Application Example 16: On the Nature of rf-GD Sputtering

Abstract

Just to give the readers a feeling of the nature of rf-GD sputtering, two rf-GDOES depth profiles are presented in Fig. E16.1. The first one, shown in Fig. E16.1a, was obtained from a mirror-polished Cu surface, covered with a monolayer of thiourea. The thiourea molecule adsorbs to the copper surface through its sulfur end, with the C–S bond oriented perpendicular to the metal surface. The adsorbed layer is disclosed clearly in the depth profile as narrow peaks, above the copper substrate, of carbon, hydrogen, nitrogen, and sulfur. Further, the positions of peaks are separated and located in the order that is expected from the orientation of the thiourea molecules adsorbed on the copper substrate. This is the first example of successful depth profiling analysis of a layer of subnanometer thickness at the atomic scale depth resolution [12], which has never been achieved by other techniques such as Auger electron spectroscopy or SIMS which rely on ion-beam sputtering for layer-by-layer removal of surface layers. The second example, shown in Fig. E16.1b, was obtained from an ultrathin air-formed oxide film covering a mirror-polished nickel surface prepared by electropolishing in a sulfuric acid solution. The chemisorption of oxygen onto a clean nickel surface and the subsequent formation of the first monolayer of NiO is very rapid. Further growth of the oxide, however, is almost insignificant at room temperature in dry atmosphere. The thickness of the film has been estimated to be ~0.3 nm [18] which is only slightly thicker than NiO monolayer. Such an ultrathin oxide film has been disclosed clearly in the depth profile as a narrow peak of oxygen which is located at the surface of nickel whose position is defined as the half-height position of the leading edge of the nickel profile. Prior to the oxygen peak, a narrow sulfur peak is observed; sulfur arises from a small amount of sulfate anions adsorbed on the electropolished nickel surface even after thorough rinsing in distilled water. Here again, the atomic scale depth resolution in rf-GDOES depth profiling analysis has been demonstrated clearly. In the depth profile, Ar profile is also included to demonstrate the extremely excellent stability of Ar plasma during the analysis. Such high stability of Ar plasma is a prerequisite not only for depth profiling analysis of ultrathin layers, but also for sample surface preparation for SEM where sputtered depth has to be controlled precisely as described below.