Chemiluminescence system for direct determination and mapping of ultra-trace metal impurities on a silicon wafer

Abstract

A highly sensitive chemiluminescence (CL) system which consumed low sample and reagent volumes in the microlitre range was developed for direct determination and mapping of ultra-trace metal contaminants on solid surfaces, such as silicon wafers or flat display panels. The analytical result of the system was confirmed with ICP-MS. The system was composed of a scanner, sensor and a wafer moving stage. The scanner, with a scanning tip made of 0.03′′ i.d. PTFE tubing, was used to collect metal impurities on the wafer surface with 5 μL of scanning solution. A coaxial sensing head of about 13 mm o.d. and 110 mm height was designed both to inject a luminescent reagent of luminol-H₂O₂ mixture and to collect the luminescence light resulting from the reaction with metal ions of Co²⁺, Fe²⁺, Cu²⁺, and Ni²⁺. Due to the almost zero background, an extremely low limit of detection of 20.8 pg/mL for Co²⁺ in 1% hydrofluoric acid (HF) was obtained from the calibration curve. In order to map the spatial distribution of the impurities, 11 cross sections of a Co-contaminated wafer were selected and scanned individually with a diluted HF solution. A contaminant level of 1.45–7.11 × 10¹¹ atoms cm⁻² was obtained for each section with an average of 4.21 × 10¹¹ atoms cm⁻², which was similar to the analytical result of 5.48 × 10¹¹ atoms cm⁻² obtained from vapor phase deposition-inductively coupled plasma-mass spectrometry (VPD-ICP-MS). Although this CL system does not have selectivity for each specific metal ion, its high sensitivity facilitates the monitoring and mapping of metal impurities of Co, Fe, Cu, etc. on the wafer directly and it can be used as an on-line inspection sensor for the first time in the semiconductor industry.