Plasma-Enhanced Atomic Layer Deposition of Silver Using Ag(fod

Aug 8, 2017 - A plasma-enhanced atomic layer deposition (ALD) process using the Ag(fod)(PEt3) precursor [(triethylphosphine)(6,6,7,7,8,8,8-heptafluoro...
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Plasma-Enhanced Atomic Layer Deposition of Silver Using Ag(fod)(PEt3) and NH3‑Plasma Matthias M. Minjauw,† Eduardo Solano,† Sreeprasanth Pulinthanathu Sree,‡ Ramesh Asapu,§ Michiel Van Daele,† Ranjith K. Ramachandran,† Gino Heremans,‡ Sammy W. Verbruggen,§ Silvia Lenaerts,§ Johan A. Martens,‡ Christophe Detavernier,† and Jolien Dendooven*,† †

Conformal Coating of Nanostructures (CoCooN), Department of Solid State Sciences, Ghent University, Krijgslaan 281 (S1), 9000 Ghent, Belgium ‡ Centre for Surface Chemistry and Catalysis (COK), Department of Microbial and Molecular Systems (M2S), Katholieke Universiteit Leuven, Celestijnenlaan 200f, Box 2461, 3001 Leuven, Belgium § Sustainable Energy, Air and Water Technology (DuEL), Department of Bioscience Engineering, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium ABSTRACT: A plasma-enhanced atomic layer deposition (ALD) process using the Ag(fod)(PEt 3 ) precursor [(triethylphosphine)(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5octanedionate)silver(I)] in combination with NH3-plasma is reported. The steady growth rate of the reported process (0.24 ± 0.03 nm/cycle) was found to be 6 times larger than that of the previously reported Ag ALD process based on the same precursor in combination with H2-plasma (0.04 ± 0.02 nm/ cycle). The ALD characteristics of the H2-plasma and NH3plasma processes were verified. The deposited Ag films were polycrystalline face-centered cubic Ag for both processes. The film morphology was investigated by ex situ scanning electron microscopy and grazing-incidence small-angle X-ray scattering, and it was found that films grown with the NH3-plasma process exhibit a much higher particle areal density and smaller particle sizes on oxide substrates compared to those deposited using the H2-plasma process. This control over morphology of the deposited Ag is important for applications in catalysis and plasmonics. While films grown with the H2-plasma process had oxygen impurities (∼9 atom %) in the bulk, the main impurity for the NH3-plasma process was nitrogen (∼7 atom %). In situ Fourier transform infrared spectroscopy experiments suggest that these nitrogen impurities are derived from NHx surface groups generated during the NH3-plasma, which interact with the precursor molecules during the precursor pulse. We propose that the reaction of these surface groups with the precursor leads to additional deposition of Ag atoms during the precursor pulse compared to the H2-plasma process, which explains the enhanced growth rate of the NH3-plasma process.

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INTRODUCTION

control over the amount of material that is being deposited.10−14 Therefore, ALD of Ag is ideally suited for the fabrication of Ag nanostructures for applications in plasmonics.15−21 Due to the enhanced catalytic and antibacterial properties of Ag at the nanoscale, Ag ALD is also an interesting technique for applications in catalysis22−24 and to fabricate antibacterial coatings.25 Unlike other noble metals such as Ru or Pt,14 only a small number of processes for ALD of Ag have been reported in the literature.26−32 The main reason for this is the difficulty of finding a suitable silver precursor that is at the same time sufficiently volatile and thermally stable.33 The first report on Ag ALD was a plasma-enhanced process using Ag(Piv)(PEt3)

Silver is best known for its use in jewelry, art, and coinage, but it is also being used on a wide scale in industry and technology. Silver has the lowest electrical resistivity, the highest thermal conductivity, and the highest reflectivity in the visible spectrum of all known metals.1 Therefore, it is being widely used in the electrical and electronics industries,2 for brazing and soldering,2 and as a reflecting layer for low thermal emissivity coatings on windows.3 Silver also finds its application because of its catalytic4 and antibacterial5 properties. There is currently a high demand for the controlled synthesis of silver nanostructures due to the excellent plasmonic properties of silver.6−9 The field enhancement8 and the propagation of plasmons6,7 are extremely sensitive to the nanometer-scale feature size and shape of metal nanostructures. Atomic layer deposition (ALD) is a thin film deposition technique which offers unparalleled conformality and atomic © 2017 American Chemical Society

Received: February 17, 2017 Revised: August 1, 2017 Published: August 8, 2017 7114

DOI: 10.1021/acs.chemmater.7b00690 Chem. Mater. 2017, 29, 7114−7121

Article

Chemistry of Materials

10−1 mbar and then opening the gate valve again. The pressure and pulse time for both the H2-plasma (20% H2 in Ar) and NH3-plasma (99.995% pure) were 5.10−3 mbar and 15 s, respectively. This pressure was attained by flowing the gas through a needle valve and keeping the gate valve to the turbo pump opened. The power of both plasmas was 100 W. SiO2 substrates were used to determine the purity, crystallographic structure, and morphology of the ALD Ag films. To avoid nucleation issues, Au substrates were used to determine the growth characteristics of the Ag ALD processes. To avoid degradation of the Ag films in air, the samples were stored in high vacuum. While the Ag layers deposited on Au were much smoother than on oxides, indicating that nucleation effects were less of an issue, they were still too rough for X-ray reflectivity (XRR) measurements of the Ag film thickness. Therefore, we determined an equivalent Ag film thickness using ex situ X-ray fluorescence (XRF). This was done by first constructing a calibration curve of the integrated Ag L peak intensity versus the XRR thickness of sputtered Ag films, and afterward using this calibration curve to extract an equivalent thickness for the ALD Ag films on the basis of XRF data. The XRR measurements were performed using a Bruker D8 diffractometer with a Cu Kα source. For the XRF measurements, a Bruker ARTAX μ-XRF spectrometer was used, and a flow of He allowed us to remove the Ar K-line (fluorescence from air) which overlaps with the Ag L-line. The crystallographic structure of the films was determined by ex situ X-ray diffraction (XRD) using a Bruker D8 diffractometer (Cu Kα source). The composition and purity of the films were determined by ex situ X-ray photoelectron spectroscopy (XPS) using a Theta Probe (Thermo Scientific) instrument. In situ Fourier transform infrared (FTIR) spectroscopy experiments were performed on a different ALD setup. This setup is described in detail elsewhere,39 and uses a Vertex V70 (Bruker) mid-IR spectrometer in reflection geometry. In these experiments, an FTIR spectrum was acquired just before each precursor and coreagent pulse during steady Ag ALD growth on SiO2 substrates. Because the ALD chamber is pumped down to base pressure before the actual measurement, we measure the surface groups on the sample without the interference of any gas-phase reactants or products. A difference spectrum is generated for each halfcycle by subtracting the IR absorption spectrum acquired before the half-cycle from the one acquired after the half-cycle. The morphology of the films was determined by using both ex situ scanning electron microscopy (SEM) and ex situ grazing-incidence small-angle X-ray scattering (GISAXS). SEM measurements were performed using a Quanta 200F (FEI) instrument. The GISAXS experiments were performed at the DUBBLE BM26B beamline of the European Synchrotron Radiation Facility (ESRF) synchrotron,40,41 using a beam energy of 12 keV and a PILATUS 1 M (DECTRIS) detector at a distance of 4.2 m from the sample. An incidence angle of 0.5° ± 0.04° was selected, which is higher than the critical angles of both Si (0.149°) and Ag (0.294°) at 12 keV.

in combination with H2-plasma, by Niskanen et al.26 This process was shown to be conformal and has a high growth rate of 0.12 nm/cycle. However, the impurity content was high and no uniformity could be achieved on large-area substrates.32 Chalker et al. reported a thermal ALD process using (hfac)Ag(1,5-COD) in combination with 1-propanol.27,28 Golrokhi et al. used the same precursor with tert-butylhydrazine as a coreagent to extend the temperature window of the process.29 Due to the low vapor pressure and insufficient thermal stability of the (hfac)Ag(1,5-COD) precursor, a liquid injection system is needed however. Masango et al. reported two thermal ALD processes using the (hfac)Ag(PMe3) precursor.30 One is a two-step process in combination with formalin, and the other is a three-step process in combination with trimethylaluminum and water. Although the (hfac)Ag(PMe3) precursor has sufficient thermal stability and volatility, both processes show very low growth rates of below 0.01 nm/ cycle due to surface poisoning with hfac ligands. Kariniemi et al. reported a PEALD process using Ag(fod)(PEt3) in combination with H2-plasma.31 The Ag(fod)(PEt3) precursor was selected for its sufficient volatility and thermal stability.33 The process has a growth rate of 0.04 nm/cycle, and the deposited films have a low impurity content.31 Although the growth rate is lower compared to that of the process using the Ag(Piv)(PEt3) precursor by Niskanen et al., the process using Ag(fod)(PEt3) displays better uniformity across large-area substrates, which is important for applications.33 The same authors also reported the conformality of the process.34 As a result of these promising features, the process by Kariniemi et al. was adopted by several research groups in recent years.16−21,35−37 Very recently, the same group reported a thermal ALD process using Ag(fod)(PEt3) in combination with dimethylamine borane (BH3(NHMe2)) as a reducing agent.32 In this work, we report a new silver ALD process using the Ag(fod)(PEt3) precursor in combination with NH3-plasma. This NH3-plasma-based process offers a 6-fold increase of the growth rate compared to the process using H2-plasma.31 In addition, the use of NH3-plasma results in easier nucleation of the Ag on oxide substrates, and overall smaller particle size after deposition of a given amount of Ag onto an oxide substrate. This difference in morphology is important for applications in catalysis22−24 and plasmonics.6−9

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EXPERIMENTAL SECTION

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The reactor used for the Ag ALD depositions is an experimental ALD reactor equipped with a turbomolecular pump and a remote inductively coupled plasma source.38 The vacuum chamber is coupled to the turbo pump through a gate valve, and has a base pressure of 10−6 mbar. The Ag(fod)(PEt3) precursor [(triethylphosphine)(6,6,7,7,8,8,8heptafluoro-2,2-dimethyl-3,5-octanedionate)silver(I), minimum 98% purity, Strem Chemicals] was heated in a stainless steel bottle, and was delivered to the vacuum chamber through stainless steel tubing and a pneumatically controlled precursor inlet valve. To ensure that the precursor vapor reaches the sample, the precursor inlet was installed ∼30 mm above the sample surface. The silver precursor bottle, tubing, inlet, and reactor walls were kept at controlled temperatures of 95, 97, 100, and 100 °C, respectively. The sample was heated to a temperature of 130 °C by clamping it to a resistively heated Cu block at the same temperature. The precursor was pulsed into the vacuum chamber using Ar carrier gas (99.9999% pure) and after the gate valve to the turbo was closed. This allowed the pressure in the vacuum chamber to be built up toward 5 mbar in 10 s. After 15 s at this fixed pressure, the chamber was pumped down by first using a rotary vane pump to pump below

RESULTS AND DISCUSSION The Ag ALD process using the Ag(fod)(PEt3) precursor in combination with H2-plasma (H2*) reported by Kariniemi et al.31 was the starting point of this study. The linearity of the H2* process at 130 °C is shown in Figure 1. By fitting a linear curve to this set of data points using the least-squares method, one can extract an equivalent growth per cycle (GPCeq) of 0.04 ± 0.02 nm/cycle, in agreement with the growth rate reported by Kariniemi et al.31 The error on GPCeq was determined as 3 times the standard error on the slope of the least-squares fit. In this work, we explored the growth of silver by combining the Ag(fod)(PEt3) precursor with other coreagents. The H2plasma was replaced by N2-plasma, NH3-plasma, and NH3-gas, and growth could be achieved with NH3-plasma (NH3*) only. The NH3* process shows a high growth rate of 0.24 ± 0.03 nm/cycle, which is 6 times larger than the growth rate found for the process using H2-plasma (Figure 1). This growth rate 7115

DOI: 10.1021/acs.chemmater.7b00690 Chem. Mater. 2017, 29, 7114−7121

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Chemistry of Materials

with the NH3* pulse time. The decrease of GPCeq with increasing H2* pulse time might be due to etching of Ag atoms by Ar+ ion bombardment on the film surface during the H2plasma (20% H2 in Ar), as the ion density in the substrate region is not completely zero for a remote plasma configuration.12 Although we would expect that the contribution of the Hn+ ions is negligible, Choi et al. reported surprisingly high Ag etch rates in pure H2-plasma, which they explained by a combined chemical/photon-excitation etching component.43 Therefore, etching by Hn+ ions might also be partially responsible for the observed decrease in GPC with increasing H2-plasma pulse time. The deposited films were polycrystalline fcc Ag for both ALD processes, as can be seen in Figure 3. The Ag(111) and

Figure 1. Equivalent Ag film thickness grown on sputtered Au at 130 °C as a function of the number of cycles for the NH3* and H2* processes. In both cases, two consecutive pulses of Ag precursor were used in each ALD cycle. The dotted lines are the linear least-squares fits to the data points, excluding the origin. The slopes of these lines are reported in the text as the GPCeq of the H2* and NH3* processes.

corresponds to approximately one monolayer of Ag atoms deposited in every cycle, as the spacing between the closepacked face-centered cubic (fcc) Ag(111) planes is 0.236 nm.42 The saturation curves for the NH3* and H2* processes at 130 °C are shown in Figure 2. Here, the GPCeq of each data

Figure 3. XRD diffractograms of Ag films grown on SiO2 after 100 cycles of the NH3* process (NH3*) and 400 cycles of the H2* process (H2*). The films have comparable equivalent thicknesses. The expected peak positions for fcc Ag are marked by the vertical bars, and the fwhm of the Ag(111) peaks is indicated using the dotted lines.

Ag(200) peaks of the film grown with the H2* process are sharper and more intense than those for the NH3* process, while the films have similar equivalent thicknesses (i.e., they contain similar amounts of Ag per surface area, as determined by XRF). The difference in width of the diffraction peaks indicates that the crystallite size is smaller for the NH3* process compared to the H2* process. The film morphology on SiO2 was investigated by SEM. Figure 4 shows the micrographs of two selected sets of samples. The first set (a, c) shows the film morphology during the initial cycles in both processes. The samples contain similar amounts of Ag, as confirmed by XRF (equivalent thickness ∼5 nm). The same holds for the second set (b, d), which shows the morphology in a later stage of each process (equivalent thickness ∼40 nm). For the H2* process, one observes initial nuclei (Figure 4c) that grow and coalesce into large Ag nanostructures that are separated from each other by narrow gaps (Figure 4d), in agreement with other reports for this process. For the NH3* process, the density of initial nuclei appears to be enhanced (Figure 4a), and continued growth results in an increase of the size of these particles without significant particle merging (Figure 4b). The smaller grain size for the NH3* process observed here is in agreement with the fwhm of the XRD peaks, as discussed above. To study the initial nucleation in more detail, ex situ GISAXS measurements were performed on the films shown in Figure 4. The acquired GISAXS patterns for samples a and c from Figure 4 are shown in parts a and c, respectively, of Figure 5. While clear scattering peaks can be observed at high qz values in the

Figure 2. Saturation curves at 130 °C, where the equivalent growth per cycle (GPCeq, here defined as the increase of equivalent Ag film thickness per ALD cycle) is plotted as a function of the number of silver precursor pulses and as a function of the plasma pulse time for the NH3* process (a, b) and the H2* process (c, d). To avoid nucleation issues, Ag ALD was carried out on sputtered Au films.

point was extracted by dividing the equivalent Ag thickness by the number of ALD exposure cycles on a gold substrate. To increase the precursor exposure during the first half-cycle, the precursor was pulsed a number of times rather than increasing the pulse time. This was done because of the static nature of the precursor pulsing and the low vapor pressure of the precursor. The time between two subsequent pulses was 120 s to ensure sufficient Ag precursor vapor buildup in the precursor bottle. It can be seen from Figure 2a,c that, for both processes, saturation of the growth rate with precursor exposure is reasonably achieved. For the H2* process, no saturation with coreagent exposure is found as the GPCeq decreases with increasing H2* pulse time, while, for the NH3* process, GPCeq does saturate 7116

DOI: 10.1021/acs.chemmater.7b00690 Chem. Mater. 2017, 29, 7114−7121

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Chemistry of Materials

= 0.6 nm−1 can be observed in the horizontal line profiles (taken at the Si Yoneda qz position, qz/qz,Si = 1)44 for the H2* and NH3* samples, respectively (Figure 5b). Because the position of the maximum is inversely related to the center-tocenter particle distance D,44 this indicates a ca. 3 times larger interparticle spacing for the H2* sample. This further implies a ca. 9 times higher particle areal density (estimated by 1/D2) for the NH3* sample, indicating an improved nucleation of this process on oxide substrates. This result is in line with the previous observations in SEM and XRD: the NH3* process results in higher densities of smaller Ag nanoparticles. In addition to lateral information, the scattering along qz can provide information about the average particle height. The vertical line profiles in Figure 5d show a first maximum at the Si Yoneda position, confirming that the films are discontinuous. For the H2* sample, two minima are observed, while only one minimum can be distinguished for the NH3* sample. This result confirms that the particle height is lower for the NH3* sample. Comparison of the GISAXS pattern of the H2* sample with simulations yields an average particle height of 14.4 nm. XPS was used to determine the atomic composition and the impurity content of the grown films, both at the surface and after sputter removal of the topmost layers (labeled as “surface” and “bulk” in Table 1, respectively). While the main bulk impurity for the H2* process is oxygen, for the NH3* process this is nitrogen. Also, for the NH3* process some carbon and phosphorus impurities were present in the bulk of the films, while they were below the detection limit for the H2* process. Note that the oxygen impurity content found for the H2* process is higher than that found by Kariniemi et al.31 This might be due to the different plasma configuration, in our case leading to less hydrogen radicals that reach the film surface to remove the oxygen. When considering the surface impurities after deposition for both processes (Table 1), it is clear that the relative concentration of impurities is higher at the surface than in the bulk. The carbon and oxygen impurities observed at the surface are probably due to the air exposure while the samples are being transferred between the ALD reactor and the XPS setup. However, the high concentration of nitrogen at the surface observed for the NH3* process cannot be easily explained by this air exposure and is probably derived from the process itself. To investigate this further, in situ FTIR experiments were performed for the NH3* process and the H2* process. The difference spectra are shown in Figure 6. It is clear that absorbance peaks mainly appear in the spectra after the precursor pulses, and have a negative counterpart in the spectra after the coreagent exposure. This means that most surface groups are generated during the precursor pulse, and they are removed during the subsequent coreagent pulse. Most of these peaks are present for both processes, which suggests that they are derived from similar adsorbed precursor ligands or

Figure 4. SEM micrographs of selected Ag samples on SiO2: (a) 20 cycles of the NH3* process, (b) 100 cycles of the NH3* process, (c) 75 cycles of the H2* process, (d) 400 cycles of the H2* process. The film in (a) contains an amount of Ag per unit area similar to that of the film in (c), and the same holds for (b) and (d). Note that the scale bar for (a) is smaller.

Figure 5. GISAXS pattern for Ag films grown on SiO2 with 20 cycles of the NH3* process (a) and 75 cycles of the H2* process (c). Horizontal and vertical cuts, indicated by the black dotted lines, were taken for both patterns, and are plotted in (b) and (d), respectively.

pattern of the H2* sample, most of the scattered intensity is concentrated at low qy and qz values for the NH3* sample. Although not very pronounced, maxima at qy = 0.2 nm−1 and qy

Table 1. Atomic Concentrations of Impurities Relative to Silver in Films Grown by the NH3* Process and the H2* Process, as Found by XPS NH3* H2*

surface bulk surface bulk

[C] (atom %)

[N] (atom %)

[O] (atom %)

[F] (atom %)

[P] (atom %)

21 4 11