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The texturization process during horizontal acidic etching of multi-crystalline silicon wafers
(2014)
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.
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.
Texturization of multi-crystalline silicon wafers for photovoltaic application comprises the removal of the saw damage and shaping the topography of the bulk surface to create a surface with a low reflectivity, the so-called texture. Etching of multi-crystalline silicon wafers is usually carried out with acid mixtures consisting of hydrofluoric acid (HF), nitric acid (HNO₃) and hexafluorosilicic acid (H₂SiF₆). The present study reveals that such acid mixtures diluted by water or modified by the addition of ammonia solution, NH₃ (added as ammonium hydroxide solution, NH₄OH) can create textures with a significantly increased surface area exceeding that obtained by standard etching mixtures by a factor of 2.5–3. This yields a significantly reduced reflectivity of the etched wafer surface. However, the addition of water or NH₃ causes a very low etching rate, which makes such mixtures inapplicable for industrial application. To overcome this disadvantage, a two-step etching regime was developed to produce surface-enlarged solar wafers within a timespan typical for industrial production lines. This procedure comprises a first step of slow etching with a NH₃-modified etching mixture to pre-shape the ascut wafer surface. The second etching step is performed with a typical HF/HNO₃/H₂SiF₆ etching mixture that finalizes the texturization. Electrical measurements made on solar cells produced from such etched wafer confirm the improved surface quality of the two-step etched wafer compared to the reference wafer.
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.