Hi, all.
When I first started using the atomic force microscope, my coworker mentioned that it is normal to have a drive of around 30% for a decent cantilever and that the closer the drive approached 100% to yield a good image was related to the life span of that cantilever. I searched for other literature to confirm this and saw similar numbers (25-35) for general cantilevers. When I first started imaging things, I didn't really pay attention to the frequency-sweep-determined drive but the images that were produced came out looking good the test structures. I don't recall having to adjust the drive that much to obtain a good image at the start. About a month back, I was working in contact-mode to image a test structure and the AFM began to screech loudly and so I proceeded to disconnect the head connection but did so without turning off the laser source via the software OR turning off the head's switch. I didn't realize this until after and proceeded to turn both things off and then try things again but the screeching continued so I decided to just stop contact mode for the time being and go back to non-contact mode. After this incident, when I pressed frequency sweep, the drive that is determined by the system is shown to be 0.1% and this has been for all 10 cantilevers I have to try. The reason I started to notice this was because I noticed I needed to increase the drive to the 30s/40s to get any sort of surface detection because nothing would happen when kept at 0.1%. Also, the resonant frequency that it selects is significantly lower than what the cantilever manufacturer's website states it should be (the AFM software selects something like 280kHz vs what it should be at 330kHz). Could the AFM been effected by the disconnecting/connecting and screeching during contact mode?
Would the AFM need to be calibrated? This is a Park Systems XE-70.
Hi, I am not familiar with your AFM system and cannot comment on the particular values you have given. Usually, screeching of an AFM system means that the topography feedback loop started ringing, thus causing large oscillations of the z-piezo. Hence, the PID parameters should be reduced significantly, first of all the I gain and then the P gain. Once the system has calmed down, one can try and increase the PID parameters again in a stepwise manner while monitoring the feedback performance via the error and the topography images. However, the AFM tip might have suffered when the system was ringing, and a new cantilever might be needed to obtain a high spatial resolution.
Not sure why the drive parameters have changed. The software might have reset some parameters to protect the piezos or the AFM tip.
In dynamic mode, one needs to set both the drive amplitude for the dither piezo and the setpoint ratio for the feedback loop. This should be done before attempting to get into feedback, ie starting the tip approach. There are a few considerations, to account for the sample surface mechanical properties, the required spatial resolution and the feedback robustness. Frequently, a value of ~75% for the setpoint ratio is a good starting point.
Hope this helps. Best wishes, Martin
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