Plasmas afterglows with \(N_2\) for Surface Treatments

This book is focused on the production of plasma active species containing N₂ which are molecules in electronic, vibrational and rotational states and dissociated atoms or radicals. The method of NO titration of N and O-atoms density and that of the percentage of afterglow resulting of N-atom recombination are detailed. The relevant kinetics equations in the afterglow and the intensity ratio method to obtain the radiative species density from that of N-atoms are elucidated. By this method, it is obtained the densities of O, H and C-atoms, of N₂(A) and N₂(X,v>13) metastable molecules, NH, NO and CN radicals. The plasmas and afterglows were obtained by a microwave supply at reduced gas pressure in the Montreal and Toulouse Univ. and at atmospheric gas pressure in the Orsay, Pau and Toulouse Univ. Interestingly enough, densities of N-atoms and N₂(A) metastable molecules in the afterglow regions, however, are measured to be very similar with each other. Production of active species is studied in N₂ and in N₂-O₂ afterglows of electrical discharges at low and atmospheric gas pressures. They are produced in microwave discharges in a large range of gas pressures from a few Torr to 100 Torr and in corona discharges at atmospheric gas pressure. The processes of bacteria decontamination in N₂-O₂ afterglows are described for low pressure microwave and atmospheric pressure corona discharges. Transmission of N-atoms through porous membranes is studied at medium pressure (10-100 Torr) in microwave afterglows.

It is shown in this chapter how to calculate the densities of atoms, molecules and radicals which are excited by electron collisions. The case of N₂ plasma is emphasized is considered. In the balance equations, the electrons create as well as destroy the N₂ excited states. Their destruction on the reactor walls (tube walls) is also considered.

Variations of N, H, O and C-atoms density have been determined along the reduced pressure flowing afterglows of microwave Ar-N₂-H₂ discharges that have been experimented in Plasmas Labs of Nancy, Montreal, Orsay and Toulouse. The active species density is obtained by NO titration for N and O-atoms and by line intensity ratios for the H and C-atoms. The results obtained by LIF measurements in Orsay and Toulouse Labs are reported for N and H –atoms. The surface cleaning by N and O-atoms has been experimented.

The sterilizing properties of cold plasmas have been demonstrated in the last decades and have raised a wide interest. The employ of N₂ microwave afterglows provides a mild platform for sterilization of thermosensitive and delicate objects, and has allowed the transfer of this technology to the industry by the development of an industrial reactor. It is the aim of this work to compile and summarize the most relevant findings regarding this topic and provide the reader of the most suitable conditions for sterilization of contaminated material, and the outreach of this technique.

In particular, the sterilisation of E-coli bacteria by N₂ microwave flowing afterglows was obtained at 5 Torr, 1 slpm, 100 w in a treatment time of 40 minutes by heating the bacteria holder at 60°C, and following this promising result, an industrial reactor (Plasmalyse) of 100 litres built up by the Satelec company has allowed the sterilization of B-Stearo spores in 30 minutes at 80°C with N₂ at 4 Torr, 1 slpm, 300 W.

Testing a more complex configuration, the transmission of N-atoms though hollow tubes, interesting the sterilization of endoscopes, was obtained in polyamide tubes of int.dia. 1.5 mm and 50 cm length, by pulsing the plasma gas: pulse 1 s - period 2s, pressure 0.9-2.3 Torr, Q=1slpm and 200 W: T_N= 3.5%.

When increasing the int.dia.to 3mm and the length to 80 cm, the transmission of N-atoms was T_N= 8.5% in the continuous discharge (4 Torr in the 5 litre reactor, 1 slpm and 150 W). It is deduced a destruction probability of the N-atoms on the tube wall:  = 1.6 10^-3.With stainless tubes of int.dia.1.5mm and length 6.5 cm, it was obtained T_N= 1.3% and = 1.6 10^-2.

Besides, in the development of conditioning materials for the sterilization process, the  N-atom transmission was tested through different membranes for sterilisation pouches where in polypropylene ones it was measured to be T_N= 0.65 at 0.1 slpm.

We report a detailed comparison between RF and microwave (HF) plasmas of N₂ as well as N₂-H₂ and N₂-CH₄ in the corresponding afterglows by comparing densities of active species at nearly the same discharge conditions of tube diameter (5-6 mm), gas pressure (6-8 Torr), flow rate (0.6-1.0 slm) and applied power (50-150 W). The analysis reveals an interesting difference between the two cases; the length of the RF plasma (25 cm) is measured to be much longer than that of HF (6 cm). This ensures a much longer residence time (10^-2s) of the active species in the N₂ RF plasma [compared to that (10^-3s) of HF], providing a condition for an efficient vibrational excitation of N₂(X,v), making the RF plasma more vibrationally excited than the HF one. As a result of high VV plasma excitation in , RF, the densities of the vibrationally excited N₂(X,v>13) molecules are higher in the RF afterglow than in the HF afterglow. Destruction of N₂(X,v) on the quartz tube wall is estimated to be very similar between the two systems as can be inferred from the Y_v destruction probability of N₂(X,v>13) on the tube wall: (2-3)10^-3  for both cases, obtained from a comparison between the density of N₂(X,v>13) along the afterglows. Interestingly enough, densities of N-atoms and N₂(A) metastable molecules in the afterglow regions, however, are measured to be very similar with each other. The measured lower density of N₂+ ions than expected in the HF afterglow is rationalized from an high oxygen impurity in the HF setup since N₂+ ions are very sensitive to oxygen impurity. In the N₂-H₂ studied gas mixtures, the N₂(X,v>13) molecules were more destroyed in RF than in HF and inversely, the NH radicals and  atoms are more populated. The TiO₂ surface nitriding in RF was compared to HF at room gas temperature (RT). First, it was produced the most rich nitriding layers with a ratio N/Ti of 0.24 in the RF N₂ late afterglow which was attributed to a low O-impurity in RF (a few ppm), compared to several 10² ppm in HF. Second, in the N₂-H₂ studied gas mixtures, the high H-atoms density appeared to be at the detriment of the nitriding layer with a N/Ti ratio reduced to a few percent in RF as for the HF case. In both RF and HF afterglows, the NH inclusion came with that of N. 

At low and atmospheric gas pressures, the production of active species in N₂ and N₂-O₂ afterglows of electrical discharges is investigated. They are generated in microwave discharges at a wide range of gas pressures ranging from a few Torr to 100 Torr, as well as in corona discharges at atmospheric gas pressure.  The active species in N₂ afterglows are N-atoms, which account for only a few percent of the total. The influence of O₂ molecules at low concentrations in low-pressure N₂ microwave plasmas and as an impurity in corona N₂ discharges was studied in depth. For microbial decontamination and transmission of N-atoms through porous membranes, the interaction of N and O-atoms with surfaces is investigated. For low pressure microwave and atmospheric pressure corona discharges, the procedures of bacteria decontamination in N₂-O₂ afterglows are outlined. In microwave afterglows, the transmission of N-atoms across porous membranes is investigated at medium pressure (10-100 Torr).

Early afterglows of N₂-H₂, Ar-N₂-H₂ and Ar-N₂-O₂ flowing microwave discharges are characterized by optical emission spectroscopy. N and O atoms, N(A) and N₂ (X,v>13) metastable molecules and N_2⁺ ion densities are determined after calibration by NO titration of N and O-atoms and band intensities measurements of NO, N₂ molecules and N₂+ ions. In N₂-xH₂ and Ar-xH₂ gas mixtures, the O-atoms are coming from O₂ impurities in the discharge.
Concentrations of these active species are compared to the ones obtained in Ar-N₂-O₂ gas mixtures in which a controlled amount of O₂ is added. The densities obtained by these line -ratio measurements are with an uncertainty of 30% for N- atoms and the order of magnitude for O-atoms and N(A) metastable molecules.

Surface plasmas treaments have been studied in Ar - gas mixtures (H₂, N₂) as film depositions in fields like microelectronics with amorphous a-SiH, iron nitriding and hard coatings with TiN deposition. Several plasmas setups are considered: sputtering plasmas for a-SiH and TiN deposition and plasmas afterglows for iron nitriding. To control the process, diagnostics by optical spectroscopy have been developed, in particular the resonant absorption method with adapted optical sources. Densities of species such as rare gas (He, Ar) metastable atoms, Al and Ti sputtered target atoms have been determined. The densities of Ti and Ti+ ground and metastable states have been specifically obtained. The behaviour of these active species by varying the plasma parameters has been related to the quality of the deposited films.

The microwave plasmas have been studied from low to atmospheric gas pressure in rare gases (mainly Ar) and gas mixtures containing N₂. At low gas pressure, it is found the surface wave plasmas, produced by patented launchers Surfatron and Surfaguide, which were developped by Michel Moisan in the Plasmas Lab. of the Montreal Univ. since 1975 [1]. At high gas pressure from a few Torr up to the atmospheric gas pressure, the SW plasmas Surfatron at low power and Surfaguide at high power are still operating. It is also given the results obtained by a resonant cavity in N₂-xO₂ with x=0-20%.

It is presently reported the detection of plasma active species obtained by optical spectroscopy.

The main used frequency was 2450 MHz .The plasma power could vary from a few Watts (surfatron) up to a few 10³ Watts (Surfaguide, resonant cavity). The plasma can be produced in a static gas flow, mainly at low gas pressure and up to several liters par minute (slpm) at high gas pressures.

The optical spectroscopy was setup first to obtain the spatial distribution of radiative species, mainly in Ar gas at low gas pressure in the perspective to characterize the SW plasmas. Then the radial distribution of Ar metastable density was obtained by resonant optical absorption.

At medium (a few Torr) up to the atmospheric gas pressure, flowing HF plasmas (resonant cavity) and SW plasmas were produced in gas mixtures with N₂ to produce N-atoms and other active species such as N₂ metastable molecules and N₂⁺ ions which are studied inside the plasma and in afterglow conditions. It is dicussed the kinetics reactions relating the N₂( X,v) , N₂(A,v’) , N₂(B,v’), N₂(C,v’), N₂⁺ (X,v’) and N₂+ (B,v’) interfering in the radiative emissions of N₂ (B,v’) , N₂ (C,v’) and N₂⁺ (B,v’) states.

At atmospheric gas pressure, the plasmas are mainly in Ar dominant gas, more easy to ionize. In these conditions, the plasmas are characterized by the electron density and the gas temperature which will be reported for specific microwave plasmas sources. Depending on plasmas parameters as powers, flow rates and exit tube diameters, the plasmas were more and less near the Local Thermodynamic Equilibrium (LTE) as demonstrated in Montreal Univ., Lisbon IST and Cordoba Univ.

Flowing plasmas and afterglows sustained by DC glow, hollow cathode arc (HCA), and microwave (surfatron) discharges have been studied to obtain a maximum density of R(He, Ne, Ar) metastable atoms and N, O, and H atoms by setting the experimental parameters such as the gas pressure and mixture, the discharge tube diameter, gas flow rates, discharge power, etc. The absolute densities of these active species have been measured by resonant absorption spectroscopy, NO titration with emission spectroscopy, and with a catalytic probe. The maximum densities of He (2³S), Ne(³P₂), and Ar(³P₂) metastable atoms obtained in these discharges at reasonable powers or power densities are a few 10¹¹-10¹² cm^-3 in the range of pressures between 0.1 and 1 Torr. The densities of N, O, and H atoms may be as large as 10¹⁵ – 10¹⁶ cm^-3 in the pressure range between 0.1 and 10 Torr providing the surface of materials facing plasma exhibit a low coefficient for heterogeneous surface recombination. A dissociation fraction of oxygen atoms is often around 10% at reasonable discharge power, more in a mixture with a noble gas. The N-atom density is limited because of the high binding energy of nitrogen molecules and thus a rather poor production rate. The H-atom density tends to be limited by significant surface losses unless the experimental systems are made from quartz glass.