This article presents a case study involving flash memory bit failures characterized by threshold voltage changes due to positive gate disturb stress. An inconsistency in failing bit behavior, which was found to be dependent on the test mode, was explored to provide an electrical explanation for the failure. The underlying defect was isolated and subsequently identified by physical analysis.

Localizing defects in one-of-a-kind failures can take days, weeks, or even months, after which a detailed physical analysis is conducted to determine the root cause. TEM and STEM play complimentary roles in this process; TEM because of its superior spatial resolution and STEM because it produces images that are easier to interpret and is less susceptible to chromatic aberrations that can occur in thicker samples. In the past, the use of STEM in FA has been limited due to the time required to switch between imaging modes, but with the emergence of TEM/STEM microscopes with computer controlled lenses, the use of STEM is increasing. This article provides an overview of STEM techniques and present examples showing how it is used to characterize subtle and complex defects in ICs.