E L S E V I E R Surface and Coatings Technology 126 (2000) 195-198 \ v n ~ v . el sev ier. nlil ocateisurfcoat

Nucleation and growth of diamond films on molybdenum

W.L. Wang"?", K.J. Liao", G.C. Gaob

aDepartment of Applied Physics, Chongqing University, Chongqing, 400044, China bDepartment of Materials Science, Chongqing University, Chongqing, 400044, China

Received 27 September 1999; accepted in revised form 11 January 2000

Abstract

The nucleation and growth of diamond films on molybdenum were investigated by scanning electron microscopy and Raman spectroscopy. Diamond films were deposited by hot-filament chemical vapor deposition via electron emission-enhanced nucle- ation, in which a dc negative voltage relative to the filament was applied to a tungsten electrode that was previously coated with diamond, by chemical vapor deposition. The electrode-to-substrate distance was one of the deposition variables. The maximum value of the nucleation density was found to be up to 10" cm-' on a pristine Mo surface by electron emission-enhanced nucleation. Electron emission-enhanced nucleation also greatly improved the quality and adhesion of diamond films on a Mo substrate. 0 2000 Elsevier Science S.A. All rights reserved.

Keywords: Diamond films; Nucleation; Emission electron; CVD

1. Introduction

In recent years, the deposition techniques for dia- mond films have progressed rapidly. The growth rate of CVD films has remarkably increased, and the quality of the films has also improved 11-41. However, one of the major problems associated with chemical vapor deposi- tion diamond on silicon or other metal substrates is low nucleation density in the absence of nucleation pre- treatments. Thus, mirror-finished silicon substrates are usually scratched with diamond powder prior to chemi- cal vapor deposition (CVD). Yugo et al. 151 firstly suggested bias-enhanced diamond nucleation in a mi- crowave plasma CVD. This in situ process not only led to a high nucleation density of diamond on untreated Si substrates, but also marked the most important

* Corresponding author. Tel.: + 86-23-651-02821; fax: + 86-23-651- 6704. E-mail address: wanluw@cgu.edu.cn (W.L. Wang)

advance in growing heteroepitaxial diamond films by a bias nucleation process in a microwave plasma CVD 161.

In this paper, the nucleation and growth of diamond films on molybdenum were investigated by scanning electron microscopy and Raman spectroscopy. The dia- mond films were deposited by hot-filament CVD via electron emission-enhanced nucleation (EEEN).

2. Experimental details

Diamond thin films were prepared by a conventional hot-filament CVD apparatus, which has been described in detail in a previous publication 171. Mirror-polished molybdenum of 20 x 20 mm was used as the substrate. The substrate was ultrasonically cleaned in acetone for 15 min and subsequently in a methanol bath for 5 min. After a short rinse in methanol, the substrate was placed on a graphite substrate holder. The chamber was pre-evacuated to torr, then hot-filament CVD

0257-8972/00/$ - see front matter 0 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 ( 0 0 ) 0 0 5 2 2 - 3

196 WL. Wang et al. /Surface and Coatings Technology 126 (2000) 195-198

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processes were performed under standard conditions. The filament temperature was kept between 2100 and 2200°C and measured by a pyrometer. The substrate temperature was set at 850°C measured by using a Pt R thermocouple mounted on the substrate holder, and calibrated by using a PtRh thermocouple attached on the substrate surface. The reaction gas was a mix- ture of H, and CH,, and was fed into the reaction chamber when the substrate temperature reached a stable value. The total gas pressure was approximately 4 x lo3 Pa. The filament was heated in a methane- hydrogen atmosphere to enhance the formation of a tungsten carbide layer covering the filament surface.

The diamond films were grown on Mo by a two-step process: nucleation and growth. The nucleation process was performed at 850°C with a methane concentration of 3.0% in hydrogen, and a negative bias of -300 V relative to the filament applied to the diamond-coated tungsten electrode of 1 mm in diameter that was previ- ously coated with diamond by chemical vapor deposi- tion, as shown in Fig. 1 [8]. The W emission electrode was set between substrate and filament. The substrate- to-filament distance was 8-10 mm. The electrode-to- substrate distance was one variable in the deposition conditions. The electrode shape was either circular or square in order to obtain a homogeneous diamond film on the substrate. The nucleation process lasted for 35 min. For the subsequent growth step, the bias voltage was turned off, and the electrode was kept away from the substrate to avoid shade. The methane concentra- tion was decreased to 1%, and all other deposition conditions remained unchanged.

+ + Bias Supplies

Gas Feed /

Itl , Filament

W electrode

Substrate Substrate Holder Heater/ Thermocouple

f, I I Assemble I I

?+7

Fig. 1. A schematic diagram of a hot-filament CVD apparatus with W electrode.

Fig. 2. SEM images of diamond nuclei on Mo substrate for 35 min: (a) with EEEN treatment; and (b) without EEEN treatment.

3. Results and discussion

Fig. 2 shows a SEM image of the nucleation stage of the 35 min. Fig. 2a,b show diamond nuclei on a Mo substrate with and without EEEN treatment. It is very clear that high nucleation density was achieved on Mo via the EEEN treatment. Most of the nuclei have joined each other after 35 min by EEEN treatment.

Fig. 3 describes the relationship between the nucle- ation density and emission current with EEEN treat- ment. The distance between the W electrode and subs- trate was approximately 5 mm. The methane concen- tration was approximately 3.0%. The emission current was increased with increasing bias voltage applied to electrode. The emission current was increased from 0 to 300 mA when bias voltage increased from 0 to - 350 V. The diamond nucleation density was drastically in- creased when the emission current was between 100 and 250 mA, while the nucleation density was quite low as the current was lower than 100 mA. Nucleation density was decreased on further increasing emission current over 280 mA or bias over - 350 V.

The experimental results indicated that the methane concentration has a significant effect on the nucleation density of diamond as shown in Fig. 4. The distance between W electrode and substrate was approximately

10"

lo9

0 107

I ,a 105

I I 103

h

0 v

v1

a 0 .C *

' in1 -- 00 SO 100 150 200 250 300

Emission Current ( mA )

Fig. 3. The relationship between the diamond nucleation density and the emission current.

WL. Wang et al. /Surface and Coatings Technology 126 (2000) 195-198 197

- 0 0.5 1.0 1.5 2.0 2.5 3.0

Methane Concentration ( YO )

Fig. 4. Changes of the diamond nucleation density with methane concentration on Mo: (a) with EEEN treatment; and (b) without EEEN treatment.

5 mm. The nucleation process lasted for 35 min at the same deposition conditions. The bias voltage and emis- sion current were kept at - 300 V and 250 mA, respec- tively. Fig. 4a,b represent the changes in nucleation density with methane concentration, with and without EEEN treatment. As can be seen, there is low nucle- ation density without EEEN treatment. The nucleation density was lower than lo5 cm-' when the methane concentration was lower than 0.5%. The nucleation density was higher than 10" via EEEN when the methane concentration exceeded 3.0%.

Fig. 5 shows the SEM micrographs of diamond films grown on Mo for 6 h with and without the EEEN stage. All deposition conditions were kept at the same values. The continuous diamond films with good facets can be obtained on Mo via EEEN treatment as shown (Fig. 5a). However, only discrete diamond grains without the EEEN process as shown (Fig. 5b), and continuous films can be formed after 12 h growth at methane concentra- tion higher than 5%.

Fig. 6 represents Raman spectra of diamond films shown in Fig. 5. Fig. 6a is the Raman spectrum of diamond films via EEEN during the nucleation process, which shows a sharp peak centered at 1332 cm-' with full width of half maximum 8.9 cm-', indicating that the obtained films are of good quality. However, there is a non-diamond carbon band at approximately 1520 cm-' without the EEEN process as shown in Fig. 6b.

Fig. 5. SEM micrographs of diamond films grown on Mo for 6 h: (a) with EEEN; and (b) without EEEN.

I 1000 1200 1400 1600 1800

Raman Shift (cm-')

Fig. 6. Raman spectra of diamond films: (a) with EEEN; and (b) without EEEN.

Fig. 7 shows the variation of scratch test adhesion force with deposition temperature. It can be seen that the adhesion force of diamond films on Mo substrate with EEEN treatment was found to be higher than 60 N as shown in Fig. 7a. However, the adhesion forces of the films on Mo without EEEN was lower than 20 N (see Fig. 7b). This means that the adhesion for the films on Mo is greatly improved via EEEN. There is little influence of temperature on the adhesion of diamond films on Mo. The film thickness was 4 pm for the two kinds of samples.

The negative bias pre-treatment was a key step in growing diamond films. In general, the bias-enhanced ion bombardment during nucleation was considered to enhance the diffusion of surface atoms of the islands, promoting the system to approach equilibrium 191. In the EEEN method, energetic ions were fully sup- pressed because the bias-field was distributed between the electrode and the filament. However, our experi- mental results show that high nucleation density up to 10'' cm-' can still be achieved via electron emission rather than using a biased substrate. Therefore, this may mean that the ion bombardment may not be a unique way to enhance the nucleation of diamond.

3 3.5 4.0 4.5 5.0 5.5 6.0

Temperature ( XIOz K )

Fig. 7. Scratch test of adhesion with temperature.

198 WL. Wang et al. /Surface and Coatings Technology 126 (2000) 195-198

The emission electrons from the diamond coatings on the electrode collide with hydrogen molecules and various hydrocarbon species, and promote their dissoci- ation into atomic hydrogen and hydrocarbon radicals [10,11]. It is important to note that the emission elec- trons ignited the plasma around the electrode at higher negative voltage (> 250 V). A distinguishable and in- tense blue light could be seen near the electrode sur- face and local filament after filament heating was stopped.

It was found that the chemical species concentration increased with increasing plasma intensity [ll]. A low nucleation density occurred without the plasma for a long time (> 8 h) even though a sufficient high nega- tive bias voltage (> 200 V) was applied to the elec- trode. In fact, the plasma and the emission current simultaneously appeared. The active chemical species in the plasma moved to the Mo substrate surface, leading to the nucleation due to their diffusion when the plasma sheath covered the surface. Over 200 mA, the nucleation density tends to approach saturation. This may be because the chemical species concentra- tion attained the saturation in the plasma. The nucle- ation density was decreased when emission current exceeded 280 mA. This can be ascribed to the fact that the scattering roles among the chemical species were enhanced with increasing species concentration, result- ing in a part of the species not arriving to the substrate surface.

4. Conclusion

High nucleation density and good-quality diamond films on Mo substrate can be achieved by emission electron-enhanced nucleation in hot-filament CVD when a dc negative voltage relative to the filament was

applied to a tungsten electrode with a diamond coating. The electrode was located between the filament and the Mo substrate. The experimental results showed that the electron emission from the diamond coatings on the electrode played a critical role during the nucle- ation process due to the excitation of an electron-en- hanced plasma. In addition, the damage to diamond caused by ion bombardment was eliminated by EEEN.

Acknowledgements

The authors wish to thank the National Natural Science Foundation of China grant 19904016 and Chongqing University for financial support of this work.

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