I'm studying brass drilling with a femtosecond laser. The holes I get are narrow (10-20 microns) and with a depth greater than 50 microns. The confocal microscope I have available is not able to detect the bottom of the hole. Any Suggestions?
Dear Jose
I think this small dimensions will never be seen by optical microscopy. Only electron microscopes are able to see. This is possible from scientific view point. I think the problem is technical. I mean how to measure with electron microscope. I suggest to make a thin film of transparent polymer over the surface you made drilling for. This film will have a negative form of the hole (Rod Shape) in the sample for which you can measure the height of the rod.
Best Regards
Hi Jose,
two suggestion to try and one comment:
(1) Use a good (!) optical microscope at large magnification and with a calibrated (!) wheel for adjusting the imaging position. Check brightfield imaging in reflection or transmission mode. By adjusting the imaging position you may be able to "focus through" the depth of your hole (always seeing a different ring shaped part in a "sharp" way) and take a note of the wheel position at the deepest position where you can see the crater bottom. The difference of the postions of your wheel for properly imaging (i) the sample surface and (ii) the crater bottom then represents your crater depth.
(2) You may check whitelight interference microscopy for directly characterizing the topography z(x,y) of your craters:
https://en.wikipedia.org/wiki/White_light_interferometry
However, the large depth-to-width ratio may reduce the amount of light (back reflected from the crater bottom) available for the measurement. You have to try..
(3) For many AFMs, the depth of 50 microns may be too large..
I dont know the shape and size of the sample where you do the holes. Neither the precision neccesary to measure the hole, but may be you can use some non destructive method like ultrasound or radiography.
Large magnifications and thus large numerical aperture us will not be able to see the ground. If all you need to see is the depth I would recommend medium magnification with small NA. A good WLIM might be your best bet.
Another idea. Use thin foils of different thickness to determine your drilling rate and depth by just looking at the other side and detecting whenever you have drilled through
If it could be possible, make some holes with the same laser parameters with overlapping and break the substrate. The depth could be measured easily under microscope.
The second suitable way would be to have a FIB ( Focused Ion beam) cross section and measure the depth of your hole. The AFM is also a good idea to measure your depth.
Hi Jose,
By using laser scanning microscope you are able to measure depth by several tens of nm accuracy.
Hi Jose
I like the first idea raised by Mohammad: for each experiment setting, you make a matrix of identical holes, and all settings on a very same substrate. You put your sample in a cold set resin and then cut the sample into 2 halves - right across holes matrices of all the settings. After polishing the cross-section, you will be able to assess, with an OM, not only hole depth but also their shapes.
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