We investigated the efficiency and formation mechanism of ammonia generation in recombining plasmas generated from mixtures of N 2 and H 2 under various plasma conditions. In contrast to the Haber-Bosch process, in which the molecules are dissociated on a catalytic surface, under these plasma conditions the precursor molecules, N 2 and H 2 , are already dissociated in the gas phase. Surfaces are thus exposed to large fluxes of atomic N and H radicals. The ammonia production turns out to be strongly dependent on the fluxes of atomic N and H radicals to the surface. By optimizing the atomic N and H fluxes to the surface using an atomic nitrogen and hydrogen source ammonia can be formed efficiently, i.e., more than 10% of the total background pressure is measured to be ammonia. The results obtained show a strong similarity with results reported in literature, which were explained by the production of ammonia at the surface by stepwise addition reactions between adsorbed nitrogen and hydrogen containing radicals at the surface and incoming N and H containing radicals. Furthermore, our results indicate that the ammonia production is independent of wall material. The high fluxes of N and H radicals in our experiments result in a passivated surface, and the actual chemistry, leading to the formation of ammonia, takes place in an additional layer on top of this passivated surface.

We measured the densities of NH and NH(2) radicals by cavity ring-down spectroscopy in N(2)-H(2) plasmas expanding from a remote thermal plasma source and in N(2) plasmas to which H(2) was added in the background. The NH radical was observed via transitions in the (0,0), (1,1), and (2,2) vibrational bands of the A(3)Pi <-- X(3)Sigma- electronic transition and the NH(2) radical via transitions in the (0,9,0) <-- (0,0,0) band of the A(2)A(1) <-- X(2)B(1) electronic transition. The measurements revealed typical densities of 5 x 10(18) m(-3) for the NH radical in both plasmas and up to 7 x 10(18) m(-3) for the NH(2) radical when N(2) and H(2) are both fed through the plasma source. In N(2) plasma with H(2) injected in the background, no NH(2) was detected, indicating that the density is below our detection limit of 3 x 1016 m-3. The error in the measured densities is estimated to be around 20%. From the trends of the NH(x) radicals as a function of the relative H(2) flow to the total N(2) and H(2) flow at several positions in the expanding plasma beam, the key reactions for the formation of NH and NH(2) have been determined. The NH radicals are mainly produced via the reaction of N atoms emitted by the plasma source with H(2) molecules with a minor contribution from the reaction of N+ with H(2). The NH(2) radicals are formed by reactions of NH(3) molecules, produced at the walls of the plasma reactor, and H atoms emitted by the plasma source. The NH radicals can also be produced by H abstraction of NH(2) radicals. The flux densities of the NH(x) radicals with respect to the atomic radicals are appreciable in the first part of the expansion. Further downstream the NH(x) radicals are dissociated, and their densities become smaller than those of the atomic radicals. It is concluded that the NH(x) radicals play an important role as precursors for the N and H atoms, which are key to the surface production of N(2), H(2), and NH(3) molecules.

The densities of NH and NH 2 radicals in an Ar-NH 3 plasma jet created by the expanding thermal plasma source were investigated for various source-operating conditions such as plasma current and NH 3 flow. The radicals were measured by cavity ringdown absorption spectroscopy using the ͑0,0͒ band of the A 3 ⌸ ← X 3 ⌺ − transition for NH and the ͑0,9,0͒-͑0,0,0͒ band of the à 2 A 1 ← X 2 B 1 transition for NH 2 . For NH, a kinetic gas temperature and rotational temperature of 1750± 100 and 1920± 100 K were found, respectively. The measurements revealed typical densities of 2.5 ϫ 10 12 cm −3 for the NH radical and 3.5ϫ 10 12 cm −3 for the NH 2 radical. From the combination of the data with ion density and NH 3 consumption measurements in the plasma as well as from a simple one-dimensional plug down model, the key production reactions for NH and NH 2 are discussed.

We measured the steady-state gas composition of plasmas produced from Ar-N 2 -O 2 mixtures and Ar-NO mixtures with quantitative mass spectrometry. In the former, mainly N 2 and O 2 , but also a significant amount of nitric oxide (NO) was formed, i.e. up to 5% of the background gas was NO. In the inverse experiment, in which NO was admixed to an argon plasma, up to 92% of the NO was converted into N 2 and O 2 . The observed molecules are mostly generated in wall association processes but also by gas phase reactions between N atoms and O 2 molecules leading to NO. The two types of plasmas show a strong mutual resemblance in the steady-state gas composition if substantial dissociation can be reached in the residence time of the gases in the plasma, i.e. ≈5% NO and ≈95% N 2 and O 2 , although the starting conditions are completely different. It seems that in first order the system prefers to produce the most thermodynamically stable molecules.

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The densities of N, NH, and NH2 radicals in a remote Ar–NH3–SiH4 plasma used for high-rate silicon nitride deposition were investigated for different gas mixtures and plasma settings using cavity ringdown absorption spectroscopy and threshold ionization mass spectrometry. For typical deposition conditions, the N, NH, and NH2 radical densities are on the order of 1012cm−3 and the trends with NH3 flow, SiH4 flow, and plasma source current are reported. We present a feasible reaction pathway for the production and loss of the NHx radicals that is consistent with the experimental results. Furthermore, mass spectrometry revealed that the consumption of NH3 was typically 40%, while it was over 80% for SiH4. On the basis of the measured N densities we deduced the recombination and sticking coefficient for N radicals on a silicon nitride film. Using this sticking coefficient and reported surface reaction probabilities of NH and NH2 radicals, we conclude that N and NH2 radicals are mainly responsible for the N incorporation in the silicon nitride film, while Si atoms are most likely brought to the surface in the form of SiHx radicals.

Abstract. A low pressure recombining Ar plasma to which mixtures of N 2 and O 2 were added has been studied to explore the relevance of surface related processes for the total chemistry. The abundances of the stable molecules N 2 , O 2 , NO, N 2 O and NO 2 have been measured by means of a combination of infrared tunable diode laser absorption spectroscopy and mass spectrometry.A gas phase chemical kinetics model was developed in chemkin to investigate the contribution of homogeneous interactions to the conversion of the feedstock gases N 2 and O 2 . At a partial pressure of N 2 plus O 2 less than 8 Pa, significant discrepancies between measured and calculated concentrations of N 2 O and NO 2 were observed, indicating that heterogeneous processes are dominating the chemistry in this pressure range. At a partial pressure of N 2 plus O 2 higher than 40 Pa and a relatively high fraction of admixed O 2 we observed a fair agreement between measured and calculated concentrations of NO molecules, indicating that homogeneous processes (notably Natom induced) are more dominant than heterogeneous processes.

A full characterization of the Ar-NH 3 expanding thermal plasma source applied in industrial processes for high-rate silicon nitride deposition is presented in terms of absolute densities of reactive species such as ions and radicals. The ion composition of the plasma was identified by mass spectrometry, while absolute ion density information was obtained by Langmuir probe measurements. N radicals were detected by threshold ionization mass spectrometry, whereas NH and NH 2 radicals were measured by cavity ringdown spectroscopy. It was found that the ion density decreases from 10 13 cm −3 in a pure Ar plasma to 10 10 -10 11 cm −3 when NH 3 is injected, with ArH + , NH + 2 , NH + 3 , NH + 4 and H + 3 being the most abundant ions. The densities of N and NH both saturate at a level of 3 × 10 12 cm −3 at NH 3 flows above 3 sccs while the density of NH 2 increases linearly with the NH 3 flow and reaches a level of 4×10 12 cm −3 . When the plasma source current is increased, the densities of N and NH increase linearly, while the NH 2 density remains approximately constant. Furthermore, it is revealed that most of the consumed NH 3 is converted into N 2 and H 2 in the plasma.