Multilayer film thickness measurement using ultrafast terahertz pulsed imaging

Abstract

Terahertz (THz) rays have higher penetration depth compared to visible rays, and hence can be effectively used to measure a geometric thickness of soft samples with no damage. Measurement of such soft samples with contact methods is a challenge in terms of accuracy and speed. In this paper, a terahertz pulsed imaging(TPI) imaga 3000 system with reflection imaging module has been employed to measure the individual soft thickness of a multilayer structure made of adhesive soft polymer films with temporal method. Automatic scan of the sample has been carried out and the echoes at each interface are sampled and then detected. The complex refractive index (RI) of the sample has been measured with terahertz pulsed spectra (TPS) 3000 with transmission module. To avoid the incorrect thickness from the measured complex RI due to dispersion, propagation constants of the sample for a given mode and polarization have been extracted by solving the Helmholtz equation, numerically. From the detected delay of echoes and the calculated propagation constants, the geometric thickness of each layer is calculated. Further thickness of an onion leaf as a biological sample was measured based on the propagation constant. Results showed good accuracy with uncertainty budget of 7% of the THz wavelength used.