Preparation of AlN and LiNbO3 thin films on diamond substrates by sputtering method

Diamond is attracting interest as a high velocity material because its elastic constant is the highest of all substances and the surface acoustic wave (SAW) velocity is more than 10 000 m/s. Although diamond is not piezoelectric, its high acoustic propagation makes it a desirable substrate for SAW devices when coupled with piezoelectric thin films such as aluminum nitride (AlN) and lithium niobate (LiNbO₃). Highly oriented AlN thin films and LiNbO₃ thin films were prepared by a reactive sputtering method on polycrystalline diamond substrates. The average surface roughness (Ra) of the AlN thin films was below 2 nm by locating the diamond substrates at the position of 100 mm from the aluminum target. The full width at half maximum of the rocking curve for the AlN(002) peak of X-ray diffraction was approximately 0.2°. SAW characteristics with an interdigital transducer (IDT)/AlN/diamond structure were investigated. The average surface roughness of LiNbO₃ thin films was 5 nm. If the highly oriented LiNbO₃ films are deposited on a diamond substrate, the IDT/LiNbO₃/diamond layered structure will be capable of wide-bandwidth application in SAW devices at high frequencies.     

Properties of diamond-like carbon films on crystalline quartz and lithium niobate

One-micron thick DLC films are deposited on Y-X cut quartz and Y-cut lithium niobate substrates using a plasma-enhanced CVD technique. From the Raman spectra, we find that the films have a small intensity ratio of ‘D’ to ‘G’ peak, indicating a high carbon sp³/sp² ratio and high hardness characteristic. The effect of accelerating a surface acoustic wave by the DLC films has been confirmed by comparing two delayed signals from two near-by delay lines. One was coated with the DLC, while the other was kept as the free quartz or lithium niobate surfaces. It is observed that the one-micron thick DLC films are able to speed up SAW by 2.4% (at 198 MHz) for DLC/quartz and 2.5% (at 430 MHz) for DLC/lithium niobate samples, respectively.     

Effects of Si3N4 and DLC passivation layers on characteristics of SAW filters

Silicon nitride (Si₃N₄) and diamond-like carbon (DLC) passivation layers were deposited on Surface Acoustic Wave (SAW) filters with Al inter-digital electrodes formed on LiTaO₃ substrates. The adhesion of the DLC film to LiTaO₃ substrate has been enhanced by introducing SiO₂ interlayer. Si₃N₄ passivation layer is found to be capable of minimizing the variations of both insertion loss and bandwidth of the SAW filters during power durability test (PDT), while DLC layers show little effect on these variations.     

Study of aluminium nitride/freestanding diamond surface acoustic waves filters

Highly oriented aluminium nitride (AlN) has been successfully deposited on silicon substrates and on the back side of unpolished, thick, freestanding, polycrystalline diamond films. Structural and electrical properties of the (0 0 2) oriented AlN films have been investigated. Using optimised AlN and chemical vapour deposited diamond films, high quality surface acoustic wave (SAW) filters were constructed by deposition of aluminium inter-digital transducers. The effects of the AlN thickness on the diamond-based SAW filter properties and the SAW propagation were investigated. A phase velocity of 17.1 km/s was observed, which is, to the best of our knowledge, the highest reported value for diamond. The dependence of the phase velocity (v) on the AlN thickness was compared to theoretical predictions. SAW filters with rather low insertion loss, high suppression and high electromechanical coefficient could be obtained. We also report on piezoelectric d₃₃ measurements of AlN films by atomic force microscopy.     

Blood compatibility of La2O3 doped diamond-like carbon films

La₂O₃ doped diamond-like carbon films (DLC) with different concentration were deposited by using Radio-Frequency magnetron sputtering. The microstructure and surface properties of DLC films were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle test. The blood compatibility of the samples was evaluated by tests of platelet adhesion. Results show the sp²-bonded C content increases with increasing of La₂O₃ concentration doped. A remarkable decrease of platelet adhered on the surface of the La₂O₃ doped DLC films was observed comparing to the Chrono flex used in clinical application, suggesting that La₂O₃ doped DLC is able to enhance its blood compatibility. The mechanism of hemocompatibility of doped films was discussed. Our results demonstrate that La₂O₃ doped DLC films are potentially useful biomaterials with good blood compatibility.     

Tribological properties of DLC films with different hydrogen contents in water environment

Diamond-like carbon (DLC) films were deposited on silicon wafers by thermal electron excited chemical vapor deposition (CVD). To change the hydrogen content in film, we used three types of carbon source gas (C₇H₈, CH₄, and a CH₄+H₂) and two substrate bias voltages. The hydrogen content in DLC films was analyzed using elastic recoil detection analysis (ERDA). Tribological tests were conducted using a ball-on-plate reciprocating friction tester. The friction surface morphology of DLC films and mating balls was observed using optical microscopy and laser Raman spectroscopy.Hydrogen content in DLC films ranged from 25 to 45 at.%. In a water environment, the friction coefficient and specific wear rate of DLC films were 0.07 and in the range of 10⁻⁸–10⁻⁹ mm³/Nm, respectively. The friction coefficient and specific wear rate of DLC film in water were hardly affected by hydrogen content. The specific wear rate of DLC film with higher hardness was lower than that of film with low hardness. Mating ball wear was negligible and the friction surface features on the mating ball differed clearly between water and air environments, i.e., the friction surface on mating balls in water was covered with more transferred material than that in air.     

Surface acoustic wave properties of natural smooth ultra-nanocrystalline diamond characterized by laser-induced SAW pulse technique

Diamond is one of the best SAW substrate candidates due to its highest sound velocity and thermal conductivity. But conventional diamond films usually express facet structure with large roughness. Ultra-nanocrystallined diamond (UNCD) films grown in a 2.45 GHz IPLAS microwave plasma enhanced chemical vapor deposition (MPECVD) system on Si (100) substrates in CH₄-Ar plasma possess naturally smooth surface and are advantageous for device applications. Moreover, highly C-axis textured aluminum nitride (AlN) films can be grown by DC-sputtering directly on UNCD coated Si substrate. However, properties of UNCD films are much complex than microcrystalline diamond films, that is because this is a very complex material system with large but not fixed portion of grain boundaries and sp²/sp³ bonding. Properties of UNCD films could change dramatically with similar deposition condition and with similar morphologies. A simple and quick method to characterize the properties of these UNCD films is important and valuable. Laser-induced SAW pulse method, which is a fast and accurate SAW properties measuring system, for the investigation of mechanical and structure properties of thin films without any patterning or piezoelectric layer.     

Electro-deposition of adherent films of H2-free quality diamond-like carbon materials on SS-304 substrates using nanocrystalline SnO2 interlayer

H₂-free diamond-like carbon (DLC) films are deposited on bare as well as nanocrystalline SnO₂-coated stainless steel (SS) 304 substrates. SnO₂ films are deposited by electrolysis of 10 mM SnCl₂ solution in nitric acid medium and then subsequently heating the sample in oxygen-ambient condition. The carbon films, on the other hand, are prepared by dissociative cleavage of methanol under high DC potential of 1 kV. Deposition conditions are optimised to get the best quality films, which are characterized by Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). It has been observed that films grown on SnO₂ interlayer are much more adherent when compared to those deposited on bare SS. Moreover, the quality of films on coated substrates is found to be much superior to those grown on bare SS. It is believed that the SnO₂ layer acts both as protective barrier against carbon diffusion into the SS substrate and also promotes nucleation due to anchorage of CH₃ and H⁺ radicals (necessary for growth of DLC film) onto its oxygen deficient sites.     

Study on the diamond-like carbon multilayer films for tribological application

In this paper, DLC multilayer films consisting of alternating layers of soft and hard carbon films were deposited on Si wafer by a plasma CVD deposition system. Different DLC multilayer films were prepared by varying the sub-layer thickness (from 1000 to 25 nm) and the ratio of hard to soft sub-layer (H/S) thickness (from 1:1 to 4:1). By using a ball-on-disk tribo-tester, the friction and wear properties of the DLC multilayer films were measured in vacuum, O₂ and dry-air environments respectively. By comparing with single-layer DLC film, the change of the multilayer structure has little influence on friction coefficient of the multilayer films. However, the wear rate of the DLC multilayer films is restricted effectively by constructed the multilayer structure in the film. The wear rate of the multilayer films is lower than that of the single film in reactive (O₂ and dry-air) environments. An DLC multilayer film with excellent wear resistance, approximately in the level of 10⁻⁸ mm³/Nm in different environments (dry-air, O₂ and vacuum), is obtained as the DLC multilayer film at a certain sub-layer thickness and ratio.     

Substrate bias effect on structure of tetrahedral amorphous carbon films by Raman spectroscopy

Diamond-like carbon (DLC) films form a critical protective layer on magnetic hard disks and their reading heads. Now tetrahedral amorphous carbon films (ta–C) thickness of 2 nm are becoming the preferred means due to the highly sp³ content. In this paper, Raman spectra at visible and ultraviolet excitation of ta–C films have been studied as a function of substrate bias voltage. The spectra show that the sp³ content of 70 nm thick DLC films increases with higher substrate bias, while sp³ content of 2 nm ultra-thin films falls almost linearly with bias increment. And this is also consistent with the hardness measurement of 70 nm thick films. We proposed that substrate bias enhances mixing between the carbon films and either the Si films or Al₂O₃TiC substrate such that thin films contain less sp³ fraction. These mixing bonds are longer than C–C bonds, which inducing the hardness decreasing of ultra-thin DLC films with bias. But for 70 nm DLC, the effect of mixing layer can be negligible by compared to bias effect with higher carbon ion energy. So sp³ content will increase for thick films with substrate bias.     

Mechanical and tribological properties of diamond-like carbon films prepared on steel by ECR-CVD process

Diamond-like carbon (DLC) films were prepared on AISI 440C steel substrates at room temperature by the electron cyclotron resonance chemical vapor deposition (ECR-CVD) process in C₂H₂/Ar plasma under different conditions. In order to prevent the inter-diffusion of carbon and improve the adhesion strength of DLC films, functionally gradient Ti/TiN/TiCN/TiC supporting underlayers were deposited on the steel substrates in advance. Using the designed interfacial transition layers, relatively thick DLC films (1–2 μm) were successfully prepared on the steel substrates without delamination. By optimizing the deposition parameters, DLC films with hardness up to 28 GPa and friction coefficients lower than 0.15 against the 100Cr6 steel ball were obtained. In addition, the specific wear rates of the films were found to be extremely low (∼10⁻¹⁷ m³/Nm). The friction-induced graphitization mechanism of DLC was confirmed by micro-Raman analysis.     

Enlarged phase velocities of ultra-wideband surface acoustic wave devices with relaxor based ferroelectric single crystal/diamond layered structure

The surface acoustic wave (SAW) characteristics of Y-cut X propagating Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (YX-PIMNT) single crystals on a diamond substrate have been theoretically calculated. The simulated results show that the phase velocity of the shear horizontal (SH) SAW may be greatly enhanced from the 1350 m/s to 3350 m/s by reducing the thickness of the PIMNT from 0.5λ to 0.05λ, with a corresponding decrease in the electromechanical coupling factor (K²) from 73.6% to 19.6%. The dispersion curves of phase velocity and K² as a function of PIMNT thickness are given for the proposed layered structure. Besides the SH SAW, there are also higher order modes that would cause unwanted responses in the pass-band of wideband SAW filters. These were suppressed by properly controlling structural parameters including top electrode thickness, thickness and Euler angle (θ) of PIMNT substrate. The calculated results demonstrate the effectiveness of this approach to enlarge the phase velocity of the SH SAW without dramatically sacrificing its K², which makes relaxor based ferroelectric single crystals promising for realizing ultra-wideband SAW devices working in ~ GHz range.     

Humidity dependence on the friction and wear behavior of diamond-like carbon film in air and nitrogen environments

Diamond-like carbon (DLC) films were deposited on Si (100) wafers by a plasma enhanced chemical vapor deposition (PECVD) technique using CH₄ plus Ar as the feedstock. The friction and wear behaviors of the resulting film sliding against Si₃N₄ balls were investigated on a ball-on-disk test rig in air and nitrogen environments at a relative humidity from 5% to 100%. The worn surface morphologies of the DLC film and the Si₃N₄ counterpart were observed on a scanning electron microscope (SEM), while the chemical states of some typical elements thereon were investigated by means of X-ray photoelectron spectroscopy (XPS). It was found that the DLC film recorded continuously increased friction coefficient and wear rate with increasing relative humidity in air. It showed linearly increased friction coefficient with increasing relative humidity in nitrogen, in this case the wear rate sharply decreased and reached the minimum at a relative humidity of 40%, which was followed by an increase with further increase of the relative humidity. The interruption of the transferred carbon-rich layers on the Si₃N₄ balls, and the friction-induced oxidation of the films in higher relative humidity were proposed to be the main reasons for the increases of the friction coefficient and wear rate. Moreover, the oxidation and hydrolysis of the Si₃N₄ ball in higher relative humidity, leading to the formation of a tribochemical film that mainly consists of silica gel on the wearing surface, were also thought to have effects on the friction and wear behaviors of the DLC films.     

Electrochemical properties of N-doped hydrogenated amorphous carbon films fabricated by plasma-enhanced chemical vapor deposition methods

Nitrogen-doped hydrogenated amorphous carbon thin films (a-C:N:H, N-doped DLC) were synthesized with microwave-assisted plasma-enhanced chemical vapor deposition widely used for DLC coating such as the inner surface of PET bottles. The electrochemical properties of N-doped DLC surfaces that can be useful in the application as an electrochemical sensor were investigated. N-doped DLC was easily fabricated using the vapor of nitrogen contained hydrocarbon as carbon and nitrogen source. A N/C ratio of resulting N-doped DLC films was 0.08 and atomic ratio of sp³/sp²-bonded carbons was 25/75. The electrical resistivity and optical gap were 0.695 Ω cm and 0.38 eV, respectively. N-doped DLC thin film was found to be an ideal polarizable electrode material with physical stability and chemical inertness. The film has a wide working potential range over 3 V, low double-layer capacitance, and high resistance to electrochemically induced corrosion in strong acid media, which were the same level as those for boron-doped diamond (BDD). The charge transfer rates for the inorganic redox species, Fe^2+/3+ and Fe(CN)₆^4−/3− at N-doped DLC were sufficiently high. The redox reaction of Ce^2+/3+ with standard potential higher than H₂O/O₂ were observed due to the wider potential window. At N-doped DLC, the change of the kinetics of Fe(CN)₆^3−/4− by surface oxidation is different from that at BDD. The rate of Fe(CN)₆^3−/4− was not varied before and after oxidative treatment on N-doped DLC includes sp² carbons, which indicates high durability of the electrochemical activity against surface oxidation.     

Optical band gap shift in thin LiNbO3 films grown by radio-frequency magnetron sputtering

Thin polycrystalline LiNbO₃ films were deposited by the radio-frequency magnetron sputtering (RFMS) method and ion-beam sputtering (IBS) method under different conditions. Study of the adsorption band edge of fabricated films reveals direct and indirect optical transition. Depending on the particular technological sputtering RFMS regime, the direct energy gap varies from 3.8 to 4.4 eV. Band tails induced by the defects formation due to the reactive plasma effect on the film structure are responsible for indirect optical transitions in the studied films. Thermal annealing has a prominent effect on trap concentration and strain in as-grown films leading to rise in direct band energy up to 4.4 eV which is close to the value for bulk LiNbO₃.     

Charge phenomena at the Si/LiNbO3 heterointerface after thermal annealing

Polycrystalline lithium niobate (LiNbO₃) films were deposited onto Si substrates by radio-frequency magnetron sputtering method in an Ar environment and an Ar+O₂ gas mixture. All as-grown films manifested ferroelectric properties with the remnant polarization of P_r =?69μC/cm² and the internal field of E_i =?2.8?kV/cm. Thermal annealing (TA) of as-grown Si-LiNbO₃ heterostructures leads to decrease in the internal field of LiNbO₃ films grown in an Ar atmosphere and results in the change of sign of E_i for the films, grown in an Ar+O₂ gas mixture. Analysis of the capacitance-voltage and current voltage characteristics of the studied heterostructures revealed that oxygen vacancies and electron trapping on border traps with energy of about E_t =?1.7?eV below the conduction band are responsible for this effect. TA results in decline of conductivity of LiNbO₃ films, affected by the phonon scattering of electrons, making these films close to single crystal lithium niobate.     

The effect of CNT content on the surface and mechanical properties of CNTs doped diamond like carbon films

The carbon nanotubes (CNTs) doped diamond like carbon films were carried out by spinning coating multi-walled carbon nanotubes (CNTs) on silicon covered with diamond like carbon films via PECVD with C₂H₂ and H₂. The results show that the I_D/I_G and sp²/sp³ ratios are proportional to the CNT contents. For wettability and hydrogen content, the increase of CNT content results in more hydrophobic and less hydrogen for CNT doped DLC films. As for mechanical properties, the hardness and elastic modulus increases linearly with increasing CNT content. The residual stress is reduced for increasing CNT content. As for the surface property, the friction coefficient is reduced for higher CNT content. For CNT doped DLC films, the inclusion of horizontal CNT into DLC films increases the hardness, elastic modulus and reduces the hydrogen content, friction coefficient and residual stress. Like the light element and metal doping, the CNT doping has effects on the surface and mechanical properties on DLC which might be useful to specific application.     

Electrochemical characterization of diamond like carbon thin films

Diamond-like carbon (DLC) thin films were deposited from pure graphite target by DC magnetron sputtering method. Experimental parameters, i.e., substrate temperature and negative bias voltage, have been changed to finely tune the chemical bonding property (sp²/sp³) of the as-deposited DLC films. The as-deposited DLC films were characterized as anode materials for Li–ion batteries and special attentions were paid to the effects of sp²/sp³ ratio on the electrochemical properties of the DLC films. The results indicated that a high fraction of sp² bonding in the DLC films is preferred for high lithium storage capacity, flat and low charge voltage plateau, and long cycling retention.     

Simulation of characteristics of KNbO3/diamond surface acoustic wave

High-frequency Surface Acoustic Wave (SAW) Devices based on diamond that has been realized to date utilize the SiO₂/ZnO/diamond structure, which shows excellent characteristics of high phase velocity over 10,000 m/s with zero temperature coefficient, and it has been successfully applied to high-frequency narrowband filters and resonators. In this paper, the characteristics of KNbO₃/diamond with the assumption that the KNbO₃ film is single crystal have been studied by theoretical calculations to find higher coupling coefficient condition. Calculations are made for the phase velocity and electromechanical coupling coefficient of the Rayleigh wave and its higher mode Sezawa waves. As a result, KNbO₃/Ground/diamond is found to offer extremely high electromechanical coupling coefficient up to 38% in conjunction with high phase velocity of 12,600 m/s. This characteristic is suitable for ultra-wide bandwidth applications in high-frequency SAW devices.