A method of forming a nozzle of a fluid ejection device, the nozzle having a straight mouth portion and a cavity portion, wherein the mouth portion is formed in a bottom surface of the substrate, and, after passivating the walls of the mouth portion, a wet etch process is applied from the bottom surface of the substrate for forming a part of the cavity portion with walls that diverge from the mouth portion, characterized in that a wet etch process is also applied from a top surface of the substrate for forming a part of the cavity portion which diverges towards the bottom surface and merges with the part that is etched from the bottom surface.