Tomographic Reconstruction of Absolute CF₂ Density in Fluorocarbon Plasma

Difluorocarbene (CF₂) governs chemical processes and etch rates in fluorocarbon plasmas, yet its absolute, spatially-resolved density has remained experimentally inaccessible. Conventional ultraviolet (UV) absorption spectroscopy and infrared laser absorption spectroscopy provide line-of-sight averages that are not easily compatible with tomographic reconstruction. In this study, we present a non-intrusive tomographic measurement of CF₂ density that merges multi-view optical emission spectroscopy (OES) with a single-wavelength UV absorption calibration. Our case study was conducted in an inductively coupled plasma (ICP) sustained in a 25-mm diameter, 10-cm long MgF₂ tube containing a 60% CF₄ and 40% Ar mixture at 100 mTorr. The plasma was driven by a single-turn coil antenna powered by a 13.56 MHz sinusoidal waveform, with an input power varied from 50 to 300 W. The measured emission spectrum revealed a broad UV continuum from 200 to 350 nm, along with four distinct peaks. Among them, the isolated 297 nm peak was selected for reconstructing the radial distribution of CF₂ relative density in the test ICP. The relative map was then converted to absolute number density using UV absorption spectroscopy. Because it requires only standard viewports and a modest optical setup, this method is readily transferrable to industrial etch reactors, providing a powerful diagnostic for real-time process monitoring and control.