Low Temperature Ag-Ag Direct Bonding Technology for Advanced Chip-Package Interconnection

摘要

Nowadays, with increasing integration density on very large scale integrated-circuits (VLSI), the pitch between joints and the joint size within the package of advanced device have been scaled down to sub-100 μm. As a result, traditional method which employs solder joint would induce more reliability and yield issues. Direct metal to metal bonding (Cu-Cu and Au-Au) have been proposed as alternative methods. However, high temperature, high bonding pressure, excellent surface finish and high vacuum are required during direct bonding, which restricts their wide acceptance by the industry. In this manuscript, a method to achieve direct Ag-Ag bonding at low temperature (210 °C) with low bonding pressure (1.38 MPa) is proposed by in-situ self-reduction of surface oxides. The temperature is lower than the reflow temperature of lead-free solder (250 °C) and the pressure is much lower than the pressure required in thermal-compression process. The preparation and characterization of surface oxides have been elaborated. During bonding, the in-situ generated Ag "wets" the surfaces and enhanced the surface diffusion, which enables the bonding of materials at atomic scale and merging of grains from two sides. The requirement for surface roughness is much lower that direct Cu-Cu bonding reported in literatures. The quality of the joint is studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and the results show that two Ag films join together to form one piece "bulk-like" polycrystalline materials without any trace of the original interface. We believe that this technique will be promising in flip chip bonding for advanced VLSI packages. In addition, this bonding technique is valuable to die attachment for high power devices as it provides the minimum heat resistance between chip and substrate.