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Low Temperature, Selective Atomic Layer Deposition of Nickel Metal Thin Films Marissa M. Kerrigan, Joseph P. Klesko, Kyle J Blakeney, and Charles H. Winter ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03074 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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ACS Applied Materials & Interfaces

Low Temperature, Selective Atomic Layer Deposition of Nickel Metal Thin Films Marissa M. Kerrigan,† Joseph P. Klesko,‡ Kyle J. Blakeney,† and Charles H. Winter*,† †

Department of Chemistry, Wayne State University, Detroit, Michigan 48202 USA

‡

Department of Materials Science and Engineering, University of Texas at Dallas, Richardson,

Texas 75080 USA KEYWORDS: nickel, precursors, atomic layer deposition, thin films, area selective deposition

ABSTRACT. We report the growth of nickel metal films by atomic layer deposition (ALD) employing bis(1,4-di-tert-butyl-1,3-diazadienyl)nickel and tert-butylamine as the precursors. A range of metal and insulating substrates was explored. An initial deposition study was carried out on platinum substrates. Deposition temperatures ranged from 160 to 220 °C. Saturation plots demonstrated self-limited growth for both precursors, with a growth rate of 0.60 Å/cycle. A plot of growth rate versus substrate temperature showed an ALD window from 180 to 195 °C. Crystalline nickel metal was observed by X-ray diffraction for a 60 nm thick film deposited at 180 °C. Films with thicknesses of 18 and 60 nm grown at 180 °C showed low rms roughnesses ( 97%, with low levels of carbon, nitrogen, and oxygen. Films deposited on ruthenium substrates displayed lower growth rates than those observed on platinum substrates. On copper substrates, discontinuous island growth was observed at ≤ 1000 cycles. Film growth was not observed on insulating substrates under any conditions. The new nickel metal ALD procedure gives inherently selective deposition on ruthenium and platinum from 160 to 220 °C.

INTRODUCTION Nickel metal films have applications as protective coatings against oxidation,1 precursors to NiSi contact materials in nanoscale devices,2-5 low electron mean free path, low resistivity conductors for future interconnects,6 magnetic materials,7 and catalysts.8-12 In microelectronics applications, nickel metal films are first grown on silicon substrates, then annealed to afford NiSi.2-5 While TiSi2 and CoSi2 have been used extensively as silicide contact materials, NiSi has advantages that include low resistivity and low silicon consumption upon formation.2-5 For catalytic applications, nickel nanoparticles are prepared by gas phase deposition on support surfaces such as SiO2, Al2O3, and carbon nanotubes that encourage agglomeration of the nickel atoms, as opposed to formation of thin, continuous films.8-12 Chemical vapor deposition (CVD) has been widely employed to deposit nickel metal films.13-16 Precursors have included nickelocenes, diketonates, and alkoxides as single-source precursors, as well as in combination with H2. Recently, the microelectronics industry has required the growth of films ranging in thickness from a few atomic layers up to 30 to 50 nm.17-19 These films must be applied to substrates with perfect conformal coverage and sub-nanometer thickness control. Atomic layer deposition (ALD) is uniquely able to provide such conformality and thickness uniformity due to the

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inherent self-limited growth.17-19 The ALD of nickel metal is challenging because nickel is an electropositive metal (Ni(II) ↔ Ni(0), E° = -0.257 V20), and because there are few co-reagents that can transform nickel ions to nickel metal at low temperatures. As a result, there has been considerable emphasis on the use of plasma-enhanced ALD, since reducing plasmas are easily generated.5,21-25 Several nickel precursors have been used, including those containing cyclopentadienyl, alkoxide, allyl, and alkyl pyrrolyliminate ligands. Plasma gases have included ammonia, hydrogen, and ammonia/hydrogen mixtures.5,21-25 While plasma-assisted ALD can afford high purity nickel metal films, energetic plasma species may damage the substrates and high aspect ratio features may be poorly coated because of reducing radical recombination on the feature walls prior to reaction with the nickel precursors.26 To avoid these issues, thermal ALD has been investigated. The precursor Ni(iPrNCMeNiPr)2 was used with H2 for the ALD of nickel metal films at 250 °C, however, a low growth rate was observed (0.09 Å/cycle).27 Moreover, Ni(iPrNCMeNiPr)2 has a thermal decomposition temperature of about 180 °C,28-30 which suggests that the growth rate at 250 °C is increased by CVD-like growth. Ni3C films were grown on silicon substrates from the proprietary precursor Bis-Ni and H2 at 220 °C.31,32 Rapid thermal annealing of the Ni3C films on silicon afforded low resistivity NiSi films. Nickel metal films were grown by thermal ALD using Ni(Me2NCH2C(Me)(Et)O)2 and ammonia at 300 °C.33 Nickel metal films were also grown by ALD using Ni(Me2NCH2C(Me)(Et)O)2 and H2 at 225 °C.34 The ALD nature of these processes employing Ni(Me2NCH2C(Me)(Et)O)2 is unclear, since closely related nickel(II) amino-alkoxide precursors undergo self-decomposition to nickel metal films at ≥ 250 °C.14 We reported nickel metal ALD with the precursors Ni((Me)(iPr)COCNtBu)2 and BH3(NHMe2).35 This process gave a growth rate of only 0.09 Å/cycle, deposition only occurred on ruthenium, and film growth ceased after 10 nm. Nickel metal ALD was reported using


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nickel(II) acetylacetonate and methanol between 250 and 300 °C.36 We recently reported nickel metal ALD using a three-step process with bis(1,4-di-tert-butyl-1,3-diazadienyl)nickel (1), formic acid, and the reducing agent 1,4-bis(trimethylsilyl)dihydropyrazine at 180 °C.37 A growth rate of 0.32 Å/cycle was observed, but this process was not investigated in detail. Another distinct approach to nickel metal films entails the ALD growth of nickel(II) oxide (NiO) films, followed by reduction of the NiO to nickel metal in a second step.7,8,10,38 For example, ALD NiO films were prepared at 165 °C with nickelocene and water, and were subsequently reduced to nickel metal with a hydrogen plasma.38

Area-selective ALD occurs when films grow preferentially on some surfaces over others.39 This approach can eliminate steps from traditional patterning technology, and thus has much promise in manufacturing. The only report of area-selective nickel ALD employed Ni(Me2NCH2C(Me)(Et)O)2 and ammonia at 300 °C on Si(100) substrates.33 Growth on the bare Si(100) substrates afforded nickel metal films. By contrast, no nickel metal films were observed on Si(100) substrates that contained octadecyltrimethoxysilane-derived self-assembled monolayers. Accordingly, the self-assembled monolayers blocked nickel metal growth. Additionally, nickel metal ALD was conducted on patterned substrates that contained alternating stripes of bare Si(100) and Si(100) with octadecyltrimethoxysilane-derived self-assembled monolayers.33 Growth of nickel metal on these substrates occurred only on the bare Si(100) regions.


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We recently described thermal cobalt metal ALD with the precursor bis(1,4-di-tert-butyl1,3-diazadienyl)cobalt (2) in combination with formic acid37,40 or alkylamines.41 These processes have high growth rates (0.95-0.98 Å/cycle), give self-limited growth at ≤ 200 °C, and afford low resistivity, high purity cobalt metal films. The process employing 2 and alkylamines is particularly desirable, because it avoids the use of corrosive and highly reactive formic acid. Since the precursor properties of 1 and 2 are very similar,42 we envisioned that an analogous approach employing 1 and alkylamines might afford nickel metal films. Herein, we report nickel metal ALD using 1 and tert-butylamine. High purity nickel metal films are obtained with a growth rate of 0.60 Å/cycle in an ALD window from 180 to 195 °C. Moreover, nickel metal grows selectively on metals and not on insulators.

RESULTS AND DISCUSSION Precursor Selection. Precursor 1 was synthesized by a previously reported procedure.42 Complex 1 crystallizes as a monomer with tetrahedral geometry at nickel.42 It sublimes at 115 °C at low pressure, shows a single step weight loss by thermogravimetric analysis, has a melting point of 185 °C, and decomposes to nickel metal at 230 °C.42 The precursor properties are therefore appropriate for use in ALD. tert-Butylamine, diethylamine, and triethylamine were evaluated as the co-reactants, because of their boiling points (46 to 89 °C) and corresponding easy delivery from bubblers. Ammonia was not tested as a co-reactant, because the reactors are not equipped to handle toxic gases. Film Growth with tert-Butylamine. Experiments to assess the ALD growth of nickel metal were performed on platinum substrates. First, self-limited growth at 180 °C was probed by


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varying the pulse length of one precursor at a time, while keeping all other conditions constant. In initial experiments, the pulse length of 1 was varied, with a constant pulse length for tertbutylamine of 0.2 s. We previously demonstrated that a tert-butylamine pulse length of 0.2 s is saturating in the growth of cobalt metal films from 2 and tert-butylamine.41 Purge lengths were 10.0 s. The growth rate reached a saturative value of 0.60 Å/cycle using ≥ 4.0 s pulses of 1 (Figure 1). Similar experiments were conducted in which the pulse lengths of tert-butylamine were varied, using a constant 5.0 s pulse of 1. A constant growth rate was obtained with ≥ 0.1 s pulse lengths of tert-butylamine (Figure S6). Saturative growth in both 1 and tert-butylamine is evidence for an ALD mechanism. Pulse lengths of 5.0 s for 1 and 0.2 s for tert-butylamine were used for all subsequent deposition experiments herein.

Figure 1. Dependence of growth rate on the pulse length of 1. 500 cycles were used with platinum substrates and a growth temperature of 180 °C. Next, experiments were carried out to explore variations in the growth rate with substrate temperatures. Platinum substrates were used with deposition temperatures of 160 to 220 °C. Precursor 1 decomposes at 230 °C,42 so depositions were not explored at > 220 °C. Figure 2


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shows the dependence of the growth rate on the deposition temperature. An ALD window17-19 was observed from 180 to 195 °C, where the growth rate was constant at 0.60 Å/cycle. Scanning electron microscopy (SEM) cross-sectional views showed continuous films with uniform thicknesses (Figures S7-S12). The higher growth rates at > 200 °C are probably because of increasingly rapid CVD-like decomposition of 1.

Figure 2. Dependence of growth rate on the deposition temperature for nickel metal ALD. Each experiment used 250 growth cycles on platinum substrates.

The nickel metal ALD process was further explored at 180 °C on platinum substrates by growing films with differing numbers of cycles. As shown in Figure 3, film thicknesses show a linear dependence on the number of cycles from 50 to 1000 cycles. The slope of the line is 0.60 Å/cycle, which matches the saturative growth rate outlined above. The x- (2.04) and y-intercepts (1.22) are close to zero, suggesting that there is negligible delay in nucleation even at 50 cycles, for which a film thickness of ~3 nm was measured. The nickel metal films were very smooth and uniform between 50 and 1000 cycles, as measured by SEM (Figures S13-15).


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Figure 3. Dependence of film thickness on number of cycles for ALD nickel metal films. The growth temperature was 180 °C and platinum substrates were used.

The grazing incidence X-ray diffraction pattern of a 60 nm thick, as-deposited nickel metal film (Figure 4) showed reflections consistent with cubic nickel metal (PDF 00-003-1051). Using the Scherrer equation, an average crystalline domain length of 13 nm was calculated. The small crystalline domain length leads to the broadened reflections observed in Figure 4.


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Figure 4. X-ray diffractogram of a nickel metal film. The film is 60 nm thick and was deposited at 180 °C on a platinum substrate. The green bars give the positions of cubic nickel metal reflections (PDF 00-003-1051). The red bar corresponds to the position of a reflection from the platinum substrate.

Nickel metal films deposited on platinum substrates at 180 °C with 300 (18 nm thick) and 1000 (60 nm thick) cycles were analyzed by atomic force microscopy (AFM). We previously reported that the root mean square (rms) roughness of the bare platinum substrate was 0.19 nm.37,40 The rms roughness of the 18 nm thick film was 0.45 nm (Figure 5a), which is 2.5% of the thickness. The rms roughness of the 60 nm thick film was 1.52 nm (Figure 5b), which is also 2.5% of the thickness. Accordingly, the 18 and 60 nm thick films produced at 180 °C have very smooth surfaces.


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(a)

(b)

Figure 5. AFM images showing 5 x 5 μm2 areas for (a) an 18 nm thick film and (b) a 60 nm thick film.


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Films with thicknesses of 18 and 60 nm were analyzed by X-ray photoelectron spectroscopy (XPS) to determine their purities and elemental compositions. These films were deposited on platinum surfaces at 180 °C (Figure S16-S26). The nickel 2p ionization intensities prior to sputtering were very weak due to the presence of adventitious contamination on the surface (Figure 6). Some surface oxidation may be present on the surface, since the films were exposed to ambient atmosphere prior to analysis. Ionizations that might arise from oxidized nickel were not assigned, due to their low intensities. After ≥ 0.5 minutes of 3 keV argon ion sputtering, ionizations consistent with elemental nickel (Ni 2p3/2 851.25 eV and Ni 2p1/2 869.99 eV) were observed. The other ionizations observed in Figure 6 at ≥ 0.5 minutes of sputtering are shake-up peaks characteristic of nickel metal and were not deconvoluted. The nickel 2p ionization region of the 18 nm thick film is shown in Figure 6. Data for the nickel 2p ionizations of the 60 nm thick film were similar. The nickel 2p ionizations exactly match the values for a standard nickel metal sample (Figure S16). XPS depth profiling results are shown in Figure 7. The 18 nm thick film was analyzed to assess whether intermixing of the nickel film and platinum substrate occurred. Significant amounts of platinum (> 10%) were observed upon sputtering for ≥ 0.5 min (Figure 7a), which suggests interfacial nickel-platinum alloy formation. Similar interfacial cobalt-platinum alloys were observed by XPS in cobalt metal films deposited on platinum using 2 and formic acid36,39 and 2 and tert-butylamine.41 These interfacial alloys may explain why cobalt and nickel metals nucleate so well on platinum substrates.37,40,41 After 3.0 minutes of argon ion sputtering, the 60 nm thick film consisted of 97.3% nickel, 0.4% platinum, 1.5% oxygen, and ≤ 0.5% carbon and nitrogen (Figure 7b). Platinum metal ionizations were not observed on the as-deposited surface of the 60 nm thick film, however, platinum (1.4%) was detected after 0.5 minutes of sputtering. The platinum concentration was ≤ 1.7% after up to 5.0


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minutes of sputtering and then increased as continued sputtering approached the underlying platinum layer. Accordingly, platinum metal is clearly present near the surface of the 60 nm thick film, at a concentration of ~1%. The surface of the as-deposited film is dominated by adventitious carbon and oxygen, which may obscure the presence of any trace platinum by reducing the intensity of the platinum signal to a level that is indistinguishable from the noise.

Ni2p

2p1/2 869.99

3.0 min 2.5 min 2.0 min 1.5 min 1.0 min 0.5 min 0.0 min

2p3/2 851.25

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210

Intensity (cps)

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900

890

880

870

860

850

840

Binding energy (eV)

Figure 6. Nickel 2p ionization region of an 18 nm thick nickel film. The substrate is platinum and the growth temperature was 180 °C.


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Atomic concentration (%)

(a)

100

Ni2p Pt4d O1s C1s N1s Si2p

90 80 70 60 50 40 30 20 10 0 0

1

2

3

Sputter time (min)

(b)

Atomic concentration (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

Ni2p Pt4d O1s C1s N1s Si2p

90 80 70 60 50 40 30 20 10 0 0

1

2

3

4

5

6

7

8

9

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Sputter time (min)

Figure 7. XPS depth profiles of (a) 18 nm and (b) 60 nm thick nickel metal films.


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Growth on Ruthenium and Copper Substrates. Nickel metal growth was explored on ruthenium and copper substrates at 180 °C for 50 to 1000 cycles. Analysis by cross-sectional SEM revealed continuous films on ruthenium substrates, except for 50 and 150 cycles, where no growth (< 2 nm) was observed (Figures S27-S30). Figure 8 shows the dependence of film thickness on the number of cycles for nickel films deposited on ruthenium and platinum. The data for growth on platinum are included for reference, and show linear behavior between 50 and 1000 cycles. On ruthenium, negligible growth was observed at 50 and 150 cycles by SEM (< 2 nm thick films). After 250 cycles, a 3.0 nm thick film was obtained, affording a growth rate of 0.12 Å/cycle in going from 150 to 250 cycles. While the growth rates were higher at 500 (0.18 Å/cycle) and 1000 (0.44 Å/cycle) cycles, these values still did not approach the saturative growth rate on platinum (Figures S27-S30). A delay in normal growth behavior was observed for the processes employing 2 and formic acid37,40 or tert-butylamine41 on ruthenium substrates. We previously proposed that this growth behavior on ruthenium substrates is due to the oxidized nature of the substrate surface,44 which causes lower growth rates until surface reactive sites can be generated.40 Cross-sectional and top-down SEM views of nickel metal films grown on copper substrates showed discontinuous island growth at ≤ 1000 cycles, and thus continuous films were not obtained (Figure S31). We previously reported that our copper substrates have oxidized surfaces, although there were identical cobalt metal film growth rates (0.98 Å/cycle) on platinum and copper.41 It is possible that the copper oxide surfaces on the copper substrates lead to poorer nucleation of the nickel metal, relative to platinum surfaces.


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Figure 8. Dependence of film thickness on the number of cycles for the process using 1 and tertbutylamine. Ruthenium and platinum substrates were used at 180 °C.

The 44 nm thick nickel metal film (180 °C, 1000 cycles) on ruthenium was investigated by XPS to determine its composition (Figures S32-S34). Top-down SEM images showed a smooth, continuous nickel film with a surface morphology comparable to that of the 60 nm thick nickel metal film deposited on platinum at the same temperature. Similar to the nickel metal films grown on platinum, nickel metal 2p ionizations were observed after argon ion sputtering for ≥ 0.5 minutes. Depth profiling was carried out to understand the purity of this film. After 2.5 minutes of argon ion sputtering, the film was composed of nickel (96.1 at%), ruthenium (1.0 at%), and carbon (2.3 at%), with oxygen and nitrogen levels at 97% nickel metal, with ≤ 1.5% each of carbon, oxygen, and nitrogen. Nickel metal film growth was explored on copper and ruthenium substrates at 180 °C. On ruthenium substrates, negligible deposition occurred after 50 and 150 cycles. Continuous nickel metal films were obtained after 250, 500, and 1000 cycles, although the growth rates at these temperatures were lower than the 0.60 Å/cycle obtained on platinum substrates. Island growth was observed on copper substrates and continuous nickel metal films were not obtained even after 1000 cycles. No nickel metal growth was present after up to 1000 cycles at 180 °C on Si(100), SiO2, Si-H, and CDO. Nickel metal deposition was not observed on these substrates in growth trials employing 250 cycles and substrate temperatures from 160 to 220 °C. Thus, nickel metal ALD occurs with inherent selectivity on metal surfaces under all conditions that were explored. Nickel metal is proposed to form upon interaction of physisorbed 1 with tert-butylamine to form adduct 3. Adduct 3 is unstable under the deposition conditions, and decomposes to nickel metal.


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EXPERIMENTAL SECTION Film growth experiments were carried out using a Picosun R-75BE ALD reactor. The carrier and purge gas was ultra-high purity nitrogen (99.999%, purchased from Airgas). The pressure of the deposition chamber was 6 to 9 Torr. Precursor 1 was synthesized by a previously reported procedure.42 All alkylamines (tert-butylamine, diethylamine, and triethylamine) were purchased commercially and were used without further purification. Precursor 1 was delivered in a Picosun solid state booster at 140 °C, while tert-butylamine was delivered with a bubbler at 20 °C. Film growth trials were conducted with substrate temperatures from 160 to 220 °C. The substrates and substrate cleaning procedures were previously reported.41 Film thickness determinations, SEM, EDS, AFM, X-ray diffraction experiments, XPS, and resistivity measurements were carried out as previously described.40,41

ASSOCIATED CONTENT Supporting Information. Film deposition and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS The authors acknowledge funding from the U.S. National Science Foundation (Grant No. CHE1607973) and EMD Performance Materials. U.S. National Science Foundation Grant Nos. CHE1427926 and DMR-0922912 are also acknowledged for the purchase of a powder X-ray diffractometer and a field emission SEM, respectively. Drs. Ravindra K. Kanjolia and Charles L. Dezelah of EMD Performance Materials are acknowledged for helpful discussions. We thank Professor Yves J. Chabal of the Laboratory for Surface and Nanoscale Modification at the University of Texas at Dallas for access to his XPS instrument. We also are also grateful to Professor Neil Dasgupta of the University of Michigan for a gift of the platinum stripe substrates.

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