Laser spectroscopy with structured-light for advanced industrial plasma metrology

Industrial plasma reactors used for semiconductor nanofabrication often have limited optical access and strong background emission. Combined with complex signals, these conditions make reliable extraction of spatially resolved, multidimensional information challenging. In this work, we apply structured light to Doppler-shift laser-induced fluorescence (LIF) and develop three variants: (i) annular-beam confocal LIF (ABC-LIF) for single-port, spatially-resolved measurements; (ii) wavelength-modulation LIF (WM-LIF) for background-and noise-resilient recovery of complex velocity distribution functions (VDFs); and (iii) vortex-beam LIF using orbital angular momentum (OAM) to obtain two-dimensional velocity information from a single laser beam. The methods are demonstrated in an industrially relevant plasma source, with quantitative performance characterization and representative VDF measurements of Ar+ and Xe+ ions. The diagnostics target ion kinetics in an industrial plasma source, but the approach is broadly applicable to other plasma systems and laser diagnostics. These methods reduce optical-port requirements, suppress background, and increase velocity-space sensitivity without modifying the discharge geometry, providing robust, background-resilient, multidimensional diagnostics for complex plasma environments.