(Invited) Selective Atomic Layer Deposition of Cobalt for Back End of Line

Monday, 2 October 2017: 14:00
Chesapeake L (Gaylord National Resort and Convention Center)
T. D. M. Elko-Hansen and J. G. Ekerdt (University of Texas at Austin)
This talk explores the area-selective atomic layer deposition (A-SALD) of cobalt on copper versus silicon dioxide and a porous low-k dielectric. Cobalt reduces copper electromigration as a capping layer on the copper metal line in back end of line applications. As device dimensions scale there is an increasing urgency to develop area-selective deposition methods that are selective for cobalt deposition on copper to eliminate the need for metal etching steps in metallization process schemes. The half reactions of bis(N-tert-butyl-N’-ethylpropionamidinato) cobalt(II) (CoAMD) and H2 are evaluated on Cu, SiO2, and a porous low-k (~2.6) dielectric. The first and second ALD half reactions of CoAMD on the respective substrates are evaluated by adsorbing the precursor on the substrates under ALD cycling conditions at 265 °C with and without H2 coreactant exposure. The adsorption studies indicate that CoAMD preferentially deposits most on Cu and least on the low-k surface. Further, CoAMD, like other amidinate precursors, readily dissociates on the Cu transition metal surface but the ultimate per-cycle coverage is self-limited by the slow desorption of amidinate ligands and fragments from the Cu surface. Co films deposited by ALD from CoAMD on Cu at 265 °C indicate that Co burrows into the lower energy Cu surface as the film grows in order to reduce the free surface energy. The Cu remains as a surfactant-like layer on the topmost Co surface up to film thicknesses of at least 16 nm. Moreover, considerable intermixing at the Co/Cu interface and Cu concentration several nm into the Co films is observed indicating high surface mobility of the two materials and Cu diffusion at polycrystalline Co grain boundaries. Finally, employing low-temperature ALD and selectively passivating the dielectric surfaces with OH targeting passivants, such as trimethylchorolosilane, leads to enhanced selectivity of CoAMD for deposition on Cu versus SiO2 and porous low-k surfaces. Depositing Co from CoAMD on Cu and the porous low-k surface at 165 °C after exposure to trimethylchlorosilane at 50 °C leads to a 30:1 preference for Co accumulation on Cu, a twelve times improvement compared to deposition on clean Cu or the low-k dielectric at 265 °C.