Design And Implementation Of Vacuum Contactless Frequency Modulated Atomic Force Microscope System

Atomic Force Microscopy（AFM）is a real-time space technique that images the surface of a sample by detecting the interaction force between the probe tip and the surface of the sample.Nowadays,frequency-modulated atomic force microscopy（FM-AFM）has rapidly developed into a powerful characterization tool at the atomic/molecular scale.Because it has true atomic resolution,and it can measure atomic force spectra,and it can observe uniform insulators.In addition,it has a wide range of applications in surface imaging of organic materials,highspeed imaging of dynamic biomolecules,chemical identification of mixed atoms and manipulation of atom etc.However,the existing commercial AFM generally adopts the amplitude modulation（AM）mode and works in the atmospheric environment or the liquid environment.The quality factor Q is less than 1000 during the measurement process,which greatly limits the high-resolution imaging.Based on the need to break through the Q limit and improve the imaging resolution,this thesis designs an atomic force microscope system in a vacuum environment.The system adopts the FM working mode,and key measurement units in the system are optimized and calibrated.The research contents are as follows:The working principle of the atomic force microscope under different working modes is studied,and the important factors affecting the imaging resolution in the FM mode are analyzed,and the variation trend of the probe detection sensitivity under different conditions is calculated and simulated.This thesis proposes a design scheme to improve the imaging resolution.The FM-AFM core measurement unit is built.Integrating key modules such as optical deflection system,probe vibration system,sample stage displacement system,scanning system in the core.To solve the problem of less than ideal resolution,a photoelectric detection system based on the principle of light deflection is designed.The design mainly focuses on optimizing and upgrading the laser alignment adjustment structure,which improved the adjustment efficiency and accuracy.Probe amplitude is calibrated by analysis of system noise.To ensure stable feedback,the inertial drive scanning system is designed to calibrate the piezoelectric sensitivity and scanning range of the piezoelectric ceramic scanning tube.The vacuum system is designed with these aspects of material selection,air extraction and structure in mind,which ensures the experimental environment of the probe cantilever with a high Q factor.It breaks through the Q limit in the atmospheric environment.And the quality factor has been increased from 600-800 to 12000-16000,effectively improving the detection efficiency.The system was tested in a vacuum environment.The sensitivity of the input signal was detected,and the experimental parameters were calibrated.And the resonance peak amplitude,phase,bandwidth and Q value of the probe vibration were measured.Compared with the performance in the atmospheric environment,the performance is obviously superior in a vacuum environment.And the noise performance of the system is measured in terms of power spectral density,noise floor and the minimum detectable Δ1)of the probe,which proves the low noise performance of the system.Then,the surface topography is measured using mica as a sample.The clear step signal can be obtained,which proves that the system has nanometer-level high resolution.In summary,the limitation of the quality factor Q is solved through the design of the vacuum non-contact FM atomic force microscope system.The influence of system noise is effectively suppressed,and the probe detection sensitivity is improved,and the system structural error is reduced.It lays the foundation for the design and optimization of the system in the ultra-high vacuum environment.