Structural Changes in Tungsten and Tantalum Wires in Catalytic

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The Journal of Physical Chemistry

Structure Changes in Tungsten and Tantalum Wires in Catalytic Chemical Vapor Deposition Using 1,3-Disilacyclobutane

I. Badran, W. H. Kan, Y. J. Shi*

Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4

* Corresponding author, email: [email protected]; Tel: 1-403-2108674; Fax: 1-403-2899488

1 ACS Paragon Plus Environment


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Abstract Metal wires (typically made of W or Ta) serve as catalysts to decompose the precursor gases to form reactive species in the technique of catalytic chemical vapor deposition. The reactions of these reactive species with the heated wire cause structure changes in the wire, which affects its catalytic properties and lifetime. Here, we report a systematic study on characterizing the structural changes in W and Ta wires when they are exposed to 1,3-disilacylobutane, a useful single-source precursor for SiC film deposition. We have shown that filament temperature, reaction time, and filament material are among the important factors in determining the nature of metal alloys formed. Formation of crystalline W2C, SiC, and W5Si3 (weak) was observed on W, whereas crystalline TaC, SiC, and Ta5Si3 (weak) were formed on Ta. While both filaments proved to form cubic crystalline 3C-SiC at low temperatures, alloying has taken different paths at higher temperatures. Between 1400 - 2400 °C, alloying in W was dominated by the formation of W2C with little contribution from WC. For Ta, the main alloy formed was TaC in the temperature range of 1400 - 2000 °C. Heating the aged Ta filament to temperatures higher than 2000 ºC tended to recover the metal wire. This same practice does not seem to work for W wires since more W2C are formed at high temperatures. It is concluded that Ta outperforms W for SiC film growth in its resistance to forming more carbides and its ability to recover at high temperatures.


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Introduction Catalytic chemical vapor deposition (Cat-CVD), also known as hot wire CVD (HWCVD), has been widely used for the deposition of silicon-containing thin films,1,2 diamond coatings,3 nanomaterials,4 and functional polymer coatings.5 The process of Cat-CVD is characterized by a high deposition rate, the scalability, and low equipment cost. In the core of this technique is the metal wire that serves as the catalyst to decompose precursor gases to form reactive species. The catalytic dissociation reactions are key to the success of the actual deposition since they initiate the gas-phase reaction chemistry and thin film growth. However, the heterogeneous reactions between the gaseous reactive species and the heated metal wire itself also result in the formation of metal alloys, such as metal silicides6-9 and metal carbides,10-12 on the wire surface. Several studies showed that the filament alloying affects the catalytic ability of the metal wire6,12 as well as the quality of the deposited thin films.11,13 A more detrimental effect from the filament alloying is the shortening of its lifetime. Depending on the filament material and the precursor gases used, the time that the filament can serve before being replaced varies from hours to months.14,15 Recently, increasing efforts have been made to recover the aged metal wires and/or to minimize the aging effect. For example, Hirunski et al. used alternative current (AC) to decrease the silicon contamination of the filament.16 Other groups used a cylindrical cavity to enclose the electrical contacts of the tungsten filament which reduced the probability of silicide formation on the colder ends.7,17,18 Annealing with H2 was found to be effective to extend the lifetime of the filament,6,8,19 however, this attempt can sometimes end up with more cracks.19 Further development of an efficient method to solve the wire aging problem needs to be built on a good



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understanding of the structural changes in the metal wires used in Cat-CVD and the mechanism leading to these structural changes. Previous studies have found that the nature of the alloys formed on the heated wire is closely related to the decomposition chemistry exhibited by the precursor gases. In the deposition of Si films using SiH4, the growth of a silicide layer on the W or Ta filament was commonly observed.6,7,9 Recently, our laboratory has studied the structural changes in W wires when they are exposed to different four-membered-ring (di)silacyclobutane molecules, including 1silacyclobutane (SCB)8 and 1,1,3,3-tetramethyl-1,3-disilacyclobutane (TMDSCB).12,20 These organosilicon compounds are potentially useful single-source precursor gases to replace the SiH4/hydrocarbon mixtures that are conventionally used for silicon carbide thin film deposition by Cat-CVD. It is interesting to find that the reactions of TMDSCB with W filament lead to the formation of tungsten carbides due to its ability to produce reactive methyl radicals on the metal wire. In contrast, the dominant decomposition pathways for SCB are the ring-opening reactions to form silene (H2Si=CH2) and silylene (:SiH2). Therefore, a tungsten silicide layer was formed on the surface of W wires when they were exposed to SCB. Depending on the relative stability of the different phases of the metal alloys, phase transformations often take place. These transitions are very sensitive to the operational temperature and the deposition time. For instance, the growth of tungsten carbides on W wires when exposed to TMDSCB was shown to be a complete carburization of the full W filament into a C-deficient W2C phase at low temperatures, followed by diffusion of carbon at higher temperatures through the growing shell to form the C-rich WC phase on the outer surface.12,20 In addition, a pure crystalline WC layer with no contamination from W2C and W was formed by treating the W wire with TMDSCB at 2400 ºC for 4 hours of deposition time.20 Tungsten monocarbide (WC) has found many industrial applications as


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heterogeneous catalysts21,22 and promising anode materials for direct methanol fuel cell (DMFC).23 Similarly, metal silicides have attracted applications in microelectronics and electrochemical nanodevices.24-26 Hence, what was once considered the “filament aging” process could be utilized to fabricate useful crystalline materials. Tantalum is known to have a longer lifetime than tungsten in HWCVD reactors.11,15,27 In the environment of silane for the deposition of Si thin films, Duan et al. found that there is a lower Si buildup on Ta than on W,15 which led to the preferred usage of Ta over W for longer lifetime. To the best of our knowledge, no reports have been found on the behavior of Ta vs. W when they are exposed to organosilicon precursors for SiC film growth. In this work, the structural changes in both W and Ta wires when they were exposed to a new cyclic organosilicon precursor, 1,3-disilacyclobutane (DSCB), were systematically investigated in the temperature range of 1100 - 2400 ºC. DSCB is considered an attractive CVD precursor gas to deposit silicon carbide thin films due to its built-in 1:1 Si:C stoichiometry that matches the one in SiC.28,29 In addition, the high ring strain of DSCB (74.1 kJ·mol-1)30 as well as its relatively weak Si-H bonds reduce its decomposition temperature on the hot wire during the Cat-CVD process.31 We have used electron microprobe analysis (EMPA), wavelength dispersive spectroscopy (WDS), X-ray diffraction (XRD), and in-situ wire resistance monitoring to understand the formation of different alloys on the wire surfaces. Crystalline metal carbides, metal silicides, and also silicon carbide have been found to form on both W and Ta wire surfaces when treated with DSCB. The effect of filament temperature and deposition time, along with the role of the unique decomposition chemistry of DSCB, on the alloy formation and phase transitions is discussed. A comparison of the structural changes in W and Ta is provided to shed light on their different behavior in forming alloys and its practical implications.



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Experimental Methods Details of the Cat-CVD reactor used to prepare the alloyed metal wires in this study has been described previously.12,20,32 Briefly, a 10 cm long W or Ta wire (99.9%, Aldrich) with a diameter of 0.5 mm was placed in a cylindrical stainless steel chamber. Prior to deposition, each new wire (W or Ta) was annealed for 60 min at 2000 ºC using 12 Torr of 1% H2 in He gaseous mixture to ensure a smooth and clean wire surface. For each deposition run, 12 Torr of 4% DSCB mixture seeded in He was introduced to the reactor chamber and allowed to react with the filament for the desired time and temperature. The filament was resistively heated by a DC power supply (Agilent, N5744A) and its temperature was measured by a two-color IR pyrometer (Chino Works). The current and voltage supplied to the filament were monitored in situ with a LabVIEW program, from which the power and resistance data can be obtained. After deposition, the wire was allowed to cool down to room temperature, before being replaced and saved under vacuum for surface analysis. The filament temperatures tested in this study range from 1100 to 2400 ºC at 200 ºC intervals. The effect of deposition time was studied for 1 to 4 hours at both 1200 ºC and 2400 ºC. The surface morphology and the film thickness of the aged metal wires were studied using an electron microprobe (JEOL JXA 8200) operating at 20 kV. WDS line scans were taken using the same microprobe. For the WDS measurements, the filaments were embedded on a sample holder using a silver conductive epoxy (MG Chemicals) and left overnight to dry. The samples were then manually polished using 600 grit (25µm) silicon carbide sandpapers wetted in distilled water for 30 min, followed by 1200 grit (15 µm) for 10 min. Final polishing was done using 1 µm alpha-alumina powders (ET Enterprises) wetted in distilled water. The samples were finally cleaned with distilled water and air sprayed. The crystalline phases of the samples were


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characterized ex situ using XRD. The XRD analysis was performed using a Bruker D8 Advance X-ray diffractometer with a Cu Kα radiation source (λ = 1.54 Å) operated at 40 kV and 40 mA. Samples were scanned in the 2θ range of 10 - 80º. Data were collected in a continuous mode with a step size of 9 s/step. The metal wires were placed over glass slides for the XRD characterization. The room-temperature data sets were refined by conventional Rietveld method using the GSAS package with the EXPGUI interface.33 A LeBail-like approach was used for the samples with highly preferred orientations, where background, scale factor, zero-point position, cell parameters, preferred orientations, and profile coefficients for pseudo-Voight/FCJ asymmetric peak shape function were refined until the convergence was achieved.

Results and Discussions Structure changes in tungsten wires upon exposure to DSCB Figure 1 shows cross-sectional secondary electron microscope images for the alloyed tungsten wires prepared at a wire temperature of 1200 ºC using 0.48 Torr of DSCB for various deposition times. Under these conditions, the filaments showed distinct outer layers, which grew steadily with deposition time. WDS measurements were recorded for the alloyed filaments. Figure 2 shows the WDS line scan for the tungsten filament exposed to DSCB at 1200 ºC for 1 h. As the scan moved away from the center of the filament towards its outer edge, an increase in intensities of the WDS peaks for C and Si was observed. This was accompanied by a drop in the tungsten peak intensity, which suggests a build up of Si/C contents on the filament surface. Figure 3 shows the XRD patterns for the alloyed tungsten wires prepared at 1200 °C. The XRD pattern for the virgin W filament shows three peaks at 2θ = 40.3° (110), 58.3° (200), and 73.2° (211) with an intensity ratio of 1.0 : 1.1 : 1.3. This represents the body-centered cubic



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tungsten (JCPDS#04-0806).34 In addition to signals from tungsten, the spectra in Figure 3 show peaks for 3C-SiC and W2C. The diffraction peaks due to the cubic 3C-SiC were observed at 35.6° (111), 60° (220) and 71.8° (311). The LeBail refinement for the XRD pattern further confirmed its space group as the cubic space group F-43m (JCPDS#29-1129)34 with cell parameters of a = 4.3635(7) Å and V = 83.08(4) Å3. The XRD peak at 35.6° also coincides with the (100) peak for hexagonal tungsten carbide WC (JCPDS#65-4539, space group P6m2).34 However, no other peaks for WC were observed at this temperature. Although the formation of cubic 3C-SiC was not observed in our previous studies using SCB8 and TMDSCB,12,20 its formation on tungsten filament under similar conditions was reported by Tabata et al.,19 who used a mixture of SiH4, CH4, H2 and N2 for the deposition of hydrogenated nanocrystalline cubic silicon carbide. The diffraction peaks of W2C were observed at 39.5° (121), 52.2° (221), 61.9° (023), and 69.8° (321), representing the orthorhombic space group Pbcn (JCPDS#65-8829).34 Further LeBail refinement supported this observation and determined the cell parameters as a = 4.785(6) Å, b = 6.074(7) Å, c = 5.245(7) Å, and V = 152.4(6) Å3. The observation of crystalline W2C is similar to our recent study using TMDSCB12,20 and other studies.10-12,19 For the W filaments exposed to DSCB for longer time of 3 hr and beyond, weak XRD peaks due to W5Si3 were also observed at 42.8° (411) and 45.1° (222) (JCPDS#65-1617).34 A close examination of Figure 3 reveals that W2C peaks intensities at 39.5°, 52.2°, 61.9°, 69.8° have decreased with deposition time, while the ones for 3C-SiC peaks at 35.6°, 60.0° and 71.9° have increased, reaching a maximum at 4 h. After 6 hours of deposition, the peaks corresponding to W2C have significantly diminished or disappeared. This observation indicates a transformation of W2C to SiC in the outer layer growth with time. Figure 4 shows the growth of the outer layer thickness as a function of the square root of deposition time. The growth obeys


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the parabolic law where the thickness is linearly proportional to the square root of time. This suggests that the growth is diffusion controlled. Similar growth behavior was observed in many other deposition studies.9,10,19,27,35,36 In the first stage of deposition (up to 3 h), the corresponding diffusion coefficient was determined to be 6.8 × 10-16 m2 s-1, which is close to the diffusion coefficient of carbon in W2C reported in the literature.37 In the second stage, between 3 and 6 h, the diffusion coefficient increases to 3.4 × 10-14 m2 s-1. This value might be considered as the diffusion coefficient of C in cubic 3C-SiC. No literature data can be found on the diffusion coefficient of 12C in 3C-SiC, however, Hong et al. reported the self-diffusion coefficient of the 14

C isotope in both α-SiC (non-cubic SiC) and β-SiC (cubic 3C-SiC).38 Assuming the

relationship on the temperature dependence of diffusion coefficient in their work could be extended to the low temperature of 1200 °C, the value of the 14C diffusion coefficient at this temperature was calculated to be 4.0 × 10-24 m2 s-1, 7.8 × 10-26 m2 s-1, and 4.8 × 10-17 m2 s-1, respectively, in the α-SiC lattice, β-SiC lattice, β-SiC grain boundary. It is noted that the diffusion coefficient of 3.4 × 10-14 m2 s-1determined in this work for the second-stage growth in Figure 4 is much closer to the literature value for the 14C diffusion in the cubic 3C-SiC grain boundary. This supports the argument that the diffusion coefficient of 3.4 × 10-14 m2 s-1 represents that of C in the grain boundary of 3C-SiC. These observations are in good agreement with a conversion from W2C to SiC at longer time of deposition, as demonstrated by the XRD analysis. In their deposition experiments using TMDSCB on W filaments, Shi et al. attributed the formation of WxC (x = 1, 2) to the precursor molecule's ability to produce methyl radicals.20 DSCB, on the other hand, does not produce methyl radicals. As shown in a previous study31 on the DSCB decomposition chemistry under practical Cat-CVD conditions, the main pathway



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involved a hydrogen elimination to form a cyclic silylene intermediate, 1,3-disilacyclobut-1ylidene (I), which quickly inserted into the Si-H bond of the parent DSCB molecule to form 1,1'bis(1,3-disilacyclobutane) (route a in Scheme 1). In addition, the cycloreversion of DSCB (route b in Scheme 1) forms two identical silenes (II), whereas the ring opening initiated by a 1,2-H shift from Si to C leads to the formation of two pairs of silene and silylene species (route c in Scheme 1). It is known that the formation of WC/W2C is due to decomposition of the gaseous precursors on the heated filament surface, followed by diffusion of carbon into the bulk tungsten.12,20,27 The silene and silylene species (I - V) are very reactive and rich in both carbon and silicon. Therefore, they are good precursors to form SiC as well as W2C. The transformation from W2C to SiC observed at 1200 ºC in our experiments is due to the fact that W2C is unstable below 1250 °C.39,40 The decomposition of the W2C layer released carbon, which allowed for the deposition of the more stable SiC on the filament surface.

Si

a -H2

H2Si

silylene addition DSCB

I SiH2

b

2 H2C

H

H

Si

Si

H2Si

SiH2

SiH2 II

H2Si DSCB

SiH multiple steps

c 1,2-H shift

H2Si

CH3

CH3SiH CH2 + SiH2 IV III H2Si

CH2 + SiH(CH3) V II

Scheme 1. Major decomposition pathways of DSCB in a Cat-CVD reactor The effect of wire temperature on the alloy formation on tungsten wires when exposed to DSCB was studied within the range of 1100 - 2400 ºC for the deposition time of 1 h. An increase


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in the peaks characteristic with W2C was observed with increasing temperature, as shown in Figure 5a. Meanwhile, the XRD peaks of 3C-SiC became weaker as the temperature was raised. The XRD peaks at 35.6° and 71.8° have completely disappeared at temperatures higher than 1600 ºC. At 2400 ºC, the XRD spectrum became dominated with the W2C peaks. The LeBail refinement performed at 2400 ºC indicated no presence of 3C-SiC or WC phases. The diminution of 3C-SiC at temperatures above 1600 ºC in our deposition experiments agrees well with a recent study by Tabata et al.,19 who managed to remove the 3C-SiC deposits on the cold parts of the W wire by heating the wire at 2000 ºC in H2 atmosphere.19 The transformation from SiC into W2C at T > 1600 ºC can be explained by a decomposition of the SiC layer into atomic Si and C, followed by carbon diffusion into the W bulk to form W2C. In order to better understand the deposition chemistry at higher temperatures, we extended the deposition time at 2400 ºC up to 4 h. Figure 5b shows the corresponding XRD pattern for the filaments prepared at this temperature. With increasing time at this high temperature, very weak peaks from WC were observed at 35.7 (100), 48.4 (101), and 64.2 (110) for the 2 hr exposure time, but quickly diminished at longer times. As can be noticed, the peaks associated with W2C (39.6°, 52.3°, 61.9°, and 69.8°) have dominated over the peaks corresponding to 3C-SiC, WC, and W for all 4 hrs. In our previous work on the alloying of W filament using TMDSCB,20 we concluded that a carbon-rich WC phase formed on the wire surface was converted to form the carbon-deficient W2C, and a pure W2C layer was formed at 2400 ºC. With longer times of deposition of 3 - 4 h, carbon diffusion into the outer W2C layer caused the re-formation of WC, and a pure WC phase was observed after 4 h of deposition at 2400 ºC. The fact that TMDSCB is an excellent methyl radical producer has affected the full carburization of the tungsten filament at high temperature. Conversely, DSCB is not able to



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produce any methyl radicals. Rather, its decomposition chemistry is ruled by silene/silylene reactive intermediates as previously shown in Scheme 1. Therefore, a conversion from W2C to WC at higher temperatures and longer deposition times was not observed with DSCB. The electrical resistance of the alloyed wires was monitored over the course of each experiment. In each run, the resistance has increased continuously with deposition time. This increase in resistance can be mainly attributed to filament alloying. Since the room-temperature resistivity of W2C (8.0 × 10-7 Ω·m) and SiC (0.01 Ω·m) are higher than that of pure tungsten (5.4 × 10-8 Ω·m),41 such temporal increase is expected. The increase in filament resistance is a common observation in filament aging studies.10,20,27 Typically, electrical resistivity increases as temperature rises for pure metals. The conductor’s resistance changes with temperature according to the following equation: − ⁄ = −

(1)

where R and R0 are resistances at temperature T and T0, respectively. The α constant is called the temperature coefficient of resistance (TCR) and can be graphically obtained by plotting − ⁄ vs. (T-T0). Figure 6 shows such a plot for the aged filaments prepared in our experiments in the temperature range of 1100 - 2200 ºC. T0 was considered to be 1100 °C, and R0 is the terminal resistance at the end of the 1 h experiment performed at 1100 ºC. Between 1100 1600 ºC, the resistance increased with a TCR of 6.5 × 10-4 °C-1. This is lower than the TCR reported for pure tungsten (5.5 × 10-3 °C-1).41 In the next stage between 1600 - 2200 °C, the TCR dropped to 8.6 × 10-5 °C-1. The decline in the TCR value can be caused by the formation of a material that has a TCR that is much lower than W, or a change in the degree of crystallinity and the filament microstructure.42 We showed that more W2C has formed in the aged tungsten


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filaments at higher temperatures in our experiments. This could cause the decline of the TCR observed in Figure 6. Structure changes in tantalum wire upon exposure to DSCB The surface chemistry between the tantalum filament and DSCB was studied using the same methodology for the experiments with tungsten. The Ta filaments exposed to DSCB for 1 h at relatively low temperatures of 1200 - 1400 ºC showed no distinct outer layer. Structural deformation started to appear at 1600 ºC, and a thin layer of 30 µm thickness was observed at 2200 ºC (See Support Information for the cross-sectional SEM images of the aged Ta filaments.). The XRD spectra for the tantalum filaments prepared at 1200 ºC (Figure 7a) showed much weaker peaks than those obtained for tungsten filaments (refer to Figure 3). The XRD pattern for the new Ta filament is composed of three peaks at 2θ = 38.5°, 55.5° and 69.6°, with an intensity ratio of 1.0 : 1.1 : 1.2. This spectrum represents the body-centered cubic tantalum (JCPDS#04-0788).34 New peaks representing tantalum silicide, Ta5Si3, at 37.0° (102) and 40.8° (211), and 42.0° (112) were observed for the space group P63/mcm (JCPDS#06-0594).34 In addition, XRD peaks characteristic of the cubic 3C-SiC were observed after 2 - 3 hours of deposition at 1200 ºC at 2θ = 35.6° (111) and 71.8° (311). The peak at 60° (220) for 3C-SiC was missing. Similar to that observed for the W filaments, the XRD peaks corresponding to the cubic crystalline SiC phase generally increase with time at 1200 ºC. Due to the poor signal-to-noise ratio and the complexity of the XRD spectra, refinement was not performed for the Ta filaments aged at 1200 ºC. Figure 7b shows the XRD spectra obtained for the Ta filaments treated with DSCB for 1 h in the temperature range of 1400 - 2400 ºC. An overall decrease in the peak intensities was observed when the temperature is increased, indicating a decline in the degree of crystallinity of



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the aged filaments. The decrease in the intensities of the XRD peaks for Ta with increasing operating filament temperature is accompanied by an increase in the TaC peaks at 2θ = 34.9° (111), 40.5° (200), 58.6° (220), 70.0° (311), and 73.6° (222). The 34.9° peak is the strongest for TaC as reported in its XRD pattern (JCPDS#35-0801).34 The formation of tantalum carbide agrees with the visual observation of the golden colour of the aged filaments.43 In addition, the peaks for Ta2C were observed at 2θ = 38.1° (101), 50.1° (102), 66.5° (103), 71.7° (112), and 72.9° (201), corresponding to the hexagonal P3m1 space group (JCPDS#32-1280).34 The crystalline TaC and Ta2C formed in our experiments are similar to those recently reported in CatCVD systems.43,44 In order to better understand the TaC/Ta2C formation, the effect of exposure time at 2400 ºC was studied up to 4 h. Figure 7c shows the XRD spectra for the Ta filaments aged at 2400 ºC at different exposure time. Clearly the peaks characteristic of 3C-SiC and TaC have almost disappeared. At the same time, the peaks for Ta2C at 33.3° (100), 38.1°, 50.1°, 59.5°, and 66.5° have dominated the spectrum after 4 h of exposure. The dominance of the Ta2C phase at high temperatures is in agreement with the results from W filament aging experiments in the previous section, where W2C dominated at 2400 ºC. Similar to what was observed for the W filaments, the resistance of the aged Ta filaments has increased as the operation temperature became higher. The TCR for the Ta filaments prepared between 1400 - 2400 ºC was obtained from eq (1) and plotted in Figure 8. Two regions existed in the TCR diagram, where the first one lay between 1400 - 2000 ºC, and the second between 2000 - 2400 ºC. As mentioned earlier, factors affecting the TCR are the degree of crystallinity, the filament microstructure, and the value of TCR for different materials. In the first region of Figure 8, the calculated TCR (1.1 × 10-4 ºC-1) is lower than the one reported for pure


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Ta (3.7 × 10-3 °C-1),41 possibly due to the formation of crystalline TaC. In the second region between 2000 - 2400 ºC, the calculated TCR (1.2 × 10-3 ºC-1) is close to that for pure Ta. This is in contrast to what has been observed for W in the previous section, where the TCR was affected by the formation of W2C. These observations, along with the decline in the intensities of the XRD peaks of TaC and Ta2C above 2200 ºC, strongly suggests that the aged tantalum filaments progress towards amorphous pure tantalum at high temperatures. The Role of Filament Material, W vs. Ta None of the W (or Ta) wires prepared in this work show wire breakage during the aging process at different temperatures (1100 - 2400 ºC) and time (0.5 - 6 h). When the XRD patterns of the tantalum filaments in Figure 7 are compared with those of tungsten in Figures 3 and 5, the aged Ta filaments exhibited weaker XRD peaks, indicating a lower degree of crystallization. At the relatively low temperature of 1200 ºC tested in this work, no Ta2C was formed on the Ta wire surface, whereas the formation of W2C was obvious on W. This is due to the difference in the required temperature to form metal carbides for the two metals according to the phase diagrams of Ta-C and W-C.40 While both filaments proved to form crystalline 3C-SiC at low temperatures, alloying has taken different paths at higher temperatures. Between 1400 - 2000 ºC, alloying in the Ta filaments was dominated by the carbon-rich TaC, while at 1400 - 2400 ºC, W alloying was dominated by the carbon-deficient W2C formation. As the filament temperatures increased, the TCR value for the Ta filaments grew into that of pure Ta, while the TCR value for the W filaments indicated the formation of more carbide. Based on these observations, it is evident that W filaments are generally more prone to forming carbides than Ta filaments. Our study also suggests that heating the aged Ta filament to temperatures higher than 2000 ºC tend to recover the metal wire by getting rid of the carbide formed on the surface. Similar phenomenon was



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observed for Ta wires with silicide layers.45 The recovering of silicide layers on aged W filaments by heating at 2100 ºC was also reported in the literature.46 However, this same practice does not seem to work for W wires with carbide layers. Therefore, similar to what has been demonstrated for metal wires used in Si film deposition, Ta should outperform W in SiC growth process due to its resistance to forming more carbides and its ability to recover at high temperatures.

Conclusions The structural changes in both W and Ta wires were systematically investigated when they were exposed to 1,3-disilacyclobutane (DSCB). Due to its built-in 1:1 Si:C ratio and high reactivity, DSCB is a potentially useful source gas for the growth of silicon carbide thin films using Cat-CVD. Crystalline metal carbides, metal silicides, and also silicon carbide have been found to form on both W and Ta wire surfaces when treated with DSCB. It has been shown that low temperature and long reaction time favored the formation of highly crystalline 3C-SiC and M5Si3 (weak) on both metals. 3C-SiC and M5Si3 (M = Ta or W) phases disappeared with increasing temperatures and short reaction time. However, Ta and W show different behavior in forming metal carbides. We have shown in this work that W filaments are generally more prone to forming carbides than Ta filaments. At a relatively low temperature of 1200 ºC, no carbide was formed on Ta, whereas W2C was formed on W. Moreover, W2C is the dominant alloy formed on W for all temperatures tested at a deposition time of 1 h, whereas alloying in Ta was dominated by TaC in the temperature range of 1400 - 2000 ºC for the same deposition time. As shown by the XRD analysis and in-situ resistance measurements, the aged Ta wires can be restored as the filament temperatures were increased to above 2000 ºC. On the contrary, more


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carbides (W2C) were formed on W at these high temperatures. Therefore, due to its resistance to forming more carbides and its ability for recovery at high temperatures, it is concluded that Ta should outperform W. The decomposition of DSCB on both W and Ta wires produces mainly a cyclic silylene species, i.e., 1,3-disilacyclobut-1-ylidene.31 This has led to the formation of crystalline SiC, metal carbides, and silicides on W and Ta observed in this work when DSCB was used as a precursor gas. Previous studies on the alloying process in W when it was treated with other precursor gases have shown the formation of tungsten silicides with SiH4 or 1-silacyclobutane (SCB) and the growth of tungsten carbides with 1,1,3,3-tetramethyl-1,3-disilacyclobutane (TMDSCB).32 Comparison of the structure changes in W/Ta wires exposed to DSCB and those to SiH4, SCB, and TMDSCB further strengthens the conclusion that the nature of the reactive species produced from the precursor gas decomposition on the hot wire is key to determine the structural changes in the metal catalysts.

Acknowledgements The financial support for this work by the Natural Sciences and Engineering Council of Canada (NSERC) and the Canadian Foundation for Innovation (CFI) is gratefully acknowledged. Access to EMPA and WDS was provided by the University of Calgary Laboratory for Electron Microprobe Analysis (UCLEMA). Access to XRD was graciously provided by Prof. Venkataraman Thangadurai at the Department of Chemistry in the University of Calgary.

Supporting Information Available: LeBail refinement of the XRD data for the tungsten filaments prepared at 1200 ºC and 2400 ºC, and cross-sectional SEM images of aged Ta



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filaments prepared at four different temperatures. This information is available free of charge via the Internet at http://pubs.acs.org.


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(17) Frigeri, P. A.; Nos, O.; Bengoechea, S.; Frevert, C.; Asensi, J. M.; Bertomeu, J. Hot Wire Chemical Vapor Deposition: Limits and Opportunities of Protecting the Tungsten Catalyzer from Silicide with a Cavity. Thin Solid Films 2009, 517, 3427-3430. (18) Honda, K.; Ohdaira, K.; Matsumura, H. A Novel Method for Suppressing Silicidation of Tungsten Catalyzer During Silane Decomposition in Cat-CVD. Thin Solid Films 2008, 516, 826828. (19) Tabata, A.; Naito, A. Structural Changes in Tungsten Wire and Their Effect on the Properties of Hydrogenated Nanocrystalline Cubic Silicon Carbide Thin films. Thin Solid Films 2011, 519, 4451-4454. (20) Shi, Y. J.; Badran, I.; Tkalych, A.; Kan, W. H.; Thangadurai, V. Growth of Crystalline Tungsten Carbides Using 1,1,3,3-Tetramethyl-1,3-disilacyclobutane on a Heated Tungsten Filament. J. Phys. Chem. C 2013, 117, 3389-3395. (21) Levy, R. B.; Boudart, M. Platinum-Like Behavior of Tungsten Carbide in Surface Catalysis. Science 1973, 181, 547-549. (22) Esposito, D. V.; Chen, J. G. G. Monolayer Platinum Supported on Tungsten Carbides as Low-cost Electrocatalysts: Opportunities and Limitations. Energ. Environ. Sci. 2011, 4, 39003912. (23) Ganesan, R.; Lee, J. S. Tungsten Carbide Microspheres as a Noble-metal Economic Electrocatalyst for Methanol Oxidation. Angew. Chem. Int. Ed. 2005, 44, 6557-6560. (24) Zhang, Y.; Geng, D.; Liu, H.; Banis, M. N.; Ionescu, M. I.; Li, R.; Cai, M.; Sun, X. Designed Growth and Characterization of Radially Aligned Ti5Si3 Nanowire Architectures. J. Phys. Chem. C 2011, 115, 15885-15889.



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(25) Banis, M. N.; Meng, X.; Zhang, Y.; Cai, M.; Li, R.; Sun, X. Spatially Sequential Growth of Various WSi2 Networked Nanostructures and Mechanisms. J. Phys. Chem. C 2013, 117, 1918919194. (26) Banis, M. N.; Zhang, Y.; Xiao, Q.; Cai, M.; Li, R.; Sun, X. Tailoring of Single-crystalline Complex Ta5Si3 Nanostructures: From Networked Nanowires to Nanosheets. Cryst. Growth Des. 2014, 14, 436-441. (27) Okoli, S.; Haubner, R.; Lux, B., Carburization of Tungsten and Tantalum Filaments during Low-Pressure Diamond Deposition. Surf. Coat. Technol. 1991, 47, 585-599. (28) Larkin, D. J.; Interrante, L. V. Chemical Vapor Deposition of Silicon Carbide from 1,3Disilacyclobutane. Chem. Mater. 1992, 4, 22-24. (29) Chaddha, A. K.; Parsons, J. D.; Wu, J.; Chen, H.; x; S; Roberts, D. A.; Hockenhull, H. Chemical Vapor Deposition of Silicon Carbide Thin Films on Titanium Carbide Using 1,3Disilacyclobutane. Appl. Phys. Lett. 1993, 62, 3097-3098. (30) Gusel'nikov, L. E.; Avakyan, V. G.; Guselnikov, S. L. Effect of Geminal Substitution at Silicon on 1-Sila- and 1,3-Disilacyclobutanes' Strain Energies, Their 2+2 Cycloreversion Enthalpies, and SiC π-Bond Energies in Silenes. J. Am. Chem. Soc. 2001, 124, 662-671. (31) Badran, I.; Shi, Y. J., Promotion of Exocyclic Bond Cleavages in the Decomposition of 1,3Disilacyclobutane in the Presence of a Metal Filament. J. Phys. Chem. A 2015, 119, 590-600. (32) Shi, Y. J., Hot Wire Chemical Vapor Deposition Chemistry in the Gas Phase and on the Catalyst Surface with Organosilicon Compounds. Acc. Chem. Res. 2015, 48, 163-173. (33) Toby, B. H., EXPGUI, a Graphical User Interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210-213.


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(34) The International Centre for Diffraction Data (ICDD) Database, Newtown Square, PA, U.S.A., 2013. (35) Peng, X. L.; Clyne, T. W., Formation and Adhesion of Hot Filament CVD Diamond Films on Titanium Substrates. Thin Solid Films 1997, 293, 261-269. (36) Roger, J.; Audubert, F.; Le Petitcorps, Y., Thermal Reaction of SiC films with Tungsten and Tungsten-rhenium alloys. J. Mater. Sci. 2008, 43, 3938-3945. (37) CRC Handbook of Chemistry and Physics, 67th Ed.; Weast, R. C. Ed.; CRC Press, Boca Raton, FL. 1986 - 1987. (38) Hong, J. D.; Hon, M. H.; Davis, R. F. Self-Diffusion in Alpha and Beta Silicon Carbide. Ceramurgia Int. 1979, 5, 155-160. (39) Goesmann, F.; Schmidfetzer, R., Stability of W as Electrical Contact on 6h-Sic - PhaseRelations and Interface Reactions in the Ternary-System W-Si-C. Mater. Sci. Eng. B 1995, 34, 224-231. (40) Alloy Phase Diagrams: ASM Handbook, Baker, H. Ed.; ASM International, Cleveland, OH, 1992, Vol. 3, pp 2-115. (41) CRC Handbook of Chemistry and Physics, 93rd Ed.; Haynes,W. M. Ed.; CRC Press, Boca Raton, FL. 2012 - 2013. (42) Shioya, Y.; Maeda, M. Analysis of the Effects of Annealing on Resistivity of Chemical Vapor-Deposition Tungsten Silicide Films. J. Appl. Phys. 1986, 60, 327-333. (43) Cheng, S. M.; Gao, H. P.; Ren, T.; Ying, P. L.; Li, C. Carbonized Tantalum Catalysts for Catalytic Chemical Vapor Deposition of Silicon Films. Thin Solid Films 2012, 520, 5155-5160. (44) Ali, M.; Urgen, M.; Atta, M. A. Tantalum Carbide Films Synthesized by Hot-filament Chemical Vapor Deposition Technique. Surf. Coat. Technol. 2012, 206, 2833-2838.



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(45) Grunsky, D.; Kupich, M.; Hofferberth, B.; Schroeder, B. Investigation of the Tantalum Catalyst During the Hot Wire Chemical Vapor Deposition of Thin Silicon Films. Thin Solid Films, 2006, 501, 322-325. (46) Matsumura, H.; Ohdaira, K. New Application of Cat-CVD Technology and Recent Status of Industrial Implementation. Thin Solid Films, 2009, 517, 3420-3423.


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Table of Content Image



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Figure Captions Figure 1. Cross-sectional secondary electron images of alloyed tungsten filaments prepared using 0.48 Torr of DSCB at 1200 °C at different deposition time of a) 0.5 h, b) 1 h, c) 2 h, and d) 4 h.

Figure 2. Wavelength dispersive spectroscopic (WDS) line scans for W, C, and Si on a tungsten filament treated with 0.48 Torr of DSCB for 1 h at 1200 °C. Counting time: 80 per point, beam current: 10 nA. pixel size, 0.70 µm.

Figure 3. XRD patterns of alloyed tungsten filaments prepared using 0.48 Torr of DSCB at 1200 °C and various exposure time of 1 - 6 h. The XRD pattern of a virgin W filament is presented for comparison.

Figure 4. Outer layer thickness as a function of the square root of exposure time for W filaments prepared using 0.48 Torr of DSCB at 1200 °C.

Figure 5. XRD patterns of alloyed tungsten filaments prepared using 0.48 Torr of DSCB a) for 1 hr at various temperatures of 1100 - 2400 °C, and b) at 2400 °C for different exposure time of 1 4 h. The XRD pattern of a virgin W filament is presented for comparison.

Figure 6. The plot to determine the temperature coefficient of resistance (TCR) of the alloyed tungsten filaments prepared using 0.48 Torr of DSCB at different temperatures of 1100 - 2200 °C. T0 = 1100 °C, and R0= 0.25 Ω.


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Figure 7. XRD patterns of alloyed tantalum filaments prepared using 0.48 Torr of DSCB a) at 1200 °C for various exposure time of 1 - 4 h, b) for 1 h at different temperatures of 1400 - 2400 °C, and c) at 2400 °C for various exposure time of 1 - 4 h. The XRD pattern of a virgin Ta filament is presented for comparison.

Figure 8. The plot to determine the TCR of the alloyed tantalum filaments prepared using 0.48 Torr of DSCB at different temperatures of 1400 - 2400 °C. T0 = 1200 °C, and R0= 0.45 Ω.



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Figure 1


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Figure 2

1000

800

Count (arb. unit)

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


C Si W

600

400

200

0 0.00

0.05

0.10

0.15

0.20

0.25

Distance from the center (mm)



SiC(311) W(211) W2C(321)

4h

W2C(023)

3000

SiC(220)

6h

W2C(121)

3500

W2C(221)

4000

W(110) W5Si3(411) W5Si3(222)

SiC(111)

4500

W(200)

Figure 3

Intensity (a.u.)

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

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2500 3h

2000 2h

1500

1h

1000 500

0.5 h pure W

0 20

30

40

50

2

60

70

80


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Figure 4 30

1

m/

mi

n 1 /2

25

pe

=3

.5 2

20

slo

Thickness (m)

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


15 1/2

slope

=0

m /m .4 9 5 

in

10

5 4

6

8

10

time

12 1/2

14

16

18

20

1/2

(min )



5000

W 2C(142)

W(211) W 2C(223)

W 2C(321) C(223)

W 2C(023)

W(200)

2400 °C

W 2C(221)

(a) 1 hour

W 2C(200) (121) W(110)

6000

W 2C(002)

Figure 5

2200 °C

2000

SiC(220)

W 5Si3(222)

1800 °C 3000

W 5Si3(411)

4000

SiC(111)

Intensity (a.u.)

2000 °C

1600 °C 1400 °C 1300 °C

1000

1100 °C W

0

W 2C(142)

80

W 2C(223)

4h

70

W 2C(321)

3000

W 2C(200)

W 2C(002)

(b) 2400 °C

60

W 2C(221)

3500

50

W 2C(023)

40 W 2C(121)

30

2h 1000

W(211) WC(110)

WC(100)

1500

W(200)

3h

WC(101)

2000

W(110)

2500

Intensity (a.u.)

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

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1h 500

W 0 30

40

50

60

70

80

2 (degrees) 5 ACS Paragon Plus Environment

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Figure 6 0.5

-1

-5 × 10 °C .6 8 = e p slo

0.4

°C

-1

0.3

=

6.

5

×

10

-4

0.2

pe

0.1

s lo

(R-R0)/R0

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


0.0 0

200

400

600

800

1000

1200

T - T0 (/°C)



Ta (211)

SiC (311)

4 hours

SiC (220)

1400

Ta (200)

(a) 1200 °C

Ta5Si3 (211) Ta5Si3 (112)

1600

SiC (111) Ta5Si3 (210) Ta (110)

Figure 7

1200

Intensity (a.u.)

3 hours 1000

2 hours 800

600

1 hour 400

200

Ta

0

Intensity (a.u.)

TaC (222)

(112) (201)

2200 °C 2500

80

Ta (211) TaC (311)

70

Ta2C (103)

Ta (200)

60

TaC (220)

2400 °C 3000

50

Ta2C (102)

(b) 1 hour

(101) Ta (110) TaC (200)

3500

40 TaC (111) Ta2C (002)

30

2000 °C 1800 °C

2000

1500

1600 °C 1000

500

1400 °C Ta

0

Ta (211)

80

Ta (110)

1500

70

Ta2C (103)

60

Ta2C (110)

Ta2C (002)

Ta2C (100)

Ta2C (101)

(c) 2400 °C 2000

50

Ta (200)

40

Ta2C (102)

30

1000 TaC (111)

Intensity (a.u.)

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

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500

0 30

40

50

60

70

80

2 (degrees)


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Figure 8 0.8

-1

0.7

×1

0

-3

°C

0.6

=1

.2

0.5

pe

0.4

slo

(R-R0)/R0

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


-1

0.3

-4 × 1 0 °C slope = 1.1

0.2

0.1

0.0 0

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400

600

800

1000

1200

T - T0 (/°C)


Structural Changes in Tungsten and Tantalum Wires in Catalytic

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