METHOD OF MAKING A THREE-DIMENSIONAL MEMORY DEVICE USING COMPOSITE HARD MASKS FOR FORMATION OF DEEP VIA OPENINGS

A method includes forming an alternating stack of first material layers and second material layers, forming an etch mask material layer containing an opening over the alternating stack, forming a non-conformal cladding liner over the etch mask material layer, where the non-conformal cladding liner includes a horizontally extending portion that overlies a horizontal top surface of the etch mask material layer and a vertically extending portion contacting a sidewall of the opening in the etch mask material layer, implanting ions of dopant atoms into the non-conformal cladding line, and performing an second anisotropic etch process that etches an unmasked portion of the alternating stack selective to the etch mask material layer and the non-conformal cladding liner. The non-conformal cladding liner provides a higher etch resistance relative to the unmasked portion of the alternating stack after the step of implanting ions than before the step of implanting ions.