Dry Plasma Alternative Removes Post-Metal Etch Residues

Post-etch photoresist and residue removal are among the challenges manufacturers face today. An undesirable side effect of photoresist strip process conditions is oxidation of post-etch polymer residues, which then become more difficult to remove and often require solvent wet cleaning.

A new process sequence that completely removes post-metal etch polymer residues, while eliminating the structural damage problems associated with the traditional wet solvent clean process, was introduced at the recent Ultra Clean Processing of Silicon Surfaces (UCPSS) symposium in Oostende, Belgium. A CF₄-based method for dry plasma removal of post-metal etch polymer residues was developed on a dual-plasma-source, dry cleaning system for use on embedded flash devices. The process was developed by researchers Maria Paola Pozzoli, senior process engineer at Novellus Systems (San Jose); and Simona Petroni, process engineer at STMicroelectronics (Agrate, Italy). The CF₄-based dry plasma in the sequence dry plasma + in situ rinse + dry plasma + DI water rinse was tested on embedded flash devices on 200 mm wafers. It was shown to provide a reliable, cost-effective alternative to solvent-based processing, with greater cleaning efficiency and less damage to device structures.

The initial approach to replace the solvent clean for the post-metal etch polymer removal was:

Step 1: CF₄/N₂/H₂ rf plasma to remove/reduce top and bottom polymers.
Step 2: CF₄/O₂/N₂/H₂ microwave plasma to remove sidewall residues.
Step 3: DI water rinse to eliminate water-soluble residues.

Residue-free metal structures achieved after the four-step clean sequence of the center (top) and the edge (bottom) of an embedded flash device. (Source: Novellus Systems)

The researchers found that the following four-step recipe sequence was necessary to effectively clean these products:

Step 1: CF₄/O₂/N₂/H₂ microwave and rf dry plasma.
Step 2: Intermediate H₂ O rinse.
Step 3: CF₄/N₂/H₂ rf dry plasma.
Step 4: Final DI water rinse.

The intermediate H₂O step was crucial in making the fluorinated polymer residues more soluble before their exposure to the second flourine-based dry plasma step and their final elimination through H₂O rinse. This intermediate H₂O treatment was achieved through an external DI water rinse, or in situ in the dry clean system using water vapor. The Figures show that the sidewalls and the top surface of the metal lines are free of any residues after this four-step sequence. Also, the four-step process demonstrates improved electrical defectivity and device yield compared with the wet clean process of record.

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