In contact-AFM mode, the tip makes soft “physical contact” with the surface of the sample. The cantilever deflection is directly proportional to the force acting on the tip, via Hook’s law. In contact-mode the tip either scans at a constant small height above the surface or under the conditions of a constant force. In the constant height mode the tip is fixed height, however in the constant-force mode the cantilever deflection is fixed and the movement of the scanner in z-direction is recorded. By, Atomic scale resolution of images is possible using contact-mode AFM. For contact mode AFM imaging, cantilever must be soft enough to be deflected by very small forces and has a high sufficient resonant frequency to not be susceptible to vibrational instabilities. Silicon Nitride tips is used for contact mode tunneling microscopy and transmission electron microscopy. A drawback of AFM as compared with SEM is the size of image. The SEM can image area on the order of square millimeters with a depth of field on the order of millimeters. The AFM can only image a maximum height on the order of 10-20 micrometers and a maximum scanning area of about 150×150 micrometers.


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