Topography and phase imaging using the torsional resonance mode

The torsional resonance mode (TR mode) is an innovative technique recently introduced for scanning probe microscopes (SPMs). In the TR mode, a cantilever tip vibrates laterally as compared to vibrating vertically in the tapping mode (TM). The tip in the TR mode remains at an almost constant height and interacts aggressively with the surface and/or the near surface because of very high torsional stiffness. In this paper, a comparative study of TM and TR modes is presented for further understanding of the mechanism of the phase angle data produced during the different tip-surface interaction modes. Topography and phase angle measurements were made on a self-assembled monolayer (SAM) sample with a two-phase structure and metal particle (MP) tapes. It is found that although surface topography images are similar using both techniques, the TR mode phase angle image provides more detailed contrast than that obtained using the TM phase technique. The experimental evidence showed that viscoelastic deformation was primarily responsible for the phase contrast and not the friction process. Next, statistical analysis of the phase angle on the MP tapes suggests that the technique can be used for evaluating the particle concentration and the uniformity of viscoelasticity and thus screening of the magnetic tapes. The reasons for the improved contrast in the phase angle imaging in the TR mode are also discussed in this paper.