A Bump Height Measurement Method Based on Optical Triangulation

2D/3D integrated circuit packaging technology is adopted in the process of wafer packaging and this technology uses wafer bumps to connect active devices. Inconsistent bump heights will cause circuit break after packaging, which will cause the whole chip to fail. Therefore, on-line detection of the height consistency of bumps is required during wafer packaging. To meet the above requirements, a fast and high-precision bump height measurement method based on optical triangulation method is studied. Based on the basic principle of oblique incidence optical triangulation method, the line beam is projected onto the chip bump, and the reflected beam on the chip surface is collected by the camera after through the imaging system. The height of the bump is calculated according to the geometric characteristics of the light stripe in the image collected by the camera and the distance between the spot on the top of the bump and the light stripe. Compared with spectral confocal method and white light interferometry, this method has the advantages of high speed and high measurement efficiency, and can meet the needs of on-line measurement of chip packaging defects. When calibrating the parameters of the chip bump height measurement system, in order to solve the difficult problem of angle calibration of projection and imaging device in the traditional triangulation method, a new calibration method is adopted: the magnification and pixel height ratio of the imaging system are used to replace the device angle to realize the indirect calibration of angle parameters. In order to accurately calibrate the pixel height ratio, a laser interferometer is used to accurately measure the height deviation of the reference plane. The center coordinate of the light strip on the reference plane is extracted by the gray centroid method to obtain the center offset of the light strip. The pixel height ratio is calculated according to the center offset of the light strip and the height deviation of the reference plane. A high-precision circular calibration plate is used to calibrate the magnification of the imaging system. The Hough circle transformation algorithm is used to extract the center coordinates of the circular pattern, and the magnification of the imaging system is calculated according to the ratio of the center distance of adjacent circles in the image to the actual distance. By substituting the calibrated system parameters into the measurement model, the accurate measurement of chip bump height can be realized. Using this method, the chip bump height is measured, and the standard deviation of height measurement is 0.58 mu m. If the same bump is measured repeatedly, the extension uncertainty is less than +/- 1 mu m. The experimental results show the accuracy of the measurement method. Compared with the traditional optical triangulation method, the chip bump height measurement model proposed in this paper is not affected by the light strip position and width, and the measurement accuracy and speed have been further improved. The research results of this paper are of great significance to realize the rapid and accurate detection of chip bumps, and have a strong reference value for the height measurement of objects with spherical structure at the top. This measurement method combined with high-precision mobile platform can further measure the bump height of chip or wafer, so as to evaluate the bump height consistency. Therefore, this measurement method has important application value for the research of industrial on-line bump height consistency measurement system.