On-line determination of sputtered depth in a radio frequency glow discharge atomic emission source by laser confocal displacement

Abstract

A new method of sputtered depth determination was evaluated for glow discharge (GD) analysis of solid samples. Although sputter analysis methods like GD may characterize samples in terms of both composition and spatial (depth) distribution of those components, quantitative analysis may suffer significantly from uncertainties in sputter rate (depth) determinations. The study described in this paper was conducted in an effort to achieve true depth profile capabilities for the radio frequency glow discharge atomic emission spectroscopy (r.f.-GD-AES) source, however the results are applicable to any sputter-type analysis method. Detailed here is a unique system by which sputter depth information may be collected simultaneously with operation of the GD plasma, with only minimal influence from sample matrix or plasma conditions. As such, the method enables the determination of real-time sputter depth. This achievement was made possible by the use of a laser confocal displacement sensor (LCDS) that optically measures the distance between a mounted sensor head and the sample surface. Very simply, the LCDS measures the displacement of the sputter-exposed surface relative to the original sample. The results outlined in this report indicate that the laser confocal technique has great potential as an advancement to depth-resolved analysis by r.f.-GD-AES. Statistical figures of merit include an experimental depth resolution of -1 μm and a sample-to-sample reproductibility of ∼5% RSD. Methods of improving further the capabilities of the system described here are also outlined.