A method of forming a hard nanomask for transferring a substantially periodic pattern into a thin film, the method comprising: forming a first layer (302, 402) of a first material; forming a topographic feature (305, 306, 405, 412) in a surface of the first layer (302, 402), wherein the topographic feature is defined by two opposing sidewalls, wherein the two opposing sidewalls are oriented substantially parallel along a first direction with the two opposing sidewalls of the topographic feature being sloped; and irradiating a surface of the topographic feature with a flow of ions from an ion beam in a stepwise manner to modify the first material by ion beam sputtering, the irradiation alternating in the stepwise manner between a first one of the two opposing sidewalls of the topographic feature and a second one of the two opposing sidewall of the topographic feature to form a hard nanomask of modified first material, the hard nanomask comprising a substantially periodic array of substantially parallel, elongated elements having a wavelike cross-section and being oriented substantially parallel along the first direction, at least some of the substantially parallel, elongated elements having the following structure in cross-section: an inner region (100, 200) of the first material, a first outer region of a second material covering a first portion of the inner region, and a second outer region of the second material covering a second portion of the inner region and connecting with the first outer region at a wave crest, wherein the first outer region is substantially thicker than the second outer region, and wherein the second material is formed by modifying the first material by ion sputtering of the first material by the flow of ions, wherein a plane of incidence of the flow of ions is substantially perpendicular to the first direction and wherein the substantially parallel, elongated elements having the wavelike cross-section are positioned on the surface of the topographic feature between the two opposing sidewalls defining the topographic feature. forming a first layer (302, 402) of a first material; forming a topographic feature (305, 306, 405, 412) in a surface of the first layer (302, 402), wherein the topographic feature is defined by two opposing sidewalls, wherein the two opposing sidewalls are oriented substantially parallel along a first direction with the two opposing sidewalls of the topographic feature being sloped; and irradiating a surface of the topographic feature with a flow of ions from an ion beam in a stepwise manner to modify the first material by ion beam sputtering, the irradiation alternating in the stepwise manner between a first one of the two opposing sidewalls of the topographic feature and a second one of the two opposing sidewall of the topographic feature to form a hard nanomask of modified first material, the hard nanomask comprising a substantially periodic array of substantially parallel, elongated elements having a wavelike cross-section and being oriented substantially parallel along the first direction, at least some of the substantially parallel, elongated elements having the following structure in cross-section: an inner region (100, 200) of the first material, a first outer region of a second material covering a first portion of the inner region, and a second outer region of the second material covering a second portion of the inner region and connecting with the first outer region at a wave crest, wherein the first outer region is substantially thicker than the second outer region, and wherein the second material is formed by modifying the first material by ion sputtering of the first material by the flow of ions, wherein a plane of incidence of the flow of ions is substantially perpendicular to the first direction and wherein the substantially parallel, elongated elements having the wavelike cross-section are positioned on the surface of the topographic feature between the two opposing sidewalls defining the topographic feature. an inner region (100, 200) of the first material, a first outer region of a second material covering a first portion of the inner region, and a second outer region of the second material covering a second portion of the inner region and connecting with the first outer region at a wave crest, wherein the first outer region is substantially thicker than the second outer region, and wherein the second material is formed by modifying the first material by ion sputtering of the first material by the flow of ions, wherein a plane of incidence of the flow of ions is substantially perpendicular to the first direction and wherein the substantially parallel, elongated elements having the wavelike cross-section are positioned on the surface of the topographic feature between the two opposing sidewalls defining the topographic feature.