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 on the first neutralization layer (NL 1 ) by a technique of self-assembly, the first block copolymer comprising first and second monomers , the first neutralization layer (NL 1 ) having an identical affinity for each of the first and second monomers ; then selectively remove the second monomer such that the layer of the first block copolymer 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 on the second neutralization layer (NL 2 ) by a technique of self-assembly, the second block copolymer comprising third and fourth monomers , the second neutralization layer (NL 2 ) having an identical affinity for each of the third and fourth monomers ; then selectively remove the fourth monomer such that the layer of the second block copolymer 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 on the first neutralization layer (NL 1 ) by a technique of self-assembly, the first block copolymer comprising first and second monomers , the first neutralization layer (NL 1 ) having an identical affinity for each of the first and second monomers ; then selectively remove the second monomer such that the layer of the first block copolymer 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 on the second neutralization layer (NL 2 ) by a technique of self-assembly, the second block copolymer comprising third and fourth monomers , the second neutralization layer (NL 2 ) having an identical affinity for each of the third and fourth monomers ; then selectively remove the fourth monomer such that the layer of the second block copolymer 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 . <EOS>