Finite Element simulations and Raman measurements to investigate thermomechanical stress in GaN-LEDs

Typical failures in microelectronics range from misalignments, non-uniform filling, delamination, missing solder bumps, large voids and cracking. Many non-destructive methods are available to inspect and detect these functional faults. Raman spectroscopy and Finite Element Modeling are applied to analyze and predict stress phenomena in gallium nitride (GaN) crystals grown on sapphire and implemented in blue light emitting diodes. The LEDs are assembled onto copper substrates by AuSn reflow soldering. The manufacturing process is simulated through FEM to investigate the stress distribution on the GaN layer. To validate the model, Raman measurements are performed to study the change in position of peaks in the spectrum in relation to the stress phenomena. Compressive stress with values of ca. 1400 MPa are recorded in the central area of the LEDs. Along the borders and at the corners, relaxation processes occur. The validation of the model allows to predict the behavior of the semiconductors in different thermal regimes, between -50 and 180 °C. The stress values do not change linearly by increasing the temperature. To take AuSn creep pohenomena in the FEM into account, after cross sectioning of the assembly, nanoindentation measurements were performed and creep deformation on the AuSn interconnection was measured at room temperature.