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.