Improvement in Signal Response of In-depth Elemental Profiles in Radio-frequency Glow Discharge Optical Emission Spectrometry by Using a Bias-current Introduction Method

The objective of this paper is to suggest a new method for obtaining an in-depth profile with better precision in 13.56-MHz radio-frequency (RF) glow discharge - optical emission spectrometry. For this purpose, a phenomenon regarding self-bias voltage in the RF plasma is focused on. The self-bias voltage is induced near the RF-loaded electrode, enabling sample atoms to be sputtered from it into the plasma. A bias current can be introduced through the electric circuit including the plasma body, by connecting an external electric device with the glow discharge lamp. The amount of the bias current would change the characteristics of the plasma for atomic emission spectrometry. When higher bias current flows, the resulting plasma extended in the whole area of the glow discharge lamp, including both the electrodes and grounded housing of the lamp, thus making the plasma unstable. However, an exposure to such a plasma could remove gaseous species, which would be entrained when the sample was exchanged, from interior parts of the lamp. This effect contributed to better response of the emission signal just after the discharge was ignited. Depth profiles of a nickel-electroplated steel plate were measured in RF-GD-OES, to investigate the effect of the bias-current introduction.