S De Benedictis

Diagnostics and decomposition mechanism in radio-frequency discharges of fluorocarbons utilized for plasma etching or polymerization

Optical emission (180–800 nm) and mass spectroscopy have been used to study the CF4, CF4+O2, C2F6, C2F6+H2, CF3Cl, and C2F4 decomposition in radio-frequency discharges. The analysis of the stable and unstable discharge products has allowed the suggestion of decomposition channels for the various gases and to classify the fluorinated gases according to their predominant etching or polymerizing characteristics on the basis of the active species present in the plasma. A new broad emission continuum centered at λ=290 nm (FWHM=66 nm) has also been identified and it has been tentatively assigned to CF+2.

Electron energy distribution functions under N2 discharge and post-discharge conditions: A self-consistent approach

Abstract Electron energy distribution functions (EDF) under N 2 discharge and post-discharge conditions have been calculated by self-consistently solving the Boltzmann equation, the vibrational master equation (including dissociation) and the kinetics of the most important electronic states of N 2 . The results show that the relaxation of EDF in the post-discharge is strongly linked to the residence time of N 2 in the discharge, which determines the initial conditions. In particular at low residence times the role of metastable states in affecting EDF through superelastic electronic collisions (SEC) overcomes the corresponding one from superelastic vibrational collisions (SVC). On the contrary, SVC dominate at long residence times, when a well-developed vibrational distribution has been built up by the discharge. At intermediate residence times both SVC and SEC affect the relaxation of EDF, which in general presents a non-monotonic behaviour.

Kinetic and spectroscopic analysis of NH3 decomposition under R.F. Plasma at moderate pressures

The plasma decomposition of NH3 has been studied as a function of the residence time, power input, and pressure. The process follows apparently zero-order kinetics, which can be interpreted on the basis of a kinetic mechanism involving as initial step the rupture of an N-H bond from vibro-rotationally excited modecules. Simultaneous spectroscopic observations of the emission light due to electronically excited NH2, NH, H, and N2 have been used to confirm the suggested mechanism and to show that NH2 and NH are successive intermediate species and that the final step of the decomposition process is the bimolecular recombination NH+NH→N2+H2.

Vibrational relaxation of N2(C, v) state in N2 pulsed rf discharge: electron impact and pooling reactions

Abstract The decay of Δv = −2 band sequence of N2 Second Positive emission system has been investigated in a N2 rf pulsed 500 Hz post discharge in the pressure range 0.1–3 Torr. The intensity of each band head of the sequence has been used to infer the N2(C, v′ = 0–4) relative vibrational distribution. The shape of the distribution varies significantly with the post discharge time and the pressure. A population inversion in the levels 0–1 and 2–3 characterizes the distributions measured at post discharge times higher than 100 μs. The distributions at the various post discharge times have been fitted by a model which takes into account three processes for the (C, v′) excitation: the electron impact from (X, v) states by fast electrons (a), from (A, w) state by slow electrons (b), and the pooling reaction N2(A, w) + N2(A, w′) (c). In the discharge the excitation is predominantly given by process (a). At 1 ms in the post discharge the electronic process (b) still survives, and, together with process (c), provides excitation to C state.

On the use of actinometric emission spectroscopy in SF6-O2 radiofrequency discharges: Theoretical and experimental analysis

A comparison of the results obtained by solving the Boltzmann equation with the experimental results from optical emissions obtained in SF6-O2 radiofrequency discharges, when N2, Ar, and He are also admitted as actinometers, has allowed us to explore the potentialities and limits of actinometry. The use of different actinometers also allowed us to monitor the evolution of the electron distribution functions as a function of the plasma parameters.

{\rm N}_{2}(A\,^{3}\Sigma _{\rm u}^{+}) density measurement in a dielectric barrier discharge in N2 and N2 with small O2 admixtures

This paper deals with the measurement of metastable state density in a dielectric barrier discharge in nitrogen and nitrogen with small admixtures of oxygen, operating in a Townsend-like discharge regime. The measurement is made by optical?optical double resonance-LIF, calibrated by a method based on the measurement of the ratio of nitrogen second positive system and NO-? emissions, and of NO density by LIF. A metastable density of the order of 1013?cm?3 was found in a nitrogen diffuse discharge. Addition of small oxygen concentrations to the discharge drives a transition to the filamentary regime that appears to be caused not by a marked decrease of the metastable density in the discharge but rather by a considerable increase of its quenching rate. Such an increase, due to collision quenching by O2 and O, strongly reduces the survival of the metastable between two discharge pulses. These observations are consistent with the idea that the diffuse regime can be due to a space charge memory effect due to the nitrogen triplet metastable, which is cancelled by the introduction of oxygen in the gas feed.

Electron-energy distribution function measurements in capacitively coupled rf discharges

Measurements by Langmuir probes have been performed in a capacitively coupled, parallel‐ plate rf discharge of He and Ar with pressures ranging from 0.1 to 2 Torr and discharge power from 10 to 50 W at 27 MHz. The first part of this paper is devoted to a discussion of the preparation of the electrostatic probe technique, for which a rf discharge is well known to be a very hostile environment, and of its intrinsic limitations, in order to assess the reliability of the measurements. Results showing a marked non‐Maxwellian character of the electron‐energy distribution function are then presented and discussed in the light of the most recent theories about the sustainment of this kind of gas discharge.

Rate constants for deactivation of N2(A)v=2–7 by O, O2, and NO

Rate constants for N2(A,v) quenching by O, for levels v=2–7, by O2 for levels v=3–7, and by NO for levels v=2–4, have been measured in this work. This is the first data set for the quenching by O of vibrational levels v>3. The results of this work are based on the measurement by laser induced fluorescence (LIF) of N2(A,v) decay in a rf pulsed postdischarge, supported by LIF measurements of NO density. O atom density is deduced by N2(A,v=0,1) decay using the known rate constants of N2(A,v=0,1) quenching by O, O2, and NO. Finally, from appropriate scaling of LIF results for the various v levels, N2(A,v) vibrational distributions are deduced, showing a quite low vibrational excitation of the triplet metastable, characterized by an average Boltzmann vibrational temperature of the order of 2000–2500 K with some superimposed structures.

OH density measurement by time-resolved broad band absorption spectroscopy in an Ar–H2O dielectric barrier discharge

We report results of a novel time-resolved broad-band absorption spectroscopy experiment for OH density measurement applied to a pulsed dielectric barrier discharge in Ar/H2O mixtures. The measurement is aimed at the calibration of our previous OH LIF measurements in the same discharge. The apparatus is simple and cheap, being based on a UV LED light source and a non-intensified, non-cooled, gateable linear CCD array as a detector. The set-up is capable of ruling out both medium/long-term drifts of the UV source and of the discharge, and discharge emission from the measurement. Performances of the set-up are discussed, together with possible improvements for its use as a standalone technique.

Excitation and decay of N2(B 3Πg,v) states in a pulsed discharge: Kinetics of electrons and long-lived species

The vibrational excitation and decay of N2(B 3Πg) state has been investigated in a N2 pulsed rf discharge. The effect of the pulsing frequency and the duty cycle on the N2 (B,v=1–12) vibrational distribution, obtained from the 1PG spectra taken at different times in discharge and afterglow, has been examined in the N2 pressure range 2.4 mTorr–1.5 Torr. The measured (B,v) distributions have been analyzed by a steady-state kinetic model taking into account the main excitation processes, like the electron impact from N2(X 1Σg+,v) and N2(A 3Σu+,v) states, the associative excitation of N2(X,v) with N2(A,v), the pooling by N2(A,v) molecules, the atomic recombination of N(4S), as well as the quenching processes. For the various processes, experimental state-to-state rate coefficients from the literature and/or calculated data sets have been used. Measured N2(A,v) and electron energy distribution functions, and estimated N2(X,v) distributions have been used as input data for the model. The model satisfactorily re...