There is a high demand for the implementation of metallic nanoparticle (NP) sintering technology for die attach in high-power electronics. The performance of this technology is superior to that of the technology involving the use of lead-free solders. Although Cu NP paste is potentially a low-cost material, it faces the challenge of oxidation during sintering. This may result in a significant deterioration of the mechanical, thermal, and electrical properties. Therefore, there are limited studies on the in-air sintering of Cu NP pastes. The present study demonstrated the in-air pressure-assisted low-temperature sintering of a commercial Cu NP paste. Furthermore, the sintering was performed without using a protective atmosphere, unlike that in most of the previously reported investigations. The sintering behavior was investigated at three levels of temperatures (200–240 °C) and five levels of pressures (5–25 MPa). The joints that were sintered at high temperatures and pressures exhibited condensed microstructures and high bonding strengths. High sintering temperatures accelerated the diffusion between Cu NPs, while high sintering pressure facilitated the removal of evaporated organic compounds and the air between NPs. This not only facilitated sintering but also prevented the oxidation of Cu. The optimal sintering conditions promoted the formation of 3D connections between the Cu NPs, thereby increasing the shear strength of the sample. The samples that were sintered at 240 °C and 10 MPa experiences the highest increase in the shear strength, furthermore, the microstructures were optimized under this condition. The shear strength of 28.1 ± 8.47 MPa was achieved under this condition, which satisfied the requirements for die attach in high power electronics applications, moreover, the sintering process was moderate and cost-effective. Therefore, the optimal sintering temperature and pressure for the in-air sintering of the Cu NP paste was concluded to be 240 °C and 10 MPa, respectively. The results indicated that in-air sintering with pressure assistance can be applied for die attach in the high-power electronics. ... Read more

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. ... Read more

We report the design, fabrication and experimental investigation of a MEMS micro-hotplate (MHP) for fast time-resolved X-ray diffraction (TRXRD) study of Cu nanoparticle paste (nanoCu paste) sintering process. The device and its system are designed to have a 60 ms minimum time interval, uniform temperature distribution and variant gas environments. A TRXRD study of nanoCu paste sintering at 200 °C in H₂-N₂ gas mixture was done using this device. With 1 sec interval, Cu₈O reduction and Cu crystallization in sintering is observed. Results can be combined with other studies to optimize material design and process development. ... Read more

We explore a methodology for patterned copper nanoparticle paste for 3D interconnect applications in wafer to wafer (W2W) bonding. A novel fine pitch thermal compression bonding process (sintering) with coated copper nanoparticle paste was developed. Most of the particle size is between 10-30 nm. Lithographically defined stencil printing using photoresist and lift-off was used to apply and pattern the paste. Variations in sintering process parameters, such as: pressure, geometry and ambient atmosphere, were studied. Compared to Sn-Ag-Cu (SAC) microsolder bumps, we achieved better interconnect resistivity after sintering at 260 °C for 10 min, in a 700 mBar hydrogen forming gas (H2/N2) environment. The electrical resistivity was 7.84 ± 1.45 μΩ·cm, which is about 4.6 times that of bulk copper. In addition, metallic nanoparticle interconnect porosity can influence the electrical properties of the interconnect. Consequently, we investigated the porosity effect on conductivity using finite element simulation. A linear relationship between the equivalent conductivity and particle overlapping ratio was found. ... Read more