On the Mechanism of Nitrogenated Carbon Film Deposition by PACVD in the Presence of Nitrogen. A Cryo‐Trapping Assisted Mass Spectrometric Study

Abstract

The well-documented effect of film inhibition/perturbation that nitrogen molecules have in the plasma-assisted (PA)CVD of a-C/N:H films in a DC glow discharge reactor under low pressure is investigated. Plasma parameters, gas-phase composition, and film characteristics are recorded using several diagnostic techniques, including the newly developed cryo-trapping assisted mass spectrometry (CTAMS). Five gas mixtures are investigated; CH 4 /H 2 , CH 4 /N 2 /H 2 , and CH 4 /Ar/H 2 as precursors for the films, at low relative concentration of the hydrocarbon molecule (methane), and H 2 /N 2 and He/N 2 as film etchers. There are found to be strong differences in the composition of reaction product between systems in which plasma/surface interactions are dominated by ion-induced processes (sputtering), and those where chemical reactions with nitrogenated species would behave as scavengers of the film precursor. The implications of these findings for the proposed mechanisms of a-C/N:H film growth are addressed. The application of the scavenger technique for the inhibition of tritium-containing carbon co-deposits in the divertor region of fusion reactors, as an international thermonuclear experimental reactor (ITER) in the carbon-dominated scenarios, is analyzed in the light of these findings.

Surface effects on the diagnostic of carbon/nitrogen low‐pressure plasmas studied by differentially pumped mass spectrometry

“…C 2 H 2 : the main production path is by hydrogen chemical sputtering of the a‐C:H at the reactor walls . Which is confirmed as no C 2 H 2 is detected in a He/N 2 plasma during the DC‐GD plasma cleaning of a‐C:H, when in a H 2 /N 2 plasma it is produced …”

“…and . C 2 H 4 is known to be a product from the chemical sputtering at the ion bombardment energy in this work . However, low C 2 H 4 is obtained because it is destroyed by nitrogen radicals, reaction 4.…”

“…HCN : it is produced by nitrogen chemical sputtering of a‐C:H layers at the reactor walls as it has been detected during DC‐GD plasma cleaning of a‐C:H films with N 2 and He or H 2. Other well‐known reaction path is the reaction of nitrogen radicals with hydrocarbons on the gas phase, reactions 3 and 4. …”

“…A Stanford Research Systems SRS Residual Gas Analyzer (RGA‐100) is used to measure the mass spectra of the products along the experiment. In order to study the minority products, the CTAMS technique was applied as described elsewhere . Briefly, it consists on the trapping of the condensable species on a liquid nitrogen cooled trap during the plasma, approximately at 100 K, and the subsequent evaporation of the condensed species by slow heating up, around 2.5 K/min.…”

“…Finally, the previously described cryo‐trap assisted mass spectroscopy (CTAMS) can provide very important additional information about the composition of the gas being monitored. The technique is based on the condensation of some species of the sampled gas to allow for the integration of small concentrations and discrimination among species with overlapping cracking pattern . Its application to the present reactive system for the detection of minority species was reported previously …”