J. Phys. Chem. B 2000, 104, 4397-4402
4397
Kinetic Study of the Diborane/Methylamine Reaction: Composition and Structure of C-B-N Films C. Go´ mez-Aleixandre,*,† A. Essafti,‡ and J. M. Albella† Instituto de Ciencia de Materiales, CSIC, Cantoblanco, 28049 Madrid, Spain, and De´ partement de Physique, Faculte´ des Sciences Semlalia, B.P. S15, UniVersite´ Cadi Ayyad, Marrakech, Morocco ReceiVed: August 23, 1999; In Final Form: December 3, 1999
C-B-N films have been deposited by chemical vapor deposition from methylamine (CH3-NH2) and diborane (B2H6) gas mixtures. For methylamine-rich mixtures (r ) [CH3-NH2]/[B2H6] ) 6), the deposition reaction is thermally activated, showing a low apparent activation energy (Ea ) 7.1 kcal/mol). In gas mixtures with lower methylamine flow rates (r ) 1), the reaction is also thermally activated in the 350-550 °C range, the apparent activation energy being slightly higher (16 kcal/mol). In the last case, for temperatures higher than 550 °C the deposition rate decreases at increasing temperatures. At these temperatures, the methylamine is decomposed to methane, ammonia, hydrogen, and hydrogen cyanide, thus changing the deposition reaction. In this temperature range (>550 °C), increasing content of methylamine in the gas mixture produces initially an increase in the deposition rate to a maximum followed by the subsequent decrease for the higher methylamine content. The composition of the deposited films has been analyzed by infrared spectroscopy (IR) and surface analytical techniques (Auger electron spectroscopy and X-ray photoelectron spectroscopy). Graphite carbon is always present in the deposited films. Metallic boron, also present in the films, decreases in the 350-800 °C range, favoring the formation of the ternary CBN compound. At temperatures higher than 800 °C, the deposited films consist mainly of a mixture of the CNB ternary material (note a new sequence of atoms in the ternary compound), in addition to boron nitride and graphite carbon. The results of the variation of the deposition rate have been explained by two simultaneous processes: (i) the chemical reactions for the C-B-N film formation, which are enhanced by the increase in the reactant concentrations as well in the deposition temperature and (ii) the parallel adsorption on the substrate surface of methane molecules, coming from the methylamine decomposition, which can disturb the deposition process.
I. Introduction C-B-N materials include numerous compounds with variable amounts of carbon, boron, and nitrogen. The addition of boron and nitrogen to the carbon is supposed to increase the otherwise low thermal stability of carbon at high temperatures. At present, there is a growing interest in the synthesis of amorphous C-B-N compounds because of the possibility of adjusting the electrical, mechanical, and optical properties of these systems by varying the C/B/N ratio. Different deposition methods (namely, physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods) have been employed to deposit this material, though PVD has failed in the preparation of C-B-N coatings with high nitrogen content. For this reason, CVD methods are being explored in order to obtain films with appropriate composition. The composition and structure of the material strongly depend on the preparation method, particularly on the corresponding precursor compound used for the deposition process. A massive deposition of the ternary C-B-N material can be obtained by pyrolitic decomposition of different precursors containing carbon, boron, and nitrogen atoms in their structure.1-3 In these processes, the chemical bonding present in the precursor compound determines the structure of the resultant C-B-N material. On the other hand, Montasser et al.4 deposited † ‡
CSIC, Cantoblanco. Universite´ Cadi Ayyad.
amorphous C-B-N:H films by plasma CVD using diborane diluted in nitrogen and ethane in the gas mixture and reported the high transparency and hardness of the films as well as their utility as mask substrates for X-ray lithography. Some attempts have been also made for using other vapor sources,5 some of them with the C-N bond included in the molecular structure, as methylamine (CH3-NH2).6 The ultimate goal in this research is the preparation of new C-B-N materials with a well-defined composition and structure, thus resulting in films with high chemical stability and good mechanical and electrical properties. In this work, C-B-N films have been deposited by lowpressure CVD from diborane and methylamine gas mixtures in order to investigate the reaction mechanisms during the formation of the film. The reaction paths of B2H6 + CH3-NH2 have been followed by studying the influence of the temperature and gas mixture composition on the deposition process and on the structure of the deposited films. Surface and bulk techniques have been used to characterize the properties of the deposits, namely, infrared spectroscopy, Auger electron spectroscopy (AES), and X-ray photoelectron (XPS) spectroscopy. II. Experimental Section Carbon-boron-nitrogen (C-B-N) films were deposited by low-pressure chemical vapor deposition (LPCVD) from methylamine and diborane (5% in H2) gas mixtures (r ) [CH3NH2]/[B2H6] ) 0.5-6) in a hot wall reactor at 267 Pa and different temperatures (350-950 °C). The samples were de-
10.1021/jp9929723 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/14/2000
4398 J. Phys. Chem. B, Vol. 104, No. 18, 2000
Go´mez-Aleixandre et al.
Figure 2. Infrared spectra of C-B-N films obtained at different temperatures (r ) 1). Figure 1. Arrhenius plot with two gas reactant flow ratios for the CH3-NH2 + B2H6 reaction.
posited onto polished (100)-oriented silicon substrates. Before the deposition, the substrates were cleaned, etched in a 10% HF solution, rinsed in deionized water, and dried by N2 to remove the native oxide layer of the surface. The samples were analyzed by using transmission infrared spectroscopy (IRS) and surface analysis techniques (Auger and X-ray photoelectron spectroscopies). The IRS spectra were recorded at room temperature by a Hitachi 270-50 infrared spectrophotometer in the 4000-400 cm-1 range with a resolution of 2 cm-1. The Auger spectra were taken in a 1.3 × 10-8 Pa residual atmosphere using an electron beam of 3 kV and 1 µA. Argon ions accelerated at 3 keV were used in the Auger depth profiles analysis for analyzing the film homogeneity. The composition obtained by AES was calibrated using the XPS results. The XPS spectra were obtained by means of a Fisons Escalab MK II instrument using unmonochromated Mg KR radiation (hν ) 1253.6 eV) as the X-ray excitation source at a pressure of ∼10-7 Pa in the analyzer. The O 1s, C 1s, B 1s, N 1s, and Si 2p regions were studied in an interval of 10 eV. The core level binding energies were calibrated to the C 1s line at 285 eV due to residual hydrocarbon. Also a profilometer Dektak 3030 was used for thickness measurements of the deposited films. III. Experimental Results III.1. Effect of Temperature on the Deposition Rate. Initially, CVD carbon-boron-nitrogen films were deposited from methylamine and diborane gas mixtures at two different flow ratios ([CH3-NH2]/[B2H6] ) 1 and 6) and a total pressure of 267 Pa. Figure 1 shows the variation of the growth rate for both ratios in an Arrhenius plot in the 350-950 °C temperature range. As can be observed, the variation of the deposition rate with the temperature strongly depends on the methylamine/ diborane ratio. For high methylamine flows (r ) 6), the deposition rate follows an activated behavior with a low apparent activation energy of 7.1 kcal/mol in the range 350-850 °C. A slightly higher activation energy of 16 kcal/mol is obtained for r ) 1 in the low-temperature range, which is followed by a decrease in the deposition rate when the temperature reaches values above 550 °C. The change in the variation of the deposition rate with temperature may be associated with the decomposition of methylamine molecules,10 which would produce a different deposition mechanism for temperatures above 550 °C. The low value of the activation energy in both cases suggests that a complex reaction mechanism takes place during the film deposition.
Figure 3. Infrared spectra of C-B-N films obtained at different temperatures (r ) 6).
Film Characterization. IR spectra of the samples deposited at different temperatures are given in Figures 2 and 3 for r ) 1 and r ) 6, respectively. As can be observed in both figures, the spectra of the samples deposited for both flow ratios in the 450-850 °C present a broad band centered at ∼1315 cm-1 (width 290-240 cm-1). In previous studies,7 we have shown that the spectra of boron nitride films are characterized by a high-intensity band centered at 1372 cm-1 with a width depending on the crystallographic phase (80 and 230 cm-1 for the hexagonal and amorphous phases, respectively). Therefore, the analyzed samples may correspond to boron nitride films containing carbon atoms incorporated in their structure, which either alter the BN bond strength or form other types of bond with similar vibration energy. For both flow ratios, the band becomes narrower and also shifts toward higher wavenumbers when the deposition temperature increases. This reduction in the bandwidth from 290 to 170 cm-1 can be due either to a more ordered structure in the deposited material or to a lower content of carbon atoms joined to the BN bonds as first neighbors. In the spectra corresponding to the films deposited at temperatures lower than 550 °C, besides the weak peak at 2500 cm-1 associated with the B-H vibration, two new bands can be also appreciated at 3300 and 2900 cm-1, indicating that N-H and C-H bonds are also present in the network. It should be noted that surprisingly both bands are not clearly enough detected in the samples deposited with a higher methylamine content. This fact has been explained by the lower thickness in the samples obtained under these conditions, the total N-H and C-H content thus being close to the infrared detection level. The presence of NH and CH radicals in the films at low temperatures supports the direct participation of methylamine molecules in this temperature range. The intensity of the bands corresponding to the hydrogenated radicals (BH, CH, and NH) diminishes at increasing temperatures, which has been already reported by other authors.8 In particular, in the C-B-N films deposited with r ) 6 at T g 650 °C, the BH content is kept below the infrared detection level.
Diborane/Methylamine Reaction
J. Phys. Chem. B, Vol. 104, No. 18, 2000 4399
TABLE 1: Composition of C-B-N Films Obtained at Different Temperatures ([CH3-NH2]/[B2H6] ) 6) T (°C)
%C
%N
%B
350 450 550 650 750 800 850 950
20.3 19.8 23.3 25.0 24.2 28.1 29.7 33.7
13.0 14.5 17.4 19.5 19.7 22.8 25.0 28.7
66.7 65.7 59.3 55.5 56.1 49.1 45.3 37.6
TABLE 2: XPS Peak Positions and Assignments for C-B-N Films spectrum
peak position (eV)
assignment (marked in bold)
C 1s
283.0 285.0 286.3 188.0 189.5 190.5 191.5 398.0 401.1
C-B-N C-C C-N and C-N-B B-B C-B-N B-N C-N-B N-B N-C
B 1s
N 1s
Auger spectroscopic results indicate a high homogeneity level in the sample composition along the deposited thickness. The composition and structure of the C-B-N films, deposited with r ) 6 in the overall temperature range, have been also analyzed by XPS. The results indicate that as the deposition temperature increases, the films are enriched in carbon and nitrogen (see Table 1), with a higher graphite C percentage in the films. This enrichment of carbon and nitrogen has been attributed to the higher reactivity of methylamine with diborane at increasing temperatures. Also these results indicate that the use of reactant gases containing carbon-nitrogen bonds allows us to obtain CBN films with high nitrogen content, compared to the films deposited by sputtering techniques. This fact has been widely discussed in the literature9 and is usually associated with the difficulties in the formation of a strong directional bond from the isolated atoms by physical methods. In this work, the bonding type among carbon, nitrogen, and boron atoms forming the films has also been determined from the analysis of the XPS spectra of the compounds obtained at different temperatures. Table 2 shows the position of the peaks in the XPS spectra of the samples (C 1s, B 1s, and N 1s), as well as the corresponding bond assignment.10 The variation of the C-B-N composition and structure with the deposition temperature can be seen in Figure 4, where the contributions to the total band have been resolved by deconvolution of the C 1s, B 1s, and N 1s spectra. At low temperatures (
