OPTICAL TECHNIQUE FOR RAPID INSPECTION OF VIA UNDERETCH AND CONTAMINATION

Sensitivity and resolution of an automated inspection system are improved and data processing loads for defect detection are reduced, increasing inspection speed, by fully illuminating an area corresponding to a nominal feature shape formed on a surface. Scanning of the illuminated area thus provides resolution of defects far smaller than the area of the illuminated spot. A preferred application of this automated inspection system is for the high speed screening of lamina or substrates having a pattern of through-holes formed therein, particularly for the formation of via connections therein. Screening for insufficient clear area of through-holes is done simply by applying a threshold to the output representing the amount of illumination transmitted and preferably reflected through the through-hole.