Paper TF-TuP15
Phase Formation Control of Sputtered Ta(N) Films

Tuesday, October 16, 2007, 6:00 pm, Room 4C

High quality Ta(N) barrier is required for the 65 nm node and beyond. Not only must this Ta(N) barrier be dense and conformal, it must have the correct phase and orientation in order to minimize electromigration (EM) and stress migration (SM) of dual damascene Cu structures. Two phases of Ta barrier can be observed after sputter deposition: a-Ta, which is body centered cubic (bcc), and beta-Ta which is distorted tetragonal. These two phases have different properties: alpha-Ta has lower bulk resistivity (15 - 30 µW cm vs. 150 - 200 µW cm) higher temperature coefficient of resistivity, and slightly higher density. Furthermore, a-Ta has a higher potential for driving <111> Cu orientation for better EM reliability. The formation of alpha-Ta by sputter deposition processes can occur through several methods. i) Small additives, such as N or O, into the Ta film can lead to a change in the crystalline structure from tetragonal beta-Ta to distorted a-Ta. ii) deposition temperature exceeding 400 °C. iii) using a base layer of TaN between the dielectric and deposited Ta film can drive the formation of alpha-Ta. More recently it has been observed that phase of deposited Ta(N) grown films largely depends on the combination of ion kinetic energy and ion-to-neutral flux ratio. Previous work has shown that low kinetic energy (< 20 eV) and high ion-to-neutral ratio (> 15) are essential to synthesizing low resistivity alpha-Ta. In this study, it was observed using selective area diffraction (SAED), that, under normal sputter deposition conditions, beta-Ta tends to form preferentially on the field, while weak alpha-Ta tends to form along feature sidewalls. Several studies various TaN underlayers to promote alpha-Ta formation throughout the feature have been conducted. Experimental and modeling studies to characterize process regimes necessary to synthesizing low resistivity alpha-Ta film will be discussed. Overall, the ability to deposit a TaN/Ta barrier and seed layer with the correct phase and orientation throughout the feature is essential in order to improve reliability of dual damascene Cu structures. Although, a TaN underlayer does promote alpha-Ta phase, as the TaN barrier size shrinks, the ability to deposit sputtered alpha-Ta throughout the feature is critical. It was demonstrated that the high ionization efficiency of the Inova HCM allows for the deposition of alpha-Ta at the feature sidewall.