1,721,091 research outputs found
Ontwikkeling van een methode voor de bepaling van het oplosbaarheidsgebied van intermetallische fasen in metaal-metaal verbindingen
No full textMy PhD research is focused on hetero-junctions of dissimilar materials, for example, the integrated and composite material structures used in micro-electronic and photovoltaic devices.The growth of intermetallic compound (IMC) layers and precipitates at and near the interfaces is the main topic to study their influence on the reliability and life time of these devices.
Research Activities:
In the frame work of my PhD research, a combined modeling approach for the growth of IMC phases will be derived and implemented based on the phase-field method for microstructure evolution simulations and the CALPHAD method for the prediction of phase equilibria and thermodynamic properties in multi-component systems. The new simulation method will be used, integrated with experimental research, to study the growth of IMC layers and precipitates and predict diffusion paths in multi-component hetero-junctions of materials with 3 and 4 components.
The major activities are listed as follows:
more extensive study of the existing phase-field models for alloys and CALPHAD approaches for Gibbs energies and diffusion mobilities
implementation and validation of coupled phase-field/CALPHAD approaches for intermediate phases with low solubility
application of the coupled phase-field/CALPHAD approach on microstructure evolution and interdiffusion in a ternary system
validation of of the coupled phase-field/CALPHAD approach on microstructure evolution and interdiffusion in a quaternary systemstatus: Publishe
Ontwikkeling van een elastoplastisch faseveldmodel voor meerfasigesystemen
No full textThe microstructure of a material largely determines its properties at the macroscale. Tailoring the microstructure to obtain desirable properties, and thereby, high performance of a material, is a challenge faced by the manufacturing sector. Increasingly, in order to avoid and reduce time-consuming and expensive experiments, computer simulations are being used to predict the microstructure of a material under the set of conditions the material is exposed to during processing or service. Phase-field modelling is one such tool that is used to simulate the evolution of the microstructure and other characteristics such as composition, strain, and stress in a material at the micron and sub-micron length scales. This is a powerful tool for simulating coupled mechanical and chemical behaviour, which is of utmost importance in solid-state multi-phase materials such as solder alloys, turbine materials, and steels, wherein the presence of strain affects phase transformations. In this work, we aimed to identify or develop a quantitative phase-field model for multi-phase systems that considers elastic and plastic deformation. To this end, we first evaluated the existing schemes for including elastic effects and found that the properties calculated using them deviated from the analytical solution. This was due to unphysical excess energy generated at the interfaces, which was an artefact of the models due to interfacial conditions that were different from local equilibrium conditions. Therefore, we developed a new scheme that takes into account local chemical and mechanical equilibrium conditions at the interface. This scheme was then validated for different configurations in 2D and 3D showing good agreement with analytical solutions. We then applied this model to the microstructure evolution in Sn-Cu/Cu solder joint, wherein the formation of brittle intermetallic phases affects the reliability of the solder joint. In order to simulate such a system using experimentally measured or theoretically estimated physical and chemical parameters from literature, the new elastic scheme developed in this work was coupled with a scheme to include plastic deformation and extended to multi-phase systems. Moreover, ab initio calculations of stiffness tensors of Cu and Cu3Sn were also performed because of large variations among existing values in literature. Simulations of intermetallic growth in the solder joint showed that strains play a significant role in the growth kinetics and composition evolution. The development of the new phase-field model in this work drives forward the growing interest and research in performing predictive simulations of microstructure evolution in complex multi-phase industrial alloys where strain plays a significant role in phase transformations.status: Publishe
Methodology Development and Experimental Determination of the Origin of Sticking Copper Droplets in Pyrometallurgical Slags
No full textCopper is one of the first metals ever extracted and used by mankind. Due to its excellent properties, such as formability, corrosion resistance, thermal and electric conductivity, copper has been an important metal throughout history and is still used in a wide variety of applications. Consequently, the copper demand remains high and is expected to continuously increase in the future. Therefore, the urge for further process optimization remains relevant for both primary and secondary copper production. Within copper producing industry, the process efficiency is to a large extent related with copper losses throughout the production process. Copper is mainly produced via pyrometallurgical processes, where sedimentation is frequently applied to obtain a phase separation between different pyrometallurgical phases (slag, matte and metal). During sedimentation, phase separation is driven by the difference in density, but is often suboptimal, leading to copper losses in the slag. Due to its economic importance, this phenomenon remains an important issue in copper production and a lot of research is already performed on this topic. Currently, it is generally accepted that there are two types of losses: chemical and mechanical losses. Chemical losses refer to copper which is dissolved in the slag in its oxidic or sulfidic form. Mechanical losses refer to entrained copper droplets in the slag which did not settle into the underlying matte or metal phase. Four main causes are frequently mentioned in literature explaining the presence of these entrained droplets:
I. Charging or tapping operations
II. Gas-producing reactions that disperse the metal into the slag
III. Precipitation of copper due to a local decrease in copper solubility in the slag
IV. The attachment of metal droplets to spinel solids in slags
The first three reasons have been extensively studied in literature. Concerning the last reason, although it has been clearly observed, limited experimental and industrial data or fundamental knowledge is available. In order to further increase our knowledge and to improve the copper recovery efficiency, it would be a significant step forward to acquire fundamental understanding concerning this phenomenon. This lack of knowledge determined the basis for this PhD-research as the goal is indeed to gain fundamental insights, knowledge and understanding concerning the phenomenon of sticking copper droplets to spinel solids in pyrometallurgical slags.
At first, a suitable methodology had to be developed to tackle this problem. A wide variety of experimental approaches have been described to study metal losses in slags in general. However, no method has focused on the phenomenon of sticking droplets. Therefore, a suitable methodology was developed starting from two complementary approaches.
First, the spinel-Cu-slag interactions were studied using the sessile drop method for copper-spinel, slag-spinel and slag-copper-spinel combinations, respectively. For this purpose, MgAl2O4 and ZnFe2O4 substrates, representing the spinel solids in industrial slags, were produced using powder based methodologies. Attempts were made to produce Fe3O4, but it appeared to be experimentally very difficult to keep this spinel structure stable throughout the substrate production, which would therefore also be a problem during the sessile drop experiment. Pure copper and copper-silver alloys (5, 12.5 and 30 wt% Ag) were selected to represent the sticking droplets and an industrial relevant synthetic (PbO-CaO-SiO2-Cu2O-Al2O3-FeO-ZnO) slag was chosen with a composition in the spinel primary phase field. To perform the sessile drop experiment, two methodologies were used: an adapted heating unit of the confocal scanning laser microscope and a set-up built around a horizontal tubular furnace.
Secondly, two experimental set-ups were developed to study the attachment of copper droplets in the slag. For these experiments, the synthetic slag was used as well. One set-up was specifically developed to study the effect of the sedimentation time, while the second set-up allowed examining the sticking droplets gradient over the slag height.
After the methodology development, the set-ups were used to study the spinel-copper-slag interactions. At first, the copper-spinel and slag-spinel interactions were evaluated with the sessile drop set-up. The copper-spinel interaction was examined by different sets of sessile drop experiments in which the wetting behaviour between Cu/Cu-Ag alloys and MgAl2O4 substrates was studied for an oxygen partial pressure range between 10-13 and 10-8 atm. For all alloys, a non-wetting behaviour was observed, with a maximal contact angle for the Cu-12.5 wt% Ag alloy. It was also observed that pure copper displays an improved wetting for an oxygen partial pressure of 10-8 atm. A less explicit effect of the oxygen partial pressure was observed for the Cu-Ag alloys, indicating a combined influence of the oxygen partial pressure and the alloy composition on the wetting behaviour. Subsequently, the effect of the composition of the spinel substrate was studied. Therefore, the wetting of pure copper on ZnFe2O4 and MgAl2O4 was evaluated using sessile drop experiments with different interaction times under a protective Ar-atmosphere. A low wettability of the copper was observed for both types of spinel substrates, but the contact angle for ZnFe2O4 (88°) was smaller than for MgAl2O4 (123°). The interaction time had no significant influence on the wetting behaviour.
The spinel-slag interaction was examined between MgAl2O4 and the synthetic slag. In contrast to copper, slag displayed a reactive wetting with the formation of an (Mg,Zn,Fe)(Al,Fe)2O4 interaction layer at the MgAl2O4-slag interface. More focus was put on the origin of the interaction layer and its evolution as a function of the interaction time. Therefore, MgAl2O4 substrates were immersed for different times in the synthetic slag (10 s, 30 s, 1 min and 3 min). Already after 10 s an interaction layer could be observed, indicating the fast nucleation and formation kinetics of this layer. Within the first minute, a significant increase in thickness of the interaction layer was observed. Subsequently, the formation speed slowed down, probably due to a local depletion of the spinel forming elements in the slag.
Additionally, an adapted sessile drop experiment was performed to study the copper-slag-spinel interaction, in which both copper and slag were molten on a MgAl2O4 substrate. The slag positioned itself in between the copper droplet and the spinel substrate. Similar to the slag- MgAl2O4 experiment, a (Mg,Zn,Fe)(Al,Fe)2O4 interaction layer was formed at the slag-spinel interface. The positioning of the slag allowed no direct interaction between the copper droplet and the MgAl2O4 substrate. At the slag-copper droplet interface, an interaction layer of copper oxide was present and copper dissolution into the slag was observed. However, the presence of small copper droplets in the slag sticking to spinel solids was also observed.
In a next step, the phenomenon of sticking droplets was studied in the synthetic slag system using the two set-ups developed in the methodology section. In both experiments, a significant part of the present mechanically entrained copper droplets was attached to spinel solids. A clear decrease of the amount of mechanical entrained droplets was noticed in the upper slag layer as a function of time, together with a variation of the amount of mechanically entrained droplets as a function of slag height. Similar observations could be made for the spinel solids present in the slag.
Within the slag system, two different types of spinel phases were present: an Al-rich spinel and an Fe-rich spinel. Throughout all experiments, it was observed that copper droplets were only attached to Fe-rich spinel solids. Furthermore, it was also observed some copper droplets were completely surrounded by spinel solids present in the slag system.
It has been noted throughout the sessile drop experiments that slag exhibits a better wetting than copper on spinel substrates. The origin of the phenomenon of the sticking copper droplets requires consequently that an energy barrier has to be overcome to break up a slag layer surrounding the spinel solid. Two different ways are suggested to explain the origin of the copper droplets sticking to spinel solids. Firstly, extensive stirring could be sufficient to remove the slag layer and induce the attachment. Secondly, it is suggested that the sticking droplets find their origin in a chemical reaction, occurring when the system evolves towards a thermodynamically more stable state. While the first assumption seems to be less likely as sticking droplets are also observed in experiments without any stirring, the second option was explored in more detail. A reaction scheme was proposed, including two possible pathways where the spinel solids form together with the copper droplets or form around the copper droplets depending on the local conditions of the system. In both pathways, redox reactions involving copper (oxides) are essential and within the applied synthetic system, Fe and Fe-oxides have a crucial role. The proposed mechanism can explain the presence of droplets sticking to spinel particles in the copper-slag-MgAl2O4 sessile drop experiments and the presence of copper droplets fully surrounded by spinel substrates.
Based on the observed interactions between the slag and copper droplet on the spinel substrates, the copper-spinel interaction seems not stable in the presence of a slag. This is in contradiction with the observed presence of small entrained sticking droplets in the slag. On the one hand, it is possible that the determining interfacial tensions in the sessile-drop set-up, where the experiment is carried out in an argon atmosphere, are different from those of sticking droplets which are fully surrounded by a synthetic slag. On the other hand, it is also possible that, if the sticking droplets originate from a chemical reaction, a reactive wetting behaviour occurs as long as the reactions takes place. This confirms the explanation that a chemical reaction mechanism might induce the phenomenon of sticking droplets.
Finally, the sedimentation behaviour of the droplet/spinel entity is evaluated. A model is developed to calculate the sedimentation rates based on an adapted form of the Stokes’ equation. Although the model is based on a number of simplifying assumptions, interesting insights are gained concerning the influence of the sticking interaction between spinel solids and copper droplets on the settling behaviour. It is noted that the attachment of the copper droplets to spinel particles hinders the complete sedimentation, since the average density of the ‘copper droplet – spinel’ entity is lower than that of the underlying copper. Additionally the presence of attached spinel particles can hinder the coagulation of smaller droplets, which also hinders the settling rate.status: Publishe
Isothermal Crystallization of Metallurgical Slags: Phase Field Simulations Combined with In Situ Experiments
No full textCrystallization of metallurgical slags occurs in many industrial pyrometallurgical processes. Important examples are the application of freeze linings, refractory degradation and the usability of the solidified slag. To extend the fundamental knowledge of these phenomena, this research focuses on the simulation and in situ observation of crystallizing minerals in oxide melts. Modeling of slag crystallization can bring valuable insight into phenomena that are difficult to measure or observe, such as solute diffusion and growth kinetics of minerals in liquid slags. In this work, the phase field technique is used because it has proven its capabilities for microstructure evolution in metals, such as solidification, coarsening and solid-state transformations. A state-of-the-art multi-phase and multi-component model is extended to simulate crystallization of oxide melts. Therefore, the model is linked with thermodynamic databases for oxides, the phase field mobility is made dependent on the interface orientation to allow faceted crystal growth, and a new boundary condition is developed to treat redox dependent behavior of multivalent cations when the melt is in contact with an oxygen-containing atmosphere. The phase field model is applied to the isothermal dendritic crystallization of Wollastonite (CaO.SiO2) in a ternary CaO-Al2O3-SiO2 melt, and the crystallization of Magnetite spinel (FeO.Fe2O3) in a FeO-Fe2O3-SiO2 melt, under an oxidizing atmosphere. Besides the simulations, the crystallization behavior of Wollastonite in a CaO-Al2O3-SiO2 melt is also observed experimentally, using a confocal scanning laser microscope (CSLM). Relating the simulation to experimental results allows an estimation of the solid-liquid interfacial energy of Wollastonite in a CaO-Al2O3-SiO2 melt, which is difficult to measure directly. The developed model is also used to answer a longstanding issue regarding ternary two-phase diffusion couples, in which multiple analytical solutions exist. Using the phase field model, this work shows that the diffusion path depends on the Gibbs energies of both phases as well as their interfacial energy.status: Publishe
Metal droplet entrainment by solid particles in slags : a combined phase field-experimental approach
Full text linkThis doctoral work investigated metal droplet entrainment by solid particles in slags with a combination of two experimental set-ups and two phase field models. The binary model with limited complexity already clarified our view of the interaction between metal droplets and nonreacting solid particles to a great extent. For example, the fact that the movement of one phase with respect to the others influenced the apparent wetting regime is very interesting for the interpretation of experimentally obtained results. Moreover, the two different types of experiments confirmed that a chemical reaction might lay at the origin of the attachment, but that it requires nucleation sites in the form of metal droplets before it takes place. However, the first phase field model assumed nonreactive solid particles. Thus, a model concerning the growth of the solid phase in a realistic quaternary oxide system was also considered. Future work needs to consider the interaction of reacting metal droplets with reacting solid particles in a realistic liquid slag
Thermodynamic and Kinetic Description of Nanowires and Nanowires Growth
No full textInitially discovered in the 60s by Wagner and Ellis, semiconducting wires have recently attracted a lot of attention. State of the art manufacturing techniques allow for the fabrication of nanometric semiconducting nanowires exhibiting physical properties very different from their bulk counterparts. Furthermore, many applications are envisioned for these objects, especially in the manufacturing of microelectronic devices in order to replace conventional transistors. Several methods have been developed to obtain nanowires: one of them is the vapour phase deposition method, following Wagner and Elliss original experiment. It is generally accepted that, in such a setup, growth proceeds through the Vapour-Liquid-Solid mechanism. The quantitative details of the processes and the mechanisms relating the growth conditions to the nanowire properties are however still difficult to characterize and control. The purpose of the present work is to gain insight in the thermodynamic properties of nanowires and in the kinetics of nanowire growth in vapour phase deposition setups. Due to the very high surface-to-volume ratio of nanowires, surface and interface properties have to be modelled and added to bulk thermodynamic descriptions. Bulk thermodynamic data is extracted from thermodynamic databases developed in the frame of the CALPHAD method. Butlers equation is used to model the surface enegy of alloys. The models allowed us to calculate phase diagrams of nanosized alloy systems and to predict phase equilibria playing an important role in the Vapour-Liquid-Solid growth of nanowires. In the kinetic modelling, the growth rate of nanowires is calculated considering adsorption and decomposition of the gaseous precursor and adatom surface diffusion as rate limiting steps. The main vapour phase deposition setups, Molecular Beam Epitaxy and Chemical Vapour Deposition, and the relative importance of the rate limiting steps depending on the growth conditions can be studied with this model.status: Publishe
Verwerkende veranderingen in mechanische spanningen in en in de buurt van Cu Through Silicon Vias (TSV's): een eindige-elementen-modelleringsstudie,,
No full textThe 3D technology, in integrated circuit applications, refers to the stacking of chips on top of each other, in order to reduce the interconnect lengths and the chip size. Through silicon vias (TSVs) filled with Cu are a key part of this technology, especially for the 3D system in package (3DSIP) devices, enabling the vertical interconnection of stacked dies. During fabrication and subsequent processing steps, the TSV undergoes several high temperature profiles which lead to thermal expansion of the Cu from a TSV due to the coefficient of thermal expansion mismatch between Cu and Si. The out-of-plane protrusion of the TSV, termed Cu pumping, which can affect the integrity and reliability of the Back-End-of-Line (BEOL) structures on top of the TSV during subsequent fabrication processes. The in-plane expansion also creates stresses in the surrounding silicon.
In this thesis, first by applying analytical methods, the mechanism of the TSV Cu pumping is studied. It is attributed to plastic deformation of Cu, due to the high temperature that the TSV is subjected to. Next, using the finite element method, Cu pumping is modeled by considering that Cu is protruding due to the fabrication processes, which include post-plating anneal, chemical mechanical polishing and BEOL deposition steps. In these models, the optimum temperature during the post-plating anneal, to better reduce the Cu pumping, has been studied. The impact of parameters such as the TSV diameter and the thickness of the dielectric liner and the diffusion barrier on stresses and Cu pumping has also been thoroughly investigated and the results are compared with the experimental data. It has been shown that, using the modeling, experimental data can be explained and a dielectric liner densification hypothesis can be verified. The impact of Cu pumping on BEOL breakdown and stress induced voiding has also been studied.
Lastly, the distribution of the Cu pumping (between different TSVs with same dimension) values observed in experimental measurements, is modeled. This phenomenon is widely attributed to the variation in the crystal structure of the Cu in the TSV. In this thesis, using phase-field modeling, the micro structure evolution of Cu in a TSV is modeled and by coupling this with an anisotropic FEM model of the TSV, the impact of grain size and its orientation distribution on Cu pumping has been analyzed. The simulations have resulted in a similar Cu pumping distribution to the ones observed experimentally. Thus using the calculated results, the source of the Cu pumping distribution is explained based on the grain sizes and orientations.status: Publishe
Vorming van intermetallische componenten in verkleinde soldeerverbindingen gebruikt voor 3D silicium-op-silicium stapeling
No full textThe third dimension, or vertical dimension, of integrated circuits, attracted more interest in the recent years since it allows achieving device density multiplication by stacking IC layers in the third dimension. To increase the functional density and obtain higher computing performance, the interconnects used in 3D integration, such as TSVs and microbumps, must be reduced in size in a reliable way. The decrease of the interconnect size and increase of the interconnect density can bring new challenges for process integration and reliability. This dissertation is therefore mainly focusing on the process development and reliability evaluation for miniaturized interconnected solder joints. The dissertation starts with a detailed introduction of 3D integration as well as an overview of different state of the art bonding/assembly approaches. Following that, different bonding/assembly approaches are compared, and their advantages and drawbacks are discussed. The motivation and challenges for the development of miniaturized solder joints are also discussed.
The microstructure of the microbumps during the product-service stage is not in the thermodynamic equilibrium state after the bonding process. At evaluated temperatures and with current flow, a new intermetallic compound (IMC) phase will replace the initial UBM and solder. This phase change affects the overall performance, including the thermal, electrical and mechanical properties, of a solder joint and can lead to failures.
With the demand for miniaturization in the modern electronics industry, the understanding of the interactions between the materials involved in the solder joint is extremely critical for reliability. In order to predict the phase transformations and the growth rate of the intermetallic materials, the electrical and kinetics parameters for different metallurgy systems, including Co/Sn, Ni/Sn, Cu/Ni/Sn and Ni/Cu/Sn, were extracted using a novel methodology, i.e. in-situ resistance measurement. This gives insight into the interfacial reaction. Based on these measurements, a recommendation of the best metallurgical system for miniaturized interconnects is given.
Besides the intermetallic growth kinetics for alternative metallurgical systems, there were also other reliability concerns related to continuing downsizing the solder joint. One of the most important ones is the impact of process variation, affecting the Sn grain size inside the solder joint. The results of a phase-field simulation study of the effect of Sn grain size on IMC morphology and solid-state interfacial reaction are discussed in this thesis. The simulation results give insight into the interfacial reaction for different metallurgical systems.
Another large challenge for miniaturized solder-based stacking is the high risk of bridging issue between the neighboring solders due to the downsizing of interconnects pitch size. Therefore, a novel low-temperature bonding strategy to avoid the solder bridging issue for fine pitch solder joints is proposed and developed. In addition, results from first reliability tests on the micro-bump structures with the recommended metallurgical system are presented and discussed.
Finally, conclusions and perspectives for further development of the fine pitch solder joint are presented.status: Publishe
Calculation of phase diagrams for lead-free solder alloys based on Bi-In-Sn-Zn
No full textstatus: Publishe
Phase-Field Simulations of Grain Growth in Materials Containing Second-Phase Particles
Full text linkPrecipitaten en inclusies hebben de eigenschap korrelgrenzen te pinnen. Wanneer een kritische korrelgrootte bereikt is, verhinderen ze verdere korrelgroei. Dit pinning-effect is van groot praktisch belang in legeringontwikkeling omdat de macroscopische eigenschappen van een legering bepaald worden door haar microstructuur.In dit werk werd het pinning-effect van tweede-fase-deeltjes bestudeerd aan de hand van computersimulaties gebaseerd op de faseveldmethode (E: phase-field method). Een bestaand faseveldmodel voor normale korrelgroei in eenfasige materialen werd uitgebreid naar materialen met tweede-fase-deeltjes. Om de benodigde rekenkracht te beperken, werden de tweede-fase-deeltjes in rekening gebracht met behulp van een plaatsafhankelijke parameter die constant is in de tijd, in plaats van met een set van faseveldveranderlijken. Dankzij deze vermindering van het aantal faseveldveranderlijken, was het mogelijk, op één enkele computer, 2D en 3D simulaties voor dunne filmen uit te voeren, voor een groot bereik van volumefracties fV van de tweede-fase-deeltjes en voor Rlim/r verhoudingen tot 20. Zulke verhoudingen worden waargenomen in gelegeerde Al-filmen (r is de gemiddelde straal van de deeltjes en Rlim is de limietwaarde van de gemiddelde straal van de korrels). Het model vereist geen veronderstellingen over de vorm van de korrelgrenzen of over het aantal deeltjes dat in contact is met een korrelgrens. De typische dimpelvorm die korrelgrenzen aannemen wanneer ze een deeltje voorbijgaan wordt automatisch weergegeven in de simulaties. De simulaties beschrijven bovendien de volledige evolutie van de korrelstructuur. Hierdoor was het mogelijk de invloed van de initiële korrelgrootte op de parameters K and b in de zenerrelatie Rlim/r=K(1/fVb) te bestuderen. Het effect van de filmdikte en de positie van de deeltjes in de film werd eveneens geanalyseerd aan de hand van 3D simulaties.status: Publishe
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