Residual stress and lattice distortion mapping of patterned devices which failed electromigration testing using the microbeam X-ray diffraction (MXRD) technique

Abstract

Spatially resolved residual stress and lattice distortion measurements have been made on experimental RF NPN devices which failed electromigration testing. The experimental devices underwent a steady state long term life test for 38272 hours at a die temperature of 100 degrees Celsius and device emitter currents of 100 mA. Residual stress contour maps of the Au interconnect metallurgy of these patterned devices were constructed with spatial resolution approaching 13 microns. The stress contour maps clearly show areas of the Au interconnect emitter lines, approximately 17 microns wide, which buckled when the local stresses exceeded the adhesion limits of these films. A stress gradient was observed going along the Au line from the emitter to the wire bonded pad which went from slightly compressive near the emitter to tensile approaching the wire bonded lead, and consistent with electromigration failures. In addition, lattice distortions as determined by integral line breadths, which were least square fitted by a Gaussian Lorentzian convolution of the instrument function, show a large continuous increase along the Au interconnect line in going from the emitter to the wire bond pad. The data was acquired using a 4-circle x-ray diffractometer which was specifically constructed to study electromigration issues in-situ. The diffractometer has a very low sphere of confusion (<5 microns) and is equipped with tapered glass capillary optics, a mechanical, electrical, and thermal stressing stage and various energy dispersive (EDX) or position sensitive (PSD) detectors.