Materials Science Forum Vols. 600-603

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: We report on the reliability of the gate oxide on C-face of 4H-SiC. Constant current stress TDDB measurement shows that QBD of the gate oxide of f200 [μm] on C-face is as much as 18 [C/ cm2], which is much larger than the typical value (0.1[C/ cm2]) of that on Si-face of 4H-SiC. The lifetimes of the gate oxide under the electric field of 3[MV/cm] are roughly evaluated from the leakage characteristics and obtained QBD. The estimated lifetimes of the gate oxide of f600 [μm] are about 900 years. TDDB measurements of MOSs on two wafers, which have different dislocation densities, show that reliability of gate oxide on C-face is insensitive to the dislocation density. Meanwhile, reliability of the gate oxide is sensitive to the surface defect density: it is significantly degraded on the wafer, which has 2000 surface defects in a whole 2-inch wafer.

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: The channel mobility and oxide reliability of metal-oxide-semiconductor field-effect transistors (MOSFETs) on 4H-SiC (0001) carbon face were investigated. The gate oxide was fabricated by using dry-oxidized film followed by pyrogenic reoxidation annealing (ROA). Significant improvements in the oxide reliability were observed by time-dependent dielectric breakdown (TDDB) measurement. Furthermore, the field-effect inversion channel mobility (μFE) of MOSFETs fabricated by using pyrogenic ROA was as high as that of conventional 4H-SiC (0001) MOSFETs having the pyrogenic-oxidized gate oxide. It is suggested that the pyrogenic ROA of dry oxide as a method of gate oxide fabrication satisfies both channel mobility and oxide reliability on 4H-SiC (0001) carbon-face MOSFETs.

Abstract: It was experimentally shown that an ONO gate dielectric carefully formed on 4H-SiC has extremely high reliability even under a negative electric field at least up to a junction temperature of 300°C, making it promising for power MOS and CMOS applications. Medium charge to failure of –30 C/cm2 was achieved for fully processed polycrystalline Si gate MONOS capacitors with an equivalent SiO2 thickness of teq = 44 nm and a 200-μm diameter. The medium time to failure of these capacitors projected for –3 MV/cm exceeds 86 and 6.3 thousand years at room temperature and 300°C, respectively. A parasitic memory action did not appear even when Eox of -6.6 MV/cm was applied for 5000 seconds.

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: We report on the effect of nitridation on the negative and positive charge buildup in SiC gate oxides during carrier injection. We observe that the incorporation of nitrogen at the SiO2/SiC interface can enhance the reliability of the interface by suppressing the generation of interface states upon electron injection but that it can also degrade the oxide by creating additional hole traps. We relate these phenomena to the passivation of atomic-level defects by nitrogen.

Abstract: We have observed variations in the instability in the threshold voltage, VT, of SiC metaloxide semiconductor field-effect transistors (MOSFETs) from various sources and/or processes due to gate-bias stressing as a function of temperature. In some cases we see a dramatic increase in the instability with increasing temperature, consistent with interfacial charge trapping or de-trapping. In other cases the temperature response is very slight, and in still other cases we actually see VT instabilities that move in the opposite direction with bias, indicating the presence of mobile ions.

Abstract: Physical and electrical properties of sol-gel derived SiO2 thick film (100-130 nm) deposited on n-type 4H-SiC have been investigated. The oxide was annealed in argon gas ambient for 30 minutes at 650, 750, 850, and 950°C, in order to optimize the oxide properties. Results indicated that the oxide is denser with a significant reduction in percentage of porosity as the annealing temperature increases, except for sample annealed at 950°C. The oxide annealed at 850°C was having values of refractive index and dielectric constant close to the values reported in thermally grown SiO2 and it has demonstrated the lowest leakage current density and total interface trap density. Viscous shear flow effect has been proposed as the main contributor for the reduction of physical properties when the oxide was annealed at 950°C.

Abstract: A novel chemical planarization method was developed for silicon carbide (SiC) and gallium nitride (GaN). This method uses catalytically generated hydroxyl radicals (OH*) to oxidize the wafer surface. OH* are generated by the reductive decomposition of hydrogen peroxide (H2O2) on the surface of the iron reference plate. An extremely flat surface without pits or scratches was obtained. Atomic force microscopy (AFM) revealed that the planarized surface had an atomic step-terrace structure, in which the step height corresponded to a single bilayer of 4H-SiC and GaN. Low electron energy diffraction (LEED) and cathodeluminescence spectroscopy showed that there was no crystallographic damage on the planarized surface.