Ultrasonic Vibration Atomic Force Microscopy Imaging And Analysis

With the rapid development of nanotechnology, we urgently need methods to describe the physical characteristics of the surface and subsurface nanostructures of a smple. Traditional3-dimensional imaging techniques, which are based on optical or acoustic principles, usually fail to obtain high-resolution images due to the diffraction limit. In scanning electron microscopy and transmission electron microscopy, the acquisition of characteristics of the subsurface nanostructures is also quite difficult.It needs to destroy samples and rebuild the complex three-dimensional structures. Scanning probe microscopy (SPM), including atomic force microscopy, has the ability to distinguish the sample topography in real-space with nanometer or even atomic resolution. However, SPM mainly images the surface structures.Acoustic atomic force microscopy (A-AFM), which combines the advantages of ultrasonic detection and high resolution of atomic force microscopy, has been demonstrated to be able to detect the embedded subsurface nanostructures. In addition, the mechanical properties such as elastic modulus and contact stiffness can also be measured. In A-AFM technique, the piezoelectric transducer whick is fixed on the cantilever or the sample, generates acoustic oscillations. The acoustic wave transmits through with the sample and then collected the cantilever, The amplitude and phase of the cantilever contain rich information on the mechanical properties of the sample and sub-surface structure at the nanometerscale.In this thesis, the following investigations on ultrasonic vibration atomic force microscopy (UV-AFM) have been carried out by theoretical analyses and experiments.(1) According to the basic theory of A-AFM, the UV-AFM system is constructed. We optimized the experimental system. The new experimental system can get better image, reduce the image drift and increase the scanning speed. Experiments were carried out to investigate the influence of the excitation frequency and environment on imaging. In order to examine the capability of subsurface imaging, a sample with prscried embedded structure, which is processed by focused ion beam is was also scanned.(2) Topographic influences on the UV-AFM amplitude image contrast have been investigated on different samples.Experimental results and a simple model analysis demonstrate that the amplitude contrast has a close correlation with the x-derivative of the topography. When the tip sample is in a single-point contact and the mechanical properties of the sample are uniform, the contrasts of the amplitude and the x-derivative of the sample topography are almost the same. In addition, the amplitude image contrast is reversed when the scan direction is altered. When the tip sample is in a multipoint contact, the amplitude at the multipoint contact region increases and the scan-direction-relevant phenomenon is also distinguishable. However, such an effect could be masked by the general amplitude increase at the multipoint contact positions. Such investigation can help to improve the accuracy of the UV-AFM data and lay the foundation for further development of UV-AFM.(3) UV-AFM probe dynamics and image characteristics with either sample excitation or probe excitation were investigated experimentally. In both acoustic excitation methods, the contact resonance frequency is usually masked by spurious frequency peaks, which can cause difficulties in accurate identification. Two approaches can help to determine the exact contact resonance frequency. One is by comparing the dynamic spectra swept at the cantilever free end and at a slight offset of the detection position. Another is by comparing the spectra measured via the two different excitation methods...