Figure 2844 (a) shows the OBIRCH signals obtained from metal 1-metal 2-via chains in PCM (process control monitor) structures of a wafer. The typical resistance of the chains was 500 Ω, while the OBIRCH signals presented the locations with in-line-test resistance of 1050 Ω. Figure 2844 (b) shows FIB (Focused-Ion-Beam) cross sectional image of this high-resistance location, indicating clearly that the via structure had been damaged by a via overetch and thus, affecting the surface resistance of the 1st metal layer.
Figure 2844. (a) OBIRCH signals obtained from metal 1-metal 2-via chains in PCM (process control monitor) structures, and (b) FIB (focused ion beam) cross sectional image of this high-resistance via (marked by the arrow). Adapted from 
 Peter Jacob and Félix Beaudoin, OBIRCH – Monitoring As A Reliability-Failure-Analysis and Screening Tool, ISTFA proceedings 2001 (ASM International), pp. 397-403.