Multiple photoresist layer process using selective hardening

Abstract

A process for selectively hardening a surface layer of a polymeric photoresist to make such surface layer opaque and insoluble in photoresist carrier solvents, where such selectivity is coextensive with the polymeric/monomeric pattern created in the photoresist. Representative hardening processes include controlled exposure to certain gas plasmas, ion bombardment, or irradiation by ultraviolet radiation of chosen wavelength range. The selectively hardened polymeric regions act as a barrier to the carrier solvent in which the polymer film is laid down and to the developer subsequently employed to remove the monomeric regions. The hardened polymeric regions further exhibit an actinic radiation barrier property preventing radiation depolymerization. In one form the process may be used in a two-layer photoresist structure, where the pinhole-covering thicker second layer is laid down and exposed before developing the monomeric regions of the thinner first layer. Thereafter, a single development operation serves to remove the monomeric regions of both layers. The actinic barrier prevents depolymerization of the monomeric regions of the first layer during the second exposure. In a second application of the selective conversion process, the actinic barrier property of the hardened surface is used to form a conformable mask on a thick, single planarizing layer of photoresist. According to this practice, the photoresist is first locally depolymerized to a fraction of its depth. After a hardening process the selectively hardened surface layer of the photoresist layer constitutes a barrier to actinic radiation. The succeeding flood illumination thereby extends the pattern formed as the monomeric region down to the substrate.