O density measurements in the pulsed-DC reactive magnetron sputtering of titanium

Using eclipse laser photo-detachment in conjunction with Langmuir probing, the density of O⁻ ions in a reactive pulsed magnetron (100 kHz, 55% duty) plasma has been determined at different times during the pulse period at a set of positions along the centre line axis of the discharge. The magnetron was operated at a fixed average power of 400 W with an oxygen partial pressure of 10% of the total pressure 1.33 Pa. The results show the plasma is weakly electro-negative, with a negative ion-to-electron density ratio α up to a maximum of 0.63.During the plasma on-phase (at all chosen measurement positions) the O⁻ density was found to reach a maximum directly after initiation of the voltage pulse decreasing weakly during the rest of the on-phase. On the transition from on-to-off phases of the pulse the negative ion density was found to fall (by 60% both close and far from the target but only 10% near the discharge centre), with the O⁻ density remaining almost constant during the rest of the afterglow.The spatial structure of the O⁻ density reveals a distinct peak 75 mm from the target close to but not at the position of the null in the magnetic field, falling by a factor of eight for increasing distances up to 30 mm both towards and away from the target. The highest O⁻ density recorded at this position was 1 × 10¹⁶ m⁻³, at a time of 2.12 μs into the pulse. From a comparison between on- and off-phase densities and using an intuitive model of the plasma, the results indicate that most negative ions are created in the bulk plasma. The density of target-borne O⁻ ions is estimated to be about 1 × 10¹⁵ m⁻³ varying little with position, possibly forming a beam-like structure.     

A Novel Method of Triple-Wire Gas Indirect Arc Welding

This paper proposes a novel triple-wire gas indirect arc welding process. The welding system consists of two power sources and three wires. The effects of the power source mode and the wire configuration on arc stability and behavior are studied. The metal transfer is analyzed and bead-on-plate welding is employed. Results show that two direct current power sources cannot produce a stable process, but the combination of a direct current with a pulsed direct current can produce a stable process. The reason is that the pulsed direct current can boost and stabilize the metal transfer. For the wire configuration, a smaller contact angle between the main wire and the side wire is more desirable. Compared with the traditional gas metal arc welding, this novel process has the advantages of high wire melting rate, low penetration depth, and low dilution rate. Compared with twin-wire gas indirect arc welding, it provides a broader range of applicable currents with sufficient heat input.     

DLC–ceramic multilayers for automotive applications

The present work explores the deposition of hard, wear resistant multilayer coatings, by magnetron sputtering onto Aluminium (Al) alloy substrates that are used in the automotive industry. Multilayer coatings have been manufactured to increase surface hardness and wear resistance for a commercial powder metallurgy Al alloys (Al 2618). The multilayer coating consisted of 25 bi-layers of Titanium Diboride (TiB₂) and diamond-like carbon (DLC). These DLC/TiB₂ coatings were fabricated, maintaining a constant composition wavelength (sum of two layers [λ] = 200 nm) for an array of ceramic fractions ranging from 75% to 95% by volume. The effect of the DLC content on the structure and performance (hardness and adhesion) of the films was investigated. The bi-layer thickness influences the failure patterns resulting from the scratch testing. This study has found hardness values of 27.8 GPa, with a critical load of 20 N and a friction coefficient of 0.47. As a result of these findings the multilayer with 10% of DLC was found to be a better compromise between high hardness (23.8 GPa) and high adhesion (critical load higher than 20 N) and with no signs of cracking during friction testing, proving to be a solution to be employed in components located in the upper valve train area of high performance vehicles.     

Characterization of iridium oxide thin films deposited by pulsed-direct-current reactive sputtering

Thin films of iridium oxide were deposited on silicon and borosilicate glass substrates by pulsed-direct-current (pulsed-DC) reactive sputtering of iridium metal in an oxygen-containing atmosphere. Optimum deposition conditions were identified in terms of plasma pulsing conditions, oxygen partial pressures, and substrate temperature. The films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and Rutherford backscattering spectroscopy. According to the results, it was possible to obtain films that are near-stoichiometric, smooth and uniform in texture. The films deposited without substrate heating were amorphous, and those deposited at substrate temperatures above 300 °C were found to have a homogeneous polycrystalline structure. The results also showed that pulsed-DC sputtered iridium oxide films were smoother and had lower micro-inclusions density than DC-sputtered films obtained under otherwise similar deposition conditions. This improvement in the film quality is at the expense of a tolerable decrease in the deposition rate.     

Coping with electrode polarization for development of DC-Driven electrical impedance tomography

An electrical impedance tomography (EIT) system design is proposed for imaging of phase distribution in gas-water two-phase flow from boundary measurement of electrical potentials in response to direct current (DC) injection. DC injection simplifies substantially the system design, but introduces problems due to polarization of injection electrodes. Electrode polarization means charge accumulation on the electrode-water interface causing a drift in the interfacial potential difference. The polarization problems are coped with by using dedicated electrodes for injection and potential measurement, and using a current source unaffected by the polarization of current-carrying electrodes (CCEs). Furthermore, the polarization of CCEs is controlled, to lessen the possible influence on the sensing electrodes (SEs), by using a short (milliseconds in width) pulse for injection with a charge balanced injection strategy. The impact of electrode polarization and the effectiveness of countermeasures introduced in the present design are discussed through comparisons of measured boundary potentials and of images reconstructed for a simple object simulating large bubbles in water.     

Porous silicon oxide sacrificial layers deposited by pulsed-direct current magnetron sputtering for microelectromechanical systems

The development of silicon oxide layers with high etch rates to be used as sacrificial layers in surface micromachining for microsystems fabrication poses a great technological challenge. In this work, we have investigated the possibility of obtaining easily removable silicon oxide layers by pulsed-direct current (DC) magnetron reactive sputtering. We have carried out a comprehensive study of the influence of the deposition parameters (total pressure and gas composition) on the composition, residual stress and lateral etch rate in fluoride wet solutions of the films. This study has allowed to determine the sputtering conditions to deposit, at high rates (up to 0.1 μm/min), silicon oxide films with excellent characteristics for their use as sacrificial layers. Films with roughness around 5 nm rms, residual stress below 100 MPa and very high lateral etch rate (up to 5 μm/min), around 70 times higher than for thermal silicon oxide, have been achieved. The structural characteristics of these easily removable silicon oxide layers have been assessed by infrared spectroscopy and atomic force microscopy, which have revealed that the films exhibit a porous structure, related to very specific sputter conditions. Finally, the viability of these films has been demonstrated by using them as sacrificial layer in the fabrication process of AlN-based microresonators.     

Synthesis of ultra-smooth and ultra-low friction DLC based nanocomposite films on rough substrates

Rough TiC/a-C films were intentionally grown on smooth surface to simulate a rough finishing of industrial substrates. Surface roughness and growth dynamics of TiC/a-C nanocomposite films deposited on such rough surfaces by non-reactive pulsed-DC (p-DC) sputtering of graphite targets at 350 kHz pulse frequency were studied using atomic force microscopy, cross-sectional scanning electron microscopy. Intensive concurrent ion impingement during the film growth at higher pulse frequency of p-DC sputtering leads to rapid smoothing of such initial rough surfaces. It was shown that rapid smoothing of initially rough surfaces with RMS roughness ~ 6 nm to < 1 nm can be effectively achieved by 350 kHz p-DC sputtering. These films exhibit dense and glassy microstructure. The surface roughness strongly influences the frictional behavior of TiC/a-C nanocomposite films where the rougher surfaces yielded higher steady state friction coefficient (CoF).The observed dynamic smoothing phenomenon was applied to obtain ultra-smooth and ultra-low friction (μ ~ 0.05) TiC/a-C:H nanocomposite films on industrial polished steel substrates by 200 and 350 kHz p-DC sputtering of Ti-targets in an argon/acetylene atmosphere.     

Comparison testing of diamond-like a-C:H coatings prepared in plasma cathode-based gas discharge and ta-C coatings deposited by vacuum arc

The method of amorphous carbon coating deposition based on decomposition of acetylene in a non-self-sustained hollow cathode pulsed-DC discharge is investigated. The discharge is maintained by the electron emission of a grid-stabilized plasma cathode based on a DC glow discharge. The method allows the gas pressure in the discharge gap and the non-self-sustained discharge parameters to be varied in a wide range. It makes it possible to optimize the properties of the deposited coating and to perform in situ the preliminary ion cleaning of sample surface and the plasma immersion ion implantation to form an interface and to improve the coating's adhesion. The 0.1-10-μm-thick a-C:H films were deposited on tungsten carbide and stainless steel substrates at a deposition rate of 0.5-8 μm/h. The coatings were investigated using the methods of atomic-force microscopy (AFM), scanning electron microscopy (SEM) and Raman spectroscopy. The arithmetic average surface roughness (9-34 nm), the friction coefficients (0.01-0.3), the density (2.2-2.4 g/cm³), the microhardness (16-75 GPa) and the internal stresses in the films (3-7 GPa) were measured. Comparison was made between the properties of the resulted a-C:H coating with the properties of the ta-C coating obtained by cathodic vacuum arc deposition.     

Pulsed-DC sputtering of molybdenum bottom electrode and piezoelectric aluminum nitride films for bulk acoustic resonator applications

Various sputtering conditions were employed to explore the feasibility of depositing a suitably textured layer of molybdenum film, as a bottom-electrode of a film bulk acoustic resonator, on a silicon substrate. A fully (110)-textured Mo film, with its full width at half maximum (FWHM) of rocking curve as low as 1.1°, could be made when a 25-nm thick primer layer of aluminum nitride (AlN) film was pre-deposited between Si and Mo. In turn, the degree of the (0002) texture of a subsequently deposited AlN piezoelectric film, about 1.4 μm in thickness, was found to be largely decided by the degree of the (110) texture of the Mo film beneath it. The residual stress of this AlN piezoelectric film also varied virtually according to the texture quality of the underlying Mo film. The optimal process condition resulted in a piezoelectric AlN thin film having a (0002) FWHM as low as 0.98°, and a slightly compressive residual stress of 439 MPa at the same time.