Volumetric characterization of CCPs via 2.5-dimensional optical emission tomography

Abstract Multidimensional diagnostics capable of mapping the in–situ, real–time characteristics of industrial plasmas remain a longstanding need. Here, we demonstrate optical emission tomography (OET) for volumetric characterization of capacitively coupled plasmas (CCPs). Using line–integrated optical intensity data, two–dimensional (2D) emission profiles parallel to the electrode surfaces are reconstructed via Phillips–Tikhonov regularization. Stacking reconstructions at multiple heights (z) above the bottom electrode yields a quasi–3D (2.5D) representation of the discharge. At a fixed 13.56 MHz radio–frequency power of 250 W, we observe significant 3D changes in plasma distribution as the working pressure is varied from 10 to 500 mTorr, including both radial and axial transitions. In particular, the radial profile evolves from center–bright, bell–shaped emission to edge-bright, hollow emission with a strong z–dependence: at z=40 mm, the edge–to–center intensity ratio [i.e., I(r=110 mm)/I(r=0 mm)] increases from ~0.4 at 10 mTorr to ~7.0 at 500 mTorr, indicating a transition from non–local to local power deposition. Furthermore, OET successfully captures point–like emission intentionally induced by a dc–biased probe tip, even in the presence of strong background plasma emission. Our results reveal that 2.5D OET enables reliable visualization of volumetric plasma behavior and offers a practical pathway toward real–time 3D, non–invasive monitoring and process control in industrial CCP reactors.