The implantation of ions into the silicon membrane masks of ion beam lithography is known to create compressive stress which leads to very serious distortion after only a few tens of exposures. In this article, we describe a new, dimensionally stable, protective coating for silicon membranes which meets the exacting requirements of very large scale integrated manufacturing. The coating is formed by first depositing a low density, low stress graphitic carbon film which is subsequently amorphized by He+ ion bombardment with a dose of about 125 mC/cm(2). We show that the stress of these bilayer membranes is stable to within experimental error (+/-1 MPa) for 20 keV He+ ion doses up to 0.53 C/cm(2), corresponding to about 8.5 million exposures in a 4X-ion projector.