A method for measuring etch depth as a function of time by differential polarimetric interferometry, comprising the following steps: - splitting a light beam into a first incident beam and a second incident beam, the first incident beam being directed towards a first area of a sample exposed to an etching treatment to form a first reflected beam and, respectively, the second incident beam being directed towards a second area of the sample to form a second reflected beam, - recombining the first reflected beam and the second reflected beam to form an interferometric beam, - polarisation splitting the interferometric beam into at least a first polarisation component and a second polarisation component, orthogonal to each other; - detecting, as a function of time (t), over a time interval (t0, t1), a first interferometric intensity signal relating to the first polarisation component and a second interferometric intensity signal relating to the second polarisation component; - calculating a first differential polarimetric interferometry signal as a function of linear combinations of the first interferometric intensity signal and of the second interferometric intensity signal; - calculating a first lower envelope function and a first upper envelope function of the first differential polarimetric interferometric signal over the time interval (t0, t1); - determining a first offset function equal to half a sum of the first lower envelope function and the first upper envelope function at each time instant (t) of the time interval (t0, t1); - determining a first normalisation function equal to half a difference between the first lower envelope function and the first upper envelope function at each time instant (t) of the time interval (t0, t1); - applying the first offset function and the first normalisation function to the first differential polarimetric interferometry signal to form a first locally-normalised differential polarimetric interferometry function at each time instant (t) of the time interval (t0, t1); and - extracting an optical phase-shift measurement as a function of time over said time interval (t0, t1), from the first locally-normalised differential polarimetric interferometry function; - calculating the etch depth as a function of time from the optical phase-shift measurement. - splitting a light beam into a first incident beam and a second incident beam, the first incident beam being directed towards a first area of a sample exposed to an etching treatment to form a first reflected beam and, respectively, the second incident beam being directed towards a second area of the sample to form a second reflected beam, - recombining the first reflected beam and the second reflected beam to form an interferometric beam, - polarisation splitting the interferometric beam into at least a first polarisation component and a second polarisation component, orthogonal to each other; - detecting, as a function of time (t), over a time interval (t0, t1), a first interferometric intensity signal relating to the first polarisation component and a second interferometric intensity signal relating to the second polarisation component; - calculating a first differential polarimetric interferometry signal as a function of linear combinations of the first interferometric intensity signal and of the second interferometric intensity signal; - calculating a first lower envelope function and a first upper envelope function of the first differential polarimetric interferometric signal over the time interval (t0, t1); - determining a first offset function equal to half a sum of the first lower envelope function and the first upper envelope function at each time instant (t) of the time interval (t0, t1); - determining a first normalisation function equal to half a difference between the first lower envelope function and the first upper envelope function at each time instant (t) of the time interval (t0, t1); - applying the first offset function and the first normalisation function to the first differential polarimetric interferometry signal to form a first locally-normalised differential polarimetric interferometry function at each time instant (t) of the time interval (t0, t1); and - extracting an optical phase-shift measurement as a function of time over said time interval (t0, t1), from the first locally-normalised differential polarimetric interferometry function; - calculating the etch depth as a function of time from the optical phase-shift measurement.