Scanning Force Microscopy (SFM)

Abstract

The scanning tunneling microscope (STM) has revolutionized the field of microscopy. Even newer scanning microscopes are being developed which are based on piezoelectric transducers and sharp probing tips that scan a surface while a feedback loop regulates the distance between the probing tip and sample. In 1986 Binniget al. invented the scanning force microscope (SFM) [4.1]. In contrast to the STM, which senses the tunneling current, the SFM senses forces between the tip and the sample (see Fig. 4.1). Forces of the order of 10−13 to 10−4N can be measured by this technique and a lateral resolution of the order of angstroms can be achieved. For comparison, typical forces in conventional profilometers are ≥ 10−4 N and the lateral resolution is in the submicron regime. Experimentally the probing tip is attached to a cantilever- type spring. The forces acting on the probing tip deflect the cantilever. A displacement sensor then measures deflections as small as 10−2 Â. The first displacement sensor proposed by Binniget al. was based on electron tunneling. Later, different sensors based on optical interferometry, beam deflection or capacitance measurements were introduced. From the beginning it was evident that SFM was not only able to simply image topography but also to detect a variety of different forces. In addition to ionic repulsion forces, also van der Waals, magnetic, electrostatic and frictional forces could be readily observed by SF.M. For a detailed description of magnetic forces the reader is referred to Chap. 5. Experimental aspects of force microscopy will be presented in Sect. 4.1. The designs of the instruments and the modes of operation will be described. In Sect. 4.2 we will give a brief description of the different forces relevant to the performance of present SFMs. Many of these concepts rely on the work of Tabor and Israelachvilideveloped in the context of the surface force apparatus (SFA) [4.2, 3, 4]. The SFA is the predecessor of SFM, the difference being that the SFM probes forces on a more local scale.