ta-C Thin Films Research Articles

Nanotribological and nanomechanical properties of 5–80 nm tetrahedral amorphous carbon films on silicon

Nanoscratch testing has been used to investigate the tribological behaviour of 5, 20, 60 and 80 nm tetrahedral amorphous carbon (ta-C) thin films deposited on silicon by the filtered cathodic vacuum arc method. The nanoscratch behaviour of the films was found to depend on the film thickness, with 60 and 80 nm films undergoing border cracking and then at higher critical load a dramatic delamination event. 5 and 20 nm films have a lower critical load for onset of border cracks but do not undergo a clear dramatic failure, and instead are increasingly worn/ploughed through until film removal as confirmed by microscopic analysis. This is consistent with the thinner films having lower stress and reduced load-carrying ability. Nanoindentation confirms that the thicker films have enhanced load support and higher measured composite (film+substrate) hardness. The 80 nm film in particular can retain appreciable load support whilst deformed during indentation, as shown by its ability to alter the critical loads for contact-induced phase transformations in the Silicon substrate during unloading.

Surface and interface morphology and structure of amorphous carbon thin and multilayer films

We review the implementation of X-ray reflection (reflectivity and scattering) techniques for the study of amorphous Carbon (a-C, a-C:H, ta-C) thin and multilayer films and in particular in the determination of the film density and surface and interface morphology, which are intrinsically significant for ultra-thin films. We present studies of various a-C and a-C:H films, which include in particular: i) the morphology of a-C/Si interface, ii) the surface morphology and density evolution during sputter growth of a-C, iii) the morphology of the sp 2-rich a-C/sp 3-rich a-C interfaces in multilayer a-C films, iv) the universal correlation between the film density and the refractive index of a-C and a-C:H films. We also compare and validate the experimental results with relative results from Monte-Carlo simulations within an empirical potential scheme. The computational results shed light on the atomistic mechanisms determining the structure and morphology of the a-C interfaces between individual sp 2- and sp 3-rich a-C layers and between a-C and Si substrates.

Application of aluminum oxide and ta-C thin films deposited at room temperature by PLD in RF-MEMS fabrication

Aluminium oxide and tetrahedral amorphous carbon thin films are deposited at room temperature using pulsed laser deposition (PLD) and their mechanical and electrical properties are investigated. As examples, the hardness of aluminium oxide films is found about 6.8 GPa and the Young's modulus 130 GPa, while the hardness of tetrahedral amorphous carbon (ta-C) layers reaches 45 GPa and its Young's modulus 650 GPa. These results allow considering the introduction of these materials in the fabrication of radio-frequency micro-electro-mechanical system (RF-MEMS). Aluminium oxide is used in RF-MEMS fabrication as well as dielectric material or structural material and nickel-doped ta-C allows the realization of localised, high value, planar, easily patterned resistances, leading to significant improvement of insertion losses of MEMS switches on electronic devices. At present, the use of ta-C thin films as structural material in RF-MEMS is limited by their high internal compressive stress.

Mechanical property optimisation of materials by nanostructuring: reduction of residual compressive stress in ta-C film

Optimisation of the mechanical properties of a tetrahedral amorphous carbon (ta-C) thin film was attempted by nanostructuring. In this study, nanoscale multilayer films of ta-C and Si incorporated ta-C:Si layers were prepared. The residual compressive stress of the multilayer films decreased drastically without a decrease in the hardness and elastic modulus, compared with the monolithic films. No significant change in the atomic bond structure was observed in Raman spectral analysis. The present results thus provide a new possibility of application of the ta-C film in harsh environments.

Evaluating the fracture properties and fatigue wear of tetrahedral amorphous carbon films on silicon by nano-impact testing

A repetitive contact technique, nano-impact testing, has been used to investigate the fracture properties of tetrahedral amorphous carbon (ta-C) thin films deposited on silicon by the filtered cathodic vacuum arc method. The impact test has shown clear differences in the resistance to impact wear of ta-C films with their thickness, for film thicknesses between 5 and 80 nm. The resistance to impact-induced fracture decreases as the film thickness increases. This may be due to the maximum shear stress being closer to the film–substrate interface, or reflect reduced toughness in the thicker films that are less able to deform as the substrate deforms plastically during the repetitive contact test. The mechanism of impact-induced failure on these thin films is (i) the initial impact stage where plastic deformation causes cracks to nucleate sub-surface, (ii) fatigue—further nucleation and growth of sub-surface cracks (with little or no change in probe depth) and (iii) crack coalescence and film fracture leading to a rapid change in probe depth as the film fails. The influence of impact load on fracture probability has been investigated. Fracture probability increases sharply as the impact load is increased from 100 to 300 μN. The greater load provides the stresses necessary to nucleate and propagate the sub-surface cracks; at low load the driving force for the cracks to coalesce and spall the coating is much reduced, so failure is less likely to occur within the test duration.

Friction and wear at nanometer scale: a comparative study of hard carbon films

Tribological properties of nanostructured carbon (ns-C) and tetrahedral amorphous carbon (ta-C) thin films were investigated by friction force microscopy. It was found that the ns-C films have a smaller friction coefficient than ta-C films for relative humidity greater than 30%. In particular, at 40% of humidity, ns-C films have lower friction coefficient (0.11±0.02) than the ta-C films (0.13±0.02), which can be attributed to both the presence of closed graphite nanoparticles and the passivation of the dangling bonds at the ns-C surface. The friction coefficient did not vary as a function of the tip scanning velocity for both films. The nanoscale wear was studied in a very low force regime, in the range of nanonewton, using an atomic force microscope (AFM) with a Si 3N 4 tip and with forces in the range of micronewton with the AFM equipped with a stainless still cantilever and a diamond tip. The ns-C provides better wear resistance compared to ta-C films in the range of forces studied. The sp 2-rich ta-C surface layer was easily scratched during the wear test in contrast to the ns-C films. The wear in ta-C in the low forces regime is attributed to the presence of this low density layer at the surface of the film due to subplantation of energetic ions during deposition while the better resistance to wear of ns-C films is attributed to its highly elastic nature.

Correlation of surface, mechanical and microproperties of tetrahedral amorphous carbon films deposited under different magnetic confinement conditions

The effect of using a magnetic field to confine and focus the carbon plasma in a filtered cathodic vacuum arc (FCVA) deposition system was investigated in the preparation of tetrahedrally bonded amorphous carbon (ta-C) thin films. The design of the magnetic field was such that the plasma can be confined into a high-density focussed spot of ∼2 cm diameter or de-focussed into a wide beam of ∼10 cm diameter. The microstructural, optical, tribological and surface properties of the ta-C films grown in the high-density magnetic field were subsequently studied in detail. Under a high-density magnetic field, ta-C thin films were deposited on Si and quartz substrates. Laser-induced surface acoustic wave (L-SAW) measurements confirmed an increase in the Young’s Modulus and the bulk density of these films as compared to ta-C films deposited under no or low magnetic field [Phys. Rev. B 48 (1993) 4777; J. Appl. Phys. 79 (1996) 7239]. XPS results showed a high >85% sp 3 content on the surface while combined EELS and Raman measurements showed high and constant sp 3 content of >85% in bulk of all deposited films. An increase in optical bandgap was observed, from 3.6 to 3.9 eV as the density of the plasma (and hence, the films) was increased. In addition, the overall surface free energy was also observed to decrease from 44 to 40 dyn/cm. This confirms that under a high external magnetic field, the carbon plasma is confined and focussed, and is thus able to deposit highly densified ta-C thin films with high optical bandgap and low sp 2 defect density. Tribological measurements showed that by comparing the high-density ta-C films with lower density ta-C films, the former have a better adhesion to the substrate, as well as a better coefficient of friction and wear rate.

Low temperature annealing in tetrahedral amorphous carbon thin films observed by13CNMR spectroscopy

For the first time to our knowledge, the 1 3 C solid-state magic angle spinning (MAS) NMR spectrum of a 99% 1 3 C enriched tetrahedral amorphous-carbon (ta-C) thin film containing a high concentration of fourfold coordinated carbon species (82%) is reported along with measured NMR spectra for the ta-C film after low temperature annealing (650 °C). Differential changes are observed for the 1 3 C MAS NMR chemical shifts and linewidths of both the fourfold (diamondlike) and threefold (graphitelike) coordinated carbon species within the thin films with increasing annealing time; however, there was no change (′2%) in the relative fourfold content. These spectral changes are associated with the large compressive stress reduction (6-8 GPa) in the carbon film. Ab initio calculations of the 1 3 C NMR chemical shift, along with shift variations as a function of atomic volume are reported for amorphous carbon and crystalline diamond. Using the observed spectral variations in the solid-state 1 3 C MAS NMR, along with the ab initio chemical shift calculations, the effect of annealing on the ta-C films is discussed and related to current models of thermal stress relaxation in ta-C thin films.

Studying the high-field electron conduction of tetrahedral amorphous carbon thin films by conducting atomic force microscopy

The high-field electron conduction of tetrahedral amorphous carbon (ta-C) thin films substrate has been studied using a conducting atomic force microscope (C-AFM). The ta-C thin films with a high concentration of sp 3 bonding (80–90%) were deposited on Si by field arc deposition (FAD). The high-field “conductance” and surface morphology were mapped simultaneously. At low bias, the “conductance” exhibits inhomogeneities on a large scale, presumably due to thickness variations or interface defects. However, at high bias, the small difference in “conductance” due to thickness variations or interface defects was buried by the high intrinsic “conductivity.” It has also been shown that high field causes electric breakdown in these films by converting sp 3 bonding to sp 2 at high electric field.

TETRAHEDRAL AMORPHOUS CARBON (Ta-C) ULTRA THIN FILMS FOR SLIDER OVERCOAT APPLICATION

Tetrahedral Amorphous Carbon (ta-C) thin film by using Filtered Cathodic Vacuum Arc (FCVA) technique has proven to be wear-resistive and corrosion resistant for a wide range of electrical, optical, and mechanical applications. Many investigations have shown that the ta-C film prepared by the FCVA technique can provide a superior ultra thin overcoat for the sliders and media compared to ECR-CVD and IBD coating technology. The ta-C film excels in terms of the film density, hardness, surface roughness and corrosion resistance. Nanofilm Technology International (NTI) has successfully developed and commercialized the FCVA coating system (FS series) for the slider overcoat application, which provides a good quality film with a high hardness (~50 GPa), low stress (2~3 GPa), low macro-particle density (~1/cm2 for particles > 0.3 μm), good uniformity (< 4%$ in 8 inch coating area) and high production repeatability (< 5%).

EFFECT OF FOCUSSED VACUUM ARC PLASMA DEPOSITION ON THE PROPERTIES OF TETRAHEDRAL AMORPHOUS CARBON FILMS

We have investigated the effect of using a magnetic field to confine and focus the plasma in a Filtered Cathodic Vacuum Arc (FCVA) deposition system used for the preparation of tetrahedrally bonded amorphous carbon (ta-C) thin films. The design of the magnetic field is such that the plasma can be confined into a high-density focussed spot or de-focussed into a lower density wide beam. Increasing the magnetic field directly increases the plasma density and thus increases the deposition rate. The ta-C films grown in the magnetic field were subsequently characterised. EELS and Raman measurement were used to measure the sp3/sp2 ratio and UV-vis spectroscopy for optical bandgap studies. The intrinsic stress and I-V characteristics of the thin films were also studied. The results show that it is possible to deposit the films at rates as high as 2.5 nm/sec without adversely affecting the material properties.

Engineering properties of fully sp3- to sp2-bonded carbon films and their modifications after post-growth ion irradiation

Amorphous carbon (a-C) and tetrahedral a-C (ta-C) thin films deposited by e-beam evaporation, magnetron sputtering and cathodic vacuum arc were studied. The techniques applied here produce films exhibiting a wide range of density (1.75–3.23 g/cm3), sp3 content (∼5–90%) and hardness (∼2–45 GPa). The films hardness (H) and elastic modulus were studied by nano-indentation employing continuous stiffness measurements (CSM) and a modelling procedure to subtract the substrate contribution. X-Ray reflectivity (XRR) and spectroscopic ellipsometry investigated the film density (ρ) and sp3 content, respectively. The combined CSM and XRR results indicate a linear correlation between H and ρ. Their dependence is expressed in terms of the binding energy, the mean atomic spacing, the covalency and parameters defining the deformation characteristics. An upper limit of ∼70 GPa was determined for the hardness of fully sp3-bonded carbon, considerably lower than that of crystalline diamond. The films were irradiated by post-growth low energy (1.5–4 keV) Ar+ ion beam. The mechanical performance of ta-C films degraded while an overall increase of the hardness of sp2-rich films was observed after ion irradiation.

Approach for a self-assembled thin film edge field emitter

A self-assembled thin film edge emitter has been fabricated. The unique fabrication process requires only a tetrahedral amorphous carbon (ta-C) thin film with one photolithography step to generate a three-dimensional structure. A high emission site density was achieved compared with that obtained from a flat ta-C film emitter using the same measurement technique. In the high field regime the emission obtained was based on the Fowler–Nordheim emission mechanism. Using the Fowler–Nordheim equation we calculated the field enhancement factor of this emitter to be 80. The potential of ta-C thin films as hard masks for silicon etching in KOH solution was also investigated.

Study of the mechanical properties of tetrahedral amorphous carbon films by nanoindentation and nanowear measurements

Abstract Nanoindentation and nanowear measurements, along with the associated analysis suitable for the mechanical characterization of tetrahedral amorphous carbon (ta-C) films are discussed in this paper. Films of approximately 100-nm thick were deposited on silicon substrates at room temperature in a filtered cathodic vacuum arc evaporation system with an improved S-bend filter that yields films with high values of mass density (3.2 g/cm3) and sp3 content (84–88%) when operating in a broad bias voltage range (−20 V to −350 V). Nanoindentation measurements were carried out on the films with a Berkovich diamond indenter applying loads in the 100 μN–2 mN range, leading to maximum penetration depths between 10 and 60 nm. In this measurement range, the ta-C thin-films present a basically elastic behavior with high hardness (45 GPa) and high Young's modulus (340 GPa) values. Due to the low thickness of the films and the shallow penetration depths involved in the measurement, the substrate influence must be taken into account and the area function of the indenter should be accurately calibrated for determination of both hardness and Young's modulus. Moreover, nanowear measurements were performed on the films with a sharp diamond tip using multiple scans over an area of 3 μm2, producing a progressive wear crater with well-defined depth which shows an increasing linear dependence with the number of scans. The wear resistance at nanometric scale is found to be a function of the film hardness.

CHARACTERIZATION OF ta-C AND GRANULAR Co-C FILMS PREPARED BY PULSED FILTERED VACUUM ARC DEPOSITION

A pulsed filtered vacuum are deposition system was used to prepare ta-C thin films and granular Co-C films. The ta-C films prepared at various substrate bias voltages were characterized using Raman spectroscopy and spectroscopic ellipsometry of which the results confirmed that these ta-C films exhibit high sp 3 fraction of over 80%. The composition of the granular Co-C films prepared by the same system was determined by non-Rutherford backscattering spectrometry. The properties of these Co-C Films, as deposited and after vacuum annealing at various temperatures, were studied using Raman spectroscopy, electrical measurements, magnetic measurements by a SQUID magnetometer, atomic force microscopy and magnetic force microscopy. It was found that the dependence of the Raman spectra of these films on annealing temperature was associated with the formation and dissociation of a cobalt carbide phase and the graphitization of amorphous carbon. The magnetic properties showed complicated composition and annealing temperature dependence. The optimum annealing temperature for the maximum coercivity was found to depend on the composition of the film. For a film of Co 65 C 35 after annealing at 623K in vacuum for one hour, the coercivity was measured to be 460 Oe at 300K and 1380 Oe at 3K. Clear MFM images of the domain structures were observed for films after annealing at sufficiently high temperature, showing that there was perpendicular magnetic anisotropy in these films. A nearly-temperature-independent electrical resistance in the range from 20K to 300K was also observed. A more detailed analysis indicated that the low temperature electrical transport is consistent with a theory for granular metal films.

Thin film TiC/TaC thermocouples

TiC and TaC thin films were investigated, for the first time, for thin film thermocouple applications. Thin films of TaC and TiC were deposited on electronic grade alumina substrates using the r.f. sputter deposition technique. Sheet resistance of the thin films was measured using a four point probe. It was observed that the sheet resistance of the films depends critically on the deposition parameters such as substrate temperature during deposition, sputter gas pressure and r.f. power used. The deposition parameters were optimized to yield the lowest sheet resistance of the thin films at room temperature. The thermoemf of the deposited films was measured as a function of temperature in a vacuum using a home made device. It was observed that thin films of TaC and TiC yield fairly high and stable thermoemf throug hout the temperature range of stability. Under the optimized deposition conditions, thin film thermocouples were fabricated. The thermocouples were calibrated against temperature and the output was measured in vacuum (pressure <10−6 Torr). TiC/TaC thermocouples yield stable output up to 1350 K, temperatures above which breakdown occurs. The thermocouple output was theoretically estimated from the thermoemf measured, and compared. It was observed that these thermocouples yield reproducible output in the temperature range of stability and hold excellent potential for high temperature thin film temperature sensor applications in vacuum or inert atmospheres.

Optical properties of tetrahedral amorphous carbon films determined by spectroscopic ellipsometry

The optical and microstructural properties of tetrahedral amorphous carbon (ta-C) thin films prepared by filtered cathodic vacuum arc (FCVA) have been investigated as a function of the ion energy. The films were characterized by spectroscopic phase modulated ellipsometry (SPME) for the first time. The optical property of the ta-C layer was derived from the Forouhi and Bloomer amorphous semiconductor model. An effective medium approximation and a linear regression analysis have been used to determine the microstructure of thin ta-C films on silicon. The microstructure of these films deposited on silicon wafers was simulated by a four-layer model consisting of a roughness layer, a ta-C layer, a graded ta-C:Si layer and the silicon substrate. The graded layer consisting of the mixture of ta-C and silicon simulates the carbon ion impinging/diffusion into the surface of the silicon substrate. The complex refraction index, N= n−ik over the range of 250–900 nm, for ta-C had been determined. The Tauc (optical) band gap as a function of ion energy was also studied.

Direct observation of sp3 bonding in tetrahedral amorphous carbon using ultraviolet Raman spectroscopy

The vibrational modes of the sp3 sites in tetrahedral amorphous carbon (ta-C) thin films are revealed directly using ultraviolet Raman spectroscopy at 244 nm excitation and are shown to produce a Raman peak centered around 1100 cm−1. In addition, the main Raman peak associated with sp2 vibrational modes is shifted upward in frequency by 100 cm−1 relative to its position in spectra excited at 514 nm. The spectra are interpreted in terms of the bonding in ta-C.

The structure of tetrahedral amorphous carbon thin films

Tetrahedral amorphous carbon (ta-C) thin films were deposited using both the filtered cathodic vacuum arc (FCVA) and the pulsed laser arc (PLA) deposition techniques. The FCVA deposited films are analysed as a function of ion energy of the deposition species and substrate temperature. The films deposited using FCVA as a function of temperature are compared with those deposited using the PLA technique. The microscopic structure of the films is examined using electron microscopy. The density, bonding hybridization and chemical composition are obtained by electron energy loss spectroscopy (EELS). The data for the diamond-like sp 3 bonding component in the ta-C films clearly show a peak which is predicted by the subplantation models proposed for the growth of amorphous carbon films. The structural modifications observed as a function of temperature can also be explained using the subplantation model. Under certain deposition conditions used in this study, nanocrystallites are observed embedded in the amorphous carbon matrix. The chemical composition of these crystallites is carbon. The microstructure of the deposited crystals is analysed in terms of lattice spacing and is indexed to graphite and cubic diamond.

Growth of TaC thin films by reactive direct current magnetron sputtering: Composition and structure

Single-phase TaC films were grown by reactive dc magnetron sputtering onto high speed steel substrates kept at 650 °C (0.22 Tm ). The films were grown in both Ar/CH4 and Xe/CH4 mixtures using target-to-substrate distances, dts, of 6 and 15 cm. For all growth conditions, plasma-probe measurements were carried out. As-deposited films were analyzed using Auger electron spectroscopy (AES), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The C content of the films, as measured with AES, was found to increase linearly with increasing CH4 partial pressure in all cases. However, both the slope of these curves and the CH4 pressure needed in order to obtain a certain C content in the films were different depending on both dts and the sputtering gas. These results are explained with arguments based on cracking of the CH4 molecules into more reactive CHn radicals in the plasma and a different gas scattering of these radicals compared to the sputtered Ta atoms. Also, the structure of the deposited films was depending on the sputtering gas and all films grown in Ar/CH4 showed a considerably higher strain level compared to films grown in Xe/CH4 discharges. This higher strain level is due to the generation of defects in the films induced by a large flux of backscattered energetic neutral Ar atoms that imping on the growing film. In the Xe case, the energy of the backscattered neutrals is not sufficient to generate defects in the films. The structure of the films was also found to depend on the C content. For over-stoichiometric films, TEM analyses showed that the grain size was only a few nanometers and that there are low density regions surrounding the grains suggesting a precipitation of the excess C. When this precipitation occurred, the preferred orientation of the films changed from (200) to (111).