Spectroscopic characterization of two-dimensional distributions of vibrational temperature and reactive nitrogen species density in low-pressure nitrogen plasma

The use of a charge-coupled device (CCD) camera, is one of the simplest methods for obtaining high-spatial-resolution plasma diagnostics owing to its ease of use and compatibility with existing setups. Although this approach is effective for atomic gas plasmas when combined with optical bandpass filters, its application in scenarios involving molecular gas plasmas, widely used in semiconductor processes, is challenging due to the limitation of spectral resolution of the filters to resolve densely overlapped molecular emission bands. Therefore, in this study, we developed a CCD-based analysis method using spectrally filtered images to enable two-dimensional mapping of vibrational temperature and reactive nitrogen species density in low-pressure nitrogen plasma. To extract the vibrational temperature, and relative species density from the observed emission intensities, the theoretical intensity ratio was calculated while taking into account the properties of the CCD camera and optical filters. The method achieved a high spatial resolution—better than 0.2 mm across an area of 100 mm × 35 mm. The results obtained exhibited over 84 % agreement with those obtained via conventional spectrum-based analysis. The vibrational temperature of N₂(B) and the relative densities of N₂(C) and N₂⁺(B) were the highest near the grounded electrode of the plasma reactor, while vibrational temperature of N₂(C) was the highest at the powered electrode surface. The results demonstrate that plasma parameters can be rapidly monitored over the entire region in a single measurement using simple optical equipment, without the need for point-by-point scanning as in the conventional method.