NOVELTY - Creating regions of differing conductivity on a substrate (400) comprises covering a first region of substrate with a mask (401); implanting ions of a first species through the mask into a second region of the substrate; performing a thermal process in a heated environment; subjecting the substrate to an etching process to expose the first region while the film remains on the second region; implanting ions of a second species into the first region, while the film remains only on the second region; and removing the film from the second region.    USE - Method of creating regions of differing conductivity on a substrate used to create an interdigitated back contact solar cell (claimed).    DETAILED DESCRIPTION - Creating regions of differing conductivity on a substrate (400) comprises:    (A) covering a first region of the substrate with a mask (401);    (B) implanting ions of a first species through the mask into a second region of the substrate;    (C) performing a thermal process in a heated environment conducive to the growth of a film, where the film is an oxide or a nitride, such that film layers of different thicknesses are grown on the substrate, where a second thickness on the second region is greater than a first thickness on the first region;    (D) subjecting the substrate to an etching process to remove an amount of oxide substantially equal to the first thickness, thus exposing the first region while the film remains on the second region;    (E) implanting ions of a second species into the first region, while the film remains only on the second region; and    (F) removing the film from the second region.    The first species comprises boron and the second species comprises phosphorus; or first species comprises phosphorus and the second species comprises boron. An INDEPENDENT CLAIM is included for creating an interdigitated back contact solar cell using a substrate.    DESCRIPTION OF DRAWING(S) - The drawings are cross-sectional views illustrating formation of interdigitated back contact solar cell.    Front surface fields (102)    Back surface field regions (204)    Substrate (400)    Mask (401)    Dopants (410, 420)