obtained as "test sample" if you ask nicely and the apps engineer is in helpful state of mind.

Best of luck!
Valery

[Probeman]

Hi Anne,

Another method that might work for you is to simply mount your double polished glass samples on a Si wafer, with only a tiny drop of isopropyl alcohol acting as the "adhesive".   I'm not sure quite what is going on as the alcohol capillaries around the sample and dries, but I'm guessing that the alcohol dissolves trace amounts of oils and other hydrocarbons already present, and as it "wicks" around the sample and dries, it makes a surprisingly strong bond. The sample can still removed using a needle gently, but it stays in place for normal handling and EPMA work, I do know from experience. Whether it will stay put for LA-ICPMS is another question, but I'll bet it will.  Worth a test I think.

Basically one places the sample on a Si wafer (which is very flat of course), and then using a needle, one applies the smallest possible drop of isopropyl alcohol to the bottom edge of the glass sample and the surface of the Si wafer as seen here:



Also if the tiny drop of alcohol is placed right in the corner of the sample, it only wicks around the bottom and sides of the sample, and  won't ooze over the top edge of the sample, so your sample surface stays uncontaminated, and of course flat and level since the dried residue has essentially no thickness.  We've often used this method for mounting micro samples in the lab of all shapes and I think it should work for your flat geological samples as well.

We have 5 x 5 mm Si wafers in the lab and one can also get 5 x 7 mm Si wafers here:

https://www.2spi.com/item/z4136sc/silicon-wafers/

Another idea that I might look into is getting 25 mm (1") round Si wafers, so they can mounted right in a standard sample holder without having to glue the Si wafer to an Al puck for mounting in the instrument's sample holder.  Here's a company that says they can provide 25mm Si wafers. I'm going to check to see how expensive they are, but on the other hand if we're using the above isopropyl mounting method, the wafers will be re-usable at least.

http://www.addisonengineering.com/silicon-wafers.html

john

[macosta]

Can you just re-polish after mounting (starting at a higher grade than usual)? IIRC FTIR samples are much thicker than what the microprobe needs.

[AnneF]

Thanks for the replies!
I like the idea of using alcohol on an Si wafer, but I need to be able to remove the carbon coat and be ready for LAICPMS analysis within a few days. I'm not sure that method will work for both EPMA and LAICPMS in short succession.

I'll test a few different methods for tweaking my epoxy mounts before making my final sample mounts.

[Anette von der Handt]

Anecdotally, I have seen people just stick their FTIR samples on carbon tape for EPMA and LA-ICP-MS analysis.

I suspect that some of the surface topography either comes from the tape that was used to place the grains on or the epoxy. There are already some good recommendations, mine would be:

To create a flat surface for small grains, a highly recommended substrate is Kapton tape. It is relatively easy to get due to its popularity in the electronic industry and 3d printing nowadays.

To find an epoxy with low shrinkage, I will refer to the excellent overview from Edinburgh https://www.ed.ac.uk/geosciences/facilities/ionprobe/technical/epoxyresins

Otherwise, I would also ask why not slightly repolish after mounting as the FTIR sections are likely thicker than what you would need for LA-ICP-MS and EPMA.

[BenjaminWade]

Hi Anne
Is there a reason you need to remove the C coat for laser. In my experience with geological samples moving from probe to laser, C coat is not too much of an issue other than affecting things visually on the reflected light image on the laser. Usually the C layer is ablated within the first second or so of acquisition which you usually get rid of anyway. Unless you are looking at specific elements with bad C polyatomic interferences?

Cheers