11 February 2000 Ž . Chemical Physics Letters 317 2000 631–636 www.elsevier.nlrlocatercplett Ultraviolet cavity ring-down spectroscopy of free radicals in etching plasmas J.P. Booth ) , G. Cunge, L. Biennier, D. Romanini, A. Kachanov Laboratoire de Spectrometrie Physique, UniÕersite Joseph Fourier-Grenoble, BP 87, F-38402 St. Martin d’Heres, France ´ ´ ` Received 29 November 1999; in final form 13 December 1999 Abstract Ž . Many reactive species of interest in technological plasmas absorb light in the UV spectral region 200–300 nm . Ž y2 y4 . Measurement of these weak absorbances typically 10 –10 for a single pass allows us to determine their absolute concentration. Low-resolution absorption spectra of these systems have previously been obtained by broad-band absorption spectroscopy. Here we present spectra obtained using laser cavity ring-down spectroscopy, which has much higher spectral resolution, and potentially higher sensitivity. Spectra were obtained for CF, CF , AlF and SiF radicals in capacitively-cou- 2 2 Ž . pled radio-frequency plasmas in fluorocarbon gases. This technique offers the possibility of real-time 1s absolute concentration measurements during wafer processing. q 2000 Elsevier Science B.V. All rights reserved. 1. Introduction Small poly-atomic radicals, such as CF and CF , 2 play an important role in determining the etch rate, selectivity and anisotropy of industrial plasma etch- w x ing processes 1–3 . Measurements of their concen- trations allow one to test models of the physical chemistry of these systems. Furthermore, simple, reliable techniques for the detection of reactive inter- mediates are needed to optimise reactor and process development, and in the long term may be useful for in-situ process control. To date a variety of tech- niques have been used. Whereas laser-induced fluo- Ž . Ž . rescence LIF offers excellent signal-to-noise SrN w x ratio and spatial and temporal resolution 4–6 , it only gives relative concentrations unless calibration ) Corresponding author. Fax: q33-4-76514544; e-mail: jean-paul.booth@ujf-grenoble.fr wx techniques are used 7 . Absorption techniques, in Ž . contrast, give the absolute line-integrated concen- tration directly, albeit with poorer spatial resolution and SrN ratios. Far-infrared diode laser absorption has been used to detect an impressive range of w x species 8–10 , but sophisticated equipment is needed. The time-honoured technique of broad-band UV absorption spectroscopy, combined with the sen- sitive multi-channel optical detectors that are cur- rently available, allows one to readily oberve single- pass absorbances in the range 10 y3 or less. Whereas Ž its spectral resolution determined by the spectro- . graph cannot match that of tunable lasers, it has the advantage that the whole absorption spectrum of a molecule can be recorded simultaneously. A substan- tial number of key species have been detected using w x this rather simple technique 11 . The recently developed laser technique of cavity Ž . w x ring-down spectroscopy CRDS 12,13 allows one 0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 01424-4