Cu wire bond parameter optimization on various bond pad metallization and barrier layer material schemes

The use of copper wire for semiconductor package assembly has been gradually gaining acceptance throughout the industry over the last decade. Although copper has several advantages over gold for wire bonding applications, the manufacturing difficulties using copper wire have made high volume, fine pitch copper bonding slow to materialize. In recent years with the spike in gold prices, copper wire has become even more attractive, and this has driven many studies on the topic.Due to the propensity for copper to work harden upon deformation, which occurs during the ball bonding process as the capillary tip smashes the ball into the bond pad, a high amount of stress is transferred into the bond pad structure. This can result in catastrophic defects such as dielectric cracking or pad cratering. The current study aims to quantify the level of underlying bond pad damage with respect to various bond pad metallization and barrier layer schemes. A first bond parameter optimization was completed on each experimental group. The results indicate that barrier layer structure and composition have a significant impact on the presence of pad cratering. The experimental group containing only TiN as the barrier material showed a high occurrence of cratering, while groups with Ti and TiW barrier metals showed no cratering, even if a TiN layer was on top of the Ti. The bond pad metal thickness, on the other hand, does not appear to play a significant role in the prevention of bond pad cratering. Metal thickness values ranging from 0.825 to 2.025 μm were evaluated, and none had bond pad cratering other than the group with TiN as the barrier metal. In addition to the first bond parameter evaluations with various bond pad and barrier metal combinations, the initial free air ball (FAB) optimization is discussed.     

Cooperative Hybrid‐ARQ Protocols: Unified Frameworks for Protocol Analysis

Cooperative hybrid‐automatic repeat request (HARQ) protocols, which can exploit the spatial and temporal diversities, have been widely studied. The efficiency of cooperative HARQ protocols is higher than that of cooperative protocols because retransmissions are only performed when necessary. We classify cooperative HARQ protocols as three decode‐and‐forward‐based HARQ (DF‐HARQ) protocols and two amplified‐and‐forward‐based HARQ (AF‐HARQ) protocols. To compare these protocols and obtain the optimum parameters, two unified frameworks are developed for protocol analysis. Using the frameworks, we can evaluate and compare the maximum throughput and outage probabilities according to the SNR, the relay location, and the delay constraint. From the analysis we can see that the maximum achievable throughput of the DF‐HARQ protocols can be much greater than that of the AF‐HARQ protocols due to the incremental redundancy transmission at the relay.     

Design and performance analysis of MAC schemes for Wireless Sensor Networks Powered by Ambient Energy Harvesting

Energy consumption is a perennial issue in the design of wireless sensor networks (WSNs) which typically rely on portable sources like batteries for power. Recent advances in ambient energy harvesting technology have made it a potential and promising alternative source of energy for powering WSNs. By using energy harvesters with supercapacitors, WSNs are able to operate perpetually until hardware failure and in places where batteries are hard or impossible to replace. In this paper, we study the performance of different medium access control (MAC) schemes based on CSMA and polling techniques for WSNs which are solely powered by ambient energy harvesting using energy harvesters. We base the study on (i) network throughput (S), which is the rate of sensor data received by the sink, (ii) fairness index (F), which determines whether the bandwidth is allocated to each sensor node equally and (iii) inter-arrival time (γ) which measures the average time difference between two packets from a source node. For CSMA, we compare both the slotted and unslotted variants. For polling, we first consider identity polling. Then we design a probabilistic polling protocol that takes into account the unpredictability of the energy harvesting process to achieve good performance. Finally, we present an optimal polling MAC protocol to determine the theoretical maximum performance. We validate the analytical models using extensive simulations incorporating experimental results from the characterization of different types of energy harvesters. The performance results show that probabilistic polling achieves high throughput and fairness as well as low inter-arrival times.     

Optimum content of Nafion ionomer for the fabrication of MEAs in PEMFCs with spray-coated Pt/CNT electrodes

To enhance the performance of a polymer electrolyte membrane fuel cell (PEMFC), a Pt catalyst was supported on carbon nanotubes (CNTs) and the optimum content of Nafion ionomer in the Pt/CNT electrode was examined by cell performance tests, cyclic voltammetry, and electrochemical impedance spectroscopy. The amount of the Pt catalyst supported on the CNTs was 34 wt.%. The Nafion content significantly changed the protonic and electronic conductivities as well as the mass transfer properties. As such, the performance of the cell was highly dependent on the content of Nafion ionomer. The results of the cell performance tests revealed that the optimum content of Nafion ionomer in the Pt/CNT electrode was about 20 wt.%.     

First-principles study of Li adsorption in a carbon nanotube-fullerene hybrid system

A carbon hybrid material system consisting of single wall carbon nanotubes (SWCNTs) and fullerene (C₆₀) has been investigated using the first-principles methods. Through combining metallic SWCNTs with C₆₀ of high electron affinity, the lithium adsorption energy on this CNT-C₆₀ hybrid system (−2.110 eV) is found to be larger than that of the pure SWCNTs (−1.720 eV). By characterizing the electronic properties of the CNT-C₆₀ system such as band structure, density of states and charge distribution as a function of the Li adsorption in comparison with SWCNT or C₆₀, it is also found that the Li adsorption takes place on the C₆₀ side preferably due to the large adsorption energy, which imparts metallic character to the C₆₀ in the CNT-C₆₀ hybrid system. Investigating various adsorption sites on the CNT-C₆₀ system in order to understand the adsorption mechanism of Li, it is found that Li atoms are preferably adsorbed at every other hexagonal or pentagonal site (next nearest neighboring sites) rather than every site (nearest neighboring sites) on the hybrid system. The possibility of Li cluster formation in this CNT-C₆₀ system does not seem to be high since the Li–Li binding is less favorable than the Li adsorption on the CNT-C₆₀ system.     

Effects of TS-1 zeolite structures on physical properties and enzymatic degradation of Poly (butylene succinate) (PBS)/TS-1 zeolite hybrid composites

The objective of this study was to investigate how the water uptake features and carrier characteristics of the TS-1 zeolite affected the physical and rheological properties, morphological parameters, and enzymatic hydrolysis of Poly (butylene succinate) (PBS). The introduction of TS-1 zeolite as catalyst was developed for the preparation of PBS/TS-1 zeolite hybrid composites (PTHC) without heavy metal toxic substance in the context on clean technology. The TS-1 zeolite can act as a catalyst as well as a reinforcement filler with the result that PTHC can show marked increases in tensile properties and elongation at breakage in the solid state. The rheological properties of PTHC with high zeolite contents showed low values of complex viscosity, as compared with PTHC with low TS-1 zeolite contents, due to the volatilization of water released from the zeolite pores during esterification. The introduction of the TS-1 zeolite in the PBS matrix was not significantly affected by changes in the size of the long period, lamella thickness, or the amorphous region, indicating that PBS chains do not penetrate into zeolite pores, as confirmed by SAXS profiles. In enzymatic hydrolysis over 90 days, the enzymatic hydrolysis rates of PTHC significantly accelerated with increasing TS-1 zeolite contents, compared with Homo PBS. This result indicated that TS-1 zeolite can act as a carrier for enzyme activation, resulting in enzymatic hydrolysis, occurring from the amorphous area on the surface into the inside of the film.     

Hydrogen Acceptor‐ and Base‐Free N‐Formylation of Nitriles and Amines using Methanol as C1 Source

An N‐formylation method using methanol as the C₁ source without a stoichiometric amount of activating reagent is described. Nitriles as well as amines can be directly used as substrates. The reaction is catalyzed by an N‐heterocyclic carbene coordinated ruthenium(II) dihydride complex, which mediates methanol dehydrogenation, nitrile reduction, and C N bond formation without any external base, hydrogen acceptor, or oxidant.

     

Relaxor/Ferroelectric Composites: A Solution in the Quest for Practically Viable Lead‐Free Incipient Piezoceramics

Recently developed lead‐free incipient piezoceramics are promising candidates for off‐resonance actuator applications with their exceptionally large electromechanical strains. Their commercialization currently faces two major challenges: high electric field required for activating the large strains and large strain hysteresis. It is demonstrated that design of a relaxor/ferroelectric composite provides a highly effective way to resolve both challenges. Experimental results in conjunction with numerical simulations provide key parameters for the development of viable incipient piezoceramics.     

Microstructure and mechanical properties of CNT/Ag nanocomposites fabricated by spark plasma sintering

Carbon nanotube/silver (CNT/Ag) nanocomposites include CNT volume fraction up to 10?vol.% were prepared by chemical reduction in solution followed by spark plasma sintering. Multiwalled CNTs underwent surface modifications by acid treatments, the Fourier transform infrared spectroscopy data indicated several functional groups loaded on the CNT surface by acid functionalisation. The acid-treated CNTs were sensitised and activated. Silver was deposited on the surface of the activated CNTs by chemical reduction of alkaline silver nitrate solution at room temperature. The microstructures of the prepared CNT/Ag nanocomposite powders were investigated by high-resolution scanning electron microscopy (HRSEM), transmission electron microscopy and X-ray powder diffraction analysis. The results indicated that the produced CNT/Ag nanocomposite powders have coated type morphology. The produced CNT/Ag nanocomposite powders were sintered by spark plasma sintering. It was observed from the microstructure investigations of the sintered materials by HRSEM that the CNTs were distributed in the silver matrix with good homogeneity. The hardness and the tensile properties of the produced CNT/Ag nanocomposites were measured. By increasing the volume fraction of CNTs in the silver matrix, the hardness values increased but the elongation values of the prepared CNT/Ag nanocomposites decreased. In addition, the tensile strength was increased by increasing the CNTs volume fraction up to 7.5?vol.%, but the sample composed of 10?vol.% CNT/Ag was fractured before yielding.