Depth profiling by pulsed glow discharge time-of-flight mass spectrometry with a combined hollow cathode cell†
A combined hollow cathode microsecond direct current pulsed glow discharge time-of-flight mass spectrometry (CHC μs-DC-PGD TOFMS) system has proved its efficiency for quantification; however, it has not been properly tested for the purpose of depth analysis until now, to the best of the authors' knowledge. The aim of the current study was to investigate the capabilities of this glow discharge source type for depth profiling. Special attention was paid to the alteration of depth resolution during the sputtering and the effect of discharge parameters on the crater shape in different types of solid samples. Both dielectric and conductive samples were tested, including: a Ni film on silicon, a Si film on borosilicate glass, and IR-reflective dielectric multilayer (mirror) coatings. Crater shapes were investigated after combined hollow cathode cell sputtering to ensure reliable profiling. For dielectric materials, including multilayer coatings, the deposition of a thin conducting layer (Ag or Ta) on the sample surface was demonstrated to result in reliable sputtering with adequate depth resolution of the profiling. This preliminary sample coating provided both the formation of required surface conductivity and reduction of the negative effect of atmospheric gases and water surface contamination at the beginning of sample sputtering. A silicon-coated borosilicate glass specimen was tested for potential quantitative profiling, showing promising results. The CHC μs-DC-PGD TOFMS system used was capable of analyzing the layers of varied thickness in the range from tens of nanometers to several micrometers. The depth resolution was 5 nm and 25 nm for conducting and dielectric layers, respectively.