Papers by Keyword: Thermal Oxide

Abstract: An area of increasing interest for SiC device processing is the processing and qualification of silicon oxides. In this article a contactless corona CV (CnCV) measurement procedure is evaluated as a way to gain more knowledge about the different processes related to oxides. A 21-point measurement pattern is used to gain information about uniformity of oxide properties. Two different types of oxides have been considered, low pressure chemical vapor deposited (CVD) oxides using tetraethylorthosilicate (TEOS) and thermally grown oxides. The two different groups have received different combinations of pre- and post-processing steps prior to measurements. As expected, low pressure CVD (LPCVD) and thermally grown SiO₂ without any post oxidation annealing (POA) showed significantly different electrical characteristics compared to the wafers that did get a POA. This difference could clearly be distinguished by CnCV, meaning that individual process steps can be analyzed without the fabrication of any test structures on the wafers. As the individual process steps can be analyzed, the uniformity of the individual steps can be accessed. Using a 21-point pattern it was possible to show that there is a non-uniformity in the LPCVD process used prior to the POA. This makes the CnCV technique suitable for in-line characterization and process monitoring.

Abstract: In this paper, the electrical properties of a thermal oxide (SiO₂) grown onto 3C-SiC layers on silicon were investigated, by monitoring the behavior of MOS capacitors. In particular, the growth rate of thermal SiO₂ was dependent on the different surface roughness condition. However, independent of the roughness a high density of positive charge was detected. The sample having the smooth surface (subjected to CMP) showed a notably improved dielectric breakdown (BD) field. However, the best BD on macroscopic MOS capacitors was still far from the ideal behavior. Additional insights could be gained employing a nanoscale characterization that revealed the detrimental role of persisting extended defects in the semiconductor. In the semiconductor region far from extended defects the nanoscale BD kinetics was nearly ideal.

Abstract: Influences of wafer-related defect and gate oxide fabrication process on MOS characteristics with gate oxides thermally grown on 4H-SiC (0001) wafer have been investigated for a realization of SiC MOS power devices. The SiC MOS characteristics depend on the gate oxide fabrication process, and are improved by the increase of DRY oxidation temperature and the applying of N2O and H2 POAs. In addition, it was clearly shown that predominant origins of SiC MOS reliability degradation are wafer-related defects such as dislocation and surface defects of epitaxial layer. Moreover, the planarization of SiC epitaxial layer surface using a CMP treatment is effective technique for the improvement of SiC MOS reliability.

Abstract: Removal of gold particles (40 nm and 100 nm) from fused silica and thermal oxide surfaces in dilute ammonium hydroxide solutions has been investigated. The particle removal efficiency (PRE) from fused silica surface has been found to be a strong function of ammonium hydroxide concentration and bath temperature. PRE increases from 0 to 85 % with increase in bath temperature from 30 to 80 °C for ammonium hydroxide concentration of 1 %. Addition of megasonic energy to the ammonium hydroxide bath at 30 °C has also shown to improve the PRE significantly. In the case of thermal oxide, the removal of gold particles is much easier compared to that from fused silica. Even for cleaning at 30 °C, the PRE for oxide surface increases from 10 to 90 % with increase in ammonium hydroxide concentration from 0 % to 4 %. Atomic force microscopy measurements reveal that an adhesion force of 10 mN/m exists between fused silica and gold particles in 4 % ammonium hydroxide solution as opposed to only repulsive force in the case of thermal oxide.

Abstract: Photo emission phenomenon and reliability of thermal oxides grown on n-type 4H-SiC (0001) wafer have been investigated using photo emission microscope. Thermal oxides were grown by dry oxidation, and treated in nitrous oxide atmosphere as followed by hydrogen post oxidation annealing. An initial photo emission phenomenon with weak intensity exists just after stress current is applied to the thermal oxide. It is confirmed that most initial emission occurred at the same position as dielectric breakdown of the thermal oxide. Also, the initial emission phenomenon was observed in the MOS capacitors broken by extrinsic defects such as threading screw dislocations and surface defects. In addition, the photo emission due to Fowler-Nordheim tunnel current through the thermal oxide has peak intensity at 2.48 eV.

Abstract: The reliability of CVD gate oxide was investigated by CCS-TDDB measurement and compared with thermally grown gate oxide. Although the QBD of thermal oxide becomes smaller for the larger oxide area, the QBD of CVD oxide is almost independent of the investigated gate oxide area. The QBD at F = 50% of CVD oxide, 3 C/cm2, is two orders of magnitude larger for the area of 1.96×10-3 cm2 at 1 mA/cm2 compared to that of thermal oxide. More than 80% of the CVD oxide breakdown occurs at the field oxide edge and more than 70% of the thermal oxide breakdown in the inner gate area. These results suggest that the lifetime of CVD oxide is hardly influenced by the quality of SiC, while the defects and/or impurities in SiC affect the lifetime of thermally grown oxide.

Abstract: Reliability of thermal oxides grown on n-type 4H-SiC substrates using an area-scaling rule has been investigated, and an influence of dislocation defects on the TDDB characteristics was examined. Using the area-scaling rule, tBD distributions of thermal oxides with different gate-area were converged to one distribution under the same breakdown factor. Finally, the reliability prediction method of thermal oxides on 4H-SiC eliminating the effect of dislocation defects has been established.

Abstract: Reliability of thermal oxides grown on the n-type 4H-SiC substrates implanted by nitrogen ion with low doping levels equal to or less than 1x1018 cm-3 has been investigated. The surface morphology becomes rough by the nitrogen implantation and the post implantation annealing. The field-to-breakdown value decreases with increase in the nitrogen concentration. The average EBD values are 11.6 MV/cm, 11.3 MV/cm and 10.7 MV/cm for the samples without the implantation and with the nitrogen implantation of doping levels of 1x1017 cm-3 and 1x1018 cm-3, respectively. The time-to-breakdown values were also degraded with the increase of the nitrogen implantation doping level. The reliability degradation of thermal oxides is caused by the implantation-induced breakdown factor.

Abstract: In this work, the correlation between thermal oxide breakdown and dislocations in n-type 4H-SiC epitaxial wafers has been investigated. Thermal oxide was grown by oxidation in N2O at 1250°C followed by annealing in NO atmosphere. The electron beam induced current (EBIC) technique was employed to find correlations between the electrically active defects in epitaxial layers and regions where the oxide breakdowns occurred. The test measurements of leakage currents in MOS devices were performed in order to correlate the leakage currents with number of defects in the epi-layer detected by EBIC technique.

Abstract: Acceleration factors in acceleration life test of thermal oxides grown on 4H-SiC(0001) wafers and influences of dislocations on oxide reliability have been investigated using time-dependent dielectric breakdown measurements. The thermal oxides are formed by dry oxidation at 1200°C followed by annealing in nitrogen atmosphere. Then, post oxidation annealing in wet ambient at 950°C or hydrogen atmosphere at 800°C were carried out for some of the oxides. Aluminum or poly-Si films with thickness of 300 nm were formed as gate electrodes. The temperature dependence of time-to-breakdown (tBD) indicates that activation energy (Ea) values for the Al-gate and Poly-Si-gate thermal oxides are 0.59 eV - 0.79 eV and 0.34 eV - 0.72 eV, respectively. Analyzing the electric field dependence of tBD, it was indicated that the values of electric acceleration parameters (β) are 2.7 cm/MV - 7.0 cm/MV and 5.8 cm/MV - 7.1 cm/MV for the Al-gate and poly-Si-gate thermal oxides, respectively. In addition, the charge-to-breakdown decreases with increase in the density of basal plane dislocation.