CF₄:O₂ surface etching for the improvement of contact resistance and high-temperature reliability in Ni/Nb ohmic contacts on n-type 4H-SiC

Abstract

The influence of CF 4 :O 2 surface etching on the formation of Ni/Nb ohmic contact to n-type 4H-SiC substrates was investigated. The specific contact resistivity extracted from the transmission line model showed that the contact with CF 4 :O 2 etching had lower contact resistance than that of the sample without the etching process. The X-ray diffraction results showed that a higher concentration of Ni 2 Si formed at the interface of the contact after the fabrication process resulted in a lower specific contact resistivity. Moreover, the depth distribution of the elements revealed that under the CF 4 :O 2 surface etching, a low density of the excess carbon atoms at the interface of the contact was observed. Transmission electron microscopy also confirmed that the low concentration of excess carbon atoms and the high density of Ni 2 Si at the interface play an important role in low specific contact resistivity as well as the high temperature reliability of the Ni/Nb/4H-SiC contacts undergoing CF 4 :O 2 treatment.

500 °C high-temperature reliability of Ni/Nb ohmic contact on n-type 4H-SiC

“…As reported in Ref. 24, after the surface treatment with CF 4 :O 2 , the Si-C bonds in the SiC substrate was broken and/ or weakened, therefore Nb was easier to react with carbon atoms at the surface of the SiC substrate, resulting in a high concentration of Nb-C compound formed at the interface of this contact after the RTA process. The Nb-C compounds act as a barrier layer that prevents the Ni from diffusing into the surface of the contact to react with Si in the substrate by the formation of Ni-Si compounds.…”

“…The fabrication process of Ni/Nb/4H-SiC ohmic contact with and without the surface etching has been reported somewhere. 21,23,24) A 336 μm thick n-type 4H-SiC (0001) bulk substrate with a doping concentration of 1.0 × 10 18 cm −3 was used in this research. The resistivity of the SiC substrate was 14.2 mΩ.cm.…”

“…Based on the results reported in Ref. 24, the surface treatment with CF 4 :O 2 can improve the high-temperature reliability of the contact, therefore after the cleaning process, the wafers were separated into two sets of samples: with and without applying surface etching. For the samples with applying surface treatment, the 4H-SiC substrate was etched to a depth of 50 nm using reactive ion etching (RIE) with CF 4 :O 2 (4:1) plasma gases.…”

“…Recently, the Ni/Nb ohmic contact to 4H-SiC with CF 4 :O 2 surface etching has been studied. 24) It has been reported that by applying the CF 4 :O 2 surface etching process, the Ni(75 nm)/Nb(25 nm)/4H-SiC not only had a low specific contact resistance but also exhibited excellent stability when operating at 400 °C. 24) However, to apply the SiC devices for many important applications, such as nuclear power plants, the reliability of the metal/SiC contact at higher ambient temperature, for example, 500 °C, should be considered.…”

“…24) It has been reported that by applying the CF 4 :O 2 surface etching process, the Ni(75 nm)/Nb(25 nm)/4H-SiC not only had a low specific contact resistance but also exhibited excellent stability when operating at 400 °C. 24) However, to apply the SiC devices for many important applications, such as nuclear power plants, the reliability of the metal/SiC contact at higher ambient temperature, for example, 500 °C, should be considered. [25][26][27][28] Tanimoto et al reported that the Ni 2 Si/4H-SiC contact showed excellent stability when operating at 300 °C, however it exhibited degradation just after 32 h aging in 500 °C ambient temperature.…”