Electromigration in Gold and Silver Nanostructures

Als Elektromigration bezeichnet man den strominduzierten Massetransport in metallischen Leitern. Sie stellt den Hauptgrund für das Versagen von integrierten Schaltkreisen dar und ist seit mehr als 50 Jahren Gegenstand intensiver Untersuchungen. In dieser Arbeit wurde die Elektromigration in polykristallinen Goldleiterbahnen und einkristallinen, selbstorganisierten Silberdrähten untersucht. Besonderes Augenmerk lag auf der hochauflösenden in-situ Beobachtung der auftretenden morphologischen Änderungen. Hierzu wurde ein neuer Versuchsstand aufgebaut und erfolgreich getestet. Während der Elektromigration kommt es in metallischen Leitern üblicherweise zur Bildung von Poren an der Kathodenseite und zur Bildung von Hügeln an der Anodenseite. Dieses Verhalten wird in der vorliegenden Arbeit detailliert untersucht. Der elektrische Ausfall der Goldleiterbahnen erfolgt über eine schlitzförmige Pore senkrecht zur Stromrichtung. Die Porenfläche nimmt im wesentlichen linear mit der Versuchszeit zu. Der Ausfall der polykristallinen Leiterbahnen erfolgt typischerweise bei einer Gesamtporenfläche von 2% bis 4 % der Gesamtleiterbahnfläche. Der Einfluss einzelner Poren in einer Leiterbahn unter massiver Strombelastung auf den elektrischen Widerstand wurde erfolgreich nachgewiesen. Die Abhängigkeit des Elektromigrationsverhalten von Leiterbahnbreite und -höhe, der Korngröße sowie der Temperatur wurde eingehend studiert. Für hochauflösende Messungen wurden spezielle Leiterbahnen mit Einbuchtungen hergestellt. Die Abhängigkeit der Elektromigration von der Stromdichte sowie der Einfluss der Messungen auf die Elektromigration ist ebenfalls Gegenstand dieser Arbeit. Bei Umkehrung der Polarität zeigt sich ein reversibles Elektromigrationsverhalten der Goldleiterbahnen, verbunden mit einer deutlichen Erhöhung der Lebensdauer. Hierbei wird ein merkliches Ausheilverhalten anhand der Widerstandsdaten der Leiterbahnen beobachtet. Weiterhin wird im Verlauf dieser Arbeit eine alternative Blechlänge bestimmt. Berechnet man das kritische Produkt aus Leiterbahnlänge und Stromdichte, ergibt sich eine gute Übereinstimmung mit Literaturdaten, welche für andere metallische Leiterbahnen erzielt wurden. Erstmalig konnten im Rahmen dieser Arbeit Messungen an einkristallinen, selbstorganisierten Silberdrähten durchgeführt werden. Überraschenderweise findet man bei diesem System ein völlig anderes Elektromigrationsverhalten als in polykristallinen Goldleiterbahnen. Der Ausfall erfolgt an der Anodenseite und nicht, wie üblich, an der Kathodenseite. Der Materialtransport in diesem System erfolgt also von der Anode in Richtung der Kathode und damit entgegengesetzt zur Bewegungsrichtung der Elektronen. Dieses Resultat deutet auf ein Überwiegen der direkten Kraft in diesem System hin.Electromigration is the current induced mass transport in metallic wires. It is the main reason for electrical breakdown in integrated circuits and has been studied for more than 50 years. In this thesis, the electromigration behavior in polycrystalline gold as well as in self-organized single crystalline silver wires are studied. To study the electromigration behavior in detail, in-situ investigations of the wires are performed in a scanning electron microscope, for which a new test rig was successfully installed. During electromigration, the development of voids on the cathode and hillocks on the anode side of the wire are observed. This behavior is studied in detail in this thesis. Electrical breakdown in the gold wires takes place due to the presence of slit-like voids perpendicular to the current direction. The void area grows linearly during the course of the experiments, and the electrical breakdown takes place when the total void area reaches a value of 2 % to 4 % of the total wire area. The influence of single voids on the electrical resistance during high current stressing is determined. The dependence of the electromigration behavior on the width and height as well as on the crystallinity and temperature of the gold wires is studied in detail. For high resolution imaging of the wires during the experiments, a special layout with arbitrary kinks is used. The dependence of electromigration effects on current density and on the influence of the measurement setup itself are also discussed in this thesis. When reversing the current direction, a reversible electromigration behavior is observed. Also, the lifetime of the wires grows considerably. According to the resistance data, a remarkable stabilization of the polycrystalline wires is observed during this experiments. Furthermore, it is possible to define an alternative Blech length according to the position of voids and hillocks in the wires. This leads also to the determination of the critical product for these gold wires, which agrees well with data from literature on other metallic systems. For the first time, it is possible to study electromigration within self-organized single crystalline silver wires. Surprisingly, the electromigration behavior is completely different from that observed for polycrystalline gold wires. Electrical breakdown takes place at the anode side of the wire, i. e. material transport takes place against the direction of the electrons. These results indicate, that the direct force might be the dominating force within this system

Deagglomeration testing of airborne nanoparticle agglomerates: stability analysis under varied aerodynamic shear and relative humidity conditions

Occupational exposure to nanomaterial aerosols poses potential health risks to workers at nanotechnology workplaces. Understanding the mechanical stability of airborne nanoparticle agglomerates under varied mechanical forces and environmental conditions is important for estimating their emission potential and the released particle size distributions which in consequence alters their transport and human uptake probability. In this study, two aerosolization and deagglomeration systems were used to investigate the potential for deagglomeration of nanopowder aerosols with different surface hydrophilicity under a range of shear forces and relative humidity conditions. Critical orifices were employed to subject airborne agglomerates to the shear forces induced by a pressure drop. Increasing applied pressure drop were found to be associated with decreased mean particle size and increased particle number concentrations. Rising humidity decreased the deagglomeration tendency as expressed by larger modal particle sizes and lower number concentrations compared to dry conditions. Hydrophilic aerosols exhibited higher sensitivities to changes in humidity than hydrophobic particles. However, the test systems themselves also differed in generated particle number concentrations and size distributions, which in turn altered the responses of created aerosols to humidity changes. The results of the present study clearly demonstrate that a) humidity control is essential for dustiness and deagglomeration testing, b) that (industrial) deagglomeration, e.g. for preparation of aerosol suspensions, can be manipulated by subjecting airborne particles to external energies, and c) that the humidity of workplace air may be relevant when assessing occupational exposure to nanomaterial aerosols

Dustiness and deagglomeration testing: interlaboratory comparison of systems for nanoparticle powders

Different types of aerosolization and deagglomeration testing systems exist for studying the properties of nanomaterial powders and their aerosols. However, results are dependent on the specific methods used. In order to have well-characterized aerosols, we require a better understanding of how system parameters and testing conditions influence the properties of the aerosols generated. In the present study, four experimental setups delivering different aerosolization energies were used to test the resultant aerosols of two distinct nanomaterials (hydrophobic and hydrophilic TiO2). The reproducibility of results within each system was good. However, the number concentrations and size distributions of the aerosols created varied across the four systems; for number concentrations, e.g., from 10(3) to 10(6) #/cm(3). Moreover, distinct differences were also observed between the two materials with different surface coatings. The article discusses how system characteristics and other pertinent conditions modify the test results. We propose using air velocity as a suitable proxy for estimating energy input levels in aerosolization systems. The information derived from this work will be especially useful for establishing standard operating procedures for testing nanopowders, as well as for estimating their release rates under different energy input conditions, which is relevant for occupational exposure

Saharan dust induces NLRP3-dependent inflammatory cytokines in an alveolar air-liquid interface co-culture model

Abstract Background Epidemiological studies have related desert dust events to increased respiratory morbidity and mortality. Although the Sahara is the largest source of desert dust, Saharan dust (SD) has been barely examined in toxicological studies. Here, we aimed to assess the NLRP3 inflammasome-caspase-1-pathway-dependent pro-inflammatory potency of SD in comparison to crystalline silica (DQ12 quartz) in an advanced air-liquid interface (ALI) co-culture model. Therefore, we exposed ALI co-cultures of alveolar epithelial A549 cells and macrophage-like differentiated THP-1 cells to 10, 21, and 31 µg/cm² SD and DQ12 for 24 h using a Vitrocell Cloud system. Additionally, we exposed ALI co-cultures containing caspase (CASP)1 −/− and NLRP3 −/− THP-1 cells to SD. Results Characterization of nebulized DQ12 and SD revealed that over 90% of agglomerates of both dusts were smaller than 2.5 μm. Characterization of the ALI co-culture model revealed that it produced surfactant protein C and that THP-1 cells remained viable at the ALI. Moreover, wild type, CASP1 −/−, and NLRP3 −/− THP-1 cells had comparable levels of the surface receptors cluster of differentiation 14 (CD14), toll-like receptor 2 (TLR2), and TLR4. Exposing ALI co-cultures to non-cytotoxic doses of DQ12 and SD did not induce oxidative stress marker gene expression. SD but not DQ12 upregulated gene expressions of interleukin 1 Beta (IL1B), IL6, and IL8 as well as releases of IL-1β, IL-6, IL-8, and tumor necrosis factor α (TNFα). Exposing wild type, CASP1 −/−, and NLRP3 −/− co-cultures to SD induced IL1B gene expression in all co-cultures whereas IL-1β release was only induced in wild type co-cultures. In CASP1 −/− and NLRP3 −/− co-cultures, IL-6, IL-8, and TNFα releases were also reduced. Conclusions Since surfactants can decrease the toxicity of poorly soluble particles, the higher potency of SD than DQ12 in this surfactant-producing ALI model emphasizes the importance of readily soluble SD components such as microbial compounds. The higher potency of SD than DQ12 also renders SD a potential alternative particulate positive control for studies addressing acute inflammatory effects. The high pro-inflammatory potency depending on NLRP3, CASP-1, and IL-1β suggests that SD causes acute lung injury which may explain desert dust event-related increased respiratory morbidity and mortality

Inhalable Saharan dust induces oxidative stress, NLRP3 inflammasome activation, and inflammatory cytokine release

Desert dust is increasingly recognized as a major air pollutant affecting respiratory health. Since desert dust exposure cannot be regulated, the hazardousness of its components must be understood to enable health risk mitigation strategies. Saharan dust (SD) comprises about half of the global desert dust and contains quartz, a toxic mineral dust that is known to cause severe lung diseases via oxidative stress and activation of the NLRP3 inflammasome-interleukin-1β pathway. We aimed to assess the physicochemical and microbial characteristics of SD responsible for toxic effects. Also, we studied the oxidative and pro-inflammatory potential of SD in alveolar epithelial cells and the activation of the NLRP3 inflammasome in macrophage-like cells in comparison to quartz dusts and synthetic amorphous silica (SAS).Characterization revealed that SD contained Fe, Al, trace metals, sulfate, diatomaceous earth, and endotoxin and had the capacity to generate hydroxyl radicals. We exposed A549 lung epithelial cells and wild-type and NLRP3-/- THP-1 macrophage-like cells to SD, three well-investigated quartz dusts, and SAS. SD induced oxidative stress in A549 cells after 24 h more potently than the quartz dusts. The quartz dusts and SAS upregulated interleukin 8 expression after 4 h and 24 h while SD only caused a transient upregulation. SD, the quartz dusts, and SAS induced interleukin-1β release from wild-type THP-1 cells>20-fold stronger than from NLRP3-/- THP-1 cells. Interleukin-1β release was lower for SD, in which microbial components including endotoxin were heat-destructed.In conclusion, microbial components in SD are pivotal for its toxicity. In the epithelium, the effects of SD contrasted with crystalline and amorphous silica in terms of potency and persistence. In macrophages, the strong involvement of the NLRP3 inflammasome emphasizes the acute and chronic health risks associated with desert dust exposure

Acute, sub-chronic and chronic exposures to TiO and Ag nanoparticles differentially affects neuronal function in vitro.

In vivo toxicokinetic studies provide evidence for the translocation and accumulation of nanoparticles (NP) in the brain, thereby causing concern for adverse health effects, particularly for effects following chronic exposure. To date, only few studies investigated the effects of NP exposure on neuronal function in vitro, primarily focusing on short-term effects. The aim of this study was therefore to investigate the effects of two common types of NP, titanium dioxide NP (TiO2NP) and silver NP (AgNP), on neuronal function following acute (0.5h), sub-chronic (24h and 48h) and chronic (14 days) exposure in vitro. Effects of NP exposure on intracellular calcium homeostasis, spontaneous neuronal (network) activity and neuronal network morphology were investigated in rat primary cortical cells using respectively, single-cell microscopy calcium imaging, micro-electrode array (MEA) recordings and immunohistochemistry. Our data demonstrate that high doses of AgNP (≥ 30µg/mL) decrease calcium influx after 24h exposure, although neuronal activity is not affected following acute and sub-chronic exposure. However, chronic exposure to non-cytotoxic doses of AgNP (1-10µg/mL) potently decreases spontaneous neuronal (network) activity, without affecting network morphology and viability. Exposure to higher doses (≥ 30µg/mL) affects network morphology and is also associated with cytotoxicity. In contrast, acute and sub-chronic exposure to TiO2NP is without effects, whereas chronic exposure only modestly reduces neuronal function without affecting morphology. Our combined findings indicate that TiO2NP exposure is of limited hazard for neuronal function whereas AgNP, in particularly during chronic exposure, has profound effects on neuronal (network) function and morphology

Effects of subchronic dietary exposure to the engineered nanomaterials SiO and CeO in C57BL/6J and 5xFAD Alzheimer model mice.

There is an increasing concern about the neurotoxicity of engineered nanomaterials (NMs). To investigate the effects of subchronic oral exposures to SiO2 and CeO2 NMs on Alzheimer's disease (AD)-like pathology, 5xFAD transgenic mice and their C57BL/6J littermates were fed ad libitum for 3 or 14 weeks with control food pellets, or pellets dosed with these respective NMs at 0.1% or 1% (w/w). Behaviour effects were evaluated by X-maze, string suspension, balance beam and open field tests. Brains were analysed for plaque load, beta-amyloid peptide levels, markers of oxidative stress and neuroinflammation

Evaluation of the neurotoxic effects of engineered nanomaterials in C57BL/6J mice in 28-day oral exposure studies.

In recent years, concerns have emerged about the potential neurotoxic effects of engineered nanomaterials (NMs). Titanium dioxide and silver are among the most widely used types of metallic NMs. We have investigated the effects of these NMs on behaviour and neuropathology in male and female C57BL/6J mice following 28-day oral exposure with or without a 14-day post-exposure recovery. The mice were fed ad libitum with food pellets dosed with 10 mg/g TiO2, 2 mg/g polyvinylpyrrolidone-coated Ag or control pellets. Behaviour was evaluated by X-maze, open field, string suspension and rotarod tests. Histological alterations were analysed by immunohistochemistry and brain tissue homogenates were investigated for markers of oxidative stress, inflammation and blood-brain barrier disruption. Effects of the NMs on tyrosine and serine/threonine protein kinase activity in mouse brains were investigated by measuring kinase activity on peptide microarrays. Markers of inflammation, oxidative stress and blood-brain barrier integrity were not significantly affected in the male and female mice following exposure to Ag or TiO2. Both types of NMs also revealed no consistent significant treatment-related effects on anxiety and cognition. However, in the Ag NM exposed mice altered motor performance effects were observed by the rotarod test that differed between sexes. At 1-week post-exposure, a diminished performance in this test was observed exclusively in the female animals. Cortex tissues of female mice also showed a pronounced increase in tyrosine kinase activity following 28 days oral exposure to Ag NM. A subsequent Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) based toxicokinetic study in female mice revealed a rapid and persistent accumulation of Ag in various internal organs including liver, kidney, spleen and the brain up to 4 weeks post-exposure. In conclusion, our study demonstrated that subacute exposure to foodborne TiO2 and Ag NMs does not cause substantial neuropathological changes in mice. However, the toxicokinetic and specific toxicodynamic findings indicate that long-term exposures to Ag NM can cause neurotoxicity, possibly in a sex-dependent manner

Airborne engineered nanomaterials in the workplace: a review of release and worker exposure during nanomaterial production and handling processes

For exposure and risk assessment in occupational settings involving engineered nanomaterials (ENMs), it is important to understand the mechanisms of release and how they are influenced by the ENM, the matrix material, and process characteristics. This review summarizes studies providing ENM release information in occupational settings, during different industrial activities and using various nanomaterials. It also assesses the contextual information - such as the amounts of materials handled, protective measures, and measurement strategies - to understand which release scenarios can result in exposure. High-energy processes such as synthesis, spraying, and machining were associated with the release of large numbers of predominantly small-sized particles. Low-energy processes, including laboratory handling, cleaning, and industrial bagging activities, usually resulted in slight or moderate releases of relatively large agglomerates. The present analysis suggests that process-based release potential can be ranked, thus helping to prioritize release assessments, which is useful for tiered exposure assessment approaches and for guiding the implementation of workplace safety strategies. The contextual information provided in the literature was often insufficient to directly link release to exposure. The studies that did allow an analysis suggested that significant worker exposure might mainly occur when engineering safeguards and personal protection strategies were not carried out as recommended