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  • Titanium Nitride Films
    Jan 04, 2018

    When sputtered or evaporated, titanium is a very reactive metal that easily form nitrides, oxides or carbides. Titanium nitride (TiN) has a NaCl structure that is stable over a broad composition interval allowing both under- and overstoichiometric phases. At low nitrogen content in an inert carrier (e.g. argon) also a Ti2N phase is possible. 


    Titanium nitride has a high hardness and a high resistance against corrosion and a low electrical resistivity somewhat lower than pure Ti. Furthermore thin TiN films can exhibit hardness much higher than and resistivity much lower than equilibrium bulk values. One of the most widespread applications of TiN films is in wear protection of cutting tools like drills and mills and of tool bits made from tool steel or high speed steel. On hard metal inserts for turning and milling TiN films often is the outermost layer in a multilayered coating. For this application CVD is the most utilized deposition method due to the possibility to coat very large batches at the same time. 


    In the microelectronics TiN is used as a gate metal in MOS structures because of the low resistivity, but also as a diffusion barrier. The stoichiometric (Ti/N = 1) TiN highly resembles gold visually and this makes it popular for decorative coatings for watches and other objects. Titanium nitride is biocompatible material and this property has given rise to a large field of applications in medicine, e.g. surgical implants. Typical properties of a commercial, tribological TiN coating (Balinit ® A) are a hardness of 2300 HV and a thermal stability up to 600 °C. The big industrial interest and the wide variety of applications for TiN thin films have often made them popular research objects where many different PVD-methods have been tested and resulting film properties have been studied. 


    Some common examples of frequently used PVD methods are electron beam evaporation, magnetron sputtering, and cathodic arc deposition. A Taiwanese group has studied TiN deposition by a reactive hollow cathode discharge ion-plating (HCD-IP) technique. In this method a RF hollow cathode is used as a high-current low-voltage electron gun for electron beam evaporation of a Ti-crucible and for simultaneous ionization of metal atoms and gas (Ar and N2 ) molecules. Typical deposition conditions are an RF power of 6 kW, a working pressure of 0.29 Pa (2.2 mTorr) and an applied DC substrate bias of -40V. 


    The preferred orientation of the obtained TiN films was for most deposition conditions especially for filmsthicker than 1 µm. The hardness of the films increased with increasing TiN texture coefficient and it was saturated at 28 GPa as the coefficient approached unity. The group has also studied the influence of ion bombardment on preferred orientation in crystalline TiN films by varying the bias voltage, the deposition power, and the nitrogen partial pressure. The ion bombardment was found to cause strain accumulation or lattice damage and the preferred orientation at low deposition temperatures is determined by which of these phenomena that dominates. The preferred orientation develops at strain accumulation and the orientation at lattice damage. The thermodynamically favorable orientation occurs when no ion bombardment is present. Further, the group investigated how the porosity of TiN films was influenced by the deposition temperature, deposition time, and ion bombardment. They conclude that long deposition times or high temperatures and a high degree of ion bombardment reduce porosity and that ion bombardment also affects grain size and preferred orientation. Dense films have either large grains or small grains with high texture coefficients. 


    Commercial techniques for reactive magnetron sputtering have frequently been applied for deposition of TiN films. Guruvenket et al. have studied the influence of ion bombardment and substrate orientation on properties of TiN films deposited on Si substrates in a DC planar magnetron system. Films deposited at a total pressure of 0.1 Pa with negative bias on Si substrates had a preferred orientation of TiN while it was TiN for films deposited on Si substrates. The grain size decreases when the bias is decreased from +20 V to negative values but then remains almost constant for bias down to -60 V. At negative bias the grains were smaller on Si than on Si. The influence of nitrogen partial pressure on the properties of reactive DC magnetron sputtered TiN films has been studied by Meng et al. Films with preferred orientation were deposited on unheated glass substrates at a total pressure of 0.8 Pa while the nitrogen partial pressure was varied from 0.08 to 0.3 Pa. The results were that the TiN texture coefficient decreased with increasing nitrogen partial pressure while the grain size increased. Other common methods for deposition of titanium nitride thin films are based on cathodic arc deposition. Two such methods were presented by Martin et al. : filtered arc deposition (FAD) and ion assisted arc deposition (IAAD). FAD has been used for TiN deposition on heated and biased Si and steel substrates (350°C) in a nitrogen atmosphere. In this setup the stress and hardness could be controlled by varying the bias. 


    In the IAAD a nitrogen ion source, that supplies N2 + ions with a fixed energy of 500 eV, is added to the FAD system. This setup allows deposition on unheated Si and carbon substrates with control over stoichiometry by the ion beam current. The deposition rates were 100 nm/min (6 µm/h) for both setups. The influence of deposition conditions on crystal and microstructure has been studied quite extensively and several models have been presented. One of these models was presented by Zhao et al. and called “Overall energy model”. The model aims to explain the evolution of preferred orientation in TiN films deposited by a biased filtered arc deposition method and is fo- cused on the ion bombardment of the film. It is based on the minimization of a total energy that is the sum of the surface energy, the strain energy, and a “stopping energy” that is defined as the density of the deposited energy of ions along a certain crystalline direction. At small film thickness the surface energy dominates over the strain energy and the preferred TiN orientation should be. At an increasing film thickness or an increasing bias the strain energy becomes dominating which leads to a preferred orientation of TiN. At a very high bias a resputtering occurs and the stopping energy becomes dominating and the TiN orientation becomes the preferred one. Other researchers have applied the Thornton structural zone model originally developed for sputtering of pure metal films also for TiN film deposition. All these findings and approaches are very important in the understanding of properties of films deposited in non-conventional systems like that used in the present PhD work.


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