AFM Training

This is my experience with the AFM Asylum. To work on it there are lots of procedures to follow and it is required a firm hand. First we have to prepare the sample on a microscope glass, attaching it via carbon tape or silver paste; the latter is better because it is more rigid.

Then we take the head of the AFM, we turn it bottom-up and we remove the holder of the cantilever. With the help of laboratory gloves, screwdrivers and tweezers we insert a new cantilever. The tips to be used are those from the boxes “TESPA”, high aspect ratio tips oscillating at 320 kHz and with a Force constant of 42 N/m. As a lab rule, the cantilevers in the boxes are put in two different positions. If they're oriented vertically they are new, if they are turned to an angle they are used but not broken.So, we unscrew the central screw of the holder and we put one of the cantilever in the aperture created, then we fix it screwing again the central screw.

We put the holder in the socket on the head of the AFM. We put the sample on the stage, verifying that the stage is at position 0. We can also switch on the antivibrating table. We clamp the sample in position with two magnets. Now we have to be sure that the legs of the AFM are long enough to avoid the tip touching the sample, so using the proper wheels we elongate the legs and we put the AFM head in position. So we reduce the lengths of the back legs, controlling that they’re at the same level with the help of a bubble. When the holder of the cantilever is near to the surface of the sample, we can decrease a little also the front leg.

At this point we can watch on the monitor where the cantilever is and we can search for it using the two screws on the back of the head (very end) that control the camera view. Then with the back wheel of the head we change the focus in order to have a good image of the cantilever. We can proceed with the alignment of the laser. So switching on the laser and using two screws, one on the back and one on the right of the head, we can control the laser spot in order to hit the cantilever. With the help of a window, Sum and Deflection Meter, we try to maximize the value of Sum; for normal silicon cantilever the value is around 3, for coated ones we arrive at 5. When the sum is maximized we try to cancel the Deflection value using the wheel to the left of the AFM head. Now the laser should be aligned.

It is time for the Thermal Tuning. This tuning shows the frequency at which the cantilever vibrates and we have to fit the peak of resonance. At this point we must do the tuning procedure for the tapping mode, that is in AC. If the final result is a perfect Gaussian, the system is correctly configured.

Now we have to extend the probe, that is engage, so we fix a set point (75% of the drive voltage, that is more or less 750 mW) and we lower the head of the AFM until it touches the surface. We can see when it touches from the sum and deflection meter, that is when the measured intensity will drop to 750 mW (or the chosen set point). Now we can start with the acquisition. Most of the parameters will be default, but can be modified during the scan to reduce noise or to better follow the profile. The sample I used consisted in a layer of gold over silicon with nanometric holes, varying from 20 nm to 500 nm.

Scanning with the AFM head we appreciated dimensions of pores down to 28 nm. Moreover we could appreciate the quality of the profile in term of roughness. The grains of the gold evaporated create “hills” up to 10 nm, while with the annealed sample we could appreciate 1-2 nm of roughness.