A detailed one-dimensional sensitivity map displaying the threshold laser pulse energy required to induce latchup in the four-terminal test structure. The inset shows the location and direction of the scan. The most sensitive location (i.e., the location requiring the least pulse energy for latchup) is found near the edge of the negative-current well. The two double peaks represent where the laser was scanned over the metal contacts. The metal lines block any incident laser light. The laser spot has a Gaussian spatial profile with a spot size on the same order as the width of the metal lines. Consequently, there is always some light that propagates past the metal lines into the device, even when the laser spot is centered on one of the lines. Thus, the devices can still be latched up by increasing the laser pulse energy; however, as shown here, this requires a considerable increase in energy.