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Influence of oxygen on the nucleation and growth of diamond films
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Thin Solid Films 303 (1997) 34-38
Influence of oxygen on the nucleation and growth of diamond films C. Gdmez-Aleixandre, M.M. Garcla, O. Sfinchez, J.M. Albella Instituto Ciencia de Materiales Madrid (CSIC), Cantoblaneo, 28049 Madrid, Spain
Received 2 July t996; accepted 6 February 1997
Abstract
The addition of oxygen to a diluted methane/hydrogen gas mixture (2% C H 4 in H 2) activated by a microwave discharge during diamond film deposition has been studied. We have observed that oxygen addition at very low concentrations ( < 2%) to the methane/hydrogen gas mixture makes the nucleation of diamond crystals more difficult, by causing a sharp decrease in the deposition rate of the diamond film. For increasing oxygen concentrations, in the [O 2] = 0.25-1% range, the diamond and graphite deposition rates remain nearly constant, showing a slight decrease in the graphite deposition rate for [0 2] = 1%. By contrast, for higher oxygen concentrations (1% < [0 2] < 2.5%) thinner films of a high quality are deposited (diamond content > 84%). These facts have been explained by an abrupt change in the chemical processes when the oxygen is fed to the CH 4 + H 2 mixture, even in a small concentration. We assume that the role of the atomic oxygen is two-fold: (i) formation of OH radicals, which etch the diamond and graphite phases at high rates, and (ii) direct etching of the initial carbon layer formed during the nucleation stage, producing CO molecules. However, for [02] > 2.5% the carbon etching rate (for all the phases) is so high that no continuous film can be deposited. In this paper we present the relative variation of the formation, in the 0-2.5% oxygen range, for both the diamond and non-diamond phases, as determined by Raman spectroscopy and scanning electron microscopy. The results have been related to the changes in the plasma composition (mainly the OH, O and CO species), as detected by optical emission spectroscopy and mass spectrometry. © 1997 Elsevier Science S.A. Keywords: Diamond; Plasma processing and deposition
1. I n t r o d u c t i o n The deposition of high quality diamond films by microwave chemical vapor deposition (MWCVD), at low power density, from C}-I 4 and H a requires a low CI-[ 4 concentration in the gas mixture ( < 0.5%[CH4]). At these conditions, the concentration of carbon-containing species in the plasma is so low that very low deposition rates are achieved. In order to increase the deposition rate of the films, numerous authors have studied the effect of the addition of oxygen containing compounds to the methane mixture, thus allowing one to use higher methane concentrations [1-5]. The presence of oxygen leads to diamond film deposition even at methane concentrations where only the graphite component is normally detected when methane and hydrogen gas mixtures are used. Oxygen is added either directly or as an integral part of the carbon carrier, such as mixtures containing CO or CO 2. The beneficial effect of the addition of oxygen-containing molecules is attributed to the enhancement of the etching of non-diamond components by hydrogen or oxygenated species (O or OH) generated in the plasma. The atomic C - H - O 0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PH $0040-6090(97)00090-4
phase diagram provides a common basis for all low-pressure CVD methods [5]. Successful diamond deposition using oxygen is restricted to a well-defined area within this diagram. Although many of these works deal with the effect of the addition of oxygen compounds, no systematic studies have been made on the separate action of the oxygen on the different stages (nucleation and growth) of the deposition process of diamond and graphite. In this work, we present the effect of oxygen on the nucleation and growth processes of diamond film deposition in the 0-2.5% oxygen range using M W C V D techniques. The deposition rates of diamond and non-diamond phases were determined by Raman spectroscopy and scanning electron microscopy (SEM). The results have been related to the changes in the plasma composition (mainly the OH, O and CO species), as detected by optical emission spectroscopy and mass spectrometry.
2. Experimental Diamond films were deposited in a M W C V D system at 2.45 GHz, excited with an ASTEX 1500 W magnetron
C, Gdmez-Aleixandre et aI. / Thin Solid Films 303 (1.997) 3 4 - 3 8
plasma source, on (100) silicon substrates which were previously scratched with I # m diamond paste. During the deposition process, the silicon substrate was placed on a graphite holder. The total pressure, substrate temperature and plasma power were maintained constant at 40 Torr (5.3 X 10 -2 bar), 900 °C (1173 K) and 1200 W, respectively. Gas mixtures were typically H 2, 2 vol.% C H 4 and 0-2.5 vol.% 02, with the total glass flow maintained at 400 sccm. The diamond content in the films has been determined from the Raman spectra by fitting the experimental spectra to a sum of gaussian and lorentzian curves [6]. The morphology of the films was studied by SEM. Also, SEM cross-sectional views have been used for determining the thickness of the diamond films. The excited species in the plasma during the deposition process were analyzed by optical emission spectroscopy (OES) in the 2 0 0 - 8 0 0 nm range using a PT Analytical Spectrometer (model PSS100), directly focused into the middle of the plasma discharge. A small flow of Ar (6 sccm) was added to act as an actinometer [7]. A portion of the plasma atmosphere in the reactor was routed to a differentially pumped quadrupole mass spectrometer (Hiden Analytical Ltd. model DSMS) in order to identify the stable species present in the reactor.
3. Results As it is known, during M W C V D process of diamond films from diluted CH 4 in H 2 gas mixtures, the deposition rate increases as the C H 4 concentration increases owing to the higher concentration of carbon containing species in the plasma. In a previous work we have determined, by Raman spectroscopy and SEM, the deposition rates of the different carbon phases (i.e. diamond and graphite) when the CH 4 content in the gas mixture increases in the 0 - 3 % range [8]. We observed an increase in the graphite component when the C H 4 concentration becomes higher, leading to lower quality diamond films. The addition of 0 2 in a very low concentration to the C H 4 4- H 2 mixture produces changes in the diamond and graphite deposition rates, as shown in Table 1. Also shown in this table are the relative intensities of the peaks corresponding to the CxH s species in the mass spectra. Other
t-"
0.6-
35
(a)
"&
diamond
v
0.4:=
0.2-
O
•
•
graphite
a9
0.0
100- (b)
o
90-
O O "O e'-
o E
80-
tm
oxygen concentration (%)
Fig. 1. (a) Diamond and graphite deposition rates, (b) diamond content in the films versus the 0 2 concentration in CH4/H 2 / 0 2 gas mixtures. oxygen-containing species such as, OH, H , O and CO are also detected in the discharge. The addition of 0.25% of 0 2 to the mixture leads to a strong decrease in the deposition rates of both, diamond and graphite, with a much larger reduction in the graphite deposition rate. Furthermore, the addition of small amounts of 0 2 to the hydrocarbon and H 2 gas mixture produces a slight increase in the intensity of the CxHy peaks in the mass spectra. Therefore, the reduction in the deposition rate must be interpreted as a consequence of new processes inhibiting the deposition mechanism, as we will see in the following. The influence of further additions of 0 2 on the diamond and graphite deposition rates as well as on the diamond content in the films are shown in Fig. l(a) and l(b), respectively. At 0 2 concentrations between 0.25 and 1%, both deposition rates remain almost constant. For 0 2 concentrations higher than 1%, the deposition rates decrease although a higher diamond content in the films is reached. Finally, when [0 2 ] > 2.5% is used no continuous films are obtained which means that the deposition rates are extremely low [9].
Table 1 Diamond and graphite deposition rates and relative intensity of mass specti6m&McC:~H>.peaks in 2% CH4/H 2 and 2% CH4/H2/0.25% 02 mixtures Gas mixture Deposition rate (tam h- I ) Relative intensity of mass spectra peaks (a.u.) (%) CH4:H 2 2:98 CH4:H2:O 2 2:97.75:0.25
Diamond 1.139 0.480
Graphite 0.510 0.I20
CH4
CH ~ 7.0 9.2
C 2H 2 6.3 7.8
C 2H 4 1.9 5.3
0.0 0.874
36
c. Gdmez-AIeixandre et al. / Thin Solid Fibns 303 (1997) 34-38
The changes in the plasma composition have been studied by OES and mass spectrometry. Fig. 2 shows the variation of the relative intensity of the major intensity peaks in the mass spectra versus the oxygen concentration. It is observed that when oxygen is progressively added to the gas mixture the signals corresponding to the C x H : species, coming from the decomposition of the CH 4 molecules continuously decrease. As for the oxygenated species (O, OH, H20, CO and CO 2) in the plasma, a different behavior is observed: (a) the intensity of the HaO and OH mass peaks initially remains fairly constant and then increase after oxygen additions higher than 1 and 2%, respectively; (b) the CO peak increases higher when the oxygen content in the gas mixture increases; (c) the atomic oxygen is only detected for [0 2] > 1%, showing an increase in its peak intensity at increasing oxygen concentrations. The main features of the emission spectra of the C H 4 / H 2 / O 2 plasma include: the hydrogen Balmer series (H a, He, H~), the activated carbon or hydrocarbon species (CH + at 360 nm and 400 nm and CH at 390 rim) and some emission in the 300-312 nm range, associated with CO and OH species. As mentioned before for the actinometer measurements we deliberately added a small amount of a noble gas to the reactive plasma and monitored the noble gas emissions concurrently with those of the reactive particles. Since the noble gas density is know, the excitation efficiency of any of its levels is determined simply by dividing the emission intensity of that level by the noble gas density. The variation of the relative intensity of the
1.2
0.8. (a) :5
/
0.4
~
OH
,r'-
._~
0.0
~
:
.
T
,
~
--.4
,
6'3
151(b)
0
'
I CO'I1"" lO4/ . . . . ..,,, /
]C.H'N. ,,'"
010c
1
2
o oxygen concentration (~A) Fig. 2. Relative intensity of the main peaks in the mass spectra versus the
02 concentration in CH4/Hz/02 mixtures.
Table 2 Variation of CO/OH, H~ and CH+ (360 nm and 400 nm) emission peaks with the oxygen content in the gas mixture [02 ] CO/O H,~ CH+ CH+ (%) (300-312nm) (656.3nm) (360 nm) (400 nm) 0.25 0.50 1.00 1.50 2.00 2.30
0.046 0.047 0.048 0.052 0.053 0.054
9.45 i0.40 8.00 8.50 7.75 %10
0.1880 0.1855 0.1860 0.1840 0.1845 0.1820
0.1685 0.1675 0.1655 0.1657 0.1650 0.1630
most important emission peaks ( C O / O H in the 300-312 nm range, H a at 656.3 nm and CH ÷ at 360 and 400 nm) with the oxygen content are given in Table 2. The CH + species, which participate in the diamond deposition process [10] show a decrease of their emission peak intensity, which can be explained by the consumption of the carbon containing species by reaction with atomic or molecular oxygen. The production of OH radicals and H 20 molecules can explain the global decrease behaviour found in the atomic hydrogen signal when the O z flow increases from 0.25 to 2.3, as shown in Table 2.
4. Discussion Within the concentration range of the gas mixture studied in this work (2% [CH 4] and 0.25-2.5% [02]), the carbon mole fraction [ C / ( C + O)] varies in the 0.30-0.80 range. We have observed that films with high diamond content, > 95% diamond content analyzed by Raman spectrometry, can be obtained from [ C / ( C + O)] = 0.33 up to 0.44. A narrower range (0.58-0.50) was previously associated with diamond deposition in the C / H / O phase diagram, proposed by Bachmann et al. [5]. The unexpected enlargement in the [ C / ( C + O)] range (low side values) can be explained by an increase in the concentration of carbon species in the reactor. This means that the real carbon species concentration is likely to be higher than that corresponding to the 2% C H 4, fed to the reactor. This effect is believed to be due to the etching of the graphite holder during the deposition process, since for the same experimental conditions, no growth of the diamond film is detected in our MWCVD system when a molybdenum plate is placed over the graphite holder. In any case, we think that the influence of oxygen must be related to the absolute oxygen concentration rather than to the relative mole fraction, which is strongly affected by the carbon species concentration. Therefore, in the following, we shall discuss the results in terms of the effect of the addition of oxygen to the CH 4 + H 2 gas mixture at increasing concentrations, firstly in a small concentration (0.25%) and then in greater amounts (0.25-2.5% range).
C. G6mez-Aleixandre er al. / Thin Solid Films 303 (1997) 34-38
4.1. Effect of the oxygen addition in a small concentration (O.25%) The observed decrease in the deposition rate of the films (diamond and graphite contributions) in Table 1 with the addition of oxygen in small concentrations to the gas mixture has been also reported by other authors [2,4]. Generally, the decrease is explained by etching processes parallel to that caused by the atomic hydrogen during the deposition. Howard et al. [3] have also pointed out that atomic oxygen can poison the growing surface by forming carbon bridges or double bonds with surface carbon atoms, which makes the growth of the films more difficult. As can be seen in Table 1, the addition of 0.25% of oxygen produces a 60% decrease in the diamond deposition rate, while an 80% reduction is detected in the graphite component rate. This result further supports the assumption that additional etching processes are acting during the deposition [11]. In addition, one should consider changes in the formation chemistry of the diamond phase in order to explain these effects. Although no signal of atomic oxygen is detected by mass spectrometry after the addition of 0.25% 02 , other oxygenated species such as OH, H20, CO and CO, are present in the discharge (see Fig. 2). It is well established during the deposition process, that OH radicals play a similar role to that of atomic hydrogen, both as etchant of the non-diamond component and as stabilizer of sp 3 bonds in the diamond structure [12]. The formation of OH radicals and H20 molecules bring about the consumption of atomic hydrogen of the plasma, as has been detected by OES. In order to discuss with more detail the influence of the oxygen on the deposition rate we can separate the diamond film deposition process into two stages: (1)nucleation; (2) growth of the continuous film. During the nucleation process, the diamond nuclei development requires a critical thickness of an amorphous carbon layer followed by its faceting through etching processes [8]. In this work, the emission peak of the CH species, which is known to participate in the amorphous carbon deposition, does not change with the addition of oxygen to the gas mixture. Hence, no appreciable changes in the deposition rate of the amorphous carbon layer can be expected. However, the high etching rate of the amorphous layer by the OH radicals present in the discharge can hinder the deposition of the critical thickness, thus producing an increase in the incubation period (time lapsed before the diamond nuclei are formed). In this respect, some authors have also reported that the addition of oxygen to the CH 4 + H 2 gas mixture produces a decrease in the nucleation density of the films [4]. Therefore, this effect of the oxygen addition on the nucleation process of the diamond crystals can explain the drastic reduction in both deposition rates (diamond and graphite). Furthermore, after the addition of O~, the CO signal is detected by mass spectrometry. The
37
CO formation could arise from the etching of the carbon layer, although some authors have also proposed the reaction with the carbon-hydrogen species present in the plasma as an alternative mechanism [3,13].
4.2. Effect of adding 02 at increasing concentrations (0.25-2.5%) It is important to note that, although the initial addition of 02 to the CH 4 + H 2 gas mixture produces a remarkable reduction in both deposition rates (diamond and graphite), during subsequent oxygen additions (up to t%), no changes in both deposition rates, and hence in the quality of the films, are detected. As can be observed in Fig. 2, in the 0.25-1% intermediate range the mass spectrum signal corresponding to the OH etchant agent does not change, whereas the C~H~ species decrease at increasing oxygen concentrations. In addition, no signal of atomic oxygen is detected in this range. Among the oxygenated species, only the CO signal continuously increases when the oxygen concentration becomes higher. These results indicate that in this range the etching processes are expected not to change with respect to the previous range (i.e. those associated with the OH radicals). The decrease in the hydrocarbon species concentration along with the increase in the CO signals further suggest that the oxidation reaction of these hydrocarbon species is taking place. However, these reaction processes seem not to affect the formation of the film since their rates remain unaltered. In the high concentration side ([O z] > 1%), the atomic oxygen signal is detected by mass spectrometry. Its peak intensity increases at increasing oxygen concentrations in the gas mixture. In this regard it is well know that the etching rate of oxygen on graphite is over 50 times larger than that of hydrogen [13] and, in addition, the OH radicals etch efficiently the non-diamond component in the films. Therefore, this enhancement in the etching process should be accompanied by an improvement in the quality of the films. This is in agreement with the results of Fig. l(b). Finally, when the oxygen concentration is higher than 2% the diamond and graphite etching rates are even higher than the corresponding formation rates, thus impeding the growth of a continuous fihn.
5. Conclusions
During diamond film deposition by MWCVD, a decrease in the formation rate along with an increase in the etching rate of the films has been detected when molecular oxygen is added to a 2% C H 4 / H 2 gas mixture. The nucleation process is supposed to be retarded by the oxygen presence, which adversely influences the initial growth of the films. On the other hand the presence of OH radicals, considered efficient etchant agents, produces an
38
C. Gdmez-Aleixandre et aL / Thin Solid Fihns 303 (]997) 34-38
increase in the etching rate of the films, thus giving rise finally to a reduction of the total deposition rate. In the 0 . 2 5 - 1 % [ 0 2 ] range the oxidation reactions o f the hydrocarbon species seems not to interfere with the chemical deposition process of d i a m o n d and graphite, whose rates remain constant in this range. However, for further oxygen additions, from 1% up to 2.5%, we have observed a decreased in both deposition rates which is attributed to the presence of atomic oxygen, i m p e d i n g the formation of a continuous film on the substrate.
References [1] W.D. Cassidy, E.A. Evans, Y. Wang, J.C. Angus, P.K. Bachmann, H, Hagemann, D, Leers and D. Wiechert, Mate~; Res. Soc. Proc., 339 (1994) 285.
[2] J.A. Mucha, D.L. Flamm and D,E. Ibottson, J. AppL Pyys.. 65 (1989) 3448. [3] W.N. Howard, K.E. Spear and M, Frenklach, Appl. Phys, Left,, 63 (1993) 2641, [4] C.-F. Chen, T.-M. Hong and S.-H, Chen, J. Appl. Phys., 74 (1993) 4483. [5] P.K. Bachrnann, D. Leers and J. Lydtin, Di~mtond Related Mater., (199I) 1. [6] D.S. Knight and W,B, White, J. Mater. Res.. 4 (1989) 385. [7] J.W. Coburn and M. Chen, J. Appt, Phys., 5t (I980) 3134. [8] C. G6mez-Aleixandre, O. S~nchez, M.M. Garc~a, L. Vfizquez and J,M. Albella, Physica Status Solidi (a), 154 (1996) 23. [9] O. Sfinchez, C. G6rnez-Aleixandre, F. Agultd and J.M. Albella, Diamond Related Mater., 3 (1994) 1183. [10] C. Gdmez-Alelxandre, O, Sfinchez and J.M Albella, J. AppL Phys., 74 (t993) 3752. [1 t] T.R. Anthony, Vacuum, 41 (1990) 1356. [12] Y. Saito, K. Sato, H. Tanaka, K. Fujita and S. Matuda, J. Mater. Sci., 23 (1988) 842. [13] Y. Muranaka, H. Yamashita, K. Sato and H. Miyadera, J. Appl. Phys., 67 (1990) 6247.
Thin Solid Films 303 (1997) 34-38
Influence of oxygen on the nucleation and growth of diamond films C. Gdmez-Aleixandre, M.M. Garcla, O. Sfinchez, J.M. Albella Instituto Ciencia de Materiales Madrid (CSIC), Cantoblaneo, 28049 Madrid, Spain
Received 2 July t996; accepted 6 February 1997
Abstract
The addition of oxygen to a diluted methane/hydrogen gas mixture (2% C H 4 in H 2) activated by a microwave discharge during diamond film deposition has been studied. We have observed that oxygen addition at very low concentrations ( < 2%) to the methane/hydrogen gas mixture makes the nucleation of diamond crystals more difficult, by causing a sharp decrease in the deposition rate of the diamond film. For increasing oxygen concentrations, in the [O 2] = 0.25-1% range, the diamond and graphite deposition rates remain nearly constant, showing a slight decrease in the graphite deposition rate for [0 2] = 1%. By contrast, for higher oxygen concentrations (1% < [0 2] < 2.5%) thinner films of a high quality are deposited (diamond content > 84%). These facts have been explained by an abrupt change in the chemical processes when the oxygen is fed to the CH 4 + H 2 mixture, even in a small concentration. We assume that the role of the atomic oxygen is two-fold: (i) formation of OH radicals, which etch the diamond and graphite phases at high rates, and (ii) direct etching of the initial carbon layer formed during the nucleation stage, producing CO molecules. However, for [02] > 2.5% the carbon etching rate (for all the phases) is so high that no continuous film can be deposited. In this paper we present the relative variation of the formation, in the 0-2.5% oxygen range, for both the diamond and non-diamond phases, as determined by Raman spectroscopy and scanning electron microscopy. The results have been related to the changes in the plasma composition (mainly the OH, O and CO species), as detected by optical emission spectroscopy and mass spectrometry. © 1997 Elsevier Science S.A. Keywords: Diamond; Plasma processing and deposition
1. I n t r o d u c t i o n The deposition of high quality diamond films by microwave chemical vapor deposition (MWCVD), at low power density, from C}-I 4 and H a requires a low CI-[ 4 concentration in the gas mixture ( < 0.5%[CH4]). At these conditions, the concentration of carbon-containing species in the plasma is so low that very low deposition rates are achieved. In order to increase the deposition rate of the films, numerous authors have studied the effect of the addition of oxygen containing compounds to the methane mixture, thus allowing one to use higher methane concentrations [1-5]. The presence of oxygen leads to diamond film deposition even at methane concentrations where only the graphite component is normally detected when methane and hydrogen gas mixtures are used. Oxygen is added either directly or as an integral part of the carbon carrier, such as mixtures containing CO or CO 2. The beneficial effect of the addition of oxygen-containing molecules is attributed to the enhancement of the etching of non-diamond components by hydrogen or oxygenated species (O or OH) generated in the plasma. The atomic C - H - O 0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PH $0040-6090(97)00090-4
phase diagram provides a common basis for all low-pressure CVD methods [5]. Successful diamond deposition using oxygen is restricted to a well-defined area within this diagram. Although many of these works deal with the effect of the addition of oxygen compounds, no systematic studies have been made on the separate action of the oxygen on the different stages (nucleation and growth) of the deposition process of diamond and graphite. In this work, we present the effect of oxygen on the nucleation and growth processes of diamond film deposition in the 0-2.5% oxygen range using M W C V D techniques. The deposition rates of diamond and non-diamond phases were determined by Raman spectroscopy and scanning electron microscopy (SEM). The results have been related to the changes in the plasma composition (mainly the OH, O and CO species), as detected by optical emission spectroscopy and mass spectrometry.
2. Experimental Diamond films were deposited in a M W C V D system at 2.45 GHz, excited with an ASTEX 1500 W magnetron
C, Gdmez-Aleixandre et aI. / Thin Solid Films 303 (1.997) 3 4 - 3 8
plasma source, on (100) silicon substrates which were previously scratched with I # m diamond paste. During the deposition process, the silicon substrate was placed on a graphite holder. The total pressure, substrate temperature and plasma power were maintained constant at 40 Torr (5.3 X 10 -2 bar), 900 °C (1173 K) and 1200 W, respectively. Gas mixtures were typically H 2, 2 vol.% C H 4 and 0-2.5 vol.% 02, with the total glass flow maintained at 400 sccm. The diamond content in the films has been determined from the Raman spectra by fitting the experimental spectra to a sum of gaussian and lorentzian curves [6]. The morphology of the films was studied by SEM. Also, SEM cross-sectional views have been used for determining the thickness of the diamond films. The excited species in the plasma during the deposition process were analyzed by optical emission spectroscopy (OES) in the 2 0 0 - 8 0 0 nm range using a PT Analytical Spectrometer (model PSS100), directly focused into the middle of the plasma discharge. A small flow of Ar (6 sccm) was added to act as an actinometer [7]. A portion of the plasma atmosphere in the reactor was routed to a differentially pumped quadrupole mass spectrometer (Hiden Analytical Ltd. model DSMS) in order to identify the stable species present in the reactor.
3. Results As it is known, during M W C V D process of diamond films from diluted CH 4 in H 2 gas mixtures, the deposition rate increases as the C H 4 concentration increases owing to the higher concentration of carbon containing species in the plasma. In a previous work we have determined, by Raman spectroscopy and SEM, the deposition rates of the different carbon phases (i.e. diamond and graphite) when the CH 4 content in the gas mixture increases in the 0 - 3 % range [8]. We observed an increase in the graphite component when the C H 4 concentration becomes higher, leading to lower quality diamond films. The addition of 0 2 in a very low concentration to the C H 4 4- H 2 mixture produces changes in the diamond and graphite deposition rates, as shown in Table 1. Also shown in this table are the relative intensities of the peaks corresponding to the CxH s species in the mass spectra. Other
t-"
0.6-
35
(a)
"&
diamond
v
0.4:=
0.2-
O
•
•
graphite
a9
0.0
100- (b)
o
90-
O O "O e'-
o E
80-
tm
oxygen concentration (%)
Fig. 1. (a) Diamond and graphite deposition rates, (b) diamond content in the films versus the 0 2 concentration in CH4/H 2 / 0 2 gas mixtures. oxygen-containing species such as, OH, H , O and CO are also detected in the discharge. The addition of 0.25% of 0 2 to the mixture leads to a strong decrease in the deposition rates of both, diamond and graphite, with a much larger reduction in the graphite deposition rate. Furthermore, the addition of small amounts of 0 2 to the hydrocarbon and H 2 gas mixture produces a slight increase in the intensity of the CxHy peaks in the mass spectra. Therefore, the reduction in the deposition rate must be interpreted as a consequence of new processes inhibiting the deposition mechanism, as we will see in the following. The influence of further additions of 0 2 on the diamond and graphite deposition rates as well as on the diamond content in the films are shown in Fig. l(a) and l(b), respectively. At 0 2 concentrations between 0.25 and 1%, both deposition rates remain almost constant. For 0 2 concentrations higher than 1%, the deposition rates decrease although a higher diamond content in the films is reached. Finally, when [0 2 ] > 2.5% is used no continuous films are obtained which means that the deposition rates are extremely low [9].
Table 1 Diamond and graphite deposition rates and relative intensity of mass specti6m&McC:~H>.peaks in 2% CH4/H 2 and 2% CH4/H2/0.25% 02 mixtures Gas mixture Deposition rate (tam h- I ) Relative intensity of mass spectra peaks (a.u.) (%) CH4:H 2 2:98 CH4:H2:O 2 2:97.75:0.25
Diamond 1.139 0.480
Graphite 0.510 0.I20
CH4
CH ~ 7.0 9.2
C 2H 2 6.3 7.8
C 2H 4 1.9 5.3
0.0 0.874
36
c. Gdmez-AIeixandre et al. / Thin Solid Fibns 303 (1997) 34-38
The changes in the plasma composition have been studied by OES and mass spectrometry. Fig. 2 shows the variation of the relative intensity of the major intensity peaks in the mass spectra versus the oxygen concentration. It is observed that when oxygen is progressively added to the gas mixture the signals corresponding to the C x H : species, coming from the decomposition of the CH 4 molecules continuously decrease. As for the oxygenated species (O, OH, H20, CO and CO 2) in the plasma, a different behavior is observed: (a) the intensity of the HaO and OH mass peaks initially remains fairly constant and then increase after oxygen additions higher than 1 and 2%, respectively; (b) the CO peak increases higher when the oxygen content in the gas mixture increases; (c) the atomic oxygen is only detected for [0 2] > 1%, showing an increase in its peak intensity at increasing oxygen concentrations. The main features of the emission spectra of the C H 4 / H 2 / O 2 plasma include: the hydrogen Balmer series (H a, He, H~), the activated carbon or hydrocarbon species (CH + at 360 nm and 400 nm and CH at 390 rim) and some emission in the 300-312 nm range, associated with CO and OH species. As mentioned before for the actinometer measurements we deliberately added a small amount of a noble gas to the reactive plasma and monitored the noble gas emissions concurrently with those of the reactive particles. Since the noble gas density is know, the excitation efficiency of any of its levels is determined simply by dividing the emission intensity of that level by the noble gas density. The variation of the relative intensity of the
1.2
0.8. (a) :5
/
0.4
~
OH
,r'-
._~
0.0
~
:
.
T
,
~
--.4
,
6'3
151(b)
0
'
I CO'I1"" lO4/ . . . . ..,,, /
]C.H'N. ,,'"
010c
1
2
o oxygen concentration (~A) Fig. 2. Relative intensity of the main peaks in the mass spectra versus the
02 concentration in CH4/Hz/02 mixtures.
Table 2 Variation of CO/OH, H~ and CH+ (360 nm and 400 nm) emission peaks with the oxygen content in the gas mixture [02 ] CO/O H,~ CH+ CH+ (%) (300-312nm) (656.3nm) (360 nm) (400 nm) 0.25 0.50 1.00 1.50 2.00 2.30
0.046 0.047 0.048 0.052 0.053 0.054
9.45 i0.40 8.00 8.50 7.75 %10
0.1880 0.1855 0.1860 0.1840 0.1845 0.1820
0.1685 0.1675 0.1655 0.1657 0.1650 0.1630
most important emission peaks ( C O / O H in the 300-312 nm range, H a at 656.3 nm and CH ÷ at 360 and 400 nm) with the oxygen content are given in Table 2. The CH + species, which participate in the diamond deposition process [10] show a decrease of their emission peak intensity, which can be explained by the consumption of the carbon containing species by reaction with atomic or molecular oxygen. The production of OH radicals and H 20 molecules can explain the global decrease behaviour found in the atomic hydrogen signal when the O z flow increases from 0.25 to 2.3, as shown in Table 2.
4. Discussion Within the concentration range of the gas mixture studied in this work (2% [CH 4] and 0.25-2.5% [02]), the carbon mole fraction [ C / ( C + O)] varies in the 0.30-0.80 range. We have observed that films with high diamond content, > 95% diamond content analyzed by Raman spectrometry, can be obtained from [ C / ( C + O)] = 0.33 up to 0.44. A narrower range (0.58-0.50) was previously associated with diamond deposition in the C / H / O phase diagram, proposed by Bachmann et al. [5]. The unexpected enlargement in the [ C / ( C + O)] range (low side values) can be explained by an increase in the concentration of carbon species in the reactor. This means that the real carbon species concentration is likely to be higher than that corresponding to the 2% C H 4, fed to the reactor. This effect is believed to be due to the etching of the graphite holder during the deposition process, since for the same experimental conditions, no growth of the diamond film is detected in our MWCVD system when a molybdenum plate is placed over the graphite holder. In any case, we think that the influence of oxygen must be related to the absolute oxygen concentration rather than to the relative mole fraction, which is strongly affected by the carbon species concentration. Therefore, in the following, we shall discuss the results in terms of the effect of the addition of oxygen to the CH 4 + H 2 gas mixture at increasing concentrations, firstly in a small concentration (0.25%) and then in greater amounts (0.25-2.5% range).
C. G6mez-Aleixandre er al. / Thin Solid Films 303 (1997) 34-38
4.1. Effect of the oxygen addition in a small concentration (O.25%) The observed decrease in the deposition rate of the films (diamond and graphite contributions) in Table 1 with the addition of oxygen in small concentrations to the gas mixture has been also reported by other authors [2,4]. Generally, the decrease is explained by etching processes parallel to that caused by the atomic hydrogen during the deposition. Howard et al. [3] have also pointed out that atomic oxygen can poison the growing surface by forming carbon bridges or double bonds with surface carbon atoms, which makes the growth of the films more difficult. As can be seen in Table 1, the addition of 0.25% of oxygen produces a 60% decrease in the diamond deposition rate, while an 80% reduction is detected in the graphite component rate. This result further supports the assumption that additional etching processes are acting during the deposition [11]. In addition, one should consider changes in the formation chemistry of the diamond phase in order to explain these effects. Although no signal of atomic oxygen is detected by mass spectrometry after the addition of 0.25% 02 , other oxygenated species such as OH, H20, CO and CO, are present in the discharge (see Fig. 2). It is well established during the deposition process, that OH radicals play a similar role to that of atomic hydrogen, both as etchant of the non-diamond component and as stabilizer of sp 3 bonds in the diamond structure [12]. The formation of OH radicals and H20 molecules bring about the consumption of atomic hydrogen of the plasma, as has been detected by OES. In order to discuss with more detail the influence of the oxygen on the deposition rate we can separate the diamond film deposition process into two stages: (1)nucleation; (2) growth of the continuous film. During the nucleation process, the diamond nuclei development requires a critical thickness of an amorphous carbon layer followed by its faceting through etching processes [8]. In this work, the emission peak of the CH species, which is known to participate in the amorphous carbon deposition, does not change with the addition of oxygen to the gas mixture. Hence, no appreciable changes in the deposition rate of the amorphous carbon layer can be expected. However, the high etching rate of the amorphous layer by the OH radicals present in the discharge can hinder the deposition of the critical thickness, thus producing an increase in the incubation period (time lapsed before the diamond nuclei are formed). In this respect, some authors have also reported that the addition of oxygen to the CH 4 + H 2 gas mixture produces a decrease in the nucleation density of the films [4]. Therefore, this effect of the oxygen addition on the nucleation process of the diamond crystals can explain the drastic reduction in both deposition rates (diamond and graphite). Furthermore, after the addition of O~, the CO signal is detected by mass spectrometry. The
37
CO formation could arise from the etching of the carbon layer, although some authors have also proposed the reaction with the carbon-hydrogen species present in the plasma as an alternative mechanism [3,13].
4.2. Effect of adding 02 at increasing concentrations (0.25-2.5%) It is important to note that, although the initial addition of 02 to the CH 4 + H 2 gas mixture produces a remarkable reduction in both deposition rates (diamond and graphite), during subsequent oxygen additions (up to t%), no changes in both deposition rates, and hence in the quality of the films, are detected. As can be observed in Fig. 2, in the 0.25-1% intermediate range the mass spectrum signal corresponding to the OH etchant agent does not change, whereas the C~H~ species decrease at increasing oxygen concentrations. In addition, no signal of atomic oxygen is detected in this range. Among the oxygenated species, only the CO signal continuously increases when the oxygen concentration becomes higher. These results indicate that in this range the etching processes are expected not to change with respect to the previous range (i.e. those associated with the OH radicals). The decrease in the hydrocarbon species concentration along with the increase in the CO signals further suggest that the oxidation reaction of these hydrocarbon species is taking place. However, these reaction processes seem not to affect the formation of the film since their rates remain unaltered. In the high concentration side ([O z] > 1%), the atomic oxygen signal is detected by mass spectrometry. Its peak intensity increases at increasing oxygen concentrations in the gas mixture. In this regard it is well know that the etching rate of oxygen on graphite is over 50 times larger than that of hydrogen [13] and, in addition, the OH radicals etch efficiently the non-diamond component in the films. Therefore, this enhancement in the etching process should be accompanied by an improvement in the quality of the films. This is in agreement with the results of Fig. l(b). Finally, when the oxygen concentration is higher than 2% the diamond and graphite etching rates are even higher than the corresponding formation rates, thus impeding the growth of a continuous fihn.
5. Conclusions
During diamond film deposition by MWCVD, a decrease in the formation rate along with an increase in the etching rate of the films has been detected when molecular oxygen is added to a 2% C H 4 / H 2 gas mixture. The nucleation process is supposed to be retarded by the oxygen presence, which adversely influences the initial growth of the films. On the other hand the presence of OH radicals, considered efficient etchant agents, produces an
38
C. Gdmez-Aleixandre et aL / Thin Solid Fihns 303 (]997) 34-38
increase in the etching rate of the films, thus giving rise finally to a reduction of the total deposition rate. In the 0 . 2 5 - 1 % [ 0 2 ] range the oxidation reactions o f the hydrocarbon species seems not to interfere with the chemical deposition process of d i a m o n d and graphite, whose rates remain constant in this range. However, for further oxygen additions, from 1% up to 2.5%, we have observed a decreased in both deposition rates which is attributed to the presence of atomic oxygen, i m p e d i n g the formation of a continuous film on the substrate.
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[2] J.A. Mucha, D.L. Flamm and D,E. Ibottson, J. AppL Pyys.. 65 (1989) 3448. [3] W.N. Howard, K.E. Spear and M, Frenklach, Appl. Phys, Left,, 63 (1993) 2641, [4] C.-F. Chen, T.-M. Hong and S.-H, Chen, J. Appl. Phys., 74 (1993) 4483. [5] P.K. Bachrnann, D. Leers and J. Lydtin, Di~mtond Related Mater., (199I) 1. [6] D.S. Knight and W,B, White, J. Mater. Res.. 4 (1989) 385. [7] J.W. Coburn and M. Chen, J. Appt, Phys., 5t (I980) 3134. [8] C. G6mez-Aleixandre, O. S~nchez, M.M. Garc~a, L. Vfizquez and J,M. Albella, Physica Status Solidi (a), 154 (1996) 23. [9] O. Sfinchez, C. G6rnez-Aleixandre, F. Agultd and J.M. Albella, Diamond Related Mater., 3 (1994) 1183. [10] C. Gdmez-Alelxandre, O, Sfinchez and J.M Albella, J. AppL Phys., 74 (t993) 3752. [1 t] T.R. Anthony, Vacuum, 41 (1990) 1356. [12] Y. Saito, K. Sato, H. Tanaka, K. Fujita and S. Matuda, J. Mater. Sci., 23 (1988) 842. [13] Y. Muranaka, H. Yamashita, K. Sato and H. Miyadera, J. Appl. Phys., 67 (1990) 6247.