• Qualitative estimation of AFM tip’s sharpness based on topography image of its surface;
• Quantitative estimation of AFM tip’s curvature radius using deconvolution algorithm.

TSD01 consists of large variety of densely packed particles with average diameter around 60 nm. Particles’ shape is not ideally round. Some of them are rather cylindrical, some ones have got vertical walls and sharp corners (like as on the REM photo). These features are necessary to collect enough statistics for further “blind” tip estimation using Deconvolution algorithm.

Qualitative tip’s shape estimation with TSD01

Qualitative tip’s shape estimation could be done by comparison of AFM images of TSD01 surface of different AFM probes. Below you may find AFM images of TSD01, performed by two cantilevers of the same model.

Looking at images’ scales, the one may notice that particles’ height on the left image is more than on the right one. That means that tip #1 penetrates deeper into gaps between particles during the scan. And the tip #1 is just the sharpest one between them both.

Quantitative tip's shape estimation: “Blind” reconstruction of AFM tip’s shape using Deconvolution algorithm

Deconvolution algorithm for AFM surface reconstruction was suggested by J.S. Villarrubia in 1997 and now it is implemented in many different AFM-image-processing programs. It also includes the part, that allows so-called "blind" tip's estimation, when analysing a scan of specific surface program draws tip's profile.

For better understanding of this method let’s imagine an ideal vertical bulk (on the image below) which width is equal to “0”. It’s not difficult to calculate that when a probe passes such an object, its image on AFM topography scan will be just the inverted probe’s shape (a dotted line).

Thus, every particle of topography imaged, which size is comparable to a tip, should hold some information about its real shape. The algorithm of “blind reconstruction” compares shapes of all particles (local maximums) of the topography scan and finds common features between them. On the basis of such comparison it outputs approximated tip’s shape. Reliability of the result depends on particles amount (it should be enough to collect statistics) and their shape (then closer to an “ideal bulk” – than better, as we have seen previously).

Knowing details of "blind" tip's estimation algorithm we could formulate the next criteria to the test sample that could be used for its realization:

• Obviously, it should be rigid;
• It should have got many particles of 20-100 nm approximate diameter, so that their size should be comparable to tip’s one;
• All the particles should densely sit on the surface. For detailed analysis each particle should contain, say 10x10 points. From the other hand, there should be 50, 100 or more particles in range of one scan so that we had enough statistics;
• As we discussed above, speaking about “ideal bulk approximation”, only AFM images of vertical objects of “zero” width hold the most exact information about tip’s shape. If even they don’t exist in nature, every vertical wall and sharp corner of our particle holds exact information about tip’s shape. Thus, the test structure for tip’s shape “blind” estimation should have got irregular shape with many vertical walls and corners.

Quantitative tip’s shape estimation with TSD01

TSD01 manufacture process determines its physical properties: all particles are being grown chaotically around small, several nanometers, cores. Thus, they have got similar, but very irregular shapes. Some of them may have got sections, close to cylindrical, some of them look to be rounder. They have got many sharp corners too. Writing several dozens of such particles during the same AFM image a tip leaves much information about its true shape from each side. And usually this statistic is enough to get reasonable tip’s images after tip’s “blind” estimation by standard deconvolution procedure.

Below you may find the results of “blind” tip’s shape reconstruction from the AFM scans of TSD01 test structure, which were posted previously:

As the one may notice, the results of tip’s shape estimation correspond to our previously-made suggestion that the probe #1 is sharper than the probe #2.

• Full diamond tip is harder than a standard silicone one. Its wear is about 10 times lower and it allows to measure surfaces that grind fast silicon needles. More of all, such cantilevers will be the best for measurements of surface elasticity properties. Their deformation during force curves processing will be minimal respectively to other-material tips;
• Full Diamond tip has got small cone angle (<25 degrees) and tip's curvature radius (<12 nm) that allows to get high quality scans of various type samples;
• Low surface energy of diamond makes FD cantilevers to be well-usable for long time scanning of sticky biological samples;
• Narrow and hard tips are also suitable for simple nanoindentation experiments with standard AFM parts;
• Experimental results have showed that Full Diamond tips are less sensitive to static charges on sample’s surface. That results in more detailed topography scans in comparison with Si cantilevers in the same conditions

Great utility of FD AFM cantilevers for precise measurements is accompanied by a detailed quality control. After gluing a tip to a lever each cantilever is being observed by SEM before shipment. Thus only good probes reach the customers.

Our colleague from University of Nebraska Medical Center, Mohtadin Hashemi, have reported about outstanding FD probes utility for measurements of sticky biological samples. He tried both FD and Si probes for scanning of fibril and globular amyloid aggregates. When standard Si tip could be used only for 2-3 hours (then its surface became too dirty, catching sample's material), FD cantilever didn't show any degradation during 7 hours continous repeated scan of the same area. Images from the both sides were taken with 3 hours difference - no tip's or sample's degradation can be noticed.

Our experiment have showed that FD cantilevers are less sensitive to effect of image distortion due to surface charges. To the left you may find an image of silver nanoparticles on mica by a standard Si cantilever in low humidity conditions. It took around 1 hour of parameters' adjustment to get this image. The engineer had to decrease contact amplitude of cantilever's oscillation 10 times in respect to free oscillations. To the right you may find a scan of the same surface, obtained by FD probe without any special adjustments. Image is more sharp and particles' sizes are much lower, as they should be in accordance to non-AFM investigation of this sample.

Chip thickness H: 0,4mm.
Reflective side coating: Au.
Tip's cone angle: < 25 degrees.
Tip's aspect ratio: > 1:5.
Tip's curvature radius: < 12 nm.
Chip has one rectangular spring.

***HA_C/FD probe could be produced with another type of HA_NC lever (235 kHz resonance frequency) by request.