Studies Of Surface Properties And Texturization Methods Of Diamond Wire Sawn Multi-crystalline Silicon Wafers

Solar photovoltaic power is an important solution to energy crisis andenvironmental pollution problem of the world. Among the current photovoltaic powersystems and other applications, nearly90%of the solar cells are of siliconwafer-based. Diamond wire saw (DWS) technology has been applied to crystallinesilicon wafer cutting production in recent years. It has the advantages over theconventional slurry wire saw (SWS) technology, including higher productivity, higherprecision enabling production of thinner wafers, less pollution and less surfacemechanical damages. The DWS technology has been gradually replacing theconventional slurry wire sawn (SWS) in production of mono-crystalline siliconwafers for solar cells. While in production of multi-crystalline silicon wafers, whichare more important to the photovoltaic industry of China, the application of DWStechnology has met serious obstacle. The current acid wet textured for SWS mc-Siwafers does not work with DWS mc-Si wafers to reduce the reflectivity to asatisfactorily low level, and the saw marks generated by the DWS process cannot beremoved.In this paper, experiments and basic, theoretical analysis researches includingphysical and chemical properties analysis and researches on the surface of DWSsilicon wafers and new etching methods exploration research were developed to solvethe etching problem. The following major research results and conclusion areobtained:1. The nature of the saw marks produced by diamond wire sawn (DWS) isshinning which makes the reflectivity of DWS mc-silicon wafers, in the visible lightrange, is an average of3%to5%higher than that of SWS mc-silicon wafers; theDWS wafer surface is actually~20%lower in both types of roughness than the SWSwafer surface； two type of saw marks on the DWS mc-silicon wafers areidentified—the sub-millimeter scale round-run fringes with the scale of0.5mm, andthe micron scale scratches with the width of0.2~5μm caused by scribing of thediamond tips. A typical surface of wafer cutting by DWS consists of three zones:distinct scratch zone, cross scratch zone and brittle split pit zone, which can bedivided into smooth area and rough area, and the smooth area accounts for33%of the silicon wafer surface, and crystal structure have changed on part of the smootharea on the surface of the DWS mc-silicon wafer---crystalline silicon have changedinto a thin layer of amorphous silicon.2. The conventional wet acid textured for SWS mc-Si wafers does not work withDWS mc-Si wafers, from HNO3-rich to HF-rich HF-HNO3-H2O solutions, andfurther attempts with a wide range of HF-HNO3-H2O ratio in the acid volumerecipes have all failed to reduce the reflectivity to a satisfactorily low level to meetthe need of the current industry standards, and the saw marks generated by the DWSprocess cannot be removed.3. Using wet alkaline etching as pre-treatment, only the crystal crystalline closeto (001) grain can be removed, and the macroscopic round-run fringe and scratch canalso be removed completely. However, this method has no effect on the otherorientation grains.4. The method by using a layer of protective organic film on silicon wafersurface before wet acid etching to increase its local selective, to lessen the saw markswhich produced by the diamond wire saw process has a certain effect, lightreflectivity is also reduced to some extent.5. The method etching by blasting corrosive vapor, which produced by thechemical reactions between the acid mixture solution of HF-HNO3-H2O and thesilicon materials, can produce corrosion pits at micron scale on the surface of MC-Siwafer cutting by DWS. And it can reduce the light reflectivity to about19%,eliminate the saw marks completely.6. The method etching by blasting corrosive vapor, which evaporated from theheat acid mixture solution of HF-HNO3-H2O, can produce corrosion pits atsub-micron scale on the surface of mc-silicon wafer cutting by DWS. And it canreduce the light reflectivity to about12%, eliminate the saw marks completely.7. The process of reaction vapor etching presents a parabolic dynamiccharacteristic, while the process of thermal vapor etching presents a linear dynamiccharacteristic.