Non-invasive, real-time measurements of plasma parameters with an industry standard spectrograph*

Plasma process control applications require acquisition of diagnostic data at a rate faster than the characteristic timescale of perturbations to the plasma. Diagnostics based on optical emission spectroscopy (OES) of intense emission lines permit rapid non-invasive measurements with low-resolution ($\sim$1~nm), fiber-coupled spectrographs, included on many plasma process tools for semiconductor processing. Here we report on rapid analysis of Ar($3p^54p \to 3p^54s$) 650-800~nm emissions with such a system to obtain electron temperatures and metastable densities in argon and argon/mixed-gas (Ar/N$_2$, Ar/O$_2$, Ar/H$_2$) inductively coupled plasmas. The OES-derived values are compared to measurements made by electric probes, white-light absorption spectroscopy, and OES measurements made with a high-resolution 0.5~m spectrometer. In a pure Ar plasma, for example, we have measured the metastable densities to better than $\pm 15\%$~accuracy within 0.25~seconds. A number of subtraction procedures have been evaluated to extract the Ar emission intensities in the presence of overlapping molecular emissions. This is especially necessary for Ar/N$_2$ plasmas, which feature intense N$_2$ molecular emissions in the wavelength range of interest.