US EPA

6683685 

Journal Article 

An assessment of a dilute hydrofluoric acid purifier wafer contamination 

Weddington, D; Bodnar, J; Jahanbani, M; Blum, R 

1999 

22 

6 

151-2, 154, 156, 158 

INSPEC:6347487 

Wet cleans account for as much as a third of all process steps in VLSI manufacturing, depending on process flow and tool configuration. A large number of these steps use dilute hydrofluoric acid (DHF) to remove silicon oxide, native or chemical oxide, metallic contamination and particles by slightly etching the silicon surface. DHF purity is critical due to the acid's tendency to deposit metal contaminants on the exposed Si surface in the active area where the gate oxides are to be grown. Noble metal ions (such as copper, zinc and iron) deposit on the Si surface by elemental reduction, which is absorption of metal species with electrochemical reduction to metal on the bare Si surface. These metallic deposits degrade the device's electrical properties by creating generation-recombination centers that affect minority carrier lifetime. The sources of metal contamination in typical processing (chemicals, equipment and incoming water) are usually low enough to allow for extended bath life. The utility of an acid purifier then is to control excursions when nonstandard events occur. Motorola evaluated the performance of an ion exchange purifier used in the DHF acid recirculating system. Tests demonstrate that the purifier can reduce the ion levels by more than 90% in the first hour.