The infrared light of the wide wavelength band that has been emitted from the SLD (Super Luminescent Diode), passes through the sensor head via the polarization maintaining fiber optic cable, and is then irradiated on the surface of the wafer. Once this is done, some of the light is reflected and the remaining light passes through the wafer and is then reflected by the reverse side.
2 reflected beams of light interfere with each other's wavelengths and then return to the spectrum unit. The intensity of the interference light is defined by the distance from the reference surface to the target and it takes an extremely large value when the distance is equal to an integral multiple of the wavelength.
Interference light is finely dispersed into individual wavelengths, and the light intensity spectral distribution is acquired from the CCD received light waveform. Additionally, wafer thickness is calculated by performing wavelength analysis such as FFT.