Transmissive laser lock-in thermography for highly sensitive and online imaging of real interfacial bubbles in wafer bonding

Online and non-destructive detection of interfacial bubbles in wafer bonding is a critical approach to enhance the yield of advanced packaging. However, the commonly used scanning acoustic microscopy (SAM) is timeconsuming and may cause additional risk due to soaking in water. The conventional thermography is suffering from low sensitivity and resolution. In this work, we demonstrate for the first time an effective method for highly sensitive and online imaging of real interfacial bubbles in wafer bonding using transmissive laser lockin thermography (LLT). The transmissive LLT system projects a modulated laser beam onto target wafer surface and records the thermal image sequence on the other side. The bubbles distinctly emerge with clear edges in the extracted lock-in phase images. To enhance the detectivity, the relationship between phase image contrast and LLT parameters was explored. As a result, we can visualize and detect the bubbles larger than 500 mu m at depth of 725 mu m even under the interference of metal pattern with a speed of 130 mm2/10 s, which is 38% faster than that of SAM. Besides, the LLT phase images contain both bubbles and metal patterns information. The ratio between bubble depth and width reaches 1.45, which is about 2-fold of the latest infrared researches. Above all, this work paves the way for high-sensitive imaging of various tiny internal defects in advanced packaging using infrared thermography.