Plurality layer thickness measuring device and method with fast fourier transform

A thickness measuring device comprising:

• a light source circuit including:
• a light source configured to irradiate light onto a surface of a substrate including a plurality of thin films;
• and
• a lens configured to magnify light emitted from the light source or reflected from the surface of the substrate;
• a detector circuit configured to measure a reflectance for each wavelength of light reflected from the surface of the substrate and passed through the lens to generate a plurality of reflectance values and derive reflectance spectrum data for each wavelength from the reflectance values;
• and
• a control circuit configured to convert the reflectance spectrum data for each wavelength into a digital signal, perform a fast Fourier transform on the converted digital signal, calculate a thickness of at least one thin film among the plurality of thin films from a result of the fast Fourier transform, and control an optical parameter of the light source circuit so that a width of a spot size generated when the light meets the surface of the substrate is less than or equal to a width of an electrode layer disposed under the plurality of thin films in a direction parallel to the surface of the substrate.