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OES study of plasma processes in d.c. discharge during diamond film deposition
Diamond and Related Materials, 3 (1994) 1385-1388
1385
OES study of plasma processes in d.c. discharge during diamond film deposition V. M . P o l u s h k i n , A. T. R a k h i m o v ,
V. A. S a m o r o d o v ,
N . V. S u e t i n * a n d M . A. T i m o f e y e v
Nuclear Physics Institute, Moscow State University. Moscow I 19899 (Russian Federation)
(Received September 24, 1993; accepted in final form March 22, 1994}
Abstract A systematic study has been performed of optical emission spectra from d.c. glow discharge during diamond film deposition. The emission intensity within the region 400-700 nm was found to be a function of methane concentration and interelectrode position. The experimentswere carried out under hydrogen pressure 160 Torr and methane concentration 0%-3%. The optical emissionhad a different character for different discharge regions (anode and cathode regions, plasma volume).The optical spectra observedwere also essentially different from the flameand jet arc emission spectra. The most intensive lines in our experimentswere emitted by hydrogen atoms, H~ and H~. Using a ratio of these lines the electron temperature in C H 4 4- H 2 plasma was estimated. The R-branch of the H2 molecular (Gi£'g+ B1Z'u+ (0-0) band in the emission spectra near 2=463 nm and vibration structures in the emission spectra of the radical C2 (d3Hga3Hu) near 2 = 515 nm were also examined. These data were used to determine a gas temperature.
1. Introduction Several techniques have been used to excite diamond growth plasmas. Optical emission spectroscopy (OES) is one of the simplest and most effective methods for studying plasma chemical processes in a gas volume, and has been used to observe microwave [ 1,2], RF [3], arcjet I-4] and flame [5] enhanced CVD of diamond films. D.c. glow discharge has been used successfully for high rate diamond film (DF) deposition [.6,7]. We used an optical emission spectroscopy method to study plasma processes in this discharge during D F deposition. The correlation between the quality of the diamond films and spectral characteristics of the discharge, noted previously 1-2], was not verified in the present work.
2. Experimental details Our d.c. plasma CVD equipment was as described previously [7] and similar to that used by Suzuki et al. [6]. The experiments were carried out under hydrogen pressure 160 Tort and methane concentration 0 % - 3 % . The gas flow rate was controlled to 500 sccm. Substrates (Mo or Si) of diameter 8 m m were placed on the anode. The gap between the electrodes was 20 mm. For these conditions the current density was 1 A cm -2 at voltage *Author to whom correspondence should be addressed.
0925 9635/94/$7.00 SSD1 0925-9635(94}00210-1
660 V. The quality of the film deposited was determined by optical microscopy, scanning electron microscopy, Raman spectroscopy and X-ray diffractometry. D.c. glow discharge in C H 4 -4-H 2 mixtures is characterized by bright light near the anode region (up to 15 mm). Optical spectra were observed within the region 4 0 0 - 7 0 0 n m using a multichannel optical spectroanalyzer (OSA). Optical signals were registered for different spatial points of discharge.
3. Results The optical emission was found to have a different character for the different discharge regions (anode and cathode regions and plasma volume). The optical spectra observed were also essentially different from the flame and arcjet emission spectra [4, 5]. A typical emission spectrum is shown in Fig. 1 (obtained from the cathode region for 2% methane concentration). Along broad H z molecular lines, the Balmer lines H , (656nm), Hp (486 nm), Hy (434 nm) and lines for the radicals Cz (516 nm), C H (431 nm) were recorded. The lines for atomic hydrogen H~ and Hp appeared to have the strongest intensity. This is unlike the arcjet [.4] and flame [5] optical spectra, in which hydrogen atomic and molecular lines are very weak, but there are strong emission lines from C z bands between 436.5 and 564.5 nm (the Swan bands) and C H bands (387-435 nm). Our spectra were quite similar to microwave discharge
~) 1994 - - Elsevier Science S.A. All rights reserved
1386
V. M. Polushkin et al. / OES study of plasma in d.c. discharge in diamond film deposition
1.0 0.9
1
H~
2
0.8 C2
~.~ 0.7
H2
0.6
0.5!
I
r
c 400
500
600 nm
Fig. 1. Overview of typical emission spectrum from d.c. glow discharge plasma (cathode region, p = 160 Torr, [CH4] =2%).
spectra [1,2]. This can be explained by the fact that electron distribution functions, which determine electron excitation rate constants, are very similar for d.c. glow and microwave discharges. Electron temperature was estimated from the relative intensity of the Balmer lines I-8]. This method is generally used for calculation of electron temperature in plasma under the local thermodynamic equilibrium. This case is realized if the population of upper excited levels is determined by collisions with electrons. For the existence of local thermodynamic equilibrium it is necessary that the electron density N~ correspond to the condition [8]:
Nc >>9 x 1016 (AE/El) 3 (kTe/El) 1/2 cm -3
(1)
where AE is the transition energy between levels, E~ is the energy of ionization and kT~ is the electron temperature. For the H a hydrogen atom line and electron temperature k T ~ l e V , the inequality (1) is Ne >~ l014 cm -3. For our experiment the electron density could be estimated from the equality N~ =j/e v~, where j is the electric current density, e the electron charge and ve the drift electron velocity, which was taken from ref. 9 with its dependence on electric field. The experimental estimation of the electron density gives N o ~ 6 x 1013 c m -3. Hence, in our case, local thermodynamic equilibrium does not really exist, and for this reason the electron temperature value given is only an estimation. However, it makes sense as an illustration of general regularity in temperature distribution in volume, and its general behavior in dependence on gas structure. Figure 2 shows the electron temperature against distance from the cathode for different methane concentrations. It can be seen
A
Fig. 2. Intergap electron temperature distribution for different methane concentrations: 0% (1), 1% (2), 2% (3), 3% (4). Typical error bars for all curves are shown on curve 1.
that the electron temperature decreases from 0.95 eV to 0.75 eV near the cathode and from 0.6 to 0.5 eV near the anode, with an increase in CH4 from 0% to 3%. This sensitivity of electron temperature to CH4 concentration can be explained by higher cross-sections of electron nonelastic scattering on methane molecules than on hydrogen ones. The electron temperature spatial distribution depends on both a high discharge energy near the cathode and the gas flow. In order to determine the gas temperature we used two means of treating spectra data. The first method was based on a determination of the vibrational temperature of the radical C2 using optical emission spectra from C2 d3Hg-a3Hu in the region of wavelength 2 = 515 nm. The vibrational temperature was determined through the i nown ratio of two relative intensities of two transitions (v', v") and (w', w") from the equality Iv,~,, _ A~,~, e x p (
Iw'w" Aw'w"
kTvibr
/
where Av,v° and Aw,w,, are the Einstein spontaneous emission coefficients, and G(v) is the energy of the anharmonic oscillator, given by
G(v) = coe (v+ 1/2) - c%x~ (v+ 1/2) 2 + coeY~(v+ 1/2) 3 + ... Our
C2
radical spectra maintained lines of transition
d3Hg-a3Hu with A v = 0 : 0 - 0 and 1-1, where v is the vibrational quantum number. Respective means G(v) were calculated according to the method described in ref. 10, and Einstein coefficients were taken from ref. 11. The second method was based on determining the rotational temperature of molecular H2 through the
V. M. Polushkin et al. / OES study of plasma in d.c. discharge in diamond film deposition
R1)~'+ spectra of the R branch of the v(~TI~',+ _g --~ --u (0--0) b a n d of the H 2 molecule in the region of wavelength £ = 463 nm. It was found that the value ln(I/S) is practically a linear function of the upper rotational energy E. The rotational temperature was determined using the incline angle of this line [12]. Here I is the relative intensity of the emission divided by the nuclear spin degeneracy, and S is equal to f+j"+l where f and j" are the rotational quantum numbers of the upper and lower levels respectively. There are several points which have to be considered in order to prove a determination of the gas kinetic temperature using the relative emission intensities of rotational lines within an electron band. The first point is that there must be enough molecular collisions to ensure that the rotational population reflects the gas kinetic temperature. The radiative decay rate of the upper level of the _(~T1)~'~+ _ g - _R1/~'~+ - - u (0--0) H 2 band is 3 x 10 v s -1 [10]. The collision rate of gas molecules is estimated to be approx. 109 s -1 [13] at a total pressure of 150 Torr and gas temperature 2000 K. Therefore it is believed that the collision rate is large enough for the rotational temperature in the excited electronic state to be considered equal to the gas kinetic temperature. The second point is that self-absorption could be a source of systematic error in this type of measurement. However, the B 1Su+ level of the H2 band has a short radiative lifetime, 8 × 10-1°s [10], which results in a small population of excited molecules to produce marked self-absorption of the H2 band. Values for the vibrational temperature T~ib and rotational temperature T~ot were found to be almost equal, Tvib~ T~ot~ 2000 K. The experimental accuracy of temperature measurement was about 150 K in both cases. This was a result of a line resolution of the electronicvibrational transition of radical C2 1 1 in the first case, and there was some deviation of registered points from the plot lnlI/Sl=f(E) in the second one. The latter deviation can be explained by serious perturbations of upper rotational levels of H 2 molecules by nearby levels with different lifetimes. This is a complicated problem known for this method. However, our results are in reasonable agreement with the data of ref. 13, which were obtained under similar conditions. The spatial distribution of lines H~ and H2 (463 nm) for pure hydrogen and 3% methane is shown in Fig. 3. The emission intensity of both lines is correspondingly six and two times greater near the anode than in volume, unlike the data reported in ref. 1, where the reverse dependence was found. Figure 4 shows the emission intensities ratio In/In2 as a function of the methane concentration. The ratio increases with increasing methane percentage, and in particular is three times greater at 3% C H 4 compared with pure H 2. This can be explained if we assume that
1387
12
5O
I0
4o
8~
V
4 2
0/I C
I
I
A C
A
0
Fig. 3. Spatial distributions of hydrogen atomic 1486 nm) and molecular (463 nm) lines: [CH4] = 0% ( 1 h [CH4] = 3 % (2).
?
10
8
L
C
A
Fig. 4. Spatial distributions of ratio of hydrogen atomic H~ to molecular (463 nm) lines intensity for different methane concentrations: 0% tl), 0.5% (2), 1% (3), 3% (4). Typical error bars for all curves are shown on curve 4.
the atomic hydrogen concentration increases with increasing methane concentration, by new H produced channels in methane containing plasma: CH4+e--*CH3 + H CH 3 + H2~CH 4 + H The decrease of IH/IH2 near the anode can be explained by atomic hydrogen recombination on the anode surface. It is been reported previously [2] that there is a correlation between the quality of diamond films (growth by microwave PECVD) and some features of the plasma emission spectra. The best diamond films have been obtained in cases where the C H (431 nm) band appeared clearly while the C2 (516 nm) emission band was almost missing [2]. We tried to verify these results and to apply this method for d.c. glow discharge diamond CVD. Figure 5 shows intergap spatial distributions of intensity of C2 (516 nm) and C H (431 nm) lines for different methane concentrations. For both lines the intensity
1388
V. M. Polushkin et al. / OES study of plasma in d.c. discharge in diamond film deposition
of a gas t e m p e r a t u r e . N o c o r r e l a t i o n was f o u n d between C H a n d C2 lines ratio a n d film quality.
10 58
m
46
Acknowledgments
_o
~4
0
o~o I C
I A
Fig. 5. Spatial distribution of ratio of radicals C2 (516 nm) to CH (430 nm) lines intensity for different methane concentrations: 0, 0.5%; C), 1%; x, 2%. increases with increasing m e t h a n e c o n c e n t r a t i o n , which is in a g r e e m e n t with the previous result [ 1 ]. H o w e v e r , we c o u l d n o t find a n y c o r r e l a t i o n between this line ratio a n d film quality. As can be seen from Fig. 5, the intensity r a t i o I c j l c n is i n d e p e n d e n t of m e t h a n e c o n c e n t r a t i o n while there is an increase in d e p o s i t i o n rate a n d a decrease in film quality. The last two p o i n t s also d e p e n d on s u b s t r a t e t e m p e r a t u r e even with no essential change of the emission spectra.
4. Conclusion A systematic s t u d y was p e r f o r m e d of O E S from d.c. glow discharge d u r i n g D F deposition. The emission intensity was studied as a function of m e t h a n e concent r a t i o n a n d interelectrode position. T h e electron t e m p e r ature in the C H 4 -t-H2 p l a s m a was e s t i m a t e d from the relative intensity of the B a l m e r lines. A v i b r a t i o n - r o t a tional structure in the emission spectra of radicals C H ( A - X ) near 2 = 4 3 0 n m a n d C 2 (d3Hg-a317u) n e a r 2 = 5 1 5 n m was o b s e r v e d a n d used for d e t e r m i n a t i o n
T h e a u t h o r s gratefully a c k n o w l e d g e A. N. O b r a z t s o v for p e r f o r m i n g the R a m a n s p e c t r o s c o p y analyses a n d S. A. D o l e n k o for useful discussions.
References 1 B. Marcus, M. Mermoux, F. Vinet, A. Campargue and M. Chenevier, Surf. Coat. Technol., 47 (1991) 608. 2 G. Balestrino, M. Marinelli, E. Milani, A. Paoletti, I. Pinter, A. Tebano and P. Paroli, Appl. Phys. Lett., 62 (1993) 879. 3 Y. Catherine, A. Pastol, L. Athouel and C. Fourrier, IEEE Trans. Plasma Sci., 18 (1990) 923. 4 K. R. Stalder and R. L. Sharpies, J. Appl. Phys., 68 (1990) 6187. 5 R. S. Yalamanchi and K. S. Harshavardhan, J. Appl. Phys., 68 (1990) 5941. 6 K. Suzuki, A. Sawabe and T. Inuzuka, Jpn. J. Appl. Phys., 29 (1990) 153. 7 V. M. Polushkin, S. N. Polyakov, A. T. Rakhimov, N. V. Suetin, M. A. Timofeyev and V. A. Tugarev, Proc. 4th Eur. Conf. on Diamond, Diamond-like and Related Materials, Albufeira, 1993, Diamond Relat. Mater., 3 (1994) 531. 8 Hans R. Griem, Plasma Spectroscopy, McGraw-Hill, New York, (transl. from Engl., Moscow, Atomizdat, 1969, pp. 223, 128). 9 Y. P. Raizer, Physics of Gas Discharge, Nauka, Moscow, 1987, pp. 80, 24. 10 K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure: Vol. IV. Constants of Diatomic Molecules, Litton Educational Publishing, 1979. 11 D. M. Cooper and R. W. Nicholls, Spectrosc. Lett., 9 (1976) 139. 12 G. Herzberg, Molecular Spectra and Molecular Structure, Vol. I, Krieger, Malabar, FL, 1989. 13 H. N. Chu, E. A. Den Hartog, A. R. Lefkow, J. Jacobs, L. W. Anderson, M. G. Lagally and J. E. Lawler, Phys. Rev. A, 44 (6) (1991) 3796.
1385
OES study of plasma processes in d.c. discharge during diamond film deposition V. M . P o l u s h k i n , A. T. R a k h i m o v ,
V. A. S a m o r o d o v ,
N . V. S u e t i n * a n d M . A. T i m o f e y e v
Nuclear Physics Institute, Moscow State University. Moscow I 19899 (Russian Federation)
(Received September 24, 1993; accepted in final form March 22, 1994}
Abstract A systematic study has been performed of optical emission spectra from d.c. glow discharge during diamond film deposition. The emission intensity within the region 400-700 nm was found to be a function of methane concentration and interelectrode position. The experimentswere carried out under hydrogen pressure 160 Torr and methane concentration 0%-3%. The optical emissionhad a different character for different discharge regions (anode and cathode regions, plasma volume).The optical spectra observedwere also essentially different from the flameand jet arc emission spectra. The most intensive lines in our experimentswere emitted by hydrogen atoms, H~ and H~. Using a ratio of these lines the electron temperature in C H 4 4- H 2 plasma was estimated. The R-branch of the H2 molecular (Gi£'g+ B1Z'u+ (0-0) band in the emission spectra near 2=463 nm and vibration structures in the emission spectra of the radical C2 (d3Hga3Hu) near 2 = 515 nm were also examined. These data were used to determine a gas temperature.
1. Introduction Several techniques have been used to excite diamond growth plasmas. Optical emission spectroscopy (OES) is one of the simplest and most effective methods for studying plasma chemical processes in a gas volume, and has been used to observe microwave [ 1,2], RF [3], arcjet I-4] and flame [5] enhanced CVD of diamond films. D.c. glow discharge has been used successfully for high rate diamond film (DF) deposition [.6,7]. We used an optical emission spectroscopy method to study plasma processes in this discharge during D F deposition. The correlation between the quality of the diamond films and spectral characteristics of the discharge, noted previously 1-2], was not verified in the present work.
2. Experimental details Our d.c. plasma CVD equipment was as described previously [7] and similar to that used by Suzuki et al. [6]. The experiments were carried out under hydrogen pressure 160 Tort and methane concentration 0 % - 3 % . The gas flow rate was controlled to 500 sccm. Substrates (Mo or Si) of diameter 8 m m were placed on the anode. The gap between the electrodes was 20 mm. For these conditions the current density was 1 A cm -2 at voltage *Author to whom correspondence should be addressed.
0925 9635/94/$7.00 SSD1 0925-9635(94}00210-1
660 V. The quality of the film deposited was determined by optical microscopy, scanning electron microscopy, Raman spectroscopy and X-ray diffractometry. D.c. glow discharge in C H 4 -4-H 2 mixtures is characterized by bright light near the anode region (up to 15 mm). Optical spectra were observed within the region 4 0 0 - 7 0 0 n m using a multichannel optical spectroanalyzer (OSA). Optical signals were registered for different spatial points of discharge.
3. Results The optical emission was found to have a different character for the different discharge regions (anode and cathode regions and plasma volume). The optical spectra observed were also essentially different from the flame and arcjet emission spectra [4, 5]. A typical emission spectrum is shown in Fig. 1 (obtained from the cathode region for 2% methane concentration). Along broad H z molecular lines, the Balmer lines H , (656nm), Hp (486 nm), Hy (434 nm) and lines for the radicals Cz (516 nm), C H (431 nm) were recorded. The lines for atomic hydrogen H~ and Hp appeared to have the strongest intensity. This is unlike the arcjet [.4] and flame [5] optical spectra, in which hydrogen atomic and molecular lines are very weak, but there are strong emission lines from C z bands between 436.5 and 564.5 nm (the Swan bands) and C H bands (387-435 nm). Our spectra were quite similar to microwave discharge
~) 1994 - - Elsevier Science S.A. All rights reserved
1386
V. M. Polushkin et al. / OES study of plasma in d.c. discharge in diamond film deposition
1.0 0.9
1
H~
2
0.8 C2
~.~ 0.7
H2
0.6
0.5!
I
r
c 400
500
600 nm
Fig. 1. Overview of typical emission spectrum from d.c. glow discharge plasma (cathode region, p = 160 Torr, [CH4] =2%).
spectra [1,2]. This can be explained by the fact that electron distribution functions, which determine electron excitation rate constants, are very similar for d.c. glow and microwave discharges. Electron temperature was estimated from the relative intensity of the Balmer lines I-8]. This method is generally used for calculation of electron temperature in plasma under the local thermodynamic equilibrium. This case is realized if the population of upper excited levels is determined by collisions with electrons. For the existence of local thermodynamic equilibrium it is necessary that the electron density N~ correspond to the condition [8]:
Nc >>9 x 1016 (AE/El) 3 (kTe/El) 1/2 cm -3
(1)
where AE is the transition energy between levels, E~ is the energy of ionization and kT~ is the electron temperature. For the H a hydrogen atom line and electron temperature k T ~ l e V , the inequality (1) is Ne >~ l014 cm -3. For our experiment the electron density could be estimated from the equality N~ =j/e v~, where j is the electric current density, e the electron charge and ve the drift electron velocity, which was taken from ref. 9 with its dependence on electric field. The experimental estimation of the electron density gives N o ~ 6 x 1013 c m -3. Hence, in our case, local thermodynamic equilibrium does not really exist, and for this reason the electron temperature value given is only an estimation. However, it makes sense as an illustration of general regularity in temperature distribution in volume, and its general behavior in dependence on gas structure. Figure 2 shows the electron temperature against distance from the cathode for different methane concentrations. It can be seen
A
Fig. 2. Intergap electron temperature distribution for different methane concentrations: 0% (1), 1% (2), 2% (3), 3% (4). Typical error bars for all curves are shown on curve 1.
that the electron temperature decreases from 0.95 eV to 0.75 eV near the cathode and from 0.6 to 0.5 eV near the anode, with an increase in CH4 from 0% to 3%. This sensitivity of electron temperature to CH4 concentration can be explained by higher cross-sections of electron nonelastic scattering on methane molecules than on hydrogen ones. The electron temperature spatial distribution depends on both a high discharge energy near the cathode and the gas flow. In order to determine the gas temperature we used two means of treating spectra data. The first method was based on a determination of the vibrational temperature of the radical C2 using optical emission spectra from C2 d3Hg-a3Hu in the region of wavelength 2 = 515 nm. The vibrational temperature was determined through the i nown ratio of two relative intensities of two transitions (v', v") and (w', w") from the equality Iv,~,, _ A~,~, e x p (
Iw'w" Aw'w"
kTvibr
/
where Av,v° and Aw,w,, are the Einstein spontaneous emission coefficients, and G(v) is the energy of the anharmonic oscillator, given by
G(v) = coe (v+ 1/2) - c%x~ (v+ 1/2) 2 + coeY~(v+ 1/2) 3 + ... Our
C2
radical spectra maintained lines of transition
d3Hg-a3Hu with A v = 0 : 0 - 0 and 1-1, where v is the vibrational quantum number. Respective means G(v) were calculated according to the method described in ref. 10, and Einstein coefficients were taken from ref. 11. The second method was based on determining the rotational temperature of molecular H2 through the
V. M. Polushkin et al. / OES study of plasma in d.c. discharge in diamond film deposition
R1)~'+ spectra of the R branch of the v(~TI~',+ _g --~ --u (0--0) b a n d of the H 2 molecule in the region of wavelength £ = 463 nm. It was found that the value ln(I/S) is practically a linear function of the upper rotational energy E. The rotational temperature was determined using the incline angle of this line [12]. Here I is the relative intensity of the emission divided by the nuclear spin degeneracy, and S is equal to f+j"+l where f and j" are the rotational quantum numbers of the upper and lower levels respectively. There are several points which have to be considered in order to prove a determination of the gas kinetic temperature using the relative emission intensities of rotational lines within an electron band. The first point is that there must be enough molecular collisions to ensure that the rotational population reflects the gas kinetic temperature. The radiative decay rate of the upper level of the _(~T1)~'~+ _ g - _R1/~'~+ - - u (0--0) H 2 band is 3 x 10 v s -1 [10]. The collision rate of gas molecules is estimated to be approx. 109 s -1 [13] at a total pressure of 150 Torr and gas temperature 2000 K. Therefore it is believed that the collision rate is large enough for the rotational temperature in the excited electronic state to be considered equal to the gas kinetic temperature. The second point is that self-absorption could be a source of systematic error in this type of measurement. However, the B 1Su+ level of the H2 band has a short radiative lifetime, 8 × 10-1°s [10], which results in a small population of excited molecules to produce marked self-absorption of the H2 band. Values for the vibrational temperature T~ib and rotational temperature T~ot were found to be almost equal, Tvib~ T~ot~ 2000 K. The experimental accuracy of temperature measurement was about 150 K in both cases. This was a result of a line resolution of the electronicvibrational transition of radical C2 1 1 in the first case, and there was some deviation of registered points from the plot lnlI/Sl=f(E) in the second one. The latter deviation can be explained by serious perturbations of upper rotational levels of H 2 molecules by nearby levels with different lifetimes. This is a complicated problem known for this method. However, our results are in reasonable agreement with the data of ref. 13, which were obtained under similar conditions. The spatial distribution of lines H~ and H2 (463 nm) for pure hydrogen and 3% methane is shown in Fig. 3. The emission intensity of both lines is correspondingly six and two times greater near the anode than in volume, unlike the data reported in ref. 1, where the reverse dependence was found. Figure 4 shows the emission intensities ratio In/In2 as a function of the methane concentration. The ratio increases with increasing methane percentage, and in particular is three times greater at 3% C H 4 compared with pure H 2. This can be explained if we assume that
1387
12
5O
I0
4o
8~
V
4 2
0/I C
I
I
A C
A
0
Fig. 3. Spatial distributions of hydrogen atomic 1486 nm) and molecular (463 nm) lines: [CH4] = 0% ( 1 h [CH4] = 3 % (2).
?
10
8
L
C
A
Fig. 4. Spatial distributions of ratio of hydrogen atomic H~ to molecular (463 nm) lines intensity for different methane concentrations: 0% tl), 0.5% (2), 1% (3), 3% (4). Typical error bars for all curves are shown on curve 4.
the atomic hydrogen concentration increases with increasing methane concentration, by new H produced channels in methane containing plasma: CH4+e--*CH3 + H CH 3 + H2~CH 4 + H The decrease of IH/IH2 near the anode can be explained by atomic hydrogen recombination on the anode surface. It is been reported previously [2] that there is a correlation between the quality of diamond films (growth by microwave PECVD) and some features of the plasma emission spectra. The best diamond films have been obtained in cases where the C H (431 nm) band appeared clearly while the C2 (516 nm) emission band was almost missing [2]. We tried to verify these results and to apply this method for d.c. glow discharge diamond CVD. Figure 5 shows intergap spatial distributions of intensity of C2 (516 nm) and C H (431 nm) lines for different methane concentrations. For both lines the intensity
1388
V. M. Polushkin et al. / OES study of plasma in d.c. discharge in diamond film deposition
of a gas t e m p e r a t u r e . N o c o r r e l a t i o n was f o u n d between C H a n d C2 lines ratio a n d film quality.
10 58
m
46
Acknowledgments
_o
~4
0
o~o I C
I A
Fig. 5. Spatial distribution of ratio of radicals C2 (516 nm) to CH (430 nm) lines intensity for different methane concentrations: 0, 0.5%; C), 1%; x, 2%. increases with increasing m e t h a n e c o n c e n t r a t i o n , which is in a g r e e m e n t with the previous result [ 1 ]. H o w e v e r , we c o u l d n o t find a n y c o r r e l a t i o n between this line ratio a n d film quality. As can be seen from Fig. 5, the intensity r a t i o I c j l c n is i n d e p e n d e n t of m e t h a n e c o n c e n t r a t i o n while there is an increase in d e p o s i t i o n rate a n d a decrease in film quality. The last two p o i n t s also d e p e n d on s u b s t r a t e t e m p e r a t u r e even with no essential change of the emission spectra.
4. Conclusion A systematic s t u d y was p e r f o r m e d of O E S from d.c. glow discharge d u r i n g D F deposition. The emission intensity was studied as a function of m e t h a n e concent r a t i o n a n d interelectrode position. T h e electron t e m p e r ature in the C H 4 -t-H2 p l a s m a was e s t i m a t e d from the relative intensity of the B a l m e r lines. A v i b r a t i o n - r o t a tional structure in the emission spectra of radicals C H ( A - X ) near 2 = 4 3 0 n m a n d C 2 (d3Hg-a317u) n e a r 2 = 5 1 5 n m was o b s e r v e d a n d used for d e t e r m i n a t i o n
T h e a u t h o r s gratefully a c k n o w l e d g e A. N. O b r a z t s o v for p e r f o r m i n g the R a m a n s p e c t r o s c o p y analyses a n d S. A. D o l e n k o for useful discussions.
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