A Replacement for RCA Cleans

The OAM cleaning process is a chemical blend of HF and an organic substance such as citric acid with ultrapure water (UPW), followed by Cl₂ gas that is dissolved by bubbling before sputtering over the wafer surface. At the same time, an O₃ gas is dissolved in this very acidic mixture. As a result, the solubility of O₃ is held at a high level without saturation of ozonator. The concentration of O₃ is steady during this cleaning step. This process is based on the oxidation-etching reaction at the wafer surface and low pH, which forms soluble chemical complexes in OAM by a rich proton medium and radicals. A high oxidation potential could prevent the oxidation-reduction phenomenon, although the high level of fresh oxidizing agent such as O₃ holds a high oxidation potential.

"Furthermore, another kind of metallic contamination is the film inclusion wherein some metals with higher oxide generation enthalpy are embedded in oxide formation, and as OAM is an etching mixture too," said Lionel Girardie, a technologist consultant based in France. "The cleaning step avoids this type of contamination by the simultaneous reaction of oxidation-etching as Dr. (W.) Kern and Dr. (D.A.) Puotinen of RCA lab proved for their SC1 mixture."

Therefore, this cleaning process could remove the different kinds of contamination of metals such as Al, Cu and even Au. One of OAM's advantages is the ability to etch a layer such as Si, SiO₂, or Cu and compound materials such as GaAs or InP using the anisotropic or isotropic etching according to the level of ozone at the boundary layer interface by the variation of O₃ gas, thereby providing smooth etching.

The OAM clean is used in the Compact Clean, a proprietary wafer surface preparation system. Girardie, formerly an engineer with SOITEC-LETI (Bernin, France), evaluated the OAM cleaning step instead of the RCA clean for single-wafer treatment.

The system uses megasonic power with a relatively high level of concentration of ozone in this mixture without creating OH radicals. The test consisted of dispersed Al₂O₃ slurries on the silicon bare wafers. For comparison, a complete batch was cleaned by three separate methods: the Compact Clean, standard cleans (DHF (0.1%) + SC1 (0.25:1:40) + SC2 (0.25:1:20) at 50°C, and the process involving six cycles of two 10 sec alternating steps of O₃ (20 ppm) and DHF (0.5%) at room temperature.

After each wafer surface cleaning, the wafer is dried with the Depletion Wave technique, Girardie's proprietary motionless single-wafer drying application, which enhances the Marangoni effect (see Semiconductor International, October 2000) — without conventional IPA/N₂ or azeotropic solution and without wafer rotation — by using a surface tension temperature gradient in spreading by a linear wafer scanning.

This cleaning technique could replace the standard cleans RCA and could be used as a wet etching process without particle contamination.