Time-resolved phase-contrast microscopy is employed to visualize spatio-temporal thermal transients induced by tight focusing of a single Ti:sapphire fs-laser pulse into a solid dielectric sample. This method relies on the coupling of the refractive index change and the sample temperature through the thermo-optic coefficient dn/dT. The thermal transients are studied on a timescale ranging from 10 ns up to 0.1 ms after laser excitation. Beyond providing direct insights into the laser–matter interaction, analyzing the results obtained also enables quantifying the local thermal diffusivity of the sample on a micrometer scale. Studies conducted in different solid dielectrics, namely amorphous fused silica (a-SiO2), a commercial borosilicate glass (BO33, Schott), and a custom alkaline earth silicate glass (NaSi66), illustrate the applicability of this approach to the investigation of various glassy materials.

Structural relaxation phenomena in binary and multicomponent lithium silicate glasses were studied upon irradiation with femtosecond (fs) laser pulses (800 nm central wavelength, 130 fs pulse duration) and subsequent thermal annealing experiments. Depending on the annealing temperature, microRaman spectroscopy analyses evidenced different relaxation behaviours, associated to bridging and non-bridging oxygen structures present in the glass network. The results indicate that the mobility of lithium ions is an important factor during the glass modification with fs-laser pulses. Quantitative phase contrast imaging (spatial light interference microscopy) revealed that these fs-laser induced structural modifications are closely related to local changes in the refractive index of the material. The results establish a promising strategy for tailoring fs-laser sensitivity of glasses through structural mobility.

The aim of this study is to get an insight into the silver film sinter kinetics and the interface formation on aluminium nitride (AlN). Therefore, systematic variations in the paste recipe where carried out and the resulting film shrinkage, blister behaviour, solderability and adhesion strength where correlated with the sintering kinetics and the microstructure of the films. The influence of the glass and bismuth(III)-oxide volume fractions on the glass-viscosity, sintering behaviour of silver pastes as well as the interfacial reactions with aluminium nitride were investigated by means of thermomechanical analysis and FESEM-cross sections of fired films. A significant role in this process play the inorganic components bismuth(III)-oxide and glass. This is because of the ability of bismuth(III)-oxide to disslove into the glass matrix, lowering the viscosity of the glass phase. Also the surface of the film changes in accordance to the glass - bismuth(III)-oxide ratio due to the changed softening behaviour and this is critical for the solderability.

Highly concentrated ceramic polymer suspensions (thick film pastes) are used in many fields of hybrid technology and micro systems packaging. Thereby, in dependence of the used printing technologies like screen printing or micro-dispensing, different requirements are set on the ceramic suspensions. For example, to reach similar structural resolutions for both technologies the suspensions can differ strongly in their solid content, their organic polymer suspension and, above all, in their viscosity. This results in the fact that it is enormously important for paste development to study the requirements and for successful deposition to adjust them by means of targeted variation of the paste components. These components are for a typical ceramic suspension: a solvent, a polymer and inorganic fillers. Such complex fluids exhibit a broad spectrum of rheological features attributable to the various components and making effective controls over their printing properties a formidably challenging task. However, understanding the rheological properties is crucial for setting an optimal high-precision printing. Previous works have tried to correlate rheological properties of commercial thick film pastes [1,2] to screen printing behavior, but these results didn't allow correlations to the particular components and no specific adjustments for the formulation of suspensions are possible. This study will show how rheological parameters can be adjusted by specific paste composition variations in order to adjust the deposition behavior according to the printing technology. Results of the pastes development, in which the particle type, shape, size, but also the polymer-solvent system were systematically varied, are shown in order to deposit high-resolution structural functional layers. Both shear thinning and thixotropic as well as viscoelastic properties of the paste systems are considered and correlated with their effects on printing performance.

The additive production of sensors on massive mechanical components for structural health monitoring (e.g., temperature, strain, and body-sound) using printing and laser technology shows great potential since the sensor structures are applied directly onto the component's surface without using adhesives or leads. The required multilayer stack of different materials can be added consecutively to build up the sensor layer by layer. Digital additive production techniques like printing and laser-induced heat treatment processes enable the manufacturing of highly individualized sensor structures down to batch size one. In this work, the principle of laser treatment of printed layers for temperature and strain sensors (Fig. 1) is introduced and first results are discussed. For the manufacturing process of insulator (glass) and conductor/resistor (silver-based) layers, microparticulate materials are deposited onto steel substrates and laser sintered. For better chemical and mechanical adhesion, the substrate surfaces are also pretreated (oxidized and roughened) using laser radiation. Combining the additive, digital and inline-capable printing, and laser sintering methods, functional layers for sensor applications on massive steel components can be realized.

The ceramic thick film technology allows the buildup of miniaturized and robust integrated multilayer circuits and sensors by means of sequential screen printing and firing of different functional materials. However, the manufacturing of integrated electronics does not succeed if the components are temperature sensitive or too large for the process in a sintering furnace. At present, large components like wind power rotors, axles, or roller bearings are monitored by vulnerable hybrid sensor systems. To implement the advantages of integrated devices, such as the direct surface contact and the high thermomechanical stability, functional ceramic-based materials are adapted or newly developed to accommodate the requirements of laser sintering techniques of printed sensor layers on structural components. In a first approach, first screen-printed thick films on steel components are investigated. The defect-free densification of functional layers crucially depends on the particular material composition and adapted laser treatment. A first generation of functional layers is presented, comprising insulating, conductive, and resistive electrical materials. The films are tested in demonstrator setups and show functional properties comparable with those of the furnace sintering technology. Herein, future aspects of material optimization and the adaption to specific application requirements are discussed.

Ziel des Forschungsprojektes ist die Entwicklung von langzeitstabilen Dickschichtwiderständen mit minimiertem Temperaturkoeffizienten des Widerstandes (TKR) für Spannungssensoren, die bei höchsten Innenwiderständen präzise Spannungsmessungen in Energieübertragungs- und Verteilungsnetzwerken zuverlässig ermöglichen. Die Präzision heutiger Systeme genügt den zukünftigen Anforderungen nicht. Für die nächste Generation der Dickschichtwiderstände für Spannungssensoren ist ein extrem kleiner TKR von 0 ± 3 ppm/K notwendig. Einen vielversprechenden Lösungsansatz für die Herstellung von robusten, intrinsisch temperaturkompensierten und kosteneffizienten Spannungssensoren bietet die Dickschichttechnologie. Dabei wird ein keramischer Funktionswerkstoff in pastöse Form überführt, bevorzugt auf keramische, isolierende Trägermaterialien mittels Siebdruck aufgebracht und anschließend gesintert. Neben der hohen Korrosionsbeständigkeit und Spannungsfestigkeit der Dickschichtwiderstände ermöglicht das Trimmen einen extrem genauen Abgleich des Nennwiderstandswertes. Es wurden Glas-RuO2- basierte Dickschichtwiderstände für Al2O3 Substrate entwickelt, welche einen niedrigen TKR aufweisen. Die Änderungen des Flächenwiderstandes und des TKR der gesinterten Schichtwiderstände sind in Abhängigkeit von der Pulverqualität des als leitfähige Phase verwendeten Oxids RuO2 sowie der Zusammensetzung des eingesetzten Glases untersucht worden. Des Weiteren wurde die Wirkung des Zusatzes von Additiven als TKR-Modifikatoren geprüft.

Using an in-situ contact resistance measurement silver pastes containing glass and aluminum additions for p- and n-type cells were investigated during rapid thermal process (RTP) firing. The glass viscosity, i.e. the formation of a liquid phase having a viscosity between 104 Pa·s and 105 Pa·s, is the crucial point for opening of the antireflection coating, regardless of the kind of wafer or aluminum addition. The maximum efficiency is obtained in an optimum between the competing effects silver precipitation and emitter corrosion and can be precisely time-resolved observed by an in-situ contact resistance measurement. The peak firing temperature determines the intensity of the interface reaction whereas the temperature related glass viscosity is crucial important for the reaction duration and acts as kinetic switch. In comparison to common pastes for p-type cells, in pastes with alumina additions for n-type cells the glass phase rearranges and depletes at the interface due to thermodynamic reasons. The rearrangement of the glass phase attenuates the reduction of silver in the interface layer and controls the solar cell contacting.

Harvesting green energy and sustainable, effective use of it is based on efficient, cost-effective power electronics. To build robust, multi-functional and highly integrated electronic modules, metallizations with low resistances and the capability to carry high current loads for inner and top layer thick-films are needed. Additionally, these layers have to be interconnectable by metallized vias, soldering and bonding. A major challenge in realization of bulky silver and copper thick-films is to allow sufficient evaporation of the gaseous products during the interface reaction. Various ways to attain evaporation possibilities via controlled shrinkage retardation of the thick metal film are examined. The influence of composition of the used solids, their grain size distributions and the use of miscellenaous organic vehicles were examined in several research projects by variation of glass amount, particle size, shrinkage measurements as well as microstructure and film properties. Through partial retardation, a thick printing copper paste for 300 μm and a pure silver paste for 100 μm metallizations could be realized. By use of suited additives, the film shrinkage could be almost completely constrained which allowed the development of non-shrinking via-fill pastes.

A silicon heterojunction solar cell based on amorphous and crystalline silicon is combined with the metal wrap through technology. In this novel solar cell concept one critical process is the via hole conditioning. Raman measurements reveal that the amorphous silicon emitter layer hardly penetrates the via holes and that thereby the via surface is not fully covered. In the conventional process sequence with via hole formation prior to wet chemical cleaning, the effective carrier lifetime is reduced by about 50 % in the vicinity of the via hole. An improved process sequence is presented, which bases on via hole formation after the thin film depositions. In this sequence, the via hole formation process is crucial for the via surface passivation. The passivation remains poor when applying a 1064 nm laser process. However, very good surface passivation is achieved with a 532 nm laser process. The lifetime reduction was below 20 % at the via hole. The superior performance of the 532 nm laser process is correlated to a smoother via surface and enhanced via sidewall oxidation. Finally, large area SHJ-MWT solar cells based on the optimized via formation process are processed and analyzed. (C) 2015 The Authors. Published by Elsevier Ltd.

Ink jet printable water based inks are prepared by a new silver nanoparticle synthesis and the addition of nanoscaled ZnO particles. For the formation of front side contacts the inks are ink jet printed on the front side of micro crystalline silicon solar cells, and contact the cell directly during the firing step by etching through the wafers' anti-reflection coating (ARC). In terms of Ag dissolution and precipitation the mechanism of contact formation can be compared to commercial glass containing thick film pastes. This avoids additional processing steps, like laser ablation, which are usually necessary to open the ARC prior to ink jet printing. As a consequence process costs can be reduced. In order to optimize the ARC etching and contact formation during firing, zinc oxide nanoparticles are investigated as an ink additive. By utilization of in situ contact resistivity measurements the mechanism of contacting was explored. Our results show that silver inks containing ZnO particles realize a specific contact resistance below 10 m Omega.cm(2). By using a multi-pass ink jet printing and plating process a front side metallization of commercial 6 x 6 inch(2) standard micro crystalline silicone solar cells with emitter resistance of 60 Omega/(sic). was achieved and showed an efficiency of 15.7%.

In this study, the contact formation of aluminum containing silver metallization pastes for boron emitters was investigated. Model pastes with varied glass composition (PbO-containing and PbO-free) and Al content were prepared. It was found, that glass viscosity as well as Al content have a strong influence on densification behavior of the pastes. The most significant effect of the aluminum addition is the change of the thermodynamic conditions in the system silver-glass-silicon. For investigations of the contact formation an in-situ-contact resistance measurement was performed. The interface morphology of the pastes in dependence on the firing temperature was investigated by means of cross section samples in SEM and EDX. Finally, n-type Si solar cells were electrically characterized and the IV-data were correlated to the interface morphology.

In this study Ag precursor equipped glasses were used to produce model pastes for front side contacts of mc-silicon solar cells. To establish the Ag precursors in the glass, a silver ion-exchange procedure of glass powders is chosen. Suchlike process can be easily integrated in the glass powder production by an additional step whereby the ready milled glass powder is dispersed in a silver salt melt for a certain temperature and time. Pastes containing Ag precursor equipped glass powders show relative efficiency increases up to 43 %. The origin for this increase is seen in two effects. First, the presence of silver ions in the paste glass partly skips the need of silver dissolution in the during the firing and accelerates the silver deposition in the contact interface. Second, a very homogeneous Ag distribution in the contact layer attenuates lateral fluctuations in the local interface reaction progress and therewith the risk of emitter shunting.

Manufacturing of ceramic multilayer applications by deposition techniques like screen printing, dispensing, or aerosol jet printing is state of the art today. However, in an industrial environment it is a complex challenge to integrate all equipment for precise paste handling and screen print of the microstructures in an existing production line. Moreover, big sized devices or items with a curved surface cannot be handled with standard screen print equipment. To overcome these limitations, a decal technology was investigated. The functional layers are stacked on non-adhesive paper by screen print, fulfilling all demands on precision in line/space ratio and layer thickness. Afterwards, a polymer varnish is printed on top of the stack, which allows to transfer the high precision thick film assembly to nearly any application surface including non-planar surfaces. Since the layers are co-fired, the sintering behavior of the materials has to be regulated. The top layer needs to remain porous until the organic burn-out and the sintering of the bottom layer is complete. Ways to obtain thick film metallization and isolation pastes which have satisfying firing behavior are examined, and the excellent functional properties are discussed. The performance of the developed thick film decals is demonstrated on the example of an alarm sensor on tempered safety glass for windows.

In this work, glasses with systematically varied compositions were manufactured and irradiated by single Ti:sapphire fs-laser pulses (800 nm, 120 fs), focused at the surface and into the bulk of the glass materials. The samples were tested for their ablation threshold fluence as well as for structural changes using μ-Raman-spectroscopy. Correlations between the glass composition, the material-ablation on the glass surface and the permanent changes of the refractive index inside the glass volume after the irradiation by fs-laser pulses were obtained. The results show, that the structural modifications found at the surface of the glasses and inside its volume are closely related. However, while the ablation threshold fluence of the glass surface primarily depends on the glass dissociation energy, the permanent refractive index change inside the volume is rather determined by its ability for absorbing the fs-laser pulses and the subsequent relaxation processes. The results of this work provide some guidance on how the glass composition can be varied in order to optimize the fs-laser induced modification of dielectrics.

The redeposited material (debris) resulting from ablation of a potassium-magnesium silicate glass upon scanning femtosecond laser pulse irradiation (130 fs, 800 nm) in air environment is investigated by means of three complementary surface analytical methods. Changes in the electronic band structure of the glass constituent Magnesium (Mg) were identified by X-ray Absorption Near Edge Structure spectroscopy (XANES) using synchrotron radiation. An up-shift of approximate to 0.8 eV of a specific Magnesium K-edge absorption peak in the spectrum of the redeposited material along with a significant change in its leading edge position was detected. In contrast, the surface left after laser ablation exhibits a downshift of the peak position by approximate to 0.9 eV. Both observations may be related to a change of the Mg coordinative state of the laser modified/redeposited glass material. The presence of carbon in the debris is revealed by micro Raman spectroscopy (mu-RS) and was confirmed by energy dispersive X-ray spectroscopy (EDX). These observations are attributed to structural changes and chemical reactions taking place during the ablation process. (C) 2013 Elsevier B.V. All rights reserved.

A large number of solar cells is metallized by printing and firing glass containing silver pastes. However, the contact formation is not fully understood so far. There is still a lack of understanding the role of the glass phase in the complex contact formation scenario because single effects could not been separately observed and evaluated up to now. To overcome this, an in-situ method to observe the contact formation via a contact resistance measurement was introduced. A special measuring device was applied to characterize two typical front side pastes, featuring a PbO-containing as well as a PbO-free glass frit during firing. The viscosity of the paste glass showed decisive influence for the etching of the anti-reflection coating (ARC). The ARC was opened immediately after entering the softening range of the respective glass, regardless of large differences in glass chemistry. Furthermore, the viscosity-temperature behaviour of the paste glass determines the intensity of the redox-reaction and related silver precipitation at the interface, which takes part between ARC opening and glass resolidification. The cooling slope was confirmed to have decisive influence on the final interface conductivity, because a crucial part of silver colloids can be formed here.