Studies On The Mechanism Of The Micro-droplet Etching Texturization And Annealing Pretreatment Of DWS Silicon Wafers

Crystalline silicon solar photovoltaic cell is a promising way to obtain solar energy.Under the two development themes of low cost and high efficiency,diamond wire saw（DWS）cutting technology has eliminated the slurry wire saw（SWS）cutting technology and gradually occupied the silicon wafer cutting market of crystal silicon cells.However,due to the difference of surface characteristics of silicon wafers caused by wafering methods,it is difficult to produce excellent uniform texture with low reflectivity in conventional wet acid etching texturization,and it is impossible to effectively remove saw marks in polycrystalline silicon wafers cut by diamond wire saw.Based on this texturization barrier,the research group developed a micro-droplet etching（MDE）texturization method previously,with the vapor condensation from thermal evaporation of HF-HNO₃ solutions.It has a good texturing effect on the DWS silicon wafers and can effectively solve the problems above.A further experimental research is conducted on the MDE texturing mechanism in this paper and the following findings are obtained:1.The conventional HF-HNO₃-H₂O bath wet etching cannot generate any texture on the polished defect-free silicon,while the MDE can do.The textures we observed on the wet textured SWS or DWS mc-Si wafers should be attributed to the preferential attack on the mechanically generated surface defects.2.When droplet etches silicon wafer,the silicon inside the droplet is located in the solid-liquid two phase environment,while the silicon wafer at the edge is in the gas-liquid-solid triple phase boundary environment.The corrosion in the interior of a droplet is similar to that in a wet bath,while the corrosion at the edge of the droplet is faster than the interior.The depth of the ravine generated by the preferential attack at the edge is much greater than the average corrosion depth in the interior of the droplet under some conditions,and the gully is deepened as the acid concentration and the temperature of the silicon wafer increase.Preferential attacks take place along the edges of the droplets,leading to a texture consisting of W-shaped micro-pits when in the MDE texturization.The production and application of the micro-dropet etching texturization have not yet been developed for special production line equipment.On the premise of not changing the conventional wet acid etching texturization equipment and process,this paper develop a silicon wafer annealing pretreatment scheme to improve the texturization effect of DWS silicon wafer effectively,and the following conclusions are obtained:1.When the silicon wafer is annealed at above 400℃for 1 hour and in wet acid etching textured at 10s,the silicon wafer can be corroded to the morphology of“small hole”.With the increase of annealing temperature,the number of“small holes”increase and the distribution become denser.2.The shape of the“small hole”in the late stage of the texturing has evolved into a large number of densely distributed circular or nearly circular etch pits,which can effectively improve the density and uniformity of the surface corrosion pit of the silicon wafer after texturing,and reduce the light reflectivity.