Contact atomic force microscopy using piezoresistive cantilevers in load force modulation mode

Scanning probe microscopy (SPM) encompasses several techniques for imaging of the physical and chemical material properties at nanoscale. The scanning process is based on the detection of the deflection of the cantilever, which is caused by near field interactions, while the tip runs over the sample's surface. The variety of deflection detection methods including optical, piezoresistive, piezoelectric technologies has been developed and applied depending on the measurement mode and measurement environment. There are many advantages (compactness, vacuum compatibility, etc.) of the piezoresistive detection method, which makes it very attractive for almost all SPM experiments. Due to the technological limitations the stiffness of the piezoresistive beams is usually higher than the stiffness of the cantilever detected using optical methods. This is the basic constraint for the application of the piezoresistive beams in contact mode (CM) atomic force microscopy (AFM) investigations performed at low load forces (usually less than 20 nN). Drift of the deflection signal, which is related to thermal fluctuations of the measurement setup, causes that the microscope controller compensates the fluctuations instead of compensating the strength of tip-surface interactions. Therefore, it is quite difficult to keep near field interaction precisely at the setpoint level during the whole scanning process. This can lead to either damage of the cantilever's tip and material surface or loosing the contact with the investigated sample and making the measurement unreliable. For these reasons, load force modulation (LoFM) scanning mode, in which the interaction at the tip is precisely controlled at every point of the sample surface, is proposed to enable precise AFM surface investigations using the piezoresistive cantilevers. In this article we describe the developed measurement algorithm as well as proposed and introduced hardware and software solutions. The results of the experiments confirm strong reduction of the AFM entire setup drift. The results demonstrating contactless tip lateral movements are presented. It is common knowledge that such a scanning reduces tip wear.