Figure 1 shows the TEM images of iodobromide crystals with different target diameters in the prepared nuclear emulsion. Although the size of the iodobromide crystals with Rtarget = 100 nm was approximately 70 nm, which is a bit smaller than the target diameter, the actual diameter of the crystals was almost the same as the target diameter. Under electron beam illumination, the silver iodobromide reacted with electrons by a reaction similar to image development as follows:
2AgBr + 2e- → 2Ag + Br2 (1)
Figure 2 shows the evolution of iodobromide crystals with Rtarget = 300 nm under electron beam illumination. Horn-shaped silver precipitates were formed by the electron beam illumination and the successive formation of small silver precipitates resulted in collapse of the crystal shape after a few minutes. Thus, although we quickly took an image after illumination by the electron beam, the shapes and sizes of the iodobromide crystals in the TEM images were somewhat influenced by the reaction shown in (1). We are currently trying to establish a precise method for the evaluation of iodobromide crystal diameter without reaction with electrons.
Figure 3 shows the XRT images of the SiC epitaxial wafers recorded on the emulsion plates with crystals of different target diameters and a commercially available plate (Ilford). The XRT image recorded on the emulsion plate with the target iodobromide crystal diameter of 100 nm is much clearer than that recorded on the other emulsion plates. In these images, linear contrasts of basal plane dislocations (BPDs) on (0001), large white circular contrasts of threading screw dislocations (TSDs) having a c-component of the Burgers vector, and small contrasts of TEDs having the Burgers vector of 1/3 < 11–20 > were observed. The contrasts of TEDs are different depending on the direction of the Burgers vector. For example, some TEDs such as TED-I were imaged as bright spots, while other TEDs such as TED-II were imaged as dark spots. The contrasts of dislocations were more clearly observed by our 100-nm emulsion plates than the other plates; in particular, dark-contrast TEDs, which are often difficult to observe with commercially available emulsion plates, were definitely observed. Figure 4 shows XRT images of TEDs with the six different Burgers vector directions recorded on our 100-nm plates and on commercially available nuclear emulsion plates. All types of TEDs were clearly observed by our 100-nm plates. In particular, the TEDs exhibiting dark contrasts were much more clearly recorded on our 100-nm plates than on commercially available nuclear emulsion plates. Furthermore, the dynamic range of the image seems to be wide and the BPD-II, which are in the substrate beneath the epitaxial layer and are difficult to image on commercially available emulsion plates, were observed by our 100-nm emulsion plates.
Figure 5 shows the XRT images of a 150-mm SiC wafer recorded on our 150 x 150 mm2 emulsion plate with Rtarget = 100 nm and the appearance of the recorded emulsion plate. Note that 7,739 microscope images were acquired and stitched together as one image. We successfully observed almost a whole 150-mm wafer in one plate with high resolution and wide dynamic range, as shown in Fig. 5(b). The contrasts of TEDs were clearly observed, and it is possible to identify the direction of the Burgers vector from the images. The exposure time for acquiring XRT by our 100-nm emulsion plates is almost the same as that for the commercially available X-ray films. With almost the same exposure time, the XRT images were much more clearly observed for our high-resolution nuclear emulsion plates than for commercially available X-ray films.
The results of the present study indicate that the small size iodobromide crystals can obtain XRT images with high resolution as well as wide dynamic range without reduction in the throughput of synchrotron experiments. Furthermore, our technology can be applied in the XRT observation of larger size wafers if a wide synchrotron X-ray beam is available. Our development will contribute to advances in electronic materials especially in the field of power electronics, in which defect characterization is important for improving the performance and yield of devices.