Catalin Vitelaru

Understanding deposition rate loss in high power impulse magnetron sputtering: I. Ionization-driven electric fields

The lower deposition rate for high power impulse magnetron sputtering (HiPIMS) compared with direct current magnetron sputtering for the same average power is often reported as a drawback. The often invoked reason is back-attraction of ionized sputtered material to the target due to a substantial negative potential profile, sometimes called an extended presheath, from the location of ionization toward the cathode. Recent studies in HiPIMS devices, using floating-emitting and swept-Langmuir probes, show that such extended potential profiles do exist, and that the electric fields Ez directed toward the target can be strong enough to seriously reduce ion transport to the substrate. However, they also show that the potential drops involved can vary by up to an order of magnitude from case to case. There is a clear need to understand the underlying mechanisms and identify the key discharge variables that can be used for minimizing the back-attraction. We here present a combined theoretical and experimental analysis of the problem of electric fields Ez in the ionization region part of HiPIMS discharges, and their effect on the transport of ionized sputtered material. In particular, we have investigated the possibility of a ?sweet spot? in parameter space in which the back-attraction of ionized sputtered material is low. It is concluded that a sweet spot might possibly exist for some carefully optimized discharges, but probably in a rather narrow window of parameters. As a measure of how far a discharge is from such a window, a Townsend product ?Townsend is proposed. A parametric analysis of ?Townsend shows that the search for a sweet spot is complicated by the fact that contradictory demands appear for several of the externally controllable parameters such as high/low working gas pressure, short/long pulse length, high/low pulse power and high/low magnetic field strength.

Spokes and charged particle transport in HiPIMS magnetrons

Two separate scientific communities are shown to have studied one common phenomenon, azimuthally rotating dense plasma structures, also called spokes, in pulsed-power E × B discharges, starting from quite different approaches. The first body of work is motivated by fundamental plasma science and concerns a phenomenon called the critical ionization velocity, CIV, while the other body of work is motivated by the applied plasma science of high power impulse magnetron sputtering (HiPIMS). Here we make use of this situation by applying experimental observations, and theoretical analysis, from the CIV literature to HiPIMS discharges. For a practical example, we take data from observed spokes in HiPIMS discharges and focus on their role in charged particle transport, and in electron energization. We also touch upon the closely related questions of how they channel the cross-B discharge current, how they maintain their internal potential structure and how they influence the energy spectrum of the ions? New particle-in-cell Monte Carlo collisional simulations that shed light on the azimuthal drift and expansion of the spokes are also presented.

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.

Ti-Ar scattering cross sections by direct comparison of Monte Carlo simulations and laser-induced fluorescence spectroscopy in magnetron discharges

A 3D Monte Carlo code (OMEGA) was developed to simulate the transport of sputtered atoms in a magnetron discharge operating in direct current mode. Collisions between the sputtered Ti atoms and the neutral process gas atoms (Ar) were modelled. Spatially resolved simulated velocity distributions of the sputtered particles parallel as well as perpendicular to the cathode surface for different operating pressures were recorded and benchmarked against experimentally obtained profiles using laser-induced fluorescence. New differential (angular and energy-dependent) cross sections for Ti-Ar elastic collisions were thereby obtained, which resulted in good agreement between modelled and experimental results. The differences with respect to commonly used extrapolated Ar-Ar cross sections to describe the Ti-Ar interaction are highlighted and discussed.

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.

Plasma reactivity in high-power impulse magnetron sputtering through oxygen kinetics

The atomic oxygen metastable dynamics in a Reactive High-Power Impulse Magnetron Sputtering (R-HiPIMS) discharge has been characterized using time-resolved diode laser absorption in an Ar/O2 gas mixture with a Ti target. Two plasma regions are identified: the ionization region (IR) close to the target and further out the diffusion region (DR), separated by a transition region. The μs temporal resolution allows identifying the main atomic oxygen production and destruction routes, which are found to be very different during the pulse as compared to the afterglow as deduced from their evolution in space and time.R. Agnese, A.J. Anderson, D. Balakishiyeva, R. Basu Thakur, 19 D.A. Bauer, A. Borgland, D. Brandt, P.L. Brink, R. Bunker, B. Cabrera, D.O. Caldwell, D.G. Cerdeno, H. Chagani, M. Cherry, J. Cooley, B. Cornell, C.H. Crewdson, P. Cushman, M. Daal, P.C.F. Di Stefano, E. Do Couto E Silva, T. Doughty, L. Esteban, S. Fallows, E. Figueroa-Feliciano, J. Fox, M. Fritts, G.L. Godfrey, S.R. Golwala, J. Hall, H.R. Harris, J. Hasi, S.A. Hertel, B.A. Hines, T. Hofer, D. Holmgren, L. Hsu, M.E. Huber, A. Jastram, O. Kamaev, B. Kara, M.H. Kelsey, S.A. Kenany, A. Kennedy, C.J. Kenney, M. Kiveni, K. Koch, B. Loer, E. Lopez Asamar, R. Mahapatra, V. Mandic, C. Martinez, K.A. McCarthy, N. Mirabolfathi, R.A. Moffatt, D.C. Moore, P. Nadeau, R.H. Nelson, L. Novak, K. Page, R. Partridge, M. Pepin, A. Phipps, K. Prasad, M. Pyle, H. Qiu, R. Radpour, W. Rau, P. Redl, A. Reisetter, R.W. Resch, Y. Ricci, T. Saab, B. Sadoulet, 3 J. Sander, R. Schmitt, K. Schneck, R.W. Schnee, S. Scorza, D. Seitz, B. Serfass, B. Shank, D. Speller, A. Tomada, A.N. Villano, B. Welliver, D.H. Wright, S. Yellin, J.J. Yen, B.A. Young, and J. Zhang

Argon metastables in HiPIMS: validation of the ionization region model by direct comparison to time resolved tunable diode-laser diagnostics

The volume plasma interactions of high power impulse magnetron sputtering (HiPIMS) discharges operated with a Ti target is analyzed in detail by combining time-resolved diagnostics with modeling of ...

Block Copolymer Elastomer with Graphite Filler: Effect of Processing Conditions and Silane Coupling Agent on the Composite Properties

The control of morphology and interface in poly(styrene-ethylene/butylene-styrene) (SEBS) composites with graphitic fillers is extremely important for the design of piezoresistive sensors for body motion or flexible temperature sensors. The effects of a high amount of graphite (G) and silane coupling agent on the morphology and properties of SEBS composites with anisotropic mechanical properties are reported. The physical and chemical bonding of silane to both G and SEBS surface was proved by EDX and TGA results; this improved interface influenced both the thermal and mechanical properties of the composite. The vinyltriethoxysilane (VS) promoted the formation of char residue and, being tightly bound to both SEBS and G, did not show separate decomposition peak in the TGA curve of composites. The mechanical properties were measured on two perpendicular directions and were improved by both the addition of VS and the increased amount of G; however, the increase of storage modulus due to orientation (from 5 to 15 times depending on the composition and direction of the test) was more important than that provided by the increase of G concentration, which was a maximum of four times that obtained for 15 wt % graphite. A mechanism to explain the influence of G content and treatment on the variation of storage modulus and tan δ depending on the direction of the test was also proposed.

Carbon and Tungsten Sputtering in a Helium Magnetron Discharge

This study reports on carbon and tungsten deposition on a heated silicon substrate under He+ bombardment in a magnetron sputtering device. The discharge was operated at constant pressure of 1.33 Pa for two discharge current intensities (200 mA and 600 mA) and target power density up to 40 Wcm-2. The deposited films were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffractometry (XRD). The topography and cross section reveals the influence of the target power density on the surface roughness, grains size and thickness of the deposited films.