An Atlas of ESD Failure Signatures in Vertical Cavity Surface Emitting Lasers
AOC herein describes a collection of material degradation features observed in Vertical Cavity Surface Emitting Lasers (VCSELs) that have been intentionally degraded with a range of electrostatic discharge (ESD) stress conditions. Failure analysis techniques employed include emission microscopy, Focused Ion Beam (FIB) microscopy and Transmission Electron Microscopy (TEM). The results have enabled higher confidence in root-cause determination for failed VCSEL devices.
Introduction
The application of sophisticated characterization techniques (e.g. emission microscopy, FIB, and TEM) to understanding VCSEL failure mechanisms has been well documented and analysts continue to add to and improve on these techniques. Typically, in the absence of gross mechanical or electrical overstress damage, these techniques often reveal that the active layers of a failed VCSEL contain a complex patterned dislocation array. Acting as non-radiative recombination sites these arrays degrade the optical and electrical properties of the laser device. However, while visually imposing, the dislocation arrays represent only the final cause, not root cause of failure. Rather, non-radiative recombination of minority carriers at a defect site in the active layers of a light emitting diode provides a positive feedback mechanism that typically leads to the self-supporting growth of a dislocation array from that defect site. Consequently, a single defect such as a dislocation propagating from the substrate is able to generate a dense dislocation array that covers the entire active region of the device. Thus, the goal in the analysis of many failed VCSELs is to find the source defect of the dislocation array. Once discovered, the next challenge is to attribute that defect to some outside event, design flaw, or growth anomaly. The emphasis of this paper is on the characterization of defects caused by outside events, primarily Electrostatic Discharge (ESD) and minor Electrical Overstress (EOS) events. The data shown below was generated with AOC’s 14um oxide confined devicesi though the principles described are applicable to other VCSEL designs.
An Atlas of ESD Failure Signatures in Vertical Cavity Surface Emitting Lasers
