TY - GEN A1 - Rietig, Anja A1 - Acker, Jörg T1 - Development and validation of a new method for the precise and accurate determination of trace elements in silicon by ICP-OES in high silicon matrices T2 - Journal of Analytical Atomic Spectrometry N2 - A new method for the accurate and precise determination of impurities in silicon was developed and statistically validated. Particular attention was paid to the correct determination of the non-metals boron and phosphorus. Instead a time-consuming open vessel digestion under mild conditions, the dissolution of silicon took place in a microwave-assisted high-pressure system. The essential innovation of the presented method is the direct use of the concentrated digestion solution for ICP-OES measurements. This approach avoids the commonly used, time-consuming method that requires the removal of silicon and acid matrix by volatilisation, which is the most critical step in the determination of boron; however, the ICP-OES measurement in such high silicon matrices requires an entirely new optimisation of the measuring conditions, including the careful selection of emission lines with respect to selectivity and, spectral and non-spectral inferences. For quantification of the impurities contents, the methods of matrix matching (MMC) and multiple standard addition (MSA) were used. After optimisation of the spike concentrations for MSA, the qualities of both methods were compared through a statistical analysis. For the metallic impurities Al, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Cu, and Zr and P, the validation was performed against certified reference materials (IPT134, IPT135, NIST57b). To validate boron, 9 silicon samples with different contents of boron from three interlaboratory comparisons were used. The new procedure allows for the determination of the impurities of 4N-silicon (12 elements). KW - impurities KW - inductively coupled plasma emission spectroscopy KW - silicon KW - microwave-assisted digestion KW - high silicon matrix KW - multiple standard addition KW - boron KW - phosphorus Y1 - 2017 UR - http://pubs.rsc.org/en/content/articlelanding/2017/ja/c6ja00241b#!divAbstract U6 - https://doi.org/10.1039/C6JA00241B VL - 32 IS - 2 SP - 322 EP - 333 ER - TY - GEN A1 - Grafe, Hans-Joachim A1 - Löser, Wolfgang A1 - Schmitz, Steffen A1 - Sakaliyska, Miroslava A1 - Wurmehl, Sabine A1 - Eisert, Stefan A1 - Reichenbach, Birk A1 - Acker, Jörg A1 - Rietig, Anja A1 - Ducke, Jana T1 - NMR investigation of boron impurities in refined metallurgical grade silicon T2 - Physica status solidi. A, Applications and Materials Science N2 - The nuclear magnetic resonance (NMR) method was applied for tracking boron impurities in the refining process of metallurgical grade (MG) silicon. From the NMR signal of the 11B isotope at an operating temperature 4.2 K, the boron concentration can be estimated down to the order of 110 wppm B. After melting and resolidification of MG-Si alloyed with Ca and Ti, a major fraction of B impurities remains in the Si solid solution as inferred from the characteristic NMR frequency. The alloying element Ti does not form substantial fractions of TiB2. Acid leaching of crushed powders of MG-Si alloyed with Ca and Ti can diminish the initial impurity content of B suggesting its accumulation in the grain boundary phases. KW - boron KW - impurity KW - intermetallic compounds KW - nuclear magnetic resonance KW - silicon KW - transition-metal diboride KW - Si-Al melt KW - removal KW - solidification Y1 - 2015 UR - http://onlinelibrary.wiley.com/doi/10.1002/pssa.201431908/full U6 - https://doi.org/10.1002/pssa.201431908 SN - 1862-6319 VL - 212 IS - 9 SP - 2031 EP - 2036 ER - TY - GEN A1 - Meinel, Birgit A1 - Koschwitz, Tim A1 - Blocks, Christian A1 - Acker, Jörg T1 - Comparison of diamond wire cut and silicon carbide slurry processed silicon wafer surfaces after acidic texturisation T2 - Materials Science in Semiconductor Processing N2 - Our work focuses on the acidic etching of silicon wafers, cut via diamond wire (DW) or silicon carbide slurry process (SP). The DW and SP as-cut wafer surface structures have a significant impact on the evolution of the two resultant and different etched morphologies. The time-dependent development of the surface morphology for mono- and multi-crystalline wafers is compared and analyzed via etch rates, reflectivity measurements and confocal microscopy. The as-cut structure of the differently sawn wafers defines a template where the etch attack preferentially occurs and predetermines the texturisation of the etched surface. Based on the experimental results it is possible to lower the reflectivity of the SP-sawn wafers by varying the acidic mixture. On the contrary, the DW-sawn wafers obtain only a small enlargement of the folded surface area during acidic texturisation and no influence of different acidic etch solutions on the reflectivity values was found. To create homogeneously texturized DW-sawn wafers of low reflectivity, an adaptation of the sawing process as well as the development of new etchants and new etch conditions is necessary. KW - Surface morphology KW - Confocal microscopy KW - Acidic etching KW - Texturisation KW - Diamond wire sawing KW - Slurry sawing KW - silicon KW - solar cell KW - chemical analysis Y1 - 2014 UR - http://www.sciencedirect.com/science/article/pii/S1369800114001875 U6 - https://doi.org/10.1016/j.mssp.2014.03.046 SN - 1369-8001 VL - 26 SP - 93 EP - 100 ER - TY - GEN A1 - Meinel, Birgit A1 - Koschwitz, Tim A1 - Heinemann, Robert A1 - Acker, Jörg T1 - The texturization process during horizontal acidic etching of multi-crystalline silicon wafers T2 - Materials Science in Semiconductor Processing N2 - Horizontal wet-chemical etching of silicon wafers in an HF/HNO3/H2SiF6 mixture is the most widely-used technique to texturize multi-crystalline silicon wafers for solar cell production. For the first time, the etch rates were determined separately for the upper and lower side during the horizontal texturization and the their different morphologies. The dependency of the surface morphology from the etch rate and etch depth is proven. Furthermore, the influence of the temperature and stirring rates on the morphological development for the upper and lower side of the wafer were examined. From temperature-dependent measurements, activation energies in the range from 17 kJ/mol to 40 kJ/mol on the upper side and from 23 kJ/mol to 40 kJ/mol on the lower side dependent from the etching time were determined. The observed results reveal a connection between the etch depth, the agitation of the etch solution, the morphology and the reflectivity of the separate wafer sides. KW - Acidic etching KW - Surface properties KW - Confocal microscopy KW - Reflectivity KW - Activation energy KW - silicon KW - HF/HNO3 mixture KW - solar cell Y1 - 2014 UR - http://www.sciencedirect.com/science/article/pii/S136980011400482X U6 - https://doi.org/10.1016/j.mssp.2014.08.047 SN - 1369-8001 VL - 26 SP - 695 EP - 703 ER - TY - CHAP A1 - Acker, Jörg A1 - Ducke, Jana A1 - Rietig, Anja A1 - Müller, Tim A1 - Eisert, Stefan A1 - Reichenbach, Birk A1 - Löser, Wolfgang ED - Oye, Harald A. ED - Brekken, Harald ED - Rong, Harry ED - Tangstad, Merete ED - Tveit, Halvard T1 - Segregation, grain boundary milling, and chemical leaching for the refinement of metallurgical-grade silicon for photovoltaic application T2 - Silicon for the Chemical and Solar Industry XII, Trondheim, 2014 N2 - The present work describes a completely new approach to the solidification refinement of metallurgical-grade silicon. The new process comprises the following steps: (i) The first step involves adding auxiliary metals to the molten silicon in order to segregate the metallic and non-metallic impurities in the secondary phase after cooling. (ii) The melt is rapidly cooled in the cellular solidification regime. This generates a Si microstructure with a defined cell size in which all cell boundaries are surrounded by the secondary phase. Furthermore, the secondary phase should form an interconnected three-dimensional network. (iii) The solids are crushed by shockwaves using electrohydraulic fragmentation techniques. The shockwaves lead to preferential crushing at the interface between the silicon and the secondary phase. (iv) The secondary phases are fast and effectively removed by microwave-assisted high-pressure leaching that was newly developed for this process. The potential of the new refinement procedure is demonstrated with auxiliary metals Ca, Al, and Ti. This new procedure yields a significant decrease in phosphorous and metal impurities. KW - silicon KW - leaching KW - hydrometallurgy KW - solar cell KW - segregation KW - etching Y1 - 2014 SN - 978-82-997357-8-0 SP - 177 EP - 188 PB - Department of Materials Science and Engineering, Norwegian University of Science and Technology CY - Trondheim ER - TY - CHAP A1 - Rietig, Anja A1 - Acker, Jörg ED - Nygaard, Lars ED - Pachaly, Bernd ED - Page, Ingrid Gamst ED - Rong, Harry ED - Tangstad, Merete ED - Tveit, Halvard T1 - A new and fast method for determination of boron, phosphorus and other trace elements in metallurgical grade silicon T2 - Silicon for the Chemical and Solar Industry XIII, Kristiansand, 2016 N2 - A new method for accurate and precise determination of non-metallic and metallic impurities in silicon was developed and statistically validated. The first step is the fast dissolution of silicon in a microwave-assisted high pressure system to minimize a loss of phosphorus. The essential innovation is the use of the concentrated digestion solution for ICP-OES measurements. This approach avoids the common removal of the silicon and acid matrix by volatilization, which can cause considerable losses of boron. Finally, for the ICP-OES measurements in such high-silicon matrices the optimum measuring conditions were determined and a careful selection of emission lines with respect to selectivity, spectral and non-spectral inferences and matrix effects was performed. The method of matrix matched calibration (MMC) is used for quantification of the impurities’ contents. For Al, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zr and P the validation was performed against certified reference materials (IPT134, IPT135, NIST57b). To validate the determination of boron 9 silicon samples of different boron contents from three interlaboratory comparisons were used. The new procedure allows the determination of impurities of 4N-silicon (12 elements) with high precision and accuracy. KW - silicon KW - ICP-OES KW - impurity KW - chemical analysis KW - boron KW - phosphorus Y1 - 2016 UR - https://www.ntnu.no/trykk/publikasjoner/Silicon%20for%20the%20chemical%20and%20solar%20industry%20XIII/ SP - 95 EP - 106 PB - Department of Materials Science and Engineering, Norwegian University of Science and Technology CY - Trondheim ER - TY - GEN A1 - Acker, Jörg A1 - Langner, Thomas A1 - Meinel, Birgit A1 - Sieber, Tim T1 - Saw Damage as an Etch Mask for the Acidic Texturization of Multicrystalline Silicon Wafers T2 - Materials Science in Semiconductor Processing N2 - The surface of multicrystalline silicon solar cells are etched by mixtures of HF, HNO3 and H2SiF6 in order to remove saw damage caused by wafer slicing, as well as to create a water surface topography that provides a low reflectance for incident light, otherwise known as the texture. Topographically analyzing wafer surfaces before and after etching has revealed that the saw damage controls the texturized wafer surface’s final topography.The first key factor is the dimension and magnitude of the plastic stress field introduced by indenting SiC grains into the wafer surface during the wafering process. The second key factor is that lattice-stressed silicon is etched at a higher rate than unstressed bulk silicon. At the wire entrance, side sharp and large SiC grains create the deepest indention pits, and therefore the deepest of the water surface stress fields. The lattice-disturbed silicon inside these pits is etched at a higher rate compared to the pit’s side walls, which are uniformly attacked across the wafer area. Consequentially, existing pits deepen, and these areas generate the wafer’s lowest reflectivity. At the wire exit side, a higher number of smaller and rounder SiC particles indent the surface and create more numerous and shallower indention pits compared to the wire entrance side. The resulting stress field is less deep, so less silicon is removed from inside of these pits during etching compared to the wire entrance side. This yields to a wafer surface region consisting of shallowly etched pits and higher reflectance. It is concluded that the saw damage acts like an etch mask in the texturization of multicrystalline silicon wafers. KW - silicon KW - texturization KW - acidic etching KW - multi-wire sawing KW - stress field KW - confocal microscopy KW - solar cell Y1 - 2018 UR - https://www.sciencedirect.com/science/article/pii/S1369800117313896 U6 - https://doi.org/10.1016/j.mssp.2017.09.039 SN - 1369-8001 VL - 74 SP - 238 EP - 248 ER - TY - GEN A1 - Acker, Jörg A1 - Sieber, Tim A1 - Langner, Thomas A1 - Herold, Steven ED - Andresen, Birger ED - Nygaard, Lars ED - Rong, Harry ED - Tangstad, Merete ED - Tveit, Halvard ED - Page, Ingrid Gamst T1 - The impact of lattice strain on the reactivity of silicon T2 - Silicon for the Chemical and Solar Industry XIV N2 - The present study is focused on the question of how lattice strain mechanically introduced into silicon alters the chemical reactivity of the silicon atoms that are affected by the strain field on a microscopic length scale. The magnitude and local distribution of lattice strain are extracted from confocal Raman microscopy measurements. The reactivity of Si is expressed by the etch rate of Si after treatment with HF–HNO3–H2SiF6 mixtures. Then, the local etch rate is calculated from the local etch depth as determined by confocal microscopy. It has been found that tensile strain leads to the highest enhancement of the etch rate, followed by a compressive strain increase in the etch rate. KW - silicon KW - etching KW - Raman spectroscopy KW - lattice strain KW - stress KW - mechanical activation KW - mechanochemistry KW - confocal microscopy Y1 - 2018 SP - 11 EP - 20 PB - The Norwegian University of Science and Technology CY - Trondheim ER - TY - GEN A1 - Langner, Thomas A1 - Sieber, Tim A1 - Acker, Jörg T1 - Etching Shapes the Topography of Silicon Wafers: Lattice-Strain Enhanced Chemical Reactivity of Silicon for Efficient Solar Cells T2 - ACS Applied Nano Materials N2 - Multiwire sawing of silicon (Si) bricks is the state-of-the-art technology to produce multicrystalline Si solar wafers. The massive indentation of the abrasive Si carbide or diamond particles used leads to a heavily mechanically damaged layer on the wafer surface. Etching the surface layer using typical HF–HNO3–H2SiF6 acid mixtures reveals an unevenly distributed etch attack with etch rates several times higher than known for bulk Si etching. The present study follows the hypothesis that lattice strain, introduced by the sawing process, leads to an increase of the etch rate and determines the topography of the etched wafer, the so-called texture. Scratches were introduced into single crystalline Si surfaces in model experiments, and the magnitude and local distribution of lattice strain were extracted from confocal Raman microscopy measurements. The essential parameter used to describe the local reactivity of Si is the local etch rate, which was derived by confocal microscopy from the local height before and after etching. It was found that the reactivity of Si increases linearly with the magnitude of lattice strain. An increase in tensile strain raises the reactivity of Si significantly higher than an increase of compressive strain. The second decisive parameter is the reactivity of the etch mixture that correlates with the total concentration of the acid mixtures. Diluted acid mixtures with a low reactivity attack only the highest strained Si, whereas more concentrated and, therefore, more reactive acid mixtures can attack even slightly strained Si. Side effects, such as the behavior of amorphous or nanocrystalline Si and the generation of highly reactive intermediary species while etching, are discussed. The presence of unevenly distributed lattice strain of different magnitude and the resulting unevenly distributed reactivity of Si explain the features of a heterogeneous etch attack observed and the resulting topography of the etched wafer surface. KW - lattice strain KW - silicon KW - Raman microscopy KW - confocal microscopy KW - etching KW - reactivity KW - solar cell KW - mechanochemistry Y1 - 2018 U6 - https://doi.org/10.1021/acsanm.8b00906 VL - 1 IS - 8 SP - 4135 EP - 4144 ER - TY - CHAP A1 - Meißner, André A1 - Acker, Jörg ED - Scheschkewitz, David ED - Kickelbick, Guido T1 - Raman spectroscopic study on the formation of Cu3Si T2 - 9th European Silicon Days, 9-12 September 2018, Saarbrücken, Germany, Book of abstracts N2 - The term “direct synthesis” is defined in the literature as the reactions between silicon and methyl chloride, hydrogen chloride and other reagents like chlorobenzene and ethyl chloride to yield various alkyl- or aryl substituted chlorosilanes.[1] These reactions have two features in common: (1) The reactivity of silicon - in terms of reaction start temperature, reaction rate and silane product distribution - is originated and controlled by the interaction with metals. (2) The reaction is fully under kinetic control since the formed silanes are the thermodynamically least stable products in the system Si-H-Cl-C.[2] Several authors consider Cu3Si as the catalytically active phase in the direct synthesis. It is assumed, that CuCl formed under the conditions of the direct synthesis reacts with Si according to Eq. 1 and 2 to yield Cu3Si.[1] (1) and (2) The present work describes a Raman microscopic study of the reaction of Si with Cu and CuCl with special emphasize given to the identification of the Cu3Si phase and the processes occurring in the surrounding bulk Si. There is one pathway in which a solid state reaction[3] between Si and CuCl leads to a massive nucleation of Cu3Si exactly at the position of the Si/CuCl solid-solid interface. The nucleation of Cu3Si creates such an enormous lattice strain so that several high-pressure modifications of Si can be identified at the reaction site and around. The second reaction pathway is controlled by a gas phase transport of CuCl at low temperatures. This transport pathway leads to a spread of Cu in nearest neighborhood close to the CuCl particles as well to a long range transport leading to a nucleation of microscopic Cu3Si precipitates away from the CuCl particles. Further studies on the reactivity of the Cu3Si containing reaction sites were performed and will be discussed in the presentation. KW - Raman spectroscopy KW - silicon KW - lattice strain KW - copper silicide KW - direct synthesis Y1 - 2018 PB - Universität des Saarlandes CY - Saarbrücken ER - TY - CHAP A1 - Langner, Thomas A1 - Sieber, Tim A1 - Acker, Jörg ED - Scheschkewitz, David ED - Kickelbick, Guido T1 - Lattice strain controls the etching of solar wafer surfaces T2 - 9th European Silicon Days, 9-12 September 2018, Saarbrücken, Germany, Book of abstracts N2 - Multi-wire sawing using an abrasive SiC slurry or diamond wires constitutes the main slicing techniques for multi- and monocrystalline silicon crystals in photovoltaics. The massive mechanical load during the sawing process creates a wafer surface layer characterized by lattice defects, pits, fractures, rifts, cracks, amorphous Si and even some high-pressure Si modifications, otherwise known as saw damage.[1] This highly defect-rich surface causes the rapid recombination of electron-hole pairs, requiring that it be removed by etching in order to manufacture solar cells and to generate a surface morphology having a low reflectivity which directly affects the solar cell’s efficiency. However, etching of the saw damage features of a heterogeneous and laterally unevenly distributed etch attack and a significantly higher etch rate compared to the underlying bulk silicon.[2,3] The present study is focused on the question of how mechanically introduced lattice strain in single-crystalline silicon alters the chemical reactivity of the silicon atoms affected by the strain field on a microscopic length scale. The magnitude and local distribution of lattice strain were extracted from confocal Raman microscopy measurements according to Ref. 4. One of the parameters used to describe the reactivity of silicon is the local etch rate, which was derived from the local removal before and after etching by confocal microscopy. Wet-chemical etching was performed with HF-HNO3-H2SiF6 acid mixtures of different concentrations. It was found, that the reactivity of silicon increased linearly with the magnitude of lattice strain. In particular, an increase in tensile strain led to a higher increase in reactivity compared to the increase observed with growing compressive strain. The second decisive parameter is the reactivity of the etch mixture. Diluted acid mixtures with a low reactivity attack only the highest strained Si, whereas more concentrated and therefore more reactive acid mixtures are able to attack even slightly strained Si. Side effects, such as the behavior of amorphous or nanocrystalline Si and the generation of highly reactive intermediary species while etching, are discussed. KW - Raman spectroscopy KW - silicon KW - lattice strain KW - etching KW - confocal microscopy Y1 - 2018 SP - S. 201 PB - Universität des Saarlandes CY - Saarbrücken ER - TY - CHAP A1 - Schönekerl, Stefan A1 - Acker, Jörg ED - Scheschkewitz, David ED - Kickelbick, Guido T1 - Behavior of electroless copper deposition onto multi-crystalline silicon in diluted hydrofluoric acid solutions T2 - 9th European Silicon Days, 9-12 September 2018, Saarbrücken, Germany, Book of abstracts N2 - The metal-assisted etching of Si is a method which has been studied for many years with regard to the creation of nanoscale surface structures, but much less research was done to elucidate the reaction processes. Most publications argue that either reduction of the metal cation could result in double charge transfer with concomitant hydrogen release or four-fold charge transfer without H2 formation [1]. Following, SiO2 is to be formed, which is subsequently converted into H2SiF6 by F-, HF2- or HF [2]. Due to the few reliable findings, own experiments were carried out. In this context, the deposition of Cu on multi-crystalline Si at various Cu2+ activities and different HF levels was investigated, and the H2 emission for these processes analysed. The results of the series of experiments indicate a different reaction behavior from the theory described above. Apparently, a slightly higher redox potential of the Cu2+/Cu+ half-cell compared to 2H+/H2 redox couple is sufficient to initiate the silicon dissolution process. The stoichiometric ratio between Cu deposition and Si dissolution process is strongly affected by the Cu2+ activity, but it is obviously not influenced by HF activity. At Cu2+ activities of < 2∙10-5 mol/kg less than one electron is nominally exchanged between metal cation and Si, and at activities of approx. 1∙10-2 mol/kg there is an almost four electron charge transfer. At activities > 1∙10-2 mol/kg the stoichiometric ratio and the Cu deposition and Si dissolution kinetics decrease, presumably due to the fact the compact Cu layer inhibits the transition of the dissolved Si into the etching solution. The shift in the stoichiometric ratio suggests the first Cu2+ based oxidative attack on Si enables the reaction of a further oxidizing agent. It is likely water will attack the silicon as second oxidant, since there is no dependence between the content of HF species in the etching solutions and the Si dissolution kinetics. This second reaction step seems to be associated with hydrogen evolution. The amount of hydrogen formation indicates that at a Cu2+ activity of < 2∙10-5 mol/kg calculative only one electron is transferred from Si to Cu2+ and H+, and at a Cu2+ activity of 1∙10-2 mol/kg four electrons in total. However, HF is necessary to convert the oxidized Si to the supposed formation of H3SiF, H2SiF2, HSiF3, and SiF4 and H2SiF6 respectively. KW - silicon KW - metal assisted etching KW - deposition KW - copper KW - dissolution Y1 - 2018 SP - S. 82 PB - Universität des Saarlandes CY - Saarbrücken ER - TY - CHAP A1 - Herold, Steven A1 - Acker, Jörg ED - Scheschkewitz, David ED - Kickelbick, Guido T1 - The influence of lattice deformations on the etch rates of potassium hydroxide on silicon T2 - 9th European Silicon Days, 9-12 September 2018, Saarbrücken, Germany, Book of abstracts N2 - One of the most common methods to analyse silicon materials is Raman-spectroscopy.[1] More recently there is a focus in the analysis of lattice deformations during sample preparations.[2] These deformations not only increase the chance of crack formation but also increases the chemical reactivity of the silicon towards etch processes or oxidation processes.[3] In this contribution, we will present our latest results on the influence of lattice deformations induced by scratches and saw damage on the etch rate of potassium hydroxide. The lattice deformation will be measured using Raman-microscopy and correlated with the height difference measured with confocal microscopy. It will be shown that the etch rate strongly depends on the lattice deformation and that the etch rate will increase by up to the factor 10 in areas with tensile deformation. KW - silicon KW - etching KW - Raman spectroscopy KW - lattice strain KW - saw damage KW - confocal microscopy Y1 - 2018 SP - S. 198 PB - Universität des Saarlandes CY - Saarbrücken ER -