Development And Application Of UAFM Measuring System Based On DSP

In recent years, with rapid development of science and technology, the observation, measurement and operation of objects in nano-sized have become a hot area. Tools for analyzing and operating nano world have been explored for a long time. Ultrasonic atomic force microscopy(UAFM) technology combined AFM with ultrasound methods. The cantilever is driven at an ultrasonic frequency, and the contact-resonance frequency of cantilever changes in the process of tip-sample contact. From this change we can acquire information about the local elastic properties of the sample surface, and achieve non-destructive test in nano or subnano scale. As a disadvantage, traditional UAFM easily damages the sample with strong contact force in contact mode. In this dissertation, the new tuning fork probe is exited at dual-frequency in a tapping UAFM, to reduce the contact force and lateral force during scanning. Finally the mechanical properties of surface and subsurface structures can be characterized.The dual-frequency UAFM measuring system is set up based on the concept of AFM, combining the new tuning fork probe with nano-positioning stage. Experiments are performed on the system. The main work includes:1) The UAFM working principle is explained to study the feasibility of tapping UAFM. The quantitative measurement of Young's modulus on traditional UAFM is analyzed through the tip-sample contact model.2) A detailed analysis about tuning fork probe is performed. Through the experiment, single excitation can not complete the measurement. So that dual-excitation system is designed.3) The frequency algorithm based on FFT is discussed, and a spectrum analysis module which uses DSP as the core is developed. Considering the experiment results,we improve the system design.4) The improvement of UAFM system is achieved by the usage of phase-locked loop and filter. The circuits for signal's superposition and separation are developed.The UAFM measuring system is set up.5) The samples with different surface and subsurface structure is measured at single point to verify the measurement principle.6) The program is written by LABVIEW software, which realizes scanning image with digital PI feedback system.