B.-O. Johansson

Mechanisms of reactive sputtering of titanium nitride and titanium carbide II: Morphology and structure

Titanium was reactively r.f. sputtered in mixed ArN2 and ArCH4 discharges onto substrates held at 775 K. The films obtained were characterized by scanning electron microscopy and X-ray diffraction and through measurements of the microhardness and electrical resistivity. The composition of the films was determined by Auger electron spectroscopy. The measurements show that the morphology of the deposits to a large extent influences the properties of the films obtained. For TiN coatings the electrical resistivity reaches the bulk resistivity only if coatings with the full bulk density are obtained. The difference observed in the lattice parameter for TiN thin film and bulk samples is explained using a grain boundary relaxation model. It is also shown that the heat of formation of the compounds plays an important role in the formation of carbide and nitride films. A high heat of formation promotes the development of large grains and dense structures.

Mechanisms of reactive sputtering of titanium nitride and titanium carbide I: Influence of process parameters on film composition

Abstract The reactive sputtering of titanium in mixed ArN 2 and ArCH 4 discharges was studied by measuring the compositions of both the deposited films and the layers formed on the target. The sputtering experiments were performed in a diode r.f. sputtering unit and the compositions were determined by means of Auger electron spectroscopy. The results show that the amount of reactive constituents incorporated into the growing films depends on whether a compound has been formed on the target. At low pressures where no such compound is formed, the incorporation of the reactive gas follows an adsorption isotherm similar to the Langmuir adsorption isotherm, and very low values of the sticking probability are observed. At higher pressures species originating from the target determine the composition of the coatings. The important process parameters were found to be the partial pressures of the reactive and inert working gases, the ion current density and the voltage applied to the target.

Mechanisms of reactive sputtering of titanium nitride and titanium carbide III: Influence of substrate bias on composition and structure

Abstract Films of TiN and TiC were grown by reactive r.f. sputtering of titanium in mixed ArN2 and ArCH4 discharges on negatively biased substrates. The compositions of the films were measured using Auger electron spectroscopy and their structure and morphology studied using X-ray diffraction and scanning electron microscopy respectively. The composition was found to be strongly affected by the bias voltage. An increase in bias both the carbon and the nitrogen contents in substoichiometric films. However, when stoichiometric TiN is formed no further change in the composition occurs. For TiC films an increase in the carbon- to-titanium ratio above 1.0 is possible. The structure is also affected by the bias voltage. For example the grain size of TiN has a maximum at a bias of 200 V. A possible explanation is a double influence of the ion bombardment. At low voltages, the adatom mobility is enhanced and the grain size increases but as the bias increases resputtering of species from the growing film surface becomes increasingly important and the grain size decreases.

Kinetics of nitride formation on titanium targets during reactive sputtering

During reactive sputtering processes formation of surface compounds on the target leads to large reductions in deposition rate. The formation of such layers during the reactive sputtering of titanium with Ar-N 2 ion beams have been studied “in situ” using Auger electron spectroscopy. The kinetics of the process were studied by monitoring the AES nitrogen signal with time. The amount of nitrogen on the target surface was estimated using a simple kinetic model. The time dependence was found to be in agreement with the exponential rate predicted by the model. To explain fully the processes occurring, chemical reactions at the surface, altering, e.g., the sticking probability and penetration of reactive ions into the target lattice have to be taken into account. It is also shown that the stable δ -TiN phase is formed on the target surface. The most important parameter for the formation of the nitride layer on the target was the ratio between the number of Ar and N 2 ions impinging. The total number of N 2 ions at a fixed N 2 -Ar ratio, i.e. the degree of ionization, was also found to be important. The energy, however, was found to have only a minor influence on the formation of the nitride layer. The resulting nitride layers during ion beam sputtering are also compared with layers formed on the target during reactive sputtering of titanium in an ordinary RF sputtering apparatus.

Effects of substrate temperature and substrate material on the structure of reactively sputtered TiN films

Abstract Stoichiometric TiN films were reactively magnetron sputtered in an Ar - N 2 atmosphere. The films were deposited at various substrate temperatures in the range 200–650°C onto two types of substrate material, high speed steel and stainless steel. The microstructure of the films obtained was investigated by the use of a transmission electron microscope and the morphology was studied in a scanning electron microscope. Measurements of the hardness were also performed. The analysis of the microstructure shows that the growth of the film is markedly influenced by the substrate material. In particular, the high speed steel substrates were found to have a considerable influence on the microstructure. The vanadium carbide particles in these steels, which have a good lattice match to TiN, stimulate a localized epitaxial growth to occur on these carbide particles. This results in a microstructure consisting of large grains surrounded by small grains. The shape of the large grains is influenced by the temperature. In the development of these large grains cracks and/or voids occur in and around the grains at substrate temperatures above 400°C and the hardness drops by about 20%. No large grains were found on films deposited onto stainless steel and their hardness increases slightly with temperature. High hardness for films deposited onto the high speed steel substrate at temperatures above 400°C can also be obtained if a substrate bias is used. Ion bombardment during film growth suppresses the formation of the large grains with voided or cracked boundaries because of a continuous renucleation process. The formation of the different microstructures is discussed in terms of surface energy minimization and thermally activated processes as surface and grain boundary migration.

Reactively magnetron sputtered Hf‐N films. II. Hardness and electrical resistivity

The microhardness and electrical resistivity have been measured on thin Hf‐N films, covering the entire composition range from pure Hf to overstoichiometric HfN. Influence of substrate bias on the properties of stoichiometric HfN films has also been studied. All films have been prepared by high‐rate reactive magnetron sputtering at a substrate temperature of 400 °C. Both hardness and resistivity increase as nitrogen is added to the α‐Hf phase. For the cubic HfN phase the hardness has a maximum, ≊3500 HV, and the resistivity a minimum, 225 μΩ cm, at a composition close to stoichiometry. However, both values are considerably higher than those reported for bulk samples. This is explained in terms of nonequilibrium growth conditions, giving rise to high densities of dislocations and interstitially incorporated nitrogen atoms. For films with a nitrogen content above 50 at. % a very high‐resistivity value is found, 2.0 Ω cm at maximum. By applying a low substrate bias voltage the resistivity of stoichiometric H...

Influence of substrate bias on composition and structure of reactively r.f.-sputtered TiC films

Abstract TiC films were grown by reactive r.f. sputtering of titanium in methane onto negatively biased substrates. The composition of the films was measured using Auger electron spectroscopy in combination with depth profiling and the structure was revealed using X-ray diffraction. The carbon-to-titanium ratio was found to be 0.98 at zero bias voltage. It increases with bias voltage up to -200 V where it reaches a maximum of 1.08. As the bias is further increased the ratio decreases and approaches 1.00 at -500 V. These results show that in addition to the reaction on the target surface that occurs during the reactive sputtering process a reaction at the substrate surface must also occur. This is in agreement with earlier reported results in which the composition of both the films and the target surfaces was determined for different methane partial pressures. The reaction at the substrate surface is discussed in terms of ionization and reaction probabilities of methane molecules and in terms of resputtering effects. Owing to the increasing ion bombardment of the films with increasing bias voltage a change in crystal structure was obtained. The grain size decreased from 400 A at zero bias to about 150 A at -500 V. A variation in the lattice parameter was also found, a maximum being reached at a bias between -200 and -300 V.

The microstructure of reactively sputtered Ti-N films

Abstract Ti-N films with nitrogen concentrations ranging from 0 to 52 at.% prepared by both d.c. magnetron and r.f. reactive sputtering are examined using transmission electron microscopy. The observed microstructures are correlated with physical properties of the films. In particular, the maximum in the microhardness and density and the minimum in the electrical resistivity are found to correspond to a fully dense structure. The sample prepared by magnetron sputtering has a very inhomogeneous and voided microstructure and a lower hardness than the r.f.-sputtered sample with a similar nitrogen content. A theory is proposed for the development of a lamellar microstructure in the films containing two phases.

Reactively magnetron sputtered Hf‐N films. I. Composition and structure

Thin films of Hf‐N, covering the entire composition range from pure Hf to overstoichiometric HfN, have been prepared by reactive magnetron sputtering. The structure of the films has been investigated by x‐ray diffraction, transmission electron microscopy, and scanning electron microscopy and the composition has been determined using Auger electron spectroscopy. A solubility of ≊30 at. % nitrogen is found in the α‐Hf region. Above 30 at. % N a multiphase structure consisting of α‐Hf, HfN, Hf3N2, and/or Hf4N3 is found, which is not in agreement with the equilibrium structure. In the mononitride region a lattice parameter of ≊4.53 A is observed. This value is slightly higher than reported bulk values due to intrinsic stress in the films. Increasing the nitrogen content above 50 at. % causes a distortion of the cubic symmetry of the lattice. This is first observed as an increase in the (111) interplanar spacing while other spacings decrease. Also a splitting of some reflections occurs at higher nitrogen conte...

Structure of reactively magnetron sputtered Hf‐N films

In spite of the technical interest in the Hf‐N system the form of the phase diagram is not yet clear, and therefore it is difficult to predict the phase composition of thin films. In this letter Hf‐N films have been prepared by reactive dc planar magnetron sputtering and the phase composition of the films investigated by x‐ray diffraction. Between the α‐Hf and HfN single phase regions a multiphase region was found consisting of α‐Hf, e‐Hf3N2, and/or ζ‐Hf4N3 and also HfN in the nitrogen rich part. For the mononitride a lattice parameter of 4.54 A was found which is slightly higher than reported bulk vlaues. This deviation is caused by intrinsic stresses in the films. If the nitrogen content is increased above that of the mononitride, the (111) interplanar distance increases further while all other interplanar distances decrease. At still higher nitrogen contents new reflections start to appear indicating a new phase. In this region the films also change from being conducting and nontransparent to electrica...