On the nature and removal of saw marks on diamond wire sawn multicrystalline silicon wafers

Abstract

Clearly visible saw marks are a significant barrier to commercial use of diamond wire sawn multicrystalline silicon wafers for solar cells. Two types of saw marks on the diamond-cut multicrystalline silicon wafers are identified—the millimeter scale round-run fringes caused by round-running of the saw wires, and the micron scale scratches caused by scribing of the diamond tips. The latter consists of smooth and shiny grooves covered by a thin layer of amorphous phase. The micro-roughness of diamond-cut wafers is actually ~25% less than that of the conventional slurry-cut wafers. The reason for the visibility of the round-run fringes to naked eyes, and for the relatively rough appearance of diamond-cut wafers, is the visual enhancement from the shiny scratches. Therefore, the key to remove the round-run fringes is to roughen the smooth grooves, as flattening the very slightly sloped fringe zones is very difficult due to lack of chemical contrast over them. Acid-etching texturization cannot remove the saw marks on the diamond-cut silicon wafers. Alkaline-etching can only remove the saw marks on grains near (001) orientation. A vapor blast etching method has been attempted. The preliminary result is encouraging—complete removal of the saw marks has been achieved, along with a good surface texture, which reduces the light reflectivity to 19%.