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Low temperature limits of diamond film growth by microwave plasma-assisted CVD
ELSEVIER
Diamond and Related Materials 5 (1996) 226-230
D IAMOND AND RELATED MATERIALS
Low temperature limits of diamond film growth by microwave plasmaassisted CVD J. Stiegler, T. Lang, M. Nyggtrd-Ferguson, Y. von Kaenel, E. Blank Laboratoire de MOtallurgie Physique, Ecole Polytechnique Fdddrale de Lausanne, CH-I015 Lausanne, Switzerland
Abstract This paper describes the low temperature deposition (270-540°C) by microwave-plasma assisted chemical vapor deposition (CVD) of diamond thin films using various gas mixtures. The substrate temperature was controlled without adjusting other deposition parameters. It has been observed that limits for low temperature growth of diamond films depend substantially on the gas system employed as well as on the carbon concentration in the particular system. Generally, a characteristic transition from crystalline growth to deposits containing a strongly increased amount of non-diamond carbon occurred when the deposition temperature was lowered. This has been defined as the low temperature limit of diamond film growth. The conventional CH4/H 2 systems have the highest transition temperatures. The addition of oxygen permits film deposition at lower substrate temperatures; the lowest have been realized in CO-rich systems. The dependency of the growth limits on crucial deposition parameters is discussed in terms of abundant gas phase species and surface processes, in particular, the role of oxygen in the gas phase and surface chemistry. Keywords: Diamond; Microwave plasma CVD; Low substrate temperature; Surface
1. Introduction Since the first successful demonstration of CVD diam o n d growth, extensive efforts have been made to exploit the exceptional properties of diamond. One of the most promising branches of this development appears to be low temperature growth of diamond ( L T G D ) films in order to extend the range of possible substrate materials for optical and mechanical applications. However, a detailed survey of the available literature on L T G D reveals that activities are relatively few in this branch, considering the great flood of publications on CVD diamond in general. It appears that there are serious obstacles, not only of an experimental nature, which make L T G D a difficult area of research. Some of these, e.g. enhancement of amorphous carbon deposition, deterioration of crystallinity and uneconomical growth rates, have recently been summarized [-1]. Despite this, promising results have been achieved using a variety of deposition methods [-2-7]. It has been claimed that excess amounts of atomic hydrogen and the addition of oxygen are crucial factors particularly for LTGD. The aim of this paper is to study different characteristic tendencies of a structural and morphological nature for 0925-9635/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0925-9635(95)00349-5
diamond film deposition, whilst the deposition temperature is lowered. The effect of important process parameters is examined, including the principal question of the minimum substrate temperature at which diamond layers can be grown.
2. Experimental Deposition was carried out in a classical microwave tube reactor onto (100) silicon. To establish conditions using low substrate temperatures (Ts), the substrate holder was fitted with an active cooling system operating with compressed air or water. This configuration enabled us to study independently the effects of important deposition parameters on L T G D , e.g. microwave power, pressure, gas mixture, etc. A reliable measurement of Ts is a basic prerequisite for LTGD. Here, it was measured directly by means of an I R pyrometer, which records thermal radiation from the substrate at approx. 8 gm wavelength through the plasma. Moreover, in-situ diagnostics of the growing film was possible, providing valuable information in real time, e.g. measurement of growth rates [8]. All films were grown to the same layer thickness (2.6-2.8 gm) to
J. Stiegler et aL/Diamond and Related Materials 5 (1996) 226-230
enable proper comparison of morphological, structural and growth-kinetic features. A high nucleation density was always attained in order to reduce artifacts induced by the nucleation period, i.e. most of the deposit grew as a film. To study LTGD, numerous deposition series were realized with different values of Ts. Each single series was characterized by a particular gas mixture, all other parameters were held constant. The gas mixtures used ranged from the conventional system CH4/H 2 through intermediate systems C H 4 / C Q 2 ( Q z ) / H 2 t o the CO-rich s y s t e m C H 4 / C O 2. The composition of these growth environments under plasma conditions could be characterized by optical emission spectroscopy (OES) [-9] and mass spectroscopy of stable species. Structural investigation was performed by Raman spectroscopy (spot size approx. 200 gm) and SEM imaging of the surface morphology. Proper criteria to distinguish between diamond and amorphous (hydrogenated) carbon deposits are lacking, partly due to the simple fact that the transition between these forms may occur continuously. In this paper diamond films are defined as those deposits which show the characteristic Raman peak of diamond. It should be emphasized that other structural investigation methods, in particular X-ray diffraction, are able to detect large amounts of diamond phase in deposits which are non-diamond according to this rather practical definition [ 10].
3. Results
Since the majority of research on CVD diamond is still carried out with conventional mixtures CH4/H2, the possibilities of this system for LTGD was evaluated first. Fig. 1 summarizes the study performed at various C H 4 f l o w s ( F M : 0.3 5 sccm) and substrate temperatures (Ts ~< 540 °C) in the form of a deposition map. The proposed division between crystalline, transitional and cauliflower structure is based on our structural investigation, shown below. It may be seen from Fig. 1 that the range of F M for diamond film deposition is progressively narrowed on lowering Ts. Accordingly, at a fixed value of FM, when Ts is decreased, the crystalline range of deposition is replaced by a cauliflower type of deposit. The border of this transition is shifted to lower T s with decreasing FM. At the transition zone, the cauliflower structure is accompanied by interference fringes due to the smooth surface. Most of these films peeled off from the silicon, apparently due to high residual stresses. In order to confirm the results of Fig. 1, Raman and SEM investigations of all films were performed. As a result, horizontal and vertical cuts through the deposition map (constant FM and Ts) reveal very similar trends. Raman spectra and selected SEM images of a
227
Total gas flow: 100 sccm, 800 W, 60 Torr [] crystalline • transition x cauliflower
x
t,) 4.
0
3
.-~
2
x
X
x
x
320
o
360
•
•
G
@
[]
[]
o
o
a
o
o
[]
o
o
[3
o
o
o
o
400
440
480
520
560
Substrate temperature (°C) Fig. 1. Deposition map of the gas system CH4/H z at low substrate temperatures.
set of samples grown with 2 sccm CH 4 are shown in Figs. 2 and 3, elucidating the general trend occurring when Ts is lowered. The crystal size is continuously diminished. When Ts is decreased, the morphology becomes dominated by small, pyramidal crystals with four {111} facets. These begin to roughen at the lowest temperatures and in a narrow range crystallinity is then completely lost and a cauliflower structure appears. The accompanying Raman spectra also reveal a continuous development and can be correlated to the morphology. The diamond peak at approx. 1333 cm -1 becomes less apparent when Ts is lowered. The signal is no longer distinguishable from the intense background when the cauliflower structure is identified by SEM. A second striking result is the strongly increasing fluorescence background, which reaches a maximum and is, in some 2.5
104
2 104 ,--:. = 1.5 104
.~
1 10 4
5000
0¸ 500
1000
1500
2000
Wavenumber (cm-1) Fig. 2. Raman spectra of films with the particular mixture 2/98 sccm CH4/H2 when the deposition temperature is lowered.
228
J. Stiegler et al./Diamond and Related Materials 5 (1996) 226-230
(a)
(b)
(c)
(d)
Fig. 3. SEM images of films grown with 2 sccm CH4/H2: (a) 540 °C; (b) 460 °C; (c) 400 °C; and (d) grown with 29.5/30.5 sccm CH4/CO 2 at 420 °C.
cases, again lowered when non-crystalline structures begin to grow. It appears that, in particular, the background feature reflects unambiguously the content of non-diamond carbon. It has often been stated that oxygen benefits diamond deposition particularly at low Ts. In a first approach, the addition of small amounts of CO2 or 02 to the C H 4 / H 2 system was studied. Even small oxygen concentrations were found to improve the film quality. To evaluate the effect on low temperature limits, four series (CH4/CO2/H2: 2/0/98, 2.6/0.6/97, 3.2/1.2/96 and 4/2/94 sccm) with varying Ts were investigated. These mixtures are characterized by a constant carbon-to-oxygen surplus. The low temperature limits were found at 420, 380, 360 and 360 °C, respectively. Essentially the same results were obtained with the use of O2 as additive gas instead of CO2. In a second approach, a CO-rich gas system was used to examine LTGD films. Fig. 4 illustrates the results for
the system CH4/CO2, revealing very similar features to the system C H 4 / H 2. Generally, the deposition limit is shifted considerably towards lower values of T s, the lowest temperature realized was 270 °C. In Fig. 5 Raman spectra of films are shown which were grown with a mixture of 29.5/30.5 s c c m C H 4 / C O 2 when Ts is reduced (see also SEM image of Fig. 3(d)). It is obvious that the diamond phase purity is greatly enhanced with respect to the conventional system. In contrast to C H 4 / H 2 mixtures, very smooth { 111 } facets appear. The crystal shape is typically determined by intersecting twin planes and renucleation processes are largely suppressed. The morphological development produced by lowering Ts is similar, even though it occurs at lower temperatures. Again, facet roughening of pyramidal crystals of reduced size marks the border of crystalline growth. Fig. 4 indicates a twofold definition of the growth limit. The correlation established by comparing Raman and SEM investigation in the CH4/(CO2/)H2 systems is
Z Stiegler et aL/Diamond and Related Materials 5 (1996) 226 230 31.5
Total gas flow: 60 sccm, 600 W• 60 Torr 31
G crystalline, diamond peak crystalline, no diamond peak transition
*
• tj ,~
G
[] D
30.5
ao~ 29"51
'i
O
28 5 t . . . .
m
oi . . . . 300
250
m
[]
i . . . . i .... 350 400
m
i .... 450
500
550
Substrate temperature (°C) Fig. 4. D e p o s i t i o n m a p o f t h e g a s s y s t e m C H 4 / C O 2 at l o w s u b s t r a t e temperatures.
29.5/30.5 sccm C H 4 / C O 2 , 600 "gV,60 T o r r
3.5 10 43 10 4 2.5 10 42 10 4 1.5 10 4
"
1 104 5000
460°(2
~y_~c-~0 500
........
I ......... 1000
g0~U- t i
-
L
1500
2000
Wavenumber (cm"1) Fig. 5. R a m a n s p e c t r a o f films w i t h t h e p a r t i c u l a r m i x t u r e 29.5/30.5 s c c m C H 4 / C O 2 w h e n t h e d e p o s i t i o n t e m p e r a t u r e is l o w e r e d .
not observed with C H 4 / C O 2 mixtures, The Raman signal of diamond completely vanishes, although the structure exhibits a crystalline character with grain sizes of several microns at lower Ts. These spectra show two prominent features at approx. 1420 and 1515cm -1 (see Fig. 5). Different investigation methods will be necessary to reveal the structure of these deposits.
4. Discussion
The analysis of the deposition experiments suggests that for a particular gas system the carbon content is not only a decisive parameter for the quality of the deposit, but it also determines the limit of crystalline growth when Ts is lowered. As reported previously [ 1],
229
deposition of non-diamond carbon is increased and the film quality deteriorates seriously• The present investigation has been performed solely by changing the substrate temperature. This leads to the supposition that there must be surface processes that control substantially the limits of LTGD. In general, crystalline or amorphous growth from the vapor phase is balanced by the rate of arrival of precursors from the gas phase and the rate at which they can diffuse across the surface, i.e. the ability to find their ways to correct sites by surface diffusion [11,12]. Minimum growth temperatures thus depend on the supersaturation of growth radicals, which is in turn correlated to the initial C H 4 concentration, as observed experimentally• Surface diffusion, described by an Arrhenius-type expression in simple vibrational diffusion theory, is increasingly hindered when Ts is lowered. Crystalline growth transforms into amorphous growth, when the rate of arrival exceeds the rate of diffusional transport at the surface• With available data of radical concentrations in the CH4/H 2 system [13], growth limits for CVD diamond can be calculated, predicting transition temperatures in the range 400-450 °C, which corresponds well with these experimental results. There have been doubts expressed that surface diffusion is operating in the case of CVD diamond, because bonding of adatoms is strong and the diamond surface is believed to be hydrogen-terminated during growth• However, recent experiments and theoretical calculations have shown that precursors can migrate via covalent bond breaking and formation in a manner equivalent to surface diffusion, whose rate is limited by temperature dependent reactions, namely H atom abstraction and addition [14-16]. A decrease in temperature should form an increasing number of lattice defects and eventually non-diamond carbon• There are additional parameters likely to influence low temperature limits• A lower deposition pressure should be equivalent to a reduction of the C H 4 c o n c e n t r a t i o n at a fixed pressure. Recent investigations in CO-rich systems reveal that the pressure dependence might be important [7]. The input microwave power is thought to be decisive, since atomic hydrogen concentrations will be appreciably influenced [9]. One fundamental role of atomic hydrogen in CVD diamond has been postulated to be preferential etching of nondiamond carbon, thereby improving the quality of the deposit [5]. The uniform thickness of the films makes it feasible to discuss morphological and structural features directly related to LTGD. From the decreasing particle size it is concluded that renucleation processes become important when Ts is lowered; this is in part responsible for the observed degradation of the film quality. Two characteristic morphologies occur in these experiments; crystalline growth with {111 } faceting and cauliflower structures at very low Ts or e x c e s s C H 4 concentrations• The former
230
J. Stiegler et aL/Diamond and Related Materials 5 (1996) 226-230
corresponds to sufficient surface diffusion to maintain crystallographic shapes. Renucleation is likely to be induced by structural defects on {111} planes [17]. There is a certain tendency of persisting columnar growth for each crystal, but not throughout the film. TEM investigations have shown that {111 } faceted films contain numerous microtwins, stacking faults and dislocations throughout the volume [18]. This may explain the generally poor quality of the deposit in the CH4/H2 system. The cauliflower type has been shown to possess a columnar structure separated by boundaries with each column consisting of massive randomly oriented nuclei [17]. This morphology is observed under conditions of high carbon supersaturation or limited surface diffusion causing extensive renucleation events. These experiments strongly support the fact that addition of oxygen is not only a useful but a necessary condition for LTGD. Even small amounts of oxygen improve crystallinity and, most importantly, make diamond film deposition possible at lower substrate temperatures. It is well established that oxygen plays a major role in controlling the gas phase chemistry [19]. The abundant OH radical is supposed to be a strong etchant of non-diamond carbon [20], a similar function is ascribed to atomic oxygen. This statement seems to be confirmed in these experiments, as small amounts of oxygen improve the film quality but lower the deposition rate. However, the kinetic analysis of LTGD with oxygen as additional gas component and the very morphological appearance of the deposits in CO-rich systems suggest that oxygen or related species must participate directly in the diamond growth process, e.g. by desorption from partially oxygen-covered surfaces [21]. Additionally, the particular importance of CO-rich gas systems has been demonstrated. Such mixtures are indeed most favorable for diamond film growth at low substrate temperatures. Deposits show the best phase purity as evaluated by Raman spectroscopy, and minimum growth temperatures are considerably lowered.
5. Conclusions The deposition of diamond thin films at low substrate temperatures has been extensively studied with the conventional CH4/H 2 gas system. A generally poor quality of the deposit has been observed owing to numerous growth defects incorporated into {111} facets and frequent renucleation. When the substrate temperature is lowered, deposition of amorphous components is strongly enhanced and the diamond phase purity deteriorates seriously. Finally, growth of crystalline diamond is thought to be inhibited by insufficient surface diffusion. The low temperature limit of crystalline growth is mainly influenced by the carbon content in the gas mixture,
allowing a reduction of the growth temperature by reduced C H 4 concentrations. In view of the numerous limitations of the CH4/H 2 system, the addition of oxygen to the gas atmosphere has been shown to play a crucial role in LTGD. Not only is the phase purity greatly improved, but also growth is possible at lower deposition temperatures. Particular importance has been attached to CO-rich gas systems which are most favorable for this approach. It is further supposed that oxygen or oxygen containing species participate directly in the diamond growth process at low substrate temperatures.
Acknowledgment The authors would like to thank Mr. B. Senior for technical assistance with the SEM. Financial support by the Kommission for die wissenschaftliche Forschung, Bern (Switzerland), is gratefully acknowledged.
References Eli Y. Muranaka, H. Yamashita and H. Miyadera, Diamond Relat. Mater., 3 (1994) 313. [2] Y. Liou, A. Inspektor, R. Weimer, D. Knight and R. Messier, J. Mater. Res., 5 (1990) 2305. [3] J. Wei, H. Kawarada, J. Suzuki and A. Hiraki, J. Cryst. Growth, 99 (1990) 1201. [4] S. Nakao, S. Maruno, M. Noda, H. Kusakabe and H. Shimizu, J. Cryst. Growth, 99 (1990) 1215. [5] Y. Muranaka, H. Yamashita and H. Miyadera, J. Appl. Phys., 69 (1991) 8145. [6] A. Yamaguchi, M. Ihara and H. Komiyama, Appl. Phys. Lett., 64 (1994) 1306. E7] C.-F. Chen, S.-H. Chen, H.-W. Ko and S.E. Hsu, Diamond Relat. Mater., 3 (1994) 443. [8] A.J. SpringThorpe and A. Majeed, J. Vac. Sci. Technol. B, 8 (1990) 266. [9] T. Lang, J. Stiegler, Y. yon Kaenel and E. Blank, Diamond Relat. Mater., 5 (1996). [10] H. Eto, Y. Tamou, Y. Ohsawa and N. Kikuchi, Diamond Relat. Mater., 1 (1992) 373. [11] P.M. Dryburgh, J. Cryst. Growth, 87 (1988) 397. [12] A.T. Voutsas and M.K. Hatalis, J. Electroehem. Soe., 139 (1992) 2659. [13] W.L. Hsu, J. Appl. Phys., 72 (1992) 3102. [14] W.J.P. van Enckevort, G. Janssen, W. Vollenberg, J.J. Schermer, L.J. Giling and M. Seal, Diamond Relat. Mater., 2 (1993) 997. [15] M. Frenklach, S. Skokov and B. Weiner, Nature, 372 (1994) 535. [16] S. Skokov, B. Weiner and M. Frenklach, J. Phys. Chem., 98 (1994) 7073. [17] W. Zhu, R. Messier and A.R. Badzian, Proe. SPIE, 1534 (1991) 230. [18] B.E. Williams and J.T. Glass, J. Mater. Res., 4 (1989) 373. [19] R. Beckmann, B. Sobisch and W. Kuliseh, Diamond Relat. Mater., 4 (1995) 256. [20] S.J. Harris and A.M. Weiner, Appl. Phys. Lett., 55 (1989) 2179. [21] R.E. Thomas, R.A. Rudder and R.J. Markunas, J. Vac. Sci. Technol. A, 10 (1992) 2451.
Diamond and Related Materials 5 (1996) 226-230
D IAMOND AND RELATED MATERIALS
Low temperature limits of diamond film growth by microwave plasmaassisted CVD J. Stiegler, T. Lang, M. Nyggtrd-Ferguson, Y. von Kaenel, E. Blank Laboratoire de MOtallurgie Physique, Ecole Polytechnique Fdddrale de Lausanne, CH-I015 Lausanne, Switzerland
Abstract This paper describes the low temperature deposition (270-540°C) by microwave-plasma assisted chemical vapor deposition (CVD) of diamond thin films using various gas mixtures. The substrate temperature was controlled without adjusting other deposition parameters. It has been observed that limits for low temperature growth of diamond films depend substantially on the gas system employed as well as on the carbon concentration in the particular system. Generally, a characteristic transition from crystalline growth to deposits containing a strongly increased amount of non-diamond carbon occurred when the deposition temperature was lowered. This has been defined as the low temperature limit of diamond film growth. The conventional CH4/H 2 systems have the highest transition temperatures. The addition of oxygen permits film deposition at lower substrate temperatures; the lowest have been realized in CO-rich systems. The dependency of the growth limits on crucial deposition parameters is discussed in terms of abundant gas phase species and surface processes, in particular, the role of oxygen in the gas phase and surface chemistry. Keywords: Diamond; Microwave plasma CVD; Low substrate temperature; Surface
1. Introduction Since the first successful demonstration of CVD diam o n d growth, extensive efforts have been made to exploit the exceptional properties of diamond. One of the most promising branches of this development appears to be low temperature growth of diamond ( L T G D ) films in order to extend the range of possible substrate materials for optical and mechanical applications. However, a detailed survey of the available literature on L T G D reveals that activities are relatively few in this branch, considering the great flood of publications on CVD diamond in general. It appears that there are serious obstacles, not only of an experimental nature, which make L T G D a difficult area of research. Some of these, e.g. enhancement of amorphous carbon deposition, deterioration of crystallinity and uneconomical growth rates, have recently been summarized [-1]. Despite this, promising results have been achieved using a variety of deposition methods [-2-7]. It has been claimed that excess amounts of atomic hydrogen and the addition of oxygen are crucial factors particularly for LTGD. The aim of this paper is to study different characteristic tendencies of a structural and morphological nature for 0925-9635/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0925-9635(95)00349-5
diamond film deposition, whilst the deposition temperature is lowered. The effect of important process parameters is examined, including the principal question of the minimum substrate temperature at which diamond layers can be grown.
2. Experimental Deposition was carried out in a classical microwave tube reactor onto (100) silicon. To establish conditions using low substrate temperatures (Ts), the substrate holder was fitted with an active cooling system operating with compressed air or water. This configuration enabled us to study independently the effects of important deposition parameters on L T G D , e.g. microwave power, pressure, gas mixture, etc. A reliable measurement of Ts is a basic prerequisite for LTGD. Here, it was measured directly by means of an I R pyrometer, which records thermal radiation from the substrate at approx. 8 gm wavelength through the plasma. Moreover, in-situ diagnostics of the growing film was possible, providing valuable information in real time, e.g. measurement of growth rates [8]. All films were grown to the same layer thickness (2.6-2.8 gm) to
J. Stiegler et aL/Diamond and Related Materials 5 (1996) 226-230
enable proper comparison of morphological, structural and growth-kinetic features. A high nucleation density was always attained in order to reduce artifacts induced by the nucleation period, i.e. most of the deposit grew as a film. To study LTGD, numerous deposition series were realized with different values of Ts. Each single series was characterized by a particular gas mixture, all other parameters were held constant. The gas mixtures used ranged from the conventional system CH4/H 2 through intermediate systems C H 4 / C Q 2 ( Q z ) / H 2 t o the CO-rich s y s t e m C H 4 / C O 2. The composition of these growth environments under plasma conditions could be characterized by optical emission spectroscopy (OES) [-9] and mass spectroscopy of stable species. Structural investigation was performed by Raman spectroscopy (spot size approx. 200 gm) and SEM imaging of the surface morphology. Proper criteria to distinguish between diamond and amorphous (hydrogenated) carbon deposits are lacking, partly due to the simple fact that the transition between these forms may occur continuously. In this paper diamond films are defined as those deposits which show the characteristic Raman peak of diamond. It should be emphasized that other structural investigation methods, in particular X-ray diffraction, are able to detect large amounts of diamond phase in deposits which are non-diamond according to this rather practical definition [ 10].
3. Results
Since the majority of research on CVD diamond is still carried out with conventional mixtures CH4/H2, the possibilities of this system for LTGD was evaluated first. Fig. 1 summarizes the study performed at various C H 4 f l o w s ( F M : 0.3 5 sccm) and substrate temperatures (Ts ~< 540 °C) in the form of a deposition map. The proposed division between crystalline, transitional and cauliflower structure is based on our structural investigation, shown below. It may be seen from Fig. 1 that the range of F M for diamond film deposition is progressively narrowed on lowering Ts. Accordingly, at a fixed value of FM, when Ts is decreased, the crystalline range of deposition is replaced by a cauliflower type of deposit. The border of this transition is shifted to lower T s with decreasing FM. At the transition zone, the cauliflower structure is accompanied by interference fringes due to the smooth surface. Most of these films peeled off from the silicon, apparently due to high residual stresses. In order to confirm the results of Fig. 1, Raman and SEM investigations of all films were performed. As a result, horizontal and vertical cuts through the deposition map (constant FM and Ts) reveal very similar trends. Raman spectra and selected SEM images of a
227
Total gas flow: 100 sccm, 800 W, 60 Torr [] crystalline • transition x cauliflower
x
t,) 4.
0
3
.-~
2
x
X
x
x
320
o
360
•
•
G
@
[]
[]
o
o
a
o
o
[]
o
o
[3
o
o
o
o
400
440
480
520
560
Substrate temperature (°C) Fig. 1. Deposition map of the gas system CH4/H z at low substrate temperatures.
set of samples grown with 2 sccm CH 4 are shown in Figs. 2 and 3, elucidating the general trend occurring when Ts is lowered. The crystal size is continuously diminished. When Ts is decreased, the morphology becomes dominated by small, pyramidal crystals with four {111} facets. These begin to roughen at the lowest temperatures and in a narrow range crystallinity is then completely lost and a cauliflower structure appears. The accompanying Raman spectra also reveal a continuous development and can be correlated to the morphology. The diamond peak at approx. 1333 cm -1 becomes less apparent when Ts is lowered. The signal is no longer distinguishable from the intense background when the cauliflower structure is identified by SEM. A second striking result is the strongly increasing fluorescence background, which reaches a maximum and is, in some 2.5
104
2 104 ,--:. = 1.5 104
.~
1 10 4
5000
0¸ 500
1000
1500
2000
Wavenumber (cm-1) Fig. 2. Raman spectra of films with the particular mixture 2/98 sccm CH4/H2 when the deposition temperature is lowered.
228
J. Stiegler et al./Diamond and Related Materials 5 (1996) 226-230
(a)
(b)
(c)
(d)
Fig. 3. SEM images of films grown with 2 sccm CH4/H2: (a) 540 °C; (b) 460 °C; (c) 400 °C; and (d) grown with 29.5/30.5 sccm CH4/CO 2 at 420 °C.
cases, again lowered when non-crystalline structures begin to grow. It appears that, in particular, the background feature reflects unambiguously the content of non-diamond carbon. It has often been stated that oxygen benefits diamond deposition particularly at low Ts. In a first approach, the addition of small amounts of CO2 or 02 to the C H 4 / H 2 system was studied. Even small oxygen concentrations were found to improve the film quality. To evaluate the effect on low temperature limits, four series (CH4/CO2/H2: 2/0/98, 2.6/0.6/97, 3.2/1.2/96 and 4/2/94 sccm) with varying Ts were investigated. These mixtures are characterized by a constant carbon-to-oxygen surplus. The low temperature limits were found at 420, 380, 360 and 360 °C, respectively. Essentially the same results were obtained with the use of O2 as additive gas instead of CO2. In a second approach, a CO-rich gas system was used to examine LTGD films. Fig. 4 illustrates the results for
the system CH4/CO2, revealing very similar features to the system C H 4 / H 2. Generally, the deposition limit is shifted considerably towards lower values of T s, the lowest temperature realized was 270 °C. In Fig. 5 Raman spectra of films are shown which were grown with a mixture of 29.5/30.5 s c c m C H 4 / C O 2 when Ts is reduced (see also SEM image of Fig. 3(d)). It is obvious that the diamond phase purity is greatly enhanced with respect to the conventional system. In contrast to C H 4 / H 2 mixtures, very smooth { 111 } facets appear. The crystal shape is typically determined by intersecting twin planes and renucleation processes are largely suppressed. The morphological development produced by lowering Ts is similar, even though it occurs at lower temperatures. Again, facet roughening of pyramidal crystals of reduced size marks the border of crystalline growth. Fig. 4 indicates a twofold definition of the growth limit. The correlation established by comparing Raman and SEM investigation in the CH4/(CO2/)H2 systems is
Z Stiegler et aL/Diamond and Related Materials 5 (1996) 226 230 31.5
Total gas flow: 60 sccm, 600 W• 60 Torr 31
G crystalline, diamond peak crystalline, no diamond peak transition
*
• tj ,~
G
[] D
30.5
ao~ 29"51
'i
O
28 5 t . . . .
m
oi . . . . 300
250
m
[]
i . . . . i .... 350 400
m
i .... 450
500
550
Substrate temperature (°C) Fig. 4. D e p o s i t i o n m a p o f t h e g a s s y s t e m C H 4 / C O 2 at l o w s u b s t r a t e temperatures.
29.5/30.5 sccm C H 4 / C O 2 , 600 "gV,60 T o r r
3.5 10 43 10 4 2.5 10 42 10 4 1.5 10 4
"
1 104 5000
460°(2
~y_~c-~0 500
........
I ......... 1000
g0~U- t i
-
L
1500
2000
Wavenumber (cm"1) Fig. 5. R a m a n s p e c t r a o f films w i t h t h e p a r t i c u l a r m i x t u r e 29.5/30.5 s c c m C H 4 / C O 2 w h e n t h e d e p o s i t i o n t e m p e r a t u r e is l o w e r e d .
not observed with C H 4 / C O 2 mixtures, The Raman signal of diamond completely vanishes, although the structure exhibits a crystalline character with grain sizes of several microns at lower Ts. These spectra show two prominent features at approx. 1420 and 1515cm -1 (see Fig. 5). Different investigation methods will be necessary to reveal the structure of these deposits.
4. Discussion
The analysis of the deposition experiments suggests that for a particular gas system the carbon content is not only a decisive parameter for the quality of the deposit, but it also determines the limit of crystalline growth when Ts is lowered. As reported previously [ 1],
229
deposition of non-diamond carbon is increased and the film quality deteriorates seriously• The present investigation has been performed solely by changing the substrate temperature. This leads to the supposition that there must be surface processes that control substantially the limits of LTGD. In general, crystalline or amorphous growth from the vapor phase is balanced by the rate of arrival of precursors from the gas phase and the rate at which they can diffuse across the surface, i.e. the ability to find their ways to correct sites by surface diffusion [11,12]. Minimum growth temperatures thus depend on the supersaturation of growth radicals, which is in turn correlated to the initial C H 4 concentration, as observed experimentally• Surface diffusion, described by an Arrhenius-type expression in simple vibrational diffusion theory, is increasingly hindered when Ts is lowered. Crystalline growth transforms into amorphous growth, when the rate of arrival exceeds the rate of diffusional transport at the surface• With available data of radical concentrations in the CH4/H 2 system [13], growth limits for CVD diamond can be calculated, predicting transition temperatures in the range 400-450 °C, which corresponds well with these experimental results. There have been doubts expressed that surface diffusion is operating in the case of CVD diamond, because bonding of adatoms is strong and the diamond surface is believed to be hydrogen-terminated during growth• However, recent experiments and theoretical calculations have shown that precursors can migrate via covalent bond breaking and formation in a manner equivalent to surface diffusion, whose rate is limited by temperature dependent reactions, namely H atom abstraction and addition [14-16]. A decrease in temperature should form an increasing number of lattice defects and eventually non-diamond carbon• There are additional parameters likely to influence low temperature limits• A lower deposition pressure should be equivalent to a reduction of the C H 4 c o n c e n t r a t i o n at a fixed pressure. Recent investigations in CO-rich systems reveal that the pressure dependence might be important [7]. The input microwave power is thought to be decisive, since atomic hydrogen concentrations will be appreciably influenced [9]. One fundamental role of atomic hydrogen in CVD diamond has been postulated to be preferential etching of nondiamond carbon, thereby improving the quality of the deposit [5]. The uniform thickness of the films makes it feasible to discuss morphological and structural features directly related to LTGD. From the decreasing particle size it is concluded that renucleation processes become important when Ts is lowered; this is in part responsible for the observed degradation of the film quality. Two characteristic morphologies occur in these experiments; crystalline growth with {111 } faceting and cauliflower structures at very low Ts or e x c e s s C H 4 concentrations• The former
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corresponds to sufficient surface diffusion to maintain crystallographic shapes. Renucleation is likely to be induced by structural defects on {111} planes [17]. There is a certain tendency of persisting columnar growth for each crystal, but not throughout the film. TEM investigations have shown that {111 } faceted films contain numerous microtwins, stacking faults and dislocations throughout the volume [18]. This may explain the generally poor quality of the deposit in the CH4/H2 system. The cauliflower type has been shown to possess a columnar structure separated by boundaries with each column consisting of massive randomly oriented nuclei [17]. This morphology is observed under conditions of high carbon supersaturation or limited surface diffusion causing extensive renucleation events. These experiments strongly support the fact that addition of oxygen is not only a useful but a necessary condition for LTGD. Even small amounts of oxygen improve crystallinity and, most importantly, make diamond film deposition possible at lower substrate temperatures. It is well established that oxygen plays a major role in controlling the gas phase chemistry [19]. The abundant OH radical is supposed to be a strong etchant of non-diamond carbon [20], a similar function is ascribed to atomic oxygen. This statement seems to be confirmed in these experiments, as small amounts of oxygen improve the film quality but lower the deposition rate. However, the kinetic analysis of LTGD with oxygen as additional gas component and the very morphological appearance of the deposits in CO-rich systems suggest that oxygen or related species must participate directly in the diamond growth process, e.g. by desorption from partially oxygen-covered surfaces [21]. Additionally, the particular importance of CO-rich gas systems has been demonstrated. Such mixtures are indeed most favorable for diamond film growth at low substrate temperatures. Deposits show the best phase purity as evaluated by Raman spectroscopy, and minimum growth temperatures are considerably lowered.
5. Conclusions The deposition of diamond thin films at low substrate temperatures has been extensively studied with the conventional CH4/H 2 gas system. A generally poor quality of the deposit has been observed owing to numerous growth defects incorporated into {111} facets and frequent renucleation. When the substrate temperature is lowered, deposition of amorphous components is strongly enhanced and the diamond phase purity deteriorates seriously. Finally, growth of crystalline diamond is thought to be inhibited by insufficient surface diffusion. The low temperature limit of crystalline growth is mainly influenced by the carbon content in the gas mixture,
allowing a reduction of the growth temperature by reduced C H 4 concentrations. In view of the numerous limitations of the CH4/H 2 system, the addition of oxygen to the gas atmosphere has been shown to play a crucial role in LTGD. Not only is the phase purity greatly improved, but also growth is possible at lower deposition temperatures. Particular importance has been attached to CO-rich gas systems which are most favorable for this approach. It is further supposed that oxygen or oxygen containing species participate directly in the diamond growth process at low substrate temperatures.
Acknowledgment The authors would like to thank Mr. B. Senior for technical assistance with the SEM. Financial support by the Kommission for die wissenschaftliche Forschung, Bern (Switzerland), is gratefully acknowledged.
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