Application of Dual-step Pulse Voltage to the Excitation Source for Improving the Depth Profiling in Glow Discharge Optical Emission Spectrometry

An excitation source driven by pulsed discharge voltage of a two-step waveform, comprising a short higher-voltage pulse and a subsequent main pulse, is suggested for the depth profiling in glow discharge optical emission spectrometry (GD-OES). Pulsation of a DC voltage can be applied effectively for analysis of several specified samples, such as a ultra-thin layer to suppress the sputtering rate; however, it might worsen the in-depth resolution due to a decrease in the sputtering rate. The main reason for this degradation is that the switching-on period of the pulsed discharge may not follow the waveform of the timing pulse and then it is delayed and distorted, due to the capacity component of the plasma itself. In the dual-step pulse method, the first-step pulse would work as a trigger for start of the switching-on period, and thus the response of the resulting plasma could be less delayed even when the duty ratio becomes smaller. As a result, the sputtering conditions were suitable for the in-depth analysis with better interface resolution as well as larger information depth. Depth profiling of an electro-plated nickel layer on a steel substrate was carried out comparatively between a conventional single-step pulse and the dual-step pulse method, yielding better interface resolution by a factor of 1.5–2 with the sputtering rate to be 2–4 times reduced.