In the development of MOS devices, a considerable interest has been directed to the use of silicon carbide (SiC). According to well-known processes, MOS devices based upon the use of SiC, such as, for example, the device of FIG. 1, are obtained by providing a substrate 10, of doped SiC, e.g., of N+ type; growing an epitaxial layer 11, of doped SiC, e.g. of N− type; depositing, on the epitaxial layer 11, an insulation layer (oxide gate 12); and forming a gate region 13, and source 14 and drain 15 regions, according to processes known in the art.
A current problem in the development of devices that use SiC is that the gate-oxide layer 12 is exposed to higher temperatures and electrical fields than in applications where silicon MOS devices are used. Consequently, it is necessary to evaluate the reliability of silicon oxide (SiO2) for large thicknesses of oxide.
Another problem is the poor electrical quality of the SiC/SiO2 interface, which is due mainly to the defects at the interface. In fact, the typical density of undesired energy states, caused by the presence of defects, at the SiC/SiO2 interface is greater, by various orders of magnitude, than at the Si/SiO2 interface. Recent studies have demonstrated that this phenomenon is due to the accumulation, during the growth of the thermal oxide of the device, of excess carbon atoms at the interface. These carbon atoms create trap states, i.e., localized areas that can attract free electric carriers. The traps can be caused by electrochemical bonds at the interface between different materials, such as SiO2 and SiC. In fact, the carbon is in the form of graphite and/or of agglomerates of sp2 orbitals, referred to as “bonded clusters”, and has energy levels distributed in the entire band of the SiC. The trap states create, then, undesired energy states, which can spoil the properties of the materials close to the interface and can reduce the performance of the respective devices. The traps can also cause an interruption of the field lines and Coulomb scattering, thus jeopardizing electrical operation of the device.
It is known that these trap states are stable to the processes of passivation in H2. For this reason, a known solution for improving the quality of the oxide and of the SiC/SiO2 interface envisages the use of processes of oxinitridation, i.e., oxidation in an environment rich in nitrogen, which cause the removal of the carbon atoms and the passivation of the free bonds of silicon. In fact, on the one hand, nitrogen creates strong bonds Si≡N with silicon, which passivate the traps at the interface; on the other hand, the nitrogen, by forming bonds N—C with carbon, removes the carbon and other compounds, such as Si—CO, from the SiC/SiO2 interface. In particular, oxinitridation obtained using NO or N2O has proven effective in reducing the states density at the interface close to the conduction band of SiC. On the other hand, even though annealing in N2O and NO furnishes nitrogen for passivating the traps, apparently it does not eliminate completely the problem of the presence of oxygen at the interface.
A solution to this problem is described in U.S. Pat. No. 7,022,378, which is incorporated herein by reference, wherein, after growing a SiO2 layer on a SiC substrate in an N2O and/or NO environment, an annealing is performed using NH3 as gas. Since this annealing gas does not contain oxygen, the oxide layer does not continue to grow during the nitridation process.
However, the use of a gas such as NO and/or N2O and NH3 for passivating the excess energy states involves considerable problems. In fact, since NO and NH3 are highly inflammable, it is not advisable to use this type of gas in traditional ovens. Instead, the use of N2O during the thermal processes brings about a lower efficiency in the passivation of the states at the interface.
A technique used for preventing the problem of the high density of energy states at the SiC/SiO2 interface without using gas as NO, N2O and NH3 is that of implanting nitrogen atoms N in the SiC layer and hence performing an oxidation. A technique of this kind is described in the article “Low Density of Interface States in n-type 4H—SiC MOS Capacitors Achieved by Nitrogen Implantation”, published in Materials Science Forum Vols. 483-485 (2005) pp. 693-696, which is incorporated herein by reference.
However, the implantation of N leads to considerable damage to the SiC surface and involves the introduction, in the process, of an additional diffusion step, useful for activating the implanted nitrogen. Furthermore, the step of diffusion is performed at temperatures higher than 1400° C., a situation that could create a further damage to the SiC.
Consequently, it is desirable to identify new strategies for overcoming the problems of the known art and improving the quality of the SiC/SiO2 interface, avoiding the use of processes of annealing in nitrogen and processes of implantation of nitrogen, and reducing to the minimum the defectiveness in the device.