The invention can provide apparatus and methods of processing a substrate using plasma generation by gravity-induced gas-diffusion separation techniques. By adding or using gases including inert and process gases with different gravities (i.e., ratio between the molecular weight of a gaseous constituent and a reference molecular weight), a two-zone or multiple-zone plasma can be formed, in which one kind of gas can be highly constrained near a plasma generation region and another kind of gas can be largely separated from the aforementioned gas due to differential gravity induced diffusion and is constrained more closer to a wafer process region than the aforementioned gas.