Micro-vision image measurement for spot arrays on silicon wafer ablated by femtosecond laser

Abstract

When using femtosecond laser micromachining center to ablate spot array on silicon wafers, it is difficult to measure the spot spacing directly by physical method because of the small geometric size between the spots. In order to quickly acquire the geometric dimensions between the spot arrays, the micro vision image measurement method is used to measure the distance of spot arrays, and a high precision measurement result is obtained. In the experiment, femtosecond laser power with 20 mW was used to ablate the spot on the surface of silicon wafer and visual measurement for the obtained spot array is realized. Firstly, a telecentric micro-vision system with a field of view of 2.528 × 1.896 mm is constructed to realize image acquisition of spot arrays. Secondly, the micro-vision system is calibrated by using the geometrical optical resolution plate, and the measurement ratio constant Kw is obtained; in the meantime, in order to improve the calibration accuracy, the resolution plate image was processed by vertical correction and pseudo-color enhancement, and the separation lines between the line pairs were extracted by Hough transform. Thirdly, in order to accurately extract the geometric center of the spot image and improve the measurement accuracy of the spacing between the spot, the R-component partitioning image of spot array is analyzed; its three-dimensional image presents a complete stereoscopic morphology of spot, which can be used as an input image for image measurement. Finally, the spot arrays were measured by image measurement method and compared with the size measured under a microscope. The results show that the maximum measurement error of micro vision measurement method is 3%, which meets the requirement of measurement accuracy.