Height Inspection of Wafer Bumps Without Explicit 3-D Reconstruction

Die bonding in the semiconductor industry requires placement of solder bumps not on PCBs but on wafers. Such wafer bumps, which are much miniaturized from their counterparts on printed circuit boards (PCBs), require their heights meet rigid specifications. Yet the small size, the lack of texture, and the mirror-like nature of the bump surface make the inspection task a challenge. Existing inspection schemes generally reconstruct every bump surface. This work addresses by how much can the task be simplified if merely the bump heights are inspected against the specification. It is assumed that ball bumps are used as the wafer bumps. An imaging setup is described that lets the peaks of the ball bumps be distinguishable in the image data. A measure is also described that reveals how well the ball bumps meet the height specification without going through explicit 3-D reconstruction. The measurement, in the form of a 3 × 3 matrix extractable from the image data, is sensitive to variations in the bump heights, but not to 2-D uncertainties in soldering the bumps onto the wafer substrate, or small variations in the placement of the wafer in 3-D. Experimental results are shown to illustrate the effectiveness of the proposed system.