Deformation of Silicon Wafers by Thermal Oxidation

Irregular mask misalignment is sometimes observed in the development of 64k MOS RAM's. It appears as if the distance between two patterns on a silicon wafer changes during several processing treatments. The possibility of deformation of the silicon wafer by thermal oxidation being the cause of this is discussed. Direct measurement of the distance between the two patterns before and after thermal oxidation and after removal of the oxide films shows that the distance is not affected by thermal oxidation, except for the expansion due to the difference between the thermal expansion coefficients of silicon and silicon dioxide. This is verified by measuring warpage before and after thermal oxidation and after removal of the oxide on one side and both sides of a wafer. However, the distance between the two patterns varied from wafer to wafer. This is apparently caused by the stress during contact printing to mark the patterns and is presumably the cause of the mask misalignment.