Abstract
A method for determination of concentration gradient of trace levels of transition metals (Cu, Zn, Cr, Co and Ni) in silicon wafer using anodic oxidation combined with microconcentric nebulizer equipped inductively coupled plasma mass spectrometry (MCN-ICP-MS) has been developed and optimized.Firstly, developing an anodic oxidation technique for uniform growth of silicon-dioxide (SiO2) layer on silicon wafer and subsequent characterization of the physical properties of the layer formed. The thickness of oxide layer was measured both by SEM and ICP-OES, and the results obtained were found to be in good agreement. The influences of current density, electrolyte composition and electrolysis time on the growth of SiO2 film on the silicon wafer by anodic oxidation were studied. Based on the experimental results, anodic oxidation of a 4-inch single crystalline silicon wafer was carried out in constant current density mode with 0.08 M HNO3 as electrolyte at room temperature. Under the anodization condition at a constant current density of 0.346 mA/cm2 and electrolysis time of 2 hr, a silicon dioxide layer with thickness of about 1000 A can be obtained.Secondly, the optimum conditions of MCN-ICP-MS for the determination of trace metal concentrations were investigated. After anodic oxidation the oxide layers were subsequently dissolved with dilute HF, and the resulted solutions were subjected to ICP-MS for the determination of analyte concentrations. The interference effects caused by the silicon matrix and acid medium in the instrumental determination step were investigated and minimized. The detection limits achievable with the present system for the determination of Co, Cr, Cu, Ni, and Zn can be down to the sub-ppb levels.Finally, the analytical system combined with anodic oxidation and MCN-ICP-MS was tested for its applicability for determination of concentration gradient of trace metals in silicon wafer. From the prelimary study on a silicon wafer heavily polluted with trace metals, it indicated that a general trend showing decrease of analyte concentration with increasing depth, which is in agreement of literature report, could be achieved. In conclusion, the established system of anodic oxidation combined with MCN-ICP-MS was found to be feasible for the determination of concentration gradient of trace metals in silicon wafer.