February_EDFA_Digital

edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 1 22 device to each of the subsequent layers in the structure. Steady state is reached approximately 10 seconds after bias is applied. In this example, the chip attach and heat spreader attach are thin, thus enabling rapid heat transfer between layers. However, they each contribute about 1.0° and 0.3°C/W, respectively, to the total thermal impedance of 2.52°C/W. This type of analysis clearly shows the con- tributions of each element in the heat conduction path. TTI SYSTEM CONFIGURATION A complete thermoreflectance-based thermal imaging (TTI) system is shown in Fig. 6. Major components include a transient imaging module (TIM), signal generator, and controller. The TIM drives the CCD camera exposure, bias signal for the DUT, and timing signal to the signal genera- tor to sync the LED illumination. These components are interconnected via a GPIB bus to the controller, which coordinates overall system management by means of the embeddedSWmodules—one for systemmanagement and another for DUT data acquisition and analysis. The 4-megapixel Si CCD camera used in this system is optimumfor illuminationwavelengths from400 to 800 nm and usable from 365 to 1060 nm. Shorter wavelengths in this range support spatial resolution of less than 250 nm. The configuration shown also includes a temperature controller for a thermal stage, which can be useful for calibration purposes, and an antivibration table. The key to achieving long-termreliability andmaximiz- ing MTTF is the ability to identify defects that may lead to a hot spot and thus to an early device failure. As shown in Fig. 1, a 10 to 20 degree hot spot could decrease MTTF by as much as two to 10 times. Fig. 5 The structure being analyzed (a) is shown above the heating curve (b). The x axis in the heating curve shows elapsed time fromthe beginning of the applied step bias pulse. From this plot, thermal impedance of the various layers in the heat flow path can be quantified. Fig. 6 Microsanj NT220 imaging system with optional accessories: probe station, thermal stage, and vibration isolation table. Spatial, temporal, and temperature resolution are 245 nm, 20 ns, and 0.1°C, respectively. Fig. 7 Comparison of MMIC gate temperatures measured using IRT and TTI, with gate tempertures calculated using finite element analysis.