A low temperature Cu-Cu direct bonding method with VUV and HCOOH treatment for 3D integration

In this paper, a low temperature Cu-Cu direct bonding method is demonstrated even at 175 degree C, which is less than solder melting point, by using fine crystalized bump surface and dedicated surface treatments. Our technique includes introducing fine crystal structure on Cu bump surface by ultra-precision cutting for bump planarization, not using conventional CMP method. This fine crystal structure can easily recrystallize at 150 degree C condition, which can provide a metallurgical interdiffusion between Cu-Cu interfaces. In addition, surface treatment is also crucial for direct Cu-Cu bonding. In this study, we investigated formic acid (HCOOH) and vacuum ultraviolet (VUV) irradiation effects on die shear strength. Cut Cu bump with VUV and HCOOH condition at 175 degree C not only showed high bonding strength, but also exhibited solid diffusion at interface while Cut Cu with HCOOH condition showed clear interface.