Study On Testing Method For Surface Cracks Of Sapphire Substrate

Sapphire is widely used in substrate materials,consumer electronics,military and other fields due to its superior performance.Among them,sapphire used as the substrate material of LED chips accounts for the largest proportion.With the development of LED chip technology,the demand for low-cost,large-sized substrate materials is becoming more and more urgent.The demand of precision manufacturing technology and quality is increasing day by day.The sapphire substrate is damaged during processing with many defects,such as cracks and edge breakages.Evaluation of its surface quality is closely related to defect detection.Crack is the defect with the feature of large size and random shapes.It directly affects the selection of subsequent processing parameters.In addition,the substrate is most likely to be broken up and cause higher processing costs due to the crack.Since sapphire substrate is translucent material,the surface crack with the feature of across scales geometric and morphological characteristics of diversity.It put forward the higher request for the analysis of characteristics of crack detection method.Sapphire substrate surface cracks are taken as the research object.According to its characteristics(length in millimeter scale,depth in micron scale,various shapes),a 2d rapid detection of the whole field and 3d high-precision detection crack measurement scheme was designed and constructed,and the characteristic parameters were extracted and analyzed.This work is helpful to deepen the understanding of crack characteristics and propagation.It will assist the study of the mechanism of crack generation and propagation.And it is of great significance for the control of processing quality and cost.The main contents are summarized as follows:(1)According to the characteristics of cracks and detection requirements,the measurement scheme is designed and the system integration is completed.A machine vision system is used to measure the 2d whole field,and fast the whole substrate to the image acquisition.The ultra-depth microscopy system is used for 3d crack detection,and to achieve high-precision detection of cracks at depths of several tens of microns.According to the special positioning requirement of the substrate and the demand of the random crack distribution and shape to the detecting light source,a kind of rotating detection table is designed.It can locate multi-specification substrates and provide a variety of light source schemes.(2)For the 2d and 3d shape features of cracks,the extraction algorithms of different feature parameters are studied to realize the evaluation,analysis and classification of cracks.According to the substrate 2d full-field image,Krisch edge detection algorithm and other algorithms are selected.Then the 2d characteristic parameters such as crack length,location coordinates,roundness and rectangularity are evaluated.Focusing synthesis algorithm is used to reconstruct the 3d tomographic image.The depth of crack is extracted by the relation between the sum of the maximum value of the focusing synthesis operator and the number of layers.Through depth-of-field synthesis of tomographic images,Hough transform is used to extract other characteristic parameters such as crack width,circumference and azimuth of the affected area.Three kinds of morphological cracks are classified according to the extracted external rectangular aspect ratio,roundness and rectangularity.(3)The surface crack evaluation system of sapphire substrate is constructed.The interface of the evaluation system is built based on MFC and OpenCV,with MATLAB to realize its software function.Based on sapphire substrate test samples,the surface cracks are measured and evaluated.Among these samples,the crack length ranges from several millimeters to several tens of millimeters,and the crack depth ranges from 30 to 50?m.The crack can be divided into bifurcation type,vertical type and polyline type.The experimental data show that the system can meet the needs of measuring and evaluating the surface crack of sapphire substrate.