Abstract
The authors use electron spectroscopy for chemical analysis and Auger electron analysis to study the interaction of Cl and F with Al thin-films deposited as thin-films on Si wafers and as Al bondpads. The motivation behind the study is F contamination being the putative source of poor throughput at wafer probe. F species stemming from NH4F and XeF2 exposure behave quite differently from HF on the Al surface. Whereas HF tends to attack the Al metal and leave an extended oxygenated-fluorinated surface, NH4F and XeF2 promote the formation of a stable, non-deliquescent fluoride salt of aluminum. HCl is far less corrosive to Al than HF, leaving a thin chlorinated-oxygenated surface. Immersion of Al thin-films in tetra-methyl-ammonium hydroxide (TMAH) and NH4OH provided non-halogenated surfaces for comparison. With exposure to air, the surface coated with the fluorinated Al salt (NH4F) adsorbs oxygen from the air to form a segregated AlF3/Al2O3 bilayer that remains stable with a total thickness on the order of 5 nm to 10 nm. Furthermore, wafers treated with NH4F display stellar throughput performance at wafer test despite having surface F contamination. A mechanical rather than chemical model is proposed to explain the improved performance at wafer probe with the immersion of wafers in a bath containing fluoride salts before wafer probe.
