Dewetting observations of ultrathin metallic films

Ultrathin metallic films like CoFe, Ta, Cu, Cr, and NiFe are widely used in magnetic devices such as magnetic random access memory (MRAM) and magnetic recording heads. Dewetting corrosions were often observed after O₂ plasma ashing in MRAM fabrications. The surface stability of these films was then examined. The results show that dewetting takes place when CoFe or Cu films are exposed to air after an O₂ plasma process. In contrast to the dewetting reported so far in organic or metallic liquid films on solid substrates, the observed dewetting does not occur in a liquid state but in a solid state. Several in situ and ex situ process methods were examined to control the dewetting. It is found that after ashing, the immediate immersion of wafer into acetone and ultrasonic cleaning some minutes after opening chamber can greatly suppress the occurrence of dewettings. Process examinations show that the heating is unimportant for the formation of the dewetting, while moisture in air may play an important role in the formation of the dewetting, acting as a necessary catalyst. Several dewetting patterns were observed, and the pattern shape depends not only on the thickness of the film, but also on the plasma parameters. Possible mechanisms responsible for the formation of these patterns are discussed.