Inverting the thickness of thin MoS₂ coating in the time-frequency domain through reflective terahertz time-domain spectroscopy

Abstract

When measuring the thickness of 8-30 μm MOS2 solid lubricating coating coated on metal substrate, the terahertz echo signals will overlap within the duration of the incident pulse, and the traditional peak detection method cannot effectively distinguish these signals. In this paper, a dual path inversion strategy based on reflection mode terahertz time domain spectroscopy (THz-TDS) data is proposed: in the time domain, wavelet de-noising cross-correlation method is used to extract sub picosecond time-of-flight delay; In the frequency domain, the coating thickness is extracted from the deconvoluted spectral phase by adaptive sliding window phase slope fitting method. In the experiment, the gold mirror reference THz-TDS system was used to measure four groups of MOS2 coating samples, and the real thickness was calibrated by scanning electron microscope (SEM) section. The measured refractive index of MOS2 in the 0.3-1.5thz band is about 3.55. The average relative error of time domain method is 7.8%, and that of frequency domain method is 3.6%. Bland-Altman analysis showed that there was a deviation of 0.74 μm between the two methods, and 93.75% of the average results of 16 groups of samples met the 95% consistency limit.