Plasma CVD of hydrogenated boron-carbon thin films from triethylboron

Low-temperature chemical vapor deposition (CVD) of B-C thin films is of importance for neutron voltaics and semiconductor technology. The highly reactive trialkylboranes, with alkyl groups of 1-4 carbon atoms, are a class of precursors that have been less explored for low-temperature CVD of B-C films. Herein, we demonstrate plasma CVD of B-C thin films using triethylboron (TEB) as a single source precursor in an Ar plasma. We show that the film density and B/C ratio increases with increasing plasma power, reaching a density of 2.20 g/cm3 and B/C = 1.7. This is attributed to a more intense energetic bombardment during deposition and more complete dissociation of the TEB molecule in the plasma at higher plasma power. The hydrogen content in the films ranges between 14 and 20 at. %. Optical emission spectroscopy of the plasma shows that BH, CH, C2, and H are the optically active plasma species from TEB. We suggest a plasma chemical model based on β-hydrogen elimination of C2H4 to form BH3, in which BH3 and C2H4 are then dehydrogenated to form BH and C2H2. Furthermore, C2H2 decomposes in the plasma to produce C2 and CH, which together with BH and possibly BH3-x(C2H5)x are the film forming species.


I. INTRODUCTION
2][3] Thermal chemical vapor deposition (CVD) methods often employ temperatures above 1000 • C for B− −C film deposition using boron trichloride (BCl 3 ) or diborane (B 2 H 6 ) as B precursors and methane (CH 4 ) as carbon precursors. 4By contrast, plasma CVD methods employ energetic plasma species to decompose precursor molecules, which lead to deposition temperatures typically below 400 • C. Plasma CVD is therefore a very attractive technique for depositing boron carbide thin films for microelectronics and neutron detectors based on neutron transparent aluminum substrates.Plasma CVD of boron carbides has been studied using boranes, such as diborane (B 2 H 6 ), pentaborane (B 5 H 9 ), decaborane (B 10 H 14 ), and methane (CH 4 ) as the carbon precursor. 5,6Carborane (C 2 B 10 H 12 ) has been used as a single source precursor for B x C in plasma CVD. 7n alternative, less explored class of boron precursors are alkylboranes.Trimethylboron B(CH 3 ) 3 (TMB), triethylboron B(C 2 H 5 ) 3 (TEB), and tributylboron B(C 4 H 9 ) 3 (TBB) were studied in thermal CVD by Lewis et al. in a pioneering paper. 8They showed that TEB gave the highest B/C ratios of up to 1.6.Our studies of TEB in thermal CVD a) Present address: IHI Ionbond AG, Industriestraße 211, CH-4600 Olten, Switzerland.b) Author to whom correspondence should be addressed: henrik.pedersen@liu.se rendered amorphous films with B/C of 4 and very low hydrogen content. 9,10We also proposed a gas phase chemical model for TEB decomposition based on β-hydrogen elimination of C 2 H 4 with H 2 assisted elimination of C 2 H 6 as a complementary route at low temperatures. 10TMB and TEB have also been tested for boronization of fusion tokamaks by deposition of amorphous, hydrogenated boron-carbon films (a-C/B:H) in H 2 − −He plasmas. 11However, the B/C ratios of these films varied between 0.2 and 0.6.We have recently demonstrated that TMB can also be used as a precursor for hydrogenated B x C thin films in plasma CVD with an Ar plasma affording films with B/C ratios of 0.4-1.9 and 10-20 at.% hydrogen. 12otivated by the high B/C ratios obtained in films from TEB in thermal CVD and the thermodynamically accessible possibility to split off the alkyl ligands by β-hydrogen elimination, we studied B− −C film deposition from TEB in CVD with an Ar plasma.Based on the characterization of the deposited films and the optical emission from the plasma, we suggest a plasma chemical model for TEB decomposition.

A. Film deposition
An in-house modified ASTEX microwave plasma CVD system, equipped with a 2500 W power supply, was used for film deposition.The inner diameter of the chamber was 14 cm and the diameter of the graphite sample holder, which is neither heated, biased, nor grounded, was 12 cm.The base pressure in the deposition chamber was 10 5 mbar, achieved by a turbomolecular pump.During film deposition, a dry rotary pump was used to pump the process gases and the pressure was controlled either by throttling the pump or by adjusting the gas flow through the CVD system.For all experiments, the plasma power was defined as the set value of power delivered to the microwave generator with the readout value of the reflected power subtracted.The reflected power was minimized by adjusting three tuning stubs on the wave guide.Further details on the plasma CVD system are given in Ref. 12.
Full 100 mm Si (100) wafers were used as substrates for the film deposition.Prior to loading, the wafers were ultrasonically cleaned, first in acetone and then in isopropanol, and finally blown dry with dry nitrogen.Prior to film deposition, the chamber was out-gassed by igniting an Ar plasma, using 99.9997% pure Ar that was further purified with an SAES Pure Gas filter, at a low Ar flow (20 SCCM) and high power (2400 W) for 15 min.Film deposition was started by adjusting the Ar flow and the plasma power to the desired values and by introducing TEB into the plasma.All depositions lasted for 1 h.The temperature of the substrates was estimated by a thermocouple that was mounted underneath the graphite sample holder.After deposition, the samples were cooled down for a few hours prior to unloading and subsequent air exposure.
Following our previous experiments with TMB, 12 which is a condensed gas at room temperature, the experiments with TEB, which is a liquid at room temperature, required installation of a bubbler system as shown in Fig. 1.The TEB bubbler was kept in a thermostat bath with a temperature of 26.4 ± 0.4 • C. Using the vapor pressure equation for TEB: log 10 (Bar) = 2.914 08-(753.261/(T(K)-112.631)), 13this temperature yields a TEB vapor pressure of 76 mbar.An Ar flow of 40 SCCM is used as carrier gas for the TEB vapor.The total pressure in the bubbler was controlled by an electronic pressure controller (EPC) and set to 500 and 700 mbar when the TEB flow was set to 7 and 5 SCCM, respectively.The TEB vapor flow was calculated using the following equation: where P vap.TEB is the vapor pressure of TEB, controlled by the temperature of the TEB liquid.P total is the total pressure in the bubbler, controlled by the pressure on EPC, and Flow TEB and Flow Ar carrier are the gas flows of TEB and Ar carrier, respectively.
In our previous plasma CVD study with TMB, 12 low TMB flows (<5 SCCM) and plasma powers (<700 W) resulted in carbon-rich and porous films.Thus, film depositions were carried out with TEB flows of 5 SCCM and 7 SCCM at plasma powers of 700 W, 1500 W, and 2400 W. The total pressure during deposition process was kept at 0.3 ± 0.05 mbar.The slight variations of the total pressure are due to the differences in TEB flow and applied plasma powers.

B. Film and plasma characterization
All deposited films were characterized by scanning electron microscopy (SEM) for morphological analysis and film thickness determination using a LEO 1550 Gemini SEM equipped with a field emission gun.Film composition was studied using Time-of-Flight Elastic Recoil Detection Analysis (ToF-ERDA) using 36 MeV iodine ions.More experimental details on the ToF-ERDA can be found elsewhere. 14,15he chemical composition and bonding states of the films were investigated by X-ray photoelectron spectroscopy (XPS, Axis UltraDLD, Kratos Analytical, Manchester, UK) using monochromatic Al (K α ) X-ray radiation (hν = 1486.6eV).XPS survey spectra and core level spectra of the B 1s, Ar 2p, C 1s, and O 1s regions were recorded on samples before and after sputter cleaning with a 500 eV Ar + beam, except for few samples where a 2 kV Ar + beam was used.The 500 eV Ar + etch was replaced by the 2 keV etch in order to reduce etching time and improve the removal of the surface oxide whilst keeping the B− −C bond structure preserved.Automatic charge compensation was applied when samples were overcharged.The core level spectra were deconvoluted using a Voigt peak shape with a Lorentzian contribution of 30% to assess the bonding configuration of the B x C films.X-ray reflectivity (XRR) was carried out for density measurements.For that, an ω/2θ scan was recorded for all samples by using Cu K α radiation and hybrid mirror optics with a parallel plate collimator on a Philips X'Pert Pro MRD diffractometer.To determine the film densities, the experimental data were fitted using the X'pert reflectivity software.The plasma composition was diagnosed by optical emission spectroscopy (OES), using a Mechelle Sensicam 900 spectrometer and with a spectral resolution (λ/∆λ) FWHM of 900 by Multichannel Instruments.Emission spectra of the plasma during the film deposition process were recorded in a wavelength range from 200 to 1100 nm.

A. Film morphology and composition
All deposited films exhibit, to the naked eye, visible uniaxial optical interference patterns due to thickness variations from the center towards the edges on the 100 mm wafer, as discussed in our previous study. 12This is likely due to the gas flow pattern in the deposition chamber.Since film uniformity is not the focus of this study, only the center area of the wafer has been further characterized.Cross-sectional SEM micrographs of films deposited with 5 and 7 SCCM TEB flows at different plasma powers are shown in Fig. 2. Since all films were deposited for 1 h, the thicknesses of the films correspond directly to variations in deposition rates.A slight increase in the rates was seen with increasing the plasma power from 700 W to 1500 W, followed by a decrease at 2400 W. XRR film density measurements show that the film density is increased from 1.55 ± 0.1 g/cm 3 to 2.20 ± 0.05 g/cm 3 when the plasma power increased from 700 W to 2400 W. This agrees with the change in film morphology from a smooth and porous to a dense and columnar structure with higher plasma power seen in Fig. 2.
Figure 3 shows the deposition rate for both 5 and 7 SCCM TEB flows which follows the same dependence on the plasma power.The deposition rates for the higher TEB flow are up to 2 times higher than those for the lower TEB flow.This study does not seek to maximize the deposition rate, and a higher deposition rate could likely be obtained in a fully optimized plasma CVD system.
The ToF-ERDA analysis reveals that all deposited films contain boron, carbon, and hydrogen.Some oxygen is found in the films and is believed to be due to surface oxidation from post-deposition air exposure.In Figs. 4 and  compositions and calculated B/C ratios are plotted for different plasma powers for films deposited with 5 and 7 SCCM TEB, respectively.The boron content increases with increasing plasma power, while the carbon content is not affected by the plasma power.This leads to a higher B/C ratio with higher plasma power.The B/C ratio is also higher for the films deposited with a lower TEB flow at the lowest plasma power studied.The amount of H incorporation in the films is found to be dependent on both the plasma power and TEB flow.The highest H content of ∼20 at.% (B/H of 2.4) is observed in films deposited at 1500 W for both high and low TEB flows.Films deposited with the lowest plasma power of 700 W have the highest oxygen content of 10-16 at.%.An increase in the plasma power to 1500 W reduces the oxygen content to ∼0.5 at.%.No oxygen was detected in films deposited with the higher TEB flow at 2400 W plasma power, while films deposited with the lower TEB flow at 2400 W contain ∼3 at.% oxygen.

B. Chemical structure of deposited films
XPS core level spectra of all deposited films were obtained after sputter cleaning with 500 eV Ar + ions.The B 1s, C 1s, and O 1s core level spectra for films deposited with 5 SCCM TEB flow are shown in Figs.6(a 16 while a very small peak at 190.9 ± 0.2 eV is assigned to B− −O bonding.This is assumed to be due to surface oxidation from post-deposition air exposure.This assumption is supported by XPS; the oxygen content of the films decreases significantly after sputter cleaning, except for the low-density films deposited at the lowest plasma power studied.The oxygen content was not affected by the sputter cleaning of low-density films.This oxygen depth profile is also seen in the ERDA measurements.We interpret this as low-density films are porous, which allows air to penetrate and oxidize deeper into the films.
The C 1s spectra [Figs.

C. Plasma characterization
The OES spectra of the Ar plasma with TEB at different plasma powers during film deposition are shown in Fig. 7.The gas flow ratio of Ar/TEB in the plasma gas mixture is 40/7.The plasma mainly consists of emission lines of excited Ar atoms [Fig.7(a)], but it should be noted that the intensity of FIG. 7. OES spectra of the plasma, the full range scan (a) showing mainly emission lines from excited Ar atoms while a zoom at the 375-675 nm range (b) shows emission lines related to the TEB decomposition products.Note the different intensity scales between (a) and (b). the H α line, where the hydrogen is assumed to emanate from the TEB molecule, is comparable with the most intensive Ar lines.The plasma power has strong effect on the intensities of the emission lines throughout the whole spectral range but not on the plasma composition as all emission lines are detected at all plasma powers with similar relative intensities.The emission lines related to TEB decomposition, H, C 2 , BH, and CH [Fig.7(b)], are the same emission lines that were found for TMB decomposition in plasma CVD with an Ar plasma, 12 except that only a single emission line from molecular H 2 at ∼603 nm is observed for TEB.

IV. DISCUSSION
An increase in plasma power is expected to yield a higher concentration of energetic species in the plasma.This should increase the dissociation of TEB, resulting in a higher concentration of film forming species and a deposition rate.Conversely, a higher concentration of energetic plasma species also increases the energetic bombardment of the film and makes the film denser, leading to an observed decrease in the deposition rate with higher plasma power (Fig. 3).Such a densification is also seen in the XRR film density measurements.It should be noted that the substrate holder has a floating potential, as it is neither biased nor grounded.This leads to a potential drop over the plasma sheath at the substrate holder and accelerates the charged species towards the film surface.It is, however, expected that most of the energetic bombardment is by neutral species in the plasma as the substrate holder is not intentionally biased.
The plasma power is also found to affect the hydrogen content in the films.Films deposited with 1500 W plasma power contain 20 at.% H, while films deposited with 2400 W plasma power contain 14-15 at.% H.This is ascribed to an enhanced energetic bombardment of the film during deposition at higher plasma power and leads to desorption of the adsorbed hydrogen.Since a solid-state neutron detector based on B x C films detects neutrons by nuclear reactions with 10 B, it is of importance to maximize the amount of boron in the films.All other atoms are unwanted, especially hydrogen, which is a powerful neutron scatterer.However, 20 at.% carbon "impurity" is accepted since it then forms B 4 C and makes the material more stable.While the amount of hydrogen in the deposited films does not meet the demands for neutron detectors, the hydrogen content has been shown to influence many material properties of B x C:H films in semiconductor applications. 18The reduction in oxygen content with increasing plasma power is ascribed to the increased film density with higher plasma power, which leads to decreased oxidation during post-deposition air exposure.
The lower B/C ratio for films deposited with a higher TEB flow, which is most pronounced for the lowest plasma power, is likely due to a higher consumption of plasma energy for the initial decomposition of the increased TEB flow.This leads to less energy available for further decomposition and energetic surface bombardment.This, in combination with the carbon content not being affected by a higher plasma power in Figs. 4 and 5, indicates that the formation of boron containing film forming species with a low amount of carbon requires more plasma energy than the formation of carbon containing film forming species.The hydrogen content is also slightly higher for films deposited with higher TEB flow, which further indicates that there is a slight difference in plasma chemistry for higher TEB flows.The increased flow consumes more plasma energy and leads to a lower degree of energetic surface bombardment during deposition and thus to a lower degree of hydrogen desorption from the film surface.At the highest plasma power studied, the plasma energy is sufficient to reach the same plasma chemistry for both TEB flows; the B/C ratio in films deposited at 2400 W plasma power is the same regardless of TEB flows.The B/C ratio in these films is 1.7, which is comparable to the highest B/C ratio measured in films deposited from TMB by plasma CVD. 12 In thermal CVD of B x C films using TEB diluted in Ar, the main decomposition path is β-hydride elimination of C 2 H 4 forming mainly BH(C 2 H 5 ) 2 and some BH 2 C 2 H 5 and BH 3 . 10owever, these molecules have a large number of vibrational modes meaning that they will lose excitation energy by vibrations instead of photon emission, making them not detectable by OES.The OES spectra of the plasma [Fig.7 (b)] suggest that in the Ar plasma TEB is further decomposed to H, C 2 (dicarbon, C= =C), BH, and CH.These are the same products identified in the decomposition of TMB by an Ar plasma. 12H, C 2 , CH, and BH are also the optically active species detected in hydrogen plasmas with small amounts of added CH 4 and B 2 H 6 . 19TMB decomposition was suggested to start by dehydrogenation of the methyl groups by an energetic plasma species or thermal energy in the plasma, followed by breakage of the B− −C bonds to form • CH radicals where at least one B− −C is broken by a hydrogen radical to form BH. BH is a singlet in its ground state with an excited triplet state ca.1.3 eV higher in energy. 20Here we write BH without specifying a singlet or triplet state.C 2 was suggested to form either by combination of two • CH radicals or by further dehydrogenation and combination of • CH radicals to atomic carbon.Given how accessible the first β-hydride elimination of C 2 H 4 from TEB is in a hot, non-ionized gas (∆G = 5.4 kJ mol 1 at 500 • C and 0.05 atm 10 ), it seems likely that this would also be a first step of TEB decomposition in a plasma.We suggest that the TEB molecule gets enough thermal energy from the plasma to undergo β-hydrogen elimination without breaking other bonds, reaction (2), to give C 2 H 4 .The next two β-hydride eliminations to render BH 2 C 2 H 5 , reaction (3), and then BH 3 , reaction (4), are less available with ∆G = +12.3and +16.5 kJ mol 1 , respectively, at 500 • C and 0.05 atm. 10 Out of the additional unfavorable reactions that are possible, such as alkane and alkene eliminations, other than those via the β-hydride mechanism, radical cleavage reactions, and H 2 eliminations, all with ∆G in the order of +200 kJ mol 1 at 500 • C and 0.05 atm, 10 the second and third β-eliminations, reactions (3) and ( 4), are the most likely paths to eliminate all B− −C bonds, FIG. 8. Schematic summary of the suggested plasma chemical decomposition mechanism for TEB in an Ar plasma.A * denotes an energetic plasma species.
BH 3 is then likely dehydrogenated by hydrogen radicals, which are abundant in the plasma, Fig. 7(b), to form BH, in the following reactions: The stable byproduct H 2 should make reactions ( 5) and ( 6) favored over direct radical cleavage to produce hydrogen radicals, as these reactions have ∆G on the order of 270-300 kJ mol 1 at 500 • C and 0.05 atm. 10 As seen from reactions ( 2)-( 4), we suggest that carbon is added to the plasma in the form of C 2 H 4 , which will break down to H, C 2 , and CH [Fig.7(b)].The same species are also optically detected when C 2 H 4 is mixed with oxygen in a flame. 21Given the high intensity of atomic hydrogen detected in the OES spectra, we propose that C 2 H 4 is dehydrogenated to C 2 H 2 , reaction (7), which is also supported by studies on C 2 H 4 plasmas, 22 C where A * is an energetic plasma species.Acetylene can then form both CH and C 2 via several reactions as described in Ref. 23.The suggested mechanism is summarized in Fig. 8.This suggested plasma chemical model accounts for the intensive H α line by the released atomic hydrogen from mainly BH 3 and C 2 H 4 , reactions ( 5)-( 7), respectively.BH, CH, and to some extent C 2 are suggested as film forming species.4][25][26] Such hydrocarbons will lose excitation energy by vibrations and are therefore not optically active or detected in this study.The fact that the hydrogen content in the deposited films decreases with increasing plasma power could be an indication that such C n H m species are active in film deposition and that their decomposition to C 2 and CH is enhanced at higher plasma powers.As the relative carbon content is not significantly affected by the plasma power (Figs. 4  and 5), it can be assumed that the β-hydrogen elimination of the first ethyl group, reaction (2), is accessible at low plasma powers leading to the formation of CH and C 2 .The increase in relative B content with higher plasma power (Figs. 4 and 5) suggests that reactions (3)-( 6), producing BH, require more energy from the plasma.However, the possibility of carbon etching by energetic plasma species as a complementary process to lower the relative amount of carbon in the films cannot be excluded.

V. CONCLUSION
Plasma CVD of B− −C thin films using triethylboron (TEB) as a single source precursor is demonstrated.The density of the films increases with increasing plasma power, which is attributed to a higher energetic bombardment during deposition, with the highest density measured to be 2.20 g/cm 3 .The boron content of the films increases with plasma power, while the carbon content in the films is not affected by the plasma power.The highest B/C ratio achieved is 1.7.The hydrogen content in the films ranges between 14 and 20 at.%.The oxygen content in the film is reduced to <1 at.% with increasing density and plasma power, rendering denser films and lower post-deposition oxidation.OES analysis shows that the TEB molecule is dissociated to BH, CH, C 2 , and H in the Ar plasma.Only the intensity of the emission lines increases with plasma power, not the number of lines detected.Based on the film composition and OES data, we suggest a plasma chemical model where the ethyl groups are split off by β-hydrogen elimination as C 2 H 4 rendering BH 3 , and these species are further dehydrogenated to BH and C 2 H 2 , which forms C 2 and CH.

ACKNOWLEDGMENTS
Financial support from European Spallation Source ERIC and the Knut and Alice Wallenberg Foundation is gratefully acknowledged.The authors would also like to acknowledge the Tandem Laboratory at Uppsala University for giving access to their ion beam facilities.The authors are also very grateful to the anonymous reviewers whose comments on the manuscript helped improving it significantly.Richard Hall-Wilton and Susan Schmidt would like to acknowledge the support of EU BrightnESS project, Grant No. 676548.Ralf Tonner is acknowledged for fruitful discussions.Nathan O'Brien is gratefully acknowledged for critically reading the manuscript.

FIG. 1 .
FIG.1.Schematics of the TEB bubbler system added to the plasma CVD system.The flow of Ar to the plasma is controlled by MFC (mass flow controller) 1, and the Ar flow into the TEB bubbler is controlled by MFC2.The total pressure in the bubbler is controlled by the EPC (electronic pressure controller).
FIG.3.Deposition rates of films deposited at 5 SCCM and 7 SCCM TEB flows are plotted as a function of plasma power.
FIG. 5. Composition of B x C films (with the calculated B/C and B/H ratios) as determined by ToF-ERDA measurements for films deposited with 7 SCCM TEB at different plasma powers.
6(b) and 6(e)] show a broad peak featuring a shoulder at the high binding energy side.Three components are fitted and assigned as C− −B and C− −C bonds at 282.9 eV and 284.7 eV ± 0.2 eV, respectively, and the C− −C− −B moiety at 283.7 eV.The presence of C− −C bonds indicates the formation of amorphous carbon in the films.However, the contribution is lowered at a TEB flow of 7 SCCM.The O 1s core level spectra [Figs.6(c) and 6(f)] show two components at 531.5 eV and 533.1 eV and are assigned as B= =O and C− −OH/C− −O− −C bonds, respectively.17