Tip Qualification

Tip quality is essential to quality AFM images. In a very general sense, an AFM image is the convolution of the tip morphology with that of the surface being imaged. Ideally an AFM probe has a radius of curvature (ROC) < 10 nm which allows for high resolution imaging of small features. When the ROC is increased due to wearing of the tip or the adhesion of a foreign object to the tip, the imaging resolution is degraded. In some cases, image artifacts may arise. The most common artifacts are self similar features. In some cases all the image structures are triangular in shape due to the fracturing of the AFM tip pyramid or ovoid in shape due to the wearing of the tip or the attachment of a foreign object to the tip. While a radical change in tip performance is readily visible, small changes often go unnoticed and these changes of more difficult to identify on certain types of samples. As such a standard surface morphology is desirable. An ideal standard should be capable of providing quantitative information about the tip quality.

The image above is a titanium surface that has very sharp crystallites emerging from the surface. This is an ideal tip qualification standard as these sharp edges can be used to deconvolute the shape of the AFM tip. By imaging such a sample the sharp edges of the crystal habits can be identified and their shape profile then used to estimate the rip ROC. This process is known as tip qualification, and can be performed just before and after a series of images to certify they are of the highest quality.

This image shows the actual tip qualification. The top left image shows a green "+" at each tip qualification point-- a sharp edge on the titanium crystallites. The bottom left image shows an estimate of the tip shape. The top right image shows the cross section of the tip 10 nm from the tip apex while the bottom right image shows it 20 nm from the tip apex. The diameters of these cross sections are estimated, and user defined upper limits are used to determine if the tip is "good", "suspect" or "bad". This tip is considered "suspect" because the diameter 20 nm from the tip apex is greater than the threshold value of 60 nm.