Illuminating Hidden Leakage Sites
As discussed previously, current leakage is one of the most prevalent failures in modern semiconductors and electronic devices. One of the most common techniques for locating current leakage is a liquid crystal, which is a quick and effective way of isolating failure sites; however, the liquid crystal has some limitations that prevent it from being useful in all cases. Liquid crystal works by using the heat generated by a leakage site to raise the temperature of the crystal to a “transition point”, where an analyst can optically observe a change in the properties of the crystal and thereby identify the leakage site. A more subtle failure may never be able to heat the liquid crystal to its transition point since smaller defects dissipate less power and therefore generate less heat. At the opposite end of the spectrum, high amounts of leakage can produce enough heat to raise the temperature of the entire device quickly enough that it is impossible to identify the transition point. To combat these shortcomings, fluorescent microthermal imaging can be used to supplement the standard liquid crystal.
Fluorescent microthermal imaging (or FMI) is similar to a liquid crystal in that it is driven by the heat produced by a leakage site. A thin layer of a UV-fluorescent, Europium based compound is painted onto the surface of a semiconductor die. The part is then placed under a light emission microscope, which uses a high-gain camera to analyze the light emitted by the fluorescent ink. When the device is powered, the heat generated by the failure increases the amount of light emitted by the ink; the power supply to the device is controlled by the microscope, and toggles on and off several times a second. The system acquires multiple images of the device, alternating between shots with the device powered up and powered down. By mathematically subtracting the power-off images from those taken with the power on, the change in fluorescence due to the heat caused by the leakage can be exactly pinpointed, thereby isolating the failure site.
Unlike liquid crystal, which is sensitive to absolute temperature (the only temperature that provides useful data is the transition point), fluorescent microthermal imaging is sensitive to temperature change. Due to the differential nature of the measurement, FMI is capable of detecting much smaller amounts of leakage current when compared to a liquid crystal. Additionally, since FMI uses very short acquisition times that prevent large amounts of heat diffusion, it can be used to isolate failures that generate large amounts of heat that are difficult to find with liquid crystal due to the rapid heating of the device.
While it may require a more complex setup to perform, fluorescent microthermal imaging is a valuable supplement to liquid crystal for isolating current leakage. The use of FMI greatly increases the number of failures that can be isolated, which in turn increases the chances of a successful analysis.