Diamond growth onCoSi(2)/Si by bias-enhanced microwave plasma chemical vapor deposition method

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Materials Chemistry and Physics 72 (2001) 172–175

Diamond growth on CoSi

2

/Si by bias-enhanced microwave

plasma chemical vapor deposition method

Mao Rong Chen, Li Chang

∗

, Der Fu Chang, Hou Guang Chen

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

Abstract

Diamond was grown on polycrystalline CoSi2/Si substrates by bias-enhanced microwave plasma chemical vapor deposition. Both of the

positive and negative biasing effects were investigated by microstructural characterization. It has been found that nucleation density can reach∼109cm−2with positive biasing, much higher than with negative biasing. Cross-sectional transmission electron microscopy shows that diamond deposited by positive biasing grows on a relatively smooth CoSi2surface, while the etching effect of ion bombardment during

negative biasing results in a rough CoSi2surface. The diamond morphology obtained with negative bias has a flat surface with a strong

Keywords: Plasma-assisted CVD; Electron microscopy; Diamond; Nucleation

1. Introduction

Diamond has many excellent physical and mechanical properties such as extreme hardness, high thermal con-ductivity, wide bandgap and optical transparency. For the realization of electronic diamond devices, it is important to synthesize the heteroepitaxial diamond films. It has been known that diamond can be grown by microwave plasma chemical vapor deposition (MPCVD) and the nucleation density of diamond could be increased by bias-enhanced nucleation (BEN) method [1–5]. The BEN method can lead to the formation of a heteroepitaxial film for which a high density of nucleation of diamond on substrate is often a necessity.

Diamond growth on Si has been reported. It is usually

found that a ␤-SiC interlayer is formed before the

dia-mond growth [5]. In the case of cemented carbides, such as WC–Co, the dissolution of carbon into the substrate in-duces graphitization of the deposit [6]. Si is a strong carbide former, while Co is hardly able to form a carbide with C in chemical vapor deposition (CVD). It will be of interest to know the role of Co and Si effects on nucleation. In the

present study, diamond films were synthesized on CoSi2

substrate by MPCVD with BEN method in a mixed gas of CH4and H2. CoSi2is a stable phase of silicide in Co–Si

bi-nary system, and is an important material in microelectronic devices. Since lattice mismatch of CoSi2 with Si is 1.2%,

∗_{Corresponding author. Tel.:}_{+886-3-573-1615; fax: +886-3-572-4727.} E-mail address: [email protected] (L. Chang).

it is possible to obtain a highly oriented or epitaxial film of CoSi2on Si. From the structural point of view, diamond

growth on CoSi2 could have a similar behavior to that on

Si. Previous work by Arnault et al. [7] shows that diamond

deposited on CoSi2 by hot-filament CVD is preceded by

the formation of a SiC layer. Gu et al. [8] have recently reported that high-quality textured diamond films can be

grown on CoSi2with BEN pretreatment. Here, we

demon-strate that the positive biasing effect on diamond deposition is different from the negative one through microstructural characterization.

2. Experimental

A CoSi2layer formed on Si (1 0 0) wafer was used as the

substrate. CoSi2 layers of 100 nm thickness were formed

by electron beam evaporation of Co on Si, followed by rapid thermal annealing (RTA) at 800◦C for 1000 s. A tube MPCVD reactor was used to deposit diamond. Before in-serting into the MPCVD reactor, the substrates were cleaned by acetone, followed by removal of oxides by HF. Mixture

of CH4and H2 was used as the gas source and d.c. power

supply was used to create the bias. The substrates were put onto a Mo holder. A tungsten needle of diameter 0.5 mm or a 3 mm diameter Mo disk was used as the electrode. The corresponding applied bias voltage on the substrate was in the range−150 to +300 V. After biasing, all the samples re-ceived the same growth conditions for 4 h. The experimental condition for deposition is listed in Table 1. Microstructural

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M.R. Chen et al. / Materials Chemistry and Physics 72 (2001) 172–175 173 Table 1

Experimental parameters

Power (W) Pressure (Torr) Concentration (%), CH4 in H2

Bias voltage (V) Time (min)

H2 plasma etching 500 20 0 0 20

Carburization 500 20 2 0 60

BEN 500 20 2 −150 to +300 10–15

Growth 500 20 1 0 240

characterization was performed by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and trans-mission electron microscopy (TEM). Cross-sectional TEM specimens were prepared by gluing the diamond face with a glass slide, followed by mechanical grinding and ion milling.

3. Results and discussion

XRD patterns reveal that the Co films were reacted with Si into CoSi2films after RTA. Though the CoSi2films are

polycrystalline, they have (1 0 0) strong preferred orienta-tions on Si (1 0 0) substrates.

3.1. Negative biasing

SEM image in Fig. 1a shows the distribution of dia-mond particles on CoSi2/Si (0 0 1) substrate with biasing at −150 V. Smooth (1 0 0) facets are clearly seen on each

par-ticle in Fig. 1b, indicating that diamond is highly oriented in (1 0 0) texture. In some cases, secondary diamond nuclei on the facets were also observed. Interestingly, deposition did not result in a continuous film. The density of dia-mond particles is estimated to be larger than 1× 106cm−2. This is an order of magnitude lower than that in diamond deposition on Si using the same experimental condition.

SEM observation reveals that CoSi2 surfaces on Si

un-covered by diamond were very rough, implying that CoSi2

might be modified by biasing. Cross-sectional TEM exami-nation on the specimen after diamond deposition shows that CoSi2has been etched into a cone shape as shown in Fig. 2.

Fig. 1. SEM images showing (1 0 0) textured diamond films grown on CoSi2/Si (1 0 0). Magnification: (a)×1000; (b) ×5000.

Fig. 2. (a) Cross-sectional TEM micrograph showing diamond deposited on CoSi2 with−150 V bias voltage and (b) the corresponding selected

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174 M.R. Chen et al. / Materials Chemistry and Physics 72 (2001) 172–175

The etching effect is mainly caused by ion bombardment during biasing. The diamond particles are seen on top of the CoSi2cones. The diamond{1 1 1} ring in selected area

diffraction pattern of Fig. 2b demonstrates that diamond sitting on those cones in Fig. 2a image is of polycrys-talline nature. No interfacial SiC layer can be observed

between diamond and CoSi2, implying that diamond may

directly nucleate on CoSi2. Also, it is noted that the Si

substrate has been etched. Hence, the strong etching effect of ions results in a small volume of CoSi2 left on top of

the cones.

3.2. Positive biasing

XRD pattern in Fig. 3 obtained from a sample pretreated with bias voltage of+300 V, shows a strong (1 1 1) diamond peak, indicating diamond is in (1 1 1) preferred orientation. SEM micrograph in Fig. 4 shows that a continuous film of diamond has been formed. The surface of the diamond film is not smooth with grain morphology of (1 1 1) facets. The grain size is rather small compared with those deposited with negative biasing. The nucleation density of diamond is es-timated in the order of 109cm−2. The enhanced nucleation density of diamond on Si with positive biasing has also been observed consistent with previous reports [9]. TEM micro-graph with the corresponding electron diffraction pattern in Fig. 5 shows that diamond was grown on CoSi2. The

inter-face between diamond and CoSi2is relatively smooth,

im-plying that the CoSi2 surface has not suffered the etching

effect during biasing. Also, no apparent interlayer between diamond and substrate can be seen. With positive biasing, most of the charged particles bombarding the substrate are electrons, which may have a negligible effect on the surface. However, electrons of the sufficiently high energy obtained by the applied bias voltage might result in an increase in

Fig. 3. XRD pattern showing (1 1 1) texture of diamond film deposited with positive biasing at+300 V on CoSi2/Si (1 0 0) substrate.

Fig. 4. SEM micrograph showing the morphology of diamond film de-posited with positive biasing at+300 V on CoSi2/Si (1 0 0) substrate.

the amount of carbon-containing radicals near the surface to enhance diamond nucleation.

The higher density of diamond deposited with positive biasing than negative one can be attributed to the smooth-ness of substrate surface. In negative biasing, the surface is very rough, approximately with one cone per micrometer. Since the cones are likely the nucleation sites for diamond, the number of cones available will limit diamond

nucle-ation. In contrast, CoSi2 surface remains to be smooth

during positive biasing, which allows diamond nucleate

Fig. 5. (a) Cross-sectional TEM micrograph showing diamond deposited with positive biasing at+300 V on CoSi2/Si (1 0 0) substrate and (b) the

corresponding selected area electron diffraction pattern showing diamond (1 1 1) ring.

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M.R. Chen et al. / Materials Chemistry and Physics 72 (2001) 172–175 175

without the site limitation. Similar behavior on Si has also been observed.

4. Conclusions

Diamond deposition on CoSi2/Si substrates with

pretreat-ment by biasing in positive and negative voltages has been investigated. Both of the positive and negative biasing en-hance the nucleation density of diamond. However, negative biasing results in a rough surface of the substrate due to etch-ing of ion bombardment. In contrast, positive biasetch-ing gives a relative smooth surface of substrate on which diamond can be grown. As a result of surface smoothness, the nucleation density of diamond obtained with positive biasing can reach 109cm−2, about two orders of magnitude higher than that obtained with negative biasing. The diamond morphology in the negative-biased film has a flat surface with a strong (1 0 0) orientation, while that in the positive one is relatively rough with (1 1 1) facets.

Acknowledgements

This work was supported by National Science Council, Taiwan, ROC, under contract of NSC 89-2216-009-006.

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