Mapping electron density with laser-collision induced fluorescence in a helium capacitively coupled radio-frequency plasma*

Electron density is a key plasma parameter influencing production of radicals and active species playing major roles in applications such as plasma etch and deposition, key processes for fabrication of microelectronics. However, accurate measurement of electron densities in low pressure RF-driven plasma reactors can be surprisingly difficult. Electrical probes perturb the plasma and can have large error factors or cannot be used due to limitations of probe theories or probe design [1]. Laser-collision induced fluorescence (LCIF) diagnostics present an alternative where the electron density can be mapped in 2D without perturbing the plasma and in regimes (density, temperature, pressures, size, duration, magnetic fields, etc.) not easily accessed with electrical probes [2]. The technique relies on the transfer of laser-excited electrons to nearby and more energetic levels with collisional excitation rates that depend minimally on the (effective) electron temperature. The measurement uncertainty can be conservatively estimated as 50%, depending primarily on the uncertainty in the rate constants or experimental calibration. We report LCIF measurements performed in a highly symmetric radio-frequency capacitively coupled plasma source known as the Budapest reference cell [3]. Studies were carried out for helium gas pressures between 50 mTorr and 1000 mTorr and peak-to-peak radio-frequency (13.56 MHz) voltages between 150 V and 350 V. A good agreement was found between the experimental and modeling results for electron density except at the lowest operating voltages and gas pressures. The (effective) electron temperature values derived by the two methods agree reasonably well within the plasma bulk.

[1] V. Godyak, J. Appl. Phys. 129, 041101, (2021).

[2] E. V. Barnat and K. Frederickson, Plasma Sources Sci. Tech. 19, 055015 (2010).

[3] B. Horváth, A. Derzsi, J. Schulze, I. Korolov, P. Hartmann, Z. Donkó, Plasma Sources Sci. Technol. 29, 055002 (2020).