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edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 1 24 temperature rise at the hot spot as shown in Fig. 9c is approximately 50°C. GaAs AND GaN HEMTS GaAs and GaN high electron mobility transistors (HEMTs) with submicron features are widely used for a wide variety of high frequency and high power wireless communication applications, including smartphones and military electronics. These applications require high reliability and long operating life. This example illustrates how TTI can be used to analyze and detect a small defect in a GaAs HEMT with submicron features. A100 × microscopeobjective andawhite light illumina- tion source are used for this example. Figure 10a shows the optical image of a failed die at 100 × magnification while Fig. 10b zooms in on the defect location. Figure 10c is the thermal image of the hot spot with an applied bias to the device. The hot spot size is 500 nm. Figure 11 displays the temperature profile through the hot spot, which indicates a temperature riseof nearly 15°C. HEMT TO DIAMOND INTERFACE In this example, thermal imaging is used to investigate the integrity of a device interface, an interface that plays a direct role in determining the device operating temperature. [8] Diamond, which has a thermal conductivity of 2000 W/mK, is proven to be an effective heat sink material for high power GaN HEMTs. This GaN-on- diamond interface, illustrated in Fig. 12, is formed by: a) bonding the GaN face to a temporary sacrifi- cial carrier; b) etching away the substrate and tran- sition layers; c) depositing a 35-nm-thick dielectric followed by a diamond layer on the backside of the GaN HEMT; and d) removing the sacrificial carrier. Figure 13 shows the resulting images of the HEMT gate region. A key finding in this analysis is the detection of three hot spots, possibly caused by gapsor imperfections in theGaN-diamond interface. CONCLUSION A relatively small temperature increase can have a significant adverse impact on long-term device reliability. Industry trends favor decreased feature sizes and increased device complexity. The result- ing power densities are making it more important than ever to possess a full understanding of device thermal behavior under all operating conditions to ensure long-term reliability. While traditional thermal analysis techniques do an adequate job of determining average temperature behavior over a region of interest, they lack the resolution to accu- rately measure temperature on a submicron scale. To gain this knowledge, it is necessary to use a thermal analysis technique with a spatial resolu- tion consistent with the device features. Traditional techniques fall short in this regard while thermal imaging systems based on the thermoreflectance principle offer the attributes and performance Fig. 13 Optical and thermal images of HEMT gate region showing hot spots caused by small voids in the HEMT-diamond interface. Fig. 12 Process steps to implement GaN-on-diamond structure. Fig. 11 Temperatureprofilepassing through thehot spot indicates ~15°C temperature rise. Fig. 10 Optical image of GaN HEMT with small defect, (a) and (b) and thermal image, (c), of resulting 500 nm hotspot. A 10-minute averaging time enables a 14-microwatt power detection.