Back-End-of-Line Cleaning

Back-end-of-line (BEOL) cleaning typically refers to the removal of residues generated during the gas phase (plasma) etching of dielectric and metal films and, in certain cases, the remaining photoresist. In BEOL cleaning, the presence of metal layers precludes the use of aggressive chemicals such as concentrated sulfuric acid peroxide (piranha), ammonium hydroxide-hydrogen peroxide (SC1), and hydrochloric acid-hydrogen peroxide (SC2) used in front-end-of-line (FEOL) cleaning. This chapter reviews the evolution of strippers followed by a discussion of chemical systems relevant to BEOL cleaning formulations. Some of the corrosion issues germane to Al and Cu are highlighted. Cleaning of wafers after chemical mechanical planarization (CMP) has become an important part of the BEOL processes. The diversity in the type of reactive gases and process parameters used in plasma etching yields residues whose compositions vary widely. In spite of this challenge, BEOL cleaning of aluminum- and copper-based structures is currently feasible with semi-aqueous fluoride formulations. Environmental concerns of solvents used in formulations have stirred considerable interest in all-aqueous formulations, which have to be tailored to be compatible with porous hydrophobic dielectric layers that are to be introduced in the very near future.