How to improve AFM scans choosing a proper cantilever
Due to various phenomena, arising during tip-sample interaction, AFM imaging may become a tricky thing. Even automation, that is implemented in many up-to-date commercial AFMs, doesn't solve all the problems. It's impossible to predict influence of thermal drift, sample's surface charges, interference of registration laser and other annoying factors.
This article is devoted to those possible issues that may arise from cantilever's quality or be solved by proper cantilever's choice.
Improvement of AFM topography acquisition
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Scratches on topography images often appear when a cantilever picks up a particle from a surface. It often happens in the semicontact mode during scans of sticky biologic samples or dusty surfaces. Typical scratch of this type is marked with a green arrow on the image to the left. It's allways parallel to fast scanning direction and it is accompanied by sudden height change at some point.
To avoid this problem the one have to minimize interaction force, which acts when a cantilever hits a sample during oscillation. To do it the next steps may help:
1) Conversion of cantilever's free amplitude from conditional units (Volts, nanoAmperes) to nanometers. It can be done using a slope of Amplitude vs Distance curves after tip-sample approach procedure when a FeedBack is on.
2) Decrease of probe's free oscillation amplitude down to 10-30 nm. Working with smaller amplitude is also good, but it may be difficult.
3) At the last turn the one can use Amplitude vs Distance curves again - to choose as big SetPoint value as possible. It should be about 90% of the Amplitude at the point where a tip starts "hitting" a surface (sharp bend between almost horizontal part, "far from a surface", and inlined part of "intermittent contact"). In our experience such Set Point is enough for maintain stable tip-sample interaction and to minimize "hitting force" the same time.
If the problem arises from
sample's sticky surface (like some biological materials), proper choice of tip's material may also improve quality of scans. According to our experience and customers' reports,
AFM cantilevers with diamond tips (FD series) demonstrate minimal sensitivity to sticky samples.
Sample's electrification is another phenomenon that can dramatically distort topography scans in the semicontact mode. In dry weather electric charges can collect on the surface of semiconductor or dielectric samples. They produce electric field, which can significantly reduce oscillation of a cantilever when it's coming towards a surface. Very often this force is so strong, that oscillation amplitude reaches SetPoint far before a tip starts "hitting" a surface. This situation may look like absolutely normal approach. But when the one tries to take a scan, he gets only a waved uniform profile, maybe with some distorted features on it at best.
Two images to the left are taken in the semicontact mode at the same area of Mica surface, overspreaded with silver nanoparticles. The upper one was obtained with standard parameters of cantilever's oscillation and SetPoint. To get clearly visible nanoparticles on the bottom image, an engineer had to increase several times oscillation amplitude and decrease SetPoint down to 5-10% of free oscillation amplitude.
The first step to overcome this parasitic effect is to conceive that it takes place at the moment. There are several attributes of sample's electrification:
1) Dry weather: the best season for surface charges collection is a snowy winter.
2) Waved topography profile with minimum of details, and they all are distorted too.
3) When the one decreases/increases SetPoint by 10%, AFM scanner will move for hundreds of nanometers. That means that no surface is under a tip and it moves freely in the force field of electrical charges.
4) Similar to (3) behaviour of Amplitude vs Distance curves. If 500 nm scanner's movement makes oscillation amplitude change just for 10-20 percents of its free-state value, most probably a tip doesn't "feel" a surface.
It's very difficult to avoid surface charges at all, when they appeared. It may be helpful to carry out AFM scans in a close chamber with high humidity atmosphere. But the result won't appear immediately. Several hours may pass before charges will be removed. Also there is a sense to ground sample's surface. Most probably, combination of these two precautions will be optimal.
Approach in the semicontact mode may be ineffective as it's difficult to predict SetPoint level of good tip-sample interaction. Instead the one can perform an approach in the contact mode, and then go to semicontact mode. Usually approach in the contact mode finish successfully even under surface charges effect. To choose proper SetPoint in the semicontact mode after approach the one may use Amplitude vs Distance curve, searching an area where curve inclination will dramatically increase. Another way - to decrease SetPoint slowly, watching over Z-scanner indicator. When a tips starts "hitting" a sample, scanner will almost stop its movement in accordance to SetPoint.
If an object of the same shape and direction repeats many time on the same scan - in 90% cases the reason is a broken apex of a tip.
The left image is an AFM topography scan of
TSD01 test sample, which consists of hemi-spherical particles of average diameter 60-70 nm. We know, that shape and direction of these particles should vary, but all the profile consists of triangular artifacts of the same orientation. It means that we don't see particles themselves, rather it's an AFM tip's deformation is observed.
Sometimes tip's damage can be even more complicated. Our engineers have met images with two, three or four particles being repeated simultaneously. So every time, when the one meets the same structure repeated over a scan, a cantilever should be exchanged to check if it was the reason of such strange artifacts.
For more detailed tip's examination several test samples from our assortment can be used:
TGT1 test grating - for whole cantilever's shape visualisation. This grating consists of 300-500 nm height needles, which shape is close to shape of a standard AFM tip. It's very convenient to compare various AFM cantilevers, using it.
TGZ3 test grating in common is intended for AFM scanners' calibration. But it may also suit well for estimation of tip's cone angle, analysing inclination of its side walls on scan. If scanner's feedback works fast, high-reliable values can be obtained.
TSD01 test sample can be used for tip's apex shape estimation. In particular, it is optimized for usage with Deconvolution algorithm and is used to check tip's curvature radius.
