Low-temperature Pressureless Copper Sintering under Formic Acid Atmosphere

With the development of power devices and the application of third-generation semiconductor materials (SiC and GaN), the electronics device tends to be high power density, high operating voltage and high operating temperature. However, traditional die bonding materials would break down at high temperature hindering the development of WBG semiconductors. Thus, sintered copper is regarded as an advanced materials due to its low price, excellent thermoelectric properties, and better resistance to electromigration. Nonetheless, the easy oxidation of copper and the high sintering temperature limit the application of sintered copper technology. In this paper, a low-temperature pressureless copper sintering method is described. Formic acid atmosphere was used to eliminate oxide particles from copper paste and oxide layer from copper substrate for high quality Cu-Cu interconnects. We also focus on the influence of temperature and atmosphere on pressureless sintered copper joints. Sintering was carried out at 210 degrees C, 240 degrees C, 270 degrees C and 300 degrees C for 30 minutes under formic acid and nitrogen. The results of shear test and scanning electron microscope (SEM) show that formic acid can significantly improve the sintering properties of copper particles. And the shear strength can reach 35.5 MPa at 240 degrees C, which is 45.5%(11 MPa) higher than nitrogen environment. In addition, single-chip devices were made for thermal resistance measurement and the results show that formic acid assisted sintered copper joints have a good thermal resistance of 0.653 K/W.