A metal gate stack having a titanium aluminum carbon nitride (TiAlCN) as a work function layer and/or a multi-function blocking/wetting layer, and methods of manufacturing the same, are disclosed. In an example, an integrated circuit device includes a semiconductor substrate and a gate stack disposed over the semiconductor substrate. The gate stack includes a gate dielectric layer disposed over the semiconductor substrate, a multi-function blocking/wetting layer disposed over the gate dielectric layer, wherein the multi-function blocking/wetting layer includes TiAlCN, a work function layer disposed over the multi-function blocking/wetting layer, and a conductive layer disposed over the work function layer.