Low temperature sintering of copper nanoparticles

Abstract

Power electronics demand miniaturization, integration, higher electrical and ther-mal conductivities. However, the traditional electronic packaging materials and technology have limitations to meet these requirements. Conventional lead-free die attach materials, like Sn-Ag-Cu solders, can not satisfy high power electronics application, due to their low operating temperatures and intermetallic defects. Therefore, material in-novation has attracted much attention in this field. Metallic particle sintering of silver nanoparticle (Ag NP) has become one of the most applied technologies in power elec-tronics industry. Furthermore, to achieve "all copper interconnect" in packaging system, and to reduce the cost further, copper nanoparticle(Cu NP)-based paste has been ex-plored recently in both material synthesis and process development. However, since Cu NPs are reactive and easy to be oxidized, it is challenging to achieve a compatible pro-cess, with profound bonding properties. In this thesis, both fundamental understanding of Cu NP-based paste sintering process and die attach process development in power electronics applications are conducted. These two important parts of research works give insight to Cu NP-based paste sintering from various aspects, but on the same physical scale. In this thesis, these knowledge and experience obtained with deep understanding of the material and process can be transferred as one of the significant information to push Cu NP-based paste into industrial application, with deep understanding of the material and processes.

First, to obtain a deeply fundamental knowledge about sintering process, both static and time-dependent characterizations need to be performed, at similar scale as in real application. X-ray diffraction (XRD) is selected due to its large detection volume and valuable material information, both qualitative and quantitive. To enable a dynamic time-resolved X-ray diffraction (TRXRD) study and in-situ sample monitoring, a MEMS-based TRXRD nanomaterial platform is firstly designed and fabricated. A gas cell is designed and fabricated to provide an environmental experimental condition, without interference with XRD measurements. Combined with gas cell and power supply, this set up can enable TRXRD characterization of nanomaterial, with large flexibility of temperature control and gas environment.

Next, with the developed characterization platform, both static and time-dependent investigations on the sintering process of a commercial Cu NPs-based paste are per-formed under different conditions. Series of XRD patterns and in-situ electrical resistance measurement are collected, followed with detailed XRD analysis and microstructure observation. These results and insights are on the one hand, a validation of the function of the developed nanomaterials characterization method and platform. On the other hand, they can be transferred to improve and guide process development and material optimization of Cu NPs-based paste.

Last but not least, the in-air pressure assisted sintering behaviors of Cu NP-based paste under various process conditions are investigated and analyzed. Based on the paste characterization results, the in-air sintering temperature range is determined and multiple pressure-assisted sintering experiments in the air are performed. As temperature and pressure increase, Cu NPs form more condensed structures with neighboring particles. Both of these parameters can accelerate the neck formation and inter-particle connection inside Cu joints.