To evaluate the reliability of ToF-SIMS to quantitatively detect trace metals on silicon wafers, purposely contaminated reference samples produced by spin coating were analysed. The spin coating method is commonly employed to produce, for instance, reference samples for TXRF analysis since the amount of metals added on the surface can be effectively controlled and laterally homogeneously contaminated samples obtained. Twin sets of wafers coated with iron, chromium, zinc, nickel, copper, calcium an potassium in the range 1010 - 1014 at/cm2 were prepared. The true surface metal densities were quantitatively evaluated by TXRF and VPD-TXRF. Samples were characterised by ToF-SIMS. The surface contaminants lateral distribution on the microscopic scale was investigated by the imaging capabilities of this technique. Calibration curves and RSFs were evaluated for ToF-SIMS trace metals quantification by comparison with data obtained by TXRF and VPD-TXRF. Linear relationships between ToF-SIMS vs. TXRF/VPD-TXRF values were found. They hold for all the elements for surface composition changes ranging on more than three orders of magnitude. Moreover, no evidences of matrix induced effects were observed on either the relative and the absolute secondary ion yield for silicon and contamination metals. Experimentally determined RSF was applied to quantify residual iron on wafers treated by different cleaning processes for IC fabrication. Different contamination removal efficiencies were shown by ToF-SIMS and lifetime (Elymat) measurements for the various cleaning recipes and treatments. A generally remarkable agreement was observed comparing these results. Not surprising, fitting the data obtained by ToF-SIMS and Elymat with a linear function resulted in a non-unitary recovery rate for iron during electrical activation required for lifetime measurements. Moreover, a non-zero offset was observed to bias the ToF-SIMS data. Possible analytical artefact and physical phenomena were investigated to explain this experimental evidence, thus obtaining clues of additional surface contamination as probably due to wafer storage or delivering to off-site laboratory for ToF-SIMS characterisation
ToF-SIMS quantification of trace metal contaminants on silicon wafer: RSF's evaluation and characterization of wet cleaning processes
Lazzeri, Paolo;Bersani, Massimo
2000-01-01
Abstract
To evaluate the reliability of ToF-SIMS to quantitatively detect trace metals on silicon wafers, purposely contaminated reference samples produced by spin coating were analysed. The spin coating method is commonly employed to produce, for instance, reference samples for TXRF analysis since the amount of metals added on the surface can be effectively controlled and laterally homogeneously contaminated samples obtained. Twin sets of wafers coated with iron, chromium, zinc, nickel, copper, calcium an potassium in the range 1010 - 1014 at/cm2 were prepared. The true surface metal densities were quantitatively evaluated by TXRF and VPD-TXRF. Samples were characterised by ToF-SIMS. The surface contaminants lateral distribution on the microscopic scale was investigated by the imaging capabilities of this technique. Calibration curves and RSFs were evaluated for ToF-SIMS trace metals quantification by comparison with data obtained by TXRF and VPD-TXRF. Linear relationships between ToF-SIMS vs. TXRF/VPD-TXRF values were found. They hold for all the elements for surface composition changes ranging on more than three orders of magnitude. Moreover, no evidences of matrix induced effects were observed on either the relative and the absolute secondary ion yield for silicon and contamination metals. Experimentally determined RSF was applied to quantify residual iron on wafers treated by different cleaning processes for IC fabrication. Different contamination removal efficiencies were shown by ToF-SIMS and lifetime (Elymat) measurements for the various cleaning recipes and treatments. A generally remarkable agreement was observed comparing these results. Not surprising, fitting the data obtained by ToF-SIMS and Elymat with a linear function resulted in a non-unitary recovery rate for iron during electrical activation required for lifetime measurements. Moreover, a non-zero offset was observed to bias the ToF-SIMS data. Possible analytical artefact and physical phenomena were investigated to explain this experimental evidence, thus obtaining clues of additional surface contamination as probably due to wafer storage or delivering to off-site laboratory for ToF-SIMS characterisationI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
