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In the variable pressure SEM, positive charges generated by collisions between incident electrons and residual gas molecules inside the chamber act to neutralize the negative charge on the specimen surface. Hitachi has developed an ultra variable-pressure detector (UVD), which can detect the charge cascade under low-vacuum conditions . In addition to this SE detector functionality, the detector can also be used for CL observation of materials.
In order to efficiently detect SEs generated from the specimen when it is irradiated by an electron beam under a low vacuum, a bias voltage is applied to the detector front edge. This forms an electric field between the detector and the specimen, which accelerates the SEs generated at the specimen surface. The SEs then collide with the residual gas molecules in the low-vacuum environment, ionizing the molecules into positive ions and electrons while simultaneously generating light. By detecting this light using the UVD during the beam scanning, images that reflect topological information about the specimen surface can be captured.
Figure 1b) shows the results of observation of a fracture surface of carbon fiber reinforced plastic (CFRP) using a UVD mounted on a model SU3800 tungsten SEM (W-SEM) system. The detailed surface roughness of the resin fracture surface and the state of the resin around the carbon fibers could be observed in the low-vacuum condition while suppressing charging. In order to respond to a wide range of needs, Hitachi have been able to provide the UVD in a range of microscopes from a W-SEM system equipped with a thermal electron gun to a field emission SEM (FE-SEM) system equipped with a Schottky electron gun.
Figure 1(a) shows a schematic diagram of the UVD. In order to efficiently detect SEs generated from the specimen when it is irradiated by an electron beam under a low vacuum, a bias voltage is applied to the detector front edge. This forms an electric field between the detector and the specimen, which accelerates the SEs generated at the specimen surface. The SEs then collide with the residual gas molecules in the low-vacuum environment, ionizing the molecules into positive ions and electrons while simultaneously generating light. By detecting this light using the UVD during the beam scanning, images that reflect topological information about the specimen surface can be captured.
Figure 1(b) shows the results of observation of a fracture surface of carbon fiber reinforced plastic (CFRP) using a UVD mounted on a model SU3800 tungsten SEM (W-SEM) system. The detailed surface roughness of the resin fracture surface and the state of the resin around the carbon fibers could be observed in the low-vacuum condition while suppressing charging. In order to respond to a wide range of needs, Hitachi have been able to provide the UVD in a range of microscopes from a W-SEM system equipped with a thermal electron gun to a field emission SEM (FE-SEM) system equipped with a Schottky electron gun.
Since information such as the crystallinity and chemical properties of a specimen can be obtained by measuring CL, which is generated by irradiating the specimen with an electron beam, it is used for analysis of specimens such as semiconductors, ceramics, minerals, and fluorescent materials. The UVD is also capable of detecting CL information. CL images can be captured using the UVD by turning off the bias voltage applied to the detector front edge. As a result, the light that carries the secondary electron information as described in the previous section can no longer be detected, and only the CL signal generated from the specimen is detected. Figure 2 shows an example of observing crystal dislocations (defects) in gallium nitride (GaN) which is used in power emiconductors with a model SU5000 FE-SEM. When a GaN substrate is irradiated by an electron beam, areas of good crystallinity generate CL and appear bright, while areas containing threading dislocations do not emit light and appear dark. The black dots in Figure 2 correspond to areas where no light has been emitted due to the presence of dislocations, and this shows that dislocations in GaN can be clearly identified.
Next, another example of CL observation for titanium dioxide (TiO2) nanoparticles is described. TiO2 nanoparticles exist as a rutile type and an anatase type, and the anatase type is used as a photocatalyst. Since the differences between these are only in the crystal structure, differentiating them by only SEM observation is difficult. However, the CL emission intensity for anatase is higher than that for rutile, and the brighter emission can be distinguished by CL observation using the UVD. Figure 3 shows the results of observing a mixture of rutile and anatase TiO2 nanoparticles using a model SU7000 FE-SEM system. Although no difference can be distinguished in the SE (a) or BSE (b) images, in the CL image (c) it can be seen that some of the particles emit light. Furthermore, the particles that emit light can be identified by overlaying the CL image on the BSE image (d). CL observation using the UVD enables a wide variety of applications such as, observation of crystal defects in semiconductors, observation of the zircon zonal structure in mineral samples, investigation of high purity alumina samples, or evaluation of fluorescent materials.
