In a method of manufacturing a semiconductor device, a gate dielectric layer is formed over a channel region in a gate space, one or more conductive layers are formed over the gate dielectric layer, a seed layer is formed over the one or more conductive layers, an upper portion of the seed layer is treated by introducing one or more elements selected from the group consisting of oxygen, nitrogen and fluorine, and a W layer is selectively formed on a lower portion of the seed layer that is not treated to fully fill the gate space with bottom-up filling approach.