Research On Key Technologies Of Ultrasonic-Based Multi-property Measurement And Microscopic Imaging For Testing Of Micro Devices

Mirco devices manufactured with high precision microstructures, such as Micro-Electro-Mechanical System (MEMS), integrated circuits (IC) and multi-layered composite materials are widely used and play an improtant role in many areas including aerospace industry, manufacturing of very large scale integrated circuits, biomedical engineering, precesion manufacturing and control, information and communication. As an indispensible part of micro manufacturing, non-destructive testing (NDT) technique with high accuracy and efficiency is of great importance to ensure the performance and reliability of mirco devices. Different from the optical imaging and scanning probe microscopy, ultrasound and X ray can penetrate into the interior of the devices and materials to achieve simultaneous testing of both suface and subsurface structures. But X ray is insensitive to some types of defects such as delamminations. There are many different ultrasonic detection methods, e.g., scanning acoustic microscopy, phased array ultrasonics, and guided wave based NDTs. Due to the high imaging resolution, scanning acoustic microscopy is suitable for testing of mirco devices.As the development of mirco devices towards material diversity, integration and stratification, there are more and more bottleneck problems with the traditional scanning acoustic microscopic imaging technique:1) the pixel value of the ultrasonic image should corresponds to the acosutic impedance of the material, so the core of scanning acoustic imaging is accurate acquisition of the acoustic impedance. However, the ultrasonic echo relies on multiple properties of the material, so how to obtain the multiple material properties inculding acoustic impedance with a high lateral resolution is a critical problem in scanning acoustic microscopyp; 2) Most mirco devices are made of multiple layers, whose properties are difficult to be simultaneously deterimined by the traditional ultrasonic signal processing methods in frequency domain. This problem is more evident with the increase of the number of layers; 3) After acquisition of the ultrasonic image, automatic detection of the defects with high efficiency and accuracy is still a problem needs to be addressed; 4) In the traditional acoustic imaging methods, high resolution and high efficiency can not be simultaneously achieved. So how to optimize the scanning efficiency and resolution is still an open problem.This thesis is divided into the following six chapters:Chapter 1 introduces the current status and developement trend of NDT for mirco devices at home and abroad. The challenges and bottleneck problems that ultrasonic based NDT technique faces are outlined. Four major research content includs simultaneous measurement of multiple local material properties, simultaneous measurement of the properties of multiple layers, fast and automatic detection of defects in large-scale ultrasonic images, and high efficient scanning acoustic microscopic imaging system. The significane of this research is briefly described.In Chapter 2, simultaneous measurement of multiple local material properties is studied. To obtain the pixel values in te ultrasonic images (which correpsonds to the acosutic impedance of the material), the technique based on V(z,t) data is proposed. A series of ultrasonic echoes is colloected and denoted as V(z,t), from which the two-dimensional acoustic reflection spectrum of the material Re(θ, ω) is obtained. On the other hand, the theoretical acoustic reflection spectrum R,(θ,ω) is calcultaed. By fitting the experimental and theoretical two-dimensional acoustic reflection spectrum, the six acoustic and geometrical properties of the material, inculding acoustic impedance are obtained, without any previous knowledge. Besides, to avoid the requirement of initial guesses of the unknown parameters in the fitting algorithm, a new method by focusing the ultrasonic beam on the front and back surfaces of the unknoewn layer in tandem is proposed to obtain the multiple properties of the material.In Chapter 3, the method for the measurement of material properties of multiple layers is studied. The instability of the traditional measurement method based on fitting of frequency spectrum is more severe with the increase of the number of layers. Ultrasonic echo processing in time domain is first proposed. Based on the non-overlapped parts of the echoes from different interfaces, the acosutic impedance, time-of-flight and acoustic attenuation coefficient can be obtained. For multi-layered material, a layer-by-layer measurement procedure is developed to obtain the acoustic properties of each layer from top to bottom recursively. Numerical simulation and experimental results validate the efficacy of this method.In Chapter 4, the fast defect detection algorithm in large-scale ultrasonic images is proposed. In many ultrasonic images, the defects such as delaminations, cracks, and bubbles can be seen as sparse and irregular two-dimensional signals, while the normal (or defect-free) regions are regular with certain periodicity. So the defect regions are the salient regions of the large-scale ultrasonic image. Phase-only Fourier-Transform (POFT) is first used to detect the salient regions and then local template matching is applied to each salient pixel for detection of the true defects. It is rigorously proved that POFT is useful to filter out most of the normal regions, and make the defect regions and some edges more salient. Experimental results show that the proposed method effectively evaluates the efficiency and accuracy of defects detection.In Chapter 5, by combining the aforementioned multi-property measurement and defects detection methods with the high efficient mechanical scanning technique, a scanning acoustic microscope is self-develped for NDT of mirco devices. For high efficient imaging, two fast mechanical scanning methods are proposed by optimally utilizing the two axis:fast raster scanning and square spiral scanning. The optimal scanning trajectory under different motion and control parameters is studied. Theoretical analysis and experimental results show that the proposed scanning method can improve the scanning efficiency by nearly 30%. Besides, multi-layer simultaneous imaging and 3D imaging mode are developed.The major study, novelties and key technologies are concluded in Chapter 6. The future research directions of the scanning acoustic microscope for NDT of mirco devices are outlined.