D. Manova

Formation of TiN, TiC and TiCN by metal plasma immersion ion implantation and deposition

Abstract Titanium nitride, titanium carbide and titanium carbonitride are well known compounds displaying a rather high hardness and melting point, thus enabling their use as hard coatings. With a titanium cathode and both nitrogen and methane gas, a series of compound TiCxNy can be prepared with the final points TiN and TiC using metal plasma immersion ion implantation and deposition. The respective properties of the films are investigated by X-ray diffraction and Rutherford backscattering spectroscopy. With increasing pulse voltage, a decreasing carbon content and growth rate is found for the TiC series, while no such effect is observed for corresponding TiN films.

Heat balance during plasma immersion ion implantation

The substrate temperature was measured during plasma immersion ion implantation (PIII) of nitrogen as a function of the pulse bias voltage and the repetition frequency. The variation of the equilibrium frequency for temperatures between 150 and 500 °C and pulse voltages between -5 and -30 kV was investigated. Using these data, the relative dose per pulse for different voltages was obtained and its voltage dependence compared with different models for the plasma sheath expansion during PIII. A higher plasma density than measured for a static plasma without pulses, due to the interaction of secondary electrons with the plasma, must be assumed. Good agreement with dose measurements of N implanted in Si was also observed. For high pulse frequencies above 1 kHz a deviation was observed, clearly showing that depletion of the ions from the plasma during the pulses leads to reduced average plasma density at high repetition rates.

Optical characterization of TiN produced by metal-plasma immersion ion implantation

The dependence of the optical properties of TiN thin films prepared by metal-plasma immersion ion implantation (Me-PIII), using a cathodic arc as the source of Ti ions, on the pulse voltage and the gas composition was investigated. High voltage pulses up to -10 kV at a duty cycle of 9% were used while the gas flow was varied between 20 and 50 sccm. For all bias voltages TiN films oriented with the (100) axis normal to the surface was obtained. For the screened plasma energy ω ps a strong dependence on the gas mixture was found, increasing from 2.6 eV for high flow/current (F N2 /I are ) ratios to more than 3.5 eV for low F N2 /I are ratios. This is correlated with a reduction of the nitrogen content as the composition changes concurrently from TiN 0.95 to TiN 0.55 .

Balancing incident heat and ion flow for process optimization in plasma based ion implantation

Plasma based ion implantation at elevated temperatures is a technology often used to obtain thick surface layers of several µm by thermally activated diffusion, e.g. nitrogen in steel, titanium or aluminium. By lowering the pulse voltage at constant temperature, the current density can be increased at a constant heat flow. However, an upper limit is given by the ratio of the diffusion rate transporting the implanted ions from the surface towards the bulk and the sputter yield. This sputtering of the surface dominates for very high current densities and limits the maximum achievable layer thickness. Different maximum current densities were found for the four investigated systems - nitrogen in different steel grades, aluminium and titanium, as well as oxygen in titanium - reflecting the varying diffusivities. Additional requirements, besides the maximum current density, as a conformal treatment for complex objects containing small holes or trenches, as well as short heating times, can be solved most effectively by pulsed voltages in the range of 2-5 kV and an additional heating of the sample. The problem of a sample cooling time of several hours after the treatment is recognized. A partial solution would be to increase the gas pressure during the cooling phase for a more effective heat dissipation.

Lateral texture evolution during formation of TiN by MePIIID

Abstract Titanium nitride was formed by metal plasma immersion ion implantation and deposition using a vacuum arc with a Ti cathode and a nitrogen backfill of the vacuum chamber. A circular substrate with a diameter of 6 cm was mounted at a distance of 39 cm from the cathode. For the substrate oriented perpendicular to the plasma stream, a deposition rate independent of the position and nearly independent of the pulse voltage was obtained. A variation of the orientation of the TiN crystallites with increasing pulse voltage was observed; [220] at 1 kV pulse at 9% duty cycle, and [200] for pulse voltages from 3 to 10 kV. Additionally, a tilt of the crystallites of up to 25° from the surface normal is measured towards the edge of the substrate. This is related with the changing angle of incidence of the ions. Using a substrate tilted at 45° towards the plasma flow, a different behaviour is observed. First, the deposition rate decreases by 40% and second, the fibre texture is changed to a biaxial texture near the edge, caused by the lower symmetry of the systems.

Filtered arc deposition and implantation of aluminium nitride

Abstract The formation and properties of AlN thin films deposited on Si and sapphire substrates were studied. A cathodic aluminium arc in nitrogen gas was used at room temperature to prepare the samples. Additionally, high voltage pulses between 0 and –10 kV with a duty cycle of 9% were applied to the substrate. Without filtering only condensed aluminium droplets were observed on the substrates while transparent films with very few droplets were produced when a filter was used to reduce the number of macroparticles. Growth rates of some 20 nm/min of pure AlN were determined with Rutherford backscattering spectroscopy. X-Ray diffraction measurements were employed to determine the crystal quality. Highly textured hexagonal AlN films with the c -axis oriented normal to the surface were found at all conditions, from no additional bias to 10 kV pulses. The surface roughness remained low over the whole voltage range with values between 0.3 nm without bias and a maximum of 5 nm at 5–7.5 kV. A rather high refractive index in the visible region was found with spectroscopic ellipsometry.

Orientation dependent sputter yield of aluminium

During nitrogen plasma immersion ion implantation (PIII) of aluminium, a strong variation of the surface structure across the surface was observed. Some of the grains exhibit a nearly flat surface, while others show a strongly corrugated relief with a high roughness. A similar effect was seen for sputtering a polycrystalline aluminium sample with 5 keV Cs ions at an incident angle of 60°. A variation of the sputter yield of up to 20% is observed, together with different surface morphologies for differently oriented grains, as determined by analysis of electron backscatter patterns (EBSP). We conclude that a small difference in the surface binding energy for different surface planes may lead to the observed variation of surface topography by a sputter yield depending on the particular grain orientation. Variations of diffusion constants and surface adsorbates with crystallographic direction may further contribute to the observed heterogeneity.

Oxygen behaviour during PIII-nitriding of aluminium

Abstract During nitrogen plasma immersion ion implantation (PIII) treatment of pure aluminium at elevated temperatures between 250°C and 500°C, a rather high oxygen concentration of up to 20 at.% is observed in the treated samples, despite a base pressure of 10 −5 Pa and a working pressure of 0.2 Pa. This behaviour is caused by oxygen uptake via surface defects created by the ion bombardment, as shown in a control experiment with Ar implantation. A thermally activated oxygen diffusion tail is observed for nitrogen implantation, which can be explained by the diffusion of Al cations in AlN.

Interplay of surface adsorption and preferential sputtering in metal plasma immersion ion implantation and deposition

Abstract Formation of hard ceramic surface layer by metal plasma immersion ion implantation and deposition (MePIIID) can be a quite complex process as quite a number of different species, including condensable metallic ions, electrons and neutral gas, which can be partially ionised, are interacting in the vacuum chamber and impinging on the surface. The resulting growth rate and chemical composition will be determined by the complex interplay of surface adsorption, ion implantation and preferential sputtering. Here, an overview of these effects is given for the important systems ZnO, TiO2, TiN and AlN prepared by MePIIID. In ZnO, the predominant mechanism is preferential oxygen sputtering. However, the oxygen adsorption from the background gas is stronger in the case of TiO2, leading to a constant Ti/O ratio beyond a threshold in the oxygen gas flow for this compound. This effect is less pronounced for TiN, where a continuously varying Ti/N ratio was found only for a varying gas flow and independent of the pulse voltage. In contrast, a constant Al/N ratio over a broad range of nitrogen gas flows was observed for AlN.

Homogeneity of metal plasma immersion ion implantation and deposition

Metal plasma immersion ion implantation and deposition (MePIIID) is a method, where metallic ions emanating from a cathodic arc are deposited onto a sample, intermittently pulsed to negative high voltage pulses. Thus, a fast coating of three-dimensional samples with excellent adhesion properties is possible at low temperatures. Albeit, the homogeneity and the ion incident angle are strongly dependent on the process conditions, i.e. cathode material, gas backfill, sample size and pulse voltage. Corresponding experiments for AlN, TiN, Ti and Al using pulse voltages of up to 10 kV and different flat sample holders are presented. In the discussion, an attempt to assess the relative importance of the process parameters is made.