Koji Ashida

Crystallographic orientation dependence of SEM contrast revealed by SiC polytypes

In low energy scanning electron microscope (SEM) with primary electron energy less than 1.0 keV, the dependence of SEM contrast on crystallographic orientation within a range of 1.0 nm in depth has been investigated by utilizing 4H-SiC (0001) as a standard sample having a definitive electron penetration depth marker layer at hexagonal sites. Reflecting the difference of the direction of topmost two Si-C bilayers stacking sequence (0.50 nm in depth), clear bright and dark SEM contrast has been observed by adjusting the sample tilting and rotation angles by a conventional Everhart–Thornley type in-chamber detector. It is revealed that the brighter signal emission arises when the incident primary electron beam direction is almost parallel to the topmost stacking sequence direction. This angular coincidence was verified separately by correlating low energy SEM contrast from 3C-SiC (111) of no hexagonal sites with its electron back scattered diffraction pattern for identifying stacking sequence direction. The ...

Investigation of the surface morphology and stacking fault nucleation on the (000-1)C facet of heavily nitrogen-doped 4H-SiC boules

The stacking fault formation during physical vapor transport growth of heavily nitrogen-doped (mid-1019 cm−3) 4H-SiC crystals was investigated. Low-voltage scanning electron microscopy (LVSEM) observations detected the stacking fault formation on the (000-1) facet of heavily nitrogen-doped 4H-SiC crystals. Stacking faults showed characteristic morphologies, and atomic force microscopy (AFM) studies revealed that these morphologies of stacking faults stemmed from the interaction between surface steps and stacking faults. Based on these results, the stacking fault formation mechanism in heavily nitrogen-doped 4H-SiC crystals is discussed.

Improving mechanical strength and surface uniformity to prepare high quality thinned 4H-SiC epitaxial wafer using Si-vapor etching technology

As a new thinning and surface planarizing process of Silicon Carbide (SiC) wafer, we propose the completely thermal-chemical etching process; Si-vapor etching (Si-VE) technology. In this work, the effects of mechanical strength and surface step-terrace structure by Si-VE are investigated on the 4° off-axis 4H-SiC (0001) Si-face substrates. The indentation hardness of Si-VE surface is superior to the conventional chemo-mechanical polishing (CMP) surface even after epitaxial growth. The transverse strength of thinned Si-VE substrate is also superior to the conventional mechanically ground substrate. The surface step-terrace structures are observed by the low energy electron channeling contrast (LE-ECC) imaging technique. The latent scratch causes bunched step lines (BSLs) with various inhomogeneous step morphologies only on the CMP surface.

Detection of Defects on SiC Substrate by SEM and Classification Using Deep Learning

In recent years, next generation power semiconductor devices using semiconductors with large band gap such as SiC (Silicon Carbide) attract attention. It is very important to detect crystal defects, surface processing defects including polishing, defects contained in the SiC substrate, defects included in the epitaxial growth film, defects caused by the device forming process, and so on. This is because elucidating the cause of the detected defect and investigating the influence on device quality and reliability lead to development of a better manufacturing method. Recently, observation with a low energy scanning electron microscope (LE-SEM) which is more accurate than C-DIC and PL has been put to practical use. As a result, crystal information of just below the outermost surface can also be obtained. However, since image processing techniques targeting SEM images of SiC substrates have not existed so far, it has not been possible to efficiently and automatically extract defects from enormous amounts of data. In this paper, we propose a method for detecting defects on SiC substrate by SEM and classifying them using deep learning.

Investigation of Run-to-Run Fluctuation in Growth Conditions of Physical Vapor Transport Growth of 4H-SiC Crystals

We investigated the run-to-run fluctuation in growth conditions of physical vapor transport growth of 4H-SiC boules through observations of surface morphology on the (000-1) facet of the boules. The boules, which were grown under the same macroscopic growth conditions, exhibited slightly different surface morphologies. This indicates that some microscopic growth parameters that influence the surface morphology fluctuate between growth runs. We have considered the C/Si ratio of the vapor sublimed from the source material as a major parameter and discussed the associated variations in the physical and surface properties of the grown crystals.

Rearrangement of Surface Structure of 4o Off-Axis 4H-SiC (0001) Epitaxial Wafer by High Temperature Annealing in Si/Ar Ambient

Mechanism of surface roughening caused by the polishing induced subsurface damage on 4o off-cut 4H-SiC (0001) substrate during thermal etching, CVD epitaxial growth, and the subsequent high temperature annealing was investigated in the wide temperature range of 1000-1800°C. Different from the previous study based on a macroscopic characterization by optical microscopy, microscopic characterization based on a scanning electron microscopy (SEM) was employed in this study. By utilizing the SEM operated under various conditions, disordered step arrangements as well as stacking faults and dislocations were imaged. The obtained results revealed that the SFs cause the fluctuation in the step kinetics, resulting in the step bunching formation during the thermal process.

Wide (0001) terrace formation due to step bunching on a vicinal 4H-SiC (0001) epitaxial layer surface

The wide (0001) terrace formation due to step bunching on a vicinal 4H-SiC (0001) epitaxial layer surface was investigated using low-energy electron channeling contrast (LE-ECC) imaging and atomic force microscopy. LE-ECC imaging revealed that step bunching resulted in the formation of wide atomically-flat (∼200 nm) (0001) terraces on the surface, and the terraces tended to form in pairs. Terraces in a pair had almost the same width and often showed the same LE-ECC; moreover, the contrast of the two terraces, either bright or dark, appeared to be determined by the orthogonal misorientation of substrates. On the basis of these results, the formation mechanism of the paired terraces with the same LE-ECC on a vicinal 4H-SiC (0001) surface is discussed herein.

SEM and ECC imaging study of step-bunched structure on 4H-SiC epitaxial layers

Step bunching on a vicinal 4H-SiC (0001) epitaxial layer surface was investigated using low-voltage electron scanning microscopy (LVSEM) and electron channeling contrast (ECC) imaging. LVSEM observations revealed that the step bunching resulted in the formation of atomically flat wide (~250 nm) terraces on the surface, and the terraces tended to form in pairs. The two terraces in paired terraces often showed the same electron channeling contrast as each other, and the contrast of the two terraces, either bright or dark, appeared to be determined by the orthogonal misorientation of substrates. On the basis of these results, the formation mechanism of the step-bunched structure on a vicinal 4H-SiC (0001) surface is discussed.

Low energy electron channeling contrast imaging from 4H-SiC surface by SEM and its comparison with CDIC-OM and PL imaging

Low energy electron channeling contrast imaging (LE-ECCI) by scanning electron microscopy (SEM) was adopted to evaluate both the direction and length of the topmost hexagonal stacking sequence of the Si–C bilayers on 4H-SiC (0001). LE-ECCI revealed the change in the dangling bond configuration at step edges depending on SiC thermal etching rate, which was difficult to be detected by optical microscope (OM) and even by atomic force microscopy (AFM). Furthermore, LE-ECCI was applied to evaluate the atomic structure of polytype inclusions in commercially available 3-inch diameter 4o off-axis 4H-SiC (0001) epitaxial wafer. The validity of LE-ECCI was discussed by comparing the one with two kinds of widely used wafer inspection methods: confocal OM with differential interference contrast (CDIC-OM) and photoluminescence (PL) imaging.

Direct Observation of the Edge Termination of Surface Steps on 4H/6H-SiC {0001} by Tilted Low-Voltage Scanning Electron Microscopy

We demonstrate a simple method for direct observation of the stacking orientation on 4H/6H-SiC {0001} surfaces by low-voltage SEM. The difference in the direction of the stacking orientation is observed as SEM contrast. By utilizing this technique, the bond configuration at {1-10n} steps can be determined by the SEM contrast.