Nondestructive characterization of sub-surface damage in rotational ground silicon wafers by laser acoustics

The present paper is concerned with the determination of the Young's modulus in rotational ground mono-crystalline silicon wafers to establish a measurement procedure with potential use in industrial applications. This procedure uses the laser-acoustic method based on surface acoustic waves. A difference measurement procedure was applied to determine the Young's modulus of single layers of less than 0.02@mm thickness. The layers were removed stepwise from the damaged wafer by reactive ion etching, allowing to identify a profile of the Young's modulus through the sub-surface region. This gave unique insights into the distribution of the damage perpendicular to the wafer surface. Several wafer sets were machined with various grinding tools, displaying the influence of tool specification, cutting speed and crystallographic orientation on the Young's modulus gradient. The measurements were performed parallel, perpendicular and 45^o to the grinding direction, revealing preferential orientations of the defects in reference to the machining directions and crystallographic orientation.