Method for fabricating first and second molds (M 1, M 2) for lithography by nano-imprinting, comprising the steps: a) provide a substrate comprising a first neutralization layer (NL 1); b) form a layer of a first block copolymer (20, 21) on the first neutralization layer (NL 1) by a technique of self-assembly, the first block copolymer (20, 21) comprising first and second monomers (20, 21), the first neutralization layer (NL 1) having an identical affinity for each of the first and second monomers (20, 21); then selectively remove the second monomer such that the layer of the first block copolymer (20, 21) comprises a series of first lithographic patterns with a random structure extending in a first direction, each of the first lithographic patterns being formed in the first monomer; c) create a first mold (M 1) comprising impressions (M 10) formed from the series of first lithographic patterns; d) provide a structured layer comprising a series of patterns, conforming to the series of first lithographic patterns and extending in the first direction, then coat the structured layer with a second neutralization layer (NL 2); e) form a layer of a second block copolymer (40, 41) on the second neutralization layer (NL 2) by a technique of self-assembly, the second block copolymer (40, 41) comprising third and fourth monomers (40, 41), the second neutralization layer (NL 2) having an identical affinity for each of the third and fourth monomers (40, 41); then selectively remove the fourth monomer such that the layer of the second block copolymer (40, 41) comprises a series of second lithographic patterns with a random structure extending in a second direction perpendicular to the first direction, each of the second lithographic patterns being formed in the third monomer; f) create a second mold (M 2) comprising impressions (M 20) formed from the series of second lithographic patterns. a) provide a substrate comprising a first neutralization layer (NL 1); b) form a layer of a first block copolymer (20, 21) on the first neutralization layer (NL 1) by a technique of self-assembly, the first block copolymer (20, 21) comprising first and second monomers (20, 21), the first neutralization layer (NL 1) having an identical affinity for each of the first and second monomers (20, 21); then selectively remove the second monomer such that the layer of the first block copolymer (20, 21) comprises a series of first lithographic patterns with a random structure extending in a first direction, each of the first lithographic patterns being formed in the first monomer; c) create a first mold (M 1) comprising impressions (M 10) formed from the series of first lithographic patterns; d) provide a structured layer comprising a series of patterns, conforming to the series of first lithographic patterns and extending in the first direction, then coat the structured layer with a second neutralization layer (NL 2); e) form a layer of a second block copolymer (40, 41) on the second neutralization layer (NL 2) by a technique of self-assembly, the second block copolymer (40, 41) comprising third and fourth monomers (40, 41), the second neutralization layer (NL 2) having an identical affinity for each of the third and fourth monomers (40, 41); then selectively remove the fourth monomer such that the layer of the second block copolymer (40, 41) comprises a series of second lithographic patterns with a random structure extending in a second direction perpendicular to the first direction, each of the second lithographic patterns being formed in the third monomer; f) create a second mold (M 2) comprising impressions (M 20) formed from the series of second lithographic patterns.