The invention relates to a method for etching a layer (20) of assembled block copolymer comprising first and second polymer phases (20A-20B), the etching method including a first step of etching by a first plasma formed from carbon monoxide or a first gas mixture comprising a fluorocarbon gas and a depolymerising gas, the first etching step being carried out so as to partially etch the first polymer phase (20A) and to deposit a carbon layer (23) on the second polymer phase (20B), and a second step (S2) of etching by a second plasma formed from a second gas mixture comprising a depolymerising gas and a gas selected among the carbon oxides and the fluorocarbon gases, the second etching step being carried out so as to etch the first polymer phase (20A) and the carbon layer (23) on the second polymer phase (20B).