February_EDFA_Digital

edfas.org 23 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 1 EXAMPLES: APPLICATIONS AND BENEFITS The following examples help illustrate the applications and benefits of TTI for evaluating the thermal behavior of devices. MMIC POWER AMPLIFIER The gate temperatureof a two-gateMMICpower ampli- fier was measured using IR thermography (IRT) and TTI. [2] The gate temperature was also calculated using finite element analysis (FEA) and compared with themeasured temperatures in Fig. 7. As shown, the agreement between TTI and FEA ismuch better than between IRT and FEA. The main reason for this discrepancy is the grossly inadequate spatial resolution of IRT (5 µm) for the gate size of the MMIC device (0.25 µm), whereas the spatial resolution of TTI at 0.3 µm was much better. Another important factor was that the emissivity of the gate surface was very low at about 0.3, which makes IRT less accurate. However, this low emissivity implies high reflectivity, which is ideal for TTI. LOGIC INTEGRATED CIRCUIT The following example demonstrates the transient analysis capability with a high speed logic IC. [6] The chip measures 1.6 x 1.1mmand is 500mmthick. Figure 8 shows the resulting thermal intensity map at four different time intervals from 0.5 to 3.0 ms after the chip is powered on. The image in Fig. 8a shows the left side of the chip heating almost immediately following the applied bias and reach- ing a peak at about 0.9ms, as shown in Fig. 8b. At 0.95ms, Fig. 8c, another region in the upper central portion of the chip, starts dissipating power. At 3.0 ms, Fig. 8d, the left side of the chip has cooled down and a hot spot is noted in the right central portion of the chip. Based on the device circuit design, the initial heating shown at 0.5 ms on the left side of the device was an anticipated event. The heating at 3.0ms, however, was not anticipated and is related to a latch-up failure, indicated by the circle on the temperature plot shown in Fig. 8d. These time-dependent thermal eventswouldnot have been detected without transient thermal analysis. Once discovered, it is the circuit designer’s role to determine what these events indicate; somemay not have an impact on device performance or device reliability, while others may lead to early catastrophic failure. MULTI-FINGER MOSFET In this example [7] , a small gate defect in a siliconmulti- finger MOSFET illustrates the spatial resolution of the thermoreflectance technique in the detection of a hot spot. The LED illuminationwavelength for this example is 470 nm. Figure 9a shows the thermal image of the device overlaid on the 50 × optical image. Figure 9b provides an enlarged view of the region with the defect. Figure 9c displays the temperature scan along the line-scan shown in Fig. 9b, which passes through the observed hot spot. The full-width-at-half-maximum hot spot size is approxi- mately 1.4 µm. An averaging time of 3 minutes for this image enables a temperature resolution of approximately 0.1°C. The Fig. 8 Time sequence of thermal image of logic IC at: (a) 0.5 ms, (b) 0.9 ms, (c) 0.95 ms, and (d) 3.0 ms. Fig. 9 Overlay of thermal image on the 50× optical image shows the defect location on theMOSFET. Figure 8(c) shows the temperature plot corresponding to the dashed line in Fig. 9(b). (a) (b) (c) (d)