A Differential and High-Sensitivity Thickness Measurement Sensor for Dielectric Solid Materials Using a Branch Line Coupler

A novel microwave sensor based on a branch line coupler (BLC) embedded with split ring resonators (SRRs) is proposed here for thickness and displacement measurement of thin dielectric layers. The SRRs introduce a transmission zero (TZ) in the BLC's transmission coefficient, which serves as the sensing parameter. The sensor exhibits a compact design due to the integration of the sensing elements (SRRs) within the host structure (BLC), eliminating the need for additional space. The sensor demonstrates a robust performance against environmental factors due to the differential operation between the direct transmission and isolation ports. This differential structure benefits from distinct TZ shifts in the transmission coefficient caused by the material under test (MUT), whereas the isolation port remains relatively unaffected. This approach also eliminates the need for two separate sensing sections, typically required for differential sensors, effectively reducing the sensor size. The proposed sensor achieves a high sensitivity of 26.5% (1/mm), surpassing the performance of the existing thickness sensors. The fabricated prototype sensor exhibits a smaller normalized sensing area compared with the existing sensors. In addition, a displacement analysis reveals a correlation between TZ variations and MUT displacement in both $x$ and $y$ coordinates, demonstrating the sensor's potential for displacement measurement. Overall, the proposed microwave sensor offers a compact, robust, and sensitive solution for thickness and displacement measurement of thin dielectric layers, making it a promising tool for various applications.