Defect inspection of copper bonding using ultrasound

Three dimensional packaging based on TSV is considered to be promising and has been accepted as the next generation technology for packaging. There are many advantages such as high speed interconnection, ultrafine pitch and high density integration. Copper bonding is one of the key bonding technologies to achieve 3D packaging with fine electrical interconnection, excellent path for thermal transfer and high mechanical reliability. The bonding interfaces are sandwiched between two wafers, leading to the defect optically opaque. Defect inspection of copper bonding becomes more difficult. In this paper, a nondestructive detection method was presented. We used 230 MHz ultrasonic transducer for copper bonding testing and captured the reflected time domain signals when the transducer scanning the copper bonding chips. The bonding layer was located in the time domain signals and the images of the bonding samples were generated by scanning acoustic microscopy. Due to the ability to deal with small sample data, the support vector machine was adopted to process the bonding images. Only ten pixel points of good bonding interface and ten pixel points of bonding interface with voids were captured to train the support vector machine, and then the whole bonding image was tested by the trained support vector machine. Meanwhile, the simple binary method with default threshold was used for the bonding image processing. The cross-section view was prepared in an epoxy mount and polished. Scanning electron microscopy analysis validated the detection results. The approach using the support vector machine shows better processing results and robustness compared to the simple binary method. Therefore, the method using ultrasound for defect inspection is effective in copper bonding.