Measurements of N2 + ions in an inductively coupled plasma using saturated cavity ringdown spectroscopy

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Abstract
This paper presents a unique study of the bulk plasma characteristics in a low pressure inductively coupled nitrogen plasma. Saturated cavity ringdown spectroscopy (sat-CRDS) has been used to determine the absolute number densities and translational temperatures of N2 +(X, ν = 0). The effect of saturation is readily accounted for by using an effective saturation parameter, S eff, and determined by a simple method employing measurements at two different gain settings of the detection system. The appropriateness of this method is confirmed by comparison with fitting individual ringdown data using a time-dependent saturation parameter, S(t), within the local approximation model for sat-CRDS; the two methods are in excellent agreement in returning absolute number densities and translational temperatures. N2 +(X, ν = 0) number densities are determined across a matrix of pressure (10−100 mTorr) and radio-frequency power (200−400 W) conditions with maximum number densities of ca. 1.3 × 1010 cm–3 while translational temperatures range from 600−1500 K.
Funding Information
  • UK Engineering and Physical Sciences Research Council (EP/P026621/1)
  • Lam Research Corporation

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