Subsurface Imaging Mechanism By Acoustic Atomic Force Microscopy

Subsurface nanoimaging in phenomenon observation of biological subcellular, defect detection of micronano device, characterization of nanocomposites, etc. is urgent demanded. Combining with ultrasonic vibration modulation atomic force microscopy (AFM) can achieve the ultra-high resolution and non-destructive testing on internal nanostructures of samples, with a good potential for application. However, using ultrasonic atomic force microscopy (UAFM) for subsurface nanoimaging is mostly for qualitative detection of internal structures, but lacks systematic analysis for imaging effect factors and quantitative methods for characterizing the detection capability. Aiming at unclear situation of mechanism on subsurface nanoimaging of UAFM, this thesis studies from theoretical and experimental aspects to figure out the association of a number of key factors on vibration detection signal and subsurface structural properties, laying the foundation for subsequent quantitative analysis of subsurface application.In the theoretical analysis, subsurface nanostructures can cause changes in the AFM tip and the sample contact action, thereby inducing ultrasonic vibration in response to the probe signal change. Therefore, the probe signals, such as vibration amplitude/phase/frequency, contain a wealth of internal structure characteristic information. Based on this idea, we establish a mechanical model of UAFM imaging. The model includes a cantilever beam between the nonlinear vibration analysis model and the role of the contact tip and the sample model. Wherein, through the cantilever nonlinear vibration theory model according to the degree of simplification compared to the actual experimental setup, we compare and analysis the simple model, tip position model, lateral force model without and with tip position, and so on four models, and expound their respective application of occasions. Subsequently, in response to vibration and contact stiffness law relations on the basis of the theoretical results, this thesis discusses the key factors of imaging parameters such as normal force, vibration modes and tip radius of the subsurface detection of effects of localized elastic modulus of resolution and sensitivity.To further explore the theoretical analysis drawn the association between the tip/sample contact stiffness associated with internal nanostructure properties, we use the finite element methods to detect typical subsurface structures containing air bubbles or spherical particles, and perform simulations and calculations. First, for the sample characteristic parameters, we study the effects of the size of internal structures, the buried depth, elastic modulus, and the coupling between adjacent structures, etc. on equivalent contact stiffness. Then, in order to optimize and select the subsurface imaging parameters, we analysis the effects of normal force, tip position, detecting spot position on imaging contrast and contact resonance frequency.In terms of experiments, in order to compare the effects of different probes or samples incentives on subsurface imaging, we built an experiment platform combining of the atomic force acoustic microscope with sample ultrasonic excitation, atomic force microscopy with probe ultrasound excitation, microscopic difference frequency mode between ultrasonic probe and the sample at the same time encourage, and free switching. We take the highly oriented pyrolytic graphite (HOPG) sample as the example, and verify the internal structures of nanoimaging capabilities of the experimental system, indicating that the three modes of operation can achieve subsurface imaging and HOPG defect detection. To achieve quantitative assessment of the subsurface characterization capability of the instrument, we design and fabricate a series of reference samples with known dimensions and depth, and perform preliminary subsurface imaging tests.To fill the gap of effect research of temperature and humidity on AFM dimensional measurements, we conduct a series of analysis experiments. We mainly investigate the quantitative effect of dimensional measurements of standard two-dimensional AFM dimensional measure results under two different controlled environments, including integral working environments of instruments and local environments of samples.