Design And Characterization Of 1D Sn-based Nanosolders & Its Applications

One of the main challenges in the practical applications of nanotechnology is the development of reliable soldering techniques at nanoscale and associated nanosolder materials in order to construct and integrate micro/nanoscale building blocks into useful devices with secure,functional and long-lasting bonding.This thesis focuses on three basic questions in future nanoscale welding techniques: the preparation of nanosolder,the development of nanoscale welding techniques and exploration nanoscale welding mechanisms.One dimensional(1D)Sn–Ag alloy nanosolder is chosen as the main study object due to its high ductility,creep resistance and thermal resistance.The main contents are as following:1.The preparation methods of 1D Sn–Ag alloy nanosolder and the size effect on its structural properties have been studied.One dimensional Sn-Ag alloy nanosolders have been successfully fabricated by a direct current electrodeposition technique into nanoporous templates.The morphology,composition,crystal structure,oxidation state,growth mechanism,melting point and wettability of theses nanosolders have been characterized at the nanoscale.It is found that 1D Sn-3.5Ag alloy nanosolders have a continuous morphology and smooth surface,and consist of matrix b-Sn and intermetallic compound Ag3 Sn.The melting point of the 1D Sn-3.5Ag nanosolders is about 215.5 °C,which is slightly lower than that of the bulk Sn-3.5Ag alloy(221 °C).The infiltration experiments reveal that 1D Sn-3.5Ag solder has really superior wettability on copper and gold substrate.2.Sn-3.5Ag nanowires have been used to weld nanopatterns at the nanoscale to test their quality as a nanosolder.The whole nanoscale electrical welding process was carried out by using two SEM in situ nanomanipulators equipped with nichrome nanoprobes.Moving a sacrificial 1D Sn-3.5Ag nanosolder across two nanowires to be joined,leaving a small bridge connecting the two nanowire ends.The solder wire melts by applying short voltage pulses to induce enough heat to fuse it and it flows onto the join.The immediate check of the joint quality shows that the resistance of welded junction is only 3.9 ?,revealing a very good ohmic contact.The mechanical strength of the SnAg solder nanojoint is strong enough to enable welded structures to be directly lifted up by a SEM nanomanipulator.The resistivity of SnAg nanowire solder was calculated to 17.5712 ??×cm by considering size effect in nanoscale.3.The welding mechanisms of 1D Sn-Ag nanosolders including melting,diffusion,wetting,recrystallization and phase transition processes have been characterized at three scales under TEM electron beam irradiation.The electron irradiation on nanowires is expected to give rise to energy transfer and induce a local increase in the temperature of the material.First,irradiation experiment at the “large” scale(micron size)in BF-TEM demonstrates that freshly prepared Sn-Ag alloy nanowires can accomplish a complete phase transformation from wires to homogeneous particles.Secondly,irradiation experiment at the “small” scale(atomic size)in HRTEM demonstrates that the solder lattice structure undergoes a cyclic variation during welding(order ? short-range disorder and long-range order ? disorder ? short-range order and long-range disorder ? order).Finally,with the protection of the oxide tube outside nanosolder,irradiation experiment at the “middle” scale(tenth of nanometer size)demonstrates the phase splitting of the homogeneous nanowire into separated Ag3 Sn and ?-Sn phases.The above results of 1D Sn-Ag investigated at three scales further improve understanding on the nano soldering mechanisms.During the nanosolder phase transformation process under electron beam irradiation,we also found that with the assistance of oxide shell covered on the Sn-based nanowires,periodic and non-periodic multilayered nanostructures are precisely sculpted and the reversibility between original homogeneous alloy phase and precipitated phases is controllable.The formation mechanism of the phase reversibility and sculpting process also works on other phase-change materials as proved by SnPb alloy nanowires.A single Sn-Ag alloy nanowire with several micron in length can be easily coded into dozens of morphology/phase status,which can produce more than 1000 barcodes.This phase tunable strategy via selective e-beam irradiation technique is believed to open up a new way of sculpting an individual nanowire with various phase status and periodicities,which may encode into a promising micro-nano identification system with the advantages of ultrahigh capacity,flexible coding,sustainable utilization and well stability.4.Two heterogeneous SnAg nanowires were welded together with a homogeneous welding spot via in situ TEM electron beam nanowelding technique.The intermetallic compound Ag3 Sn is inclined to converge into the welding spot.We found that not only eutectic Sn-3.5Ag nanosolder,but also silver rich nanosolder,oxidized nanosolder and even heterogeneous nanosoder can form homogeneous welding spot.The welding mechanisms found here changed our focus on developing useful welding techniques and on characterizing the properties,instead of hindered by the challenges on preparing homogeneous welding materials.