Accurate Thickness Measurement Based on Dispersion Compensation via Terahertz Time-Domain Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) has shown significant potential in thickness detection. The time-of-flight (TOF) method is most widely used for judgment. However, the method only refers to peak information, and waveform information, such as deformation, is simply ignored. Consequently, errors are introduced when testing samples with high dispersion and large absorptive properties. As a result, the signals are broadened, and the peaks become smooth or even disappear, which makes the TOF method fail. In this article, we propose a dispersion compensation (DC) algorithm to solve the above problems. Thickness can be accurately determined by nondispersive signals in the spatial domain. Amplitude compensation is used for materials with large absorption. The DC algorithm’s error is less than 1% of the TOF algorithm’s error in simulations. While measuring LiNbO $_{3}$ and water film, errors are, respectively, reduced by 96.24% and 50% compared with using the TOF method, and corresponding deviations are 0.2% and 1.6% compared with the reference thickness. Moreover, the DC algorithm is noniterative, stable, and requires much less calculations than the fitting method. The DC method drastically improves the accuracy by overcoming inherent drawbacks of the TOF method. It can be applied in fields such as nondestructive testing and in-line quality control because of noncontact and real-time advantages.