Tomographic Reconstruction of Capacitively Coupled Plasma Structure Using Multi-Directional Imaging

In recent years, non-intrusive diagnostic techniques for analysis of spatial plasma structure have become important, with applications such as plasma uniformity improvement and process conditions optimization. We have previously proposed a method to reconstruct 3D plasma structure using multi-view imaging and tomographic techniques. In our previous reports [1, 2], we demonstrated structure analysis using images from a few directions in axisymmetric and non-axisymmetric capacitively-coupled plasmas (CCPs). In this study, to improve the accuracy, a new plasma apparatus which enables image acquisition from multi-direction is proposed. A periscope tube with a small lens at its tip is installed on a rotatable top plate of the cylindrical vacuum chamber, allowing plasma imaging from arbitrary directions while maintaining vacuum. A 40 MHz CCP is produced inside a cylindrical liner (radius; 180 mm, height: 40 mm) using a RF electrode at the bottom of the chamber. For the reconstruction, rectangular voxels (10 mm × 10 mm parallel to the electrode and 1 mm perpendicular) are supposed in the plasma space to focus on the sheath structure. After careful spatial-pixel calibration, reconstruction is performed using Tikhonov–Phillips regularization method. The multi-directional imaging setup significantly improves spatial resolution and reconstruction accuracy.

[1] M. Kyuzo, et al., Jpn. J. Appl. Phys. 63 (2024) 056004.

[2] R. Izumi, et al., ISPlasma2025 / IC-PLANTS2025, Aichi, Japan, March 2025, 05P-14