Utilizing Bath/Wafer Contamination Correlation to Validate a Pre-Gate Cleaning Strategy
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433 Mat. Res. Soc. Symp. Proc. Vol. 477 01997 Materials Research Society
EXPERIMENTAL In order to test the metals removal performance of our chosen clean sequence, we adopted the strategy of contaminating various baths and investigating the correlation between bath contamination and wafer surface contamination. We used several analytical techniques and methods in this evaluation. Each technique and method is summarized below. Total X-Ray Fluorescence (TXRF) A Technos TREX 610-T TXRF instrument was used. The analysis area is 10 mm in diameter, and the penetration depth is about 30A for glancing angles below the critical angle. Vapor Phase Decomposition (VPD) VPD was used to concentrate contamination on a Si wafer in a small area. Quantification is achieved using certified standards. The procedure was performed by placing a wafer into an enclosed polyethylene box. 200 ml ppt (parts per trillion) grade HF is placed in a cup on the bottom of the chamber. HF vapor etches the oxide layer. The wafers remain in the box for 15 min droplet is to assure complete etching of the oxide. If TXRF is to be performed, then a 200 Wl rolled over the entire wafer surface and dried with a heat lamp. Inductively Coupled Plasma (ICPMS) A Varian Liberty 200 was used for analysis of the heavily contaminated baths. Both an HP 4500 and a Varian Ultramass were used. Results were calculated by the method of standard additions. The ICPMS was used for clean baths and in combination with VPD for wafer surface analysis. VPD/ICPMS VPD and ICPMS were used in conjunction to analyze extremely low levels of contamination. 500 W1of a solution containing HF and H20 2 is rolled over the surface of the wafer, after VPD, and collected in a pre-weighed Teflon vial. The weight of the solution is measured, and the sample is diluted and weighed again. The sample is then analyzed by quadropole ICPMS. A cross-flow nebulizer with an uptake rate of 100 pA/min is used for delivering the sample to the plasma. Detection limits for VPD/ICPMS on 150mm wafers (atoms/cm 2): Ni (58) = 8.0e+08 Al (27) = 5.0e+08 Mg (24) = 1.0e+08 Ca (40) = 1.0e+09 Cr (52) = 4.0e+08 K (39) = 1.0e+09 Cu (63) = 5.0e+08 Na (23) = 4.0e+08 Li (7) = 1.0e+08 Fe (56) = 7.0e+08 Zn (64) = 1.0e+09 VPD/TXRF The dried VPD spot was analyzed for metal contamination. Detection limits are on the order of 5.OE+9 atoms/cm 2 .This technique is faster than VPD/ICPMS, but is incapable of quantifying sodium and aluminum. Bath Contamination A Universal Plastics manual hood in our clean room laboratory was used to intentionally contaminate wafers and analyze their surface. The HF tank was a standard PVDF recirculated bath with temperature control. The SC I tank was a stagnant quartz bath with temperature control. In order to contaminate the wafers, they were dipped for 5 min with a 7 min cascade 434
rinse and dried. Known concentrations of Al, Ca, Cu, Fe, Mg, and Zn were added to dilute HF baths and two concentrations of SC 1 solutions at two different temperatures. The baths were analyzed using ICPMS, while wafers
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