Control of interaction force in constant-height contact mode atomic force microscopy

Abstract

Contact mode is a versatile and widely used technique for imaging samples using the Atomic Force Microscope (AFM). When contact mode imaging is performed in constant-height mode, it enables linear and faster response but leads to uncontrolled tip-sample forces. Here, a control strategy based on magnetic actuation is proposed to achieve high-bandwidth control of the tip-sample forces in constant-height contact mode AFM. A magnetic particle attached to the AFM probe is actuated by an external solenoid and employed for force regulation. A quasi-static model has been proposed and employed to develop the control strategy. Likewise, the contact natural-frequency, which decides the limit of achievable speed, has been shown to be significantly higher relative to the free probe and to be relatively insensitive to the particle size. Subsequently, a setup is developed to validate the control strategy and demonstrate reduction of tip-sample force variation by over a factor of 12 compared to conventional constant-height mode operation. Likewise, in comparison with conventional contact mode AFM, an improvement of linearity by over a factor of 9 and improvement in response speed by a factor of 100 have been demonstrated while imaging hard samples. The system has been shown to image topography at speeds of 2.44 frames per second while regulating the interaction force. Finally, the stiffness of a sample has also been characterized using the developed system and simultaneous estimation of topography has also been demonstrated. They are shown to agree well with theoretical expectations.