Methodology to analyse small silicon samples by glow discharge mass spectrometry using a thin wafer mask

C Modanese,L Arnberg,M Di Sabatino

doi:10.1016/j.mex.2015.10.005

Abstract

Glow discharge mass spectrometry (GDMS) is widely used for trace element analysis of bulk solid samples. The geometry of the GD source limits the minimum size of the sample, which for the instrument used in this work (ThermoElementGD) is 20mm in diameter. From time to time, there is the need to analyse smaller samples with this technique, and we present here a methodology to analyse samples of 9–20mm diameter through the use of thin masks.Thin masks have been previously used mostly as secondary cathode for the analysis of non-conducting materials, with hole size smaller than the area of the glow discharge. The use of masks in this work includes the following customization:•The choice of highly-pure Si as mask material, to decrease the chance of interferences with the Si samples.•The use of a hole in the mask of the same size as the discharge area. This implies that the mask material is not sputtered, thus decreasing chances for contamination from the mask itself.

Highlights

Glow discharge mass spectrometry (GDMS) is widely used for trace element analysis of bulk solid samples
Elemental analyses of silicon by direct current GDMS are generally carried out on bulk samples, which act as the cathode
The as-received Ta sheets are not as flat as the silicon wafers, i.e. they present curvature of the surface. This implies that the physical contact between the flat surface of the Si sample and the slightly curved surface of the Ta mask may lead to a small gap

Summary

Method details

Elemental analyses of silicon by direct current (dc) GDMS are generally carried out on bulk samples, which act as the cathode (see Fig. 1). Smaller samples may be of interest for analysis, e.g. due to the lab-scale size of the material’s production process. Few alternatives, such as using a smaller anode diameter, have been suggested to address this issue. The thickness of the mask has an impact on the distance between the anode and the sample surface (cathode), being $200 mm at the beginning of the discharge. 3. In order to avoid contamination from the laser cutting onto the internal ring in the mask, each mask was etched with the following procedure: I. Removal of organic residues by cleaning in RCA1 solution (ratio 5:1:1 – H2O:NH3:H2O2), 10 min. The mask was stored in a clean box and handled only with gloves

The roughness of the wafers has been calculated as: roughness

Method validation

Findings

Â 109 Æ 2 Â 108 108 Æ 8