Ion absorbing stencil mask coatings for ion beam lithography

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. (C) 1997 American Vacuum Society.