Gross fracture pits at the intersection of two single scratches during grinding of silicon

Abstract

Infeed grinding is used in the industry for grinding thick silicon wafers. The grinding process being a series of overlapping scratches, leaves behind the surface and sub-surface damages in the silicon. Continuous in-feed grinding of a thick wafer when used to produce thin wafer leads to breakage upon release from the fixture. This happens because of not only damage induced by a single scratch, but also due to cumulative damage induced as the grinding proceeds with several overlapping and intersecting scratches. In a single scratch, depths below a critical depth can cause material removal without gross fracture and pits; however, it is not clear if this applies to multiple intersecting scratches while grinding silicon. In this study, the grinding operation is performed for a very short period of time, gradually removing the material over only a partial region of the wafer surface while leaving the remaining portion unground. This produces multiple intersecting grinding scratches of increasing depth. Scanning electron microscopy and surface profile measurements of these scratches show that while in a single scratch, material is removed without gross fracture and pitting; when two such scratch intersects, it results in gross fracture at the site of their intersection. Such gross fractures at intersections can happen even at a very shallow depth of scratch as low as 28 nm. This gross fracture at the intersections act as local sites to initiate wafer damage and material removal. Based on the results, we attempted to redefine the critical depth of cut for grinding operation. The study paves the way to manufacture thin damage-free wafers needed for the next generation thin silicon solar cells.