Design and analysis of a monolithic flexure atomic force microscope

Abstract

This thesis details the design, manufacture, and testing of a sub-nanometer accuracy atomic force microscope. It was made to be integrated into the Sub-Atomic Measuring Machine (SAMM) in collaboration with the University of North Carolina at Charlotte (UNCC). The microscope uses a tuning fork sensor to gauge its proximity to the sample surface. The sensor is fixed to a stage that is guided to move in one degree of freedom by a monolithic flexure. A piezoelectric actuator drives the moving stage while three capacitance sensors provide a non-contact direct measurement of the displacement. A decoupling flexure prohibits the error motions of the actuator from propagating into the moving stage. A digital control system uses closed loop control to regulate the vertical displacement of the stage. The positioning system demonstrated a 450 Hz -3db closed loop bandwidth and 0.249 RMS noise positioning. A new probe named after its inventor Dr. Terunobu Akiyama is implemented in a feedback control system that adjusts the displacement of the stage in order to maintain a constant gap between the probe and the sample. The system displayed an 8.3 nm RMS positioning noise when set to measure a stationary block of aluminum. The dynamics of the feedback control loop indicate that the system can operate at 27 Hz upon application of a proportional controller. Advanced methods to self excite the tuning fork sensor at resonance by use of a phase locked loop are explored. Follow-up work to integrate the atomic force microscope into the SAMM stage, diminish the electrical noise in the tuning fork, and to implement the phase locked loop circuit are suggested.