Tailoring the Properties of Atomic Layer Deposited Nickel and Nickel

Article pubs.acs.org/JPCC

Tailoring the Properties of Atomic Layer Deposited Nickel and Nickel Carbide Thin Films via Chain-Length Control of the Alcohol Reducing Agents Mouhamadou Sarr,† Naoufal Bahlawane,† Didier Arl,† Manuel Dossot,‡ Edward McRae,§ and Damien Lenoble*,† †

Centre de Recherche Public Gabriel Lippmann, 41, rue du Brill L-4422 Belvaux, Luxembourg Laboratory of Physical Chemistry and Microbiology for the Environment, UMR CNRS-Université de Lorraine 7564, 405 rue de Vandoeuvre 54601 Villers-lès-Nancy, France § Institut Jean Lamour, UMR 7198 CNRS-Université de Lorraine, FST, BP 70239, 54506 Vandoeuvre-lès-Nancy, France ‡

S Supporting Information *

ABSTRACT: Atomic layer deposition (ALD) of nickel and nickel carbide is reported starting from nickel acetylacetonate and a primary alcohol. The sequential reactions of both reactants with the adsorbed species are shown to be self-limited. Use of propanol or ethanol as reducing agents yields the formation of the technologically relevant carbon-Ni3C thin films, whereas the carbon content with use of methanol is less than 5 atom %. These metallic nickel thin films are electrically conductive and feature a soft ferromagnetic behavior. The thermally stable cubic lattice of nickel was grown at 300 °C with methanol as a reducing agent while the metastable hexagonal structure was obtained at lower temperatures. The morphology and the structure of the films were investigated with use of scanning electron microscopy and X-ray diffraction. The films are nanocrystalline featuring an average crystallite size of ∼10 nm. Hydrogen-free ALD of nickel is particularly appealing for the deposition of (i) conformal coatings on hydrogen-sensitive substrates such as highly reducible oxides and metals with high capacity to form hydrides and (ii) 3D nanomaterials with high aspect ratio.

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INTRODUCTION In integrated electronic devices, electromigration and signal delay cause significant limitations. Copper exhibits a low electrical resistivity and a high electromigration resistance, which justify its widespread use as an interconnect metal.1 Nevertheless, reducing the size of interconnects is accompanied by a considerable increase of the resistivity.2 Several alternatives have been explored to address this limitation, including the use of nickel as an adhesion promoting layer via alloying with copper.3 Furthermore, Ni silicide thin films are used to address contact issues in nanoscale devices.4,5 Although several vaporand liquid-based techniques have been developed for the deposition of nickel, the atomic layer deposition (ALD) mode remains particularly advantageous for applications, such as 3D advanced CMOS devices, requiring conformal coatings.6 The growth of metallic thin films by CVD and ALD relies on the use of inorganic, metal−organic, or organo−metallic precursors. Regarding the metal−organic family, nickel films have principally been prepared with use of amidinate, cyclopentadienyl, and β-diketonate precursors.7−9 Maruyama et al.10 have obtained polycrystalline Ni films by CVD, using the H2-reduction of Ni(acac)2 at 250 °C. Besides the conventional H2-reduction route, Utriainen et al.11 have proposed a two-step process in which the CVD of NiO is © 2014 American Chemical Society

followed by a reduction step under hydrogen. The reduction of metal−organic precursors with atomic hydrogen is an efficient approach for the ALD of metals although it presents a limit in terms of step coverage on high aspect ratio structured surfaces.12 Ammonia possesses good reducing properties for the gas phase deposition of metals but the obtained films usually exhibit high nitrogen contamination.13 The advantages and disadvantages of other strong and mild reducing agents have been discussed in a recent review.14 There is still a strong demand for alternative approaches for the ALD of metals for the elaboration of 3D devices with complex shapes. Furthermore, the deposition of metals at the atomic scale without using hydrogen is of interest to widen its application scope toward structured surfaces with high aspect ratio and to improve manufacturing safety. Here, we propose, for the first time to our knowledge, the ALD of nickel via an alcohol reduction of nickel acetylacetonate. The reaction mechanism between these two reactants has been reported by Premkumar et al.15 under pure CVD conditions only. Mass spectrometric and infrared investigations Received: July 9, 2014 Revised: September 8, 2014 Published: September 10, 2014 23385

dx.doi.org/10.1021/jp5068318 | J. Phys. Chem. C 2014, 118, 23385−23392

The Journal of Physical Chemistry C

Article

Renishaw Raman spectrometer with five excitation wavelengths (325, 442, 532, 633, and 785 nm). A “Sartorius Cubis microbalance” with an accuracy of around 1 μg was used. The weight of the substrate was initially determined (mini), and assumed to remain unchanged throughout the deposition process, and then the total weight after deposition (mfin) for more than 10 measurements have been taken and the average value is used. A simple difference measurement was used to determine the growth rate of the film assuming that the film uniformly covers the entire surface of the substrate. The other possibility is the use of a Quartz Crystal Microbalance (QCM), but QCM is known to be very sensitive to the variation of the temperature. Since the decomposition of primary alcohols is exothermic, the temperature at the surface varies when pulsing the alcohol, which makes the measurement hardly exploitable and reliable. Vibrating Sample Magnetometer (VSM) measurements were performed on two of the films.

showed the oxidative dehydrogenation of alcohol molecules upon reaction with the chemisorbed Ni(acac)2.16 The use of alcohols as reducing agents in ALD was first demonstrated by Chalker et al.:17 propanol was used to reduce (hexafluoroacetyl acetonateto)silver(I) (1,5-cyclooctadiene) for the deposition of silver nanoparticles at temperatures below 150 °C. The capacity of an alcohol to undergo a dehydrogenation reaction depends strongly on its structure and on the reaction temperature.18 The dehydrogenation of alcohols can be catalyzed by transition metal surfaces,19−21 or by adsorbed transition metal complexes.22,23 Such a chemical route will be exploited here for the ALD of nickel starting from nickel acetylacetonate. The chemisorption of Ni(acac)2 on silica surfaces is made possible by the ability of the partially ionized −OH surface groups to accept the axial ligand of the Ni(II).24 This step leads to a nickel cation coordinated by one acetylacetonate ligand at the surface and the release of one acetylacetone molecule. The formation of acetylacetone is due to the strong interaction of the proton of the surface −OH with the quasi-π electron system of the acetylacetonate ligand.25 The chemisorbed Ni(acac) moiety is expected to undergo a further reduction upon interaction with alcohols. For primary alcohols the reaction takes place as follows, where R = H or CnH2n+1 (with n = 1, 2, ...):16,26

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RESULTS AND DISCUSSIONS Figure 1 shows the ALD saturation curves at 250 °C with use of the three primary alcohols. The deposition was performed by

Ni(acac)2 + R−CH 2−OH → Ni + 2Hacac + R−CHO

The objective of this study is to show that nickel metal thin films can be grown in ALD mode with use of readily available and safe reactants. The feasibility of this reaction route is investigated by using methanol, ethanol, and propanol as reducing agents. The microstructure, chemical composition, and morphology of the obtained films are studied as a function of the alcohol and the deposition temperature. The electrical resistivity was measured for the films obtained with methanol while the magnetic properties were comparatively measured for the films made by using methanol and ethanol.

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EXPERIMENTAL SECTION The ALD of nickel was performed on a silicon oxide (SiO2) layer thermally grown on (100) silicon at 1100 °C. The SiO2 thickness was measured by ellipsometry to be 50 nm, a thickness sufficient to exclude the formation of silicides.27 The substrates were ultrasonically cleaned in acetone and in ethanol prior to deposition. The coatings were performed by using the as-received nickel acetylacetonate and alcohols (Sigma-Aldrich) in a Beneq TFS200 setup. The sublimation hot source was used at 180− 200 °C for nickel acetylacetonate, while the alcohols were evaporated at room temperature. The purge of the reactor was ensured with a continuous nitrogen flow of 300 sccm. One ALD cycle comprised alternate pulses of Ni(acac)2 (pulse time tNi) and the alcohol (pulse time tAlcohol), separated by a reactor purge time (tP). The purge time was fixed at 2 s, while the pulse times of Ni(acac)2 and alcohol were varied from 0.1 s to 4 s. The growth temperature was investigated from 250 to 300 °C. The morphology of the films was inspected with use of a “Helios Nanolab 650” scanning electron microscope (SEM). The composition and the crystalline phase identification were assessed by X-ray Photoelectron Spectroscopy (XPS, Kratos Axis Ultra DLD) and X-ray Diffraction (XRD, Brüker D8Discover). Standard database JCPDS cards were used for XRD peak identifications. The Raman spectra were done by using a

Figure 1. Growth rate per cycle as a function of the used alcohol at 250 °C with tNi = tP = 2 s.

using Ni(acac)2 pulses of 2 s and 300 sccm of N2 for the purge. Irrespective of the alcohol used, the profiles show saturation after 2 s of exposure. The saturation behavior provides evidence that the reaction of the alcohol with the chemisorbed precursor is self-limited. Varying the temperature from 250 to 300 °C has no significant effect on the growth rate (Figure S1, Supporting Information). Figure 1 shows that the growth rate significantly increases with the length of the alcohol chain. Interestingly Premkumar et al. noted a similar enhancement of the growth rate with the length of the alcohols’ alkyl chain in the case of cobalt-CVD starting from Co(acac)2.15 The trend observed in Figure 1 is tentatively attributed to the incomplete dissociation of the alcohol leading to an incorporation of carbon and the consequential increase in the film mass. This will be correlated below to the composition of the films. In fact, the three films comprise both nickel and carbon, but with highly contrasting atomic ratios. No oxygen was detected despite its presence in both Ni(acac)2 and the alcohols used as reactants. The reduction of adsorbed nickel acetylacetonate by alcohols occurs via hydrogen atom being withdrawn from the alcohol 23386



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Figure 3. XPS core level spectra of Ni 2p and C 1s of films grown by using various alcohols as reducing agents at 300 °C at 400 s of etching time.

Figure 2. XPS depth profiles of films grown by using (a) C3H7OH, (b) C2H5OH, and (c) CH3OH as reducing agents at 300 °C.

of minimal carbon incorporation and can lead to an unambiguous growth of a metal film. The core level spectra displayed in Figure 3 have been corrected by using the C 1s peak position. Carbon is mainly present at the surface for methanol reduction, while it is the dominant element in films obtained by using ethanol and propanol reduction. This difference explains the noisier peak obtained with methanol than for the others. Both types of carbons are supposed to present the same binding energy in the XPS analysis. The fine structure of the core level Ni 2p peak of all analyzed films presents the characteristics of reduced nickel. The binding energy of Ni 2p3/2 in the metallic state is reported at 852.8 eV, which differs significantly from that in oxide phases (854.6 eV for NiO; 855.7 eV for Ni2O3; and 856.45 eV for Ni(OH)2).31−34 It is worth mentioning that the XPS spectrum of Ni 2p3/2 of metallic nickel features a satellite at +6 eV that was attributed to a predominant surface plasmon loss and a contribution from interband transitions.31 This is perfectly in line with the spectrum acquired for Ni grown starting from Ni(acac)2 and methanol. The use of ethanol or propanol as reducing agent yields films in which the XP-Spectra feature a subtle difference. As can be seen in Figure 3, the satellite is shifted by 1 eV toward higher binding energies. A similar effect was observed for carbon−nickel composites grown by a sputtering process.35 The authors attributed this to the presence of carbon in the

group. The reactivity of primary alcohols has already been related to their chain length. It has been demonstrated22 that propanol has the ability to easily give up a proton compared to the other investigated alcohols. This explains the high growth rate per cycle obtained with propanol since carbon composition in the film made by ethanol and propanol has only a little difference. A high amount of carbon (70−80%) both at the surface and within the films was detected when ethanol and propanol were used as reducing agents (Figure 2a,b). This contrasts significantly to the composition of the films obtained by using methanol as a reducing agent. In that case, the films feature nickel purity above 95% and the carbon contamination is mainly present near the surface as displayed in Figure 2c. The overall carbon contamination behavior in the film is in line with the higher growth rate obtained with ethanol and propanol reactants relative to methanol (Figure 1). In agreement with previous studies, it is assumed that the alcohol reduction of adsorbed nickel acetylacetonate occurs via hydrogen atoms being withdrawn from the alcohol groups.28 It has been shown that the dehydrogenation of primary alcohols yields hydrogen, CO, CO2, aldehydes, and other fragments coming from their decomposition.29,30 The use of methanol, which has only one alkyl radical, is therefore of interest in terms 23387



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Figure 4. XRD profiles (a) of Ni films grown at 300 °C by using Ni(acac)2 as precursor and methanol, ethanol, or propanol as reducing agents with 2 s of tNi and 4 s of tAlcohols. The tP was 2 s and the deposition was pursued for 3000 cycles. The Raman spectra of these films (b) are collected by using an excitation wavelength of 442 nm. Changes of the Raman spectrum as a function of the excitation wavelength are illustrated for the film grown by using ethanol as reducing agent (c). The asterisk shows the vibration of atmospheric O2 molecules.

Figure 5. (a) XRD patterns and core-level XP-Spectra (b) of Ni films grown at different temperatures with use of Ni(acac)2 as precursor and methanol reducing agent with 2 s tNi and 4 s of methanol exposure times. tP was 2 s, and the deposition was done for 3000 cycles. The structural transition (c) upon heat treatment is monitored by XRD for films grown at 285 °C. The inset illustrates the coexistence of the hcp and fcc phases after treatment at 300 °C.

nickel crystalline phase, which modified the core-hole screening of the less-localized 4sp electrons. Based on the XPS study it

can be concluded that nickel is present as a carbide phase when the films are grown with use of ethanol or propanol as reducing 23388



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Figure 6. SE-micrographs of nickel film with use of Ni(acac)2 and methanol at 300 °C. The scale bar corresponds to 300 nm. The deposition was pursued for 3000 cycles with use of 2 s tNi and 4 s of tAlcohol, with 2 s of tP.

Figure 8. Growth rate per cycle as a function of the Ni(acac)2 exposure time at 300 °C with use of methanol as a reducing agent with 3 s of tAlcohol and 2 s of tP.

agents, while it is present in the metallic phase when methanol is used. The XRD patterns of the nickel films grown at 300 °C are shown in Figure 4a. Referring to PDF no. 06−0697, the observed diffraction peaks can be assigned to (110), (006), and (113) planes of the rhombohedral Ni3C, which confirms the conclusions drawn from the XPS analysis. This result agrees with the formation elsewhere observed of cobalt carbide as a result of the reaction of cobalt acetylacetonate with primary alcohols under CVD conditions.36 In the latter case, the CVD process yielded either metallic or carbide phases of cobalt depending on the type of alcohol used and on the deposition temperature. The use of methanol yields pure Ni films as shown by the XPS analysis. For these films, the XRD pattern, Figure 4a, can unquestionably be attributed to the cubic face centered lattice (PDF no. 04−0850) of metallic nickel. Raman analysis of the films obtained with the various alcohols was performed and the results are displayed in Figure 4b. Disregarding the peak attributed to the oxygen of air (1556 cm−1, peaks indicated by stars in Figure 4b), the films grown by

the reaction of Ni(acac)2 with ethanol or propanol exhibit two bands at 1370 (D band) and 1590 cm−1 (G band). In nanocrystalline graphite or amorphous carbon, these would correspond respectively to the zone-center E2g phonon mode and the disorder-induced mode.37 The characteristic energy dependence of the D-band is displayed in Figure 4c. The presence of these two bands thus provides evidence of the amorphous or very disordered nature of the excess carbon present in the films. Neither of these bands is observed in the films obtained from the reaction of Ni(acac)2 with methanol. This is in line with the XPS and XRD analyses that evidence the growth of an almost carbon-free metallic film in this case. Based on the XPS and XRD analyses, we confirm that the ALD starting from Ni(acac)2 and ethanol or propanol yields nickel carbide−carbon thin films and the Raman confirms that carbon is present under its amorphous nature, while the reaction of Ni(acac)2 with methanol yields metallic nickel coatings. The influence of the growth temperature on the chemistry and structure of the grown films is displayed in Figure 5, panels a and b, using methanol reduction. The

Figure 7. VSM hysteresis loop for Nickel films deposited with use of Ni(acac)2 as precursor and methanol (a) and ethanol (b) as reducing agents at 300 °C. 23389



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Figure 9. NanoSIMS images (a), particle diameter distribution (b) and SEM images (c) of Ni nanoparticles on aluminum oxide by ALD with use of Ni(acac)2 (tNi: 1 s) and methanol (tAlcohol: 1 s); tP: 2 s at 300 °C and 1000 cycles.

nickel sample: it is typical for soft magnetic materials, with a saturation magnetization and a coercivity of 196 (0,25 T) and 18 kA·m−1 (22 mT), respectively. The saturation magnetization is lower than that of bulk materials (484 kA·m−1) because of the dependence of the magnetic properties on the thickness and the microstructure of the films.38,39 It has been shown that increasing the thickness enhances the saturation magnetization.39 Figure 7b shows a weak magnetic signal with the presence of a high background noise level of the hysteresis loop compared to that of the nickel film. This is in line with the weaker ferromagnetic behavior of nickel carbide, as observed by Chen et al.40 Methanol as the reducing agent thus enables the ALD of metallic nickel films either in the stable, fcc, or the metastable polymorph (hcp) depending on the growth temperature. The effect of the surface exposure to Ni(acac)2 in this case was investigated at 300 °C while fixing the methanol exposure time at 3 s. As depicted in Figure 8, the growth rate increases with the precursor exposure time to reach a saturation plateau at 2 s. This behavior provides evidence about the self-limiting character of the reaction, which allows excluding the occurrence of the precursor thermolysis in a CVD-like growth. This is clearly advantageous for the growth of highly conformal films with precise and reproducible thicknesses. Here, the films are mostly composed of metallic nickel (carbon contamination less than 5%) and feature the fcc structure. Therefore, it is possible to convert the results of the gravimetric monitoring to growth rates in nanometers/cycle. Based on the data presented in Figure 8, it can be concluded that pulse times of nickel precursor during the ALD process should be set at 2 s or greater to be in the surface saturation conditions. The conformality of nickel-ALD, using methanol as a reducing agent, was demonstrated by the application of the coating on the surface of an anodized aluminum oxide membrane as shown in Figure 9. The porous Anodic Aluminum Oxide was commercially purchased at VWR Company under the name “Anodisc 13” with a product number of 514-0523. SEM micrographs show clearly the

combined XRD and XPS analyses provide evidence that the films grown at ≤285 °C contain metallic nickel in the hexagonal close-packed structure (hcp). The recorded core level XPS Ni 2p3/2-satellite at +6 eV characterizes the change of electron bending energy due to the surface plasmon of metallic nickel. This is observed also for films grown at 260 °C. Therefore, the observed XRD peaks are assigned to the hcp structure of nickel. Figure 5c shows the evolution of the film made at 285 °C upon heat treatment under nitrogen. Starting from 40 °C, the temperature was increased to 300 °C at a rate of 1 deg/s. For the diffractograms of Figure 5, a waiting time of 10 min was set for each temperature before recording the spectrum. While the hexagonal structure dominates at low temperature, this metastable phase undergoes a structural phase transformation at 300 °C. In this transition, the fcc structure is formed at a temperature that coincides with its formation with ALD as shown in Figure 5a. After heat treatment at 300 °C, both the fcc and hcp phases coexist with a clear dominance of the cubic phase as shown in the inset of Figure 5c. The morphology of the 60 nm thick nickel film, grown at 300 °C, is shown in Figure 6. The SE-Micrograph shows a continuous film formed by compact grains. The grain size is large compared to the crystallite size calculated by using the Scherrer formula from the fcc-Ni(111) obtained by XRD. The average in-plane crystallite size was estimated to be 10 nm, which suggests the agglomerated nature of the compact grains observed by SEM in Figure 6. A room temperature electrical resistivity of 27(±3) μΩ·cm was measured with a four-probe technique for films with a thickness ranging between 15 and 60 nm. This result is comparable with the electrical resistivity of CVD-grown films using the hydrogen reduction of Ni(acac)2.10 The magnetic properties were measured for films deposited with use of ethanol and methanol, which allows comparing the behavior of the nickel carbide and metallic nickel films, respectively. The VSM measurements were performed on both films with a thickness of around 60 nm. Figure 7a shows the hysteresis loop obtained at room temperature for the metallic 23390



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presence of nickel along the 200 μm depth of the membrane (see Figure S3, Supporting Information). The structured substrate features ∼180 nm diameter wide aligned-vertical channels and a wall thickness of ∼30 nm with an overall membrane thickness of 200 μm; these membranes correspond to a pore aspect ratio of ∼950. As illustrated on Figure 8, deposition at 300 °C yields the growth of homogeneously dispersed Ni nanoparticles on the inner walls of the channels throughout the membrane. The average diameter of the Ni nanoparticles is estimated to be ∼10 nm (Figure 9b). Figure 9a shows nanoSIMS analysis performed by a NanoSIMS-50 instrument (Cameca).41 These images clearly show the presence of nickel and aluminum oxide in the membrane even if the lateral resolution of the instrument with use of cesium as primary ion beam is around 50 nm. This result demonstrates the ability of the ALD technique to coat complex shapes with a metallic deposit by using methanol as a reducing agent. The resulting structure is particularly relevant for heterogeneous catalysis or catalyzed growth of hierarchical nanostructures.

CONCLUSIONS In this work, the ALD of nickel was performed at ≤300 °C with use of nickel acetylacetonate as precursor and alcohols as reducing agents. The three primary alcohols used, irrespective of their alkyl chain lengths, are shown to reduce nickel acetylacetonate, while films obtained by using ethanol or propanol are identified as carbon−Ni3C composites. The use of methanol yields an almost pure metallic nickel with high electrical conductivity and soft magnetic behavior compared to the nickel−carbide films known for their weak ferromagnetic behavior. Films grown at low temperatures,

Tailoring the Properties of Atomic Layer Deposited Nickel and Nickel

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