Research On Key Techllologies Of Simultaneous Characterization For Thin Layer With Acoustic Microscopy

As a functional material, thin layer plays an enormous impact on the modern industry and the national economy, thus the non-destructive characterization of thin layer has been strongly demanded during the processes of thin layer development, preparation and application. Ultrasonic nondestructive testing is widely used in the field of material property characterization and quality control of thin layer due to its unique advantages, and shows the trends of simultaneous characterization. With the rapid development of acoustic microscope technique, the high resolution characterization has been possible and it is very suitable for the characterization of thin layer. Therefore, the characterization technique based on the acoustic microscopy has shown great potential and would be the mainstream. Thus, with the support of National Natural Science Foundation-funded project, the simultaneous characterization of material and geometrical properties of thin layer with acoustic microscopy is studied.On the basis of theoretical model of wave propagation and the proposed reflection coefficient measurement technique, the material properties, including thickness, density, longitudinal and transverse velocities, and attenuation, are determined with an inverse algorithm utilizing least square method to minimize the sum of squared deviations between theoretical and measured reflection spectrum at normal and oblique incidence. The detailed context and innovative points of this dissertation are as following:In chapter one, the importance of non-destructive testing on the development, preparation and application of thin layer is briefly described. Then, The current research status of non-destructive testing associating with thin layer and its development in the further are summarized. Finally, the characterization of thin layer with acoustic microscopy is systematically generalized. Meanwhile, the problems are clarified and the research direction is pointed out. The content and chapter arrangement of the dissertation are also given.In chapter two, the theoretical reflection spectrum of layered structure at normal incidence is firstly established by analyzing wave propagation. A reformulation of the Thomson-Haskell method is presented to calculate the acoustic reflection coefficients of layered structures of arbitrary configurations. Finally, the simulation studies verify the validity and feasibility of the proposed method. In Chapter three, the spatial and temporal structure of the acoustic field is analyzed on the basis of wave propagation model, and an experimental method based on angular spectrum to evaluate the acoustic coefficient as a function of the incident angle θ and frequency co is presented with acoustic microscopy. Then, the Z-axis sampling interval is briefly discussed. The measurements of the reflection coefficients of the layered structures based on the proposed technique with self-developed scanning acoustic microscopy are performed. Two substrates of aluminum and Plexiglas, four stainless plates with various thicknesses of100μm,150μm,200μm, and250μm, and two typical multilayer structure are applied. The measured results are consistent with the corresponding theoretical calculations to verify the validity and feasibility of the proposed technique.In chapter four, a kind of simultaneous measurement method for the thickness, density, sound velocity and attenuation using V(z,t) data recorded by time-resolved acoustic microscopy is proposed. The theoretical reflection spectrum of thin layer at normal incidence is established as a function of three dimensionless parameters. The measured reflection spectrum R(θ,ω) is obtained from V(z,t) data and the measured thickness is derived from the signals when the lens focusing on the front and back surface of thin layer, which are picked up from the V(z,t) data.The density, sound velocity and attenuation are then determined by the measured thickness and inverse algorithm utilizing least squares method to fit the theoretical and measured reflection spectrum at normal incidence. Thus, the proposed method allows to simultaneously measuring the thickness, density, sound velocity and attenuation of thin layer. An experimental example using a point-focusing transducer with nominal frequency of around50MHz is conducted with250μm stainless steel plate. The measurement results validate the proposed method. The results demonstrate that the thickness, density and sound velocity can be measured with a percentage biases less than5%and the sound attenuation is close to true value.In chapter five, an ultrasonic technique for simultaneous determination of the complete set of acoustical and geometrical properties of thin layer using two incident angles is presented.The theoretical model of the two-dimensional spectrum of the thin layer is calculated as a function of six parameters:thickness, density, longitudinal and transverse velocities and attenuation, using the reformulation of Thomson-Haskell method.Then, The experimental spectrum can be measured by V(z,t) technique with acoustic microscopy. The full set of the properties can be derived inversely with minimization of the difference between the calculated and experimentally measured reflection spectrum of the thin layer at two angles:one at normal incidence and the other at oblique incidence. By introducing a two-step inversion algorithm, the searching process in six-dimensional space is transformed to two searching processes in three-dimensional space. The sensitivity of the two-dimensional spectrum to individual properties and its stability against experimental noise are studied. Meanwhile, an experimental example is performed with250μm stainless steel plate. The measured errors are less than4%, validating the proposed method.In chapter six, the research results and the innovative points of this dissertation were summarized, and the future research works are also forecast.