An electrical fuse is formed on a semiconductor substrate and a first dielectric layer is formed over the electrical fuse. At least one opening is formed by lithographic methods and a reactive ion etch in the first dielectric layer down to a top surface of the electrical fuse or down to shallow trench isolation. A second dielectric layer is deposited by a non-conformal deposition. Thickness of the second dielectric layer on the sidewalls of the at least one opening increases with height so that at least one cavity encapsulated by the second dielectric layer is formed in the at least one opening. The at least one cavity provides enhanced thermal isolation of the electrical fuse since the cavity provides superior thermal isolation than a dielectric material.