C. Corbella

Growth of hydrogenated amorphous carbon films in pulsed d.c. methane discharges

Abstract We report the preparation of hydrogenated amorphous carbon (a-C:H) films from asymmetrical bipolar pulsed d.c. methane discharges. The films were deposited at 10 Pa of pressure on c-Si substrates placed onto an electrode powered by a pulsed d.c. generator. The asymmetrical bipolar pulsed d.c. voltage waveform consisted of a fixed positive pulse amplitude of 40 V followed by a variable negative pulse whose peak amplitude was varied from −400 up to −1400 V. Pulse frequencies of 100, 125, 150 and 200 kHz were used at a constant positive pulse time of 2 μs. In addition, a series of a-C:H samples were grown from r.f. capacitive discharges at bias voltages from −200 to −800 V. Pulsed d.c. a-C:H films 1-μm-thick were deposited at growth rates up to 60 nm/min and with internal compressive stress values between 1 and 1.5 GPa. Fourier transform-infrared and Raman analyses revealed the diamond-like character of the films. The effects of pulse parameters on the growth and structural properties of the films are discussed and compared to those of films obtained by conventional r.f. plasma-enhanced chemical vapour deposition.

Effects of gas pressure and r.f. power on the growth and properties of magnetron sputter deposited amorphous carbon thin films

Abstract We discuss the effects of both the electrical power supplied to an r.f. magnetron discharge and the Ar gas pressure on the growth of amorphous carbon (a-C) films on silicon substrates by sputtering of a graphite target. The power applied to the magnetron cathode was provided in a continuous wave mode as well as in a pulsed mode where the amplitude of the r.f. signal was square-wave modulated. In the continuous wave deposition mode the power was varied from 100 to 300 W at a fixed pressure of 0.2 Pa, and the pressure from 0.2 to 2 Pa at a constant power of 300 W. The pulsed mode processes were performed by varying the r.f. peak power from 200 to 400 W at 100 Hz of modulating frequency and 20% of duty cycle. At 0.2 Pa pressure the deposition rate increased from 1 to 6 nm/min with increasing power, and decreased to 2 nm/min as pressure was increased up to 2 Pa. The compressive stress was approximately 3 GPa for films grown at low pressure and low power, and decreased below 1 GPa at higher pressure or power. Raman analysis revealed that increasing pressure favours the growth of a-C films with more disordered sp 2 domains, whereas the increase in r.f. power first leads to a reduction and then to an increase in the number and clustering of sp 2 sites into ordered rings. The friction coefficient measured using a ball-on-disk tribometer ranged between 0.1 and 0.2, being the films deposited at higher power levels that possessed the lowest values. These results were discussed in terms of the effects induced by pressure and power on the energy and flux of the species impinging the film-growing surface.

Plasma parameters of pulsed-dc discharges in methane used to deposit diamondlike carbon films

Here we approximate the plasma kinetics responsible for diamondlike carbon (DLC) depositions that result from pulsed-dc discharges. The DLC films were deposited at room temperature by plasma-enhanced chemical vapor deposition (PECVD) in a methane (CH4) atmosphere at 10 Pa. We compared the plasma characteristics of asymmetric bipolar pulsed-dc discharges at 100 kHz to those produced by a radio frequency (rf) source. The electrical discharges were monitored by a computer-controlled Langmuir probe operating in time-resolved mode. The acquisition system provided the intensity-voltage (I-V) characteristics with a time resolution of 1 μs. This facilitated the discussion of the variation in plasma parameters within a pulse cycle as a function of the pulse waveform and the peak voltage. The electron distribution was clearly divided into high- and low-energy Maxwellian populations of electrons (a bi-Maxwellian population) at the beginning of the negative voltage region of the pulse. We ascribe this to intense stoc...

Comparative study of metal/amorphous-carbon multilayer structures produced by magnetron sputtering

Abstract The present study discusses the structural and mechanical properties of metal/amorphous carbon (a-C) multilayer structures of a-C/Mo/a-C/…/Mo/substrate and a-C/W/a-C/…/W/substrate deposited by magnetron sputtering. To this end, the effects of deposition parameters on multilayered nanometric structures were examined. Samples consisted of structures with alternate metallic and a-C layers with thicknesses in the nanometric range (1–3 nm). The films were deposited on crystalline silicon and glass substrates at room temperature using two opposing magnetron sputtering heads, which allowed the alternate deposition of the metallic and a-C films on the substrates placed on a directional holder. The substrate negative bias voltage was varied between 40 and 300 V and the process was performed at Ar pressures in the range 0.2–2 Pa. The structural and morphological properties and local order of the layers and interfaces were analysed by transmission electron microscopy and X-ray diffraction. Mechanical stress, and critical load for coating failure were measured by profilometry and the microscratch technique, respectively, and the results are discussed in terms of the deposition conditions and the multilayer nanostructure. Potential applications of films based on metal/a-C multilayers include the production of hard, protective, wear-resistant coatings for corrosion-resistant and high temperature-resistant applications.

Modifying surface properties of diamond-like carbon films via nanotexturing

Diamond-like amorphous carbon (DLC) films have been grown by pulsed-dc plasma-enhanced chemical vapour deposition on silicon wafers, which were previously patterned by means of colloidal lithography. The substrate conditioning comprised two steps: first, deposition of a self-assembled monolayer of silica sub-micrometre spheres (~300 nm) on monocrystalline silicon (~5 cm2) by Langmuir–Blodgett technique, which acted as lithography template; second, substrate patterning via ion beam etching (argon) of the colloid samples (550 eV) at different incidence angles. The plasma deposition of a DLC thin film on the nanotextured substrates resulted in hard coatings with distinctly different surface properties compared with planar DLC. Also, in-plane anisotropy was generated depending on the etching angle. The samples were morphologically characterized by scanning electron microscopy and atomic force microscopy. The anisotropy introduced by the texture was evidenced in the surface properties, as shown by the directional dependences of wettability (water contact angle) and friction coefficient. The latter was measured using a nanotribometer and a lateral force microscope. These two techniques showed how the nanopatterns influenced the tribological properties at different scales of load and contact area. This fabrication technique finds applications in the industry of microelectromechanical systems, anisotropic tribological coatings, nanoimprint lithography, microfluidics, photonic crystals, and patterned surfaces for biomedicine.

Anisotropic surface properties of micro/nanostructured a-C:H:F thin films with self-assembly applications

The singular properties of hydrogenated amorphous carbon (a-C:H) thin films deposited by pulsed DC plasma enhanced chemical vapor deposition (PECVD), such as hardness and wear resistance, make it suitable as protective coating with low surface energy for self-assembly applications. In this paper, we designed fluorine-containing a-C:H (a-C:H:F) nanostructured surfaces and we characterized them for self-assembly applications. Sub-micron patterns were generated on silicon through laser lithography while contact angle measurements, nanotribometer, atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to characterize the surface. a-C:H:F properties on lithographied surfaces such as hydrophobicity and friction were improved with the proper relative quantity of CH4 and CHF3 during deposition, resulting in ultrahydrophobic samples and low friction coefficients. Furthermore, these properties were enhanced along the direction of the lithography patterns (in-plane anisotropy). Finally, self...

Ion energy distributions in bipolar pulsed-dc discharges of methane measured at the biased cathode

The ion fluxes and ion energy distributions (IED) corresponding to discharges in methane (CH4) were measured in time-averaged mode with a compact retarding field energy analyser (RFEA). The RFEA was placed on a biased electrode at room temperature, which was powered by either radiofrequency (13.56 MHz) or asymmetric bipolar pulsed-dc (250 kHz) signals. The shape of the resulting IED showed the relevant populations of ions bombarding the cathode at discharge parameters typical in the material processing technology: working pressures ranging from 1 to 10 Pa and cathode bias voltages between 100 and 200 V. High-energy peaks in the IED were detected at low pressures, whereas low-energy populations became progressively dominant at higher pressures. This effect is attributed to the transition from collisionless to collisional regimes of the cathode sheath as the pressure increases. On the other hand, pulsed-dc plasmas showed broader IED than RF discharges. This fact is connected to the different working frequencies and the intense peak voltages (up to 450 V) driven by the pulsed power supply. This work improves our understanding in plasma processes at the cathode level, which are of crucial importance for the growth and processing of materials requiring controlled ion bombardment. Examples of industrial applications with these requirements are plasma cleaning, ion etching processes during fabrication of microelectronic devices and plasma-enhanced chemical vapour deposition of hard coatings (diamond-like carbon, carbides and nitrides).

Properties of W/a-C nanometric multilayers produced by RF-pulsed magnetron sputtering

Abstract Single-layer W and a-C thin films and multilayer W/a-C coatings were deposited by either continuous or pulsed RF magnetron sputtering. The films were deposited on silicon substrates using two opposite RF magnetron sputtering heads that allowed single or alternate deposition of W and a-C. Multilayer films consisted of 15 bilayer structures with bilayer period in the nanometric (8–16 nm) range, whereas single-layer films were between 140 and 380 nm thick. Transmission electron microscopy showed the regular structure of the W/a-C multilayers with well-defined interfaces. X-Ray diffraction measurements revealed the presence of α and β phases of tungsten and provided the bilayer period of the multilayer structures by means of the modified Bragg law. High reflectivity of X-rays was found in the multilayer samples at low incident angles. Multilayer coatings had less stress than single-layer W or a-C films. Nanoindentation measurements indicated an enhancement of hardness and elastic modulus, up to 18 and 191 GPa, respectively, in the multilayer W/a-C coatings as the bilayer thickness decreased to 8 nm. Furthermore, adherence to the substrate was improved compared to a-C single films. All coatings showed low friction coefficients (0.03–0.07).

Tribological Properties of Fluorinated Amorphous Carbon Thin Films

1.1 Amorphous carbon characteristics The peculiar electronic configuration of carbon atoms, 1s2 2s2 2p2, and the small energy difference between their 2p and 2s orbitals, compared to the binding energy of the carbon bonds, allow the electrons to rearrange in s and p mixed orbitals that enhance the binding energy with other atoms. This process is called hybridization and produces three different types of orbitals: sp = s + p, sp2 = s + p + p and sp3 = s + p + p + p. Each different bonding state corresponds to a certain structural arrangement: sp bonding gives rise to chain structures (with two σ bonds and two π bonds), sp2 bonding conforms onto planar structures (three σ bonds and one π bond) and finally sp3 bonding produces tetrahedrical structures (four σ bonds). The p orbitals that form π bonds overlap less than the orbitals forming σ bonds. The reduced overlapping makes π bonds weaker than σ bonds. However, a number of scenarios are possible. Sometimes, as in ethene (C2H4), a σ and π bond combine producing a stronger bond between carbon atoms. This is called a double bond: C=C. Triple bonds consist of a σ bond and two π bonds, as in ethyne (C2H2). Although chemically stronger thanks to double bonds, the mechanical stability obtained with sp2 hybridization in solids is limited, due to the planar geometry. Instead, sp3 hybridization allows the creation of a three dimensional network of σ bonds. Due to this variety of possible bonding configurations, carbon has a number of allotropes: graphene (sheet of sp2 bonded carbons: σ bonds plus delocalized π bonds), carbon nanotubes and fullerenes (graphene sheets rolled over themselves forming cylinders or spheres, respectively), graphite (Bernal stack of graphene sheets), diamond (network of sp3 bonded carbons) and amorphous carbon (cross-linked and non-organized carbon matrix with a mixture of sp2 and sp3 bonds). It is to the modification of the latter with fluorine that this chapter is devoted to. The International Union of Pure an Applied Chemistry (IUPAC) defines amorphous carbon as “A carbon material without long-range crystalline order”. It also states that “Short range order exists, but with deviations of the interatomic distances and/or interbonding angles with respect to the graphite lattice as well as to the diamond lattice.” Depending on the ratio of sp2 and sp3 bonds in the matrix, amorphous carbon (a-C) presents a variety of well-reviewed mechanical properties [Silva, 2003]. Tetrahedral amorphous carbon films (ta-C or TAC) present the highest hardness, with a high degree of sp3 bonding and without hydrogen. It is almost exclusively deposited by filtered cathodic vacuum arc