Thickness determination of subnanometer layers using laser ablation inductively coupled plasma mass spectrometry

Laser ablation inductively coupled plasma mass spectrometry has been applied to determine the thickness of subnanometer (subnm) metallic layers. Metallic Nd was deposited onto Si wafers with 0.5, 1, 3, and 6 nm thickness and covered by a 10 nm Al coating. Integrated ion signals corresponded to the layer thickness, indicating that external calibration provides accurate data. Utilizing sensitivity ratios obtained from ablation of a glass standard reference material (SRM) and the Al layer as reference, the deviations between prepared and measured layer thickness were less than 10%. A further attempt was made to determine the layer thickness with the absolute sensitivity obtained from ablation of a glass SRM. The approach also yielded a linear correlation between determined and actual layer thickness, but bias and interday variability were significantly higher. This is assumed to result from imprecise determination of the crater size when ablating the glass SRM. This study shows that subnm layers of metals can be analyzed for their thickness either using reference materials of identical composition or using sensitivity ratios when one layer can be used as a reference. Absolute determination of the layer thickness is less accurate but can circumvent the lack of reference materials of similar composition.