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Deposition of molybdenum carbonitride thin films
from
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Hsin-Tien Chiu, Wen-Yu Ho, and Shiow-Huey Chuang
Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30050, Republic of China
(Received 6 December 1993; accepted 28 December 1993)
Mo(NBu')2(NHBut)2 is used as a single-source precursor to deposit thin films of cubic
phase molybdenum carbonitride, MoQNy (x: 0.2-0.55, y: 0.1-0.47), by chemical vapor deposition on silicon substrates. In general, the C/Mo ratios increased from 0.2 to 0.55 and the N/Mo ratios decreased from 0.47 to 0.1 with increasing the temperature of deposition from 773 to 923 K. Based on the elemental composition and the composition of the gas phase products, it is proposed that the carbon atoms were incorporated through /3-methyl activation of the ligands.
Transition metal carbides and nitrides are interest-ing materials with many useful physical and chemical properties.1 Many research results of growing thin films
of early transition metal carbides, nitrides, and carbo-nitrides from single-source precursors by chemical vapor deposition (CVD) have been published.2"7 Recently,
we have shown that tungsten nitride thin films can be prepared employing a bistertbutylimido complex of tungsten, W(NBut)2(NHBut)2.8 In this communication,
we wish to report our preliminary findings applying an analogous molybdenum complex, Mo(NBut)2(NHBu')2,
1, as a single-source precursor of CVD.
1, synthesized according to a literature route with slight modification,9 can be vaporized easily at 323 K
in vacuum. Deposition of thin films on silicon and glass substrates was performed in a low-pressure cold-wall reactor at 723-923 K and 5 - 2 5 Pa depending on the carrier gas flow rate. Initial attempts to grow thin films on silicon substrates were difficult, possibly due to lack of suitable nucleation sites. Thus, we applied hydrogen plasma to etch the substrates before the de-positions because our previous experience indicated that this procedure can assist thin film growth, probably by increasing nucleation sites on the surface.10 After this
process, grey metallic shining thin films with good ad-hesion to the substrates (Scotch tape test) were obtained. The surface morphology was smooth with very fine grains as shown by the scanning electron micrograph (SEM) in Fig. 1. Thin films with similar properties were grown on glass substrates without difficulty.
X-ray diffraction (XRD) studies, such as the one shown in Fig. 2, showed major C u Ka diffraction peaks
at angles 20 equal to 36.99-37.35°, 43.19-43.52°, 62.38-63.07°, and 74.74-75.47° for the films. These peaks, characteristic of (111), (200), (220), and (311) reflections of cubic structures, are close to the values of y - M o2C (37.77°, 43.69°, 63.39°, and 75.72°)
and y - M o2N (37.38°, 43.44°, 63.10°, and 75.72°).
031927 20KV K£0.0K 1.50um
FIG. 1. SEM photograph of a thin film deposited at 773 K.
High resolution x-ray photoelectron spectra (XPS) of a typical sample deposited at 773 K showed ma-jor signals assignable to the binding energies of the following electrons, Mo3rf5/2 and Mo3rf3/2 (228.4 and
231.6 eV), Mo3/,3/2 and Mo3pi/2 (394.4 and 412.0 eV),
Cls(282.6 eV), Nl s (397.0 eV), and Ols (530.2 eV), after
some of the contaminated surface layers were removed by Ar+ sputtering. Auger depth profiling of the films,
such as the one shown in Fig. 3, showed uniform dis-tributions of Mo, C, and N atoms within the films. O concentration was low except at the surface. The XPS and Auger data indicate that the films are molybdenum 1622 J. Mater. Res., Vol. 9, No. 7, Jul 1994 © 1994 Materials Research Society
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Communications
0.8
60 80 20 (degree)
FIG. 2. XRD pattern of a thin film deposited at 823 K.
carbonitride of a formula MoQNy. Bulk elemental
com-position of the films was determined by wavelength dispersive spectroscopy (WDS). C/Mo and N/Mo ratios of the films, derived from the WDS studies, are plot-ted against the temperature of deposition and shown in Figs. 4 and 5, respectively. Figure 4 shows that with in-creasing temperature of deposition from 773 to 923 K, the C/Mo ratios increased from 0.2 to 0.5 for the films grown without carrier gas. When either H2 or Ar was
em-ployed as the carrier gas, which increased the pressure within the reactor, the C/Mo ratios increased from 0.45 at 773 K, a relatively high value, to 0.55 at 923 K. As shown in Fig. 5, the N/Mo ratios decreased from 0.47 to 0.1 with increasing temperature of deposition from 773 to 923 K for the films deposited without carrier gas. When H2 or Ar was employed as the carrier gas, the
N/Mo ratios lowered from 0.35 at 773 K to 0.1 at 923 K. These data suggest that both temperature and pressure of deposition influenced the C/Mo and the N/Mo ratios significantly while using either H2 or Ar as the carrier
gas affected the elemental compositions only slightly.
I
o
o
Etch Time (min)
FIG. 3. Auger depth profile of a thin film deposited at 773 K.
o 0.4 0.2
-e
o i i o 1 __ ^ ^ o — e —- 8
no carrier gas Ar 10 seem H2 10 seem 773 8 7 3 973 T (K)FIG. 4. Effect of temperature of deposition on C/Mo ratio of the films.
Volatile products, analyzed by a residual gas ana-lyzer (RGA), were identified to be H2, CH4, HCN, N2,
CH3CN, and Me2C=CH2. To account for the origins of
these molecules, possible reaction pathways are proposed in Eqs. (1) and (2) below.
(1)
\ H CH3 -M0-N-C-CH3 \ CH3 -Mo=N-d-CH3 \ H CH3 -M0-N-C-CH3 CH3 \ CH3 -Mo=N-C-CH3 / CH3 7-H activation > /3-methyl activation > ^(surface) \ -M0-NH2 + Me2C=CH2 \ -Mo=NH \ -Mo +MeCN + HCN + ( / (2)Analogous reaction pathways have been proposed previously.8 In Eq. (1), y-hydrogen activation of the
0.8 0.6 o 0.4 0.2 O no carrier gas • Ar 10 seem O — H2 10 seem 773 873 973 T ( K )
FIG. 5. Effect of temperature of deposition on N/Mo ratio of the films.
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tertbutyl groups of both amido and imido ligands, fol-lowed by C—N bond dissociation, is suggested to be the origin of Me2C=CH2, and it appears to be unlikely
to lead carbon atoms into the films through this pathway. On the other hand, as suggested in Eq. (2), MeCN and HCN are formed via /3-methyl activation of the tertbutyl groups by successively removing two and three methyl groups, respectively, followed by M o - N bond dissocia-tion. These methyl groups may either combine hydrogen atoms on the surface to cause the evolution of CH4 or
un-dergo further C - H bond activation to form the carbidic atoms in the films. As shown in Fig. 6, lM+/lMe2c=CH2 >
the ratios of the intensities of the ions formed in RGA from an evolved compound M and Me2C = CH2 are
relatively constant for M = MeCN and HCN between 773 and 923 K, the temperature range employed for the deposition. This suggests that the amounts of the tertbutyl groups decomposed via Eq. (1) have an almost constant proportion to those decomposed via Eq. (2) between 773 and 923 K. Thus, it is reasonable to as-sume qualitatively that lMe2c=CH2+ is proportional to
the amount of 1 decomposed at these temperatures. Consequently, the value of lM+/lMe2c=CH2 m Fig- 6
can be correlated to the amount of M evolved versus the amount of 1 decomposed at a given temperature. As shown in Fig. 6, lH2+/lMe2c=CH2 increases with
increasing temperature of deposition, suggesting that the films deposited below 773 K still contain some H atoms. When the temperature rose, these atoms combined to form hydrogen molecules. Thin films of metal nitrides containing H atoms have been grown by CVD from M(NR2)4 (M: Ti, Zr, Hf; R: Me, Et) and NH3 at
low temperatures.11 On the other hand, with increasing
temperature of deposition, IcH4+/lMe2c=cH2 decreases.
CM I CJ II o CM 673 773 873 9 7 3 T(K)
FIG. 6. Effect of temperature of decomposition on lM+/lMe2c-CH2
ratios. For each M, the ratio at 673 K is normalized to 1. Each scale on the vertical axis is 0.5.
This trend correlates well with the variation of the C/Mo ratios with the temperature of deposition and supports the carbidic atom formation process proposed above. At a high pressure of deposition, removal of the methyl groups as methane molecules from the surface was hampered because diffusion of the gas was difficult. This rationalizes why the C/Mo ratios were high when either H2 or Ar was employed as the carrier gas. It has
been shown that molybdenum carbide can be prepared by reacting molybdenum nitride with methane at tem-peratures above 670 K, along with nitrogen evolution.12
Compared to other early transition metal nitrides such as niobium nitride, molybdenum nitride is relatively unstable at high temperatures.13 Complete decomposition
of molybdenum nitride can be achieved below 1073 K. This explains why lN2+/lMe2c=CH2 increases (Fig. 6),
although not significantly, and N/Mo decreases with increasing temperatures of deposition.
This study showed that 1 can be employed as a single-source precursor to molybdenum carbonitride thin films. CVD of thin films from other imido complexes of early transition metals is under investigation.
ACKNOWLEDGMENTS
We thank the National Science Council of Taiwan, the Republic of China (NSC-83-0208-M-009-033) for support and the Instrument Center of the NSC for sample analyses.
REFERENCES
1. L. E. Toth, Transition Metal Carbides and Nitrides (Academic Press, New York, 1971), and references therein.
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