Tapping Mode Atomic Force Microscopy And Phase-locked Detection

After all the mode of Atomic Force Microscope(AFM) are analyzed, this dissertation introduces the design of AFM in Tapping mode. In this system, The vibration of microprobe is detected using the combination of DMT(Derjaguin-Muller-Toporov) model and light probe point-diffraction theory. Then the information of the sample is measured by means of detecting and controlling signal of the phase delay, which is generated by the force between the tip of microprobe and sample. Lastly it uses the computer to control two-dimension precise scanning table, and gets the microscopic pattern of the sample by sampling the phase information. In the thesis one feedback system is reported, the circuit design can be referred by the next researcher.The important content of the thesis is the design and research of the phase lock system. It also includes the research of the microprobe used in tapping mode AFM and its harmonic frequency, the design and fabrication of phase lock system design, the elementary design ofthe feedback system, and the program designing and optimizing.To the tapping mode AFM,the force varying is detected by the microprobe. Then it becomesthe key part of the whole system. Andthe performance ofthe system is dependent on the sensitivity of the microprobe. So research and selection of the microprobe becomes to one key part of thethesis.Phase lock system is used widely in weak signal detecting because of the high nature of noise-immune. It is also suitable to AFM system. In the original system, it used the SRS830(made by Stanford Research System) as the phase lock system. However, it is not fit to be a product for the high price, and its frequency band is too low(below 102KHz) to drive the new miciopiobe (its highest harmonic frequency is about 400KHz), and it is not fit to be used in the feedback system, etc.( The detail can be seen in the chapter 3) The phase lock system, which is design by ourselves', can be used widely( its frequency band reach 30~500KHz), and it costs only 10% of the SRS830, and its volume is small enough to be used in most situation. So it is very suitable to be used in tapping mode system. Adapting to the phase lock system and the frequency synthesis, the sampling and scanning program is designed and optimized.