A Low-Profile Half-Mode Substrate Integrated Waveguide-Based Sensor for Contactless Sheet Resistance Characterization

Transparent conductive nanofilms are the critical components in products, such as including electrochromic glass, solar panels, touchscreens, and photovoltaics. Precise measurement of their conductivity is necessary for ensuring quality control procedures, process monitoring, and product inspection before installation. This research discusses a half-mode substrate integrated waveguide (HMSIW)-based sensor, operating from 5.5 to 18 GHz, to provide a low-profile and noncontact solution for measuring sheet resistance (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R} _{s}$ </tex-math></inline-formula>) of conductive thin films. Full-wave simulations and experiments illustrated that the transmission coefficient (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S} _{{21}}$ </tex-math></inline-formula>) of an HMSIW with a gap inserted in the transverse plane depends on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R} _{s}$ </tex-math></inline-formula> of a conductive thin film placed within this gap. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R} _{s}$ </tex-math></inline-formula> of nine samples of indium tin oxide (ITO) films, ranging from 2.841 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$441.4~\Omega $ </tex-math></inline-formula>/sq, was measured using the presented technique, and the results were compared with the standard four-point probe (4PP) method at the dc range. The experimental results agree well with the proposed fitting formulas that describe the relation between relative <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S} _{{21}}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R} _{s}$ </tex-math></inline-formula>, with a maximum absolute relative error of 7.9%. Additionally, the study provides a detailed analysis of the HMSIW sensor design, outlines the calibration process, and highlights its advantages over traditional <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R} _{s}$ </tex-math></inline-formula> measurement methods, such as its low cost, noncontact measurement capability, and compact form factor, making it a more efficient alternative to the available techniques.