The mechanism for zirconium oxide atomic layer deposition using bis(methylcyclopentadienyl)methoxymethyl zirconium and H(2)O was examined using ab initio calculations of hydrolysis energies to predict the order of ligand loss. These predictions were tested using in situ mass spectrometric measurements which revealed that the methyl ligand, and 65% of the methylcyclopentadienyl ligands are lost during the zirconium precursor adsorption. The remaining 35% of the methylcyclopentadienyl ligands and the methoxy ligand are lost during the subsequent H(2)O exposure. These measurements agree very well with the predictions, demonstrating that thermodynamic calculations are a simple and accurate predictor for the reactivities of these compounds. (c) 2007 American Institute of Physics. (DOI: 10.1063/1.2824814

Elam, J. W.

Elliott, Simon D.

Faia, M. C.

Hupp, J. T.

Pellin, M. J.

Zydor, Aleksandra

Applied Physics Letters

Cork Open Research Archive

Title Mechanism for zirconium oxide atomic layer deposition usingbis(methylcyclopentadienyl)methoxymethyl zirconiumAuthor(s) Elam, J. W.; Pellin, M. J.; Elliott, Simon D.; Zydor, Aleksandra; Faia,M. C.; Hupp, J. T.Publication date 2007Original citation Elam, J. W., Pellin, M. J., Elliott, S. D., Zydor, A., Faia, M. C. andHupp, J. T. (2007) 'Mechanism for zirconium oxide atomic layerdeposition using bis(methylcyclopentadienyl)methoxymethyl zirconium',Applied Physics Letters, 91(25), pp. 253123. doi: 10.1063/1.2824814Type of publication Article (peer-reviewed)Link to publisher'sversionhttp://aip.scitation.org/doi/abs/10.1063/1.2824814http://dx.doi.org/10.1063/1.2824814Access to the full text of the published version may require asubscription.Rights © 2007 American Institute of Physics.This article may bedownloaded for personal use only. Any other use requires priorpermission of the author and AIP Publishing. The following articleappeared in Elam, J. W., Pellin, M. J., Elliott, S. D., Zydor, A., Faia,M. C. and Hupp, J. T. (2007) 'Mechanism for zirconium oxideatomic layer deposition usingbis(methylcyclopentadienyl)methoxymethyl zirconium', AppliedPhysics Letters, 91(25), pp. 253123 and may be found athttp://aip.scitation.org/doi/abs/10.1063/1.2824814Item downloadedfromhttp://hdl.handle.net/10468/4375Downloaded on 2018-08-23T18:51:42ZMechanism for zirconium oxide atomic layer deposition usingbis(methylcyclopentadienyl)methoxymethyl zirconiumJ. W. Elam and M. J. PellinS. D. Elliott and A. ZydorM. C. Faia and J. T. HuppCitation: Appl. Phys. Lett. 91, 253123 (2007); doi: 10.1063/1.2824814View online: http://dx.doi.org/10.1063/1.2824814View Table of Contents: http://aip.scitation.org/toc/apl/91/25Published by the American Institute of PhysicsArticles you may be interested inSurface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water processJournal of Applied Physics 97, 121301 (2005); 10.1063/1.1940727Mechanism for zirconium oxide atomic layer deposition usingbis„methylcyclopentadienyl…methoxymethyl zirconiumJ. W. Elama and M. J. PellinArgonne National Laboratory, Argonne, Illinois 60439, USAS. D. Elliott and A. ZydorTyndall National Institute, Lee Maltings, Cork, IrelandM. C. Faia and J. T. HuppDepartment of Chemistry, Northwestern University, Evanston, Illinois 60208, USAReceived 19 October 2007; accepted 22 November 2007; published online 20 December 2007The mechanism for zirconium oxide atomic layer deposition usingbismethylcyclopentadienylmethoxymethyl zirconium and H2O was examined using ab initiocalculations of hydrolysis energies to predict the order of ligand loss. These predictions were testedusing in situ mass spectrometric measurements which revealed that the methyl ligand, and 65% ofthe methylcyclopentadienyl ligands are lost during the zirconium precursor adsorption. Theremaining 35% of the methylcyclopentadienyl ligands and the methoxy ligand are lost during thesubsequent H2O exposure. These measurements agree very well with the predictions, demonstratingthat thermodynamic calculations are a simple and accurate predictor for the reactivities of thesecompounds. © 2007 American Institute of Physics. DOI: 10.1063/1.2824814Atomic layer deposition ALD is a thin film growthmethod using alternating, self-limiting reactions betweengaseous precursors and a solid surface to deposit materials inan atomic layer-by-layer fashion.1 Zirconium oxide ZrO2 isa promising high-dielectric constant replacement for SiO2 infuture microelectronic devices,2 and also has applications inphotovoltaics3 and catalysis.4 ALD is an attractive methodfor preparing ZrO2 thin films because it affords precise thick-ness control and superb conformality.5 Understanding theALD mechanism is important because the mechanism affectsthe growth rate and purity of the films. Additionally, amechanistic understanding can guide proper precursor selec-tion. In this study, ab initio calculations are performed topredict the order in which the ligands are lost during ZrO2ALD. These predictions are tested using in situ quadrupolemass spectrometry QMS.We focus on the heteroleptic precursor, bismethyl-cyclopentadienylmethoxymethyl zirconiumZrMeCp2MeOMe, abbreviated as ZrL4, with ligandsL=MeCp, Me, and OMe. Heteroleptic precursors facilitatemechanistic studies and allow precursor fine tuning. The Zrand Hf versions of this precursor are thermally stable to500 °C Ref. 6 and produce high quality dielectric films.7Using H2O as oxygen source, the expected ALD reaction isZrL4gas + 2H2Ogas→ ZrO2solid + 4HLgas. 1Equation 1 provides no information about the surface-mediated mechanism of growth or about the order of ligandrelease. This information is relevant because steric hindrancebetween the ligands remaining after the ZrL4 pulse will dic-tate the ALD growth rate.8At the start of the ZrL4 pulse, the growing surface iscovered with hydroxyls surf-OH that provide protons H+.The adsorption of ZrL4 produces ligands on the surfaceL−=MeCp−, Me−, and OMe−, which can combine with pro-tons and desorb as HL. The kinetics of this elimination reac-tion will be determined by the relative bond strengths9 ofZr–L versus H–L at the surface, and by surface propertiessuch as the O–H strength and H+ diffusion rate. To comparedifferent ligands, it is adequate to compute the differentZr–L versus H–L bond enthalpies and to ignore effects thatare specific to surface geometry.10We define the gas-phase Brønsted basicity BB of L−relative to OH− asEBB = E, for L− + H2O→ HL + OH−. 2The more negative the EBB, the stronger the BB of L− andthe stronger the H–L bond. We compute the change in inter-nal energy neglecting entropy/temperature effects. We like-wise define the Lewis basicity LB of L− relative to OH− asELB =14E, for 4L− + ZrOH4→ ZrL4 + 4OH−. 3Stronger Lewis bases with strong Zr–L bonding show morenegative ELB. Combining these equations, Ehyd=EBB−ELB, whereEhyd =14E, for ZrL4 + 4H2O→ ZrOH4 + 4HL .4Negative Ehyd corresponds to an exoergic hydrolysis reac-tion at T=0 K. Ehyd thus reflects the relative strengths ofZr4+ and H+ bonding to L−, using H2O and ZrOH4 as com-mon reference molecules. Equation 4 is thus a model reac-tion for HL elimination whenever surf-OH and surf-L arepresent. The resemblance of Eq. 4 to the overall growthreaction with H2O as precursor Eq. 1 is coincidental,since Eq. 1 contains no useful mechanistic information.The species in Eq. 4 were modeled as isolated mol-ecules in vacuum. The ground state electronic wavefunctionof each molecule was calculated self-consistently withinKohn-Sham density functional theory DFT using TURBO-MOLE Ref. 11 with the B-P86 functional,12 an atom-centered SVP basis set,18 and a 28-electron effective coreaElectronic mail: jelam@anl.gov.APPLIED PHYSICS LETTERS 91, 253123 20070003-6951/2007/9125/253123/3/$23.00 © 2007 American Institute of Physics91, 253123-1potential on Zr.13 All species were closed shell. Uncon-strained optimization of the molecular geometry was carriedout on the DFT potential energy hypersurface, but a vibra-tional analysis was not carried out. This method has beenapplied previously to heteroleptic Zr precursors.14The calculated energetics are shown in Table I. The com-puted BB values decrease as MeCpOMeMe. The com-puted LB values are similar for Me and OMe but larger forMeCp. Applying Eq. 4, Ehyd increases as OMeMeCpMe. We therefore predict that Me ligands will be elimi-nated first during the ZrL4 pulse, followed by MeCp ligandsif there are sufficient surf-OH. The OMe ligands along withsome MeCp should be eliminated during the H2O pulse.To test these predictions, ZrO2 ALD was monitored byQMS Ref. 15 Stanford Research Systems RGA300 in aviscous flow reactor16 at 350 °C using alternating exposuresto ZrMeCp2MeOMe Epichem for 3 s and de-ionizedH2O for 1 s with 5 s purge periods between exposures. TheZrMeCp2MeOMe was vaporized at 95 °C. We verifiedthat these conditions yield self-limiting ZrO2 ALD using el-lipsometric analysis of films deposited on silicon. The QMSsignals arise from reactions occuring on the hot walls of thereactor, and no substrate is installed during these measure-ments.The top three solid traces in Fig. 1 present the m /z=79,m /z=16, and m /z=31 QMS signals recorded during ZrO2ALD. The dotted lines at the bottom of the figure show thedosing times for the ZrMeCp2MeOMe and H2O precur-sors with a high value designating an exposure to theindicated precursor. The middle portion of the graphbetween 30 and 92 s shows 4 12 ALD cycles in which theZrMeCp2MeOMe and H2O precursors are pulsedsequentially. Between 0 and 30 s, only theZrMeCp2MeOMe precursor is pulsed to measure thebackground for this compound. Similarly, only the H2O ispulsed between 92 and 120 s to evaluate the H2O back-ground. Note that during the background measurements, botha 1 s exposure followed by a 5 s exposure are used to main-tain the same timing sequence as in the ALD cycles.During the ZrO2 ALD cycles, m /z=31 peaks are onlyobserved during the H2O exposures and the correspondingbackground is small 15% , indicating that the methoxyligand –OMe is released exclusively during the H2O reac-tion. Similar results were obtained monitoring m /z=32 inagreement with the cracking pattern for methanol17 producedby the reaction of methoxy ligands with the hydroxylatedsurface.The m /z=16 trace in Fig. 1 shows the amount of CH4released during the ZrO2 ALD along with the correspondingbackground measurements performed as described above.Peaks in the m /z=16 signal are observed when dosing boththe ZrMeCp2MeOMe and H2O precursors. However,while the ZrMeCp2MeOMe background is negligible atm /z=16, the H2O background and ALD signals are identicalwithin the experimental error. Consequently, CH4 is only re-leased during the ZrMeCp2MeOMe exposures of theZrO2 ALD.The m /z=79 signals attributed to methylcyclopentadieneMeCpH formed during the ZrO2 ALD in Fig. 1 reveal thatMeCpH is released during both of the precursor exposures.Similar results were obtained using m /z=80, and 77consistent with the cracking pattern for HCpMe.17 Integra-tion of the m /z=79 peaks shows that, while theZrMeCp2MeOMe background is negligible, 44% of thesignal observed during the H2O exposures is background.After background correction, we conclude that 65±10%of the CpMe ligands are eliminated during theZrMeCp2MeOMe exposures, and the remaining35±14% are released during the subsequent H2Oexposures.FIG. 2. Illustration of proposed ZrO2 ALD mechanism.TABLE I. Computed energies kJ/mol for ligands in model ALD reactions.The Brønsted basicity EBB is from Eq. 2, the Lewis basicity ELB fromEq. 3, and the hydrolysis energy Ehyd=EBB−ELB from Eq. 4. Theligand with the most negative Ehyd is predicted to be eliminated first.Ligand Elimination product EBB ELB EhydMe CH4 −46.9 +176.6 −223.5MeCp MeCpH +274.7 +375.1 −100.4OMe MeOH +160.9 +152.5 +8.4FIG. 1. QMS signals for m /z=31 MeOH, m /z=16 CH4, and m /z=79MeCpH measured during ZrO2 ALD. The ZrMeCp2MeOMe andH2O doses are indicated by the dotted lines at the bottom. TheZrMeCp2MeOMe and H2O background signals are measured beforeand after the 4 12 , consecutive ZrO2 ALD cycles as indicated.253123-2 Elam et al. Appl. Phys. Lett. 91, 253123 2007The calculations and measurements suggest themechanism for ZrO2 ALD in Fig. 2. In step A,ZrMeCp2MeOMe reacts with the hydroxylated surfacereleasing the Me ligand as CH4 and one or more of theMeCp ligands as MeCpH. This modified surface is exposedto H2O in step B, liberating any remaining MeCp ligandsas MeCpH, and all of the OMe ligands as MeOH. TheQMS measurements indicate that, on average, 1.3 MeCpligands are removed in step A, so that 30% of theZrMeCp2MeOMe molecules react with three hydroxylsand release both MeCp ligands in step A.Following the ZrMeCp2MeOMe adsorption, thesurface is covered with MeCp and OMe in the ratio 2:3.Consequently, the steric bulk of these ligands will limit theALD growth rate. However, because the Me ligand is elimi-nated before saturation, replacing the Me with a bulkier alkylgroup should not affect the growth rate.The QMS measurements follow the ligand release pat-tern suggested by the Ehyd calculations in Table I. Thisagreement supports our assertion that a simple comparison ofbond strengths captures the essential information for predict-ing the ALD mechanism. Furthermore, we have identifiedthe important precursor properties: strong affinity of Me forH+ of surf-OH, weak bonding of MeCp to Zr, and similarbonding of OMe to Zr and H.The submitted manuscript has been created by the Uni-versity of Chicago—Argonne, LLC as Operator of ArgonneNational Laboratory “Argonne” under Contract No. DE-AC02-06CH11357 with the U.S. Department of Energy. Thework at Northwestern is supported by the U.S. Departmentof Energy, Basic Energy Sciences Program under Grant No.DE-FG02-87ER13808. The work at Tyndall was supportedby the European Commission under the sixth Frameworkproject REALISE NMP4-CT-2006-016172, http://www.tyndall.ie/realise.1M. Ritala and M. Leskelä, in Handbook of Thin Film Materials, edited byH. S. Nalwa Academic, San Diego, 2001, Vol. 1, p. 103.2G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys. 89, 52432001.3D. B. Menzies, Q. Dai, L. Bourgeois, R. A. Caruso, Y. B. Cheng, G. P.Simon, and L. Spiccia, Nanotechnology 18, 125608 2007.4A. Wootsch, C. Descorme, S. Rousselet, D. Duprez, and C. Ternplier,Appl. Surf. Sci. 253, 1310 2006; C. P. Nicholas and T. J. Marks,Langmuir 20, 9456 2004.5K. Kukli, M. Ritala, J. Aarik, T. Uustare, and M. Leskelä, J. Appl. Phys.92, 1833 2002; M. Putkonen, J. Niinistö, K. Kukli, T. Sajavaara, M.Karppinen, H. Yamauchi, and L. Niinistö, Chem. Vap. Deposition 9, 2072003; K. Kukli, J. Niinistö, A. Tamm, J. Lu, M. Ritala, M. Leskelä, M.Putkonen, L. Niinistö, F. Song, P. Williams, and P. N. Heys, Microelec-tron. Eng. 84, 2010 2007.6S. Rushworth, K. Coward, H. Davies, P. Heys, T. Leese, L. Kempster, R.Odedra, and P. Williams, Surf. Coat. Technol. 201, 9060 2007.7J. Niinistö, M. Putkonen, L. Niinistö, F. Q. Song, P. Williams, P. N. Heys,and R. Odedra, Chem. Mater. 19, 3319 2007.8R. L. Puurunen, Chem. Vap. Deposition 9, 249 2003.9Although reaction rates are governed by activation energies rather thanbond enthalpies, the relative activation energies are likely to follow thesame trends as bond enthalpies in this set of similar reactions.10S. D. Elliott, Surf. Coat. Technol. 201, 9076 2007.11R. Ahlrichs, M. Bär, M. Häser, H. Horn, and C. Kölmel, Chem. Phys. Lett.162, 165 1989.12A. D. Becke, Phys. Rev. A 38, 3098 1988; J. P. Perdew, Phys. Rev. B33, 8822 1986.13D. Andrae, U. Haussermann, M. Dolg, H. Stoll, and H. Preuss, Theor.Chim. Acta 77, 123 1990.14A. Zydor, S. D. Elliott, T. Leese, F. Song, and S. Rushworth, ECS Trans.11, 113 2007.15J. Niinistö, A. Rahtu, M. Putkonen, M. Ritala, M. Leskelä, and L. Niinistö,Langmuir 21, 7321 2005; A. Rahtu and M. Ritala, J. Mater. Chem. 12,1484 2002.16J. W. Elam, M. D. Groner, and S. M. George, Rev. Sci. Instrum. 73, 29812002.17Eight Peak Index of Mass Spectra Mass Spectometry Data Centre, TheRoyal Society of Chemistry, Cambridge, UK, 1991, Vol. 1, p. 1.18A. Schäfer, H. Horn, and R. Ahlrichs, J. Chem. Phys. 97, 2571 1992; K.Eichkorn, F. Weigend, O. Treutler, and R. Ahlrichs, Theor. Chem. Acc.97, 119 1997.253123-3 Elam et al. Appl. Phys. Lett. 91, 253123 2007

Mechanism for zirconium oxide atomic layer deposition using bis(methylcyclopentadienyl)methoxymethyl zirconium

Abstract

Similar works

Full text

Available Versions

Cork Open Research Archive

Crossref