Study On The Solar Cell Wafer Surface Quality Inspection Based On The Fringe Reflection Technique

Phase Measuring Deflectometry(PMD) utilizes fringe patterns distorted by the reflection of the tested specular surface to infer the surface gradient. The 3-Dimentional surface shape of the measured object can be then acquired according to the integration of gradient. PMD has the advantages of non-contact, high accuracy and sensitiveness and set-up simplicity, and it has a good prospect in the surface quality inspection on the mirror and mirror-like such as eyeglass and automotive body paintings. This thesis focuses on the surface quality inspection of solar cell by using PMD. The influence of the specular light effects to the extracted phase accuracy is discussed, and a suitable method, which applies the HDRI(High Dynamic Range Imaging) technology, is proposed to solve this problem. In addition, the PMD system calibration and the phase ambiguity are studied and solar cells are tested by applying the PMD technology. The main work is as follows: 1. A quality-guided phase unwrapping algorithm is studied for the purpose of high accuracy phase measurement. The algorithm is suitable for objects with holes or big height variance, and has a good performance for phase unwrapping of shadow area. The approach is more robust than the non-guided path-following algorithm. It is superior to the traditional spatial phase unwrapping algorithm when unwrapping the phase modulated by the object with high height.2. The calibration of solar cell surface inspection system based on PMD is studied. The precision of 3D reconstruction is closely related to the accuracy of the system calibration. Camera calibration is discussed in this thesis. To calibrate the distance between the LCD(Liquid Crystal Display) screen and the reference plane, the Hough transform is used to detect the circles center pixels, and the circle centers in world coordinates are measured according the spatial position. A mirror plane and two sphere surfaces with the radii 20000 mm and 5000 mm respectively are tested, respectively. The results show that the root mean square errors of height(RMSE) were 2.061×10-5mm, 6.189×10-5mm and 1.002×10-3mm, respectively.3. The phase ambiguity effect of PMD and the ways to eliminate this problem are analyzed. The error distribution introduced by the inaccurate placement of tested specimen is analyzed. A ray tracing method is used to build the explicit analytical model of phase, gradient and the absolute surface height. The influence of height and slope variation of surfaces on the measurement accuracy was simulated, and the corresponding experimental work was conducted. The results show that the phase caused by the specimen height can be ignored if the ratio of the distance between the LCD screen and the reference plane and the object height comes to more than 200.4. Experiments of the solar cell wafer defects detecting are carried out. The surface characteristics and defects of solar cell are analyzed and categorized. A solar cell with defects of fingerprint stains and scratches is inspected by applying the PMD system set up in this thesis. In addition, the system measurement accuracy is analyzed.