Analysis of Tip Shape : Nanoanalysis using Atomic Force Microscopy (AFM) : Analytical Nanoprobes : Research : Surface & Nanoanalysis : Science + Technology : National Physical Laboratory

Analysis of Tip Shape

Many quantification issues in scanning probe microscopy (SPM) and AFM demand an accurate measure of the tip shape at its extremity. Material properties such as the Young's modulus require the tip-sample contact area to be known. Even for simple imaging, the tip has to be as sharp as possible for maximum resolution. In the Surface and Nano-analysis team at NPL we developed methods and materials, which can be used for the rapid examination of tips at the nanometre level.

The method and surface used to characterise the tip depends on what region of the tip you are interested in characterising.

SEM image of the InP sample used for characterising AFM tips.

One sample we have developed to characterise AFM tips is an sputtered Indium phosphide, which when sputtered becomes enriched with nano regions of indium at the surface. When the InP is heated, the sputtering forms small, sharp filaments, of a suitable geometry, strong enough to define tips. The preparation and use of this material is detailed in:

M.P. Seah, S.J. Spencer, P.J. Cumpson and J.E. Johnstone, Surface and interface Analysis, 29, 782 (2000)

These samples can be used rapidly between analyses to check for tip wear, tip crashes and resolution in the AFM itself. There are several tip modification issues in AFM. We have found that tips can become chisel shaped perpendicular to a persistent scan direction and sharpening of tip can be achieved by post manufacturer treatment. Tips can shear completely after a crash and, finally, tips can be improved by cleaning them with detergents and/or UV- ozone in a commercial cleaner.

Sputtered InP has advantages and disadvantages over colloidal gold reference samples. They include the ability to withstand relatively high lateral loadings (ideal for contact mode tip characterisation) and can be imaged in high humidity environments. However, the features have a wider statistical distribution of radii and shape compared with gold colloids.