A method is provided for forming copper material over a substrate. The method includes forming a barrier layer over a substrate. Then, a depositing-soaking-treatment (DST) process is performed over the barrier layer. A copper layer is formed on the cobalt layer. The DST process includes depositing a cobalt layer on the barrier layer. Then, the cobalt layer is soaked with H2 gas at a first pressure. The cobalt layer is treated with a H2 plasma at a second pressure. The second pressure is lower than the first pressure.