VCSEL Diode Failure Analysis

Gideon Analytical Labs received several VCSELs (vertical-cavity surface-emitting laser) with diminished light capacity were submitted for failure analysis. A VCSEL is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers which emit from surfaces formed by cleaving the individual chip out of a wafer. Thus, all the light is emitted from one spot, the laser aperture. The devices were tested on a curve tracer to determine whether any junction problems (junction inversion, leakage current, facet oxidation, etc.) were present. There was no difference in the curve trace of a good and a failed device.

The laser avalanches at the same voltage as the suspected devices. If something were wrong with the junction, leakage current would be evident. Having received information about ESD leakage current in the reverse direction, Gideon measured both good and bad VCSELs and found no distinguishable difference (measuring down to 100 picoamps) between the two devices. The VCSELs were enclosed in a hermetic T0-46 can with a slanted anti-reflective glass coating optically radiating in the region of 795 nanometers. The gold ball bond is connected to the laser via a gold trace line which makes contact to the upper Bragg P-type reflector. The small circle (the laser aperture) in the middle is where the light emanates from. The circumference of the circle was scanned (see white arrow below) using SEM-EDS to determine whether this area has undergone any changes. The spectrum is shown below. The red arrow points to the laser aperture. Through a process of investigation, experimentation, and gas analysis, Gideon determined the diminished light capacity per unit current input was due to a high temperature and possible absorption of hydrogen gas. Hydrogen is a poisoning agent for VSCELs; it alters the lattice structure. If this has occurred on the aperture (the eye), it inhibits the current. If the current is inhibited, the light intensity diminishes.

Internal Structure is shown below with the link here. http://www.physics.ucc.ie/pres/SS01-L2-temporal/sld002.htm Gideon Analytical Labs can solve production and manufacturing problems that detract from yields and performance degradation. In class 3 environments, reliability is the main concern after the product has been debugged. We can solve many of these problems and fix the problem on single source military suppliers. 845-255-5356