In Situ Coating Thickness Measurement of Parylene Using a Capacitive Sensor

With parylene coatings, conventional layer thickness measurement methods such as gravimetry, reflectometry, and cross-sectional microscopy are performed post-process and do not allow real-time monitoring or control. This paper presents a novel in situ measurement method based on the capacitance change in interdigitated copper electrodes fabricated on a printed circuit board (PCB). As parylene deposits on this sensor, the effective permittivity above the electrodes increases from ϵr≈1 (vacuum) to ϵr=2.1–3.2 (parylene), causing a measurable capacitance change. Finite element simulations were performed to model the relationship between layer thickness and sensor capacitance. Experimental validation with parylene C and F-VT4 demonstrated good agreement between simulation and measurement. Four consecutive parylene C runs with 60 g raw material showed reproducible capacitance increases of 49–53 pF, corresponding to layer thicknesses of 20–25 µm, verified by cross-sectional microscopy. Two coating runs were performed with parylene F-VT4 with target layer thicknesses of 2.5 µm and 5 µm. They show particularly good agreement with the simulation. The proposed method enables real-time process monitoring and provides a foundation for closed-loop control of parylene CVD processes.