Article pubs.acs.org/cm
Studies on Thermal Atomic Layer Deposition of Silver Thin Films Maarit Mak̈ ela,̈ *,† Timo Hatanpaä ,̈ † Kenichiro Mizohata,‡ Kristoffer Meinander,‡ Jaakko Niinistö,† Jyrki Raï san̈ en,‡ Mikko Ritala,† and Markku Leskela†̈ †
Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland Department of Physics, University of Helsinki, P.O. Box 43, FI-00014 Helsinki, Finland
‡
ABSTRACT: The growth of Ag thin films by thermal atomic layer deposition (ALD) was studied. A commercial Ag compound, Ag(fod) (PEt3), was applied with a reducing agent, dimethyl amineborane (BH3(NHMe2)). A growth rate of 0.3 Å/cycle was measured for Ag at a deposition temperature of 110 °C. The purity of the particulate, polycrystalline Ag thin films was studied with time-of-flight elastic recoil detection analysis (TOF-ERDA) and X-ray photoelectron spectroscopy (XPS). TOF-ERDA showed only small amounts of impurities in the film deposited at 110 °C, the main impurities being oxygen (1.6 at. %), hydrogen (0.8 at. %) and carbon (0.7 at. %). In addition to the conventional ALD process, the idea of activation of the amineborate inside the ALD reactor was tested. A catalytic Ru surface was utilized to convert BH3(NHMe2) into possibly even more reducing species inside the reactor without contaminating the catalysts with a growing film.
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INTRODUCTION Atomic layer deposition (ALD) process for Ag is desired for various applications. Photonics and catalysis are especially attractive applications for ALD, which is an excellent method to produce uniform and conformal thin films on complex structures. The common thin film deposition methods used to produce Ag thin films comprise physical and chemical vapor deposition techniques that result in thin films with excellent properties but are incapable of producing conformal coatings like ALD.1−5 The basis of ALD is in precursors. The most important requirements for an ALD precursor include sufficient volatility, no self-decomposition at the used temperatures, and sufficient reactivity toward surface groups formed by the other precursor to ensure growth also at low temperatures with complete reactions.6 With Ag precursors the major challenge has been in combining sufficient volatility and thermal stability. Limited thermal stability of the precursors has been seen in the previous ALD publications on Ag. Self-limiting growth is expected only at a very narrow temperature range and always at deposition temperatures below 200 °C.7−11 Considering thermal ALD of Ag the challenge is not only in finding a proper Ag precursor, but also in finding a reactive enough coreactant especially because the Ag precursors require the deposition temperature to be below 200 °C. In ALD the basic principle is that the reaction of a chemisorbed metalprecursor and a coreactant should be aggressive and thermodynamically favored, i.e., the reaction should have as negative Gibbs free energy change as possible.12 True reducing agents used in metal ALD have been, e.g., SiH4, Si2H6, B2H6, and Zn.13 However, there are limitations in safety and in deposition temperatures related to these reducing agents. In recent years very sophisticated reducing agents including 2methyl-1,4-bis(trimethylsilyl)-2,5-cyclohexadiene and 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine have been presented.14 © 2017 American Chemical Society
Another new reducing agent, dimethylamine borane (BH3(NHMe2)), was introduced for deposition of copper and transition metal thin films.15,16 BH3(NHMe2) is an interesting precursor for ALD because it sublimes at very low temperature (32 °C in this study) leaving room for usage of various metal precursors and deposition temperatures. Moreover it does not contain oxygen making it attractive not only for transition metal ALD but also for noble metal ALD including Ru for interconnects. Previous publications have shown, however, limitations in the growth when applying BH3(NHMe2).15,16 A catalytic Ru surface and a nucleation process was required to ensure sufficient growth of metals including Cu, Ni, Co, Fe, Mn, and Cr. Nucleation process was described to consist of 50 cycles of 20 s pulse of a metal precursor, 5 s purge, 1 s pulse of BH3(NHMe2) and 10 s purge. The nucleation process resembled normal ALD cycle but had a longer metal precursor pulse and purge times. In the film growth the metal precursor pulse was typically 4 s or longer, and the pulse length of BH3(NHMe2) was 1−2 s.16 No clear reason for the required nucleation step was given. When Co, Mn, Ni, and Cr were grown, the growth terminated when the Ru surface was covered with the transition metal. A hypothesis about formation of reductive surface groups from BH3(NHMe2) on the Ru surface was presented. Therefore, BH3(NHMe2) can be considered to be a “surface selective” reducing agent. Previous ALD publications on Ag are either plasmaenhanced ALD (PEALD) 7,8 or liquid injection ALD (LIALD)9,10 processes with one exception.11 In PEALD the reducing agent (like H2, NH3) is activated by plasma to form radicals which are very reactive also at low deposition Received: September 21, 2016 Revised: February 24, 2017 Published: February 24, 2017 2040
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PANalytical X́ Pert PRO MPD X-ray diffractometer. The measurements were performed in parallel beam geometry with copper Kα (λ = 1.5406 Å). Four point probe measurements (Keithley 2400 SourceMeter with a Cascade Microtech four-point probe) were done for electrical characterization. Composition of one particulate Ag thin film deposited at 110 °C was analyzed with time-of-flight elastic recoil detection analysis (TOFERDA).23 This measurement was performed with 35 MeV 79Br ions. In addition to TOF-ERD analysis, the composition of the Ag surface layers was analyzed by X-ray photoelectron spectroscopy (XPS), using an Argus Spectrometer (Omicron NanoTechnology GmbH, Taunusstein, Germany) operating at a pass energy of 20 eV. Three samples deposited at 110 °C were illuminated with X-rays emitted from a standard Mg source (Kα-line) at a photon energy of 1253.6 eV. Binding energies were calibrated using the C 1s peak (284.8 eV) of ambient hydrocarbons, and peak fitting was done using the CasaXPS software (www.casaxps.com).
temperatures. PEALD ensures growth of high quality thin films even at temperatures where thermal ALD processes are not possible.17 A major concern with metal PEALD processes is limited conformality of the films due to radical recombination on metal surfaces.18,19 In LIALD the metal precursor is dissolved into a solvent to ensure lower evaporation and deposition temperatures.20 In LIALD one potential concern is the treatment of solvent vapor in exhaust. An ALD Ag process with a thermally stable Ag precursor and a reducing agent would therefore be favored over the PEALD and LIALD processes. Masango et al.11 report two alternative thermal ALD processes for particulate Ag thin films. In the other one trimethylphosphine(hexafluoroacetylacetonato)silver(I) ((hfac)Ag(PMe3)) was applied with formalin. This process worked best at the deposition temperature of 200 °C. Very long precursor pulses and purge times were used (600−300−300− 300 s) because the particulate Ag films were deposited on the silica gel. The growth rate was relatively low (0.07 Å/cycle). The other process, a three step process, applied (hfac)Ag(PMe3), trimethylaluminum, and water. This process deposited particulate Ag thin films already at 110 °C, but again a very low Ag mass gain per ALD cycle was reported. In this study Ag was deposited by alternate pulsing of a metal precursor and a reducing agent. Ag depositions were performed by combining the metal precursor, triethylphosphine(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)silver(I) (Ag(fod) (PEt3)) (ref 7), with a reducing agent BH3(NHMe2). The process produced particulate Ag thin films. Evidences of self-limiting growth characteristics were obtained. However, no typical linear increase of film thickness as a function of applied cycles was confirmed. This was attributed to complex reducing chemistry of BH3(NHMe2). In fact it was speculated that BH3(NHMe2) reacts on Ag surface and forms even more reducing species than BH3(NHMe2) itself. The same effect was confirmed with Ru metal. In these experiments Ru pieces were inserted inside the ALD reactor. The metal pieces were only in contact with BH3(NHMe2) and were not contaminated by the growing film. A clear enhancement of Ag growth on Si and glass was confirmed.
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RESULTS AND DISCUSSION Film Deposition. Ag thin films were deposited at 104− 130 °C. In each experiment 1000 cycles were applied with 1 s Ag(fod) (PEt3) pulse, 1 s purge, 1 s BH3(NHMe2) pulse, and 1 s purge. The growth rate stabilized to 0.30 Å/cycle at temperatures 110−120 °C (Figure 1). At 130 °C the growth
Figure 1. Growth rate of Ag at deposition temperatures 104−130 °C. In each experiment 1000 cycles were applied with 1 s precursor pulses and 1 s purges.
rate was already higher and the film was slightly more nonuniform. At very low deposition temperatures like here there is a risk of applying insufficient purge times. This would lead to CVD like growth. To exclude this possibility, one Ag thin film was deposited with 5 s purges instead of 1 s purges. The growth rate was the same so it could be concluded that the purge time did not affect the growth. To confirm ALD like growth, the effect or precursor pulse length on the growth was studied with both precursors at 110 °C (Figure 2). When the pulse length of Ag(fod) (PEt3) was varied, the growth rate saturated to 0.3 Å/cycle. However, 4 s pulses resulted in slightly higher growth rate than the shorter pulses. When the pulse length of BH3(NHMe2) was varied, there seemed to be saturation of growth with 2 and 3 s pulses. However, with 4 s pulses the growth rate was lower than expected. Anyhow, the process can be concluded to be reasonably well saturative at 110 °C. All the thin films were uniform when deposited at 110 °C with 1 s Ag(fod) (PEt3) and BH3(NHMe2) pulses (Figure 3). Also thin films deposited with longer precursor pulses at 110 °C were uniform. The effect of number of cycles on film thickness was studied. Typically in an ALD process the film thickness increases linearly as a function of number of cycles, or in other words the
EXPERIMENTAL SECTION
Ag thin films were deposited in an ASM Microchemistry F120 ALD reactor which is a cross-flow cassette reactor. In the reaction chamber two substrates of 5 × 5 cm2 size were inserted.21 The substrates were Si(100) and glass. Purging between precursor pulses was done with nitrogen (AGA 99.999%). Pressure in the reaction chamber was about 5 mbar during the deposition. Ag(fod) (PEt3) and BH3(NHMe2) were loaded to glass boats and inserted to the reactor. Source temperatures were 95 °C for Ag(fod) (PEt3) and 32 °C for BH3(NHMe2). The precursor vapors were transported to the substrates with nitrogen flow and they were pulsed with inert gas valving.21 In some experiments Ru pieces with a total area of 3 × 5 cm2 cut from a sputtered Ru wafer were inserted to the hot end of the source tube of BH3(NHMe2). The same Ru pieces were used in all the experiments, and no degradation of the catalytic effect was noticed. One deposition was done with H2 (AGA 99.999%) as the reducing agent with a flow rate of about 15 sccm. To define the nominal film thickness of the particulate Ag samples on Si(100) thin films were analyzed with energy dispersive X-ray spectroscopy (EDS; Oxford INCA 350 Energy spectrometer) connected to a Hitachi S-4800 field emission scanning electron microscope (FESEM). Bulk density of Ag (10.49 g/cm3) was used to calculate the film thickness from the k ratios measured for Ag Lα-lines using a program GMRFilm.22 FESEM was used to study morphology of the films. Crystal structure of the Ag thin films was studied with 2041
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Figure 4. Effect of cycle number on Ag growth rate. Depositions were performed at 110 °C with 1 s Ag(fod) (PEt3) and BH3(NHMe2) pulses. Purge time was 1 s.
BH3(NHMe2) decomposes on catalytic Ru surface and forms reactive surface groups which are strong reducing agents. In the current study a catalytic metal, Ag, was deposited and the amount of Ag clearly affected the growth. To study more the hypothesis about decomposition of BH3(NHMe2) on catalytic surfaces, Ru pieces were placed in a source line of BH3(NHMe2). This is possible in the F120 ALD reactor used where the precursor is placed in a glass boat into a glass tube and transported with N2 into a cross-flow reaction chamber.21 It should be emphasized that only BH3(NHMe2) was in contact with Ru and there was no film growth on Ru. Ru in the source line had a clear impact on the growth rate of Ag by raising it to a level of 0.33−0.35 Å/cycle from 0.3 Å/cycle when in both cases 1000 cycles were applied (Figure 5).
Figure 2. Growth rate of Ag as a function of (a) Ag(fod) (PEt3) and (b) BH3(NHMe2) pulse length. Deposition temperature was 110 °C. The other precursor pulse was 1 s.
Figure 3. Uniformity of the Ag thin films deposited with different cycle numbers resulting in different thicknesses. Depositions were performed at 110 °C.
Figure 5. Effect of Ru catalyst in the source line of BH3(HNMe2) on the growth rate of Ag. 1000 cycles were applied with pulsing sequence 1/1/1/1 s.
growth rate is independent of the number of cycles. In the current Ag process this was not confirmed. With 100 and 300 cycles the growth rate of Ag was still the same but then the growth rate started to increase, until again with 1500, 1750, and 2000 cycles the growth rate remained the same (Figure 4). 1500 cycles resulted into an Ag film thickness of about 55 nm. The growth was thus slower on the starting surfaces (Si and glass) than on Ag itself. It was concluded that the Ag surface catalytically affected the growth. In addition to depositing on Si and glass, one experiment was done on previously deposited Ag. 1000 cycles were applied on a 73 nm thick Ag coating at 110 °C. The growth rate of the second Ag layer was 0.34 Å/ cycle which was noticeably higher than on Si in which case 1000 cycles resulted in 0.29 Å/cycle. Growth rate of the second layer was also very close to the expected growth rate for Ag films thicker than 55 nm (0.36 Å/cycle). Previous publications have shown that a catalytic surface may affect the growth of a metal when BH3(NHMe2) is applied as a reducing agent. Kalutarage et al. 15,16 presented that
Enhanced growth rate is close to the growth rate achieved on Si when more than 1500 cycles were applied (0.36 Å/cycle, Figure 4). At a deposition temperature of 130 °C differences in growth rates with and without Ru are nondistinguishable, most likely because of thermal decomposition of Ag(fod) (PEt3). The growth rates achieved in the current study can be compared to the growth rates reported for the PEALD process with the same metal precursor and plasma activated hydrogen. In this PEALD process self-limiting growth was confirmed at 120 °C with a growth rate of 0.33 Å/cycle. At 140 °C the growth rate was already higher (0.38 Å/cycle).7 The results are consistent and differences in temperatures are most likely related to the reactor designs. The relatively similar growth rates in thermal ALD and PEALD with different reducing agents, suggest that the Ag(fod) (PEt3) is mostly determining the growth rate. No matter whether it is adsorbed molecularly or after dissociation of the phosphine adduct, the adsorbate is 2042
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Figure 6. FESEM images of Ag thin films deposited with different number of cycles at 110 °C resulting in different thicknesses (6, 29, and 105 nm). Pulsing sequence was 1 s precursor pulses with 1 s purges.
compared to growth rates achieved without activation of BH3(NHMe2). It remains to be studied whether the species formed from BH3(NHMe2) permit ideal ALD like growth and what would be the temperature limits for this. One should also explore if this kind of activation can be applied to the growth of other metal thin films as well. Film Properties. All the Ag thin films deposited on Si or glass had visually matte finish which is a sign of rough microstructure. FESEM studies showed that the thin films were particulate (Figure 6). Thicker films were confirmed to consist of particles with different sizes: in between very large particles there were also smaller ones. Particle size distribution is a result of coalescence and continued nucleation. Coatings stored in laboratory air turned yellow over time. Also some delamination was noticed within days. This is why the samples were mostly stored in a desiccator. Some thin films were also deposited on sputtered Ru surfaces. On Ru the microstructure of the films was at least as rough as that on Si or glass (Figure 7).
quite bulky which limits the adsorption density and thereby causes the relatively low growth rate. According to the observation on the enhancement of the Ag growth when BH3(NHMe2) was in contact with Ru, and literature refs 15 and 16, it is likely that BH3(NHMe2) adsorbs on Ru and reacts. Not only reductive surface groups are formed on the catalytic surface, but also species that desorb from the Ru surface are created. These species can be transported into the ALD reaction chamber. Because the Ru catalyst caused enhancement of the growth, desorbed species appear to be even more reactive (or reducing) than BH3(NHMe2) itself. Considering the literature concerning BH3(NHMe2),24 formation of [Me2N-BH2]2 and H2 on Ru and Ag surface can be proposed. Dehydrogenation of BH3(NHMe2) to [Me2NBH2]2 and H2 is known to occur at elevated temperatures (>100 °C) in the melt or even at lower temperatures with a metal catalyst (when BH3(NHMe2) is dissolved into a solvent). However, commonly it is accepted that without any metal catalyst the dehydrogenation is an intermolecular process and is relatively unlikely to happen in a gas phase in a glass tube almost irrespective of temperature, referring e.g. to conditions inside an ALD reactor when the source temperature of BH3(NHMe2) is below 100 °C and there are no catalytic substrates.24 In theory it is possible that when Ru was inserted in the source tube of BH3(NHMe2), BH3(NHMe2) adsorbed on the Ru surface and produced [Me2N-BH2]2 and H2 which both are potentially reducing25 and can contribute to the ALD metal growth. To study the role of the possibly forming H2, Ag deposition was attempted at 110 °C by pulsing 1500 cycles of Ag(fod) (PEt3) (1 s) and 15 sccm of H2 (4 s). This resulted in only 1 nm Ag film. This shows clearly that H2 cannot reduce chemisorbed Ag(fod) (PEt3) at this temperature. The results presented in the current study do not show how powerful a reducing agent BH3(NHMe2) actually is. It is possible that Ag growth would not occur without minor decomposition of Ag(fod) (PEt3) that is required to launch the growth of Ag by creating Ag nuclei which activate BH3(NHMe2). This kind of a minor decomposition of a metal precursor is speculated to be essential also in noble metal processes that utilize O2 where self-limiting noble metal growth has been confirmed on noncatalytic surfaces although it should not be possible without activation of O2 by a catalytic surface.13,26 Until in situ characterization of the growth mechanism is performed, the mechanism how a catalytic surface affects BH3(NHMe2) can only be speculated. Catalysis occurred on Ru but also on growing Ag after a certain thickness. In this study it was shown that it is possible to activate BH3(NHMe2) in a source line to species that increase the growth rate of Ag
Figure 7. FESEM images of Ag thin films deposited on (a) Si and (b) sputtered Ru surface at deposition temperatures 110 °C. Both substrates were deposited at the same time. 1000 cycles were applied.
All the Ag thin films were nonconductive even if the nominal thickness was close to 100 nm. This is a result of the particulate structure of the films and gaps between the islands. Other thermal Ag ALD processes have similarly resulted in particulate films, at least with the nominal thicknesses mentioned in the publications. Ag PEALD processes are reported to produce conductive films despite of rough microstructure.7−11 Rough microstructure of Ag thin films is explained by the high mobility of Ag atoms which leads to rough films already at low temperatures. When the current thermal ALD process is compared to the PEALD process with the same metal precursor, it is noticed that both processes produce particulate coatings. However, as already mentioned, the PEALD films are conductive after a certain film thickness.7 This could be explained by differences in nucleation. Metal PEALD processes can, in general, show decreased nucleation delays compared to thermal ALD processes.17 2043
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Chemistry of Materials The Ag thin film with a nominal thickness of 55 nm deposited at 110 °C was measured to consist mostly of Ag (96.9 at. %) as analyzed by TOF-ERDA. The main impurities in that film deposited with 1 s precursor pulses were oxygen (1.6 at. %), hydrogen (0.8 at. %), and carbon (0.7 at. %). To complement TOF-ERDA analysis, XPS spectra of Ag samples deposited at 110 °C were measured and analyzed. The main impurities in the films, as high-lighted by the XPS survey spectrum in Figure 8, were found to be carbon, oxygen,
Cubic crystal structure of Ag (International Centre for Diffraction Data (ICDD) file card no. 00-004-0783) was identified with XRD (Figure 9). Even the thinnest Ag deposit
Figure 9. XRD patterns of Ag thin films with different thicknesses. The thin films were deposited at 110 °C.
with a nominal thickness of 2 nm showed a small hump in the direction of (111). In this Ag film crystallites are so small that the intensity of the peak (111) is low.
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CONCLUSIONS Uniform, particulate, and polycrystalline Ag thin films were deposited by pulsing alternatively Ag(fod) (PEt 3) and BH3(NHMe2). A growth rate of 0.3 Å/cycle was achieved with 1 s Ag(fod) (PEt3) and BH3(NHMe2) pulses at 110 °C. The process showed self-limiting growth characteristics. However, it was observed that the Ag surface catalytically enhanced the growth of Ag on itself. This phenomena was attributed to complex reducing chemistry of BH3(NHMe2). This observation led to an idea of activating BH3(NHMe2) on a catalytic Ru surface before BH3(NHMe2) entered the reaction chamber. Ru was in contact only with BH3(NHMe2) and no film was growing on Ru. Ru in the source tube of BH3(NHMe2) was confirmed to enhance the growth of Ag by raising the growth rate from 0.3 Å/cycle to the level of 0.33−0.35 Å/cycle. It stayed unclear what species were created on the Ag or Ru surface but a hypothesis about the formation of [Me2N-BH2]2 was presented. Activation of BH3(NHMe2) can be useful when it is applied to other metal ALD processes.
Figure 8. XPS survey spectrum of a 55 nm thick film, showing peak positions for the main elements and impurities. The insets show high resolution scans of the Ag 3d peaks and Ag (MNN) Auger line. Dashed lines in the Auger spectrum indicate literature values expected for both metallic Ag (357.9 eV) and Ag2O (356.6 eV).
chlorine, phosphorus, and fluorine. High resolution spectra of the Ag 3d peaks and Ag (MNN) Auger peaks (insets in Figure 8) were similar for all of the deposited film thicknesses. The sharpness of the asymmetric Ag 3d peaks indicated a metallic Ag(0) state, which was confirmed by the kinetic energy of the M4VV Auger peak (357.9 eV), showing that the films mainly consisted of metallic Ag.27 The carbon concentrations were measured to be between 20 and 40%, typical for the surface sensitive XPS signal, which is dominated by adventitious carbon. Similarly, increased silicon and oxygen contents were noticed when the films did not completely cover the native oxide of the substrates. Neither TOF-ERDA nor XPS analysis showed any boron impurities in the films. Presented TOF-ERDA and XPS results can be compared to TOF-ERDA results obtained for the Ag thin films deposited with the same metal precursor and plasma-activated hydrogen.7 An Ag thin film deposited at 120 °C contained about 85 at. % silver, 7 at. % hydrogen, 3 at. % carbon, 3 at. % oxygen, 0.9 at% phosphorus, 0.5 at% fluorine, and 0.7 at. % nitrogen. These values were estimated to be the upper limits for the impurities because of high surface roughness with respect to the film thickness (17 nm). Taking into account that the thermally grown films were stored in a desiccator to prevent postdeposition contamination whereas the PEALD films were stored in laboratory air, the impurity levels of the Ag thin films deposited with these two processes are relatively similar. Higher impurity levels of PEALD films can be a result of contamination after deposition. The chlorine impurities in the thermally deposited Ag thin films which were measured by XPS but not detected by TOF-ERDA are speculated to locate at the surface of the samples and are most likely caused by contamination from sample boxes or laboratory equipment during postdeposition handling.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: maarit.makela@helsinki.fi. ORCID
Maarit Mäkelä: 0000-0001-8383-2854 Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding from the Finnish Centre of Excellence in Atomic Layer Deposition (Academy of Finland) is gratefully acknowledged.
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