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J. Phys. D: Appl. Phys. 37 No 22 (21 November 2004) L35-L39

doi:10.1088/0022-3727/37/22/L01

PII: S0022-3727(04)81055-6

RAPID COMMUNICATION

Improvement of nanocrystalline diamond film growth process using pulsed Ar/H2 /CH4 

microwave discharges 

P Bruno1,2

, F Bénédic1,3

, F Mohasseb1, G Lombardi

1, F Silva

1and K Hassouni

1 

1Laboratoire d'Ingénierie des Matériaux et des Hautes Pressions, UPR 1311 CNRS, Université

Paris 13, 93430 Villetaneuse, France2IMIP-CNR Sezione di Bari w/o Dipartimento di Chimica, Universita' di Bari, Via Orabona 4,

70126 Bari, Italy

3Author to whom any correspondence should be addressed.

Email: benedic@limhp.univ-paris13.fr 

Received 20 May 2004, in final form 29 September 2004

Published 28 October 2004

Abstract. For the first time nanocrystalline diamond (NCD) films were deposited by the pulsed

microwave plasma assisted chemical vapour deposition process starting from an Ar/H2 /CH4 gas

mixture. Comparisons with continuous mode deposition gave evidence for the improvement in

film quality when the microwave power was modulated with a pulse repetition rate in the range50-1000 Hz A reduction in grain size and surface roughness, especially at low pulse repetition

rate, accompanied by a decrease in soot particle formation was observed. A thermo-chemical

plasma model, developed for pulsed Ar/H2 /CH4 microwave discharges, provides evidence for the

fact that the pulsed mode permits the enhancement of the mole fraction of the C2 dimer assumed

to be the growth precursor of NCD. This may be responsible for a high secondary nucleation rate

improving the nanostructure of the film in pulsed discharges.

1. Introduction 

The effects of the pulsed mode on the deposition of polycrystalline diamond (PCD) films in

H2 /CH4 microwave discharges has been a subject that has attracted great interest in the lastdecade. The use of a pulsed wave (PW) was mainly motivated by the existence of two additional

degrees of freedom, compared to the continuous wave (CW) regime, namely the pulse repetition

rate and the duty cycle. The latter parameter represents the ratio of the pulse duration to the time

between two successive pulses, i.e. the pulse period. Thus, depending on the experimental device

and the operating conditions, it has been reported that, compared to the CW process, the pulsed

mode permits to improve the purity of diamond [1-4]. This has been attributed, in particular, to

an enhancement of the production of H atoms [5, 6], which is an sp2

phase-etching species for the

considered growth process [7]. The increase in the growth rate has also been noticed in pulsed

ECR plasmas [2]. The possibility of controlling the chemical kinetics and energy dissipation in

the plasma by varying the pulse repetition rate and/or the duty cycle in relation to PCD growth

optimization has been previously discussed [8, 9]. For instance, in a pulsed H2 /CH4 microwave

discharge ignited in a moderate pressure/high power tubular quartz reactor, it has been reported

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that a low duty cycle of less than 20% favours the production of H atoms, whereas the CH3 

radical, which is the PCD growth precursor [10], is mainly produced for duty cycles above 50%.

As a general rule, the production or consumption of active species may often be enhanced,

compared with a continuous discharge with the same average power, by an optimal choice of the

duty cycle and pulse repetition rate.

The synthesis of nanocrystalline diamond (NCD) films has been demonstrated by the microwave

plasma assisted chemical vapour deposition (MPACVD) process in various environments: Ar/C60 

[11], Ar/CH4  [12], Ar/H2 /CH4  [13], or H2 /CH4 mixture with high CH4 concentration [14].

Nevertheless, to date, all the studies reported in the literature were performed using only the CW

regime. Similarly to PCD deposition, the use of the pulsed mode should be considered as a way

of improving the NCD growth process in terms of growth rate, phase purity or film

nanostructure. Moreover, it is well known that powder formation may occur in hydrocarbon

plasmas suitable for thin film deposition. This is the case, for example, in Ar/CH4 and Ar/C2H2 

RF-discharges used for DLC deposition [15] or in Ar/H2 /CH4 microwave discharges employed

for NCD growth [16], where soot particle formation has been reported. Dust formation may be

due to homogeneous or heterogeneous nucleation mechanisms and is responsible in most of thecases for process instability and deposit contamination. The use of pulsed plasmas, as already

reported for other processes [17, 18], could be an efficient way to limit soot formation observedin continuous Ar/H2 /CH4 microwave discharges employed for NCD growth.

In this paper, we report for the first time the deposition of NCD films in pulsed Ar/H2 /CH4 

discharges. We focus on the influence of the pulse repetition rate on the characteristics of the

films assessed by ex situ analysis, supporting some of the experimental behaviours with those

predicted by a plasma thermo-chemical model developed for pulsed discharges.

2. Experimental 

The microwave (MW) reactor used for NCD deposition has been extensively described

elsewhere [19]. Briefly, the deposition set-up consists of a quartz bell jar low-pressure chamber

surrounded by a metallic cage that forms a resonant cavity at a frequency of 2.45 GHz. The

discharge, confined in the centre of the bell, is generated by a MW-power supply with a

maximum output power of 1200 W, working either in pulsed or continuous mode. Silicon wafers,

pre-treated in an ultrasonic bath with a suspension of 45 µm grain size diamond powder in

ethanol [20], were used as substrates. The surface temperature, controlled with an additional

heating system, was monitored by a bichromatic infrared pyrometer.

The MW discharges were pulsed by modulating the input power with a square wave, varying therepetition rate in the range 50-1000 Hz, which was previously investigated for the improvement

of the PCD film growth [3-5]. Since the present study focuses on the influence of the pulse

repetition rate, the duty cycle was kept at a constant value of 50%. The peak power was set to

1000 W in order to maintain a constant time-averaged power of 500 W. It enabled us to maintain

the substrate temperature approximately constant for all the conditions, as well as to allow

comparisons with NCD films elaborated in continuous mode under 500 W. Each growth cycle

lasted for 4 h and the substrate temperature was in the range 820-850°C. The MW plasma was

generated in an Ar/H2 /CH4 gas mixture in the ratio 96 : 3 : 1, a total gas flow rate of 250 sccm

and a pressure of 200 mbar. These conditions correspond to those commonly employed for the

synthesis of nanodiamond films by the MPACVD process, and the values set for this study were

previously optimized in continuous mode for the deposition system involved [21].

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3. Results and discussion 

The relative growth rate was estimated by dividing the measured deposition rate obtained for all

the NCD films by the one corresponding to the film elaborated in a CW plasma. The

measurements indicate that the relative growth rate is greater than 0.5 in pulsed mode, and in

particular it is higher than 1 at low pulse repetition rate (50 Hz). Since in the pulsed mode theMW power is injected for 2 h (50% duty cycle), as opposed to 4 h in the continuous mode, these

results suggest that the power modulation induces an appreciable increase of the growth

precursors density and/or favours a possible growth during the post-discharge phase.

Raman spectroscopy was carried out in the UV domain using a 363.8 nm laser excitation in order

to enhance the scattering signal coming from sp3

phases with respect to sp2

bondings. The spectra

for CW and PW modes are presented in figure 1. They mainly exhibit typical features of NCD

films, with lines centred at 1140 and 1332 cm- 1

, characteristic of trans-polyacetylene in NCD

[22, 23] and diamond, respectively. Other sp2

contributions attributed to graphite D and G bands

and trans-polyacetylene may be easily recognized through the bands located around 1350 and

1550 cm - 1 [24]. The figure shows a significant increase in diamond peak intensity with the pulserepetition rate, due to a transition towards diminishing nanocrystalline features of the diamond

films.

Figure 1. UV-Raman spectra of NCD films elaborated in PW and CW

modes.

This evolution is more quantitatively assessed in figure 2 where the full width at half maximum

(FWHM) of the diamond peaks is reported as a function of the pulse frequency. Indeed,

increasing the pulse repetition rate from 50 to 1000 Hz leads to a decrease in the FWHM of the

diamond peaks from 17 to 12 cm- 1

. This result, together with the evolution of the peak intensity,

indicates that the grain size increases and that the sp3

carbon phase is enhanced. The FWHM

obtained under pulsed mode is typically higher than the continuous mode value, and evolvestoward this value when the frequency is increased.

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Figure 2. Diamond Raman peak FWHM as a function of pulse repetition rate

compared with CW mode.

This behaviour is well supported by XRD analysis performed using the CuKα 1  radiation (λ  =

1.54056 Angstrom), with an incident x-ray angle of 10°. Figure 3 shows the diffraction spectra of 

the films obtained under pulsed mode at different frequency values along with the one obtained

in CW mode. The 1 1 1 , 2 2 0 , 3 1 1 and 4 0 0 reflections are clearly visible for all the

experimental conditions, showing that the films are composed of crystalline cubic diamond; there

is no evidence for the presence of crystalline graphite. It is worth noting that in the PW mode the

diffraction peaks become sharper when the frequency increases, which indicates an increase in

the crystallite size.

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Figure 3. XRD pattern of NCD films elaborated in PW and CW modes.

Figure 4  depicts the grain size behaviour estimated from the FWHM of the 111 diffraction

peak using the Scherrer law [25]. It significantly increases from 10 to 20 nm as the frequency

increases from 50 to 1000 Hz, approaching the value of CW mode at high repetition rates. This

confirms that the MW power modulation leads to a reduced crystallite size, suggesting an

enhancement of the secondary nucleation rate, especially at low repetition rates.

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Figure 4. Grain size as a function of pulse repetition rate compared with the

CW mode, estimated from the Scherrer formula applied on the 111

diffraction peak.

The surface morphology of the films was examined by SEM and the use of an AFM in tapping

mode allowed us to evaluate the film roughness (Rms). The SEM micrographs of the films grown

at 50 Hz and under CW mode are shown in figure 5. They corroborate the nanocrystalline nature

of the films, but also reveal a significant topographical modification when passing from the CW

to the PW mode. Smooth surfaces can be observed without crystallized grains for both films, but

the one obtained in CW mode exhibits a surface roughness (Rms ≈ 40 nm) higher than the one

estimated for 50 Hz (Rms ≈ 20 nm). This surface quality improvement is consistent with XRD

and Raman characterizations.

Figure 5. SEM micrographs of NCD films elaborated: (a) in pulsed plasma

under 50 Hz pulse repetition rate and 500 W time-averaged MW power; (b)

in continuous plasma under 500 W.

The final point of our experimental investigations concerns dust particle formation. The pulsedregime apparently limits the production of soot particles in the Ar/H 2 /CH4 MW discharges. This

was supported by qualitative observations that differentiate between the CW and PW modes:

under the PW mode (i) no incandescent particles were visible during the process through orange

luminescence, and (ii) no black deposit on the quartz bell was observed after deposition. Further

experimental and theoretical investigations will be the subject of future work, and will include

the quantification of the reduction of dust particles in pulsed discharges.

In order to understand the secondary nucleation rate augmentation induced by modulating the

MW power, especially at low frequencies, we developed a thermo-chemical plasma model able

to describe the pulsed Ar/H2 /CH4 discharges obtained in a MW cavity system. This model, based

on previous ones [8,  26], enabled us to predict the time evolution of plasma composition, gastemperature and electron average energy, with the assumption of quasi-homogeneous discharges.

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Since the C2 dimer is recognized to be the growth precursor for NCD films [11, 13], we focused

on its time variation and time-averaged mole fraction as a function of the repetition rate.

In figure 6 we show the C2 mole fraction ( ) and the gas temperature (T g) evolutions as a

function of time, at 50 and 500 Hz. For the lower frequency value, reaches two maxima, oneduring the pulse (t on) and one while the MW power is off (t off ), and ranges from 5 × 10

- 6to 2 ×

10- 3

. At 500 Hz the mole fraction maximum is reached at the end of  t off  and evolves from 4 ×

10- 4

to 10- 3

. For both frequencies the temperature evolves in the same manner, i.e. it increases

during t on and decreases during t off . The ranges estimated are 3000-5200 K at 50 Hz and 3700-

4000 K at 500 Hz. All these values should be compared to the values obtained in continuous

mode under 500 W, i.e. and T g = 3900 K. As already discussed [26], the

production of C2 within the discharge is mainly governed by the gas temperature. The production

of C2 from C2H2 conversion is enhanced at relatively high gas temperatures but too high a value

for T g leads to a strong conversion of the dimer into the C atom.

Figure 6. Time variation of C2 mole fraction and gas temperature T g calculated for pulsed Ar/H2 /CH4 (96 : 3 : 1) discharges under 200 mbar

pressure and 500 W time-averaged microwave power for 50 and 500 Hzpulse repetition rates.

It is worth noting that, as illustrated for the two particular frequencies considered here, the PWmode allows us to increase at least temporarily the C2 mole fraction with respect to the CW

mode. Indeed, for 50 and 500 Hz, reaches values above the one obtained in continuous mode

during both t on and t off . Besides, at high frequencies the C2 mole fraction remains at appreciable

values even while the MW power is off. These results may corroborate the experimental

observations suggesting that the growth process could be maintained during the  post-discharge 

and that the pulsed mode induces an increase of the growth precursors.

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To elucidate the latter observation, the time-averaged C2 mole fraction calculated on a pulse

cycle (t on + t off ) is presented in figure 7 as a function of the pulse repetition rate along with the

value of calculated for continuous mode. The averaged production of the C2 dimer is

noticeably increased with respect to CW mode for pulse repetition rates above 50 Hz, with the

highest values obtained around 100 Hz. Also, the C2 mole fraction tends to approach thecontinuous value at high frequency. Since high secondary nucleation rates and good

nanocrystalline features are expected when C2 dimer production is enhanced, the shape of the

curve in figure 7 is, from a qualitative point of view, in satisfactory agreement with experimental

investigations. Indeed, it has been pointed out that the highest growth rate and lower grain sizes

are obtained in PW mode at a low frequency (50 Hz), and that NCD features approach those of 

continuous mode at high frequencies. However, the fact that the growth rates measured for

frequencies ranging from 100 to 1000 Hz are lower than the one estimated for CW mode,

whereas the C2 mole fraction values are higher, must be highlighted. It should be kept in mind

that the model does not take into account species transport, especially near the substrate surface,

and that other preponderant carbon-species such as C2H2 or C, and etching species such as the H

atom, should be carefully considered.

Figure 7. C2 time-averaged mole fraction calculated for pulsed Ar/H2 /CH4 

(96 : 3 : 1) discharges under 200 mbar pressure and 500 W time-averaged

microwave power, as a function of pulse repetition rate, compared with CW

mode.

4. Conclusion 

To summarize, for the first time diamond films with nanocrystalline features were achieved in

pulsed Ar/H2 /CH4 MW discharges. The pulse repetition rate effects were investigated.

Comparisons with continuous mode prove that the MW power modulation improves the film

quality in terms of grain size, especially at low frequencies. This is consistent with first hand

calculations performed using a thermo-chemical plasma model that show an enhanced productionof the C2 dimer at low frequencies, which should favour a high secondary nucleation rate.

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Besides, the pulsed mode seems to be a promising method to reduce soot particle formation

usually observed in CW operation. Further investigations are required in order to quantify the

dust particle reduction and to improve the understanding of mechanisms involved in pulsed mode

through thorough plasma diagnostics.

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