Development of an atomic force microscope operating in DC non-contact mode

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The purpose was development of atomic force microscope operating in DC non-contant mode. In beginning, I have made a force feedback system with a seesaw shape lever. The lever deflection was detected by an optical lever method. The sensitivity of force measurement was -10^<-10>N.However the complex structure of the seesaw lever has caused intolerable thermal drift and a low frequency response. To overcome these problems, a novel force-feedback system using a micro-cantilever commercially available was proposed.Since the cantilever deflection is detected by a simple optical fiber interferometer, the thermal drift is reduced. And the high resonant frequency of the micro-cantilever allows the high-speed scanning of samples. The optical fiber end was coated by gold thin film. The electrostatic force between the fiber end and the gold coated micro-cantilever is balanced with the atomic force between the probe tip and sample.Some force curves were measured by using this force feedback system … More . The curves were quite different from those of conventional method in which the interaction force is determined from cantilever deflection without feedback system. Especially the maximum of the attractive force in approaching was in the same order of the maximum in going back. The stability of the system was improved, so that the force resolution was -10^<-11>N.However the frequency response was still poor because the control system used for the measurement was poor. Then I could not obtain an AFM images by scanning the sample surface.A discussion to design the feedback control system with a high frequency response was made. A simple dynamic model of cantilever was proposed in case that the probe tip was in the field of interaction force. Supposing a PID controller, the control parameters were estimated to realize the high frequency response of the feedback system.A simulation result shows a possibility of frequency response of higher than 1kHz. I will actually make the controller and will attempt to operate the AFM with non-contact DC mode. Less