Cai-Fu Li

Low Temperature Sintering Cu6Sn5 Nanoparticles for Superplastic and Super‐uniform High Temperature Circuit Interconnections

Brittle intermetallics such as Cu6 Sn5 can be transformed into low cost, nonbrittle, superplastic and high temperature-resistant interconnection materials by sintering at temperatures more than 200 °C lower than its bulk melting point. Confirmed via in situ TEM heating, the sintered structure is pore-free with nanograins, and the interface is super-uniform.

Synthesis of variously shaped magnetic FeCo nanoparticles and the growth mechanism of FeCo nanocubes

By varying the molar ratio of Fe2+/Co2+ and the concentration of PEG-400 and cyclohexane, as well as the reaction temperature and time, FeCo alloy nanoparticles with different morphologies have been synthesized successfully, in which the iron content could be adjusted in the range of 17–84 at%. Accompanying the decrease of the Fe2+/Co2+ molar ratio from 5 : 1 to 1 : 5, the shapes of FeCo nanoparticles change from sheets, cubes and spheres to flower-like particles, which were confirmed by X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy. Typically, the growth direction of nanosheets is along the [110] direction exposing (110) surface planes, while FeCo nanocubes have a perfect square shape with clear-cut edges and expose {100} surface facets. It is found that the combined action of PEG-400 and cyclohexane plays a vital role in regulating the growth rate of different crystal facets. Under the assistance of PEG-400 and cyclohexane, the growth of perfect cubes with some intermediates can be observed. Their addition effects were discussed, and a “step-by-step surface wrapping” growth mechanism was proposed to illustrate the formation of nanocubes. Among different nanoparticles, Fe75Co25 nanocubes (250 ± 10 nm) have the highest saturation magnetization, which is measured to be 250 emu g−1.

Fabrication of nanocrystalline SnO2 using electron stimulated oxidation

Fabrication of nanocrystalline SnO₂ using electron stimulated oxidation was investigated by in situ transmission electron microscopy. SnO₂ nanocrystals ranging from several to dozens of nanometers were transformed from single crystalline tin under 200 keV electron irradiation. This process includes crystallization of the surface amorphous SnO₂ layer and oxidation of the inner tin crystal substrate. On stimulation by electron irradiation, newly formed SnO₂ is supposed to act as a catalyst to oxidize the tin atoms underneath with lattice oxygen, and then be re-oxidized by absorbed oxygen from the residual gas of the microscope. This provides a new method to fabricate nanocrystalline SnO₂ materials and structures.

Identification of IMCs at SAC/Fe-Ni interface during solid-state aging at 125°C

The high temperature storage test (HTST) was conducted on the SnAgCu/Fe-Ni solder joints. The microstructural evolution during aging at 125°C was observed by both Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). During the reflow process, FeSn₂ layer and rod-like (Cu,Ni)₆Sn₅ grains were formed. During the aging at 125°C, dispersed (Cu,Ni)₆Sn₅ with two distinct morphologies were newly formed at the outer side of FeSn₂ layer which have island shape and small rod-like shape respectively. Through TEM analysis, it was confirmed that the newly formed (Cu,Ni)₆Sn₅ IMCs also has hexagonal primitive crystal structure. It was proved that the hexagonal structure was formed even below 189°C with the existence of Ni element. As the aging time increased, rod-like grains started to interconnect with each other and the island shape grains were progressed into a continuous IMC layer. The formation and the size of η-(Cu,Ni)₆Sn₅ grains were determined by the diffusion flux of Cu and Ni elements. At the SAC/Fe-Ni interface during the reflow process, the Cu element was mostly supplying from the solder, thus the (Cu,Ni)₆Sn₅ were mostly floating within the solder. Inversely, during the long-time aging, diffusion of Cu element came mostly from Cu pad at the bottom of Fe-Ni UBM while partly from the solder. Due to the restriction of substrate, island shape grains grew next to the FeSn₂ layer and its size decreased giving to the diffusion barrier effect of Fe-Ni UBM. Moreover, the small rod-like grains were usually growing upon the island shape grains, so the decreased supply of Cu and Ni element would further reduce the dimension of the grain size of formed (Cu,Ni)₆Sn₅.