Development of a force sensor for atomic force microscopy using piezoelectric thin films

Abstract

A piezoelectric force sensor has been developed to overcome the complexity of the force detector of the conventional non contact atomic force microscope (AFM). The sensor consists of a cantilever with a piezoelectric thin film deposited on one side. The layer is formed on a thermal SiO₂ microbeam and is then encapsulated by a thin passivation film. The piezoelectric film is c-axis oriented ZnO deposited by RF magnetron sputtering, or lead-zirconate-titanate (PZT) prepared by the sol-gel method. The cantilever is excited near its first natural resonance and forces are monitored by detecting the change in the admittance of the layer induced by the piezoelectric effect. The gradient of the force between the tip of the vibrating cantilever and the sample when in close proximity modifies the vibration amplitude, hence inducing a change in the admittance of the layer. A typical sensor with a length of 200 mu m and a width of 50 mu m had a calculated spring constant of 8.7 N m^-1 and a first resonance frequency of 72.5 kHz. We have measured a sensor signal as a function of the tip-sample spacing and verified the feasibility of the sensor for a non contact AFM.