3D Optical Emission Tomography of a Capacitively Coupled Plasma: Technique and Scientific Rational

Although increasing demands of high-throughput, high spatial and temporal uniform plasmas in advanced semiconductor, optical diagnostics has been limitedly utilized for EPD and residual gas analysis. While optical emission tomography (OET) is routinely used for 2D cross-sections, true volumetric reconstructions remain scarce, limiting insights into the volumetric nature of plasma uniformity. In this contribution, we demonstrate a 3-D OET technique for an argon capacitively coupled plasma (CCP). Line-integrated emission data were acquired at multiple-stacked planes using a linear CMOS detector that was translated normal to the electrode gap. The 3-D emission distribution was reconstructed via Phillips–Tikhonov regularization. Our 3-D observation reveals a pronounced pressure-driven transition in the radial profile. As the operating pressure increases from 10 mTorr to 500 mTorr, the radial emission profile evolves from a central-peaked "bell" to a center-dimmed "hollow" distribution, with hollowing first evident in planes adjacent to the upper grounded electrode. This shift reflects the higher electron–neutral collision rate at higher pressures, which prevents high-energy electrons generated near the edge from reaching the center. These behavior—obtainable only through full 3-D tomography— demonstrates the method's necessity for diagnosing and ultimately controlling volumetric uniformity in CCP reactors.