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This work investigates the thermal stability and barrier characteristics of two species of silicon carbide dielectric films, α-SiCN with a dielectric constant of 4.9 and α-SiC with a dielectric constant of 3.8. The TDDB lifetime of Cu damascene metallization structure is greatly improved by using a α-SiCN/α-SiC bilayer dielectric stack as the etching stop layer (ESL). This improvement is presumably due to the α-SiC dielectric's lower leakage current, absence of nitridation on Cu surface, and better adhesion on Cu as well as OSG intermetal dielectric (IMD), though the α-SiC film has a very slow deposition rate. We believe that the α-SiCN/α-SiC bilayer dielectric is a favorable combination for the ESL because α-SiCN can protect α-SiC from plasma attack during the photoresist stripping.