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Diagnostic of a pulsed CH4–H2 plasma to improve microwave plasma assisted chemical vapour deposition process for diamond synthesis
Surface and Coatings Technology 146 – 147 (2001) 586–592
Diagnostic of a pulsed CH4–H2 plasma to improve microwave plasma assisted chemical vapour deposition process for diamond synthesis L. De Poucques, G. Henrion*, J. Bougdira, R. Hugon ´ et Applications, CNRS-UMR 7040, Faculte´ des Sciences, Universite´ Henri Poincare, ´ Nancy 1, FLaboratoire de Physique des Milieux Ionises 54506 Vandoeuvre les Nancy Cedex, France
Abstract A microwave plasma assisted chemical vapour deposition (MWPACVD) process used for diamond growth was studied under continuous wave (CW) and pulsed mode. Depending on the plasma conditions, it is shown that the discharge exhibits two different regimes. One is characteristic of a resonant cavity and allows a stationary wave to be created in the centre part of the reactor. Thus the discharge can be placed in the middle of the tubular reactor, just above the treated substrate. Within the second regime, the microwave power is absorbed as soon as it enters the reactor, thus implying a displacement of the plasma ball on the side wall of the reactor. These two regimes are explained by considering the wave propagation according to the plasma parameters, especially the electron density and the collisions through the gas pressure. It is pointed out that the pulsed mode allows the input peak power to be higher than in CW operation and consequently to increase the H-atom density in the close vicinity of the substrate. Experiments with specific conditions have shed light on contradictory results dealing with pulsed MWPACVD processes. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapour deposition; Plasma processing and characterisation; Diamond
1. Introduction Owing to its physico-chemical properties, diamond is an excellent material for applications in optics and microelectronics. To achieve high purity and high quality diamond thin film deposition, one of the best methods consists in using microwave plasma assisted chemical vapour deposition (MWPACVD). Moreover, the use of a pulsed power supply is known to enhance the diamond deposition process w1–3x. Recent work w4,5x (hereafter referred to as I) have shown that using a pulsed microwave power supply greatly improves the growth rate and the quality — in terms of diamond to graphite ratio — of the diamond films deposited by MWPACVD. The correlation between the plasma measurements and the film analysis showed that the best quality diamond films were obtained when pulsing microwave power with a repeti* Corresponding author. Tel.: q33-83912734; fax: q33-83273498. E-mail address: [email protected] (G. Henrion).
tion rate and duty cycle fixed at 500 Hz and 50%, respectively. It was also established that these optimum conditions corresponded to the higher H-atom density in the discharge phase while the concentration of C2 in the early afterglow was minimised. On the other hand, Laimer et al. w6–8x (hereafter referred to as II) have also studied a similar reactor working with pulsed microwave power at a repetition rate of 200 Hz, and 1 and 5 kHz. From their studies, they concluded that the pulsed regime does not improve the diamond deposition process as compared with the continuous regime. As a consequence, they considered that only the average input power must be taken into account for the production of reactive species. Thus, their conclusion is inconsistent with our results. This contradiction seems to be connected to the experimental conditions that are different in both cases. Indeed, the experiments reported in II were performed using different conditions than those used in I. The power pulse repetition rate investigated in I ranged from 250 Hz to 1 kHz; the working gas pressure used in I was twice
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 4 3 6 - 0
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that used in II and the input peak power was lower in I (600 W) than in II (800 W). Moreover, it is shown in II that pulsing the discharge does not allow the H-atom density to be increased with respect to the continuous regime while the corresponding input peak power is two times higher (800 W in pulsed mode compared with 400 W in CW mode). In our studies, it was clearly shown that the pulsed mode allows the H-atom density in the discharge to be increased by a factor of 2 with respect to the continuous mode. Consequently, it clearly appears from these contradictory results that the experimental working conditions, especially the time parameters of the power pulse, the input peak power and the gas pressure, greatly influence the behaviour of the discharge. This paper aims to provide an insight into the changes occurring in the plasma according to the working conditions used and to explain the influence of the power pulse time-parameters (repetition rate and duty cycle) on the H-atom density. 2. Experimental arrangement The entire experimental arrangement has been described previously w4,5x. Briefly speaking, the plasma reactor consists of a tubular quartz tube (50 mm in diameter, 350 mm in length) that intersects a rectangular 2.45-GHz wave guide. Three tuning stubs allow the reflected power to be minimised and a short-circuit stub ensures the plasma to be centred just above the silicon substrate. The gas mixture composition is ensured by mass flowmeters that are computer controlled in order to maintain both the H2 yCH4 ratio and the total pressure at constant values. When operating in pulsed mode, the 0–1200-W microwave power supply is driven by a waveform generator, thus allowing the adjustment of the output peak power, the pulse duration and the pulse repetition rate. The plasma characterisation is carried out by means of time-resolved optical emission spectroscopy (TROES) and laser induced fluorescence (LIF). The collected light is measured by a photomultiplier through a 500-mm focal length monochromator. Photographs of the plasma ball were also taken through the top window of the quartz tube, along the tube axis. 3. Results and discussion For the purpose of the present work, we were particularly interested in the variation of H-atom ground state population. The time-resolved measurements of H-atoms have been carried out by means of two photons LIF through the following scheme w9x: HŽns1.q2hcyl1™HŽns3.™HŽns2.qhcylf where lls205 nm and lfs656.3 nm are the laser and
Fig. 1. Time variation over a pulse period of the relative concentration of H-atom ground state for various power pulse repetition rates. The straight lines correspond to average values. Ps8000 Pa (60 torr); gas mixturesCH4 (0.5)qAr (3)qH2 (96.5). Duty cycles50%; microwave power as reported in Table 1.
fluorescence photon wavelengths, respectively. Though the working pressure is rather high, the quenching of the excited state has been neglected. In fact, all the measurements of H-atoms have been normalised to those obtained in CW operation. As a consequence, the quenching process has no effect on the relative density variation. On the other hand, the space-resolved evolution of the discharge has been determined through the variation of the Ha line as measured by means of emission spectroscopy. Such a measurement of the Hatom excited state is sufficient to study the displacement of the discharge over a pulse period. 3.1. Influence of the pulse repetition rate In order to study the influence of the pulse repetition rate on the H-atom density, we chose to maintain the duty cycle at a constant value of 50%. In the same way, the input peak power was adjusted according to the frequency so as to maintain the time-averaged power at a rather constant value of 300 W, thus ensuring the sample temperature to be kept at a value of 8508C, whatever the power pulse parameters. The influence of the pulse frequency on the H-atom ground state density during both the discharge and the afterglow is reported in Fig. 1. All the measurements have been normalised to the corresponding measurement obtained under CW operation. Analysis of Fig. 1 clearly shows that the pulsed mode leads to an increase in H-
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atom density as compared with the CW regime. In fact, the time-averaged H-atom concentration is improved by a factor of 1.2–1.3 and the relative density value at the end of the discharge phase is close to twice that reached in CW operation. Photographs of the corresponding plasmas are presented in Fig. 2. It can be seen that the volume of plasma remains nearly constant whatever the power pulse repetition rate is, and it is slightly larger in pulsed mode than in CW mode. This is significant of the average power density which is roughly equivalent for each case. On the other hand, the instantaneous peak power density in pulsed mode is twice that of CW mode, this results in the relative value of the H-atom concentration in the discharge is doubled in the pulsed regime compared with the CW one. 3.2. Influence of the duty cycle The power pulse repetition rate was fixed at a value of 500 Hz. As for the above mentioned study, the peak power was adjusted according to the experimental conditions in order to keep the sample temperature at a constant temperature of 8508C. The influence of the duty cycle on the H-atom relative density is reported in Fig. 3. As for Fig. 1, the density values have been normalised to the corresponding ones obtained under CW conditions. It can be noted from Fig. 3 that the relative H-atom density exhibits a maximum (both for instantaneous and time-averaged values) when the plasma is operated with a duty cycle fixed at 50%. Nevertheless, the corresponding top views of the plasma (Fig. 4) clearly show that the plasma volume depends on the duty cycle. Indeed, the size of the plasma ball is enlarged for the duty cycle fixed at a value of 25% while it remains nearly constant for duty cycles greater than or equal to 50%. In fact, when decreasing the duty cycle, the input power density is increased to keep the temperature at a constant value. As a consequence, the dissociation of hydrogen molecules increases, thus implying an increase in H-atom density. On the other hand, by using a short plasma on-time (duty cycles25%), the high peak power (1200 W) implies the plasma to diffuse toward the wall faster than with higher duty cycles. It follows that the plasma volume increases and thus the microwave power is partly absorbed as soon as it enters the discharge tube. Consequently, the H-atom density measured in the centre part of the quartz tube decreases. This expansion of the plasma ball is more pronounced when working with different conditions (lower pressure, longer discharge and afterglow times, higher input peak power) as shown in Fig. 5. Though the discharge seems to be nearly symmetrical with respect to the tube axis (Fig. 5a), the spectroscopic measurement of H atoms (Fig. 5b) clearly shows that the plasma has mainly moved toward the input power side of the discharge
tube. In the case of lower pressure, the shift of plasma toward the wall is so pronounced that it is impossible to locate the plasma elsewhere, whatever the position of the short-circuit stub. 3.3. Discussion As pointed out by the above results, two different discharge regimes can be encountered which depend on the power–pressure parameters. The first regime (hereafter referred to as reg1) corresponds to the working conditions that allow the plasma to be centred in the discharge tube by adjusting the short-circuit stub. In that case, the discharge tube works as a resonant cavity. The second regime (hereafter referred to as reg2) corresponds to the condition for which the plasma ball is shifted toward the tube wall, on the input power side. In that case, it is not possible to obtain a stationary wave in the centre part of the discharge tube. In order to explain these two different discharge regimes, let us look at the amplitude of the wave transmitted through the discharge tube, which can be expressed from the complex refraction index of the plasma w4,10x: NUsaqjb
(1)
with S
Tas y2µŽ Tbs v 1 1
v2p 1y 2 2 q v qvc
. yŽ
v2p 1y 2 2 v qvc
U
2
v2c v2p q 2 2 v v qv2c
2
. Ž .∂
1y2
vp2 v 2a Žv qv2c . c
V
2
(2) and the electric field can be written: a.x b.x ™ . EsE0eyjvŽty c .UeyvŽ c ™ x
(3)
In Eq. (2), vp and nc are the plasma frequency and the collision frequency, respectively. The first exponential term in Eq. (3) represents the propagation term while the second exponential is the amplitude of the transmitted wave. Fig. 6 represents the computed amplitude of the wave transmitted across the plasma diameter as a function of the gas pressure and the electron density, with ncs3.61=107*P sy1 w11x. Fig. 6 shows the working zone of our reactor which was observed experimentally by comparing the plasma luminosity; the plasma is more luminous when working at high power–low pressure than by using high pressure– low power. From Fig. 6 it is possible to distinguish and to explain the two different regimes, reg1 and reg2. 3.3.1. Low power–high pressure In this case, the electron density is low and the amplitude of the transmitted wave is close to 1. The wave propagates toward the short-circuit stub on which
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Fig. 2. Top window photographs of the plasma. Same conditions as in Fig. 1 (a) f s250 Hz; (b) f s500 Hz; (c) f s1000 Hz; (d) CW operation. Fig. 4. Top window photographs of the plasma. Same conditions as in Fig. 3. (a) 25%; (b) 50%; (c) 75%; (d) CW operation. Fig. 5. Pulsed plasma at different pressure and power plasma. Ps4000 Pa; powers855 W; gas mixturesH2 (97)–Ar (3). (a) Time-average photograph of the plasma. (b) Time and space variation of Ha line emission intensity.
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the short-circuit stub has henceforth no influence on the localisation of the discharge (reg2). In other words, with the present working conditions, the discharge changes from reg1 to reg2 in 1 ms. 3.4. Why it is better to work with pulsed mode than with CW
Fig. 3. Time variation over a pulse period of the relative concentration of H-atom ground state for various power pulse duty cycles. The straight lines correspond to average values. Ps8000 Pa (60 torr); gas mixturesCH4 (0.5)qAr (3)qH2 (96.5). Pulse repetition rates500 Hz; time-averaged microwave powers300 W.
it is reflected. The reflected and incident waves combine to form a stationary wave, the maximum of which being located at the desired position by adjusting the position of the short-circuit stub. Reg1 is reached.
One of the main advantages in using pulsed power supply is the possibility to increase the H-atom density in the vicinity of the sample surface. In fact, when working in CW mode, the H-atom density reaches a saturation value for an input power greater than 450 W. On the other hand, the pulsed operation allows a higher input peak power and the H-atom density to be increased up to 30% with respect to the CW regime (Fig. 7). The saturation in H-atom density measured in CW mode is significant of the plasma delocalisation. Indeed, when the discharge runs under reg2 (high power) the dissociation of molecular hydrogen mainly occurs on the power input side of the quartz tube (Fig. 8a) and the H-atom density in the centre part cannot be increased anymore; this is of course not optimum for the diamond deposition process. As can be observed in Fig. 8b, the plasma delocalisation also occurs in pulsed mode, owing to the transition from reg1 to reg2 (Fig. 5). Nevertheless, the plasma decay during the afterglow (especially the cooling of the neutral gas and the decrease in electron density) allows the peak power to be higher (300 W more than in CW mode) that is to increase the power
3.3.2. High power–low pressure In this case, the electron density is high and the amplitude of the transmitted wave is close to 0. The wave is nearly completely absorbed by the plasma. It follows that the wave does not reach the short-circuit stub that cannot influence the discharge, whatever its position. Consequently, the discharge is mainly located on the input power side of the discharge tube as soon as the electron density has reached a high enough value. Reg2 is attained. 3.3.3. Transition from reg1 to reg2 (refer to Fig. 5) During the first millisecond of the discharge phase, the electron density is still rather low so that the discharge runs according to reg1. In Fig. 5b, the maximum amplitude of the stationary wave has been located on the short-circuit side wall of the discharge tube. Over this first millisecond, the electron density increases and becomes high enough so that the electrons diffuse in the radial direction toward the power input size; thus the plasma now absorbs the main part of the incident power. The wave can no longer reach the short-circuit stub and the stationary wave disappears. The discharge is shifted to the right hand side (refer to Fig. 5a) and
Fig. 6. Variation of the transmitted amplitude vs. electron density and gas pressure. Electron density is noted on each corresponding curve in units 1015 my3.
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Fig. 8. Time average photographs of the plasma for various working conditions. CW operation: (a) 300 W; (b) 600 W; (c) 900 W. Pulsed operation 500 Hz; 50%; peak power: (d) 600 W; (e) 900 W; (f) 1200 W.
Fig. 7. Variation of H-atom density vs. input power. The measurement in pulsed mode are done at the end of the discharge regime (ts1 ms).
density since the volume of plasma is equivalent in CW and in pulsed regime (comparison of Fig. 8a,b). Consequently, the H-atom density can be increased by a factor of 1.3 as compared with CW mode.
thus leading to an increase in H-atom density up to 1.3 times as compared with CW mode, provided that the gas pressure be high enough to make the reactor work as a resonant cavity. Moreover, it has been clearly pointed out that, depending on the working conditions, the plasma exhibits two different regimes, one of which implies a strong delocalisation of the plasma ball toward the input power side of the discharge tube. This phenomenon has been explained by considering the propagation of the microwave along the plasma diameter as a function of the plasma density. The presented results coupled with specific experiments have allowed us to
3.5. Specific conditions used by Laimer et al. wIIx Fig. 9 represents photographs of the plasma working with the conditions described in II that are: average microwave power: 400 W; duty cycle: 50%; gas pressure: 4000 Pa. From these photographs and from the above discussion, it clearly appears that the conditions used in II make the plasma work in reg2 whatever the conditions. Thus the H-atom density cannot be increased in the centre part of the discharge tube and is rather constant for all the power pulse repetition rates considered in II. This explains the conclusion drawn by these authors for their particular discharge parameters which is in contradiction with our previous results. 4. Conclusion From observations and time- and space-resolved measurement of the H-atom density in a hydrogen plasma, we have shown that using a pulsed power supply can improve the diamond deposition process through an increase in H-atom concentration in the vicinity of the substrate. In fact, the pulsed mode allows the input peak power to be higher than in CW operation,
Fig. 9. Photographs of plasmas working with the conditions reported in II: gas pressures4000 Pa; time-averaged powers400 W. (a) CW operation; (b) 200 Hz — 50%; (c) 500 Hz — 50%; (d) 2000 Hz — 50%.
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shed light on the conclusion drawn by other authors which were in contradiction with our previous results. References w1x M. Noda, H. Kusakabe, K. Taniguchi, S. Maruno, Jpn. J. Appl. Phys. 33 (1994) 4400. w2x A. Hatta, K. Kadota, Y. Mori, T. Ito et al., Appl. Phys. Lett. 66 (1995) 1602. w3x A. Hatta, H. Suzuki, K. Kadota, H. Makita, T. Ito, A. Hiraki, Plasma Sources Sci. Technol. 5 (1996) 235. w4x L. De Poucques, Caracterisation ´ ´ d’une decharge micro-onde ´ dans le melange ´ pulsee CH4–H2 en vue de son optimisation ` de films de diamants. PhD thesis, University pour la synthese ´ Nancy 1, France, 2000. Henri Poincare,
w5x L. De Poucques, J. Bougdira, R. Hugon, G. Henrion, P. Alnot, J. Phys. D: Appl. Phys. 34 (2001) 896. w6x J. Laimer, M. Shimokawa, S. Matsumoto, Diamond Relat. Mater. 3 (1994) 231. w7x J. Laimer, S. Matsumoto, Plasma Chem. Plasma Process. 14 (1994) 117. w8x J. Laimer, S. Matsumoto, Int. J. Refract. Met. Hard Mater. 14 (1996) 179. w9x J. Amorim, G. Baravian, M. Touzeau, J. Jolly, J. Appl. Phys. 76 (1994) 487. w10x J.R. Roth, Industrial Plasma Engineering, Vol. 1: Principles, chap 13, IOP publishing, Bristol and Philadelphia, 1995. w11x Y.P. Raizer, Gas Discharge Physics chap. 2, Springer Verlag, Berlin-Heidelberg-New York, 1991.
Diagnostic of a pulsed CH4–H2 plasma to improve microwave plasma assisted chemical vapour deposition process for diamond synthesis L. De Poucques, G. Henrion*, J. Bougdira, R. Hugon ´ et Applications, CNRS-UMR 7040, Faculte´ des Sciences, Universite´ Henri Poincare, ´ Nancy 1, FLaboratoire de Physique des Milieux Ionises 54506 Vandoeuvre les Nancy Cedex, France
Abstract A microwave plasma assisted chemical vapour deposition (MWPACVD) process used for diamond growth was studied under continuous wave (CW) and pulsed mode. Depending on the plasma conditions, it is shown that the discharge exhibits two different regimes. One is characteristic of a resonant cavity and allows a stationary wave to be created in the centre part of the reactor. Thus the discharge can be placed in the middle of the tubular reactor, just above the treated substrate. Within the second regime, the microwave power is absorbed as soon as it enters the reactor, thus implying a displacement of the plasma ball on the side wall of the reactor. These two regimes are explained by considering the wave propagation according to the plasma parameters, especially the electron density and the collisions through the gas pressure. It is pointed out that the pulsed mode allows the input peak power to be higher than in CW operation and consequently to increase the H-atom density in the close vicinity of the substrate. Experiments with specific conditions have shed light on contradictory results dealing with pulsed MWPACVD processes. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapour deposition; Plasma processing and characterisation; Diamond
1. Introduction Owing to its physico-chemical properties, diamond is an excellent material for applications in optics and microelectronics. To achieve high purity and high quality diamond thin film deposition, one of the best methods consists in using microwave plasma assisted chemical vapour deposition (MWPACVD). Moreover, the use of a pulsed power supply is known to enhance the diamond deposition process w1–3x. Recent work w4,5x (hereafter referred to as I) have shown that using a pulsed microwave power supply greatly improves the growth rate and the quality — in terms of diamond to graphite ratio — of the diamond films deposited by MWPACVD. The correlation between the plasma measurements and the film analysis showed that the best quality diamond films were obtained when pulsing microwave power with a repeti* Corresponding author. Tel.: q33-83912734; fax: q33-83273498. E-mail address: [email protected] (G. Henrion).
tion rate and duty cycle fixed at 500 Hz and 50%, respectively. It was also established that these optimum conditions corresponded to the higher H-atom density in the discharge phase while the concentration of C2 in the early afterglow was minimised. On the other hand, Laimer et al. w6–8x (hereafter referred to as II) have also studied a similar reactor working with pulsed microwave power at a repetition rate of 200 Hz, and 1 and 5 kHz. From their studies, they concluded that the pulsed regime does not improve the diamond deposition process as compared with the continuous regime. As a consequence, they considered that only the average input power must be taken into account for the production of reactive species. Thus, their conclusion is inconsistent with our results. This contradiction seems to be connected to the experimental conditions that are different in both cases. Indeed, the experiments reported in II were performed using different conditions than those used in I. The power pulse repetition rate investigated in I ranged from 250 Hz to 1 kHz; the working gas pressure used in I was twice
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 4 3 6 - 0
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that used in II and the input peak power was lower in I (600 W) than in II (800 W). Moreover, it is shown in II that pulsing the discharge does not allow the H-atom density to be increased with respect to the continuous regime while the corresponding input peak power is two times higher (800 W in pulsed mode compared with 400 W in CW mode). In our studies, it was clearly shown that the pulsed mode allows the H-atom density in the discharge to be increased by a factor of 2 with respect to the continuous mode. Consequently, it clearly appears from these contradictory results that the experimental working conditions, especially the time parameters of the power pulse, the input peak power and the gas pressure, greatly influence the behaviour of the discharge. This paper aims to provide an insight into the changes occurring in the plasma according to the working conditions used and to explain the influence of the power pulse time-parameters (repetition rate and duty cycle) on the H-atom density. 2. Experimental arrangement The entire experimental arrangement has been described previously w4,5x. Briefly speaking, the plasma reactor consists of a tubular quartz tube (50 mm in diameter, 350 mm in length) that intersects a rectangular 2.45-GHz wave guide. Three tuning stubs allow the reflected power to be minimised and a short-circuit stub ensures the plasma to be centred just above the silicon substrate. The gas mixture composition is ensured by mass flowmeters that are computer controlled in order to maintain both the H2 yCH4 ratio and the total pressure at constant values. When operating in pulsed mode, the 0–1200-W microwave power supply is driven by a waveform generator, thus allowing the adjustment of the output peak power, the pulse duration and the pulse repetition rate. The plasma characterisation is carried out by means of time-resolved optical emission spectroscopy (TROES) and laser induced fluorescence (LIF). The collected light is measured by a photomultiplier through a 500-mm focal length monochromator. Photographs of the plasma ball were also taken through the top window of the quartz tube, along the tube axis. 3. Results and discussion For the purpose of the present work, we were particularly interested in the variation of H-atom ground state population. The time-resolved measurements of H-atoms have been carried out by means of two photons LIF through the following scheme w9x: HŽns1.q2hcyl1™HŽns3.™HŽns2.qhcylf where lls205 nm and lfs656.3 nm are the laser and
Fig. 1. Time variation over a pulse period of the relative concentration of H-atom ground state for various power pulse repetition rates. The straight lines correspond to average values. Ps8000 Pa (60 torr); gas mixturesCH4 (0.5)qAr (3)qH2 (96.5). Duty cycles50%; microwave power as reported in Table 1.
fluorescence photon wavelengths, respectively. Though the working pressure is rather high, the quenching of the excited state has been neglected. In fact, all the measurements of H-atoms have been normalised to those obtained in CW operation. As a consequence, the quenching process has no effect on the relative density variation. On the other hand, the space-resolved evolution of the discharge has been determined through the variation of the Ha line as measured by means of emission spectroscopy. Such a measurement of the Hatom excited state is sufficient to study the displacement of the discharge over a pulse period. 3.1. Influence of the pulse repetition rate In order to study the influence of the pulse repetition rate on the H-atom density, we chose to maintain the duty cycle at a constant value of 50%. In the same way, the input peak power was adjusted according to the frequency so as to maintain the time-averaged power at a rather constant value of 300 W, thus ensuring the sample temperature to be kept at a value of 8508C, whatever the power pulse parameters. The influence of the pulse frequency on the H-atom ground state density during both the discharge and the afterglow is reported in Fig. 1. All the measurements have been normalised to the corresponding measurement obtained under CW operation. Analysis of Fig. 1 clearly shows that the pulsed mode leads to an increase in H-
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atom density as compared with the CW regime. In fact, the time-averaged H-atom concentration is improved by a factor of 1.2–1.3 and the relative density value at the end of the discharge phase is close to twice that reached in CW operation. Photographs of the corresponding plasmas are presented in Fig. 2. It can be seen that the volume of plasma remains nearly constant whatever the power pulse repetition rate is, and it is slightly larger in pulsed mode than in CW mode. This is significant of the average power density which is roughly equivalent for each case. On the other hand, the instantaneous peak power density in pulsed mode is twice that of CW mode, this results in the relative value of the H-atom concentration in the discharge is doubled in the pulsed regime compared with the CW one. 3.2. Influence of the duty cycle The power pulse repetition rate was fixed at a value of 500 Hz. As for the above mentioned study, the peak power was adjusted according to the experimental conditions in order to keep the sample temperature at a constant temperature of 8508C. The influence of the duty cycle on the H-atom relative density is reported in Fig. 3. As for Fig. 1, the density values have been normalised to the corresponding ones obtained under CW conditions. It can be noted from Fig. 3 that the relative H-atom density exhibits a maximum (both for instantaneous and time-averaged values) when the plasma is operated with a duty cycle fixed at 50%. Nevertheless, the corresponding top views of the plasma (Fig. 4) clearly show that the plasma volume depends on the duty cycle. Indeed, the size of the plasma ball is enlarged for the duty cycle fixed at a value of 25% while it remains nearly constant for duty cycles greater than or equal to 50%. In fact, when decreasing the duty cycle, the input power density is increased to keep the temperature at a constant value. As a consequence, the dissociation of hydrogen molecules increases, thus implying an increase in H-atom density. On the other hand, by using a short plasma on-time (duty cycles25%), the high peak power (1200 W) implies the plasma to diffuse toward the wall faster than with higher duty cycles. It follows that the plasma volume increases and thus the microwave power is partly absorbed as soon as it enters the discharge tube. Consequently, the H-atom density measured in the centre part of the quartz tube decreases. This expansion of the plasma ball is more pronounced when working with different conditions (lower pressure, longer discharge and afterglow times, higher input peak power) as shown in Fig. 5. Though the discharge seems to be nearly symmetrical with respect to the tube axis (Fig. 5a), the spectroscopic measurement of H atoms (Fig. 5b) clearly shows that the plasma has mainly moved toward the input power side of the discharge
tube. In the case of lower pressure, the shift of plasma toward the wall is so pronounced that it is impossible to locate the plasma elsewhere, whatever the position of the short-circuit stub. 3.3. Discussion As pointed out by the above results, two different discharge regimes can be encountered which depend on the power–pressure parameters. The first regime (hereafter referred to as reg1) corresponds to the working conditions that allow the plasma to be centred in the discharge tube by adjusting the short-circuit stub. In that case, the discharge tube works as a resonant cavity. The second regime (hereafter referred to as reg2) corresponds to the condition for which the plasma ball is shifted toward the tube wall, on the input power side. In that case, it is not possible to obtain a stationary wave in the centre part of the discharge tube. In order to explain these two different discharge regimes, let us look at the amplitude of the wave transmitted through the discharge tube, which can be expressed from the complex refraction index of the plasma w4,10x: NUsaqjb
(1)
with S
Tas y2µŽ Tbs v 1 1
v2p 1y 2 2 q v qvc
. yŽ
v2p 1y 2 2 v qvc
U
2
v2c v2p q 2 2 v v qv2c
2
. Ž .∂
1y2
vp2 v 2a Žv qv2c . c
V
2
(2) and the electric field can be written: a.x b.x ™ . EsE0eyjvŽty c .UeyvŽ c ™ x
(3)
In Eq. (2), vp and nc are the plasma frequency and the collision frequency, respectively. The first exponential term in Eq. (3) represents the propagation term while the second exponential is the amplitude of the transmitted wave. Fig. 6 represents the computed amplitude of the wave transmitted across the plasma diameter as a function of the gas pressure and the electron density, with ncs3.61=107*P sy1 w11x. Fig. 6 shows the working zone of our reactor which was observed experimentally by comparing the plasma luminosity; the plasma is more luminous when working at high power–low pressure than by using high pressure– low power. From Fig. 6 it is possible to distinguish and to explain the two different regimes, reg1 and reg2. 3.3.1. Low power–high pressure In this case, the electron density is low and the amplitude of the transmitted wave is close to 1. The wave propagates toward the short-circuit stub on which
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Fig. 2. Top window photographs of the plasma. Same conditions as in Fig. 1 (a) f s250 Hz; (b) f s500 Hz; (c) f s1000 Hz; (d) CW operation. Fig. 4. Top window photographs of the plasma. Same conditions as in Fig. 3. (a) 25%; (b) 50%; (c) 75%; (d) CW operation. Fig. 5. Pulsed plasma at different pressure and power plasma. Ps4000 Pa; powers855 W; gas mixturesH2 (97)–Ar (3). (a) Time-average photograph of the plasma. (b) Time and space variation of Ha line emission intensity.
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the short-circuit stub has henceforth no influence on the localisation of the discharge (reg2). In other words, with the present working conditions, the discharge changes from reg1 to reg2 in 1 ms. 3.4. Why it is better to work with pulsed mode than with CW
Fig. 3. Time variation over a pulse period of the relative concentration of H-atom ground state for various power pulse duty cycles. The straight lines correspond to average values. Ps8000 Pa (60 torr); gas mixturesCH4 (0.5)qAr (3)qH2 (96.5). Pulse repetition rates500 Hz; time-averaged microwave powers300 W.
it is reflected. The reflected and incident waves combine to form a stationary wave, the maximum of which being located at the desired position by adjusting the position of the short-circuit stub. Reg1 is reached.
One of the main advantages in using pulsed power supply is the possibility to increase the H-atom density in the vicinity of the sample surface. In fact, when working in CW mode, the H-atom density reaches a saturation value for an input power greater than 450 W. On the other hand, the pulsed operation allows a higher input peak power and the H-atom density to be increased up to 30% with respect to the CW regime (Fig. 7). The saturation in H-atom density measured in CW mode is significant of the plasma delocalisation. Indeed, when the discharge runs under reg2 (high power) the dissociation of molecular hydrogen mainly occurs on the power input side of the quartz tube (Fig. 8a) and the H-atom density in the centre part cannot be increased anymore; this is of course not optimum for the diamond deposition process. As can be observed in Fig. 8b, the plasma delocalisation also occurs in pulsed mode, owing to the transition from reg1 to reg2 (Fig. 5). Nevertheless, the plasma decay during the afterglow (especially the cooling of the neutral gas and the decrease in electron density) allows the peak power to be higher (300 W more than in CW mode) that is to increase the power
3.3.2. High power–low pressure In this case, the electron density is high and the amplitude of the transmitted wave is close to 0. The wave is nearly completely absorbed by the plasma. It follows that the wave does not reach the short-circuit stub that cannot influence the discharge, whatever its position. Consequently, the discharge is mainly located on the input power side of the discharge tube as soon as the electron density has reached a high enough value. Reg2 is attained. 3.3.3. Transition from reg1 to reg2 (refer to Fig. 5) During the first millisecond of the discharge phase, the electron density is still rather low so that the discharge runs according to reg1. In Fig. 5b, the maximum amplitude of the stationary wave has been located on the short-circuit side wall of the discharge tube. Over this first millisecond, the electron density increases and becomes high enough so that the electrons diffuse in the radial direction toward the power input size; thus the plasma now absorbs the main part of the incident power. The wave can no longer reach the short-circuit stub and the stationary wave disappears. The discharge is shifted to the right hand side (refer to Fig. 5a) and
Fig. 6. Variation of the transmitted amplitude vs. electron density and gas pressure. Electron density is noted on each corresponding curve in units 1015 my3.
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Fig. 8. Time average photographs of the plasma for various working conditions. CW operation: (a) 300 W; (b) 600 W; (c) 900 W. Pulsed operation 500 Hz; 50%; peak power: (d) 600 W; (e) 900 W; (f) 1200 W.
Fig. 7. Variation of H-atom density vs. input power. The measurement in pulsed mode are done at the end of the discharge regime (ts1 ms).
density since the volume of plasma is equivalent in CW and in pulsed regime (comparison of Fig. 8a,b). Consequently, the H-atom density can be increased by a factor of 1.3 as compared with CW mode.
thus leading to an increase in H-atom density up to 1.3 times as compared with CW mode, provided that the gas pressure be high enough to make the reactor work as a resonant cavity. Moreover, it has been clearly pointed out that, depending on the working conditions, the plasma exhibits two different regimes, one of which implies a strong delocalisation of the plasma ball toward the input power side of the discharge tube. This phenomenon has been explained by considering the propagation of the microwave along the plasma diameter as a function of the plasma density. The presented results coupled with specific experiments have allowed us to
3.5. Specific conditions used by Laimer et al. wIIx Fig. 9 represents photographs of the plasma working with the conditions described in II that are: average microwave power: 400 W; duty cycle: 50%; gas pressure: 4000 Pa. From these photographs and from the above discussion, it clearly appears that the conditions used in II make the plasma work in reg2 whatever the conditions. Thus the H-atom density cannot be increased in the centre part of the discharge tube and is rather constant for all the power pulse repetition rates considered in II. This explains the conclusion drawn by these authors for their particular discharge parameters which is in contradiction with our previous results. 4. Conclusion From observations and time- and space-resolved measurement of the H-atom density in a hydrogen plasma, we have shown that using a pulsed power supply can improve the diamond deposition process through an increase in H-atom concentration in the vicinity of the substrate. In fact, the pulsed mode allows the input peak power to be higher than in CW operation,
Fig. 9. Photographs of plasmas working with the conditions reported in II: gas pressures4000 Pa; time-averaged powers400 W. (a) CW operation; (b) 200 Hz — 50%; (c) 500 Hz — 50%; (d) 2000 Hz — 50%.
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w5x L. De Poucques, J. Bougdira, R. Hugon, G. Henrion, P. Alnot, J. Phys. D: Appl. Phys. 34 (2001) 896. w6x J. Laimer, M. Shimokawa, S. Matsumoto, Diamond Relat. Mater. 3 (1994) 231. w7x J. Laimer, S. Matsumoto, Plasma Chem. Plasma Process. 14 (1994) 117. w8x J. Laimer, S. Matsumoto, Int. J. Refract. Met. Hard Mater. 14 (1996) 179. w9x J. Amorim, G. Baravian, M. Touzeau, J. Jolly, J. Appl. Phys. 76 (1994) 487. w10x J.R. Roth, Industrial Plasma Engineering, Vol. 1: Principles, chap 13, IOP publishing, Bristol and Philadelphia, 1995. w11x Y.P. Raizer, Gas Discharge Physics chap. 2, Springer Verlag, Berlin-Heidelberg-New York, 1991.