New Technology for Removing Post Etch Residues
Post via etch residues with significant amount of Ti (top), and SEM photo after DFC assisted cleaning (bottom).
Staff -- Semiconductor International, 7/1/1998
Dr. Vladimir Starov, director of corporate technology and co-workers of GaSonics International (San Jose, Calif.), recently developed a dry cleaning method that demonstrated impressive results for post etch residue removal. The densified fluid cleaning (DFC) method is based on application of densified gases at 200-400 psi pressure and 30-40°C. This dry cleaning method lends itself to removal of metallics and particles from wafers. The technologists found that DFC, in conjunction with microwave downstream plasma, accomplishes positive results in post via etch residue removal (see Figure). These post etch residues were not successfully removed without application of DFC. Starov stated, "We have also been successful with cleaning up after polysilicon etch. In this case, the DFC-assisted processing reduces the oxide loss." Electrical tests conducted to assure the quality of the device structures verified no degradation of contact resistance after application of DFC.
DFC is a hybrid method that uses a gaseous source, yet performs the cleaning in the liquid phase. The process begins with a chosen gas of sufficient pressure. The gas is initially delivered to the wafer in the gaseous phase. The pressure then builds above saturation vapor pressure at the wafer temperature, and the cleaning gas starts condensing on the wafer. At this point, a thin layer of liquid is formed on the wafer surface. During the cleaning, the gas/liquid flows over the wafer surface and carries away the impurities in the liquid phase. The chamber is then decompressed and the liquid evaporates, leaving no residue. The process provides benefits of lower quantities of chemicals used and possible reuse of the process gas through recirculation.
DFC is looked to as an option in removing materials from high-aspect ratio structures including veils, sidewall and post etch polymer residues. GaSonics studied the effects of DFC, working together with microwave downstream plasma technology for a variety of cleaning steps, including applications in post via etch, post metal etch and post poly etch.
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| Post via etch residues with significant amount of Ti (top), and SEM photo after DFC assisted cleaning (bottom). |
Process adjustment is usually accomplished by altering the cleaning sequence or the cleaning chemistries. Compared to plasma cleaning alone, GaSonics has a broader spectrum of cleaning technologies that combine DFC with plasma treatments. According to Starov, "We now can successfully remove a greater variety of post etch residues using much lower temperatures. And most important, no additional solvent clean and rinse is required after DFC."
