Chuantong Chen

Quantitative analysis on size dependence of eutectic temperature of alloy nanoparticles in the Ag–Pb system

The eutectic transition of Ag–Pb alloy nanoparticles was investigated using in situ transmission electron microscopy. It was found that the eutectic temperature of Ag–Pb alloy nanoparticles decreased with decreasing particle size. A linear relationship was obtained for the eutectic temperature as a function of the reciprocal of the particle radius. Theoretical calculations based on the theory of thermodynamics also suggested a linear relationship between the eutectic temperature and the inverse radius. The calculated results were in good agreement with the experimental observations.

In situ observations of crystalline-to-liquid and crystalline-to-gas transitions of substrate-supported Ag nanoparticles

The phase transitions of silver nanoparticles on graphite and alumina substrates have been investigated using in situ high-resolution transmission electron microscopy. It was revealed that no crystalline-to-liquid transition, only a crystalline-to-gas transition, took place in graphite-supported silver nanoparticles prior to the complete disappearance of the nanoparticles; crystalline-to-liquid transition did take place in alumina-supported silver nanoparticles. The substrate-induced difference in the phase transition behavior is discussed on the basis of a thermodynamic model.

Highly reliable and highly conductive submicron Cu particle patterns fabricated by low temperature heat-welding and subsequent flash light sinter-reinforcement

Submicron Cu particle ink was developed to successfully achieve highly reliable and highly conductive Cu patterns on low-cost, transparent, and flexible substrates by an optimized two-step sintering process involving low temperature heat-welding and subsequent flash light sinter-reinforcement. The Cu ink contains a special additive of the Cu–amino complex made from copper(II) formate and 2-amino-2-methyl-1-propanol solvent. Low temperature heat-welding promotes the decomposition of the Cu–amino complex into fresh metallic Cu particles, which as nano-welders can in situ weld those big submicron Cu particles. The subsequent flash light sintering further reinforces the connection between big Cu particles with the assistance of these active nano-welders and strengthens the adhesion between sintered Cu patterns and polymer substrates due to the local soft-effect. The achieved resistivities of sintered Cu patterns on polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and polyimide (PI) substrates are 26.5 μΩ cm, 15.9 μΩ cm and 7.2 μΩ cm at a low welding temperature of 140 °C for 10 min and subsequent flash light energies of 1080 mJ cm−2, 1273 mJ cm−2 and 2073 mJ cm−2, respectively, at which the same electrical properties cannot be obtained from either pure nano-Cu or submicron Cu particle ink as reported in previous research studies. Moreover, bending fatigue and oxidation-resistance tests indicate that the sintered Cu patterns have superior mechanical and environmental stability. Finally, flexible and foldable LED circuits and flexible dipole antennas were successfully fabricated to demonstrate the applicability of the sintered Cu patterns for printed electronic devices. It should be noted that this method opens a new way for making highly reliable and highly conductive Cu patterns on low-cost, transparent, and flexible substrates with big Cu particles instead of nanoparticles under a suitable sintering process, which may largely decrease the cost and enhance the application of Cu inks for flexible electronic devices.

Mechanical Deformation of Sintered Porous Ag Die Attach at High Temperature and Its Size Effect for Wide-Bandgap Power Device Design

The mechanical properties of sintered Ag paste with microporous structure have been investigated by tensile and shear tests, focusing on the temperature-dependent plastic deformation at various temperatures from 25°C to 300°C, corresponding to the target operating temperature range of emerging wide-bandgap semiconductor devices. Specimens were prepared by sintering hybrid Ag paste consisting of microflake and submicron spherical Ag particles, simulating a typical bonding process for power semiconductor die attach. Mechanical tests revealed that the unique microstructure caused a brittle-to-ductile transition at temperature of around 160°C, remarkably lower than that of bulk Ag. The obtained Young’s modulus and shear modulus values indicate obvious softening with increasing temperature, together with a remarkable decrease in Poisson’s ratio. These plastic behaviors at elevated temperature can be explained based on Coble creep in the microporous network structure. Fracture surfaces after tensile and shear tests indicated unique features on scanning electron microscopy, reflecting the variation in the ductile behavior with the test temperature. Furthermore, these temperature-dependent mechanical parameters were employed in three-dimensional finite-element analysis of the thermomechanical stress distribution in wide-bandgap semiconductor module structures including Ag paste die attach of different sizes. Detailed thermal stress analysis enabled precise evaluation of the packaging design for wide-bandgap semiconductor modules for use in high-temperature applications.

Silver nanowires with a monoclinic structure fabricated by a thermal evaporation method

Silver nanowires with a monoclinic structure (mono-Ag NWs) were fabricated by a thermal evaporation method for the first time. The crystal lattice parameters of the mono-Ag NWs were calculated using the UnitCell program. They are as follows: a = 0.303 nm, b = 1.140 nm, c = 0.292 nm, and beta = 118.5 degrees. In situ annealing experiments revealed that the as-prepared mono-Ag NWs transited to fcc-Ag NWs during annealing at approximately 1173 K for 60 s.

Printable and Flexible Copper–Silver Alloy Electrodes with High Conductivity and Ultrahigh Oxidation Resistance

Printable and flexible Cu-Ag alloy electrodes with high conductivity and ultrahigh oxidation resistance have been successfully fabricated by using a newly developed Cu-Ag hybrid ink and a simple fabrication process consisting of low-temperature precuring followed by rapid photonic sintering (LTRS). A special Ag nanoparticle shell on a Cu core structure is first created in situ by low-temperature precuring. An instantaneous photonic sintering can induce rapid mutual dissolution between the Cu core and the Ag nanoparticle shell so that core-shell structures consisting of a Cu-rich phase in the core and a Ag-rich phase in the shell (Cu-Ag alloy) can be obtained on flexible substrates. The resulting Cu-Ag alloy electrode has high conductivity (3.4 μΩ·cm) and ultrahigh oxidation resistance even up to 180 °C in an air atmosphere; this approach shows huge potential and is a tempting prospect for the fabrication of highly reliable and cost-effective printed electronic devices.

Self-healing of cracks in Ag joining layer for die-attachment in power devices

Sintered silver (Ag) joining has attracted significant interest in power devices modules for its ability to form stable joints with a porous interconnection layer. A function for the self-healing of cracks in sintered porous Ag interlayers at high temperatures is discovered and reported here. A crack which was prepared on a Ag joining layer was closed after heating at 200 °C in air. The tensile strength of pre-cracked Ag joining layer specimens recovers to the value of non-cracked specimens after heating treatment. Transmission electron microscopy (TEM) was used to probe the self-healing mechanism. TEM images and electron diffraction patterns show that a large quantity of Ag nanoparticles formed at the gap with the size less than 10 nm, which bridges the crack in the self-healing process. This discovery provides additional motivation for the application of Ag as an interconnection material for power devices at high temperature.

Low-Stress Design for SiC Power Modules with Sintered Porous Ag Interconnection

The mechanical properties of sintered porous Ag-paste are investigated by tension test and shear test in the temperature range from 25 °C to 300°C. Stress-strain curves of sintered porous Ag-paste are measured at different temperatures. The Poissons ratio, which is calculated by Youngs modulus and shear modulus, decreased from 0.31 at 25 °C to 0.11 at 300 °C. In addition, 3D finite element model (FEM) is constructed for six types of module structures under thermal cycling analysis, which focuses particularly on the stress of sintered porous Ag-paste at different temperatures to find an optimized low-stress structure for the long-term reliability. The obtained results in this study suggested that the sintered porous Ag-paste may survive longer and continue to functions at high temperature variations because of the large plastic deformation.

Dry-growth of silver single-crystal nanowires from porous Ag structure

A fabrication method of single crystal Ag nanowires in large scale is introduced without any chemical synthesis in wet processes, which usually generates fivefold twinned nanowires of fcc metals. Dense single-crystal nanowires grow on a mechanically polished surface of micro-porous Ag structure, which is created from Ag micro-particles. The diameter and the length of the nanowires can be controlled simply by changing the temperature and the time of the heating during the nanowire growth in air. Unique growth mechanism is described in detail, based on stress-induced migration accelerated by the micro-porous structure where the origin of Ag nanowires growth is incubated. Transmission electron microscopy analysis on the single crystal nanowires is also presented. This simple method offered an alternative preparation for metallic nanowires, especially with the single crystal structure in numerous applications.

A novel mechanism of silver microflakes sinter joining

In this paper, we describe a novel mechanism of Ag microflakes sintering which is not yet reported. It involves the generation of Ag amorphous layer, the nucleation and recrystallization of Ag nano particles and dynamic replication of the above-mentioned steps. Transmission electron microscopy (TEM) is utilized to characterize the Ag microflakes sinter joining, which reveals sintering is under the combination of this novel mechanism and low-temperature diffusion bonding.