Pulsed microwave plasma-assisted chemical vapour deposition of diamond

Int. J. of Refractory Metals &Hard Materials 14 (1996) 179-184 Copyright Q 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0263-4368/96/$15.00 ELSEVIER

Pulsed Microwave Plasma-Assisted Chemical Vapour Deposition of Diamond J. Laimer” & S. Matsumoto National Institute for Research in Inorganic Materials, l-l Namiki, Tsukuba, Ibaraki, 305, Japan (Received 10 November

1994; accepted 8 December

1994)

Abstract: In a pulsed microwave

discharge used for plasma CVD of diamond, operating with a dilute mixture of methane and argon (as actinometer) in hydrogen, emissions of H, Ar, H,, CH and C, were observed by time-resolved optical emission spectroscopy. At constant microwave average power, the influence of repetition rate on the concentration of atomic hydrogen and on the morphology and microstructure of diamond films was studied in detail. At high pulse repetition rates ( 2 1 kHz), the atomic hydrogen concentration becomes almost time-independent and approaches the continuous wave value. At low repetitition rates, the average concentration of atomic hydrogen is lower in pulsed plasmas compared to continuous wave conditions, but the atomic hydrogen concentration during the pulse-on-time is increased. However, growth rate as well as film quality, as assessed by scanning electron microscopy and Raman spectroscopy, is almost unaffected by pulsed deposition conditions in the range of 50 Hz-20 kHz.

INTRODUCTION

perature variation occurs.4s5 In this case, however, the time dependence of radical concentrations gains importance. It should be pointed out that some of the results obtained by conventional microwave plasma CVD were not performed in a real continuous wave (CW) mode. From our own experience we know that in Japan some microwave generators used for diamond deposition generate not CW but pulses with a frequency of 50 (60) or 100 (120) Hz. Also, in the USA, such types of generators are used for diamond deposition,6 which show only duty cycles of 30-40% at 60 Hz. It is not clear how many results on diamond deposition reported so far have in reality been achieved using pulsed microwave discharges.

Since the announcement of the growth of polycrystalline diamond coatings in a microwave discharge,’ this method, operated in a ‘continuous’ wave mode, became the workhorse in the research of diamond synthesis at low pressure/ low-temperature conditions. Recently, microwave pulsed discharges attracted some attention. Basically two operation conditions can be distinguished. In the first case, the discharge is maintained for a certain time interval until the substrate reaches a specified temperature level, and then the plasma is immediately turned off. 2,3 The substrate is cooled to room temperature (which takes several minutes) before another cycle is initiated. In the second case the time duration of each cycle is so short (ns-ms range) that no substantial substrate tem-

EXPERIMENTAL

Plasma-assisted CVD with 2.45 GHz microwave plasma excitation in continuous wave mode as well as in pulse mode was investigated. A sche-

*Permanent address: Institut fi.ir Allgemeine Physik, Technische Universitgt Wien, Wiedner HaupstralSe 8-10, 1040 Wien, Austria. 179

J. Laimer, S. Matsumoto

180

matic illustration of the experimental set-up used for the deposition experiments is shown in Fig. 1. As a more detailed description is given elsewhere,5 only a brief description will be given. The reactor consists basically of a vertically oriented quartz tube intersecting a microwave cavity. Silicon substrates were positioned 15 mm below the centre of the cavity on a quartz susceptor. Besides 0.5% methane (CH, purity 99.97%) diluted in hydrogen (H, purity 99*999%), used as source gases for the usual deposition experiments, a small flow of argon (Ar purity 99.999%) was added to the H, + CH, mixtures to act as an actinometer, sensing changes in the discharge electron density and electron energy distribution, and concomitant changes in excitation efficiency. 7,8 A total gas flow rate of 100 seem at a total pressure of 30 torr was used. All experiments were performed at an average power of 400 W. Because of the lack of an additional heating or cooling of the substrate, the substrate temperature was primarily determined by the microwave power. The substrate temperature was monitored by an optical pyrometer. The effect of pulsing on film quality and plasma emission was investigated at pulse repetition rates from 20 kHz to 50 Hz - also in comparison to the continuous wave mode. Prior to deposition, Si( 100) substrates were ultrasonically scratched in a diamond-alcohol

slurry, rinsed with alcohol and distilled water and then blown dry in nitrogen. The morphology of the films was studied with scanning electron microscopy (SEM). Surface morphology and morphology of fracture cross-sections were observed. Raman characterization of the diamond films was performed using the 5 14.5 nm line of an argon laser with a spot size of 200 hum at an incident laser power of 300 mW. The scattered light was detected in conventional backscattering geometry. Time-resolved optical emission spectroscopy (OES) was performed on the same apparatus already used for the diamond deposition experiments, but during special dummy runs. An illustration of the experimental set-up used for OES is shown in Fig. 2. Plasma emission from the centre of the microwave discharge was collected. A more detailed description is given elsewhere.” In order to investigate all spectral features of interest, two measurements in different spectral range were necessary. In the ranges of 402-668 nm the atomic lines of H (H, and HP), the molecular bands of CH (430 nm system), C, (Swan system at 516 nm) and H, (570-630 nm, Fulcher band, etc.), and the hydrogen continuum (dissociative decay of Hz) were observed. In the range of 549-8 14 nm the atomic lines of hydrogen (H, ) and argon (750 and 811 nm), and the molecular band of H, were observed.

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Quartztube Power Monitor Pulse Microwave Generator 2.45 GHz 5kW

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Experimental apparatus used for pulse microwave plasma CVD.

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Pulsed microwave plasma-assisted chemical vapour deposition of diamond

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Relative concentrations of atomic H were recorded as the ratio of the Bahner a-line (H,: 656.3 nm) to the Ar 750.4 nm line. Although the excitation thresholds for the two differ by 1.5 eV, these transitions were used since they are strong and frequently used for this purpose.10-12 It was observed that the discharge conditions changed with time, most probably due to changes of the reactor surface exposed to the plasma, which was expressed in a variation of the HJAr (750.4 nm) ratio for the same experimental parameters. Therefore, a standard measurement condition was used to check the reactor condition before each experiment and only experiments will be reported which showed a deviation of less than + 2% from a specified H,/Ar (750.4 nm) ratio.

RESULTS AND DISCUSSION

From earlier investigations we know that in first order the microwave average power is responsible for the substrate temperature, similar to the microwave power in the CW modee5 In the parameter range investigated, the microwave peak power had only a small effect on deposition conditions as well as on fihn quality. The effect of the pulse repetition rate, at a microwave average power of 400 W and peak power of 800 W, on the morphology is shown in Fig. 3. The surface morphology (left side) and morphology of the fracture cross-section (right side) is shown for 200 Hz, 1 kHz, 5 kHz and CW. The fii thickness and, therefore, the growth rate (all experiments were performed at a constant deposition time) can

set-up used for OES.

be assessed from the cross-sectional view. Figure 4 shows the related Raman spectra. As can be seen from the fracture cross-sections, the growth rate does almost not depend on pulse repetition rate. The morphology is also almost unaffected by the pulse repetition rate. Slight changes in morphology (bigger sized crystals) were only observed at very low pulse repetition rates (200 Hz). The dark appearance of the fracture crosssection in Fig. 3(c) (5 kHz) is caused by a different measurement condition of the SEM and is not related to properties of the diamond coating. The main features of the Raman spectra are the diamond peak at 1332 cm- ’ and a weak broad peak at about 1500-1600 cm-i, which is related to amorphous carbon. It can be seen that the Raman spectra are similar for CW and 200 Hz. At higher pulse repetition rates the diamond peak declines, however, the difference should not be overestimated. Time-resolved atomic hydrogen concentrations were determined during the pulse-on-times at pulse repetition rates of 50 Hz-20 kHz for a peak power of 800 W and for CW (Fig. 5). The relative atomic hydrogen concentration is normalized for the CW condition (H,/Ar = 1) in order to compare the results of the pulsed experiments more easily with CW. The relative atomic H concentration is varying strongly at 1 kHz and only weakly at 5 kI-Iz. At even higher pulse repetition rates the relative atomic H concentration becomes more and more independent of time and approaches the CW value. At lower pulse repetition rates the relative atomic H concentration reaches a saturation value and remains constant during

J. Laimer, S. Matsumoto I

Fig. 3. SEM micrographs of the surface (left side) and the fracture cross-section (right side) of diamond films prepared by pulse microwave plasma CVD at a pulse repetition rate of (a) 200 Hz, (b) 1 kHz, (c) 5 kHz and (d) in the continuous wave mode. All other parameters were held constant (average power 400 W, peak power 800 W).

183

Pulsed microwave plasma-assisted chemical vapour deposition of diamond

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of the relative atomic hydrogen Fig. 5. Dependencies concentration on the dimensionless parameter t/T (time/ pulse repetition time) at a peak power of 800 W for three pulse repetition rates and continuous wave (average power is in all cases 400 W). Values are normalized for CW (H,/Ar (750.4 nm)= 1).

800 1000 1200 1400 1600 1800

Raman

shift

(cm-‘)

Fig. 4. Raman spectra of diamond films prepared by pulse microwave plasma CVD at a pulse repetition rate of (a) 200 Hz, (b) 1 kHz, (c) 5 kHz and (d) in the continuous wave mode. All other parameters were held constant (average power 400 W, peak power 800 W).

most of the pulse-on-times. However, there is a strong increase in relative atomic H concentration from almost zero at the beginning of the pulse. By assuming an exponential decay, the relative atomic H concentration should be also almost zero during most of the pulse-off-time. From these data, mean relative atomic H concentrations (average of a cycle) can be determined. Although fairly intensive emissions of CH and C2 were observed, their excitation mechanism does not fulfil the precondition for actinometry, which is direct electron impact excitation of the neutral species. The excitation of these species could be caused also by the chemiluminescence or recombination. Therefore, it was not possible for us to obtain reliable data for these species from our measurements. As can be seen from our results, pulsing does affect the relative atomic H concentration only at lower pulse repetition rates. In this range the atomic H concentration becomes higher during pulse-on-times, but lower in the average over one cycle compared to CW. Under such conditions the saturation value of CH/H, is 1.3 X the CW

value and the saturation value of C/H, is 1.7 x the CW value, whatever that means for the concentrations of carbon containing species. Also, the ratio of HJH, changes. It becomes 1.08 X the CW value, which means that the electron temperature is lower. At high pulse repetition rates the results obtained by optical emission spectroscopy can explain the results of the deposition experiments. The concentration of atomic H (maybe also of other important radicals) in the gas phase as well as the substrate temperature (because of the thermal inertia of the substrate) is almost unaffected by the pulse repetition rate. At low pulse repetition rates, growth of diamond occurs only during the pulse-on-times, as radical concentration diminishes quickly after the end of the pulse. However, radical concentrations seem to be higher during the pulse as no decrease in deposition rate can be observed. The slight change in morphology at lower pulse repetition rates can be caused by either changed conditions during the growth (pulse-on-time) or by quickly alternating growth and non-growth conditions. However that may be, this is exactly the condition under which diamond deposition experiments were performed using microwave generators with line frequency modulation (pulse repetition rates from 50 to 120 Hz). CONCLUSIONS Our investigations growth of diamond

clearly demonstrated in pulsed microwave

that dis-

J. Laimer, S, Matsumoto

184

charges at pulse repetition rates up to 20 kHz is comparable to CW conditions, as long as the same average power is used. For low pulse repetition rates, which frequently occur in experiments using low-cost microwave generators containing magnetrons, the diamond film quality and growth rate neither deteriorates nor improves. Therefore, it can be concluded that pulsing of the microwave discharge does not have any advantage over CW conditions for diamond growth. On the other hand, a low ripple of the microwave power is not required, as diamond growth does not improve with a more continuous power output. However, pulsed microwave discharges are a useful tool for investigating time dependencies and delivering insight into the behaviour of chemical species in the plasma chemical process.

REFERENCES Kamo, M., Sato, Y., Matsumoto, S. & Setaka, N., J. (iyst. Growth, 62 (1983) 642. Ong, T. P. & Chang, R. P. H., Appf. Phys. Lett., 55 ( 1989) 2063.

Muranaka, Y., Yamashita, H. & Miyadera, H., J. Vuc. Sci. Technol., A9 ( 199 1) 76. Akluft, M. & Brock, D., In Proc. of the 2nd Inr. Symp. on Diamond Films, Vol. 91-98. The Electrochemical Society, Pennington, PA, USA, p. 39. 5. Laimer, J. & Matsumoto, S., Diamond Relat. Muter., 3 (1994)231.

6. Brewer, M. A., Brown, 1. G., Dickinson, M. R., Galvin, J. E., MacGill, R. A. & Salvadori, M. C., Rev. Sci. Instrum., 63 (1992) 3389.

7. Donnelly, V. M., Plasma Diagnostics, Vol. 1, ed. 0. Auciello & D. L. Flamm. Academic Press, San Diego, CA, USA, 1989. 8. Pastel, A. & Catherine, Y., J. Phys. D, 23 ( 1990) 799. 9. Laimer, J. & Matsumoto, S.. Plasma Chem. Plasma Proc.. 14 (1994)

ACKNOWLEDGEMENT One of the authors gratefully acknowledges a Science and Technology Agency Fellowship provided by the Japanese Government.

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10. Tsuji, K. & Hirokawa, K., Thin Solid Fifms, 205 ( 1991) 6. 11. Ropcke, J. & Ohl, A., Contrib. Plasma Phys., 31 ( 1991) 669. 12. Mucha, J. A., Flamm, D. L. & Ibbotson, D. E., J. Appl. Phys., 65 ( 1989) 3448.