Plasma treatment effects on hydrogenated amorphous carbon films prepared by plasma-enhanced chemical vapor deposition

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Journal of Physics and Chemistry of Solids 69 (2008) 505–508

Plasma treatment effects on hydrogenated amorphous carbon ﬁlms

prepared by plasma-enhanced chemical vapor deposition

Jun Wu

a

, Ying-Lang Wang

b,c,

, Cheng-Tzu Kuo

a

a_{Department of Materials Science and Engineering, National Chiao-Tung University, Hsin-Chu, Taiwan} b_{Department of Material Science, National University of Tainan, Taiwan}

c

Department of Applied Physics, National Chiayi University, Chiayi, Taiwan

Abstract

When hydrogenated amorphous carbon (a-C:H) films are deposited by a radio frequency (RF; 13.56 MHz) glow discharge system, their properties can be significantly affected by RF power input at deposition. Furthermore, the hydrogen content in the a-C:H films will be decreased in the postdeposition plasma treatment. This study investigated the effects of plasma input at deposition and postdeposition plasma treatment on the resulting film properties of a-C:H films. Atomic force microscopy (AFM) was employed to detect the surface roughness of the plasma-enhanced chemical vapor deposition (PECVD) a-C:H films. Raman spectroscopy was employed to determine the hydrogen concentration as well as the tetrahedral and trigonal bondings associated with C–H bond. The Raman analysis results suggested the occurrence of a higher degree of structural order in the sp2lattice of the well plasma-treated a-C:H films.

Keywords: A. Amorphous materials; A. Semiconductors; A. Thin ﬁlms; B. Plasma deposition

1. Introduction

There has been a growing interest in hydrogenated amorphous carbon (a-C:H) thin films due to their unique properties, such as high hardness and dielectric strength, chemical inertness to both acids and alkaline, transparency for infrared light, and a low coefficient of friction [1]. Hydrogenated amorphous carbon (a-C:H) thin films are applied as protective coating on magnetic recording disks, optical devices, etc. The properties of these alloys are determined by the hybridization of carbon atoms, by the relative concentrations of different bonds, and by the distribution of hydrogen among different types of carbon atoms. Being deposited by radio frequency (RF) plasma from hydrocarbons, the film properties are strongly influenced by the energy of the film forming particles. The energy is, in turn, determined by the deposition parameters. In order to obtain the desired physical properties during deposition, it is important to understand

the relationship between the observed physical properties, the chemical structure, and their correlation with the preparation conditions.

There exist different kinds of carbon bonds in these films: sp3, sp2, and sp1 as well as various C–H bonding configurations[2]. The properties of these films are closely related to the sp2/sp3ratio, the H content, and the degree of short-range order in the structure. In this paper, we will describe the effects of plasma ion energy on the surface roughness and chemical bonding ratio of a-C:H thin films. Atomic force microscopy (AFM) was employed to detect the surface roughness of the plasma-enhanced chemical vapor deposition (PECVD) a-C:H films. The sp2/sp3ratio in the bulk of the films was characterized by Raman spectroscopy. The effect of postplasma treatment on the a-C:H films was also investigated from the evolution of Raman spectra.

2. Experimental

The experiments were performed on a number of amorphous hydrogenated carbon ﬁlms prepared by PECVD. The a-C:H ﬁlms were deposited on silicon

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Corresponding author. Department of Material Science, National University of Tainan, Taiwan. Tel.: +886 6 5051400; fax: +886 6 5051273.

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substrates in a 13.56 MHz RF ethane glow discharge system. The pressure was kept at 6 Torr, while the plasma power was varied from 0.8 to 2.4 kW. The substrate temperature during deposition was kept at 4001, while the postplasma treatment was conducted in situ after the deposition process, with argon ions at RF power of 1.8 kW and 6.0 Torr of chamber pressure. The surface topogra-phies were examined using AFM (DINanoScope III), with the scan size of each AFM image being 10 mm 10 mm. The mean surface roughness (Ra) of the ﬁlms was obtained from AFM scan. The Raman spectra was obtained in the backscattering conﬁguration, and collected using a spectro-meter coupled with an optical microscope. The measure-ment was performed in the 1000–2000 cm 1region by using a single-stage spectrometer (Action SprectraPro 500I). Each sample was illuminated with an ion argon laser of

wavelength 514.5 nm, focused to a 50 mm round spot. A longitudinal optical peak at 1590 cm 1is the character-istic of crystalline graphite and is called the G peak. The shoulder peak at around 1380 cm 1caused by disorder is called the D peak. These two spectral features will provide information on sp2 clustering in the amorphous matrix. The ﬁlm thickness and refractive index were determined by a spectroscopic ellipsometer at a 74.91 incident angle and 633 nm wavelength.

3. Results and discussions

The ion energy at the deposition of PECVD a-C:H films will modify both surface topography and film structure. The modification can be ascribed to numerous phenomena, such as sputtering, implantation, diffusion,

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Fig. 1. Three-dimensional AFM images of PECVD a-C:H ﬁlms deposited at (a) 0.8 kW and (b) 2.4 kW of RF plasma. The scanning size is 10 mm 10 mm. J. Wu et al. / Journal of Physics and Chemistry of Solids 69 (2008) 505–508

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and redeposition effects[3]. AFM measurements have been performed to investigate a more detailed surface topogra-phy.Fig. 1 shows the AFM analysis of surface roughness for a-C:H films deposited at 0.8 and 2.4 kW. AFM result indicates that the surface roughness of the a-C:H films increases with the RF plasma power input during film deposition. Significant roughening of the surface is observed for a-C:H films deposited at 2.4 kW. Fig. 2 illustrates the correlation of mean surface roughness (Ra) and the RF power energy at deposition for PECVD a-C:H films. A gradual increase in the roughness of film surface was observed at RF power between 0.8 and 1.8 kW. Nevertheless, as the RF power at deposition of PECVD a-C:H film is above 1.8 kW, the surface roughness increases at a higher ramp rate. At relatively low ion energy, the surface topography is reconstructed. This gradual increase of surface roughness indicates persistent destruction of chemical bonds, which develops internal surfaces in the clustered carbon network. At too high the RF plasma, when very energetic carbon ions were used, the strong sputtering effects can lead to effective destruction of the carbon network during its formation, and, consequently, mounding roughening at the surface was observed[4,5].

For the a-C:H, both the ﬁlm and the impinging ions are rich in hydrogen. During subsurface accommodation, hydrogen impedes formation of new C–C bonds and so limits the amount of carbon–carbon bonding. Increasing the RF power can break the C–H bonding more easily thus resulting in the decrease in the hydrogen content and a transition from tetrahedral bonding to trigonal bonding. Fig. 3demonstrated the Raman intensity of PECVD a-C:H ﬁlms deposited at various RF plasma power. With increasing RF power, the Raman spectra evolve continu-ously to higher peak intensities, and thus result in a gradual shift of the G line peak position to lower wave numbers. The changes in the shape of the spectra along with the

relative peak intensities, reflect the structural changes in the films, which are mainly due to an increasing sp2 and therefore also the decrease of hydrogen concentration with increasing ion energy [6]. The observed shift of the G line can be ascribed to the modification of disorder and the clusterization of a mainly sp2-bonded carbon network with a high concentration of implanted carbon atoms [7]. Though it was reported elsewhere that ion inhibits the graphitization either directly, through the destruction of the graphitic cluster, or indirectly, through a densification of the matrix structure [8], the decrease of Id/Ig peak intensity ratio with increasing RF plasma for a-C:H films still indicates that the degree of graphitization for the a-C:H films depends strongly on the ion energy chosen for the film deposition.

Fig. 4compares the Raman spectra in the vicinity of the D peak (wave number1380 cm 1) of PECVD a-C:H ﬁlm as-deposited and subjected to in-situ postplasma treatment.

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0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 0.7 0.8 0.9 1.0 1.1 Mean Roughness ( Ra ) RF Power (K Watts)

Fig. 2. Variation of the mean surface roughness of PECVD a-C:H ﬁlms with the RF plasma.

1200 1300 1400 1500 1600 1700

RF power 2.4 KW RF Power 1.8 KW RF Power 1.2 KW

Intensity (Arb. units)

Wavenumber (cm-1₎

Fig. 3. Raman spectra of PECVD a-C:H ﬁlms deposited at (a) 1.2 kW, (b) 1.8 kW, and (c) 2.4 kW of RF plasma power.

1300 1350 1400 1450 1500

As - deposited a-C:H Postplasma - treated a-C:H

Intensity (Arb. units)

Wavenumber (cm-1₎

Fig. 4. Raman spectra of PECVD a-C:H ﬁlms (a) as-deposited and (b) 1.8 kW postplasma-treated.

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Raman spectra showed that the postplasma treatment results in the decrease of the D-line peak intensity, indicating an increase of sp2coordination thereby generat-ing more ordered graphite. Since hydrogen atoms will preferentially be bonded to C (sp3) atoms in a-C:H films [9], the increase of sp2 carbon fraction also implicates the loss of hydrogen. In the postplasma treatment of a-C:H films, the energetic particles persistently destroy the loosely bounded C–H chemical bonds, thus allowing a more stable C–C bonding to be formulated. The observed evolution of the Raman spectra of the postplasma-treated a-C:H films suggests the thermodynamic instability of the as-deposited PECVD C:H films. In the case of postplasmtreated a-C:H films, the amount of unbounded and loosely bounded molecules in the structure can be reduced significantly, and a structure relaxation process can proceed to a thermo-dynamic minimum of energy.

4. Conclusions

In this study, the effects of plasma ion energy on the surface roughness and chemical bonding ratio were investigated. The surface roughness increases with increas-ing RF plasma power. The increase of RF plasma power also leads to the increase of sp2bonding and the decrease of

hydrogen concentration in PECVD a-C:H ﬁlms. The postplasma treatment destroys the loosely bounded C–H chemical bonds, thus reducing the hydrogen content, and helps develop a more stable C–C bonding.

Acknowledgments

The authors gratefully acknowledge the auspices ex-tended by the National Science Council (NSC) of Taiwan. References

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