An ultrahigh vacuum dynamic force microscope for high resonance frequency cantilevers

We present the design of an ultrahigh vacuum dynamic force microscope incorporating a heterodyne Doppler interferometer and a superheterodyne circuit with an intermediate frequency of 10.7MHz. The method allowed the use of a low-noise narrow-band analog phase-locked loop with a voltage controlled crystal oscillator for demodulating the frequency shifts caused by the interaction force gradients between the tip and the sample at the intermediate frequency. The system could be used for a conventional cantilever operating in its fundamental and higher modes, as well as for small or stiff cantilevers with high resonance frequency up to 100MHz. A preliminary measurement was demonstrated by the observation of the Si(111)−7×7 reconstructed surface with the second resonance of 1.6MHz with subangstrom amplitudes.