Production and application of reactive plasmas using helicon-wave discharge in very low magnetic fields

Thin Solid Films 506 – 507 (2006) 550 – 554 www.elsevier.com/locate/tsf

Production and application of reactive plasmas using helicon-wave discharge in very low magnetic fields G. Sato *, T. Kato, W. Oohara, R. Hatakeyama Department of Electronic Engineering, Tohoku University, Sendai 980-8579, Japan Available online 6 September 2005

Abstract In order to explore new application fields of a helicon-wave discharge, we have investigated the production of helicon-wave plasmas in very low magnetic fields (0 – 10 mT) and the resultant nanocarbon creation using methane and/or hydrogen. The reactive plasmas are effectively produced by the helicon wave around 3 mT independently of gas species in the wide range of pressures (0.1 – 10 Pa), where hydrocarbons and atomic hydrogens are generated. Using the helicon-wave reactive plasma as a precursor source for plasma-enhanced chemical vapor deposition, well-aligned carbon nanotubes and nanowalls are found to be formed even in a very low gas pressure of 0.7 Pa. D 2005 Elsevier B.V. All rights reserved. Keywords: Helicon-wave discharge; Plasma enhanced chemical vapor deposition; Nanostructures; Carbon

1. Introduction Radio-frequency (rf) plasmas have been produced so far by electromagnetic fields using many type of electrodes or antennas supplied by rf powers in the megahertz range. These are classified into capacitively and inductively coupled plasmas (CCPs and ICPs) by types of the coupling between the plasma and the rf fields, respectively [1]. The rf discharge easily attains a high density compared with a dc discharge, and it is incorporated into material processing. CCPs and ICPs are typically employed without a magnetic field, where discharges are considered to be effective in the presence of the magnetic field. A heliconwave discharge occurring with the application of the magnetic field to ICPs has been investigated as a promising plasma source because a high density (1012 – 1013 cm 3) is obtained under a low gas pressure of a few hundred millipascals [2,3]. The helicon-wave discharge is sustained by an electromagnetic wave which is launched from an antenna and propagates along magnetic-field lines, and the absorption mechanism of the wave is proposed to be due to the coupling to Trivelpiece –Gould modes. The * Corresponding author. E-mail address: [email protected] (G. Sato). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.049

helicon-wave discharge is identified by density jumps observed when the magnetic field or the rf power are increased. Specifically, the density jumps are caused when the rf field excited by the antenna satisfies the heliconwave dispersion relation by increasing the magnetic field or the plasma density. In addition to those sharp jumps, another kind of density peak is observed in a very low magnetic field of a few millitesla. The density peak in the low magnetic field attains to the order of 1011 cm 3 and the low magnetic field is welcomed for processing applications. We have investigated the density peak in an argon plasma using the phased helical antenna [4]. It is found that the helicon wave is effectively excited and produces the plasma even in the low magnetic field when the axial wavelength of the antenna is matched to that of the helicon wave. Using the antenna exciting the rf field with an axial component, the introduction of only a low magnetic field to ICPs enables the helicon-wave discharge to occur [4]. When the rf power is increased on the helicon-wave discharge performed in the strong magnetic field with several gases including molecular gases, on the other hands, the threshold power of the density jump and the ultimate density depend on gas species [5]. From the viewpoint of applications of the density-peak phenomenon in the low magnetic field, it is important to investigate the

G. Sato et al. / Thin Solid Films 506 – 507 (2006) 550 – 554

discharge characteristics using various gases, especially molecular gases which are extensively used in reactive ion etching (RIE) or chemical vapor deposition (CVD). Carbon nanotubes (CNTs) [6] are known as a new member of carbon allotropes and have attracted great interest for their remarkable properties such as high mechanical strength, electrical conductivity, and so on. The formation of CNTs has been developed by the various CVD methods using methane (CH4), hydrogen (H2), etc. Especially plasma-enhanced CVD (PECVD) methods [7 – 11] are preferable in the sense of the vertically well-aligned nanotube growth under the condition of lower substrate temperature compared with other CVD methods. In a recent work, it is reported that the substrate temperature in thermal CVD method for performing the CNT synthesis can be reduced under the condition of lower gas pressure [12]. The helicon-wave plasma has been used for PECVD of SiO2 films [13], fluorinated amorphous carbon thin films (a-C:F) [14], and BN films [15], but the application to a CNT synthesis has not been reported. The CNT synthesis using helicon-wave plasmas is expected to decrease the synthesis temperature due to the sufficient dissociation of CH4 and the reduction of stable precursors; furthermore, the working gas pressure lower than that in any conventional methods has a great meaning in the way of the synthesis-mechanism elucidation of CNTs. In this experiment, the density-peak phenomenon in the low magnetic field using molecular gases is investigated, where CH4, H2, and a mixture of them are used. Considering that the reactive plasma with the mixture gas has continually been used for the formation of diamond films or amorphous-carbon films by PECVD, here we attempt to perform the synthesis of CNTs.

Matching Box

551

B0

Optical Emission Spectroscopy System

Pyrex Tube CH4/H2

Substrate

to T.M.P. Heater

Phased Helical-Antenna Langmuir Probe Fig. 1. Schematic diagram of experimental setup.

and z directions, and the rf field rotates in the electrongyration (m = + 1) and ion-gyration (m = 1) directions, respectively. Plasma parameters and optical emissions from neutral and charged particles are obtained by a Langmuir probe and an optical fiber, respectively, which are kept 5 cm downstream from the entrance of the process chamber. An electrode surface of the Langmuir probe is usually covered by an insulating thin film in a reactive gas, such as CH4, plasma. In our experiment, the discharge gas is changed every 5 min during the measurement and a DC bias voltage of 200 V is applied to the surface, which is resultantly kept clean by Ar ion sputtering in the Ar plasma. The optical emission is transmitted to an optical emission spectroscopy (OES) system and analyzed. A substrate made of Ni for a CNT synthesis is set on a heater, which is situated at the same position of the probe and the fiber. The substrate is constantly heated up to a desirable temperature Tsub, where a dc voltage V sub is applied for the purpose of CNT-synthesis enhancement. Field emission scanning electron microscopy (FE-SEM, Hitachi S-4100) is utilized for the structural investigation of the raw soot.

2. Experimental setup 3. Plasma characteristics The experimental apparatus is schematically shown in Fig. 1. A Pyrex discharge tube with the outer diameter of 10 cm and the length of 40 cm is attached on the axis to a large stainless-steel vacuum chamber with an inner diameter of 26.3 cm and a length of 89 cm. A phased helical antenna is directly wound on the discharge tube, which can excite spatially and temporally rotating electromagnetic fields with azimuthal mode number )m) = 1 by supplying temporally phased four rf powers to the four helical components [4]. The rf powers up to 2 kW in total at a frequency of x/ 2p = 13.56 MHz are provided to the antenna through an impedance matching circuit. The other end of the tube is terminated by a flat insulator, through which a mixture gas of methane (CH4) and hydrogen (H2) is introduced. The base pressure of 10 4 Pa is kept and the operating pressure is in the range of 0.01– 10 Pa. A uniform magnetic field B 0 below 11 mT is applied along the z-axis by solenoid coils. The positive and negative signs of B 0 denote that the direction of the magnetic-field lines corresponds with + z

Fig. 2 gives the electron density (n e) dependence on the magnetic field for several rf powers ( P rf = 300, 1000, and 2000 W) in the cases of (a) H2 ( P H2 = 0.4 Pa) and (b) CH4 ( P CH4 = 0.1 Pa). In both cases, the electron density increases with increasing B 0 for m = +1, and the density has a peak at B 0 = 2 – 4 mT. The peak density becomes higher with increasing in P rf, where the density for P rf = 2000 W attains up to one order of magnitude larger than that at B 0 = 0 mT, i.e., ICP mode. On the other hand, the density increase is not observed for m = 1 independently of P rf. The electron temperature and the space potential are almost constant (Te ¨ 4 eV and / s ¨ 50 V) in both gas cases when the density peaks are observed. The maximal electron density as a function of gas pressure for P rf = 1000 W is shown in Fig. 3(a). The density increases with increasing in the gas pressure in both the gas cases, but the helicon-wave discharge becomes degraded for P H2 > 2 Pa and 0.7 Pa. In general, the helicon-wave

G. Sato et al. / Thin Solid Films 506 – 507 (2006) 550 – 554

ne (× 1011 cm–3)

2

(a) 300 W 1000 W 2000 W

1

ne (× 1011 cm–3)

0 4

(b) 2

0

–10

–5

0

5

10

B0 (mT) Fig. 2. Electron density dependence on magnetic field for various rf powers P rf with (a) hydrogen (H2) and (b) methane (CH4). Gas pressures are P H2 = 0.4 Pa and P CH4 = 0.1 Pa.

discharge is preferentially put into practice in a relatively low gas pressure, where the collision frequency between neutral particles and electrons is usually less than x/2p. Plasma waves are inhibited from propagating under the condition of such frequent collisions because of the strong damping, and the helicon wave is no exception to this principle. P H2 yielding the maximal density is higher than P CH4 yielding that because the electron mean free path in the H4 plasma is longer than that in the CH4 plasma. In order to identify neutral and ion species in the CH4 and H2 plasmas, spectral lines of visible emission in the wavelength range of 350 – 750 nm are measured under the same conditions of Fig. 3(a). In the case of the CH4 plasma, line emissions from Ha, Hh, CH, and C2 are strongly observed at the wavelengths of 656.2 nm, 486.1 nm, 431.5 nm, and 516 nm

Emission Intensity

ne (× 1011 cm–3)

1.5

(a)

CH4 H2

1 0.5 0 10

(b) CH4

5

Hβ

Hβ

H2

CH

H2

C2

0

10–1

100

Pressure (Pa)

101

(Swan bands), respectively. The emissions from Hh, CH, and C2 are plotted as a function of the gas pressure in Fig. 3(b), where the emissions from H2 and Hh are also plotted in the case of the H2 plasma. The gas pressures of yielding the emission peaks correspond to those of the density peaks for all species except C2. On the other hand, the emission from C2 has a peak at the higher pressure than other species because there are abundant C2 generated by the combination of C atoms in the higher pressure region in which the collisions are more frequent. The relationship between the electron density and the emissions in the H2 and CH4 plasmas are described in Fig. 3(c) and (d), respectively. The emissions from Hh and H2 in the H2 plasma are in proportion to the electron density independently of the gas pressure, while the relationship in the CH4 plasma shows hysteresis curves, and the emissions in low pressures are stronger than that in high pressures when the electron density is kept constant. Especially in the latter case, the relationship between the population of excited species and the electron density is not clear, and the more detailed studies are necessary for optimization of the discharge parameters because complicated processes of dissociation and recombination take place in the discharge with polyatomic molecules, like CH4. When PECVD is performed with reactive gases, a mixture of several kinds of gases is used. The plasma parameters and the emissions from Hh, CH, and H2 depending on the CH4 concentration [CH4] (CH4/(CH4 + H2)) are plotted in Fig. 4, where the mixture gas pressure is kept constant P Gas = 0.7 Pa for P rf = 1000 W and B 0 = 3 mT. When the CH4 concentration is changed, Te and / S are almost unchanged, Te ¨ 3 eV and / S ¨ 50 V, as shown in Fig. 4(a). Although n e is decreased just slightly around [CH4] = 0.5, it is found that all plasma parameters are approximately independent of the CH4 concentration.

Emission Intensity Emission Intensity

552

10 Hβ

(c)

5 H2

0 10 Hβ

5

(d) CH C2

0

0

0.5

1

1.5

ne (× 10 cm ) 11

–3

Fig. 3. Electron density (a) and line emissions (b) depending on gas pressure in H2 and CH4 plasmas at P rf = 1000 W and B 0 = 3 – 4 mT. Relationships between the electron density and the emission in the (c) H2 and (d) CH4 plasmas, which are obtained from (a) and (b).

G. Sato et al. / Thin Solid Films 506 – 507 (2006) 550 – 554

φS

ne

1

4

Te

Emission Intensity

0 1

Te (eV), φS (× 10 V)

11

8

(a)

–3

ne (× 10 cm )

2

0

(b)

0.5

Hβ

H2 CH

0

0

0.5

1

CH4 Concentration Fig. 4. (a) Plasma parameters [plasma density (?), electron temperature (>), and plasma potential (‚)] and (b) optical emission [Hh (h), H2 (>), and CH (?)] depending on the CH4 concentration (= CH4/(CH4 + H2)) under the conditions of P Gas=0.7 Pa, P rf = 1000 W, and B 0 = 3 mT.

Fig. 4(b) indicates the peak intensity variation of Hh, H2, and CH for different CH4 concentrations. As can be appreciated, the peak intensity of CH monotonously increases and that of Hh decreases when the CH4 concentration increases. By contrast, the peak intensity of H2 remains nearly constant independently of the CH4 concentration, which means that the H2 emission is given off from H2 molecules which are not only supplied from the outside but also generated due to dissociation of CH4. Even if the helicon-wave discharge activates the dissociation of methane molecules, the generation of molecular hydrogens is dominant since its onset energy is relatively lower than that of other dissociation processes of CH4 [16]. Thus, it is difficult to obtain a large number of atomic hydrogens from only CH4. In general, the atomic hydrogen is assumed to play a vital role in the deposition of carbon allotropes [17], and mixing hydrogen gas is necessary in order to secure the sufficient atomic hydrogen.

553

growth of vertically well-aligned MWNTs directly onto a cylindrical rf-electrode [9]. The mixture gas pressure is about a few tens of pascals in almost all cases, and the lowest pressure reported is 5 Pa [11]. However, our condition of the mixture gas pressure is only 0.7 Pa, which is much lower than that in the previous experiments. Since the CNT growth rate in our experiment is about 0.2 Am/min, which is similar to those of previous reports, it can be said that the helicon plasma is available in the point of view of the efficient use of material gases. In addition to MWNTs, the carbon nanowalls (CNWs) [18] are observed to be formed at P rf = 1000 W, B 0 = 4 mT, CH4/H2 = 10:0, / sub = 0 V, and Tsub = 600 -C. CNWs consist of wellaligned carbon sheets with a thickness in the range of nanometers as shown in an inset of Fig. 5(b) and are considered to be prospective as an electron emitter as well as CNTs. Thus, the helicon-wave discharge is for the first time demonstrated to be used as PECVD source for the synthesis of various kinds of carbon nanomaterials.

5. Conclusion In summary, we have demonstrated the characteristics and the application of a helicon-wave plasma to a plasmaenhanced chemical vapor deposition source in low magnetic field (0– 10 mT). The helicon wave effectively produces the reactive plasma independently of the gas species in a wide pressure range (0.1 – 10 Pa) only applying the magnetic field about 3 mT to an inductive-coupled discharge. Molecular gases, methane and hydrogen, are dissociated to hydrocarbon and atomic hydrogen in the plasma, which can play the role of precursors for the nanocarbon synthesis. In the helicon-wave reactive plasma, well-aligned carbon nanotubes and nanowalls are found to be formed even in a very low gas pressure of 0.7 Pa which is up to one order of magnitude lower than that in conventional plasmas. The application of the helicon-wave plasma to the nanocarbon synthesis is demonstrated for the first time.

4. Carbon nanotube synthesis The carbon nanotube synthesis is performed using the Ni plate as a catalyst in the helicon-wave discharge for 10 min under the condition of P rf = 1000 W, B 0 = 4 mT, and P Gas = 0.7 Pa (CH4/H2 = 3:7). The substrate bias voltage and temperature measured directly by an embedded thermocouple are kept at / sub = 300 V and Tsub = 850 -C, respectively. It is confirmed that individually separated and one-way aligned multi-walled carbon nanotubes (MWNTs) are produced as typically presented in Fig. 5(a), diameters and lengths of which are about 65 nm and 2 Am. Up to now, the CNT synthesis has been performed by the PECVD methods mentioned above by many researchers, while we have also reported the large-area

Fig. 5. Typical FE-SEM images of carbon materials on the Ni plate. (a) Carbon nanotubes formed at / sub = 3000 V, Tsub = 850 -C, P rf = 1000 W, B 0 = 4 mT, and P Gas = 0.7 Pa (CH4/H2 = 3:7). Inset shows the focus of CNT with the diameter of about 65 nm. (b) Carbon nanowalls formed at / sub = 0 V, Tsub = 600 -C, P rf = 2000 W, B 0 = 4 mT, and P Gas = 0.7 Pa (CH4/ H2 = 10:0).

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Acknowledgement The authors thank H. Ishida, T. Hirata, K. Tohji, and K. Motomiya for technical supports. This work was supported by the Sasakawa Scientific Research Grant from The Japan Science Society and Tohoku University 21st Century COE (Center of Excellence) Program.

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