Investigations of the effect of hydrogen, nitrogen or oxygen on the in-depth profile analysis by radiofrequency argon glow discharge-optical emission spectrometry

The effect of adding either hydrogen, nitrogen or oxygen (from 0.5 to 10% v/v) to an argon radiofrequency glow discharge (rf-GD) for optical emission spectrometric measurements has been investigated. Changes to the dc-bias voltage developed for conductive samples, to the crater shapes produced (both in homogeneous conductive materials, as austenitic stainless steels, and in a glass sample), and to the depth resolution for thin films on a glass substrate have been measured, operating the rf-GD at constant pressure and constant delivered power. Concerning the dc-bias voltages, the observed general effect was an increase of the voltage with increasing added hydrogen. For nitrogen and oxygen additions, an enhancement of the dc-bias was also observed, as compared to pure argon, but just in the interval 2-10% v/v of added gas. Experiments related to the crater shapes showed that working at 40 W and 600 Pa the craters produced in stainless steels increased their convexity with increasing percentages of any of the three molecular gases assayed. However, such convexity was reduced by working at lower delivered powers. The studies on adding hydrogen, nitrogen or oxygen to the argon rf-GD on the crater shapes produced in a homogeneous glass sample showed very good crater shapes in the glass at 20 W and 450 Pa, for most of the assayed plasma gas compositions in the interval 0.5-5% molecular gas/argon. Finally, qualitative profiles of two glass samples covered with multilayers, in the order of nanometres (6-27 nm), were measured for different plasma gas compositions and their relative depth resolution calculated. Results show that the plasma gas mixtures under investigation should be considered, at least for qualitative in-depth profile analysis, as they seem to offer a great potential to improve depth-resolution in rf-GD work.