Keywords: 
• MgB_2超导薄膜 /
• 电子束蒸发 /
• 超导成相

Abstract: The effects of annealing temperature, annealing time and film thickness on the properties of MgB_2 thin films prepared by ex-situ annealing of Mg/B muhilayer have been studied. The films with the thickness of 250 nm annealed at temperatures as low as 400 ℃ exhibited a superconducting transition. The optimal annealing condition of 250 nm thick MgB_2 films regarding to the transition temperature, T_c, was 750 ℃ for 20-30 rain. T_c became lower as the film thickness decreased and the T_c of thinner MgB_2 films was influenced more greatly by the annealing temperature. With reducing the thickness of each single layer, we were able to fabricate the 20 nm thick MgB_2 superconducting film with a T_c~(onset) of 31 K.