Contact scanning mode AFM for nanomechanical testing of free-standing structures

A technique for performing nanomechanical testing of free-standing structures using contact scanning mode atomic force microscopy (AFM) has been developed and implemented. The technique consists of performing a contact mode scan of an area of a cantilever test structure as well as of an area that does not deform. The use of two different values of the force during the scan allows the effect of the topography of the test structure to be eliminated. Also note that in this constant force scan, the bending moment in the test cantilever changes continuously with the position of the applied force, allowing both the stress and strain to change even though the force remains constant. With the dimensions of the structure known, material properties (i.e. the elastic modulus) can be determined from the relationship between the applied force and the structural deflection. This method is not susceptible to nonlinearities in the AFM photodetector and allows for piezo drift during testing to be quantified and corrected. 50 nm thick chromium cantilever structures were fabricated for testing using electron-beam lithography. The determination of the elastic modulus gave consistent results as the AFM tip was scanned along the cantilever. As is the case with AFM fixed position force–displacement measurements, the accuracy of the results is affected by uncertainties in the measured AFM cantilever stiffness and by uncertainties in the test structure dimensions. This testing method can be performed by any AFM capable of scanning in the contact mode without requiring specialized software. Fabrication of the cantilevers requires only a single-mask process.