Development of a flat-cutoff sensor for non-invasive plasma density measurement in plasma processing

Real-time plasma diagnostics have garnered significant attention within the semiconductor processing field due to their potential to improve process yield and detecting process abnormality. In this study, we developed a bar-type flat-cutoff sensor (BCS) [1-3], which is embedded into the chamber wall and electrode to measure electron density during plasma processing. Our investigation includes optimizing the sensor's structure and analyzing its measurement characteristics of plasma density through the utilization of a plasma equivalent circuit model considering the structure of BCS, electromagnetic simulation, and experiment. It was confirmed that BCS measures the electron density near the plasma-sheath boundary from the chamber wall or wafer. Furthermore, the BCS enables measuring of electron density, even when various types of wafers are positioned on it, and a shift in cutoff frequency was observed in extreme cases such as thick dielectric and wafers with metal layer. A dielectric layer thicker than 1 mm caused a low-frequency shift of the cutoff frequency. Conversely, wafers with conductive layers induced a high-frequency shift of the cutoff frequency, which is dependent on the plasma density.