Materials Science Forum Vols. 778-780

Abstract: The physical and electrical properties of p⁺-Si/n-4H-SiC and n⁺-Si/n-4H-SiC heterojunctions fabricated by using surface-activated bonding (SAB) were investigated by scanning electron microscopy (SEM), current-voltage (I-V) and breakdown characteristics measurements at raised ambient temperatures. The I-V characteristics for the reverse bias voltages of the two junctions were compared with the expectations based on Frenkel-Poole, and trap-assisted tunneling models. The results of calculations using the trap-assisted tunneling model were close to the measurements.

Abstract: A novel abrasive-free polishing method called catalyst-referred etching (CARE) has been developed. CARE can be used to chemically planarize a silicon carbide (SiC) surface with an etching agent activated by a catalyst. Platinum (Pt) and hydrofluoric (HF) acid are used as the catalyst and etchant, respectively. CARE can produce an atomically flat and crystallographically highly ordered surface of 4HSiC (0001) with a root-mean-square roughness of less than 0.1 nm regardless of the cut-off angle. However, industrial use of CARE is difficult because of HF acid usage. In this study, pure water was investigated as an alternative etchant to HF acid. We examined CARE using pure water by applying it to the planarization of a 4HSiC substrate and observed a feasible performance. The removal mechanism is considered to be the dissociative adsorption of water molecules to the SiC bonds of the topmost Si atom, namely the hydrolysis of the back bond, and the catalysis of Pt is considered to enhance the reaction. CARE with pure water is expected to represent a breakthrough method for surface processing of SiC, and will be widely applied in industrial processes such as planarization after high temperature processing in device fabrication.

Abstract: We have developed a novel abrasive-free planarization method, which we term catalyst-referred etching (CARE). In silicon carbide (SiC) CARE, Pt is used as a catalyst and HF solution is used as an etchant. CARE produces a crystallographically undamaged and smooth SiC surface. To understand the removal mechanism at the topmost surface of SiC in the CARE process, we performed first-principles reaction path simulations using the simulation tool for atom technology (STATE) program package. These calculations are based on the density functional theory within the generalized gradient approximation of Perdew et al. The barrier height of the dissociative adsorption of HF on a SiC surface was evaluated by the climbing image nudged elastic band method. We present simulation results for the initial stages of the etching process. The reaction barrier height for adsorption of the first HF is 1.2 eV.

Abstract: An anisotropic etching process for mesa structures using fluorinated plasma with hydrogen addition was developed in an electon cyclotron resonance setup. The evolution of the mesa morphology was studied in dependence on the gas composition, the applied bias and the pressure. The achieved side wall slope approached 90° with a negligible trenching. The aspect ratios of the fabricated structure in the developed residue free ECR plasma etching process were between 5 and 20.

Abstract: The C-face (0001) 4H-SiC surface morphology produced by etching using chlorine trifluoride gas was studied, focusing on the influence of the off-orientation. The etching pit at the 4^o off-oriented surface was formed at a temperature higher than 973 K, which was higher than 623 K for the on-axis surface. At 1073 K, the hexagonal-shaped etching pits were observed after the etching at the chlorine trifluoride gas concentration of less than 3 %. In the temperature range lower than 900 K, the mirror surface could be maintained after the etching. Thus, the mirror surface and the pitted surface are expected to be formed on the 4^o off-oriented surface by means of appropriately adjusting the parameters, such as the temperature and the chlorine trifluoride gas concentration.

Abstract: A SiC dry etching reactor using chlorine trifluoride (ClF₃) gas was designed and evaluated with the help of numerical calculations and experimental results. The etching rate was about 16 μm/min when the ClF₃ gas concentration, the total flow rate and the SiC substrate temperature were 90%, 0.3 slm and 500 °C, respectively. The gas stream above the substrate surface was concluded to significantly affect the etching rate profile.

Abstract: This study focuses on the effects of a high temperature anneal after dry etching of trenches (post-trench anneal, PTA) on 4Hsilicon carbide (4H-SiC). We aim at the optimum 4H-SiC post-trench treatment with respect to the fabrication and the operation of a trenched gate metal oxide semiconductor field effect transistor (Trench-MOSFET). PTA significantly reduces micro-trenches, also called sub-trenches [, in the corners of the bottom of the trench. This is highly beneficial in case the etched trench sidewall is used as the channel of a Trench-MOSFET. However, PTA is also shown to cause a slight enlargement of the trench width along with a considerable increase of the substrate surface roughness. In addition, X-ray photoelectron spectroscopy (XPS) depth profiles indicate an increased carbon atom concentration at the 4H-SiC surface after the high temperature PTA. The non-stoichiometric surface composition affects the quasi-static capacitance-voltage (QSCV) behavior of MOS structures using a deposited gate oxide (GOX). We assume that a sacrificial oxidation directly after the PTA could restore a stoichiometric 4H-SiC surface.

Abstract: High temperature (>1000 °C) chemical etching using molten KCl or molten KCl+KOH as the etchant has been carried out to remove the mechanical-polishing (MP) induced damage layer from 4H-SiC surface. Atomic force microscopy observations have shown that line-shaped surface scratches that have appeared on the as-MPed surface could be completely removed by KCl-only etching or by KCl+KOH etching (KCl:KOH=99:1 in weight) at ~1100 °C. Between the two recipes, KCl+KOH etching has shown a higher etch rate (6~7 times) and is able to remove ~9 μm and ~36 μm-thick damage layer from the Si (0001) and the C(000-1) surface, respectively. Besides, KCl+KOH etching seems to have formed a Si (0001) surface covered with atomic steps while KCl-only etched surface is featured with nanometer-scale pores.

Abstract: In recent years, silicon (Si) has been mainly used in power devices, but the limit of its performance is being reached. Therefore, silicon carbide (SiC) power devices have been attracting attention because they enable the fabrication of devices with low power consumption. To reduce the on-resistance in vertical power transistors, backside thinning is required after device processing. However, it is difficult to thin a SiC wafer with a high removal rate by conventional mechanical processing because its high hardness and brittleness cause cracking and chipping during thinning. Therefore, we have attempted to thin a SiC wafer using plasma chemical vaporization machining (PCVM), which is plasma etching using atmospheric-pressure plasma. In this study, we describe a machining property using a newly developed slit electrode that is composed of two parts and has a slit that allows for a new gas to pass.

Abstract: Diamond abrasives are generally used to machine silicon carbide (SiC) single crystals because of the high hardness of those crystals. Although Chemo-Mechanical Polishing (CMP) employs abrasives softer than the SiC single crystals together with oxidizing agents in order to avoid mechanical damage to the surface of SiC single-crystal wafers, none has reported so far the use of abrasive wheels other than diamond for grinding large SiC single-crystal wafers. The current study revealed that a novel grinding technique using non-diamond abrasives such as ceria (CeO₂) can efficiently machine large SiC single-crystal wafers of 100 mm in diameter due hypothetically to the nature of newly named tribo-catalytic abrasives, and is promising to minimize the surface damage prior to the final CMP step.