Small-amplitude dynamic force microscopy using a quartz cantilever with an optical interferometer

We propose dynamic force microscopy (DFM) that employs a quartz cantilever in combination with an interferometric deflection sensor. The high stiffness of the quartz cantilever allows DFM to be performed at oscillation amplitudes as small as 1 A. DFM topographic images can be obtained for a Si(111)-(7 x 7) surface even when a blunt tip is used at room temperature. The low-noise interferometer with a deflection noise floor of n = 15 fm Hz(-1/2) exhibits a high force sensitivity in force spectroscopy. By subtracting the huge van der Waals force contribution, we obtain a short-range force curve that is in good agreement with the theoretical curve of the covalent bonding force. Simultaneous measurement of the tunneling current during DFM imaging is also demonstrated.