Enhancement of the Stability of Ti and Ni Ohmic Contacts to 4H-SiC with a Stable Protective Coating for Harsh Environment Applications

We report on wafer-level measurements of the long-term stability of Ti and Ni ohmic contacts to n-4H-SiC during thermal treatments in air or air/moisture environments up to 500 degrees C. Contact metallizations with and without a sputtered Ti (20 nm)/TaSix (200 nm)/Pt (150 nm) diffusion barrier stack and Ti (20 nm)/TiN (10 nm)/Pt (150 nm)/Ti (20 nm) interconnects were compared. A protective coating consisting of a SiOx (250 nm)/SiNy (250 nm) stack deposited by plasma-enhanced chemical vapor deposition (PECVD) was used. The stability of the contact metallizations during long-term thermal treatments in air and air/moisture was studied. The best performance was achieved with Ti ohmic contacts without the Ti/TaSix/Pt stack. This system successfully withstood 1000 h thermal treatment at 500 degrees C in air followed by 1000 h at 500 degrees C in air/10% moisture. After the aging, the contact failure ratio was below 1% and the specific contact resistivity amounted to (2.5 +/- 1.1) x 10(-4) Omega cm(2). Scanning electron microscopy (SEM) cross-sectional analysis indicated no degradation in the contact metallization, demonstrating the effectiveness of the SiOx/SiNy protective coating in preventing oxidation of the contacts. These results are very promising for applications in harsh environments, where the stability of ohmic contacts is crucial.