Limitation of the TiN/Ti layer formed by the rapid thermal heat treatment of pure Ti films in an NH₃ ambient in fabrication of submicrometer CMOS flash EPROM IC's

In this study, the technical feasibility and limitation of the TiN film formed by the rapid thermal heat treatment (RTHT) in an ammonia ambient for the fabrication of 0.85-μm CMOS flash electrically programmable read only memory (EPROM) integrated circuits having a contact aspect ratio of approximately 1.5, were investigated. When the as-deposited thickness of the Ti film was less than 130 Å, all memory contacts exhibited an “electrically open” contact signature (>4750 Ω/contact). Transmission electron microscopy (TEM) cross section examination of a failed contact chain structure indicated that the failure mechanism could be a physical separation at the W-plug/silicon interface. For the Ti films with thickness between 130 and 240 Å, the contact resistance dropped to a value between 59 and 68 Ω/contact. Furthermore, the contact failure rate (contacts exhibiting “electrically open” signature) decreased as the thickness of the Ti film increased. In this case, the failure mechanism appeared to be the direct contact between the TiN layer and the silicon at the contact interface. This condition was created during the multistep nitridation process where approximately 240 to 250 Å of the Ti film was converted to TiN. For the memory devices having Ti films thicker than 240 Å, all memory contacts were electrically and mechanically stable, and exhibited equivalent or higher circuit yields compared to devices having a sputter deposited TiN/Ti bilayer film (control group)