Theoretical analysis on the effects of crystal anisotropy on wiresawing process and application to wafer slicing

Most of the crystals sliced using wiresaw are anisotropic to an extent. The effect of crystal anisotropy on the process of slicing using wiresaw is studied and presented in this paper. A method is proposed to determine the direction of approach (DOA) which will give a better surface finish and reduce deviation from the desired surface normal by maintaining symmetry in material removal rates on the two sides of the wire. The effect of cleavage anisotropy on wiresaw slicing is also studied. If the DOA is perpendicular to a cleavage direction, then the longitudinal direction of the wire aligns with the cleavage direction which increases the tendency of wafer breakage, resulting in lower yield of the wafers. This can be easily avoided by choosing an appropriate DOA. Theoretical analysis is carried out using the proposed methods for slicing silicon wafers. Recommendations are made for three most commonly sliced orientations of silicon: (100), (110) and (111). DOA can be any direction for (100) and (110) wafers from the symmetry point of view but preferred DOAs do exist for these wafers from cleavage point of view. For (111) crystal there are exactly six DOAs with symmetry. However, these six DOAs do not lie in the preferred zones suggested by cleavage criterion. It is suggested that in such situations the symmetry criterion should be given precedence over the cleavage criterion during wiresawing process, as the semiconductor industry has strict tolerances in place for surface normal deviation and flatness.