Spontaneous formation of sub-4 nm nanocrystalline alloy via polymorphic phase transformation

Abstract

A new phase-transformation-induced path to spontaneous formation of extreme nanograin structure is reported. In-situ-heating-mode-microscopy exhibited a substantial grain-growth of Cu 6 Sn 5 . During cooling, the grain-growth continued, but it spontaneously switched to grain-refinement mode on phase transformation through ∼ 180°C from η-Cu 6 Sn 5 to η'-Cu 6 Sn 5 , ending up with an extremely small nanograin size of ∼ 2.5 nm. The cooling cycling always restores the nanograin size regardless of thermal exposure history, making this to be the first demonstration to stabilize the nanograin with its own spontaneous behavior. The Young's Modulus was significantly reduced by ∼ ×3, and the elongation was remarkably increased by ∼ ×8 to ∼ 9%. IMPACT STATEMENTMonitoring cooling and phase transformation at atomic level revealed a new path to manufacture and stabilize extremely refined sub-4 nm grains by taking advantage of the spontaneous phase transformation behavior.

Formation of Cu-=SUB=-6-=/SUB=-Sn-=SUB=-5-=/SUB=- intermetallic in Cu/Sn thin films

“…Thus, it is shown that the η-Cu 6 Sn 5 phase is formed at the activation temperature T in ≈ 95 • C and passes into the η ′ -Cu 6 Sn 5 phase after film ageing at room temperature for one month. Our results are in compliant with the work by Y. Zhong [14], in which the formation of Cu 6 Sn 5 nanocrystalline alloys by means of a polymorphic reversing phase transformation η ↔ η ′ , as well as with the work by Zhang [15], in which there was a phase transformation η-Cu 6 Sn 5 → η ′ -Cu 6 Sn 5 in whiskers during ageing at room temperature for 1−40 days.…”

Robust Cu–Cu Bonding with Multiscale Coralloid Nano-Cu3Sn Paste for High-Power Electronics Packaging

“…In recent years, Cu–Sn IMC nanoparticles (NPs) have been utilized to form full-IMC joints, which can reduce the processing time under a normal soldering temperature. For example, a full-Cu 6 Sn 5 joint with a shear strength of 17.37 MPa can be obtained only after 20 min using a hot pressing sintering technique with Cu 6 Sn 5 IMC NPs. , Compared with other commercially used high-temperature interconnection materials (high Pb alloy, nano-Ag, and nano-Cu), Cu 6 Sn 5 IMC NPs present the merits of low cost, low sintering temperature, high electromigration resistance, antioxidation, low anisotropy, and a similar thermal expansion coefficient with Cu. − Apart from Cu 6 Sn 5 IMC NPs, the Cu 3 Sn IMC NPs also have the advantage of a higher MP (676 °C), higher electrical conductivity, higher fracture toughness, higher elastic modulus, especially higher antioxidation, and lower anisotropy, , making them more suitable for high-temperature semiconductor applications. The key properties of Cu 3 Sn compared with other materials are listed in Table S1 and Figure S1.…”

“…For example, the interface of the full-Cu 6 Sn 5 joint will be completely transformed from the Cu 6 Sn 5 /Cu 3 Sn/Cu structure into the Cu 3 Sn/Cu structure in the subsequent high-temperature service process. This will inevitably form the “Kirkendall voids” at the interface of Cu 3 Sn/Cu. − In addition, the crystal structure of Cu 6 Sn 5 will change from hexagonal η-Cu 6 Sn 5 to monoclinic η′-Cu 6 Sn 5 at 186 °C . The allotropic transformation will induce a volume change and thermal stresses, which cause the reliability problem during the high-temperature service process of power devices.…”

“…26−28 In addition, the crystal structure of Cu 6 Sn 5 will change from hexagonal η-Cu 6 Sn 5 to monoclinic η′-Cu 6 Sn 5 at 186 °C. 23 The allotropic transformation will induce a volume change and thermal stresses, which cause the reliability problem during the high-temperature service process of power devices. Compared with the unstable interface formed in the full-Cu 6 Sn 5 joints, a stable Cu 3 Sn/Cu interface can be formed by direct Cu−Cu bonding with Cu 3 Sn IMC NPs.…”