ARE ARTIFACTS IN SCANNING NEAR-FIELD OPTICAL MICROSCOPY RELATED TO THE MISUSE OF SHEAR FORCE

It is becoming clear that a significant proportion of scanning near-fi eld optical microscopy (SNOM) images are artefacts caused by adjusting the tip-sample separation when scanning. An example that occurred wit h shear-force regulation is given in this work and it is shown that co nstant-height imaging provides a partial solution to the problem. The problem could be intrinsic to closed-loop imaging or occur through usi ng shear-force regulation incorrectly. An operational protocol is pres ented which shows that for maximum sensitivity, shear forces should be measured off resonance. Simultaneous tunnelling measurements have bee n used to demonstrate that with this protocol, shear-force measurement s can provide non-contact operation in the case of differential interf erometer-based systems. It has been observed that differences in subst rate material produce significant phase changes in the probe oscillati on. Here, the phase shift would have resulted in the probe-sample sepa ration being approximately 7 Angstrom smaller when the probe resided o ver aluminium islands on the test specimen. Whether this error is resp onsible for the observed optical artefacts is still being determined. (C) 1998 published by Elsevier Science B.V. All rights reserved.