AVS2010 Session SE+PS-MoA: Pulsed Plasmas in Surface Engineering

High power impulse magnetron sputtering (HIPIMS) is one of the youngest magnetron sputtering technologies. It provides new parameter space and new level of control of deposition parameters which are unattainable by conventional sputtering or cathodic arc evaporation technologies.

HIPIMS utilises a short (impulse) gas discharge with duration of ~100 µs and duty cycles of <1% allowing it to access high peak power densities of 3000 Wcm^-2 at voltages of several hundred volts and current density of 1-4 Acm^-2. Within each HIPIMS pulse the discharge is ignited through a hot electron ionisation wave and then develops into a cold metal plasma. The properties of the target material such as sputter yield, secondary electron emission coefficient, atomic mass and ionisation potential determine the power dissipated in the discharge, the density of plasma and the transport from the target to the substrate. The lifetime of both gas and metal ions spans over several milliseconds after the pulse often extending to the next pulse, thus creating a constant bombardment of ions at the substrate.

The degree of metal ionisation is controlled by the peak power density dissipated at the target and reaches 50% for Ti and 70% for Ta. The high metal ionization fraction of the HIPIMS technology has been utilised in applications for metalizing high-aspect vias with depth-to-width ratio of 30:1 achieving 10% bottom coverage for Ti, Ta and Cu films. The technology has been upscaled to a production cycle for through-silicon via (TSV) interconnects on 200 mm wafers.

HIPIMS pretreatment can implant metals and rare earths in substrates whilst maintaining the crystalline character, promoting local epitaxial growth over large lateral areas and excellent adhesion. This enables the introduction of oxidation- and corrosion- barriers at the coating-substrate interface.

Reactive sputtering in Ar and N₂ atmosphere in HIPIMS are characterised with a strong dissociation of the nitrogen molecule. In conditions of high power density, the N¹⁺ : N₂⁺ ratio and Ti¹⁺:Ti⁰ ratio can exceed 1 thus promoting a fully dense intercolumnar boundaries in TiN films and increase their hardness. A preferred growth orientation of (200) is observed even without substrate biasing.

Nanoscale multilayer (superlattice) structured coatings based on CrAlYN/CrN have been grown with very low waviness and strongly improved density. These coatings provide excellent oxidation resistance and reduced fatigue deficit of aerospace turbine blades.

Nanocomposite coatings based on CrAlSiN were also deposited by HIPIMS for applications in high-temperature oxidation protection. Closer packing and reduced misorientation of nanocrystals as well as increased size of nanoclusters in which they are grouped are crucial mechanisms crucial in enhancing the film hardness.

Growth of films by condensation from the vapor phase frequently proceeds far from thermodynamic equilibrium giving rise to metastable configurations with unique attributes which are largely determined by the energy of the film forming species. One way of transferring energy to the growing film is via bombardment by ionized species which are present in plasma assisted physical vapor deposition (PVD) techniques. High power impulse magnetron sputtering (HiPIMS) is a novel plasma assisted PVD technique in which large fluxes of energetic ions are made available at the growing film. This is achieved by applying the power to the target in short unipolar pulses of low duty cycle (<10%) and frequency (<10 kHz). This mode of operation results in the generation of ultra dense plasmas (electron densities 10¹⁸-10¹⁹ m^-3) and a subsequent high degree of ionization for both gas atoms and sputtered material. HiPIMS has been extensively used for the deposition of polycrystalline elemental and compounds films facilitating control over their microstructure, phase composition, optical, mechanical and electrical properties. In the present talk the use of HiPIMS for the deposition of amorphous and nanocrystalline carbon and metal nitride based films is demonstrated. Discharges are generated using a variety of experimental parameters with respect to the pulse width, pulsing frequency, composition and pressure of the gas atmosphere. Time-averaged and time-resolved plasma diagnostics reveal that the variation of the above mentioned process parameters allows for control over the flux, the energy and the nature of the bombarding ionized species. Growth of films at those conditions enables to tune their bonding properties, their microstructure and their crystallinity and through this tailor important functional attributes such as their mechanical performance and high temperature stability.