Detectability of automotive power MOSFET on-resistance failure at high current induced by Wafer Fab process excursion

• Characterization of on-resistance drift on low voltage vertical power N-MOSFETs dedicated to micro-hybrid automotive application.
• A complete Failure Analysis, including electrical and physical investigations, describes the process used to determine the failure mechanism.
• A minor Wafer Fab process variability was demonstrated to be at the origin of nanometric layer of oxide residue, identified as the origin of the failure.
• This defect has created a resistive contact in the source areas of the power VDMOSFET, generating on-resistance increase.

This paper presents the meaningful consequence of a minor Wafer Fab process variability, generating on-resistance (RDSon) drift on low voltage vertical power N-MOSFETs dedicated to micro-hybrid automotive application. The research challenge concerned the fact that failure was not detected in production but at customer level; therefore, it was determinant to understand the root cause of this failure mode to drive corrective actions in order to secure customer by detecting and delivering correct parts. The originality of that paper concerns the necessity to use complementary Failure Analysis (FA) investigations, needed to determine the origin of the failure without any possibilities to perform any fault localization. First, assembly investigations are presented to discriminate if the failure was coming from customer or provider. Then, electrical characterizations at high current confirmed the RDSon failure. Thus, strategic physical destructive FA approaches were combined in order to discriminate all potential root causes, using physical step-by-step deprocessing, FIB milling cross-sections and TEM analysis. Finally, a nanometric layer of oxide residue was identified as the origin of the failure and corresponded to a wafer Fab process defect, induced by a lack of oxide overetch at spacer definition process step. These results allowed implementing corrections and improving test screening to protect customer.