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Structural behavior of direct-current sputtered and thermally evaporated molybdenum thin films
150
Thin Solid Films, 249 (1994) 150-154
Structural behavior of direct-current sputtered and thermally evaporated molybdenum thin films S. Kacim Departement de Chimie, Facultk des Sciences, Semlalia Bd Prince My Abdellah BP., S 15 40001 Marrakech, Morocco
P. Delcambe, L. Binst, M. Jardinier-Offergeld and F. Bouillon Laboratoire de Chimie Analytique et Minbrale, Facult~ des Sciences C.P. 160, Universit~ Libre de Bruxelles, 50 Av F.D. Roosevelt, 1050 Bruxelles, Belgium (Received January 17, 1994; accepted March 9, 1994)
Abstract Mo films were deposited on sintered polycrystalline MgO. These films exhibited, depending on deposition conditions, two different structures, namely the normal body-centered cubic and an abnormal face-centered cubic. The analysis performed by electron diffraction and Auger electron spectroscopy indicated that the fcc structure assumed by molybdenum is an impurity-stabilized phase rather than a simple polymorphic transformation or dimolybdenum nitride (yMo2N) as previously stated by some authors.
1. Introduction
2. Experimental techniques
Thin films are of considerable technological interest. They are used in optical coatings, corrosion protection and semi-conducting devices. It has generally been observed that the properties of a thin film are related to its microstructure which is determined by the deposition parameters, such as the kinetic energy of different species arriving at the substrate surface and the substrate material itself, its crystal structure and temperature. The control of these deposition parameters and the knowledge of the crystallographic nature of the deposit-substrate interface are very important to understanding and improving the requ.ested property. Because of its refractory properties, molybdenum is a promising material for the above-mentioned applications.Its deposition as thin films was thus extensively studied using various substrates and different techniques of elaboration [1-12]. Some of these studies have reported the occurrence of a face-centered cubic (fcc) modification of molybdenum [1-4]. Unfortunately, their conclusions are inconsistent. The aim of this work is to provide some enlightenment on the discrepancy between previous authors about the origin of the crystallographic transformation of molybdenum.
Mo films were prepared using either thermal evaporation or d.c. sputtering. The two operations were carried out in the same installation which consists essentially of a pyrex bell-jar and a pumping system consisting of a rotatory pump associated with an oil diffusion pump. Evaporated films were obtained by self-resistance heating of a spiral molybdenum filament (Johnson Matthey, purity 99.97%). A movable shutter was used for protection from deposition during the heating of the
0040-6090/94/$7.00 SSDI 0040-6090(94) 06128-8
source.
Sputtered films were deposited by d.c. glow discharge in a pure argon (Air liquide, 99.99%) between two planar electrodes. The cathode was a high-pressure compacted disc of molybdenum (99.9%). Argon was introduced in the chamber at the desired pressure through a variable leak valve and was continuously pumped during sputtering to avoid the build-up of contaminating gases. Prior to deposition, the cathode surface was cleaned up by applying a glow discharge between the target and the shutter for 2 min. The sputtering conditions were: voltage, 2500 V; current density, 2 - 3 mA cm-2; argon pressure, 1.33-6.66 Pa. In all cases, the substrate was 60 mm away from the molybdenum source and was heated by a halogen lamp
(~ 1994 - - Elsevier Science S.A. All rights reserved
S. Kacim et al. / Structural behavior of d.c. sputtered and thermally evaporated Mo tl,in films
151
placed in a copper block. The temperature was monitored by a N i - N i C r thermocouple. The characterization of each film was done by reflexion and transmission high energy electron diffraction and Auger electron spectroscopy (AES). The film thickness was estimated by optical interferometry.
3. Results and discussion
The experimental results relative to thermal evaporation and sputtering are summarized in Table 1 and Table 2, respectively. Simultaneous examinations of these tables allowed us to make the following remarks. --Depending on experimental conditions, molybdenum can assume either a fcc or a body-centered cubic (bcc) structure. Their corresponding THEED patterns are shown in Figs. l(a) and l(b). - - T h e adoption of the fcc structure for evaporated films is not dependent on temperature or film
(a)
(b)
Fig. 1. THEED patterns characteristic of (a) Mo bee and (b) Mo fcc structures.
thickness, as reported by Denbigh and Marcus [2] and Chopra et al. [3], but mainly on the average deposition rate which seems to be a determinant factor. In fact, fcc
TABLE 1. Thermally evaporated films deposited onto compacted polycrystalline MgO Run number
Substrate temperature (K)
Evaporation time (min)
Film thickness (nm)
Deposition rate (nm min -I)
1
298
40
48
1.20
2
298
5
59
11.80
3 4 5 6 7 8 9 10 11
693 723 773 773 863 913 963 973 983
10 30 30 40 4 60 53 15 10
82 45 62.7 35 50 70 96.5 94.5 91.3
8.20 1.50 3.24 0.87 12.50 1.17 1.80 6.30 9.13
Remarks*
amorphous-like Mo fcc amorphous-like Mo bcc Mo bcc Mo fcc Mo bcc + Mo fcc Mo fcc Mo bcc Mo fcc Mo fcc Mo bcc + Mo fcc Mo bcc
*Mo bcc and Mo fcc denotes that molybdenum has a bcc and a fcc structure respectively. TABLE 2. Sputtered films deposited onto compacted MgO substrates Run number
Substrate temperature (K)
Sputtering time (rain)
Film thickness (nm)
Deposition rate (rim rain -l)
Current density (mAcm -2)
Remarks*
1
353
60
120
2
2.50
2 3 4 5 6 7 8 9 10 11 12
533 643 723 763 773 803 823 873 883 893 973
30 30 30 30 40 60 30 40 60 30 20
52 70 62 40.2 75 95 66 74 126 64 96
1.73 2.33 2.07 1.34 1.87 1.58 2.20 1.85 2.10 2.13 1.80
2.5-3.00 2.50-3.50 2.50 2 2.50 2.25 2.50 2.50 2.50-2.75 2.75 2.25-2.50
amorphous-like Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc
*Mo fcc denotes that Mo has a fcc structure.
152
S. Kacim et al. / Structural behavior of d.c. sputtered and thermally evaporated Mo thin films
and bcc single phases were favoured by low and rapid deposition rates, respectively, while a mixture of both phases was observed for intermediate rates. - - T h e sputtered films systematically adopt a fcc structure, but it is interesting to note that their corresponding deposition rates are relatively low and do not exceed 2.4 nm min-'. The manifestation of the Mo fcc phase in thermally evaporated as well as sputtered films has been reported by several authors [1-4], but their conclusions about its composition and nature are inconsistent and even contradictory. Aggarwal and Goswami [1] were the first to observe it in thermally evaporated films, and attested that it was a pure polymorphic transformation of the metal. The same structure was obtained by Bykov et al. [13] when they bombarded bcc molybdenum films either with argon, neon or nitrogen ions with medium energy. Their results were explained in terms of radiation defect storage. More detailed investigations on the nature of the fcc molybdenum were undertaken by Chopra et aL [3] and Nagata and Shoji [4]. Based on their experimental observations, especially chemical analysis of the film nitrogen content which was determined using the micro-Kjeldhal technique to be less than 1 at.%, Chopra et al. concluded that the observed Mo fcc structure is a pure polymorphic transformation. However, Nagata and Shoji who analyzed the residual gases using a quadripole mass filter, attested that it was a cubic dimolybdenum nitride [14] (TMo2N). The measured lattice parameters reported by the above-mentioned authors for this fcc phase, are 0.419 nm and in the range 0.415-0.423 nm respectively. Recently, Lin and coworkers [8, 9] studied the effect of deposition rate and substrate bias on the resistivity (p) of Mo and etablished a relationship between p and oxygen contamination. The pronounced diminution of resistivity was interpreted as a consequence of an oxygen-stabilized molybdenum phase formation. Their phase, which is probably identical to that observed by Saito et al. [ ! 5] but different from ours, corresponds with Mo30 with a fl-tungsten structure [16]. The question which arises is whether the Mo fcc structure is a pure polymorphic transformation or an impurity-stabilized phase, and we shall now try to answer this. In agreement with the results of Nagata and Shoji [4], the measured lattice constant for this phase varies from 0.415 to 0.422 nm. This allows the exclusion of the case of polymorphic transformation, in contrast with the suggestions of Aggarwal and Goswami [ 1], Denbigh and Marcus [2] and Chopra et al. [3], because if it was so, only one single value would have been found for the fcc lattice parameter.
Since the fcc phase was found in sputtered as well as thermally evaporated films, a radiative action [13] can also be excluded. Furthermore, our hypothesis is supported by the two following remarks. --Assuming that molybdenum atoms have the same radius as in the normal bcc lattice, namely 0.136 nm, then the calculated lattice constant of the fcc molybdenum would be 0.385 nm, which is smaller than the measured values (0.415-0.422nm). The observed lattice expansion, of about I0%, can be explained if we admit an insertion in fcc octahedral sites of non-metal elements such as nitrogen and carbon which have atomic radii slightly higher than the dimensions of these spaces. If nitrogen or carbon atoms enter these interstitial positions, the calculated lattice constants can be 0.414 and 0.426 nm respectively. These values are very close to those observed on our films. Indeed, carbon and nitrogen were observed to form metallic compounds with transition elements. In the case of molybdenum, cubic MozC [17,18] and 7Mo2N [14] have a fcc structure with lattice parameters of 0.414 and 0.4155 nm respectively. - - T h e formation of this fcc phase is favored by low deposition rates, independent of the deposition method used (thermal evaporation or glow discharge sputtering). The film is thus exposed to the impurities for a longer period of time. From the above arguments, it can be stated that the fcc structure assumed by Mo is not an allotropic transformation but an impurity-stabilized phase. Another question then arises regarding the nature of the impurities which are responsible for this stabilization. To answer this new question we undertook a qualitative analysis of our films by AES. Typical AES spectra for the fcc phase are shown in Figs. 2(a) and 2(b). For comparison, we have also reported that of the bcc molybdenum in Fig. 2(c). The spectrum of the bcc phase shows pratically only M N N Mo transitions while the two first spectra indicate, in addition, the presence of K L L transitions corresponding to C, O and N. These impurities were expected to be originating from residual gas decompositions, but the abundance of carbon is probably due to the diffusion pump oil contamination. Moreover, the Auger spectra (Figs. 2(a) and 2(b)) show that carbon has a "carbidic structure". Indeed, the Auger spectrum for carbon, which corresponds to the K L L group of transitions where L states are the 2s and 2p valence states, is very sensitive to the type of bond hybridization. Thus, each type of carbon bond is associated with a distinctive Auger lineshape in the derivative mode ( d N ( E ) / d E ) which acts as a fingerprint for the chemical bonding state of the carbon [19,20]. Using the intermediate coupling scheme, Siegbahn et al. [21] predicted six possible transitions for quasimetallic carbon (Table 3).
S. Kacim et al. / Structural behavior of d.c. sputtered and thermally evaporated Mo thin films
M,o
C . ,,-,.,,-~
~N
153
TABLE 3. Possible transitions of quasi-metallic carbon*
0
State of carbon 2s° 2p2 2s1 2p~ ion
2s2 2p°
Auger transition KLjL~ KLtL2 KLlL3 KL2L2 KL2L3 KL3L3 Energy (eV) 243 252 258 265 266 267 *Final state of carbon ion after Auger transition.
(a) o
100
2C)0 Mo
(b) o
100
(c)
C
200
MO
1~o
4'00 560
300
6()0
EnergyleV~'
N
i
i
300
400
C
2bO 300
N
460
I
500
600
Energyfe~l
0
560
660
EneravleV'l
Fig. 2. Typical AES spectra obtained on (a), (b) Mo fcc films and (c) Mo bcc films. Recording conditions: Ep, 2.5 kV; modulation, 3.5 V.
However, for transition metal carbides, only three transitions are generally observed for carbon. The case of Ni3C was studied by Coad and Rivi6re [22]. According to these authors, the valence band of nickel carbide is sufficiently narrow such that the Ll(2S) level is outside this band, whereas the L2 and L3 levels are inside it. Thereby only KL~LI (~-249eV), KLIL2.~ ( ~-257 eV) and KL2.3L2.3 ( - 2 7 0 eV) transitions were observed. Since carbon forms metallic carbides with transition elements, it is in a bonding situation much closer to that of a metal and the valence band would be expected to be narrow. Then we speculate that the model used for the interpretation of the Ni3C Auger spectrum can be extended to all transition metal carbides. F r o m the above observation we can think of the cubic Mo2C prepared by Lander and Germer [17,18] being produced by thermal decomposition of molybdenum carbonyl as well as by direct action of carbon monoxide on the metal. However, the existence of such a carbide, the formula of which was assigned by similarity with ;~MozN, has been contested by several investigators [23-25] who did not exclude an eventual stabilization by oxygen. Their idea finds its support in the results of Rudy et al. [26]. According to these later authors who described a cubic MoC as a high temperature phase, several rapid quenchings from at least 2273 K were required to keep it pure until room temperature. The phase thus obtained presents a homogeneous domain which varies from 39.7 to 43 at.% carbon with lattice parameters of 0.42666 and 0.4281 nm, respectively. So far, we have considered the AES-detected impurities on the qualitative viewpoint. To estimate rough compositions of our films we have used the method based on sensitivity factors [27]. The results relative to the three analyzed samples, gathered in Table 4, show that the total concentration of impurities does not exceed 10 at.% in the case of Mo bcc. However, when the fcc phase is obtained the composition of the samples corresponds approximately with Mo2X (where X = C, N, O). The total concentration in the latter case is great enough to produce the conversion of Mo bcc to Mo fcc phase. Indeed polymorphic transformation of the Mo bcc lattice to the M o fcc lattice can be induced by implantation as low as 24 at.% C and 26 at.% N, as calculated by Grechnev et al. [28].
S. Kacim et al. / Structural behavior of d.c. sputtered and thermally evaporated Mo thin films
154
TABLE 4. Compositions of the three analyzed samples (Fig. 2) estimated on the basis of relative sensitivity factors of elements [27], except for carbon which manifests a carbidic structure, where a sensitivity factor 3.5 times greater than that of graphite is used [20] Sample
a b c
Total concentration of impurities
Composition of the sample (atomic fractions) Mo
C
N
O
C+N+O
0.708 0.688 0.910
0.117 0.056 0.009
0.113 0.158 0.024
0.062 0.098 0.057
0.292 0.312 0.090
Taking into account the enormous differences between lattice constants reported by Rudy et al. [26] and those measured on our films on the one hand and the AES-detected impurities on the other, the cases of oxycarbides and even oxycarbonitrides can be anticipated. In fact, if we consider a homologous series of isotypic compounds MC, MN and MO (where M = Ti, V, Nb) [29], we can note that, for each metal, the lattice constant decreases from carbide to nitride and then to oxide. So, the introduction of nitrogen or oxygen into the carbide lattice induces contraction of the unit cell. Indeed Ferguson et al. [30] have isolated a series of cubic oxycarbides whose composition varies from 18.9 to 26.4 at.% oxygen and lattice constants of 0.4181 and 0.4169 nm, respectively. Lux and Ignatowicz [31] obtained, by pyrolysis of Mo(CO) 6 in a vacuum, another oxycarbide which were formulated as MOC0.4600.98 with a lattice parameter of 0.4187 nm. All these oxycarbides have crystal parameters lower than those measured by Rudy et al. [26] for MoC but are hardly identical to ours. Hence, it is very likely that the Mo fcc phase is an impurity-stabilized phase formed mainly as oxycarbides or oxycarbonitrides.
4. Conclusion Mo thin films have been deposited by either thermal evaporation or d.c. sputtering. Their physico-chemical characterization allowed us to confirm the existance of a fcc modification of this metal when it is prepared as thin films. However, this new structure is not a pure allotropic transformation because the measured lattice parameter is not constant but varies in the range 0.415 to 0.422 nm. It is not necessarily dimolybdenum nitride either, because, in most cases, the Auger spectra do not show the presence of nitrogen in the films when the electron diffraction patterns were characteristic of the fcc structure. On the basis of the above considerations and the identified impurities, it can be stated that non-metal
elements such as carbon, nitrogen and oxygen are responsible for the stabilization of this Mo fcc structure as interstitial compounds.
References 1 P. S. Aggarwal and A. Goswami, Proc. Phys. Soc. B, 70 (1957) 708. 2 P. N. Denbigh and R. B. Marcus, J. Appl. Phys., 37(1966)4235. 3 K. L. Chopra, M. R. Randlett and R. H. Duff, Phil. Mag., 16 (1967) 261. 4 S. Nagata and F. Shoji, Jpn. J. Appl. Phys., /0(1971) I1. 5 J. E. Grow, M. Stongin, R. S. Thompson and O. F. Kammerer, Phys. Lett. A., 30 (1969) 161. 6 P. H. Schmidt, R. N. Castellano, H. Barz. A. S. Cooper and E. G. Spencer, J. Appl. Phys., 44 (1973) 1833. 7 G. Beddies, W. Bretschneider and P. Bogdanowa, Wiss. Z.d. Tu. Karl-Max-Stadt, 29 (1987) 247. 8 J. S. Lin, R. C. Budhani and R. F. Bunshah, Thin Solid Films, 153 (1987) 359. 9 T. T. Bardin, J. G. Pronko, R. C. Budhani, J. S. Lin and R. F. Bunshah, Thin Solid Films, 165 (1988) 243. 10 F. Meyer, E. Velu, C. Pellet, C. Schwebel and C. Dupas, Rev. Phys. Appl., 23 (1988) 933. 11 J. M. Slaughter, A. Shapiro, P. A. Kearney and C. M. Falco, Phys. Rev. B, 44 (1991) 3854. 12 T. J. Vink, M. A. J. Somers, J. L. C. Daams and A. G. Dirks, J. Appl. Phys., 70 (1991) 4301. 13 V. N. Bykov, V. A. Troyan, G. G. Zdorovsteva and V. S. Khaimovich, Phys. Status Solidi (A), 32 (1975) 53. 14 G. Hagg, Z. Phys. Chem. B, 7(1930) 339. 15 Y. Saito, K. Mihama and R. Uyeda, Jpn. J. Appl. Phys., 19 (1980) 1603. 16 G. Hagg and N. Sch6nberg, Aeta. Crystallogr., 7(1954) 351. 17 J. J. Lander and L. H. Germer, Trans AIME, 175 (1948) 648~ 18 J. J. Lander and L. H. Germer, Met. Teeh., 14 (1946) 14. 19 T. W. Haas, J. T. Grant and G. J. Dooley III, J. Appl. Phys., 43 (1972) 1853. 20 S. Craig, G. L. Harding and R. Payling, Surf Sei., 124(1983) 591. 21 K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Harmin, J. Hedman, G. Johansson, T. Bergmark, S. Karlsson, I. Lindgren and B. Lindberg, ESCA--Atomic Molecular and Solid State Structure Studied by Means of Electron Spectroscopy, North-Holland, Amsterdam, 1967. 22 J. P. Coad and J. C. Rivi~re, Surf Sci., 25 (1971) 609. 23 R. Kieiffer and F. Benesovsky, Hartstoff, Springer-Verlag, Vienna, 1963. 24 J. C. Schuster and H. Nowotny, Monatsh. Chem., 110 (1979) 321. 25 E. K. Storms, The Refractory Carbides, Academic Press, New York, 1967, Chapt. 8. 26 E. Rudy, S. Windisch, A. J. Stosick and J. R. Hoffman, Trans. Met. Soc. AIME, 239 (1967) 1247. 27 L. E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach and R. E. Weber, Handbook of Auger Electron Spectroscopy, 2nd edn., Phys. Electr. Div. Inc, Eden Prairie, MN, 1976. 28 G. E. Grechnev, G. G. Kvachantiradze, N. A. Panuashvili and K. S. Kharebov, Soy. Phys. Tech. Phys., 36(1991) 178. 29 H. J. Goldschmidt, Interstitial Alloys, Butterworth, London, 1967. 30 I. F. Ferguson, J. B. Ainscough, D. Morse and A. W. Miller, Nature, 4339 (1964) 1327. 31 H. Lux and A. Ignatowicz, Chem. Ber., 101 (1968) 809.
Thin Solid Films, 249 (1994) 150-154
Structural behavior of direct-current sputtered and thermally evaporated molybdenum thin films S. Kacim Departement de Chimie, Facultk des Sciences, Semlalia Bd Prince My Abdellah BP., S 15 40001 Marrakech, Morocco
P. Delcambe, L. Binst, M. Jardinier-Offergeld and F. Bouillon Laboratoire de Chimie Analytique et Minbrale, Facult~ des Sciences C.P. 160, Universit~ Libre de Bruxelles, 50 Av F.D. Roosevelt, 1050 Bruxelles, Belgium (Received January 17, 1994; accepted March 9, 1994)
Abstract Mo films were deposited on sintered polycrystalline MgO. These films exhibited, depending on deposition conditions, two different structures, namely the normal body-centered cubic and an abnormal face-centered cubic. The analysis performed by electron diffraction and Auger electron spectroscopy indicated that the fcc structure assumed by molybdenum is an impurity-stabilized phase rather than a simple polymorphic transformation or dimolybdenum nitride (yMo2N) as previously stated by some authors.
1. Introduction
2. Experimental techniques
Thin films are of considerable technological interest. They are used in optical coatings, corrosion protection and semi-conducting devices. It has generally been observed that the properties of a thin film are related to its microstructure which is determined by the deposition parameters, such as the kinetic energy of different species arriving at the substrate surface and the substrate material itself, its crystal structure and temperature. The control of these deposition parameters and the knowledge of the crystallographic nature of the deposit-substrate interface are very important to understanding and improving the requ.ested property. Because of its refractory properties, molybdenum is a promising material for the above-mentioned applications.Its deposition as thin films was thus extensively studied using various substrates and different techniques of elaboration [1-12]. Some of these studies have reported the occurrence of a face-centered cubic (fcc) modification of molybdenum [1-4]. Unfortunately, their conclusions are inconsistent. The aim of this work is to provide some enlightenment on the discrepancy between previous authors about the origin of the crystallographic transformation of molybdenum.
Mo films were prepared using either thermal evaporation or d.c. sputtering. The two operations were carried out in the same installation which consists essentially of a pyrex bell-jar and a pumping system consisting of a rotatory pump associated with an oil diffusion pump. Evaporated films were obtained by self-resistance heating of a spiral molybdenum filament (Johnson Matthey, purity 99.97%). A movable shutter was used for protection from deposition during the heating of the
0040-6090/94/$7.00 SSDI 0040-6090(94) 06128-8
source.
Sputtered films were deposited by d.c. glow discharge in a pure argon (Air liquide, 99.99%) between two planar electrodes. The cathode was a high-pressure compacted disc of molybdenum (99.9%). Argon was introduced in the chamber at the desired pressure through a variable leak valve and was continuously pumped during sputtering to avoid the build-up of contaminating gases. Prior to deposition, the cathode surface was cleaned up by applying a glow discharge between the target and the shutter for 2 min. The sputtering conditions were: voltage, 2500 V; current density, 2 - 3 mA cm-2; argon pressure, 1.33-6.66 Pa. In all cases, the substrate was 60 mm away from the molybdenum source and was heated by a halogen lamp
(~ 1994 - - Elsevier Science S.A. All rights reserved
S. Kacim et al. / Structural behavior of d.c. sputtered and thermally evaporated Mo tl,in films
151
placed in a copper block. The temperature was monitored by a N i - N i C r thermocouple. The characterization of each film was done by reflexion and transmission high energy electron diffraction and Auger electron spectroscopy (AES). The film thickness was estimated by optical interferometry.
3. Results and discussion
The experimental results relative to thermal evaporation and sputtering are summarized in Table 1 and Table 2, respectively. Simultaneous examinations of these tables allowed us to make the following remarks. --Depending on experimental conditions, molybdenum can assume either a fcc or a body-centered cubic (bcc) structure. Their corresponding THEED patterns are shown in Figs. l(a) and l(b). - - T h e adoption of the fcc structure for evaporated films is not dependent on temperature or film
(a)
(b)
Fig. 1. THEED patterns characteristic of (a) Mo bee and (b) Mo fcc structures.
thickness, as reported by Denbigh and Marcus [2] and Chopra et al. [3], but mainly on the average deposition rate which seems to be a determinant factor. In fact, fcc
TABLE 1. Thermally evaporated films deposited onto compacted polycrystalline MgO Run number
Substrate temperature (K)
Evaporation time (min)
Film thickness (nm)
Deposition rate (nm min -I)
1
298
40
48
1.20
2
298
5
59
11.80
3 4 5 6 7 8 9 10 11
693 723 773 773 863 913 963 973 983
10 30 30 40 4 60 53 15 10
82 45 62.7 35 50 70 96.5 94.5 91.3
8.20 1.50 3.24 0.87 12.50 1.17 1.80 6.30 9.13
Remarks*
amorphous-like Mo fcc amorphous-like Mo bcc Mo bcc Mo fcc Mo bcc + Mo fcc Mo fcc Mo bcc Mo fcc Mo fcc Mo bcc + Mo fcc Mo bcc
*Mo bcc and Mo fcc denotes that molybdenum has a bcc and a fcc structure respectively. TABLE 2. Sputtered films deposited onto compacted MgO substrates Run number
Substrate temperature (K)
Sputtering time (rain)
Film thickness (nm)
Deposition rate (rim rain -l)
Current density (mAcm -2)
Remarks*
1
353
60
120
2
2.50
2 3 4 5 6 7 8 9 10 11 12
533 643 723 763 773 803 823 873 883 893 973
30 30 30 30 40 60 30 40 60 30 20
52 70 62 40.2 75 95 66 74 126 64 96
1.73 2.33 2.07 1.34 1.87 1.58 2.20 1.85 2.10 2.13 1.80
2.5-3.00 2.50-3.50 2.50 2 2.50 2.25 2.50 2.50 2.50-2.75 2.75 2.25-2.50
amorphous-like Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc Mo fcc
*Mo fcc denotes that Mo has a fcc structure.
152
S. Kacim et al. / Structural behavior of d.c. sputtered and thermally evaporated Mo thin films
and bcc single phases were favoured by low and rapid deposition rates, respectively, while a mixture of both phases was observed for intermediate rates. - - T h e sputtered films systematically adopt a fcc structure, but it is interesting to note that their corresponding deposition rates are relatively low and do not exceed 2.4 nm min-'. The manifestation of the Mo fcc phase in thermally evaporated as well as sputtered films has been reported by several authors [1-4], but their conclusions about its composition and nature are inconsistent and even contradictory. Aggarwal and Goswami [1] were the first to observe it in thermally evaporated films, and attested that it was a pure polymorphic transformation of the metal. The same structure was obtained by Bykov et al. [13] when they bombarded bcc molybdenum films either with argon, neon or nitrogen ions with medium energy. Their results were explained in terms of radiation defect storage. More detailed investigations on the nature of the fcc molybdenum were undertaken by Chopra et aL [3] and Nagata and Shoji [4]. Based on their experimental observations, especially chemical analysis of the film nitrogen content which was determined using the micro-Kjeldhal technique to be less than 1 at.%, Chopra et al. concluded that the observed Mo fcc structure is a pure polymorphic transformation. However, Nagata and Shoji who analyzed the residual gases using a quadripole mass filter, attested that it was a cubic dimolybdenum nitride [14] (TMo2N). The measured lattice parameters reported by the above-mentioned authors for this fcc phase, are 0.419 nm and in the range 0.415-0.423 nm respectively. Recently, Lin and coworkers [8, 9] studied the effect of deposition rate and substrate bias on the resistivity (p) of Mo and etablished a relationship between p and oxygen contamination. The pronounced diminution of resistivity was interpreted as a consequence of an oxygen-stabilized molybdenum phase formation. Their phase, which is probably identical to that observed by Saito et al. [ ! 5] but different from ours, corresponds with Mo30 with a fl-tungsten structure [16]. The question which arises is whether the Mo fcc structure is a pure polymorphic transformation or an impurity-stabilized phase, and we shall now try to answer this. In agreement with the results of Nagata and Shoji [4], the measured lattice constant for this phase varies from 0.415 to 0.422 nm. This allows the exclusion of the case of polymorphic transformation, in contrast with the suggestions of Aggarwal and Goswami [ 1], Denbigh and Marcus [2] and Chopra et al. [3], because if it was so, only one single value would have been found for the fcc lattice parameter.
Since the fcc phase was found in sputtered as well as thermally evaporated films, a radiative action [13] can also be excluded. Furthermore, our hypothesis is supported by the two following remarks. --Assuming that molybdenum atoms have the same radius as in the normal bcc lattice, namely 0.136 nm, then the calculated lattice constant of the fcc molybdenum would be 0.385 nm, which is smaller than the measured values (0.415-0.422nm). The observed lattice expansion, of about I0%, can be explained if we admit an insertion in fcc octahedral sites of non-metal elements such as nitrogen and carbon which have atomic radii slightly higher than the dimensions of these spaces. If nitrogen or carbon atoms enter these interstitial positions, the calculated lattice constants can be 0.414 and 0.426 nm respectively. These values are very close to those observed on our films. Indeed, carbon and nitrogen were observed to form metallic compounds with transition elements. In the case of molybdenum, cubic MozC [17,18] and 7Mo2N [14] have a fcc structure with lattice parameters of 0.414 and 0.4155 nm respectively. - - T h e formation of this fcc phase is favored by low deposition rates, independent of the deposition method used (thermal evaporation or glow discharge sputtering). The film is thus exposed to the impurities for a longer period of time. From the above arguments, it can be stated that the fcc structure assumed by Mo is not an allotropic transformation but an impurity-stabilized phase. Another question then arises regarding the nature of the impurities which are responsible for this stabilization. To answer this new question we undertook a qualitative analysis of our films by AES. Typical AES spectra for the fcc phase are shown in Figs. 2(a) and 2(b). For comparison, we have also reported that of the bcc molybdenum in Fig. 2(c). The spectrum of the bcc phase shows pratically only M N N Mo transitions while the two first spectra indicate, in addition, the presence of K L L transitions corresponding to C, O and N. These impurities were expected to be originating from residual gas decompositions, but the abundance of carbon is probably due to the diffusion pump oil contamination. Moreover, the Auger spectra (Figs. 2(a) and 2(b)) show that carbon has a "carbidic structure". Indeed, the Auger spectrum for carbon, which corresponds to the K L L group of transitions where L states are the 2s and 2p valence states, is very sensitive to the type of bond hybridization. Thus, each type of carbon bond is associated with a distinctive Auger lineshape in the derivative mode ( d N ( E ) / d E ) which acts as a fingerprint for the chemical bonding state of the carbon [19,20]. Using the intermediate coupling scheme, Siegbahn et al. [21] predicted six possible transitions for quasimetallic carbon (Table 3).
S. Kacim et al. / Structural behavior of d.c. sputtered and thermally evaporated Mo thin films
M,o
C . ,,-,.,,-~
~N
153
TABLE 3. Possible transitions of quasi-metallic carbon*
0
State of carbon 2s° 2p2 2s1 2p~ ion
2s2 2p°
Auger transition KLjL~ KLtL2 KLlL3 KL2L2 KL2L3 KL3L3 Energy (eV) 243 252 258 265 266 267 *Final state of carbon ion after Auger transition.
(a) o
100
2C)0 Mo
(b) o
100
(c)
C
200
MO
1~o
4'00 560
300
6()0
EnergyleV~'
N
i
i
300
400
C
2bO 300
N
460
I
500
600
Energyfe~l
0
560
660
EneravleV'l
Fig. 2. Typical AES spectra obtained on (a), (b) Mo fcc films and (c) Mo bcc films. Recording conditions: Ep, 2.5 kV; modulation, 3.5 V.
However, for transition metal carbides, only three transitions are generally observed for carbon. The case of Ni3C was studied by Coad and Rivi6re [22]. According to these authors, the valence band of nickel carbide is sufficiently narrow such that the Ll(2S) level is outside this band, whereas the L2 and L3 levels are inside it. Thereby only KL~LI (~-249eV), KLIL2.~ ( ~-257 eV) and KL2.3L2.3 ( - 2 7 0 eV) transitions were observed. Since carbon forms metallic carbides with transition elements, it is in a bonding situation much closer to that of a metal and the valence band would be expected to be narrow. Then we speculate that the model used for the interpretation of the Ni3C Auger spectrum can be extended to all transition metal carbides. F r o m the above observation we can think of the cubic Mo2C prepared by Lander and Germer [17,18] being produced by thermal decomposition of molybdenum carbonyl as well as by direct action of carbon monoxide on the metal. However, the existence of such a carbide, the formula of which was assigned by similarity with ;~MozN, has been contested by several investigators [23-25] who did not exclude an eventual stabilization by oxygen. Their idea finds its support in the results of Rudy et al. [26]. According to these later authors who described a cubic MoC as a high temperature phase, several rapid quenchings from at least 2273 K were required to keep it pure until room temperature. The phase thus obtained presents a homogeneous domain which varies from 39.7 to 43 at.% carbon with lattice parameters of 0.42666 and 0.4281 nm, respectively. So far, we have considered the AES-detected impurities on the qualitative viewpoint. To estimate rough compositions of our films we have used the method based on sensitivity factors [27]. The results relative to the three analyzed samples, gathered in Table 4, show that the total concentration of impurities does not exceed 10 at.% in the case of Mo bcc. However, when the fcc phase is obtained the composition of the samples corresponds approximately with Mo2X (where X = C, N, O). The total concentration in the latter case is great enough to produce the conversion of Mo bcc to Mo fcc phase. Indeed polymorphic transformation of the Mo bcc lattice to the M o fcc lattice can be induced by implantation as low as 24 at.% C and 26 at.% N, as calculated by Grechnev et al. [28].
S. Kacim et al. / Structural behavior of d.c. sputtered and thermally evaporated Mo thin films
154
TABLE 4. Compositions of the three analyzed samples (Fig. 2) estimated on the basis of relative sensitivity factors of elements [27], except for carbon which manifests a carbidic structure, where a sensitivity factor 3.5 times greater than that of graphite is used [20] Sample
a b c
Total concentration of impurities
Composition of the sample (atomic fractions) Mo
C
N
O
C+N+O
0.708 0.688 0.910
0.117 0.056 0.009
0.113 0.158 0.024
0.062 0.098 0.057
0.292 0.312 0.090
Taking into account the enormous differences between lattice constants reported by Rudy et al. [26] and those measured on our films on the one hand and the AES-detected impurities on the other, the cases of oxycarbides and even oxycarbonitrides can be anticipated. In fact, if we consider a homologous series of isotypic compounds MC, MN and MO (where M = Ti, V, Nb) [29], we can note that, for each metal, the lattice constant decreases from carbide to nitride and then to oxide. So, the introduction of nitrogen or oxygen into the carbide lattice induces contraction of the unit cell. Indeed Ferguson et al. [30] have isolated a series of cubic oxycarbides whose composition varies from 18.9 to 26.4 at.% oxygen and lattice constants of 0.4181 and 0.4169 nm, respectively. Lux and Ignatowicz [31] obtained, by pyrolysis of Mo(CO) 6 in a vacuum, another oxycarbide which were formulated as MOC0.4600.98 with a lattice parameter of 0.4187 nm. All these oxycarbides have crystal parameters lower than those measured by Rudy et al. [26] for MoC but are hardly identical to ours. Hence, it is very likely that the Mo fcc phase is an impurity-stabilized phase formed mainly as oxycarbides or oxycarbonitrides.
4. Conclusion Mo thin films have been deposited by either thermal evaporation or d.c. sputtering. Their physico-chemical characterization allowed us to confirm the existance of a fcc modification of this metal when it is prepared as thin films. However, this new structure is not a pure allotropic transformation because the measured lattice parameter is not constant but varies in the range 0.415 to 0.422 nm. It is not necessarily dimolybdenum nitride either, because, in most cases, the Auger spectra do not show the presence of nitrogen in the films when the electron diffraction patterns were characteristic of the fcc structure. On the basis of the above considerations and the identified impurities, it can be stated that non-metal
elements such as carbon, nitrogen and oxygen are responsible for the stabilization of this Mo fcc structure as interstitial compounds.
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