A multilevel interconnect structure in a semiconductor device comprises a first insulating layer (2) formed on a semiconductor wafer (1), a Cu interconnect layer (4) formed on the first insulating layer (2), a second insulating layer (6) formed on the Cu interconnect layer (4), and a metal oxide layer (5) formed at an interface between the Cu interconnect layer (4) and the second insulating layer (6). The metal oxide layer (5) is formed by immersion-plating a metal, such as Sn or Zn, on the Cu interconnect layer (4) and then heat-treating the plated layer in an oxidizing atmosphere.