Semi-Aqueous Cleaning Approaches the Forefront

Maria A. Lester, Associate Editor -- Semiconductor International, 3/1/2001

Chemical cleaning for post metal etch is continually challenged as devices shrink. For example, tungsten plugs exposed during the metal-cleaning step are subject to tungsten-plug erosion when cleaned with sidewall polymer removal chemistries. Standard amine-based and hydroxylamine-based chemistries have shown good cleaning performance, but need high temperatures and 15-45 min process times. Ozone, super-critical cleaning and dry etch need more research and development. The integration/introduction of semi-aqueous cleaning (SAC) chemistries for post metal etch cleaning is another option. EKC Technology Inc. (Hayward, Calif.) designed fluoride chemistries for both wet benches and spray tools and the evolving single-wafer spray tools. These chemistries can be used for low- or room-temperature processes with shorter process times, are rinseable in water and can reduce water-rinse volumes.

Researchers studied the effects of cleaning metal structures with SAC chemistries such as the EKC 600 series 640 SACs with low concentrations of fluoride (pH <9.5). The focus was to determine metal integrity when looking at 1) process latitude of the cleaning chemicals; 2) the effect of DI water rinsing; 3) the effect on isolated vs. dense metal structures; and 4) the effect of using different intermediate rinses. The study showed that DI water rinsing could have a considerable effect on metal integrity. In particular, use of CO₂ sparging in the DI water, usually used to prevent galvanic corrosion after cleaning chemistries, can cause linewidth loss in metal structures when compared with DI water without CO₂. "We show that the traditional CO₂ sparge step in the rinse water is not a good option; the rinse water pH moves to the acid side, which will 'bother' the very clean Al metal surface," said EKC's Robert Small. "Normally, CO₂ can be used with limited success for traditional amine-based chemistries."

In addition, buffered intermediate rinse solutions (pH ~4) that work with hydroxylamine-based chemistries were found to be ineffective, and could enhance corrosion when used with certain semi-aqueous chemistries. Results also indicate that there are intermediate rinse chemistries (pH 7-8) that can also be used for fluoride-based chemistries. The effect of pH adjustments of the intermediate rinse solutions can be developed for device structures that are very sensitive to the chemistry and DI rinse sequence. "These unusual fluoride chemistries can clean various types of metal structures (also vias) under very mild conditions and short process times," Small said. "With SAC chemistries, water rinses can be as short as 30 seconds or up to two minutes. The low concentration of fluoride and organic solvents that are readily soluble in water make this possible."

Results shown are before wet cleaning with etch and in situ only (left), after 3 min of SAC chemistry at 27°C and 3 min DI QDR (center), and after 3 min SAC chemistry at 27°C followed by intermediate rinse (pH 7.5) for 3 min and 3 min of QDR (right). (Source: EKC Technologies)

The Figure demonstrates results where the cleaning chemistry time and temperature are kept constant. The left image shows the sample after metal etch and in situ ash. The center image shows the effect of varying the DI water rinse time after the SAC chemistry. The right image shows the effect of raising the pH (7.5) of the commercially available intermediate rinse.

These chemistries could fill the gap needed for current metal-cleaning requirements, and show potential for dual-damascene copper-cleaning applications. But the industry first needs to let go of its dependence on hydroxylamine-based residue removers.

For additional information on clean processing, go to www.semiconductor.net/clean

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