Carbon material deposition by remote RF plasma beam

Surface and Coatings Technology 180 – 181 (2004) 238–243 Carbon material deposition by remote RF plasma beam B. Mitua, S. Vizireanua, C. Petcua, G. Dinescua,*, M. Dinescua, R. Birjegab, V.S. Teodorescuc a National Institute for Lasers, Plasma and Radiation Physics, Magurele MG-16, Bucharest, Romania b Zecasin S.A., Splaiul Independentei 202, Bucharest, Romania c National Institute for Materials Physics, Magurele, Bucharest, Romania Abstract Hydrogenated and nitrogenated carbon films were obtained by PECVD with carbon species supplied by acetylene gas injected into an argon or argonynitrogen remote plasma generated by an expanding radiofrequency discharge. The properties of carbon material like the nature of bonds, morphology, surface topography, crystallinity are presented. The reaction pathways linking the precursors with the depositing species, as revealed by emission spectroscopy, are discussed. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Remote plasma deposition; Carbon material; Hydrogenatedynitrogenated carbon films; Emission spectroscopy 1. Introduction Thin films and structured carbon materials are of large interest for various industrial fields w1x. The involvement of these materials in applications as anti-scratching protective coatings, anti-friction and anti-sticking layers, in optical and medical devices, thermal management of high density microelectronics components, asks for better properties such as thermal and chemical stability, wear resistance w2x, transparency w3x, conductivity, biocompatibility w4x, etc. Consequently, the deposition and characterization of films and structured carbon is an intensive research field. Particularly, plasma methods, due to their versatility in managing the deposition conditions, are largely investigated w5–7x. In this paper results concerning properties and the deposition process of hydrogenated and nitrogenated carbon thin films and structures using a remote radiofrequency (RF) plasma system are presented. The peculiarity of the deposition system w8,9x is that the discharge and deposition regions are spatially separated and material growth occurs downstream, being sustained by species transported along the plasma flow. The discharge was generated in argon, nitrogen or their mixture, while the carbon species were either from gas (acetylene *Corresponding author. Tel.: q40-21-4574470; fax: q40-214574243. E-mail address: [email protected] (G. Dinescu). injected in the remote plasma) or solid surfaces (graphite electrodes). The grown material was studied by specific techniques: infrared spectroscopy (FTIR) for unraveling the chemical bonds, AFM for surface topography, SEM for material morphology, X-ray diffraction (XRD) for structure. As the nitrogenated material produced from graphite electrodes in nitrogen plasma beam was studied in detail elsewhere w10x we focused on deposition from gaseous precursors. The reaction plasma pathways leading to deposition were investigated by optical emission spectroscopy (OES) technique. The main radiative species assisting the deposition were identified. The correlation between dependencies of the deposition rates and emission intensities upon experimental parameters (gas composition, power) are discussed in term of radicals contributing to deposition process. 2. Experimental The experimental set-up used for the deposition of carbon containing thin films is presented in Fig. 1. It consists in a double chamber plasma beam system w9x sustained by a RF (13.56 MHz) power supply. In the active chamber, the discharge is generated in flowing gas (argon, nitrogen) between two parallel electrodes (16 mm diameter). It expands into the deposition chamber as a plasma beam through an aperture (;2 mm 0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.10.147 B. Mitu et al. / Surface and Coatings Technology 180 – 181 (2004) 238–243 239 Fig. 1. The experimental set-up. diameter) performed in the bottom electrode, which acts as nozzle. During the plasma processing the deposition chamber (20 cm diameter, 120 cm length) is continuously pumped by a tandem consisting of a mechanical rotary pump and a Roots pump. The pressure in the reactor is established in the range of 0.1–2 mbar by the balance between the input flow value and the pumping speed. For deposition of hydrogenated carbon, the discharge was fed with argon at flow rates of 100–1000 sccm (sccm, standard cubic centimeters per minute), while for the deposition of hydrogenatedynitrogenated carbon, the discharge was operated either in nitrogen or argony nitrogen mixture. Two different techniques can be used for the introduction of carbon species in the discharge. First, as usual in plasma assisted CVD techniques, the remote plasma was fed with acetylene gas injected at 1 cm downstream the nozzle, at flows between 1 and 20 sccm, leading to hydrogenated deposits. Second, the nitrogen containing discharge was operated with graphite electrodes in order to obtain non-hydrogenated nitrogenated carbon material w10x. Si(1 0 0) polished wafers, transparent to IR radiation, were used as substrates. Their temperature was set in the range 50–500 8C. The film properties were investigated by several techniques. The absorption in the infrared region 400– 4000 cmy1 (FTIR) was measured over 32 average scans using a Nicolet Magna 550 apparatus, with a set resolution of 2 cmy1. The film structure and morphology were investigated by scanning electron microscopy technique, using a JEOL-200 CX microscope. The SIMS investigation was performed by means of a VEGASIM (VG SIMS LAB) instrument using Csq ions with an incident beam spot size 100=100 mm2. The XRD spectra were recorded with a diffractometer (model DRON3) using a Co Ka filtered radiation (lCo Kas ˚ under the following conditions: acceleration 1.7902 A) voltage 30 kV, current 30 mA. A step scanning mode Fig. 2. Carbon in depth profile of a film obtained at 150 W, 50 8C, in steps of 10 min duration and variable gas composition. was applied in the range 2uCo Kas5–708 with a step of 0.18 for 5 s at each step. OES was used to identify the plasma species formed in the discharge. The plasma optical radiation was focused by a quartz lens and directed via a quartz optical fiber to a grating monochromator (SPM-2, 1200 lines per mm). A photomultiplier–amplifier–acquisition system was used to detect and record the incoming light. 3. Results and discussions The deposition rates were calculated from profilometry measurements. Deposition rates of nitrogenated carbon films obtained from graphite electrodes were in the range of 0.3–2 nm sy1 w10x. In the case of acetylene fed deposition, larger rate values, in the range of 1–20 nm sy1, were found. The rate increases with the acetylene flow, the applied RF power (50–300 W) and decreases with the distance. The higher rates obtained in the acetylene fed experiments show that the deposition rate of nitrogenated carbon from graphite surfaces is limited by the carbon availability. Even higher deposition rates were reported for remote deposition of carbon layers from acetylene, as example ;100 nm sy1 in thermal arc remote plasma w11x, but at larger power (1–5 kW) and larger gas flows (6000 sccm argon, 600 sccm acetylene). The material quality and deposition rates depend on the experimental conditions. As example, a carbon film was continuously deposited for 30 min at 100 W RF power, 50 8C substrate temperature, 1 mbar pressure, but with step variation of gas composition (AryN2 y C2H2 in sccm) at every 10 min, as follows: first gas fed composition was 500y2y2, second composition was 1000y2y2 and third composition was 1500y2y2. The 240 B. Mitu et al. / Surface and Coatings Technology 180 – 181 (2004) 238–243 absorption band in the region 3000–3100 cmy1 (which is related to the stretching vibrations modes of sp2 C–H bonds), one can found an indication of a preferential bonding of the carbon in C_ C_ C ambient rather than C_ CH2. For some of the deposited samples, an absorption band centered on 3300 cmy1 and one approximately 2200 cmy1 are also present. The assignment of these peaks is not straightforward, since they can be correlated either to sp C–H and C^ C bonds, respectively, or to N–H and C^ N bonds w6x. However, the peak at 2200 cmy1 is a clear indication of the existence of C atoms in sp hybridization, as well. This can be detrimental in respect to hardness properties of the deposit. Fig. 3. FTIR spectrum obtained from a film deposited at 100 W, 500 8C, mass flow rate ratios (sccm) of AryN2 yC2 H2 : 1000y2y1, pressure 0.6 mbar. in-depth distribution of the carbon atoms, as observed by SIMS technique is presented in Fig. 2, and reflects the change of gas composition. The lines show the averaged relative concentration of carbon for the respective steps. Considering that the profiling does not depend much on the material quality we conclude that the deposition rate (range of 1–3 nm sy1) increases and the relative carbon content decreases with the increase of acetyleneyargon ratio in the fed gas. 3.1. Material characterization 3.1.1. Infrared absorption spectroscopy (FTIR) In Fig. 3 an FTIR absorption spectrum of a sample deposited at 100 W RF applied power at a precursor ratio AryN2yC2H2 equal to 1000y2y1 is presented. The bands at 1376 and 1453 cmy1 have been attributed to C–CH3 symmetric and asymmetric deformation, respectively, according to w12x. The presence in the film of carbon atoms in sp3 hybridization is reinforced by the absorption features in the region 2800–3000 cmy1, where the peaks at 2872 and 2955 cmy1 correspond to sp3 CH3 symmetric and asymmetric stretching vibration, while the one at 2926 cmy1 is related to sp3 CH2 asymmetric stretching w13x. The broad band between 1500 and 1750 cmy1 is related to the conjugated C_ C double bonds in various configurations (e.g. the graphitic G Raman band approximately 1570 cmy1 that becomes IR active due to the incorporation of nitrogen in the film w14x and the C_ C stretching vibration at 1630 cmy1). It proves the existence of sp2 C atoms in the deposited film. Also, C_ N absorption band can contribute to the above mentioned band in the region 1500–1600 cmy1. By correlating the strong band at approximately 1600 cmy1 with the absence of any 3.1.2. Film morphology and surface topography Depending on the plasma conditions various film morphologies and surface topographies were obtained from smooth films to particulates. We will focus on the films for which the FTIR spectra were presented in previous section. In Fig. 4a an SEM image of a film deposited in the conditions: RF power 100 W, gas composition AryN2 yC2H2: 1000y2y1 (sccm), pressure 6=10y1 mbar, substrate temperature ;500 8C. Apparently, the morphology is granular. Nevertheless a fracture in the film, as shown in the inset of Fig. 4 reveals rather columnar morphology, with columns of 150–300 nm in diameter. In addition, the surface of a sample deposited in similar conditions was investigated by the AFM technique, in contact mode. The AFM image is presented in Fig. 5 and supports the SEM results. The surface roughness was calculated to be Rrmss11.5 nm. The peculiarity of the deposition at the above mentioned parameters is the presence of particulates on some of the film zones. Images of such particulates are presented in Fig. 6. They are carbon microstructures like cauliflower showing morphology similar to the film. Fig. 4. SEM image of a film deposited at 100 W, 500 8C, mass flow rates (sccm) of AryN2yC2H2: 1000y2y1, pressure 0.6 mbar. B. Mitu et al. / Surface and Coatings Technology 180 – 181 (2004) 238–243 241 Fig. 5. AFM image of a film deposited at 100 W, 500 8C, mass flow rates (sccm) of AryN2yC2H2: 1000y2y1, pressure 0.6 mbar. Their aspect and placement with respect to the film indicate that their nucleation is independent on the substrate surface. Possible, particles removed by sputtering or erosion from the stainless steel electrodes provided the nucleation seeds for these carbon microstructures. 3.1.3. X-ray diffraction The XRD patterns (Fig. 7) assigned for the formation of amorphous carbon matrix (very broad peak extending between 2uCos308–508). For deposition in the conditions RF power 100 W, gas composition AryN2 yC2H2: 1000y2y1 (sccm), pressure 6=10y1 mbar, substrate temperature ;500 8C the carbon graphite (0 0 2) peak could be also detected, accounting for the formation of carbon crystalline nanoparticles embedded in the amor˚ and the phous matrix. The d0 0 2 spacing is 3.34 A ˚ The average crystallite (grain) size Lc is only 85 A. average crystalline size was estimated according to Scherrer formula. It describes the broadening of (0 0 2) peak due to small constituent crystallites (grains) along c direction or stating in other words to lower average thickness of parallel pseudographitic layer if a layered structure for the carbon nanoparticles is assumed. The presence of only (0 0 2) peak suggests a columnar growing morphology with columns oriented perpendicularly to the substrate surface, as SEM results suggested. However, the absence of either (h k o) or other (h k l) reflections, which would have much smaller intensities for nanotubes as well as for graphite makes uncertain from this XRD pattern to discriminate between graphite and other forms of structured carbon. Fig. 6. SEM image of particulates grown at 100 W, 500 8C, mass flow rate ratios (sccm) of AryN2yC2H2: 1000y2y1, pressure 0.6 mbar. shown in Fig. 8. The spectrum exhibits the molecular bands of CH(A2DyX2P, Dvs0) and C2(d3Pgya3Pu) Swan spectral systems with the band heads at 473.7 nm (Dvsq1), 516.5 nm (Dvs0) and 563.5 nm (Dvsy 1). In addition, the emission of the CN Violet (B28qy X28q) system, Dvs0 sequence with the (0, 0) band head at 388.3 nm was observed. The nitrogen bands of the second positive system N2(C3PuyB3Pg, Dvs0, "1, "2) and in a lesser extent of the first negative system N2q(B28uyX28q g , Dvs0) were also present. The formation of emitting species is explained by the energetic conditions of the electron population in our plasma. The electron temperature in the plasma in the proximity of the injection point is approximately 2 eV w1,3x and the high-energy electrons from the tail of 3.2. Reaction pathways from the precursor gas to the depositing species The addition of C2H2 into the remote AryN2 plasma, leads to deposition of hydro-nitrogenated carbon material, as shown above. A typical emission spectrum is Fig. 7. X-ray diffractogram obtained from a carbon film deposited at 100 W, 400 8C, mass flow rate ratios (sccm) of AryN2yC2H2: 1000y2y1, pressure 0.6 mbar. 242 B. Mitu et al. / Surface and Coatings Technology 180 – 181 (2004) 238–243 A parametric study of the bands intensity on the precursor flows and on the applied RF power was performed. In Fig. 9 left and right the dependencies of the band intensities upon C2H2 flow and RF power are shown. An increase of the bands is observed with tendency of saturation at high C2H2 flow and high RF values. This behavior may be explained by the quenching of the excited species due to the increase of the radicals density and, respectively, due to the limited amount of acetylene available for dissociation. Along the flow axis a decrease of the emission is observed. Taking into account that the deposition rate decreases with distance as well, and increases with tendency to saturation with applied RF power and acetylene flow, these features suggest that the radicals observed contribute to the growing process. Fig. 8. Emission spectrum of a remote plasma generated at 100 W, in AryN2yC2H2 (mass flow rate ratios 1000y2y2) at pressure 0.6 mbar and 4.5 cm from the injection point. distribution function dissociate and ionize directly the C2H2 molecules by collisional processes. The C2H2q ions dissociate in a second step by very fast dissociative recombination reaction with slower electrons producing CH and C2 radicals w15x. N atoms are formed in similar processes as CH radicals but starting from N2 molecules. Further, the CN radical is formed by reactive processes of N atoms with CxHy radicals w16x. An additional contribution to dissociation comes from the Penning processes with Ar ions and N ions w17x. As concerning the excitation of the formed radicals, they are partially produced directly in the excited states by the above reactions or are excited afterwards by energy transfer and electronic collisions. Mainly at larger distances the excitation can be provided by electrons with low energies, as all CN, CH and C2 bands are low lying states with excitation thresholds of only 2–3 eV. 4. Conclusions Carbon materials were downstream deposited by a remote RF plasma beam from acetylene precursor injected in argonynitrogen flowing plasma. Deposition rates in the range 1–20 nm sy1 were obtained. The material is hydrogenatedynitrogenated with a large degree of sp3 bonds. The growing morphology is columnar, with columns diameter in the range of 150–300 nm. Smooth films and particulates with morphology similar to the films were grown. The deposited material is mainly amorphous, but crystalline nanoparticles embedded in an amorphous matrix are obtained, as well. 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