Computational study on transamination of alkylamides with NH3 ...

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ARTICLE IN PRESS3590Y.S. Won et al. / Journal of Crystal Growth 311 (2009) 3587–3591N2Selected bond lengths and angles4. ConclusionsN1TaTa-N1 (Å) 2.00 (avg.)Ta-N2 ( Å) 1.78N1-Ta-N2 ( ° ) 109.35 (avg.)ΔH ° 298 (kcal/mol)Ta-N1 93.0Ta-N2 138.9Fig. 4. Optimized geometry of Ta(NH 2 ) 3 (NH) following complete transamination.N1 denotes nitrogen in the amido ligand and N2 denotes nitrogen in the imidoligand.of related precursors with NH 3 in TaN ALD executed at even lowertemperature (250 1C) [1,6–8]. However, the relatively higherbarrier for the elimination of tert-butylamine via transaminationis consistent with the observation of residual carbon content(15 at%) in the TaN ALD films using TBTDET and NH 3 at 250 1C[1,8]. The lack of difference in the carbon content (15 at%) in theALD films using the same MO source (TBTDET) with H 2 plasma[1,8] also indirectly supports the slower transamination of tertbutylimidoligand as a cause of residual carbon. ALD experimentsusing PEMAT and NH 3 , in contrast, produced much lower carboncontent (7–8 at%) in the deposited films 1 because the fiveequivalent alkylamido ligands of PEMAT are exchanged easilyvia transamination with NH 3 .To evaluate how Ta–N bonds were affected by transamination,the optimized geometry of the (NH 3 ) 3 Ta(NH), in which all theligands have been exchanged via transamination, and its calculatedbond dissociation enthalpies are shown in Fig. 4. Theincrease of bond dissociation enthalpies by 12–16 kcal/mol upontransamination is consistent with survival of the imido ‘‘Ta–N’’fragment (Ta–N2 in Fig. 4) during deposition but the strongeramido Ta–N bonds (Ta–N1 in Fig. 4) could also have a greatertendency to be carried on the film deposition. A dielectric TaN xfilm (x41, e.g., Ta 3 N 5 ) with very high resistivity would then bemore likely to result if ammonia is employed [2,9,10]. The natureof TaN x has been confirmed to range from highly conductive (Tarich)to insulating (N-rich) according its stoichiometry [9,10,29].As the film grows, NH 2 ligands resulting from transaminationare expected to be easily redistributed over vacant sites on thesurface to provide the ligands for chemisorption of additional Taspecies to the surface. The thermal stability of NH 2 (ad) on the Si(10 0) reconstructured surface was computationally demonstratedby Widjaja and Musgrave [30] using the Si clusterapproximation. The remnant Ta–N bond was then preserved todeposit metallic TaN films. The adatoms, NH 2 (ad)s, however,contribute to the deposition of dielectric N-rich Ta 3 N 5 films [2].The concentration of NH 2 (ad) adatoms on the surface would besufficiently high for them to be incorporated into the depositedfilms in the absence of an additional reducing agent. It is notedthat NH 3 is also a source of NH 2 (ad) adatoms by the dissociationinto NH 2 (ad) and H(ad) [30]. TaN films grown by MOALD as wellas MOCVD generally demonstrated high film resistivity, whichcircumvents their usefulness as diffusion barriers. The ALD of TaNfilms using TBTDET and H 2 plasma (instead of NH 3 ) demonstrateda huge improvement in the film resistivity [1] possibly due to theaddition of active reducing H 2 radicals. The amorphous phaseformation with the use of NH 3 in TaN MOALD [1] is also likelyattributed to the presence of stable NH 2 (ad) adatoms on thesurface especially at the low temperature employed in ALD,occupying active reaction sites and blocking the properly stackedadsorption of Ta MO species.The transamination of alkylamide ligands in MO precursorswith NH 3 for the film growth of transition metal nitrides wascalculated to be facile under MOCVD conditions and would beaccessible at the very low temperature employed in MOALD aswell. Transamination is a likely pathway for removal of carboncontainingligands. The calculated strength of Ta–NH 2 bonds wasincreased after the transamination consistent with an increasednitrogen content and deposition of N-rich dielectric films in thepresence of NH 3 .AcknowledgementL.M.-W. and T.J.A. thank the National Science Foundation forsupport under NSF-CRC grant CHE-0304810.Appendix 1. Supporting InformationSupplementary data associated with this article can be foundin the online version at doi:10.1016/j.jcrysgro.2009.05.003.References[1] H. Kim, J. Vac. Sci. Technol. B 21 (6) (2003) 2231.[2] M.H. Tsai, S.C. Sun, H.T. Chiu, C.E. Tsai, S.H. Chuang, Appl. Phys. Lett. 67 (8)(1995) 1128.[3] M.H. Tsai, S.C. Sun, C.P. Lee, H.T. Chiu, C.E. Tsai, S.H. Chuang, S.C. Wu, ThinSolid Films 270 (1-2) (1995) 531.[4] S.-L. Cho, K.-B. Kim, S.-H. Min, H.-K. Shin, S.-D. Kim, J. Electrochem. Soc. 146(10) (1994) 3724.[5] J.W. Elam, M. Schuisky, J.D. Ferguson, S.M. George, Thin Solid Films 436 (2003)145.[6] O. van der Straten, Y. Zhu, K. Dunn, E.T. Eisenbraun, A.E. 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