- Email: [email protected]
Annihilation of nucleation sites during diamond CVD
200
Diamond and Related Materials, 1 (1992) 200-204
Elsevier Science Publishers B.V., Amsterdam
Annihilation of nucleation sites during diamond CVD Jeoung Woo Kim Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology. P.O. Box 150. Cheongryang. Seou1130-650 (Korea)
Young-Joon Baik and Kwang Yong Eun Hard Materials Lab., Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650 (Korea)
Abstract To investigate the annihilation behavior of nucleation sites during diamond deposition on silicon substrate, the effectsof thermal annealing and erosion with atomic hydrogen on the defecthealing have been observed.The nucleationdensity variations according to the deposition time with 0.3% and with 1% methane in hydrogen have been shown, and compared with those effects.Nucleation sites disappear by thermal annealing and by erosion with atomic hydrogen.The annihilation rate of nucleation sites is more rapid at higher substrate temperature and depends strongly on the concentrationof atomic hydrogen.In real diamond depositionprocessing, nucleation sites are annihilated dominantlyby erosion with atomic hydrogenduring the deposition with 0.3% methane in hydrogen, while only the effectof thermal annealingexists during the deposition with 1% methane in hydrogen.
1. Introduction The nucleation density of deposited diamond can be increased greatly by mechanically damaging (surface treatment) the substrate surface with SiC or diamond powder [,1-4]. The effect of surface treatment has been generally explained to generate surface defects that can be preferred nucleation sites of diamond [2, 4-6], even though the nature of nucleation sites has not yet been identified. The nucleation rate of diamond depends strongly on the density of surface defects, and thus the nucleation behavior is affected by the annihilation of defects during the deposition. Kamo et al. [6] reported previously that the nucleation rate decreased after deposition for 6 h on Si substrate, which was explained by the elimination of nucleation sites by thermal annealing. Kim et al. [-7] observed that the nucleation density and growth rate showed their maxima between the substrate temperatures of 900 and 1000 °C, while the substrate temperature of maximum nucleation density was lower than that of growth rate. The difference was considered to come from the more rapid annihilation of nucleation sites at higher substrate temperature. These observations suggest the necessity of understanding the defect healing during the deposition for the control of nucleation behavior of diamond deposition. The purpose of this study was to investigate the annihilation behavior of nucleation sites during the deposition process. It has been reported already that the effect of surface treatment disappeared by chemical
0925 9635/92/$05.00
etching [8], ion beam etching [9], ion implantation [-3], thermal annealing [6], and hydrogen plasma etching [4]. Thermal annealing and erosion with atomic hydrogen among these are expected to occur in real deposition processing. However, there was no quantitative study about these effects on the defect healing except that of Yugo et al. [4] showing the size variation of scratch by etching in hydrogen plasma. In the present study, the annihilation of nucleation sites by thermal annealing and by erosion with atomic hydrogen was observed, respectively. Based on the result that the annihilation rate depended strongly on the concentration of atomic hydrogen, a model experiment was done. It was composed of two-step depositions between which the methane concentration in hydrogen was changed. Since the saturation density of deposited diamond particles was different according to the methane concentration, it was possible to distinguish the main annihilation process during diamond deposition.
2. Experimental details Silicon (100) substrates mechanically damaged with SiC powder of a size between 60 and 90 mesh were used as specimens. The SiC powder was placed in a cylinderical steel vessel, on the round wall of which the substrates were fixed by Scotch tape. Then it was sealed and rotated around the cylinder axis. By this treatment method, the density of diamond particles after deposition was uni-
201
d. W. K ira et al. ,' Annihilotion o[ nucleation sites during CVD
form among specimens, which implied that the density of nucleation sites was almost the same. The surface damaged substrates were cleaned with alcohol in an ultrasonic cleaner and dried by a nitrogen gas gun. As reported previously [8], the density of diamond particles after deposition increased with the surface treatment time. The treatment time was determined for the particle densities to be around 107/cm 2, where less impingement between particles was observed. To investigate the variation of the annihilation behavior due to the amount of surface damage, surface treatment with SiC powder of size of 7 mesh was also done, by which the particle density' around 2 x l 0 8 / c m 2 was obtained. However, when the particle density was higher than 108/cm 2, there could be some counting error because of impingements between particles. The surface treated substrates were thermally annealed or eroded with atomic hydrogen under the conditions listed in Table 1. Afterwards, diamond was deposited under the following condition: 1% CH4 in H 2, 940 C of substrate temperature, 2100 ~'C of filament temperature, 30 mbar (3 kPa) of total pressure, and 100 sccm of total flow rate, where well-defined crystalline diamond was obtained. Since the nature of nucleation sites was not known, the density of diamond particles after deposition was considered to represent the density of nucleation sites before deposition. The deposition apparatus was similar to that reported previously [10], except that we used a cold wall type instead of a hot wall type in this study. The distance between the filament and the substrate was l0 mm. The filament temperature was measured by an optical pyrometer. The substrate temperature was measured by Pt/Pt-I 3Rh thermocouple which was attached to the surface of substrate holder. Four specimens were deposited at each experimental condition. The density of particles was counted by scanning electron microscopy (SEM) with a magnification of 3000, where the particles larger than 0.3 Iam were observed clearly. About 1000 particles were counted per specimen and the average value among specimens was used as a particle density. During annealing, the surface treated specimens were enveloped in a quartz tube evacuated below 10 2 mbar (1 Pa) and heated in a furnace. Many Si slices were put in together to avoid the evaporation of the damaged
surface. It took 2 min for the specimens to arrive at the annealing temperature, and the specimens were air cooled after annealing. The annealing time means the duration period at the annealing temperature. Erosion with atomic hydrogen was conducted in a new clean deposition apparatus with identical condition. The concentration of atomic hydrogen was varied by changing the filament temperature [-11]. During the erosion, the flow rate of hydrogen was 100 sccm and the pressure of reaction chamber was 30 mbar (3 kPa). It took 1 min for the substrate temperature to arrive at the desired value after turning on the filament and the substrate heating element. The erosion time was measured just after turning on the filament. The conditions of the two-step depositions to investigate the actual contribution of the two annihilation processes are shown in Table 2. Diamond was nucleated with 1 or 0.3% CH4 in H2 for several periods of deposition (first depositionl. Then the CH4 concentration was changed to 0.3 or 1%, respectively, and further deposition conducted (second deposition). At this time, the substrate temperature was 915 C , and the other deposition variables were the same as mentioned above. Between the first and the second deposition, the filament and the substrate heating element were turned off maintaining vacuum.
3. Results and discussion Figure 1 shows the variations of the diamond particle density with annealing time at the annealing temperatures of 850 and 980 ~'C. The substrates were surface treated with SiC powder of size between 60 and 90 mesh. The particle density decreased with increasing annealing time. ]-'he particle density after annealing for 0 min was lower lhan that with no annealing, which showed that the annihilation of nucleation sites occurred during TABLE 2. The experimental conditions of the two-step depositions Surface treatment
First deposition + Second deposition
SiC, 60 90 mesh lh
0.3% CH,,, 5 60 rain + 1.0% CH 4, 2 h 1.0% C H , , , 5 6 0 m i n + 0 . 3 % CH 4 , 4 h
TABLE I. The conditions of thermal annealing and erosion Annealing
Erosion
Temperalure [T~) (C)
Time Ih)
Substratc temperature (T~) (C)
Filament temperature [ TF) ~C)
Erosion time Imin)
850 980
0 2 0 2
850 980
1600 2000 1600-2000
I 10 1 1()
J. W. Kim et al. / Annihilation of nucleation sites during C VD
202
• No Annealing • Annealed at 850°C o Annealed at 980*C
~'10
E 0
c D
rl
i
i
i
i
i
i
30
15 Annealing
Time
(min)
Fig. I. The variations of the particle density with annealing time at the annealing temperatures of 850 and 980 °C. The substrates were surface treated with SiC powder of size between 60 and 90 mesh.
heating the substrate up to the annealing temperature. The healing rate of nucleation sites was more rapid at a higher annealing temperature. On the other hand, the particle densities were the same after annealing for 30 min at both annealing temperatures. This implies that no more defect healing would occur by annealing afterwards. What is to be noted is that the particle density after annealing to the saturation was still higher than 104/cm 2 that was reported to be obtained on the mirror polished Si substrate [9]. The effect of annealing was similar when the substrates were mechanically damaged more strongly. Figure 2 shows the variation of the particle density with annealing time when the substrates were surface treated with SiC powder of size of 7 mesh. The particle density decreased with increasing annealing time, and no more annihilation of nucleation sites occurred after 2 h. On the contrary, the particle density after annealing to the saturation was
very high, and the annealing time necessary for the saturation was longer in comparison with the results shown in Fig. 1. Furthermore, the ratio of the particle density with annealing to the saturation to that without annealing was higher in the case of strong surface damage. These results show that the density and size of defects of the types to be annealed out increase with the surface damage, even though the nature of these is not known. When the substrate surface was eroded with atomic hydrogen at high substrate temperature, the annihilation of nucleation sites occurred not only by erosion but also by thermal annealing. Figure 3 shows the variation of the particle density with erosion time at several filament temperatures. The particle density decreased with increasing erosion time, and the annihilation rate was faster at higher filament temperatures. The decreasing profile with erosion time for the filament temperature of 1600 °C was similar to that of the thermal annealing at the same annealing temperature shown in Fig. 1. This implies that there was no erosion effect at the filament temperature of 1600 °C. Celii et al. [11] observed that the concentration of atomic hydrogen decreased as the filament temperature decreased, and that no atomic hydrogen was detected at the spot 8 mm away from the filament when the filament temperature was 1600 °C. Based on their results, Fig. 3 shows that the erosion rate increases with the concentration of atomic hydrogen, and that erosion does not occur significantly without atomic hydrogen. On the other hand, the particle density on the substrate eroded at the filament temperature of 2000 °C for 10 rain was lower than that obtained after annealing for a longer time. It was the same when the surface treatment was done with SiC powder of size of 7 mesh as shown
I0'
E
• No Annealing o Annealed at 980°C
'
• No erosion a Eroded at 1600°C Tr = Eroded at 1800°C "IF
E
o Eroded at 2000=C "IF
0
I,
lO c-
'~ 10 ' C %}
o
s
1o
i
-0 13_
O_
4
10 i
i
,
0 Annealing
i
i
3 Time
i
,
6 (h)
Fig. 2. The variation of the particle density with annealing time at the annealing temperature of 980 'C. The substrates were surface treated with SiC powder of size of 7 mesh.
0
Erosion
4
Time
8
12
(min)
Fig. 3. The variations of the particle density with erosion time and filament temperature at the substrate temperature of 850 °C. The substrates were surface treated with SiC powder of size between 60 and 90 mesh.
J. 14". Kim et al.
Amfihilation ol m~cleumm ,sites during C V D
in Fig. 4. The annihilation rate was higher at higher substrate temperatures, and very fast in comparison with that of annealing. The difference between the amounts of annihilated nucleation sites by annealing and by erosion seems to come from the difference of reaction mechanism. The annihilation of defects occurs by surface and/or bulk diffusion of the atoms of substrate material during thermal annealing. Therefore, its rate is slow and some types of defect can remain after annealing [12]. But the damaged surface layer is removed during erosion, which results in the elimination of every kind of defect. The change of the scratch size during erosion with atomic hydrogen, reported by Yugo et al. [4], is an example of the erosion process. Generally, a mixed gas of hydrocarbon and hydrogen is acti~ ated during diamond deposition. With the addition of hydrocarbon, the mole fraction of atomic hydrogen, thus the reactivity of it with Si substrate, decreases. Therefore, the results shown in Figs 3 and 4 presenl the maximum amounts of defect annihilation by erosion with atomic hydrogen, and do not show whether the nucleation sites are actually annihilated by erosion during the deposition. On the other hand, the annealing of nucleation sites seems to occur during the deposition because of the high substrate temperature necessary for diamond deposition. In order to investigate the actual annihilation behavior of nucleation sites during diamond deposition, the nucleation density variations with deposition time during the deposition with 1% and with 0.3% CH,t in H= were observed. The density of deposited diamond particles did not increase any more after deposition for 2 h [7]. The saturation density was 8 x IW/cm 2 and 3 x 107.'cm ~ after deposition with 0.3% and 1% CHa in He, respectively, in this experiment. In addition to these saturation
• No
10'-
203
densities, the particle densities measured after the two step depositions are shown in Fig. 5, where the deposition time represents the deposition period of the first deposition. Since the difference between the saturation densities was fairly large, the particle density was expected to be higher than 8 x 10~"cm 2 when the second deposition was performed with 1% C H 4 in H 2 after the first deposition with 0.3% CH,~ in H> However, the experimental result in this case was different from the expectation. The particle densities were almost the same, regardless of the deposition time of the first deposition, as the saturation density obtained after a long deposition with 0.3% CH,, in H,. This result obviously shows that the nucleation sites were eliminated within 5 rain of the deposition with 0.3% CH,, in H 2. The elimination of nucleation sites within 5 rain supports that the annihilation of nucleation sites occurred dominantly by erosion with atomic hydrogen during the deposition with 0.3% CH,, in H 2, since the substrate temperature of this experiment was 915 C and the erosion rate was fast at high substrate temperature as shown in Fig. 4. In the case of the first deposition with 1% CH4 in H2 followed by the second deposition with 0.3% CH,, in H 2, the particle density increased with increasing the deposition time of the first deposition as shown in Fig. 5. No more significant nucleation would occur during the second deposition with 0.3% CH,~ in H 2, because the nucleation sites were eliminated within 5 rain of the deposition with 0.3% CHa in H2. Therefore, the measured particle density shows the density of nucleated diamond particles just after the first deposition. The increase of the nucleation density with deposition time during the deposition with 1% CH~ in H e implies that the annihilation rate of nucleation sites was so slow that
erosion
o Eroded
ot
850°C
TE
• Eroded
at
980°C
TE
•
&'-"
1~CH4,
2h
o 0.3~CHi, 4h • 0.3~CH4 + 1~CH4 ~ 1~CH4 + 0.3~CH4
O
{
T
t
{ m c
C)
c~ lo U
ck
° i[
10 7 i
i
4
f
8
I
O EL
i
12
Erosion Time (min) Fig. 4. The variations of the particle density with erosion time at the substrate temperatures of 850 and 980 'C. The filament temperature was 2000 C. and the substrates were surface treated with SiC powder of size of 7 mesh.
i
,
0
i
20
,
i
40
,
i
60
,
80
Deposition Time (min)
Fig. 5, The variations of the nucleation density after two step depositions with the deposition time of the first deposition with 0.3% or I% CH., in H,.
204
J. W. Kim et al. / Annihilation of nucleation sites during CVD
nucleation continued to occur. These results compared with those of Fig. 3, supports that there was no erosion effect during the deposition with 1% CH 4 in H2.
tion with 1% C H 4 in H 2. Since diamond is usually deposited with a CH4 concentration higher than 0.5%, the lower substrate temperature is beneficial to prevent the annihilation of nucleation sites.
4. Conclusions
References
The nucleation behavior of diamond deposition depends strongly on the annihilation of surface defects. The effects of thermal annealing and erosion with atomic hydrogen on the defect healing in real deposition processing were investigated. The amount of annihilated nucleation sites increased with increasing annealing time, and its rate was faster at higher annealing temperatures. In the case of erosion with atomic hydrogen, the annihilation rate increased with increasing both the concentration of atomic hydrogen and the substrate temperature. Furthermore, it was very fast in comparison with annealing. However, it was hard to investigate the density and/or morphology change of every kind of surface defect during annealing or erosion accurately, and thus the identification of preferred nucleation sites still remains as a basic problem. In real deposition processing, nucleation sites were annihilated dominantly by erosion with atomic hydrogen during the deposition with 0.3% CH4 in Ha, while only the effect of thermal annealing existed during the deposi-
I K. E. Spear, J. Am. Ceram. Soc., 72 (1989) 171. 2 Y. Mitsuda, Y. Kojima, T. Yoshida and K. Akashi, J. Mater. Sci., 22 (1987) 1557. 3 K. Higuchi, S. Noda and O. Kamigaito, J. Jpn Soc. Powder & Powder Metall., 36 (1989) 139. 4 S. Yugo, A. Izumi, T. Kanai, T. Muto and T. Kimura, Some Observations on Nucleation Sites in Diamond Growth By Plasma CVD. 2nd Int. Conf. on New Diamond Science and Technology, Washington, D.C., 1990. 5 B. V. Spitsyn, L. L. Bouilov and B. V. Deryaguin, J. Cryst. Growth, 52 (1981) 219. 6 M. Kamo, New Dia., 15 (1989) 50 (in Japanese). 7 J. W. Kim, Y. J. Baik and K. Y. Eun, In Y. Tzeng, M. Yoshikawa, M. Murakawa and A. Feldman (eds) Application of Diamond Films and Related Materials, Elsevier, Amsterdam, The Netherlands, 1991, p. 399. 8 Y. Mitsuda, T. Yoshida and K. Akashi, Proc. 8th Int. Syrup. Plasma Chem., Tokyo, 1987, p. 2469. 9 K. Hirabayashi, Y. Taniguchi, O. Takamatsu, T. Ikeda, K. Ikoma and N. Iwasaki-Kurihara, Appl. Phys. Lett., 53 (1988) 1815. 10 S. Matsumoto, Y. Sato, M. Tsutsumi and N. Setaka, J. Mater. Sci., 17 (1982) 3106. 11 F. G. Celii and J. E. Butler, Appl. Phys. Lett., 54 (1989) 1031. 12 R. Stickler and G. R. Booker, Phil. Mag., 8 (1963) 859.
Diamond and Related Materials, 1 (1992) 200-204
Elsevier Science Publishers B.V., Amsterdam
Annihilation of nucleation sites during diamond CVD Jeoung Woo Kim Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology. P.O. Box 150. Cheongryang. Seou1130-650 (Korea)
Young-Joon Baik and Kwang Yong Eun Hard Materials Lab., Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650 (Korea)
Abstract To investigate the annihilation behavior of nucleation sites during diamond deposition on silicon substrate, the effectsof thermal annealing and erosion with atomic hydrogen on the defecthealing have been observed.The nucleationdensity variations according to the deposition time with 0.3% and with 1% methane in hydrogen have been shown, and compared with those effects.Nucleation sites disappear by thermal annealing and by erosion with atomic hydrogen.The annihilation rate of nucleation sites is more rapid at higher substrate temperature and depends strongly on the concentrationof atomic hydrogen.In real diamond depositionprocessing, nucleation sites are annihilated dominantlyby erosion with atomic hydrogenduring the deposition with 0.3% methane in hydrogen, while only the effectof thermal annealingexists during the deposition with 1% methane in hydrogen.
1. Introduction The nucleation density of deposited diamond can be increased greatly by mechanically damaging (surface treatment) the substrate surface with SiC or diamond powder [,1-4]. The effect of surface treatment has been generally explained to generate surface defects that can be preferred nucleation sites of diamond [2, 4-6], even though the nature of nucleation sites has not yet been identified. The nucleation rate of diamond depends strongly on the density of surface defects, and thus the nucleation behavior is affected by the annihilation of defects during the deposition. Kamo et al. [6] reported previously that the nucleation rate decreased after deposition for 6 h on Si substrate, which was explained by the elimination of nucleation sites by thermal annealing. Kim et al. [-7] observed that the nucleation density and growth rate showed their maxima between the substrate temperatures of 900 and 1000 °C, while the substrate temperature of maximum nucleation density was lower than that of growth rate. The difference was considered to come from the more rapid annihilation of nucleation sites at higher substrate temperature. These observations suggest the necessity of understanding the defect healing during the deposition for the control of nucleation behavior of diamond deposition. The purpose of this study was to investigate the annihilation behavior of nucleation sites during the deposition process. It has been reported already that the effect of surface treatment disappeared by chemical
0925 9635/92/$05.00
etching [8], ion beam etching [9], ion implantation [-3], thermal annealing [6], and hydrogen plasma etching [4]. Thermal annealing and erosion with atomic hydrogen among these are expected to occur in real deposition processing. However, there was no quantitative study about these effects on the defect healing except that of Yugo et al. [4] showing the size variation of scratch by etching in hydrogen plasma. In the present study, the annihilation of nucleation sites by thermal annealing and by erosion with atomic hydrogen was observed, respectively. Based on the result that the annihilation rate depended strongly on the concentration of atomic hydrogen, a model experiment was done. It was composed of two-step depositions between which the methane concentration in hydrogen was changed. Since the saturation density of deposited diamond particles was different according to the methane concentration, it was possible to distinguish the main annihilation process during diamond deposition.
2. Experimental details Silicon (100) substrates mechanically damaged with SiC powder of a size between 60 and 90 mesh were used as specimens. The SiC powder was placed in a cylinderical steel vessel, on the round wall of which the substrates were fixed by Scotch tape. Then it was sealed and rotated around the cylinder axis. By this treatment method, the density of diamond particles after deposition was uni-
201
d. W. K ira et al. ,' Annihilotion o[ nucleation sites during CVD
form among specimens, which implied that the density of nucleation sites was almost the same. The surface damaged substrates were cleaned with alcohol in an ultrasonic cleaner and dried by a nitrogen gas gun. As reported previously [8], the density of diamond particles after deposition increased with the surface treatment time. The treatment time was determined for the particle densities to be around 107/cm 2, where less impingement between particles was observed. To investigate the variation of the annihilation behavior due to the amount of surface damage, surface treatment with SiC powder of size of 7 mesh was also done, by which the particle density' around 2 x l 0 8 / c m 2 was obtained. However, when the particle density was higher than 108/cm 2, there could be some counting error because of impingements between particles. The surface treated substrates were thermally annealed or eroded with atomic hydrogen under the conditions listed in Table 1. Afterwards, diamond was deposited under the following condition: 1% CH4 in H 2, 940 C of substrate temperature, 2100 ~'C of filament temperature, 30 mbar (3 kPa) of total pressure, and 100 sccm of total flow rate, where well-defined crystalline diamond was obtained. Since the nature of nucleation sites was not known, the density of diamond particles after deposition was considered to represent the density of nucleation sites before deposition. The deposition apparatus was similar to that reported previously [10], except that we used a cold wall type instead of a hot wall type in this study. The distance between the filament and the substrate was l0 mm. The filament temperature was measured by an optical pyrometer. The substrate temperature was measured by Pt/Pt-I 3Rh thermocouple which was attached to the surface of substrate holder. Four specimens were deposited at each experimental condition. The density of particles was counted by scanning electron microscopy (SEM) with a magnification of 3000, where the particles larger than 0.3 Iam were observed clearly. About 1000 particles were counted per specimen and the average value among specimens was used as a particle density. During annealing, the surface treated specimens were enveloped in a quartz tube evacuated below 10 2 mbar (1 Pa) and heated in a furnace. Many Si slices were put in together to avoid the evaporation of the damaged
surface. It took 2 min for the specimens to arrive at the annealing temperature, and the specimens were air cooled after annealing. The annealing time means the duration period at the annealing temperature. Erosion with atomic hydrogen was conducted in a new clean deposition apparatus with identical condition. The concentration of atomic hydrogen was varied by changing the filament temperature [-11]. During the erosion, the flow rate of hydrogen was 100 sccm and the pressure of reaction chamber was 30 mbar (3 kPa). It took 1 min for the substrate temperature to arrive at the desired value after turning on the filament and the substrate heating element. The erosion time was measured just after turning on the filament. The conditions of the two-step depositions to investigate the actual contribution of the two annihilation processes are shown in Table 2. Diamond was nucleated with 1 or 0.3% CH4 in H2 for several periods of deposition (first depositionl. Then the CH4 concentration was changed to 0.3 or 1%, respectively, and further deposition conducted (second deposition). At this time, the substrate temperature was 915 C , and the other deposition variables were the same as mentioned above. Between the first and the second deposition, the filament and the substrate heating element were turned off maintaining vacuum.
3. Results and discussion Figure 1 shows the variations of the diamond particle density with annealing time at the annealing temperatures of 850 and 980 ~'C. The substrates were surface treated with SiC powder of size between 60 and 90 mesh. The particle density decreased with increasing annealing time. ]-'he particle density after annealing for 0 min was lower lhan that with no annealing, which showed that the annihilation of nucleation sites occurred during TABLE 2. The experimental conditions of the two-step depositions Surface treatment
First deposition + Second deposition
SiC, 60 90 mesh lh
0.3% CH,,, 5 60 rain + 1.0% CH 4, 2 h 1.0% C H , , , 5 6 0 m i n + 0 . 3 % CH 4 , 4 h
TABLE I. The conditions of thermal annealing and erosion Annealing
Erosion
Temperalure [T~) (C)
Time Ih)
Substratc temperature (T~) (C)
Filament temperature [ TF) ~C)
Erosion time Imin)
850 980
0 2 0 2
850 980
1600 2000 1600-2000
I 10 1 1()
J. W. Kim et al. / Annihilation of nucleation sites during C VD
202
• No Annealing • Annealed at 850°C o Annealed at 980*C
~'10
E 0
c D
rl
i
i
i
i
i
i
30
15 Annealing
Time
(min)
Fig. I. The variations of the particle density with annealing time at the annealing temperatures of 850 and 980 °C. The substrates were surface treated with SiC powder of size between 60 and 90 mesh.
heating the substrate up to the annealing temperature. The healing rate of nucleation sites was more rapid at a higher annealing temperature. On the other hand, the particle densities were the same after annealing for 30 min at both annealing temperatures. This implies that no more defect healing would occur by annealing afterwards. What is to be noted is that the particle density after annealing to the saturation was still higher than 104/cm 2 that was reported to be obtained on the mirror polished Si substrate [9]. The effect of annealing was similar when the substrates were mechanically damaged more strongly. Figure 2 shows the variation of the particle density with annealing time when the substrates were surface treated with SiC powder of size of 7 mesh. The particle density decreased with increasing annealing time, and no more annihilation of nucleation sites occurred after 2 h. On the contrary, the particle density after annealing to the saturation was
very high, and the annealing time necessary for the saturation was longer in comparison with the results shown in Fig. 1. Furthermore, the ratio of the particle density with annealing to the saturation to that without annealing was higher in the case of strong surface damage. These results show that the density and size of defects of the types to be annealed out increase with the surface damage, even though the nature of these is not known. When the substrate surface was eroded with atomic hydrogen at high substrate temperature, the annihilation of nucleation sites occurred not only by erosion but also by thermal annealing. Figure 3 shows the variation of the particle density with erosion time at several filament temperatures. The particle density decreased with increasing erosion time, and the annihilation rate was faster at higher filament temperatures. The decreasing profile with erosion time for the filament temperature of 1600 °C was similar to that of the thermal annealing at the same annealing temperature shown in Fig. 1. This implies that there was no erosion effect at the filament temperature of 1600 °C. Celii et al. [11] observed that the concentration of atomic hydrogen decreased as the filament temperature decreased, and that no atomic hydrogen was detected at the spot 8 mm away from the filament when the filament temperature was 1600 °C. Based on their results, Fig. 3 shows that the erosion rate increases with the concentration of atomic hydrogen, and that erosion does not occur significantly without atomic hydrogen. On the other hand, the particle density on the substrate eroded at the filament temperature of 2000 °C for 10 rain was lower than that obtained after annealing for a longer time. It was the same when the surface treatment was done with SiC powder of size of 7 mesh as shown
I0'
E
• No Annealing o Annealed at 980°C
'
• No erosion a Eroded at 1600°C Tr = Eroded at 1800°C "IF
E
o Eroded at 2000=C "IF
0
I,
lO c-
'~ 10 ' C %}
o
s
1o
i
-0 13_
O_
4
10 i
i
,
0 Annealing
i
i
3 Time
i
,
6 (h)
Fig. 2. The variation of the particle density with annealing time at the annealing temperature of 980 'C. The substrates were surface treated with SiC powder of size of 7 mesh.
0
Erosion
4
Time
8
12
(min)
Fig. 3. The variations of the particle density with erosion time and filament temperature at the substrate temperature of 850 °C. The substrates were surface treated with SiC powder of size between 60 and 90 mesh.
J. 14". Kim et al.
Amfihilation ol m~cleumm ,sites during C V D
in Fig. 4. The annihilation rate was higher at higher substrate temperatures, and very fast in comparison with that of annealing. The difference between the amounts of annihilated nucleation sites by annealing and by erosion seems to come from the difference of reaction mechanism. The annihilation of defects occurs by surface and/or bulk diffusion of the atoms of substrate material during thermal annealing. Therefore, its rate is slow and some types of defect can remain after annealing [12]. But the damaged surface layer is removed during erosion, which results in the elimination of every kind of defect. The change of the scratch size during erosion with atomic hydrogen, reported by Yugo et al. [4], is an example of the erosion process. Generally, a mixed gas of hydrocarbon and hydrogen is acti~ ated during diamond deposition. With the addition of hydrocarbon, the mole fraction of atomic hydrogen, thus the reactivity of it with Si substrate, decreases. Therefore, the results shown in Figs 3 and 4 presenl the maximum amounts of defect annihilation by erosion with atomic hydrogen, and do not show whether the nucleation sites are actually annihilated by erosion during the deposition. On the other hand, the annealing of nucleation sites seems to occur during the deposition because of the high substrate temperature necessary for diamond deposition. In order to investigate the actual annihilation behavior of nucleation sites during diamond deposition, the nucleation density variations with deposition time during the deposition with 1% and with 0.3% CH,t in H= were observed. The density of deposited diamond particles did not increase any more after deposition for 2 h [7]. The saturation density was 8 x IW/cm 2 and 3 x 107.'cm ~ after deposition with 0.3% and 1% CHa in He, respectively, in this experiment. In addition to these saturation
• No
10'-
203
densities, the particle densities measured after the two step depositions are shown in Fig. 5, where the deposition time represents the deposition period of the first deposition. Since the difference between the saturation densities was fairly large, the particle density was expected to be higher than 8 x 10~"cm 2 when the second deposition was performed with 1% C H 4 in H 2 after the first deposition with 0.3% CH,~ in H> However, the experimental result in this case was different from the expectation. The particle densities were almost the same, regardless of the deposition time of the first deposition, as the saturation density obtained after a long deposition with 0.3% CH,, in H,. This result obviously shows that the nucleation sites were eliminated within 5 rain of the deposition with 0.3% CH,, in H 2. The elimination of nucleation sites within 5 rain supports that the annihilation of nucleation sites occurred dominantly by erosion with atomic hydrogen during the deposition with 0.3% CH,, in H 2, since the substrate temperature of this experiment was 915 C and the erosion rate was fast at high substrate temperature as shown in Fig. 4. In the case of the first deposition with 1% CH4 in H2 followed by the second deposition with 0.3% CH,, in H 2, the particle density increased with increasing the deposition time of the first deposition as shown in Fig. 5. No more significant nucleation would occur during the second deposition with 0.3% CH,~ in H 2, because the nucleation sites were eliminated within 5 rain of the deposition with 0.3% CHa in H2. Therefore, the measured particle density shows the density of nucleated diamond particles just after the first deposition. The increase of the nucleation density with deposition time during the deposition with 1% CH~ in H e implies that the annihilation rate of nucleation sites was so slow that
erosion
o Eroded
ot
850°C
TE
• Eroded
at
980°C
TE
•
&'-"
1~CH4,
2h
o 0.3~CHi, 4h • 0.3~CH4 + 1~CH4 ~ 1~CH4 + 0.3~CH4
O
{
T
t
{ m c
C)
c~ lo U
ck
° i[
10 7 i
i
4
f
8
I
O EL
i
12
Erosion Time (min) Fig. 4. The variations of the particle density with erosion time at the substrate temperatures of 850 and 980 'C. The filament temperature was 2000 C. and the substrates were surface treated with SiC powder of size of 7 mesh.
i
,
0
i
20
,
i
40
,
i
60
,
80
Deposition Time (min)
Fig. 5, The variations of the nucleation density after two step depositions with the deposition time of the first deposition with 0.3% or I% CH., in H,.
204
J. W. Kim et al. / Annihilation of nucleation sites during CVD
nucleation continued to occur. These results compared with those of Fig. 3, supports that there was no erosion effect during the deposition with 1% CH 4 in H2.
tion with 1% C H 4 in H 2. Since diamond is usually deposited with a CH4 concentration higher than 0.5%, the lower substrate temperature is beneficial to prevent the annihilation of nucleation sites.
4. Conclusions
References
The nucleation behavior of diamond deposition depends strongly on the annihilation of surface defects. The effects of thermal annealing and erosion with atomic hydrogen on the defect healing in real deposition processing were investigated. The amount of annihilated nucleation sites increased with increasing annealing time, and its rate was faster at higher annealing temperatures. In the case of erosion with atomic hydrogen, the annihilation rate increased with increasing both the concentration of atomic hydrogen and the substrate temperature. Furthermore, it was very fast in comparison with annealing. However, it was hard to investigate the density and/or morphology change of every kind of surface defect during annealing or erosion accurately, and thus the identification of preferred nucleation sites still remains as a basic problem. In real deposition processing, nucleation sites were annihilated dominantly by erosion with atomic hydrogen during the deposition with 0.3% CH4 in Ha, while only the effect of thermal annealing existed during the deposi-
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