Materials Science Forum Vols. 821-823

Abstract: The flatness of a silicon carbide wafer in terms of bow and warp is the result of the combination of factors both material and process related. Sub-surface damage (SSD) from the wafering process steps can be considered as a thin film under compressive stress on the wafer surface. SSD is generally decreased with each subsequent processing step after the multiwire saw. Single-sided process steps can produce very different levels of SSD on opposing wafer surfaces, leading to high bow and warp values. The present study investigates the effects of SSD on wafer flatness at various process steps as well as methods to minimize shape effects due to SSD during and after processing.

Abstract: In this work, we present a planarization concept for epitaxial SiC trench structures involving reactive ion etching (RIE) and inductive coupled plasma (ICP) dry etching. The general idea is to transfer the flat surface from spun-on BCB/photo-resist layers to deposited silicon dioxide and finally to bulk SiC by applying process conditions with the same etch rate for the different materials. In this way several microns of unwanted material can be removed and planar SiC surfaces are obtained. With this method trench structures filled by epitaxial re-growth can be planarized with smooth surfaces and good homogeneity over the wafer. Cost-efficient device manufacturing can be achieved by using standard semiconductor process equipment. This technology makes it possible to manufacture advanced epitaxial SiC material structures for devices such as trench JBS diodes and double-gate trench JFETs.

Abstract: A SiC dry etcher using chlorine trifluoride (ClF₃) gas was evaluated, particularly about the etching rate distribution. At 100%, the etching rate was high in the center region and was low in the outer region. However, that at 20% showed the opposite profile. This difference was considered to be due to the chlorine trifluoride gas distribution which was built above the gas distributor.

Abstract: The 1/f noise has been investigated for the first time at 300 and 77 K in high-quality 4H-SiC Schottky diodes. It is shown that, that at 77 K, the dependence of the spectral noise density on current, S_I(I), differs fundamentally between the cases of the current flowing through the main part of the diode area with a comparatively high barrier and the current flowing through the nanosized patches with a comparatively low barrier.

Abstract: The influence of stacking faults (SFs) and triangular defects (TDs) on the electrical properties of 4H-SiC Schottky barrier diode (SBD) were investigated. The SF types and locations were distinguished and mapped by using room-temperature photoluminescence (PL) and optical microscope. SBDs were fabricated including the location of SF’s and TD’s. The effects of the types of defects and its area portion in the fabricated SBDs were also investigated. Based on the present data, 3C-TD has more harmful effect rather than the other SFs. The fabricated SBDs including SFs showed that increase of area portion of SF’s also resulted increase of specific on resistance of SBDs.

Abstract: Leakage current points in 4H-SiC Schottky barrier diodes (SBDs) were found to be in the same location as rough surface morphologies. Rough surfaces after activation annealing at the Schottky interface of diodes were removed by chemical mechanical polishing (CMP) or catalyst-referred etching (CARE). As a result, this study succeeded in reducing the leakage current and barrier height variation of 1.2 kV SBDs by eliminating surface roughness.

Abstract: Vertical Schottky diodes have been fabricated on low C content Si_1-xC_x and 3C-SiC epilayers epitaxially grown on Si(001) substrates. Significant leakage current was observed in 3C-SiC diodes under reverse bias, masking any rectifying behavior. This issue is far less pronounced in Si_1-xC_x based Schottky diodes which demonstrate a clear critical breakdown. Leakage current is shown to be greater in relaxed Si_1-xC_x layers. While crystalline Si_1-xC_x is not currently a viable material for high power electronics it is useful for assessing the impact lattice mismatch and crystalline quality has on the behavior of rectifiers.

Abstract: The charge induced in SiC-SBDs with different epi-layer thicknesses by ion incidence was measured to understand the mechanism of heavy-ion-induced anomalous charge collection in SiC-SBDs. SiC SBD of which epitaxial-layer thicknesses is close to ion range show larger anomalous charge collection than SBD with thicker epi-layer although the former one has lower electric field than the later one. The gains of collected charge from the SBDs suggest that the impact ionization under 0.16 - 0.18 MV/cm of the static electric field in depletion layer is not dominant mechanisms for the anomalous charge collection. It is suggested that the epitaxial-layer thickness and ion-induced transient high electric field are key to understand the anomalous charge collection mechanisms in SBDs.

Abstract: High voltage 4H-SiC Ti Schottky junction barrier schottky (JBS) diode with breakdown voltage of 1700 V and forward current of 5 A has been fabricated. A low reverse leakage current below 3.8×10^-5 A/cm² at the bias voltage of -1700 V has been obtained. The forward on-state current was 5 A at V_F = 1.7 V and 15.8 A at V_F = 3 V. The active area is 1.5 mm × 1.5 mm. The turn-on voltage is about 0.9 V. The on-state resistance is 3.08 mΩ·cm². The doping and thickness of the N-type drift layer and the device structure have been performed by numerical simulations. The SiC JBS devices have been fabricated and the processes were in detail. The die was assembled in a TO-220 package. The thickness of the N- epilayer is 17 µm, and the doping concentration is 3.2 × 10¹⁵ cm⁻³. The number of floating guard p-rings was chosen to be 25, the distance between the rings was chosen to be 0.7 µm ~ 1.3 µm and the width of the p-rings is 2.5 µm. We use the PECVD Si_xN_y/SiO₂ as the passivation dielectric and a non photosensitive polyamide as the passivation in the end. The reverse recovery current I_r was 1.26A and the reverse recovery time T_rr was 26ns when the diode was switched from 5A forward current to a reverse voltage of 700V. The reverse recovery electric charge Q_rr of 16nC was obtained.

Abstract: W-SBD show exceptional reliability from 200 to 500K, however, its barrier analysis has never been performed thoroughly down 81K. This paper shows our study of Schottky barrier and Richardson coefficient was extracted for different temperature ranges. We observed fluctuation in function of the temperature. We analyse this phenomenon and compare it to literature for other barriers. Measurements of reverse characteristics up 1200V have been performed from 81 to 450K. It confirms that partial ionization influence on the drift doping impacts on the barrier height.