Kim S. Siow

Are Sintered Silver Joints Ready for Use as Interconnect Material in Microelectronic Packaging

Silver (Ag) has been under development for use as interconnect material for power electronics packaging since the late 1980s. Despite its long development history, high thermal and electrical conductivities, and lead-free composition, sintered Ag technology has limited market penetration. This review sets out to explore what is required to make this technology more viable. This review also covers the origin of sintered Ag, the different types and application methods of sintered Ag pastes and laminates, and the long-term reliability of sintered Ag joints. Sintered Ag pastes are classified according to whether pressure is required for sintering and further classified according to their filler sizes. This review discusses the main methods of applying Ag pastes/laminates as die-attach materials and the related processing conditions. The long-term reliability of sintered Ag joints depends on the density of the sintered joint, selection of metallization or plating schemes, types of substrates, substrate roughness, formulation of Ag pastes/laminates, joint configurations (i.e., joint thicknesses and die sizes), and testing conditions. This paper identifies four challenges that must be overcome for the proliferation of sintered Ag technology: changes in materials formulation, the successful navigation of the complex patent landscape, the availability of production and inspection equipment, and the health concerns of Ag nanoparticles. This paper is expected to be useful to materials suppliers and semiconductor companies that are considering this technology for their future packages.

Mechanical properties of nanocrystalline copper and nickel

Abstract The present review paper traces the development of nanocrystalline copper (Nc-Cu) and nanocrystalline nickel (Nc-Ni) and their mechanical properties. The objective is to summarise the various results available in the literature. The mechanical properties discussed are elastic modulus, Poissons ratio, hardness, yield stress, ultimate tensile stress, strain/elongation to failure, superplasticity, creep, fatigue and fracture properties. The review is limited to bulk nanocrystalline materials. Shortcomings and limitations of the various studies that have been conducted are also highlighted. The present compilation is expected to be useful for researchers engaged in experimental work and computer modelling in this area.

Identifying the Development State of Sintered Silver (Ag) as a Bonding Material in the Microelectronic Packaging Via a Patent Landscape Study

Sintered silver joint is a porous silver that bonds a semiconductor die to the substrate as part of the packaging process. Sintered Ag is one of the few possible bonding methods to fulfill the operating conditions of wide band-gap (WBG) power device technologies. We review the current technology development of sintered Ag as a bonding material from the perspective of patents filed by various stakeholders since late 1980s. This review addresses the formulation of sintered pastes (i.e., nano-Ag, hybrid Ag, and micron Ag fillers), innovations in the process and equipment to form this Ag joint. This review will provide the insights and confidence to engineers, scientists from universities and industry as well as investors who are developing and commercializing the sintered Ag as a bonding material for microelectronic packaging. [DOI: 10.1115/1.4033069]

Pitting corrosion of duplex stainless steels

This study showed the complex interactions of alloying elements and their prevailing microstructure on the pitting corrosion. This interaction was illustrated by electrochemical tests of three grades of wrought duplex stainless steels. It was shown that SAF2507 had the highest pitting potential, followed by SAF2205 and SAF2304. However, SAF2205 had higher corrosion potential than SAF2507. SAF2205 and SAF2507 were immune to pitting, while SAF2304 was susceptible to pitting. It was also found in the experiment that the austenite‐ferrite interface was the most susceptible to corrosion, followed by the austenite and finally the ferrite phase.

Enhancing the biocompatibility of the polyurethane methacrylate and off-stoichiometry thiol-ene polymers by argon and nitrogen plasma treatment

Our studies focused on improving the biocompatibility properties of two microfluidic prototyping substrates i.e. polyurethane methacrylate (PUMA) and off-stoichiometry thiol-ene (OSTE-80) polymer by Ar and N2 plasma treatment. The contact angle (CA) measurement showed that both plasma treatments inserted oxygen and nitrogen moieties increased the surface energy and hydrophilicity of PUMA and OSTE-80 polymer which corresponded to an increase of nitrogen to carbon ratios (N/C), as measured by XPS, to provide a conducive environment for cell attachments and proliferation. Under the SEM observation, the surface topography of PUMA and OSTE-80 polymer showed minimal changes after the plasma treatments. Furthermore, ageing studies showed that plasma-treated PUMA and OSTE-80 polymer had stable hydrophilicity and nitrogen composition during storage in ambient air for 15days. After in vitro cell culture of human umbilical vein endothelial cells (HUVECs) on these surfaces for 24h and 72h, both trypan blue and alamar blue assays indicated that PUMA and OSTE-80 polymer treated with N2 plasma had the highest viability and proliferation. The polar nitrogen moieties, specifically amide groups, encouraged the HUVECs adhesion on the plasma-treated PUMA and OSTE-80 surfaces. Interestingly, PUMA polymer treated with Ar and N2 plasma showed different HUVECs morphology which was spindle and cobblestone-shaped respectively after 72h of incubation. On the contrary, a monolayer of well-spread HUVECs formed on the Ar and N2 plasma-treated OSTE-80 polymers. These variable morphologies observed can be ascribed to the adherence HUVECs on the different elastic moduli of these surfaces whereby further investigation might be needed. Overall, Ar and N2 plasma treatment had successfully altered the surface properties of PUMA and OSTE-80 polymer by increasing its surface energy, hydrophilicity and chemical functionalities to create a biocompatible surface for HUVECs adhesion and proliferation.

Plasma polymerized carvone as an antibacterial and biocompatible coating

Antibacterial coating is important to prevent the colonization of medical devices by biofilm forming bacteria that would cause infection and sepsis in patients. Current coating techniques such as immobilization of antimicrobial compounds, time-releasing antibiotic agents and silver nanoparticles, require multiple processing steps, and they have low efficacy and low stability. We proposed a single-step plasma polymerization of an essential oil known as carvone to produce a moderately hydrophobic antibacterial coating (ppCar) with an average roughness of <1nm. ppCar had a static water contact angle of 78°, even after 10days of air aging and it maintained its stability throughout 24h of LB broth immersion. ppCar showed promising results in the live-dead fluorescence assay and crystal violet assay. The biofilm assay showed an effective reduction of E. coli and S. aureus bacteria by 86% and 84% respectively. ppCar is also shown to rupture the bacteria membrane for its bactericidal effects. The cytotoxicity test indicated that the coating is not cytotoxic to the human cell line. This study would be of interest to researcher keen on producing a bacteria-resistance and biocompatible coating on different substrates in a cost-effective manner.

Electrical conductivity of porous silver made from sintered nanoparticles

Electrical conductivity of open cell porous silver (Ag) with sub-micrometer features was studied. Porous Ag was formed from annealing Ag nanoparticles at 150°C up to 5 minutes. Porous Ag is a network of cylindrical ligaments joined at their ends to spherical vertices. Electrical conductivity of porous Ag was ~20% of bulk value after 5 mins annealing. Kelvin cells (truncated octahedrons) with cylindrical ligaments and spherical vertices (CLSV) were used to compute electrical conductivity which is in agreement with experimental data, without any fitting parameter. Results of the CLSV model are also in agreement with the well-established Koh-Fortini empirical relation.

Characterization of sulfate and phosphate containing plasma polymer surfaces

This paper describes three methods for producing sulfated and phosphated surfaces using plasma-based technologies, namely plasma treatment, plasma polymerization, and plasma activation followed by chemical grafting. Plasma treatment using sulfur dioxide (SO2) produced sulfur-containing groups while plasma polymerization using triisopropyl phosphite (TIP) as the monomer created phosphated surfaces. The plasma-plus-grafting technique involved deposition of an amine plasma polymer followed by grafting with vinyl sulfonate or vinyl phosphonic acid via Michael addition. The various oxidation states and surface charges of chemical groups present on the surfaces were assessed by the surface analytical techniques X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). The stability and ageing mechanism of these plasma surfaces were also characterized.

Three-Dimensional Finite Element Method Simulation of Perforated Graphene Nano-Electro-Mechanical (NEM) Switches

The miniaturization trend leads to the development of a graphene based nanoelectromechanical (NEM) switch to fulfill the high demand in low power device applications. In this article, we highlight the finite element (FEM) simulation of the graphene-based NEM switches of fixed-fixed ends design with beam structures which are perforated and intact. Pull-in and pull-out characteristics are analyzed by using the FEM approach provided by IntelliSuite software, version 8.8.5.1. The FEM results are consistent with the published experimental data. This analysis shows the possibility of achieving a low pull-in voltage that is below 2 V for a ratio below 15:0.03:0.7 value for the graphene beam length, thickness, and air gap thickness, respectively. The introduction of perforation in the graphene beam-based NEM switch further achieved the pull-in voltage as low as 1.5 V for a 250 nm hole length, 100 nm distance between each hole, and 12-number of hole column. Then, a von Mises stress analysis is conducted to investigate the mechanical stability of the intact and perforated graphene-based NEM switch. This analysis shows that a longer and thinner graphene beam reduced the von Mises stress. The introduction of perforation concept further reduced the von Mises stress at the graphene beam end and the beam center by approximately ~20–35% and ~10–20%, respectively. These theoretical results, performed by FEM simulation, are expected to expedite improvements in the working parameter and dimension for low voltage and better mechanical stability operation of graphene-based NEM switch device fabrication.

TRIZ technique to produce stable plasma modified surfaces with high density of reactive chemical functionalities

Plasma modification is used to alter surface chemical and physical properties of substrates to improve specific characteristics including adhesion and biocompatibility. Such stable and highly cross-linked coatings produced via high discharge power, however, results in a low concentration of reactive chemical functionalities on the surface. Here, we carried out the Theory of Inventive Problem Solving (TRIZ) process consist of functional analysis, cause and effect chain analysis and physical contradiction to arrive at the relevant inventive principles to solve this issue. Then, we selected relevant examples from the literature to demonstrate the application of these proposed inventive steps in solving this issue. This paper is expected to be useful to engineers/scientists who are interested to use the TRIZ technique to generate novel ideas in solving chemical or materials science related engineering problems.