Gheorghe Popa

Dielectric barrier discharge technique in improving the wettability and adhesion properties of polymer surfaces

A dielectric barrier discharge (DBD) in helium at atmospheric pressure was used to improve the polymer surface wettability as a first condition for ensuring a good adhesivity in particular for a subsequent immobilization of selected biological macromolecules (heparin, drugs, enzymes, etc.) on these surfaces. The DBD was analyzed by electrical measurements and optical emission spectroscopy. The polymer surface was characterized by thermodynamic parameters which may predict the adhesion properties, the adhesion work and the surface polarity, and also by its morphology. The results show that the DBD treatments improve the wettability and thus the adhesive properties due to the creation of functional groups and less due to a physical adsorption induced by an expected larger area of the treated surfaces. Dimensions of grains/crystallites are decreased on the treated surface, but a significant and systematic modification of the surface roughness was not observed.

Roughness modification of surfaces treated by a pulsed dielectric barrier discharge

Local modifications of surface roughness are very important in many applications, as this surface property is able to generate new mechano-physical characteristics of a large category of materials. Roughness is one of the most important parameters used to characterize and control the surface morphology, and techniques that allow modifying and controlling the surface roughness present increasing interest. In this respect we propose the dielectric barrier discharge (DBD) as a simple and low cost method that can be used to induce controlled roughness on various surfaces in the nanoscale range. DBD is produced in helium, at atmospheric pressure, by a pulsed high voltage, 28 kV peak to peak, 13.5 kHz frequency and 40 W power. This type of discharge is a source of energy capable of modifying the physico-chemical properties of the surfaces without affecting their bulk properties. The discharge is characterized by means of electrical probes and, in order to analyse the heat transfer rate from the discharge to the treated surface, measurements of temperature distribution on the surface are performed. Influence of DBD on the roughness of surfaces with various properties, a semiconductor (tin oxide), a dielectric (polyvinylchloride) and a metallic (silver) surface, respectively, are investigated. Modifications of the surface morphology are detected by atomic force microscopy images, statistic roughness parameters and contact angle measurements. Results show an important increase of roughness and porosity of the thin films after DBD treatment, depending on the type of the material (semiconductor, dielectric and metallic). In the case of dielectric surfaces, this new morphology is correlated with adhesion work estimations. DBD treatments should be a convenient tool to induce a controlled roughness of various types of materials.

Two-dimensional fluid approach to the dc magnetron discharge

A two-dimensional (r, z) time-dependent fluid model was developed and used to describe a dc planar magnetron discharge with cylindrical symmetry. The transport description of the charged species uses the corresponding first three moments of the Boltzmann equation: continuity, momentum transfer and mean energy transfer (the last one only for electrons), coupled with the Poisson equation. An original method is proposed to treat the transport equations. Electron and ion momentum transport equations are reduced to the classical drift?diffusion expression for the fluxes since the presence of the magnetic field is introduced as an additional part in the electron flux, while for ions an effective electric field was considered. Thus, both continuity and mean energy transfer equations are solved in a classical manner. Numerical simulations were performed considering argon as a buffer gas, with a neutral pressure varying between 5 and 30?mTorr, for different voltages applied on the cathode. Results obtained for densities of the charged particle, fluxes and plasma potential are in good agreement with those obtained in previous studies.

Time resolved metal line profile by near-ultraviolet tunable diode laser absorption spectroscopy

Pulsed systems are extensively used to produce active species such as atoms, radicals, excited states, etc. The tunable diode laser absorption spectroscopy (TD-LAS) is successfully used to quantify the density of absorbing species, but especially for stationary or slow changing systems. The time resolved-direct absorption profile (TR-DAP) measurement method by TD-LAS, with time resolution of μs is proposed here as an extension of the regular use of diode laser absorption spectroscopy. The spectral narrowness of laser diodes, especially in the blue range (∼0.01 pm), combined with the nanosecond fast trigger of the magnetron pulsed plasma and long trace recording on the oscilloscope (period of second scale) permit the detection of the sputtered titanium metal evolution in the afterglow (∼ms). TR-DAP method can follow the time-dependence of the temperature (Doppler profile) and the density (deduced from the absorbance) of any medium and heavy species in a pulsed system.

On the Secondary Discharge of an Atmospheric-Pressure Pulsed DBD in He with Impurities

The secondary discharge was induced at the end of a slow-falling voltage flank, when a semisine monopolar pulse voltage excites the dielectric-barrier discharge. Formation and properties of the secondary discharge with respect to different dielectric materials such as glass, ceramic, and polyethylene theraphtalate were studied. The tunable diode laser absorption spectrometry (at 777.194 nm) was used to analyze the time-space distribution of the density of the atomic oxygen in metastable state (35S2 rarr 35P3) in addition to both discharge voltage and discharge current versus time. The secondary discharge is always formed, and its amplitude, as well as the amplitude of the main discharge, depends on surface properties of the dielectric barrier.

On the anisotropy and thermalization of the metal sputtered atoms in a low-pressure magnetron discharge

Space-resolved velocity distributions of titanium atoms sputtered in a direct-current (dc) magnetron discharge working at low pressure (0.4 Pa) were investigated using the laser-diode–induced fluorescence technique. A blue-light laser diode, covering the Ti 3d24s2-3d2(3F)4s4p(1P0) transition λ0=398.289 nm, was used to excite neutral Ti atoms and to measure their Doppler absorption profile through the induced fluorescence signal. Taking advantage of the very narrow laser linewidth, energetic and thermalized atoms can be clearly distinguished. Moreover, flux distributions of sputtered atoms along the laser direction were deduced and discussed.

Detailed Characterization of 2-Heptanone Conversion by Dielectric Barrier Discharge in N2 and N2/O2 Mixtures

The products of 2-heptanone conversion by dielectric barrier discharge plasma are analyzed under different conditions: alternating current (ac) or pulsed mode of excitation, variable energy, variable composition of the carrier gas. The efficiency of the conversion is higher using a pulse excitation mode than an ac mode. With a small oxygen percentage (about 2-3%) added to nitrogen, 2-heptanone is about 30% more efficiently removed than in pure nitrogen, while the 2-heptanone removal decreases with an oxygen percentage higher than 3%. A new analysis method, based on chemical ionization mass spectrometry, is used for volatile organic compound detection along with chromatography. Several products issued from 2-heptanone conversion with ac excitation are identified in nitrogen and in air, and a chemical scheme is proposed to explain their formation and their treatment by the discharge. It appears that byproducts are issued not only from oxidation reactions, but also from C-C bond cleavage by collisions with electrons or nitrogen excited states.

Tunable diode-laser induced fluorescence on Al and Ti atoms in low pressure magnetron discharges

Two different blue light laser diodes were used to investigate two types of atoms, namely Ti with resonance transition centred at λ0(Ti) = 398.289 nm and Al with λ0(Al) = 394.512 nm. Tunable diode-laser induced fluorescence offers local information on two groups of sputtered particles—non-thermalized and thermalized. The anisotropic velocity distribution functions (vdfs) are characterized probing the plasma along two directions: parallel to the target, vr, and perpendicular to it, vz. Measurements were performed in two plasma reactors both having planar magnetron cathodes with circular symmetry but with Ti and Al targets of different magnet strengths and diameters. The similar results of the vdf space dependence for these magnetron systems confirm the general behaviour of sputtered species transport. These similarities are related to the circular geometry and fundamentals of sputtering whereas differences are due to each specific sputtered element. The experimental results also show the effect of current density on the shape of vdf for Ti and Al. An increase in the current intensity implies a linear increase in the relative density of energetic sputtered atoms while the group of thermalized ones appears unaffected in the high current density regime.

Space-resolved velocity and flux distributions of sputtered Ti atoms in a planar circular magnetron discharge

The tuneable diode-laser-induced fluorescence (TD-LIF) technique was used to investigate both velocity and flux distributions of ground-state titanium atoms sputtered from a planar circular magnetron target working at a low pressure (0.4 Pa) and a high pressure (4 Pa). TD-LIF Doppler profiles were measured in front of the racetrack: (i) normal (vz) and parallel (vr) to the target at several distances (z), and (ii) for different angles (α) at 10 mm from the target, addressing the same local volume. The space variation of the velocity and flux distribution functions was deduced with high accuracy from the Doppler shift. Thus it was possible to determine the energetic over thermalized relative atom flux ratio going away from the cathode. The energy distribution function of Ti sputtered atoms in the normal direction is in good agreement with the extended Thompsons formula only for the collisionless sputtering regime, namely in the vicinity of the target and at low gas pressures. Moreover, TD-LIF measurements into the plasma but close to the racetrack centre with different angles of the laser beam show that the angular dependence of the energetic particles is well described by the projection of the normal velocity on each investigated direction. Hence, in this particular plasma region (10 mm in front of the racetrack), the actual angular velocity distribution is mainly governed by the component perpendicular to the target.

On the HiPIMS benefits of multi-pulse operating mode

High power impulse magnetron sputtering (HiPIMS) technology is particularly interesting for its high ionization efficiency of the sputtered target elements. However, one of its major drawbacks is the typically lower deposition rate, compared to direct current magnetron sputtering (DCMS), mainly due to the metal ion back-attraction. Using HiPIMS with very short pulses (less than 5 µs), it is possible to partially overcome the deposition rate limitation. In this contribution our focus is the optimization of the HiPIMS process with respect to DCMS, based on the physical understanding of the plasma state when using sequences of pulses, instead of general single high power pulse. Sequences of consecutive very short high-power pulses significantly increase the deposition rate compared to standard HiPIMS (s-HiPIMS), as proven by quartz crystal microbalance measurements and by scanning electron microscopy cross section images of the deposited films. Tailoring the sequences of multi-pulses (m-HiPIMS), experimental results undoubtedly show that tungsten deposition rate is at least 50% higher than in the s-HiPIMS, for the same average power. This finding is explained via tuneable diode laser absorption spectroscopy measurements of the sputtered W atoms correlated with electrostatic probe ion detection and time evolution of the plasma potential measured at the substrate position. Moreover, poly-crystalline thin film structure changes to a mainly preferential orientation (2 0 0) and film roughness is drastically reduced as provided by x-ray diffraction and atomic force microscopy, respectively.