Thermal microstrains measured by atomic force microscopy

Abstract

Microdeformations observed in atomic force microscopy (AFM) images on the surface of metallic films when an electrical current (dc) flows through the film were analyzed on a film–substrate system. The electrical current flow causes an increase in temperature on the thin film and a step in the AFM topography while imaging. Experiments realized with AFM explain the nature of such a step and allow one to quantify these microdeformations. Theoretical bending models mainly derived from modifications of the Stoney equation support our experimental results. Theoretical predictions show reasonable agreement with AFM measurements, and demonstrate that the major contribution to the observed deformation step (about 90%) is due to the thermal bending effect of the AFM cantilever. The remaining deformation is due to the nature of bimaterial effects in the film–substrate system and thermal expansion of the substrate, with the expansion of the film being negligible. The possibility of electric and magnetic effects in the AFM cantilever (due to current flow through the film) is also discussed. The existence of a strong thermal effect on the AFM cantilever is outlined. Hence, a slight variation in temperature during AFM measurements could turn out to be misinterpretation of the results obtained. Consequently, we recommend strict control of the temperature during AFM imaging, in order to improve the reliability and accuracy of the instrument.