Abstract: A method for fabricating very narrow superconducting metallic lines on a substrate using ion-implantation and etching techniques. The method permits lines to be produced which are much smaller than those fabricated by conventional masking and etching techniques. It makes the fabrication of very small Josephson and other superconducting devices possible. Also since lines are formed in metals, they have high conductivity, so are useful as ordinary conductors at high temperature or when the technique is utilized with non-superconducting materials. The method includes the steps of depositing a selected metal film on a substrate, applying a photoresist or other masking pattern and exposing, etching away the exposed region, ion-implanting the edge of the resulting pattern, removing the photoresist and etching away the unimplanted portion of the metal leaving an ultra-narrow line pattern.