Conference-ISSS-4-XPS study of ultrathin carbon films prepared by filtered cathodic vacuum arc

Diamond like carbon (DLC) films have attracted increasing attention in a variety of industries due to their excellent mechanical and chemical properties. To meet the demands from the emerging industries, such as micro electro mechanical systems (MEMS), a considerable number of research studies have been conducted to advance the technology for thin DLC films fabrication. However, it has been reported that the properties of DLC films vary with thickness, which necessitate further understanding of the mechanism of initial film growth. To elucidate the mechanism of carbon film growth, the present study employs X-ray photoelectron spectroscopy (XPS) to characterize two types of amorphous carbon films fabricated by cathodic vacuum arc discharge and radio-frequency magnetron sputtering. The results indicated that wide range of energy distribution of carbon ions in the cathodic arc discharge caused a decline in the sp ratio with decreasing thicknesses. [DOI: 10.1380/ejssnt.2006.129]


I. INTRODUCTION
Diamond like carbon (DLC) films have attracted increasing attention in a variety of industries due to their excellent mechanical and chemical properties.To meet the demands from the emerging industries, such as micro electro mechanical systems (MEMS), a considerable number of research studies have been conducted to advance the technology for thin DLC films fabrication.
However, it has been reported that the properties of DLC films vary with thicknesses [1,2].Sundararajan et al. showed that hard carbon overcoats weakened below a thickness of 5nm using nanoindentation.Liu et al. used atomic force microscopy (AFM) to show that the friction coefficient of cathodic arc carbon films increased with decreasing thickness.The former study attributed the decrease in wear-resistance to deformation of the substrate caused by low load-carrying capacity of ultrathin carbon films.The latter study pointed out that the Si content of the carbon films increased with decreasing thickness due to the diffusion from the Si substrate, which increased friction coefficient.
In the present study, we observe sp 3 ratio of DLC films for various thicknesses.The sp 3 ratio is a key parameter in determining carbon quality that can be correlated to the above-mentioned variation in DLC film properties with thickness.The purpose of the present study is to determine the dependence of sp 3 ratio of DLC films on the film thickness.XPS is used to determine sp 3 ratio of DLC films since this technique has been shown to be a powerful tool for characterizing ultrathin films [3] and differentiating sp 3 and sp 2 bonding on the basis of binding energy gap [4,5].Non-hydrogenated DLC films studied were of two types, namely ta-C and a-C, which are deposited by cathodic arc discharge and radio frequency (RF) magnetron sputtering, respectively.The energy gap between the incoming carbon atoms of the two methods will help us to understand the mechanism of how the properties of DLC films vary at small thickness.

II. EXPERIMENTAL
ta-C films were prepared by filtered cathodic vacuum arc (FCVA) deposition.The FCVA system is schematized in Fig. 1.In FCVA, the depositing carbon ions are generated in the arc discharge between the graphite cathode and anode.The carbon ions in the arc discharge have hyperthermal energies (several tens eV), which leads to the formation of hard carbon films.The mechanism of hard film formation is explained by subplantation theory [6].Hyperthermal carbon ions collide with the surface of the growing films and penetrate into the films to a depth of a few nm, where the high compressive stress allows carbon atoms to form sp 3 bonding with surrounding atoms.One shortcoming of deposition by arc discharge is that it generates numerous particulates several microns in size.FCVA was developed to prevent generated particulates from reaching the substrate [7,8].In the present study, an FCVA system made by Nanofilm Technology Institute (NTI) was used.The arc current was 30 A, and base pressure was 8 × 10 −5 Pa.No working gas was introduced during deposition.a-C films were prepared by radio frequency (RF) magnetron sputtering.The graphite target was sputtered by Ar ions at an RF power of 200 W, Ar mass flow rate of 50 sccm, and pressure during sputtering of 0.6 Pa.In sputtering, the depositing atoms have an energy of several eV, lower than that for FCVA [9].Through a comparison of nonhydrogenated DLC films formed by the above two processes, one can verify the influence of ion energy on film quality.
For both a-C and ta-C, Si(100) wafers were used as substrates.The thicknesses of samples were measured by spectroscopic ellipsometry and X-ray fluorescence (XRF).The film quality was assessed by XPS.In the C1s spectra obtained by XPS, there is an energy gap between electrons involved in sp 3 and sp 2 bonding of 0.9±0.1 eV [4,5], which can give the ratio of sp 3 to sp 2 .Information on depth was obtained in XPS by tilting substrate surface, i.e., by varying take-off angle of photoelectrons.For fitting, three Gaussians were used, corresponding to sp 3 , sp 2 , and C-O bonding.While C-O peak can be further deconvoluted to more details, only one Gaussian was used here since the intensity of C-O peaks was small.sp3 ratio, given by sp 3 /(sp 2 +sp 3 ) was used as an indicator of film quality.The spectra measured were calibrated by Au 4f 7/2 (84.0 eV).Thermal evaporation was used to deposit Au through masks on films prior to the measurements.Measurements were conducted by monochromated Al K α at a voltage of 15 kV, a current of 15 mA, and a pass energy of 20 eV.Typical C 1s spectra are shown in Fig. 2 for both ta-C and a-C.

III. RESULTS AND DISCUSSION
This section will show the thicknesses and depth dependence of the sp 3 ratio of ta-C and a-C films, which will form the basis of the discussion of the mechanism of DLC film degradation.
In Fig. 3, sp 3 ratios of DLC films having various thicknesses are shown for both ta-C and a-C films.The spectra were obtained at a take-off angle of 90  showed a characteristic dependence on thickness.The sp 3 ratios of ta-C films declined with decreasing thickness.For a-C films, the sp 3 ratios were independent of thickness.
Figure 4 shows the dependences of the sp 3 ratio, as measured by XPS, on take-off angles, i.e., the angle which the photoelectrons escaping from the surface form with the horizontal axis.The films measured about 20 nm in thickness.When the take-off angles is 90 • , the direction of photoelectrons is normal to the surface.By decreasing the take-off angles, the depth from which photoelectrons can escape from surfaces can be reduced.The figures reveal that for ta-C films, the sp 3 ratio is lower in surface region than in the film body.In contrast, sp 3 ratios of a-C films are constant with the take-off angle.The structure of a-C films seems to be uniform in the depth direction.
To discuss the above observations, the mechanism of film formation needs to be considered.The most significant difference between two non-hydrogenated carbon films used here is the energy of depositing carbon atoms or ions.As mentioned above, carbon ions generated in FCVA system have hyperthermal energies, several tens eV [7,10], much larger than those in RF sputtering.Thus, former ions can create sp 3 bonding through subplantation.That has been a good explanation of the mechanism of how high ratios of sp 3 bonding can be created by FCVA.However, when it comes to ultrathin films, other effects in arc discharge should be taken into account.These are (i) contribution of low-energy ions in FCVA, (ii) insufficient surface compressive stress, and (iii) mixing at the interface with Si substrate.
Let us now discuss these three effects.(i) Contribution of low-energy ions: not all ions have sufficient energy to create sp 3 bonding.Due to the Gaussian distribution of energies [10], depending on the average temperature of arc discharge and the mean number of collisions the ions would experience, there must be some portion of lowenergy carbon ions [1].In the case of both FCVA and RF sputtering, low-energy ions contribute to film growth by simply colliding with the growing surface, moving about, and then stopping without penetration, which can create sp 2 -rich surface layer.(ii) Insufficient compressive stress: calculations by Jaeger et al showed that compressive stress in the vicinity of surface is not high enough to create high sp 3 bonding [11].Thus it is difficult for sp 3 bonding to form in the vicinity of surface.According to the mechanisms discussed here, the carbon films formed by hyperthermal ions inevitably have sp 2 -rich surface layer, whose thickness might be determined by the energy distribution in the arc discharge.Thus, with decreasing thickness, the fraction of the surface layer to the whole thickness will increase, hence decreasing the sp 3 ratio.iii) Mixing with Si at the interfaces [1]: Due to the high energy of carbon ions, thermal spikes caused by the collisions are large compared with the sputtering method.Significantly large number of thermal spikes will enhance the diffusion at interfaces between C and Si, resulting in a C-Si mixing layer which may have a lower sp 3 ratio.
Above all, the wide range of energy distribution of carbon ions in FCVA might lead to the decline in the sp 3 ratio with decreasing thickness.Low-energy ones create sp 2 rich layer on surface while high-energy ones simultaneously induce sp 3 bonding in film body and diffusion between Si and C. In contrast, nothing significant occurred in carbon films fabricated by RF sputtering, which involves lower kinetic energies.

IV. CONCLUSIONS
In this study, we characterize the structural differences between ta-C and a-C films by XPS, and verified that the http://www.sssj.org/ejssnt(J-Stage: http://ejssnt.jstage.jst.go.jp) sp 3 ratio of ta-C films declined with decreasing thickness, while those of a-C films changed little.The mechanism of DLC film formation suggested that the decline in the sp3 ratio was caused by the formation of the graphitic surface layer and subsequent mixing with Si at the interfaces due to the impact of high-energy carbon ions

FIG. 4 :
FIG.4:The dependence of sp 3 ratio on take-off angles.sp 3 ratio of ta-C films declined when measured in surface region, while that of a-C films was kept almost constant.