Setpoint, resonance frequency and PID parameters...

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Ashley Slattery replied on Thu, Nov 29 2012 4:43 PM

Suggested by Ang Li

Hi Igc,

I have found that for sharp, delicate probes there are two approaches that you can use which work well for me.

Note that in both of these methods I will set the scan size to zero (to minimise any lateral forces on engage, which I think was recommended by someone here and seems to work effectively).

PEAKFORCE TAPPING

My preferred option for delicate probes is PeakForce tapping, mainly because the force control is very good.

Set the laser spot as close to the free end of the cantilever such that it s on the cantilever entirely and doesn't spill over the edge.

Set ScanAsyst to "off" and set the "peakforce setpoint" to zero and the gain to 10. (note that "peakforce setpoint" is different to "peakforce engage setpoint" which I introduce next)

Go to Microscope-> Engage Settings and set the "peakforce engage setpoint" to 0.05V, and reduce the engage gain if you want a slower, more careful engage (not always necessary, I usually use a value of 1 if I want to be extra careful).

While you are in this menu, make sure you turn "smart engage" to off if you have a dimension fastscan, this seems to be far from gentle to the probe.

Try to engage, you will likely get a false engage; in which case you should withdraw and increase the peakforce engage setpoint slightly, then try to engage again.

(there is something in the manual about changing the SPM engage step when the peakforce setpoint is altered, I'm not sure how much to change this value for a given change in engage setpoint, so I've just been leaving it as the same value. Maybe one of the Bruker guys can weigh in on whether this is required or not.)

Basically, repeat this procedure of increasing the engage setpoint until the probe engages on the surface, this minimises the engage force and protects your probe.

Once engaged, you can determine whether the probe is in contact and change the peakforce setpoint to achieve very light interaction, best to keep an eye on the "heartbeat" or force curves while you do this.

The biggest problem you will face with this procedure is laser interference (and possibly noise from other sources), otherwise this mode is excellent.

The false engages you will experience will be due to the deflection signal changing during the engage, as a result of this noise. So you need a peakforce engage setpoint that is slightly higher than the noise and fluctuations in the deflection signal during the engage.

I find that for highly reflective probes or for surfaces which aren't reflective, I have managed to engage at or below 0.05V. In other cases, I need to go well above 0.15V or even 0.3V, in these cases I prefer to use tapping mode if possible.

TAPPING

Tapping mode is actually much simpler to explain, due to the preset "Soft Tapping" mode.

Simply start this mode in soft tapping, tune with the default values and engage.

After a successful engage, the tip will be off the surface. Decrease the setpoint slowly until the tip comes into contact, then increase the scan size slowly and adjust gains/setpoint etc...

I find that tapping mode's engage is much more forgiving with regards to laser inteference, so this is the mode I usually use if I have to use a large engage setpoint with peakforce tapping.

Keep in mind also that certain cantilevers will be more suited to certain modes. I like peakforce tapping because I can use soft levers and limit the force on the tip, but if your cantilevers are very stiff you may find that they are better suited to tapping mode.

Like wise, very soft cantilevers generally wont work in tapping mode.

In either case, if your tips are very expensive, I would try this with a scanasyst air probe first. These often have very sharp tips (below 10nm) and you need to engage and image very softly to preserve the tip sharpness, which is good practice.

Hopefully this helps,

Ash