Abstract
Today the physical vapor transport process is regularly applied for the growth of bulk SiC crystals. Due to the required high temperature of up to 2400 °C, and low gas pressure of several Mbar inside the crucible, the systems are encapsulated by several layers for heating, cooling and isolation inhibiting the operator from observing the growth. Also, the crucible itself is fully encapsulated to avoid impurities from being inserted into the crystal or disturbing the temperature field distribution. Thus, once the crucible has been set up with SiC powder and the seed crystal, the visible access to the progress of growth is limited. In the past, X-ray radiography has allowed this limitation to be overcome by placing the crucible in between an X-ray source and a radiographic film. Recently these two-dimensional attenuation signals have been extended to three-dimensional density distribution by the technique of computed tomography (CT). Beside the classic X-ray attenuation signal dominated by photoelectric effect, Compton effect and Rayleigh scattering, X-ray diffraction resulting in the crystalline structure of the 4H-SiC superimposes the reconstructed result. In this contribution, the achievable material contrast related to the level of X-ray energy and the absorption effects is analyzed using different CT systems with energies from 125 kV to 9 MeV. Furthermore the X-ray diffraction influence is shown by the comparison between the advanced helical-CT method and the classical 3D-CT.
Highlights
Latest results of the in situ computed tomography (CT) of bulk 4H-SiC crystals [1] show that it is possible to analyze the three-dimensional surface geometry of 3” boules as shown in Figure 1 even during growth applying the Physical Vapor Transport (PVT) process.Due to the high penetration depth of the 3” SiC boule in combination with the relatively lowX-ray energy applied, the achievable image quality in terms of density fidelity and contrast resolution is questioned
The experimental signal distribution corresponds to an average X-ray energy level defined by the maximum acceleration of the electrons inside the X-ray tube given by the tube voltage, the Tungsten target and the pre-filtration setting described by the thickness of an output window and metal filter sheet
Tin (Sn) setting corresponds best to a 60 kV mono-energetic attenuation characteristic, the 220 kV and 2 mm Sn setting to the 100 kV mono-energetic and the 450 kV and 1 mm Sn to a 200 kV
Summary
Latest results of the in situ computed tomography (CT) of bulk 4H-SiC crystals [1] show that it is possible to analyze the three-dimensional surface geometry of 3” boules as shown in Figure 1 even during growth applying the Physical Vapor Transport (PVT) process.Due to the high penetration depth of the 3” SiC boule in combination with the relatively lowX-ray energy applied, the achievable image quality in terms of density fidelity and contrast resolution is questioned. Latest results of the in situ computed tomography (CT) of bulk 4H-SiC crystals [1] show that it is possible to analyze the three-dimensional surface geometry of 3” boules as shown in Figure 1 even during growth applying the Physical Vapor Transport (PVT) process. Due to the high penetration depth of the 3” SiC boule in combination with the relatively low. X-ray energy applied, the achievable image quality in terms of density fidelity and contrast resolution is questioned. It is expected that the detection of voids or surface inhomogeneity is limited at the high penetration regions of the crystal. In previous contributions [2,3], the image quality of the in situ CT scan has been addressed, considering the peripheral furnace and vacuum chamber structure. Technique to possibly increase X-ray energies and crystal sizes.
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