Analysis of residual stresses in laser trimmed alumina microelectronic substrates

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Virginia Tech


The research presented here investigates the effects of laser trimming on the state of stress in alumina Al₂O₃ hybrid microelectronics substrates. Evaluation of stress was performed using x-ray diffraction residual stress analysis and dynamic strain measurements using strain gages before and after laser trimming. X-ray diffraction measurements were carried out in both the longitudinal and transverse directions on the front and back sides of the substrates. The dynamic strain measurements were performed in situ with strain gages attached to the bottom of the substrates while the substrates were trimmed with a 400 watt YAG laser.

The substrates were characterized using optical microscopy, scanning electron microscopy / energy dispersive x-ray analysis (SEM/EDAX), electron probe microanalysis (EPMA) and electron spectroscopy for chemical analysis (ESCA). The results from these characterization steps gave results for fractography (optical), surface and bulk composition (SEM/EDAX), chemical composition (ESCA) and phase analysis (EPMA).

Results show that laser trimming produces stress gradients which are generally tensile in nature and could have deleterious effects on the mechanical integrity of the substrates if used in hybrid microelectronic applications. Furthermore the stress distribution across the substrates was found to be uniformly distributed showing no peak stresses near the heat affected zone (HAZ) boundary. Phase analysis determined that the substrates contained a magnesium aluminum spinel phase (MgAl₂O₄) and that the glass and pore phases are randomly distributed in the substrates. This could have some overall effect on the state of residual stress in the substrates after they have been laser trimmed.