Article pubs.acs.org/cm
Substrate Selectivity of (tBu-Allyl)Co(CO)3 during Thermal Atomic Layer Deposition of Cobalt Jinhee Kwon,*,† Mark Saly,‡ Mathew D. Halls,§ Ravindra K. Kanjolia,‡ and Yves J. Chabal*,† †
Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States SAFC Hitech, Haverhill, Massachusetts 01832, United States § Materials Design Inc., San Diego, California 92127, United States ‡
ABSTRACT: Tertbutylallylcobalttricarbonyl (tBu-AllylCo(CO)3) is shown to have strong substrate selectivity during atomic layer deposition of metallic cobalt. The interaction of tBu-AllylCo(CO)3 with SiO2 surfaces, where hydroxyl groups would normally provide more active reaction sites for nucleation during typical ALD processes, is thermodynamically disfavored, resulting in no chemical reaction on the surface at a deposition temperature of 140 °C. On the other hand, the precursor reacts strongly with Hterminated Si surfaces (H/Si), depositing ∼1 ML of cobalt after the first pulse by forming Si−Co metallic bonds. This remarkable substrate selectivity of tBu-AllylCo(CO)3 is due to an ALD nucleation reaction process paralleling a catalytic hydrogenation, which requires a coreactant that acts as a hydrogen donor rather than a source of bare protons. The chemical specificity demonstrated in this work suggests a new paradigm for developing selective ALD precursors. Namely, selectivity can be achieved by tailoring the ligands in the coordination sphere to obtain structural analogues to reaction intermediates for catalytic transformations that exhibit the desired chemical discrimination. KEYWORDS: selective deposition of metal, atomic layer deposition, cobalt thin film
1. INTRODUCTION Cobalt is an important transition metal for giant magnetoresistance (GMR) applications, spintronics, and microectronics technology.1−5 CoSi2, for example, is considered as an alternative contact material to TiSi2 because of its wider silicidation window and superior thermal and chemical stability.6,7 CoSi2 is fabricated by Co film deposition onto silicon gate contacts followed by annealing. Deposition of cobalt is typically performed by physical vapor deposition (PVD) although some chemical vapor depositon (CVD) routes have been explored.8,9 PVD and CVD suffer from poor step coverage in deep contact holes, resulting in poor conformality. This intrinsic constraint raises the need to develop a process for suitable atomic layer deposition (ALD) of Co for nanoscale devices.10−14 ALD proceeds entirely through surface reactions, and the characteristics of deposited films can be mainly determined by the initial reactions of gas-phase precursors with the substrate. In general, hydrophilic OH-terminated substrates are more favorable for initiating ALD because of the higher reactivity of hydroxyl groups with the metal precursors than hydrophobic H-terminated surfaces.15−17 This study presents a completely opposite case, in which the cobalt complex (tertbutylallylcobalttricarbonyl (tBu-AllylCo(CO)3)) is substantially more reactive with H-terminated Si than OH-terminated SiO2. This unexpected selectivity for the ALD of Co can simplify patterning in contact applications and be utilized for nanostructure © 2012 American Chemical Society
fabrications, without the need for depositing blocking layers.13,14
2. EXPERIMENTS AND RESULTS Float-zone grown, double-side polished Si(111) substrates (lightly doped, ρ ∼ 10 Ω cm) with thin native oxide are cleaned by the standard RCA18 method to produce OH-terminated oxide surfaces. Atomically flat monohydride Si(111) surfaces (H/Si(111)) are obtained by etching in HF (∼20%, 30 s) followed by a 2-min dip in NH4F (∼49%).19 After thorough rinsing with deionized water and blow-drying with nitrogen (N2), the sample is loaded into the reactor with the base pressure of 10−4 Torr. The Si substrate is kept at 140 °C during ALD growth, that is, during alternative pulses of tBu-AllylCo(CO)3 and dimethylhydrazine separated by N2-purge. The tBu-AllylCo(CO)3 precursor is exposed by a 2-s pulse using ultrapure N2 carrier gas (flow rate = 10 sccm, P ∼ 20 mTorr) through an ampule kept at 35 °C. The dimethylhydrazine is introduced by a 1-s pulse, drawn directly from the reservoir at room temperature (9 Torr vapor pressure). In-situ infrared absorbance spectroscopy is carried out after every half ALD cycle, using a Thermo Nicolet 6700 interferometer that is coupled to the reactor via external optics. Single-pass transmission geometry is used with two incidence angles, (1) close to the Brewster angle and (2) normal incidence, to help distinguish polarization of infrared vibrational modes in absorbance spectra.20,21 Investigation of surface morphology, impurity Received: September 28, 2011 Revised: December 13, 2011 Published: February 14, 2012 1025
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attributed to the substrate Si phonon modes as shown in the inset of panel b. These large absorption variations are observed even though the sample temperature is well controlled during measurements and thus not due to variations in Si phonon absorption. Instead, this large negative absorption of the Si phonons (involving optical and acoustic transverse/longitudinal combination modes) is more likely a result of either (i) a variation of the electromagnetic field in the underlying Si, expected with the formation of a dielectrically denser overlayer expected for cobalt silicide metallic bonding22 or (ii) etching of silicon in the vicinity of the surface. Rutherford backscattering spectrometry (RBS) measurements indicate that the cobalt surface density is 7.9 × 1014 Co atoms/cm2 on H/Si(111) after the first tBu-AllylCo(CO)3 exposure. This density is very close to that of Si atom surface density of the ideal Si(111) face (7.8 × 1014) and, as such, is approximately a complete monolayer. On the other hand, RBS measurements show that the Co surface density on SiO2 is negligible. This observation is strong support for the interpretation of the infrared absorbance spectra in Figure 1, confirming the strong selectivity of the Co precursor toward H- versus OH-terminated Si surfaces. Figure 2 shows that the Co growth mode on H/Si(111) during the initial few ALD cycles is different from that of later ALD cycles. After the first t Bu-AllylCo(CO) 3 pulse, terminal carbonyl groups, perpendicular to the surface at 2020 cm−1 (CO⊥) and parallel to the surface at 1900−2000 cm−1 (CO||) remain on the surface, together with organic ligands (CHx, 2800−3000 cm−1) and a monolayer of Co (Figure 2a, “1st Co”). The absorption at 1643 cm−1 corresponding to the CC stretching vibration is observed only after the first tBuAllylCo(CO)3 pulse. It is accompanied by the disappearance of the surface hydrogen band (Si−H at 2083 cm−1), indicating that the very initial surface ALD reaction proceeds by a mechanism facilitating transfer of the surface hydrogen to the allyl group, without immediate elimination of the tBu-Allyl ligand from the surface. The subsequent dimethylhydrazine (“1st DMHz”) adds its own methyl groups (N−CH3) by reacting with the carbonyl that is perpendicular to the surface (CO⊥) at 2020 cm−1. This indicates that the concentration of C and N impurities at the very interface between the growing Co film and Si substrate is higher due to this lack of ligand exchange. Up to 3 ALD cycles, the surface-bound CO⊥ provides weak reaction sites both to tBu-AllylCo(CO)3 and dimethylhydrazine, suggesting slow reaction during the nucleation period. After this nucleation period, the ligand exchange, typical for ALD process, is observed for later ALD cycles. Figure 2b shows that the carbonyl CO bonded to metallic Co0 at 2070 and to Co1+ at 2170 cm−1 and organic ligands (2800−3000 cm−1) after the Co precursor are removed by the subsequent dimethylhydrazine pulse. The ligand exchange shown in the differential IR absorbance spectra in Figure 2b suggests that the bulk of growing Co has lower concentration of C and N impurities. XPS measurements on a 20-ALD cycle Co film were then carried out using sputtering to investigate the impurity depth profile, as shown in Figure 3. In Figure 3, the sputtering time is increased from the top to the bottom spectra, and the top spectra are measured after the outermost layer has been removed to eliminate the effects of air exposure suffered during transport to the XPS chamber. The spectra clearly show the presence of metallic Co0, characterized by a binding energy of 778.3 eV (for Co 2p3/2) with a spin−orbit splitting of 14.9 eV. The C and N concentrations decrease with sputtering time, but the O concentration is quite persistent all the way to the Co/Si interface. After normalization of the area of each element with respect to Co (Figure 4), the concentrations of C, N, and O impurities are observed to be highest at the very interface of Co/Si and to decrease in the bulk of the Co film. The oxygen concentration increases slightly near the Co surface due to oxidation of the thin Co film after exposure to air. This finding is consistent with IR absorbance in Figure 2 showing a lack of ligand exchange during the first few ALD cycles, hence resulting in the accumulation of C and N at the Si/Co interface. Figure 2a shows that the carbonyl species with absorption parallel to the surface at 1900−2000 cm−1 observed after the first tBu-AllylCo(CO)3 exposure is not removed during the subsequent ALD cycles, thus
concentration, and oxidation states of cobalt is carried out by using atomic force microscopy (AFM) and X-ray photoemission spectroscopy (XPS) with a monochromatized Al Kα line (1486.5 eV) of PHI 5600, respectively. Co surface density is measured with Rutherford backscattering spectroscopy (RBS) with 2 MeV He+ ions. The detector is placed at 160° backscattering angle with respect to the surface normal, and Co bulk density of 8.9 g/cm3 is assumed for Co thickness estimation. The resistivity of Co films is measured with a Jandel multiheight four-point probe in combination with a Guardian surface resistivity meter Model #SRM-232. Figure 1 shows infrared absorbance spectra of (a) hydroxylated SiO2 and (b) H/Si(111) surfaces maintained at 140 °C after a 2 s exposure
Figure 1. Infrared absorbance spectra of hydroxylated SiO2 (a) and H/Si(111) (b) after the first tBu-AllylCo(CO)3 (structure shown as the inset of panel a) exposure at 140 °C referenced to each initial surface. The inset of panel b is the zoom-in of Si bulk phonons manifested in the data (black) in the circled region, and typical Si phonon modes (red) are presented for comparison. to tBu-AllylCo(CO)3 referenced to the respective initial surfaces. The most striking feature is the absence of reactivity of the precursor, t Bu-AllylCo(CO)3, with the oxide surface and the strong reactivity with the H-terminated silicon surface. For the hydroxylated SiO2, which would usually provide the most favorable reaction sites for initial nucleation during ALD, the spectrum is featureless except for the small variation at 1000−1100 cm−1. Importantly, there is no characteristic signature of −OH loss at 3740 cm−1 necessarily observed during chemical reaction with hydroxyl, and no ligand-related vibrational modes (i.e., below detection limit). (Figure 1a). On the other hand, the usually less reactive H-terminated Si surface strongly reacts with tBu-AllylCo(CO)3, as evidenced by the complete loss of the Si−H stretching mode at 2083 cm−1 (Figure 1(b)). The spectrum in Figure 1b also features a strongly tilted baseline, characteristic of broadband absorption associated with scattering, that is, an increasing absorption with increasing frequencies. The scattering in the absorption spectrum suggests that metallic Co−Si bonds with higher index of refraction are formed. In addition, absorption features below 1500 cm−1 (the region highlighted with a dashed ellipse) are 1026
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Figure 3. X-ray photoemission spectra of Co 2p (a), O 1s (b), N 1s (c), and C 1s (d) of the Co film after 20 ALD cycles. The top spectra of each element are measured after the outermost layer is removed by sputtering to minimize the effect of air exposure for ex situ measurement, and the sputtering time increases toward the lower spectra in each panel.
Figure 4. Normalized concentrations of C, N, and O with respect to Co as a function of sputtering time. precursor. The silicon phonon modes manifested in the infrared absorbance spectrum in Figure 1b also indicate that modification of the Si surface is very likely. For comparison, ALD of Co performed with another cobalt carbonyl complex (μ2-η2-(tBu-acetylene) dicobalthexacarbonyl (CCTBA)) does not show any variation in the silicon phonon mode region. After 0.7 monolayer (ML) of metallic Co is deposited by the first CCTBA exposure of H/Si(111) at 140 °C, the infrared absorption is free from silicon phonon modes, although scattering is clearly observed through its characteristic high-frequency absorption.23 Increasing the Co surface density to 1.2 ML with CCTBA still does not lead to a variation of the Si phonon absorption, indicating that it is not simply a role of metallic monolayer that gives rise to variations in the bulk silicon phonon modes in IR spectra. It is most likely that a combination of metallic properties from Co−Si bonds and substrate etching by tBu-AllylCo(CO)3 leads to the observed variations in the silicon phonon mode absorption.22 After three ALD cycles, the film starts to show some granular structures on the surface as seen in Figure 5b, indicating island formation during the nucleation period. The RMA roughness increases with ALD
Figure 2. Differential infrared absorbance spectra with the Brewster incidence during initial 1−3 (a) and 8−10 (b) ALD cycles with t Bu-AllylCo(CO)3 (nth Co) and dimethylhydrazine (nth DMHz) on H/Si(111) referenced to each preceding treatment. The gray spectrum of the carbonyl stretching region after the first tBu-AllylCo(CO)3 (“1st Co”) in panel a is measured with normal incidence of infrared beam, that is, with sensitivity only vibrational modes parallel to the surface. contributing to the higher concentration of C and O at the Co/Si interface in Figure 4. The concentration of impurities in bulk Co is lower, as expected for the ALD process shown in Figure 2b. The AFM image obtained after 1-cycle Co deposition is shown in Figure 5a, showing a typical triangular patterns associated with the Si(111) crystallographic structure. Together with the very high Co surface density right after the first tBu-AllylCo(CO)3 pulse, the rich triangular patterns in the AFM image suggest that the initial surface reactions might accompany surface Si etching by the gas phase metal 1027
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surface reaction sites to enable efficient exploration of the possible surface reaction pathways.24 The DFT simulations were performed using the NWChem 5.0 suite of electronic structure programs25 within the generalized-gradient approximation, using the Perdew−Burke−Ernzerhof (PBE) functional,26 along with Ahlrich’s split-valence polarized double-ζ basis set (pVDZ).27 Upon interaction of the allylCo(CO)3 precursor with the H/Si surface, there are various reaction pathways that could lead to the formation of a surface Si−Co linkage. The relative energies of potential surface reaction products were calculated to identify the lowest energy ALD nucleation pathway. The thermodynamically favored surface reaction leads to the adsorption of the allylCo(CO)3 precursor without immediate ligand elimination. The overall chemistry results in the reduction of the allyl ligand by the surface hydrogen and the formation of a Si−Co bond: (CH2CRCH2)Co(CO)3 + H*−Si(111) → (CH2CRCH*H2)(CO)3 Co−Si(111)
Figure 5. 3 × 3 μm2 AFM images of Co films after 1 (a), 3 (b), 20 (c), and 30 (d) ALD cycles with rms roughness of 0.20 nm, 0.26 nm, 2.5 nm, and 0.7 nm, respectively.
where * denotes the surface hydrogen and RH or tbutyl. This surface product is formed in the very initial reaction, which may be followed by reactions leading to subsequent ligand reorganization and transfer. Cobalt−carbonyl complexes Co(CO)3 are known to be active homogeneous catalysts for the hydrogenation of unsaturated functional groups, in conjunction with a hydrogen donor.28 Hydrosilanes, which have the general formula, H−SiR3 exhibit reactivities that closely parallel that of molecular hydrogen in interacting with transition metal centers, such as cobalt, and are used as coreactants in processes such as hydrosilylation.29 It is well established that reactions involving hydrogen transfer catalyzed by Co(CO)3 complexes proceed through a cobalt hydrocarbonyl intermediate, HCo(CO)3, as shown in Scheme 1 for the Co-ALD nucleation process examined here.28,29
cycles, and it reaches 2.5 nm by the 20th ALD cycle (Figure 5c). However, the roughness eventually decreases beyond 20 cycles, as shown Figure 5d, featuring much smoother films around 30 ALD cycles with RMA value of 0.7 nm. RBS measurements reveal that the growth rate is approximately 0.5 Å/cycle (Figure 6) after a nucleation period (∼10 cycles). This
Scheme 1. Co-ALD Nucleation Process on H-Terminated Si
Figure 6. Growth rate of Co on H/Si(111) at 140 °C with tBuAllylCo(CO)3 and dimthylhydrazine. nucleation period, most evident during the first 5 cycles, is consistent with the differential infrared absorbance spectra in Figure 2 that show a lack of ligand exchange during the initial nucleation cycles. The resistivity of thicker Co after 800 cycles (∼40 nm thick) is lower than the detection limit of the instrument, and considering 6 μΩ·cm as the bulk Co resistivity, this value corresponds to a sheet resistance of 0.11 Ω/sq, which is at the lower end of the detection limit of our instrument, confirming the metallic property of Co grown by thermal ALD process.
In this process, the Si−H surface hydrogen is transferred to the allyl ligand through a reaction intermediate, characterized by a rearrangement of the allyl ligand from η 3 to η 1 coordination, formation of the Si−Co interface linkage and transfer of the surface hydrogen to the cobalt center. The hydrogen is then transferred to the allyl ligand producing the final surface product complex with the general formula R3Si− Co(allylH)(CO)3. In this scheme, the tBu-AllylCo(CO)3 precursor structure is analogous to a Co(CO)3 catalyst
3. DISCUSSION To gain insight into the atomistic details and reaction energetics underlying the substrate selectivity of tBu-AllylCo(CO)3 toward H/Si(111) versus OH/SiO2, calculations based on density functional theory (DFT) were carried out. The smaller allylCo(CO)3 precursor was adopted, along with truncated cluster models of the H/Si(111) and OH/SiO2 1028
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precursor tBu-AllylCo(CO)3 and the H/Si or OH/SiO2 surface requires activation of the catalytic Co metal center by transfer of a hydrogen. Silicon is less electronegative, and oxygen is more electronegative than hydrogen (Pauling, 1.92, 3.61 versus 2.30). Therefore, in chemical reactions, Si−H species serve as neutral hydrogen or as hydride donors, whereas hydroxyl O−H groups act as a source of bare protons. The difference in surface hydrogens is revealed in the atomic charges calculated using DFT, where the electrostatic potential (ESP) fit charge for hydrogen is −0.06 and +0.36 for H/Si and OH/SiO2, respectively. The chemical selectivity for H/Si over OH/SiO2 in this ALD nucleation process reflects the significant difference in the nature of hydrogen on H/Si and OH/SiO2 substrates. The above demonstration and analysis of substrate specific Co ALD should prompt new efforts to develop tailored precursors for the selective deposition of other technological materials. Through the appropriate selection and design of ligands, new ALD precursors can thus be constructed as mechanistic analogues mimicking chemical intermediates for chemical reactions that exhibit the desired chemical selectivity. In this work, substrate specific ALD of Co has been demonstrated using the cobalt carbonyl complex, tertbutylallylcobalttricarbonyl (tBu-AllylCo(CO)3). The precursor exhibits remarkable selectivity for H-terminated Si over OH-terminated SiO2. First-principles simulations have shown that this bias is due to the nucleation reaction being thermodynamically favored on the H/Si surface and hindered on the OH/SiO2 surface, reflecting the differences in the hydrogen donor capability of these substrates. These results suggest new avenues for the development of ALD precursors with tailored substrate specificity that will enable patterned ALD for nanostructure fabrication.
prebound to the allyl-ligand reactant, and the H/Si(111) substrate can be viewed as an extended hydrosilane, which acts as the hydrogen-donor in the reaction.30 The reaction energetics for the Co-ALD nucleation reaction (Scheme 1) was evaluated on both surfaces for comparison. The optimized reaction intermediates and product structures and corresponding enthalpies for the allylCo(CO)3 reaction on H/Si and OH/SiO2 surfaces are presented in Figure 7. As seen
Figure 7. Optimized reaction intermediates and product structures (Scheme 1) and relative energies calculated at the PBE/pVDZ level of theory for the ALD nucleation reaction using allylCo(CO)3 at the OH/SiO2 and H/Si(111) surfaces. Atoms are identified by element color as follows: H, white; O, red; Si, gold; C, gray; and Co, green.
there, the difference in relative energies for the Si and SiO2 surface reactions is dramatic and provides a foundation for the observed substrate selectivity of tBu-AllylCo(CO)3 for cobalt ALD. The reaction of allylCo(CO)3 at the H/Si surface is energetically favorable with the formation of the product complex being exothermic by 0.58 eV. In clear contrast, reaction at the hydoxylated-SiO2 surface is found to be thermodynamically hindered, being endothermic by 0.65 eV. The formation of the hydrocarbonyl intermediate and the reaction product complex is more favorable on the H/Si surface than the OH/SiO2 by 0.83 and 1.23 eV, respectively. The overall reaction enthalpy for the full tBu-AllylCo(CO)3 precursor is calculated to be −0.24 eV and +0.66 eV for the H/Si and OH/SiO2 substrate, respectively (0.9 eV more favored on H/Si). Another potential reaction pathway leading to the formation of the Si−Co(tBu-AllylH)(CO)3 product complex is a two-step radical mediated process involving (1) hydrogen abstraction by the gas-phase precursor (producing a Si· reaction site on the H/Si surface), followed by (2) adsorption of the hydrogenated precursor to form the surface product. Hydrogen abstraction from H/Si by the precursor is calculated to be energetically disfavored by 2.48 eV, which is considerably larger than the formation energy of the surface hydrocarbonyl reaction intermediate (0.91 eV). For, an additional point of comparison, the energy for surface Si−H homolytic bond dissociation is computed to be 3.46 eV. The higher energies required for radical mediated pathways indicates that they would not be competitive with the surface adsorbed reaction pathway presented here. In a reaction that is exactly analogous to a known catalytic hydrogenation using a cobalt carbonyl catalyst, the ALD nucleation reaction between the coordinatively saturated
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AUTHOR INFORMATION
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ACKNOWLEDGMENTS This work at the University of Texas at Dallas was supported by the National Science Foundation (Grant No. CHE-0911197). The authors are grateful to L. Wielunski for the RBS measurements carried out at Rutgers University.
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REFERENCES
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