Thinning device with a wet etching apparatus and a monitoring device for monitoring a thinning of at least one semiconductor wafer in the wet etching apparatus, the monitoring device comprising the following: - a light source designed for emitting coherent light of a light wave band for which the semiconductor wafer is optically transparent, - a measuring head arranged without contacting a surface to be etched of the semiconductor wafer, the measuring head being designed for irradiating the semiconductor wafer with the coherent light of the light wave band and for receiving a radiation reflected by the semiconductor wafer, - a spectrometer, - a beam splitter via which the coherent light of the light wave band is directed onto the measuring head and the reflected radiation is directed onto the spectrometer, and - an evaluation unit, wherein the thinning device comprises a coupling unit, a control unit of the wet etching apparatus and a dispenser for an etching fluid for thinning the semiconductor wafer, wherein the coupling unit is connected to the evaluation unit and to the control unit and is designed to stop the etching process on reaching a preset final thickness (d z) of the thinned semiconductor wafer, characterised in that - the evaluation unit is designed to determine a thickness d(t) of the semiconductor wafer and the thickness (d f (t)) of an applied etching film of etching fluid from the radiation reflected by the semiconductor wafer during the thinning of the semiconductor wafer, using a method selected from the group consisting of a one-dimensional spatially coded Fourier-Domain optical coherence tomography (1D-se FDOCT) method, a one-dimensional time-coded Fourier-Domain optical coherence tomography (1D-te FDOCT) method and a one-dimensional spatially coded Time-Domain optical coherence tomography (1D-se TDOCT) method, and in that - the control unit comprises a regulator designed to regulate a quantity of the etching fluid to be dispensed by the dispenser as a function of the thickness (d f (t)) of the applied etching film of etching fluid and as a function of the thickness reduction (Δd(t)) of the semiconductor wafer. - a light source designed for emitting coherent light of a light wave band for which the semiconductor wafer is optically transparent, - a measuring head arranged without contacting a surface to be etched of the semiconductor wafer, the measuring head being designed for irradiating the semiconductor wafer with the coherent light of the light wave band and for receiving a radiation reflected by the semiconductor wafer, - a spectrometer, - a beam splitter via which the coherent light of the light wave band is directed onto the measuring head and the reflected radiation is directed onto the spectrometer, and - an evaluation unit, - the evaluation unit is designed to determine a thickness d(t) of the semiconductor wafer and the thickness (d f (t)) of an applied etching film of etching fluid from the radiation reflected by the semiconductor wafer during the thinning of the semiconductor wafer, using a method selected from the group consisting of a one-dimensional spatially coded Fourier-Domain optical coherence tomography (1D-se FDOCT) method, a one-dimensional time-coded Fourier-Domain optical coherence tomography (1D-te FDOCT) method and a one-dimensional spatially coded Time-Domain optical coherence tomography (1D-se TDOCT) method, and in that - the control unit comprises a regulator designed to regulate a quantity of the etching fluid to be dispensed by the dispenser as a function of the thickness (d f (t)) of the applied etching film of etching fluid and as a function of the thickness reduction (Δd(t)) of the semiconductor wafer.