  By using the dry etching process of tantalum (Ta) film, we had obtained
transmon qubit with the best lifetime (T1) 503 us, suggesting that the dry
etching process can be adopted in the following multi-qubit fabrication with Ta
film. We also compared the relaxation and coherence times of transmons made
with different materials (Ta, Nb and Al) with the same design and fabrication
processes of Josephson junction, we found that samples prepared with Ta film
had the best performance, followed by those with Al film and Nb film. We
inferred that the reason for this difference was due to the different loss of
oxide materials located at the metal-air interface.
