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

doi:10.1016/j.tsf.2005.08.049

Thin Solid Films

Volumes 506–507, 26 May 2006, Pages 550-554

https://doi.org/10.1016/j.tsf.2005.08.049 Get rights and content

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.

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 helicon-wave 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 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 helicon-wave 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 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], [8], [9], [10], [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.

Section snippets

Experimental setup

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

Plasma characteristics

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 B₀ for m = + 1, and the density has a peak at B₀ = 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 B₀ = 0 mT, i.e., ICP mode. On the other hand, the

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, B₀ = 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 ϕ_sub = − 300 V and T_sub = 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

Conclusion

In summary, we have demonstrated the characteristics and the application of a helicon-wave plasma to a plasma-enhanced 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,

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.

References (18)

F.F. Chen et al.
IEEE Trans. Plasma Sci.
(1997)
I.-H. Kim et al.
Thin Solid Films
(1996)
M.A. Lieberman et al.
Principles of Plasma Discharges Materials Processing
(1994)
R.W. Boswell et al.
IEEE Trans. Plasma Sci.
(1997)
G. Sato et al.
Y. Sakawa et al.
Phys. Plasmas
(1999)
S. Iijima
Nature
(1991)
O.M. Küttel et al.
Appl. Phys. Lett.
(1998)
V.I. Merkulov et al.
Appl. Phys. Lett.
(2001)

There are more references available in the full text version of this article.

Cited by (9)

Low temperature growth of nanocrystalline TiO2 films with Ar/O<inf>2</inf> low-field helicon plasma
2011, Surface and Coatings Technology
Citation Excerpt :
High-density helicon plasmas have been applied to many forms of plasma processing (etch, deposition, etc.) [39–41]. However, only a few examples on the use of low magnetic field mode for processing applications can be found in the literature such as the deposition of carbon nanotubes [42], RF negative ion sources [43] and plasma thruster [44]. The present paper studies the potential of using low-field helicon plasma for a sputtering deposition process of titanium dioxide.
TiO₂ thin films were deposited on silicon wafer substrates by low-field (1 < B < 5 mT) helicon plasma assisted reactive sputtering in a mixture of pure argon and oxygen. The influence of the positive ion density on the substrate and the post-annealing treatment on the films density, refractive index, chemical composition and crystalline structure was analysed by reflectometry, Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD). Amorphous TiO₂ was obtained for ion density on the substrate below 7 × 10¹⁶ m^− 3. Increasing the ion density over 7 × 10¹⁶ m^− 3 led to the formation of nanocrystalline (~ 15 nm) rutile phase TiO₂. The post-annealing treatment of the films in air at 300 °C induced the complete crystallisation of the amorphous films to nanocrystals of anatase (~ 40 nm) while the rutile films shows no significant change meaning that they were already fully crystallised by the plasma process. All these results show an efficient process by low-field helicon plasma sputtering process to fabricate stoichiometric TiO₂ thin films with amorphous or nanocrystalline rutile structure directly from low temperature plasma processing conditions and nanocrystalline anatase structure with a moderate annealing treatment.
Fabrication of carbon nanowalls using electron beam excited plasma-enhanced chemical vapor deposition
2008, Diamond and Related Materials
Carbon nanowalls (CNWs), two-dimensional nanostructured carbon, were synthesized by an electron beam excited plasma-enhanced chemical vapor deposition (EBEP-CVD) employing a mixture of CH₄ and H₂ at a total pressure of 2–4 Pa. High growth rate of 32 nm/min for the synthesis of well-defined, vertically aligned CNW film was attained at the total gas pressure of 4.0 Pa. CNWs were characterized by scanning electron microscopy, transmission electron microscopy, and micro-Raman spectroscopy. The EBEP-CVD proved to have a great potential for fabricating carbon nanostructures.
Complex of Facilities for Studying Interaction of Hydrogen Isotopes with Materials
2023, Fusion Science and Technology
Carbon: The black, the gray and the transparent
2017, Carbon: The Black, the Gray and the Transparent
Controlling parameters determining technological properties of a helicon discharge system
2013, Problems of Atomic Science and Technology
Reactive ion etching of carbon nanowalls
2011, Japanese Journal of Applied Physics

View all citing articles on Scopus

View full text