Spatially resolved, excited state densities and neutral and ion temperatures in inductively coupled argon plasmas

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Abstract
Spatially resolved, line integrated, excited state densities, and neutral and ion temperatures have been measured in inductively coupled argon plasmas. Absorption spectroscopy was used to measure the line integrated density and temperature of the argon 1s5, 1s4, 1s3, and 1s2 energy levels. Laser‐induced fluorescence was used to confirm the neutral temperatures and to measure argon metastable ion temperatures. For rf powers between 50 and 300 W and pressures of 4–50 mTorr, the line integrated density of the 1s5 energy level varied between 1×1016 and 2×1016 m−2. The densities of the 1s4, 1s3, and 1s2 levels were approximately 4–10 times smaller. In the center of the plasma, the ion and neutral temperatures were identical, between 550 and 1000 K for plasma powers between 30 and 240 W and pressures between 4 and 50 mTorr. The neutral temperature had a maximum in the center of the discharge and decreased towards the edge of the discharge. However, the ion temperature increased to between 3000 and 4000 K at the edge of the discharge. Ion drift velocity in the radial direction was between 1×105 and 2×105 cm/s at the edge of the plasma. No significant changes in the spatial density distribution or temperature were observed when either a rf bias was applied to the lower electrode or when the stainless‐steel lower electrode was covered with a bare silicon wafer. The addition of nitrogen to the argon discharge resulted in the density of the 1s5 state decreasing by a factor of 2 and the density of the 1s4 state decreasing by a factor of 10. Implications of these measurements on the radial electric fields, radiation trapping, and the energy transport in the plasma are discussed.