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Does the chips thickness of the AFM probe chip affects the laser alignment?
I have some problems with some new contact mode tips. ND-CTCR2-2 tip from advanced diamond technologies. I uses them in a MM8 setup with NSV controller.
I have the following issues:
The sum signal is low beside the fact that the cantilever is coated with an Al reflection layer.
The reflected laser beam which I check with the paper card (MM style laser alignment) shows several diffraction maxima next to the main spot. A standard cantilever with comparable width shows just one spot.
If the tips in contact with the surface on can see that the laser spot around the cantilever is much bigger compared it a standard TM cantilever is used.
It seems to that the reason for this issue is the chip of the new cantilever. It has the following dimensions: 3.6 x 1.5 x 0.5 mm which means that the thickness of the chip is almost doubled compared with a “standard” silicon AFM tip chip which has a thickness of 0.3 mm. The length of the cantilever is 500 microns
Is it possible that this offset in height has the effect the laser is not focused correctly on the cantilever and a large part of the beam is reflected form the substrate? It does not help to move the laser beam a little bit away from the cantilever end which normally eases problems with substrate reflections.
My questions:
Do you agree that the problems can arouse through the thick substrate chip?
Is there a possibility to focus the laser beam on the cantilever?
Thank you very much
Oliver
Oliver,
The cantilever die thickness very much effects the performance of the optical system in the AFM. In most cases, the cantilever holders are designed with the expected chip thickness in mind. For example, since most air levers are silicon (and made on 300um substrates) the air cantilever holders are designed for this thickness.
Historically many fluid levers were nitride on glass (500 um substrates), so many fluid cells were designed for that thickness. Bruker’s SNL probes are nitride cantilevers with silicon tips on silicon substrates (300um) so we have a separate set of fluid cantilever holders for them.
The thickness offset causes the beam to be out of focus, and potentially larger than the lever, causing low reflection and/or tip sample optical interference – the symptoms you describe.
However, the “poor spot” is more indicative of a cantilever backside roughness, likely brought about by your diamond coating.
To isolate, can you check an identical cantilever that is uncoated? If you have the same problem, it is a substrate thickness issue. In this case see if you can get a 300um lever coated. If you need low k or nitride, try an SNL (contact Probes for a sample).
If an uncoated probe works, he problem is either the diamond coating or the Al coating. Again try the diamond uncoated with Al to isolate. If it is the diamond you will have to work with your vendor on a smoother deposition. Same for optimizing the Al coating if that proves the culprit.
Worse case you can use an old fluid cell that holds the 500um substrates to hold the probe in air.
After that it is possible to refocus the laser, but first please be sure to isolate it to die thickness, and try all these other options first.
Hope this helps,Steve
The thickness difference of 200 um should not have a significant impact. The depth of focus of the laser spot is larger than that anyway. The effect you are seeing is, as Steve pointed out, most probably due to a poor cantilever coating than anything else. If you use the old style SiN probes you get fine signals I assume and they too had a 500 um thick substrate. So, to answer your questions:
1. No, I dont think this is a problem in air.
2. There is no need for it.
Stefan