Paper NS-TuM5
Ambient Dynamic Mode AFM Non-contact Operating Regime Determination

Tuesday, October 16, 2007, 9:20 am, Room 616

Dynamic mode AFM, which uses a cantilever vibrating near its natural mechanical frequency, is the most popular AFM imaging method in the ambient. As AFM tip approaches the sample, it enters the non-contact regime first due to attractive force interactions and, as tip continues to descend, it starts to contact surface intermittently.¹ During tip descend, the vibration amplitude continuous to decrease. Therefore, using a feedback loop that maintains a constant amplitude set-point during AFM imaging, the AFM can be operated in either non-contact mode or intermittent contact mode and the set-point of former is higher than that of the latter. Non-contact mode is desirable, because it affords higher spatial resolution and longer tip life.² Hence, accurate determination of the non-contact amplitude operating range is needed. A unique characteristic of phase signal of cantilever is that, in non-contact regime, phase changes monotonically with tip-to-sample distance, but as soon as tip makes intermittent contact with sample, the phase signal abrupt jumps in the other direction.¹ Therefore, non-contact regime can be determined by approaching tip to surface in open-loop condition, monitoring the amplitude and phase signals at the same time and determining the amplitude corresponding to phase reversal point. However, this popular method causes tip and sample damage and inaccurate non-contact regime determination. A new method is reported here. In this method, the tip approach is stopped at the start of non-contact interaction regime and the feedback loop is turned on; the tip descend is then resumed by lowering the amplitude set-point continuously and at the same time monitoring the phase signal; as soon as the phase signal makes a sudden discontinuous reversal, the intermittent contact mode is reached. By recording the amplitude set-point at the onset of intermittent contact, the non-contact regime can be determined more precisely. It is shown that this range depends on tip material, sample material and the cantilever initial vibration amplitude. Therefore, this method can be used to image sample surface properties and to optimize non-contact operating parameters.

¹ R. Garcia and A. S. Paulo, Phys. Rev. B 60, 4961 (1999)
² C. Wang et al., Proceedings: the 2005 International Conference on Characterization and Metrology for ULSI Technology, 194 (2005).