- Email: [email protected]
Effect of additional gas on diamond deposition by DC PACVD
ELSEVIER
Materials Science an I Engineering A209 (1996) 399 404
Effect of additional gas on diamond deposition by DC PACVD Jae-Kap Lee, Young-Joan Baik, Kwang Yang Eun Division of Ceramics, Korea Institute of Science (, 'ld Techl1oloK)" PO Boy 13/, Cheol1KrranK, Seoul 130-650, South Korea
Abstract The effect of the addition of Ar and nitrogen gas on the deposition of diamond using a direct current (DC) plasma of CH c H 2 gas mixture is investigated, The DC plasma is generat:d by applying a voltage between 820 and 900 V and the resulting current is between 5,5 and 6 A, The addition of Ar makes th: plasma unstable and is limited to 5'/i, owing to the plasma extinction at higher Ar concentrations. Growth rate and non-diamo ld carbon content in the diamond film increase with the Ar amount slightly irrespective of deposition temperature. An optical emi~ sion spectrum shows the increase of emission of C2 and CH while showing a constant average electron energy of the plasma. On he contrary, by adding nitrogen, the morphology is changed to a ball-like diamond shape as well as growth rate of diamond film is decreased abruptly. The optical emission intensities of C2 and hydrogen also show an abrupt drop with the nitrogen addition. The role of plasma species in the deposition behaviour of diamond is also discussed. Ke)'lmrds: Diamond deposition; Argon gas; Nitrogen gas
1. Introduction
Since the success of diamond synthesis fro n gas phase [1], a hydrocarbon and hydrogen gas mixtl re has been used as a basic gas system, Activation of a gas mixture by either plasma or thermal energy is nel essary for diamond synthesis, Although the synthesis 11echanism of diamond and the gas chemistry during the diamond synthesis are not clarified yet, atomic 1ydrogen has been recognized empirically as crucial 10 diamond synthesis [2], In order to increas: the supersaturation of the atomic hydrogen, high frequency electric plasma or thermal plasma instead of 10" temperature plasma has been preferred. As a result, the growth rate has been increased by an order of )nagnitude using thermal plasma [3], Another method to change plasma character stic is the addition of noble gases. Noble gases can ellhance ionization rate and, consequently, deposition rate in lots of CVD systems [4]. Studies on the effect of nobl,' gases on the diamond deposition have also shown the t they can change the electron energy distribution of 1)Iasma and, thus, growth rate and non-diamond carbon C Jntent in diamond [5]. The effect of Ar on the didmond deposition was greatest among noble gases [5,6]. fhis is 0921-5093;96/$15.00© 1996 -
SSDI 0921-5093(95)10143-8
Elsevier Science SA All rig Its reserved
because its ionization potential is similar to that of hydrogen. This result has been observed in a microwave plasma assisted chemical vapour deposition system (MW PACVD) [5,6], the average electron energy of which is higher than that of low frequency plasmas, In this paper, we studied the effect of Ar on the characteristic of DC plasma and deposition behavior of diamond. A DC plasma generated at several ten torr is not frequently used for diamond synthesis due to its inefficiency of gas activation. However, the DC plasma used in this study is generated above 100 torr and very close to an equilibrium thermal plasma. The gas activation is, thus, expected to be sufficient irrespective of the frequency. It is difficult to use the DC plasma stably for diamond deposition because its operating condition is very close to a glow to arc transition boundary. The addition of noble gases can be one of the method inhibiting an arc generation, No study on the effect of noble gases using the DC plasma has been conducted for their role in plasma stabilization and diamond deposition yet. We selected Ar among noble gases because its effect is expected to be greatest. Oxygen and nitrogen have also been added to a methane hydrogen gas mixture for diamond deposition, Small amount of oxygen not only reduces defect con-
400
J.-K. Lee et al. / Materials Science and Engineering A209 (1996) 399-404
tent in diamond but also changes the growth rate [7,8]. We have already confirmed its effect on the diamond synthesis by DC PACVD method [9]. The oxygen now is a common additive to the gas mixture for diamond synthesis. Nitrogen has drawn attention recently, owing to its function as a dopant or the possibility of forming CN compound [10,11]. It has been also suggested that nitrogen can act a role of hydrogen in diamond synthesis [11]. The study on the effect of nitrogen on the diamond deposition using a high energy thermal plasma is thus meaningful for finding a new material or a new gas system for diamond synthesis. An optical emission spectroscopy (OES) was employed to study the effect of nitrogen on the DC plasma of CH c H 2 . Comparing OES results with those of growth rate measurement and Raman spectroscopy, we tried to find the relation between plasma characteristics and diamond deposition behaviour.
emission of high density of thermal electron from cathode material. The impact of Ar ions in the plasma to the cathode can be a main cause of cathode temperature rise. A variation of plasma species due to Ar addition can be observed by an optical emission spectroscopy. A typical optical emission spectrum from the DC plasma of CH 4 and H 2 is shown in Fig. 1. The major species observed in this spectrum are H atom (Balmer series, H~ :656.2 nm, Hfi :486.1 nm, H;.:434.0 nm), C 2 radical (Swan band, 563.6, 516.5 and 474.7 nm), CH radical (431.4 nm) and excited H 2 molecule (581.0 nm). Although its intensity was very weak, a peak around 216 nm could be observed, probably coming from CH 3 and/or C2 H 2 emission. However, the wavelengths of two emissions were too close to distinguish each other. The changes of emission intensities of C2 , CH and H atom at different concentration of Ar is shown in Fig. 2. The methane concentration was 3%. The carbon
2. Experimental A DC PACVD system used in this study has been described in detail previously [9]. A plasma was excited by a DC power supply with voltage between 820 and 900 V and current between 5.5 and 6.0 A. Ar and nitrogen were added up to 5% to a CH 4 -H 2 gas mixture separately. The total gas flow and the methane concentration kept constant at 150 sccm and 3% respectively; an amount of hydrogen equivalent to that of the added gas was reduced. A pressure of 140 mbar was used for all deposition processes. Diamond films were deposited on Mo substrate with thickness of several ten micrometers. They were peeled off after cooling to room temperature. The thickness of free standing films was measured using a digital micrometer. Scanning electron microscopy and Raman spectroscopy were employed for the characterization of the morphology and quality of diamond. An optical emission spectroscopy was used to analyse the plasma species using an DIGISEM 350 emission spectrometer of Sofie instrument. A spectrum range used was from 200 to 700 nm.
H~
CH
35~O;--~-:;4~OO:;--~-:;47;;50:;--~·5*'OO:;--~·5:t50:;--~"""tr600
Wave length (nm) Fig. I. A typical optical emission spectrum of DC plasma of 3'f"CH 4 -
H2
3. Results and discussion
3.1. The effect of Ar addition Ar has been used to control plasma density or energy distribution to investigate the possibility of enhancing reaction for deposition. However, the addition of Ar to DC plasma of CH 4 - H 2 gas was observed to make the plasma unstable: the plasma decreased its size with Ar addition, transited to arc and extinguished above 5% of Ar. The plasma-arc transition is caused mainly by
a
2
3
4
5
Ar concentration (vol. %) Fig. 2. The changes of emission intensities of C 2 , CH and H atom with Ar concentration.
J.-K. Lee et al.
401
Materia s Science and Engineering A209 (1996) 399-404
containing species, C 2 and CH, show an increase in the emission intensities while the hydrogen atom shows nearly constant emission intensity. In comparison with the previous result for MW plasma [5], we can find a nearly similar behaviour of variation of emission intensities between two plasmas: for example, abOl t 30(/'() and 50% increase in C 2 emission intensity for the MW plasma and DC plasma respectively and aim lSt no variation in H emission intensity with the addi ion of 5(% Ar. (The emission intensity of H increased above YYo in the MW plasma [5], but we could not (dd Ar above 5% to the DC plasma). The ratio of Hili H" an indication of average electron energy, was not va 'ied by adding Ar. This implies that the electron energy distribution remains constant independently of Ar ac dition. The variation of the emission intensity, related to the concentration of the excited species, can be cal sed by either a variation of the ground state species con :entration or a variation of excitation efficiency of th 11 species. The excitation efficiency is dependent 'lll the electron energy distribution in the plasma. E ut the latter possibility can be ruled out in the DC )Iasma owing to the constant HldHx value. Hence, ~ e conclude that the concentration of ground state C 2 and CH species in the plasma increased by adding Ar. The variation of plasma species is closely rellted to the behaviour of diamond deposition such as ill< lrphology, growth rate and defect concentration in dii mond. No significant variation of the morphology of di amond film was observed within the experimental rangl of Ar addition up to 5%. Fig. 3 shows a typical mrface morphology of diamond film grown at CH 4 ind Ar concentrations of 3% and 50;;1 respectively. (1(0) and (111) facets are on the surface and the texture (rientation is (110). Moreover, the grain size remaint d constant with the increasing Ar amount at c· mstant methane concentration. The effect of Ar addil ion on the nucleation and morphology of diamond .s thus insignificant. However, the growth rate and quality of d'amond film was changed by Ar addition. Fig. 4 she ws the measured growth rate at different Ar concentrati on and deposition temperature. We can see that the inc) ease of the growth rates also depends on the depositic n temperature. A typical Raman spectrum of diamo ld film grown at CH 4 and Ar concentration of 3% (nd 3'% respectively and a variation of full width at hal·' maximum value (FWHM) of diamond peak at vari JUS Ar concentration are shown in Fig. 5. Only a d amond peak at 1332 cm - 1 without any broad peak around 1500 cm - 1 was observed for all diamond films However, the variation of FWHM value and S/N ra.io (the ratio of diamond peak to background intensity shows more defective diamond film growing at hig ler Ar concentration. This result is consistent with the observation of OES and those of previous reports 5]: the
Fig. 3. Surface morphology of diamond film grown at 3%CH 4 3'y',Ar H 2 and 1290 °C for 20 h.
--.~:
24
-.&.--.
,-.,
--2-E
3%CH 4
1210 0 C
---e- 1250 0 C
'~~:~~
£
20
- /
~
0
1290 C
-'0-
(])
I II
....
£
j
0 .... (9
16
;7"
~~ ..
~
:::=::::===-
Ar concentration (vol %) Fig. 4. The change of growth rate with Ar concentration at various deposition temperature.
higher the concentration of carbon containing species such as C ll the higher the growth rate.
3.2. The effect of nitrogen addition
The chemistry of plasma with the addition of nitrogen to CH c H 2 mixture is more complex compared with that of Ar, because chemical reactions between nitrogen, carbon and hydrogen occur. We calculated by SOLGAS program the equilibrium concentration of a N 2 -CH c H 2 mixture at various N 2 concentration and at 5000 K and 140 mbar. The temperature is assumed as that of the DC plasma. The order of abundancy of gas species is H, C, N, CN, N 2 , CH, C 2 , etc. We selected some of the species, which were observed on OES spectrum and plotted the change of the normalized concentrations at various nitrogen amount in Fig. 6. The normalized concentrations of C 2 , CH and atomic hydrogen decrease with the increase of nitrogen while that of CN increases.
402
J.-K. Lee et al. / Materials Science and Engineering A209 (1996) 399-404
The optical spectrum of 3%CH 4 -5%N 2 -H 2 and the variation of respective peak intensities at various nitrogen amounts are shown in Fig. 7. The intensities of C2 and hydrogen emission dropped abruptly at nitrogen concentration below I % and then decreased at a slow rate, while that of CH emission decreased gradually. The intensity of CN emission, however, increased with the increase of nitrogen. The OES result shows a tendency similar to the calculated one except the initial drop of C2 and hydrogen. From this similarity, we can regard the DC plasma as being close to equilibrium. The ratio Hfi/H x also decreased with the addition of nitrogen. Hence, the average electron energy becomes lower than that without nitrogen. As a result of this the plasma temperature decreases, which makes dissociation or ionization of molecules become less active. Nitrogen, thus, acts as a sink of carbon and coolant of plasma. Such effect of nitrogen on the DC plasma is
1.08 ,-~-r-~---'r-~---,r-~---,r-~---,r-,1.6 -.-: CH 30/0CH 4 , 5000 K, 140 mbar
--:C 2
§ ~
1.04
- ..-: H
0.00
~
0.92
£
..... iii ..... o' ::J ::J
0.8
~::~~_.
I
o
::J
~ l00~.~~_Z/ ~
()
1.2
~:
CN
-y-
I~:
....... 3 o
0.4
CD
?f?
---
t-~-T-~-2~~-3~~-4T-~-5~-J
00
N2 concentration (vol %) Fig. 6. The variation of equilibrium concentrations with N 2 concentration calculated by SOLGAS program.
::J
co ......-
::J
~
~
.iii
c::
-
~ .iii
Q)
c::
c::
Q)
c::
CN
CN
CH
I 1200
1300
1400
1500
1700
1600
Raman shift (cm- 1 )
(a)
350
(a)
---------------.
7
400
450
500
550
600
Wave length (nm)
16 2.0
~
1.6 6
1.2
-/ l.-~_.l....-~_.l....-~_.l....-~_.l....-~_.l....--'
(b)
.!::l
8
.....
Q)
::J
.£;
CD
::J
1j Q)
E .... o
C/l
234
0.4
c::
4
0.6
5
t-~-T-~-*"2 -~--;!;-3-~-47-~-5=--J
Ar concentration (vol %) ( b)
Fig. 5. (a) A Raman spectrum of diamond film grown at 3'!'oCH 4 3%Ar-H 2 and (b) a variation of FWHM of diamond peak and intensity ratio of diamond peak to background with Ar concentration.
~
m
0.8
Z
------------o
-
12
c::
ro
0.8 5
'iii
0
N2 concentration (vol %)
Fig. 7. (a) A typical optical emission spectrum of 3%CH c 5%N r H 2 plasma and (b) the changes of emission intensities of C 2 • CH, Hand CN species.
J.-K. Lee 1'1 al.
ivfal1'rial Scimce and Enf;in1't'rinK A209 (1996) 399-404
very consistent with a deposition behaviour to )e explained in the following paragraph. All diamond films grown with the nitrogen ad dition show ball-like feature as shown in Fig. 8 (a). Gen?rally,
(a)
~
.iii
c
2c
1200
1300
1400
1500
Raman shift
1700
1600
(cm- 1 )
(b) Fig. 8. (a) Surface morphology for diamond film grown at : '/',CH 4 5'Y<,N 2 -H 2 and 1250 °C for 20 h and (b) its Raman spectru 11.
-.---e· -A-
.-.. £:
-'0-
1140°C 1180 0 C 1220 0 C 1270 0 C
E
403
the diamond film having this kind of morphology contains a large amount of non-diamond carbon phase. Analysis by Auger electron spectroscopy on the surface of the film showed no nitrogen in the film irrespective of the nitrogen content in the gas and deposition temperature. X-ray diffraction pattern showed only diamond peaks. However, Raman spectrum does not show a definite diamond peak as shown in Fig. 8 (b). The spectrum seems to consist of three broad peaks, 1332 cm I, 1350 cm - 1 and 1560 cm 1; therefore, it is considered as a mixture of diamond and disordered graphite peaks although small peak shifts are observed. Growth rates decreased with the addition of nitrogen as shown in Fig. 9. They show a sudden drop with nitrogen addition below I(Yo. This observation as well as the morphology variation shows that even a small amount of nitrogen affects the plasma chemistry significantly. Comparing with the emission intensity variation shown in Fig. 7 (a), we can find a good correspondence: the intensities of C 2 and H also show sudden drop. The high concentration of C 2 species is generally considered as being related to high growth rate; this is consistent to the present result. However the criteria that C 2 is a precursor forming a non-diamond carbon phase is contrary to the present observation. Despite the decrease of the C 2 emission intensity, the content of the non-diamond carbon phase increases. The criteria that C 2 is a precursor forming non-diamond carbon phase was made from a CH 4 -H 2 system. As the methane concentration increases, the emission intensity of C 2 always increases. For this reason C 2 was considered as a cursor forming the non-diamond carbon phase. However, we believe that this analogy is unreasonable and hydrogen is rather a good standard in the OES of the plasma for determining how much the diamond film contains the non-diamond carbon phase. The important species that has to be considered also in a CH 4 -H 2 system is the atomic hydrogen, because it has been considered as inhibiting the non-diamond carbon phase. Moreover, the role of C 2 and even CH 3 and C 2 H 2 in determining diamond quality is not clear even until now. The abrupt decrease of the atomic hydrogen emission intensity is also consistent with the experimental observation.
2;
Q)
I1l L..
4. Conclusions
•
I
N2 concentration (vol. %) Fig. 9. The change of growth rate with nitrogen concentJ ation at various deposition temperature.
We investigated the effect of Ar and nitrogen on diamond deposition by DC PACVD method. The effect of Ar was not significant except a small enhancement of deposition rate and defect formation in the diamond film. The OES observation showed that only carbon containing species, C 2 and CH, increased with Ar addition. Nitrogen, however resulted in a
404
J.-K. Lee et al. / Materials Science and Engineering A209 (1996) 399--404
significant change in both deposition behavior and optical emission spectrum. Clear diamond facets changed to spherical morphology with nitrogen addition and diamond quality became worse. Deposition rate was also decreased. The OES observation showed a definitive relation between C2 emission intensity and the deposition rate and between H emission intensity and diamond quality.
Acknowledgements The authors are grateful to Ministry of Science and Technology for financial support.
References [I] S. Matsumoto, Y. Sato, M. Kamo and N. Setaka, Jpn. J. Appl. Phys., 21 (1983) Ll83. [2] R.C. DeVries, Ann. Rev. Mater. Sci., 17 (1987) 161. [3] S. Matsumoto, M. Hino and T. Kobayashi, App. Phys. Lett., 51 (1987) 737. [4] J.c. Knight, R.A. Lujan. M.P. Rosenblum, R.A. Street, O.K. Bieglesen and J.A. Reimer, App. Phys. Lett., 38 (1981) 331. [5] W. Zhu, A. Inspector, A.R. Badzian. T. McKenna and R. Messier, J. Appl. Phys., 68 (1990) 1489. [6] H.C. Shih, c.P. Sung and W.L. Fan, Surf Coat. Technol., 54/55 (1992) 380. [7] T. Kawato and K. Kondo, Jpn. 1. Appl. Phys., 26 (1987) 1429. [8] Y.-J. Baik and K.Y. Eun, Thin Solid Films, 212 (1992) 156. [9] J.-K. Lee, W.-S. Lee, Y.-J. Baik and K.Y. Eun, J. Korean Inst. Met. Mater., 32 (1994) 742. [10] A.Y. Liu and M.L. Cohen, Science, 245 (1989) 841. [II] A. Badzian, T. Badzian and S.-T. Lee, Appl. Phys. Lett., 62 (1993) 3432.
Materials Science an I Engineering A209 (1996) 399 404
Effect of additional gas on diamond deposition by DC PACVD Jae-Kap Lee, Young-Joan Baik, Kwang Yang Eun Division of Ceramics, Korea Institute of Science (, 'ld Techl1oloK)" PO Boy 13/, Cheol1KrranK, Seoul 130-650, South Korea
Abstract The effect of the addition of Ar and nitrogen gas on the deposition of diamond using a direct current (DC) plasma of CH c H 2 gas mixture is investigated, The DC plasma is generat:d by applying a voltage between 820 and 900 V and the resulting current is between 5,5 and 6 A, The addition of Ar makes th: plasma unstable and is limited to 5'/i, owing to the plasma extinction at higher Ar concentrations. Growth rate and non-diamo ld carbon content in the diamond film increase with the Ar amount slightly irrespective of deposition temperature. An optical emi~ sion spectrum shows the increase of emission of C2 and CH while showing a constant average electron energy of the plasma. On he contrary, by adding nitrogen, the morphology is changed to a ball-like diamond shape as well as growth rate of diamond film is decreased abruptly. The optical emission intensities of C2 and hydrogen also show an abrupt drop with the nitrogen addition. The role of plasma species in the deposition behaviour of diamond is also discussed. Ke)'lmrds: Diamond deposition; Argon gas; Nitrogen gas
1. Introduction
Since the success of diamond synthesis fro n gas phase [1], a hydrocarbon and hydrogen gas mixtl re has been used as a basic gas system, Activation of a gas mixture by either plasma or thermal energy is nel essary for diamond synthesis, Although the synthesis 11echanism of diamond and the gas chemistry during the diamond synthesis are not clarified yet, atomic 1ydrogen has been recognized empirically as crucial 10 diamond synthesis [2], In order to increas: the supersaturation of the atomic hydrogen, high frequency electric plasma or thermal plasma instead of 10" temperature plasma has been preferred. As a result, the growth rate has been increased by an order of )nagnitude using thermal plasma [3], Another method to change plasma character stic is the addition of noble gases. Noble gases can ellhance ionization rate and, consequently, deposition rate in lots of CVD systems [4]. Studies on the effect of nobl,' gases on the diamond deposition have also shown the t they can change the electron energy distribution of 1)Iasma and, thus, growth rate and non-diamond carbon C Jntent in diamond [5]. The effect of Ar on the didmond deposition was greatest among noble gases [5,6]. fhis is 0921-5093;96/$15.00© 1996 -
SSDI 0921-5093(95)10143-8
Elsevier Science SA All rig Its reserved
because its ionization potential is similar to that of hydrogen. This result has been observed in a microwave plasma assisted chemical vapour deposition system (MW PACVD) [5,6], the average electron energy of which is higher than that of low frequency plasmas, In this paper, we studied the effect of Ar on the characteristic of DC plasma and deposition behavior of diamond. A DC plasma generated at several ten torr is not frequently used for diamond synthesis due to its inefficiency of gas activation. However, the DC plasma used in this study is generated above 100 torr and very close to an equilibrium thermal plasma. The gas activation is, thus, expected to be sufficient irrespective of the frequency. It is difficult to use the DC plasma stably for diamond deposition because its operating condition is very close to a glow to arc transition boundary. The addition of noble gases can be one of the method inhibiting an arc generation, No study on the effect of noble gases using the DC plasma has been conducted for their role in plasma stabilization and diamond deposition yet. We selected Ar among noble gases because its effect is expected to be greatest. Oxygen and nitrogen have also been added to a methane hydrogen gas mixture for diamond deposition, Small amount of oxygen not only reduces defect con-
400
J.-K. Lee et al. / Materials Science and Engineering A209 (1996) 399-404
tent in diamond but also changes the growth rate [7,8]. We have already confirmed its effect on the diamond synthesis by DC PACVD method [9]. The oxygen now is a common additive to the gas mixture for diamond synthesis. Nitrogen has drawn attention recently, owing to its function as a dopant or the possibility of forming CN compound [10,11]. It has been also suggested that nitrogen can act a role of hydrogen in diamond synthesis [11]. The study on the effect of nitrogen on the diamond deposition using a high energy thermal plasma is thus meaningful for finding a new material or a new gas system for diamond synthesis. An optical emission spectroscopy (OES) was employed to study the effect of nitrogen on the DC plasma of CH c H 2 . Comparing OES results with those of growth rate measurement and Raman spectroscopy, we tried to find the relation between plasma characteristics and diamond deposition behaviour.
emission of high density of thermal electron from cathode material. The impact of Ar ions in the plasma to the cathode can be a main cause of cathode temperature rise. A variation of plasma species due to Ar addition can be observed by an optical emission spectroscopy. A typical optical emission spectrum from the DC plasma of CH 4 and H 2 is shown in Fig. 1. The major species observed in this spectrum are H atom (Balmer series, H~ :656.2 nm, Hfi :486.1 nm, H;.:434.0 nm), C 2 radical (Swan band, 563.6, 516.5 and 474.7 nm), CH radical (431.4 nm) and excited H 2 molecule (581.0 nm). Although its intensity was very weak, a peak around 216 nm could be observed, probably coming from CH 3 and/or C2 H 2 emission. However, the wavelengths of two emissions were too close to distinguish each other. The changes of emission intensities of C2 , CH and H atom at different concentration of Ar is shown in Fig. 2. The methane concentration was 3%. The carbon
2. Experimental A DC PACVD system used in this study has been described in detail previously [9]. A plasma was excited by a DC power supply with voltage between 820 and 900 V and current between 5.5 and 6.0 A. Ar and nitrogen were added up to 5% to a CH 4 -H 2 gas mixture separately. The total gas flow and the methane concentration kept constant at 150 sccm and 3% respectively; an amount of hydrogen equivalent to that of the added gas was reduced. A pressure of 140 mbar was used for all deposition processes. Diamond films were deposited on Mo substrate with thickness of several ten micrometers. They were peeled off after cooling to room temperature. The thickness of free standing films was measured using a digital micrometer. Scanning electron microscopy and Raman spectroscopy were employed for the characterization of the morphology and quality of diamond. An optical emission spectroscopy was used to analyse the plasma species using an DIGISEM 350 emission spectrometer of Sofie instrument. A spectrum range used was from 200 to 700 nm.
H~
CH
35~O;--~-:;4~OO:;--~-:;47;;50:;--~·5*'OO:;--~·5:t50:;--~"""tr600
Wave length (nm) Fig. I. A typical optical emission spectrum of DC plasma of 3'f"CH 4 -
H2
3. Results and discussion
3.1. The effect of Ar addition Ar has been used to control plasma density or energy distribution to investigate the possibility of enhancing reaction for deposition. However, the addition of Ar to DC plasma of CH 4 - H 2 gas was observed to make the plasma unstable: the plasma decreased its size with Ar addition, transited to arc and extinguished above 5% of Ar. The plasma-arc transition is caused mainly by
a
2
3
4
5
Ar concentration (vol. %) Fig. 2. The changes of emission intensities of C 2 , CH and H atom with Ar concentration.
J.-K. Lee et al.
401
Materia s Science and Engineering A209 (1996) 399-404
containing species, C 2 and CH, show an increase in the emission intensities while the hydrogen atom shows nearly constant emission intensity. In comparison with the previous result for MW plasma [5], we can find a nearly similar behaviour of variation of emission intensities between two plasmas: for example, abOl t 30(/'() and 50% increase in C 2 emission intensity for the MW plasma and DC plasma respectively and aim lSt no variation in H emission intensity with the addi ion of 5(% Ar. (The emission intensity of H increased above YYo in the MW plasma [5], but we could not (dd Ar above 5% to the DC plasma). The ratio of Hili H" an indication of average electron energy, was not va 'ied by adding Ar. This implies that the electron energy distribution remains constant independently of Ar ac dition. The variation of the emission intensity, related to the concentration of the excited species, can be cal sed by either a variation of the ground state species con :entration or a variation of excitation efficiency of th 11 species. The excitation efficiency is dependent 'lll the electron energy distribution in the plasma. E ut the latter possibility can be ruled out in the DC )Iasma owing to the constant HldHx value. Hence, ~ e conclude that the concentration of ground state C 2 and CH species in the plasma increased by adding Ar. The variation of plasma species is closely rellted to the behaviour of diamond deposition such as ill< lrphology, growth rate and defect concentration in dii mond. No significant variation of the morphology of di amond film was observed within the experimental rangl of Ar addition up to 5%. Fig. 3 shows a typical mrface morphology of diamond film grown at CH 4 ind Ar concentrations of 3% and 50;;1 respectively. (1(0) and (111) facets are on the surface and the texture (rientation is (110). Moreover, the grain size remaint d constant with the increasing Ar amount at c· mstant methane concentration. The effect of Ar addil ion on the nucleation and morphology of diamond .s thus insignificant. However, the growth rate and quality of d'amond film was changed by Ar addition. Fig. 4 she ws the measured growth rate at different Ar concentrati on and deposition temperature. We can see that the inc) ease of the growth rates also depends on the depositic n temperature. A typical Raman spectrum of diamo ld film grown at CH 4 and Ar concentration of 3% (nd 3'% respectively and a variation of full width at hal·' maximum value (FWHM) of diamond peak at vari JUS Ar concentration are shown in Fig. 5. Only a d amond peak at 1332 cm - 1 without any broad peak around 1500 cm - 1 was observed for all diamond films However, the variation of FWHM value and S/N ra.io (the ratio of diamond peak to background intensity shows more defective diamond film growing at hig ler Ar concentration. This result is consistent with the observation of OES and those of previous reports 5]: the
Fig. 3. Surface morphology of diamond film grown at 3%CH 4 3'y',Ar H 2 and 1290 °C for 20 h.
--.~:
24
-.&.--.
,-.,
--2-E
3%CH 4
1210 0 C
---e- 1250 0 C
'~~:~~
£
20
- /
~
0
1290 C
-'0-
(])
I II
....
£
j
0 .... (9
16
;7"
~~ ..
~
:::=::::===-
Ar concentration (vol %) Fig. 4. The change of growth rate with Ar concentration at various deposition temperature.
higher the concentration of carbon containing species such as C ll the higher the growth rate.
3.2. The effect of nitrogen addition
The chemistry of plasma with the addition of nitrogen to CH c H 2 mixture is more complex compared with that of Ar, because chemical reactions between nitrogen, carbon and hydrogen occur. We calculated by SOLGAS program the equilibrium concentration of a N 2 -CH c H 2 mixture at various N 2 concentration and at 5000 K and 140 mbar. The temperature is assumed as that of the DC plasma. The order of abundancy of gas species is H, C, N, CN, N 2 , CH, C 2 , etc. We selected some of the species, which were observed on OES spectrum and plotted the change of the normalized concentrations at various nitrogen amount in Fig. 6. The normalized concentrations of C 2 , CH and atomic hydrogen decrease with the increase of nitrogen while that of CN increases.
402
J.-K. Lee et al. / Materials Science and Engineering A209 (1996) 399-404
The optical spectrum of 3%CH 4 -5%N 2 -H 2 and the variation of respective peak intensities at various nitrogen amounts are shown in Fig. 7. The intensities of C2 and hydrogen emission dropped abruptly at nitrogen concentration below I % and then decreased at a slow rate, while that of CH emission decreased gradually. The intensity of CN emission, however, increased with the increase of nitrogen. The OES result shows a tendency similar to the calculated one except the initial drop of C2 and hydrogen. From this similarity, we can regard the DC plasma as being close to equilibrium. The ratio Hfi/H x also decreased with the addition of nitrogen. Hence, the average electron energy becomes lower than that without nitrogen. As a result of this the plasma temperature decreases, which makes dissociation or ionization of molecules become less active. Nitrogen, thus, acts as a sink of carbon and coolant of plasma. Such effect of nitrogen on the DC plasma is
1.08 ,-~-r-~---'r-~---,r-~---,r-~---,r-,1.6 -.-: CH 30/0CH 4 , 5000 K, 140 mbar
--:C 2
§ ~
1.04
- ..-: H
0.00
~
0.92
£
..... iii ..... o' ::J ::J
0.8
~::~~_.
I
o
::J
~ l00~.~~_Z/ ~
()
1.2
~:
CN
-y-
I~:
....... 3 o
0.4
CD
?f?
---
t-~-T-~-2~~-3~~-4T-~-5~-J
00
N2 concentration (vol %) Fig. 6. The variation of equilibrium concentrations with N 2 concentration calculated by SOLGAS program.
::J
co ......-
::J
~
~
.iii
c::
-
~ .iii
Q)
c::
c::
Q)
c::
CN
CN
CH
I 1200
1300
1400
1500
1700
1600
Raman shift (cm- 1 )
(a)
350
(a)
---------------.
7
400
450
500
550
600
Wave length (nm)
16 2.0
~
1.6 6
1.2
-/ l.-~_.l....-~_.l....-~_.l....-~_.l....-~_.l....--'
(b)
.!::l
8
.....
Q)
::J
.£;
CD
::J
1j Q)
E .... o
C/l
234
0.4
c::
4
0.6
5
t-~-T-~-*"2 -~--;!;-3-~-47-~-5=--J
Ar concentration (vol %) ( b)
Fig. 5. (a) A Raman spectrum of diamond film grown at 3'!'oCH 4 3%Ar-H 2 and (b) a variation of FWHM of diamond peak and intensity ratio of diamond peak to background with Ar concentration.
~
m
0.8
Z
------------o
-
12
c::
ro
0.8 5
'iii
0
N2 concentration (vol %)
Fig. 7. (a) A typical optical emission spectrum of 3%CH c 5%N r H 2 plasma and (b) the changes of emission intensities of C 2 • CH, Hand CN species.
J.-K. Lee 1'1 al.
ivfal1'rial Scimce and Enf;in1't'rinK A209 (1996) 399-404
very consistent with a deposition behaviour to )e explained in the following paragraph. All diamond films grown with the nitrogen ad dition show ball-like feature as shown in Fig. 8 (a). Gen?rally,
(a)
~
.iii
c
2c
1200
1300
1400
1500
Raman shift
1700
1600
(cm- 1 )
(b) Fig. 8. (a) Surface morphology for diamond film grown at : '/',CH 4 5'Y<,N 2 -H 2 and 1250 °C for 20 h and (b) its Raman spectru 11.
-.---e· -A-
.-.. £:
-'0-
1140°C 1180 0 C 1220 0 C 1270 0 C
E
403
the diamond film having this kind of morphology contains a large amount of non-diamond carbon phase. Analysis by Auger electron spectroscopy on the surface of the film showed no nitrogen in the film irrespective of the nitrogen content in the gas and deposition temperature. X-ray diffraction pattern showed only diamond peaks. However, Raman spectrum does not show a definite diamond peak as shown in Fig. 8 (b). The spectrum seems to consist of three broad peaks, 1332 cm I, 1350 cm - 1 and 1560 cm 1; therefore, it is considered as a mixture of diamond and disordered graphite peaks although small peak shifts are observed. Growth rates decreased with the addition of nitrogen as shown in Fig. 9. They show a sudden drop with nitrogen addition below I(Yo. This observation as well as the morphology variation shows that even a small amount of nitrogen affects the plasma chemistry significantly. Comparing with the emission intensity variation shown in Fig. 7 (a), we can find a good correspondence: the intensities of C 2 and H also show sudden drop. The high concentration of C 2 species is generally considered as being related to high growth rate; this is consistent to the present result. However the criteria that C 2 is a precursor forming a non-diamond carbon phase is contrary to the present observation. Despite the decrease of the C 2 emission intensity, the content of the non-diamond carbon phase increases. The criteria that C 2 is a precursor forming non-diamond carbon phase was made from a CH 4 -H 2 system. As the methane concentration increases, the emission intensity of C 2 always increases. For this reason C 2 was considered as a cursor forming the non-diamond carbon phase. However, we believe that this analogy is unreasonable and hydrogen is rather a good standard in the OES of the plasma for determining how much the diamond film contains the non-diamond carbon phase. The important species that has to be considered also in a CH 4 -H 2 system is the atomic hydrogen, because it has been considered as inhibiting the non-diamond carbon phase. Moreover, the role of C 2 and even CH 3 and C 2 H 2 in determining diamond quality is not clear even until now. The abrupt decrease of the atomic hydrogen emission intensity is also consistent with the experimental observation.
2;
Q)
I1l L..
4. Conclusions
•
I
N2 concentration (vol. %) Fig. 9. The change of growth rate with nitrogen concentJ ation at various deposition temperature.
We investigated the effect of Ar and nitrogen on diamond deposition by DC PACVD method. The effect of Ar was not significant except a small enhancement of deposition rate and defect formation in the diamond film. The OES observation showed that only carbon containing species, C 2 and CH, increased with Ar addition. Nitrogen, however resulted in a
404
J.-K. Lee et al. / Materials Science and Engineering A209 (1996) 399--404
significant change in both deposition behavior and optical emission spectrum. Clear diamond facets changed to spherical morphology with nitrogen addition and diamond quality became worse. Deposition rate was also decreased. The OES observation showed a definitive relation between C2 emission intensity and the deposition rate and between H emission intensity and diamond quality.
Acknowledgements The authors are grateful to Ministry of Science and Technology for financial support.
References [I] S. Matsumoto, Y. Sato, M. Kamo and N. Setaka, Jpn. J. Appl. Phys., 21 (1983) Ll83. [2] R.C. DeVries, Ann. Rev. Mater. Sci., 17 (1987) 161. [3] S. Matsumoto, M. Hino and T. Kobayashi, App. Phys. Lett., 51 (1987) 737. [4] J.c. Knight, R.A. Lujan. M.P. Rosenblum, R.A. Street, O.K. Bieglesen and J.A. Reimer, App. Phys. Lett., 38 (1981) 331. [5] W. Zhu, A. Inspector, A.R. Badzian. T. McKenna and R. Messier, J. Appl. Phys., 68 (1990) 1489. [6] H.C. Shih, c.P. Sung and W.L. Fan, Surf Coat. Technol., 54/55 (1992) 380. [7] T. Kawato and K. Kondo, Jpn. 1. Appl. Phys., 26 (1987) 1429. [8] Y.-J. Baik and K.Y. Eun, Thin Solid Films, 212 (1992) 156. [9] J.-K. Lee, W.-S. Lee, Y.-J. Baik and K.Y. Eun, J. Korean Inst. Met. Mater., 32 (1994) 742. [10] A.Y. Liu and M.L. Cohen, Science, 245 (1989) 841. [II] A. Badzian, T. Badzian and S.-T. Lee, Appl. Phys. Lett., 62 (1993) 3432.