A Study Of Texturization Of Diamond Wire Sawn Multicrystalline Silicon Wafers By Vapor Etching

Diamond wire sawing of silicon wafers is faster,with better quality of the wafers cut,and less burdern to environment.It has been applied in production of monocrystalline silicon wafers.However for wafering of multicrystalline silicon,its application is hindered,as the conventional technology of for texturization of multicrystalline silicon wafers fails with the diamond wire sawn multicrystalline wafers.We put forward two kind of low cost vapor etching method to solve the problem,the thermal evaporation method and the reaction method.The former has the etching gas generated by heating up HF/HNO₃ mixed acid;the latter has the etching gas generated by reaction of silicon with HF/HNO₃ mixed acid.The works carried out include designing and fabrication of three types of experiment equipment for vapor etching texture,investigations of the effects of vapor source heating temperature,flow intensity of vapor and preheat temperature on etching rate,microstructure and light reflectivity of the textured surfaces,and discussions on the mechanisms involved.The results showed that the etching depth formed by thermal evaporation method increases with time linearly and the etching rate is about 1μm/min.The formation of corrosion pits is not affected by the surface cutting marks and surface state of diamond wire saw silicon wafers during the stage of vapor etching.This method can eliminate the cutting marks and eventually a kind of corrosion pits whose sizes close to the nanoscale distribute along the surface of silicon wafer.The effect of reducing reflection is very good of this morphology and the lowest reflectivity can be closely to 10%.But this kind of nanoscale structure might be not compatible with the subsequent solar cell technologies.Improving the flow intensity of the thermal evaporation method through designing of pooling airflow can improve the etching uniformity of the wafer and tending to form the microscale corrosion pits which can be compatible with the subsequent solar cell technologies.The effect of reducing reflection is obviously superior to that of wet acid etching method acting on slurry-cut silicon wafers.What’s more,it can completely eliminate the apparent cutting marks of the diamond wire saw silicon wafers.The best relative ratio of HF and HNO₃ is HF: HNO₃ = 1:4 for the mixed acid.Increasing the proportion of HNO₃ can make etch pits at wafer becoming shallow while increasing the proportion of HF can lead to serious selective etching in the place of dislocation outcrop.The best temperature of mixed acid is about 90℃ while the best temperature of initial preheating is about 60℃ during the thermal evaporation method.The etching rate of the reaction method is not stable and the maximum rate can reach 8μm/min.It can get good texturization with ball type pits.Its effect of reducing reflection is obviously better than that of wet acid texturization applied on the slurry-cut wafers.And this method is able to completely eliminate the surface sawing marks of the diamond wire sawing silicon wafers and it has the optimal results at temperature of 70℃ of the initial preheating.But its reaction process depends on adding silicon lead to instantaneous etching rate is too high,and then late to pause,which lead to stable continuous control is difficult.The added silicon reacts with mixed acid strongly in the reaction method and this is an exothermic reaction.It made the temperature rising rapidly and produced evaporation gas at the same time.The reaction etching mechanism was similar to the thermal evaporation method.These two methods of vapor etching system were not complete dry.There should have a certain density of steam condense on the surface of silicon wafer to form the dispersed droplets.Etching effect would be the same as wet etching leading to texturing failure when the density is too big while it cannot produce effective etching when the density is too small.Because the wafer temperature can directly control the density of steam which condenses on the surface,it had an optimal value.This study found two kinds of vapor etching methods expected to solve the problems of texturization for diamond wire sawing of multicrystalline silicon wafers.And we promoted the understanding of the relevant technologies and mechanisms by improving experiment devices and experiment research methods.It had important reference value on the industrialization development of the vapor etching for texturization and solving the problems of texturization of diamond wire sawing of multicrystalline silicon wafers.