txrf (total reflection x-ray fluorescence)

TXRF (Total Reflection X-Ray Fluorescence) detects trace metallic contamination on wafer surfaces at extremely low concentrations for process monitoring and qualification. **Principle**: X-ray beam strikes wafer surface at very shallow angle (below critical angle for total reflection). Fluorescence from surface contaminants detected while substrate signal suppressed by total reflection geometry. **Sensitivity**: Detects metallic contamination down to 10^8 - 10^10 atoms/cm² (parts per trillion level). Orders of magnitude more sensitive than conventional XRF. **Total reflection**: Below critical angle (~0.1 degrees for Si), X-ray beam penetrates only top ~5nm of surface. Surface-sensitive technique. Minimal substrate background signal. **Elements detected**: Transition metals (Fe, Ni, Cu, Cr, Zn, Ca, K, Na) that are common clean room and process contaminants. **Applications**: Incoming wafer qualification, process tool monitoring, clean qualification, chemical purity verification, contamination excursion investigation. **Measurement**: Automated wafer scanning at multiple points. Maps contamination across wafer surface. **VPD-TXRF**: Vapor Phase Decomposition concentrates surface contaminants into a small droplet, which is then measured by TXRF. Improves sensitivity by 100-1000x. **Process monitoring**: Each process tool monitored for metallic contamination using witness wafers. Excursions trigger tool qualification. **Specifications**: Advanced fabs specify <10^10 atoms/cm² for critical metals on incoming wafers. **Vendors**: Bruker, Rigaku, Technos.