Negative ions in a radio-frequency plasma in CF₄

Abstract

Experiments to study negative ion densities in a radio-frequency CF4 plasma have been carried out using a photodetachment technique. Electrons are photodetached from the negative ions using the pulse of a Nd-YAG laser at the tripled (355 nm) or the quadrupled (266 nm) frequency. The photodetached electrons are detected by a microwave method as a sudden increase of the electron density in the plasma. The negative ion density, which consists mainly of F- is found to be typically four times higher than the stationary electron density at a pressure of 13 Pa, an RF power of 15 W and a CF4 flow of 15 SCCM. The measured decay of the detached electrons after the laser pulse has been interpreted in terms of electron attachment and ambipolar diffusion. The results demonstrate the possibilities for use of this technique to evaluate attachment coefficients in active plasmas. The attachment rate constant for CF4 is found to be (7+or-1)*10-17 m3 S-1 at RF powers of 15 W. The electron diffusion coefficient is 0.13+or-0.12 m2 s-1 at standard conditions of 1 Torr and 300 K.

Hessian of the solution of the Hamilton-Jacobi equation in the theory of extremal problems

“…density is about four times the field-averaged stationary electron density. The results agree with previous data obtained in the 3WMP mode [ 5 ] . Since the results in the 4 0 s~ mode do not depend on the specific photodetachment cross section of the different species, because of the saturation effect, this agreement confirms that the photodetachment cross section used to obtain these results is correct.…”

“…Full knowledge of the relative spatial distribution f ( r , z) of the electrons together with a measured value of (n,(t))f will yield values for nc(r, z). If we assume that in the case of the TMoZn mode, f(r, 2) = p ( r ) q ( z ) ( 5 ) it follows that…”

“…Extra electron decay has been studied using the 3WMP mode [5]. Under these conditions the observed electron decay against time can be approximated by an exponential expression This apparently points to the fact that in this nonsaturated mode the extra electron density profile corresponds to a fundamental mode of the solution of a diffusion equation.…”

“…This extra shift comes on top of the resonance frequency shift resulting from the stationary electron density. First investigations on the evaluation of the negative ion density by this technique have been reported recently [ 5 ] . Values of these densities were deduced from an extrapolation of the measured time-varying electron density to a time immediately after the laser pulse.…”

A laser photodetachment technique for the measurement of H− in a high frequency traveling wave discharge

“…The radiation of Ruby ͑694 nm͒ 3,4 and Nd-yttrium-aluminum-garnet ͑YAG͒ lasers ͑1064, 532, and 355 nm͒ 5 with fixed wavelengths as well as tunable dye lasers ͑600-750 nm͒ 6 have been used for the purpose. Various techniques have been used to detect the change in electron density following photodetachment, namely the shift in resonance frequency of a microwave cavity or a resonant emissive probe, [7][8][9] the change in the standing wave pattern of a microwave interferometer, and the change in the saturation current of a positively biased Langmuir probe. 1,4,5,10,11 The latter technique, the so-called probe based laser photodetachment technique, provides high spatial and temporal resolution 1,4,11 while resonant cavity or microwave interferometry methods provide only spatially averaged density values.…”