Establishing the Relationship between Substrate Bias Voltage and Formation Process of Single Component Ion-Plasma ’ s Film Based on Tin by Electric-Arc Evaporation

Influence of substrate bias voltage on temperature conditions, formation stages, structure formation processes and prevailing orientation of titanium nitride films in the course of electric-arc evaporation was investigated. Increase in substrate bias voltage was found to accelerate considerably the formation stages of poly-crystalline TiN films with prevailing crystallographic orientation (111). Degree of prevailing orientation and crystallinity increases with substrate bias voltage. Optimum temperature range of polycrystalline (111)TiN films is 645-725 K. [DOI: 10.1380/ejssnt.2011.34]


I. INTRODUCTION
The operating properties depend on ion-plasma films determined by the actual structure depend on combined action of many process parameters, in particular, when producing films by electric-arc evaporation: the arc current, the partial pressure of the operating gas mixture, the negative bias voltage value, the percent ratio of the reactive gas and the inert gas, the cathode-to-substrate distance.Specifically structured films with improved and stable operating properties may be produced by optimizing the above deposition process parameters, developing the mode of controlling formation and structuring process of polycrystalline films, the phase composition and prevailing crystallographic orientation direction of phases, the temperature of the film during deposition, its physical, mechanical and chemical properties [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19].
The aim of this work is to examine the influence of substrate bias voltage on the temperature conditions, pattern and formation stages, structure formation processes, prevailing orientation direction, phase composition and mechanical properties of TiN films during electric-arc evaporation.

II. EXPERIMENTAL
Steel 3 test samples with TiN films were produced using industrial NNV-6.6-I4 unit by means of one electricarc evaporator with VT-1-00 titanium cathode at varying substrate bias voltage.Prior to the process, the resistance heater was turned on in the central part of the vacuum chamber for 20 min to remove any residual moisture and gases off the vacuum chamber walls, then, to increase TiN film adhesion strength, the surfaces of the test samples was treated by ion cleaning and heating and titanium underlayer was deposited.The test sample temperature after ion cleaning, un-derlayer deposition, and the film temperature after each 10 min at total film deposition duration of T d = 30 min was measured us-ing "Termix" infrared non-contact pyrometer.The structural features of the films produced were examined using scanning * Corresponding author: kameneva@pstu.ru   after the film deposition process.

III. EXPERIMENTAL RESULTS AND DISCUSSION
The process parameters of substrate surface pretreatment prior to the film deposition (ion cleaning and TiN underlayer deposition) are given in Table I, the process parameters of electric arc evaporation and the microhardness of TiN film-substrate composition (hereinafter "the composition microhardness") in Table II.X-ray diffraction phase analysis of TiN film areas established that, regardless of the bias voltage value, a two-phase film comprising main cubic TiN phase with prevailing (111) crystallographic orientation and auxiliary hexagonal TiN 0.3 phase (Fig. 1, and Tables III and IV) is formed on the substrate.
In spite of dissimilar internal stresses arising in the film due to maximum deviation of the interplanar space from the tabular value and minimum peak width of

2) Globular substructure primary ordering stage
This is a structural transition from globular to granular substructure.As the film temperature increases due to increase in the deposition process duration, variously oriented space (3D) granular structures distributed statically uniformly over the film surface are observed to form, due to ordering of film with globular structure (Fig. 3(a)).It should be noted that combined increase in film temperature and ion energy due to increase in its heating rate and bias voltage value, correspondingly, facilitate increase in 3D structure-to-substrate contact area, like at the globular stage (Fig. 3(b)).The tops of 3D structures have flattened domelike shape (Fig. 3(a)).Minimum diameter and min-imum height of a 3D structure correspond to a film formed at 200 V and 250 V (Fig. 3 The stage conditions are as follows: film heating rate and its tem-perature of 13.5 K/min and 650 K, correspondingly.The substrate contact area and height of 3D structures with granular substructure increase, the shape and orientation with respect to the substrate remaining the same (Figs.4(a) and (b)).As 3D structures coalesce, the grain boundary migrates towards smaller ones and the structure surface is being textured (Fig. 4(a), an increased fragment is given on the right).Defects formed at the globular stage (Fig. 2(a)) result in discontinuity (incoherence) of 3D structures as the latter coalesce (Fig. 3(b)).Conditions for primary ordering as pseudofaces 100 at the surface of structures with granular substructure corresponds to film heating rate V heat = 14.5 K/min (Fig. 4(c)).Maximum increase to V heat = 15.0K/min and of film temperature to 745 K facilitates coalescence of aggregates of the above structures with granular substructure to rod-like structures of up to 10.0 µm (Fig. 4(d)).Primary polycrystalline (Fig. 5(a)) and seed crystallites with faces 100 (Fig. 5(b)) nucleate on the film surface when optimum values of film temperature (660-705 K) and energy of ions bombarding the film surface, fur to higher rate of its heating 14.0-15.0K/min, and substrate bias voltage increase up to 150 V, correspondingly.Seeds of polycrystalline component of the film unidirectional with respect to the substrate and distributed uniformly over it are formed during film deposition at substrate bias voltage of 200 V and film heating rate of 14.2 K/min (Fig. 5(b)).

5) Polycrystalline film island formation stage
The stage of nucleation of the polycrystalline component of the film passes into the stage of formation of polycrystalline structures-islands on the film surface (Figs.6(c) to (e)).Growth of film temperature facilitates finally a multiple increase in diameter of 2D islands.Substrate bias voltage of U b = 200 V was found to result in multiple increase of number and diameter of islands, and decrease of their height and crystallite diameter (Fig. 6(c)).Repetition of the stage of formation of 3D structures with granular substructure on the surface of polycrystalline islands should be noted (Figs.6(b), (d),  and (e)).At film temperatures of <725 K and absence of preliminary nucleation stage either islands with globular structure and fractal surface geometry (Fig. 6(a)), or islands with disordered polycrystalline structure (Fig. 6(b)) are formed.

6) Continuous polycrystalline film formation stage
Electron microscopic examination of the film surface structure showed that a continuous film with cellular structure is formed in the range of temperatures and heating rates of T = 675-715 K, V filmheat = 12.5-15.0K/min.Cellular structure is known to occur often in films formed for different reasons: stresses (most often), dissatisfactory surface treatment, residual island structures or signs http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/)  of fractality (for metals).Cell diameter is observed to decrease as the above values increase (Fig. 7(a)) and structure passing into granular form (Fig. 7(b)).We may assume that increase in the film temperature promotes stabilization of its structure.
Continuous TiN films formed at any bias voltages have a characteristic feature of ion-plasma films: peculiar "shortage" of material for filling intercolumnar voids due to partial or complete formation synchronicity loss of columnar structures and resulting in loss of continuity (and, therefore, coherency) (Fig. 7(c)).Minimum crystallite size within 20 nm (Fig. 7(c)-an increased fragment is given on the right) and minimum film discontinuity size are char-acteristic of formation conditions U b = 200 V. Maximum discontinuity size was found in a film formed at U b = 250 V (Fig. 8(e)).

7) Stage of formation of rod-like structures on continuous film surface
Regardless of technological and thermal conditions, rod-like microstructures are formed on the film surface after 30 min due to coalescence of 2D and 3D structures with globular or granular substructure (Fig. 8).The largest rod-like structures with globular substructure are formed if bias voltage of 80 V is applied to the substrate during deposition (Fig. 8(a)).At minimum voltage in-crease of up to U b = 100 V, besides structures with globular or granular substructure, a two-layer structure with columnar substructure at the substrate and globular substructure at film surface is formed (Fig. 8(b)).At voltages U b = 150 V, 200 V the structures have ordered columnar substructure, their size is reduced to 20 µm (Figs.8(c) and (d)).Numerous and large structures, maximum number of surface discontinuities are characteristic of films formed at 250 V (Fig. 8(e)).Examination of fine structure of continuous film surface found granular structure whose fineness decreases as the film temperature grows.
Formation stages of TiN film were established depending on substrate voltage value, film temperature and heating rate (Fig. 9).

IV. SUMMARY
As bias voltage and energy of ions bombarding the substrate increase, the rates of film formation stages are observed to increase due to the film heating rate increasing, the temperature range of film formation shifting towards higher temperatures.
Change in temperature conditions of formation of the film is reflected on the process of structuring the film.http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology Volume 9 (2011) Change in phase composition is reduced to increase the volume fraction of cubic phase (111) TiN and decrease the hexagonal phase TiN 0.3 in the film.Change in mechanical properties of the film is directly depended on technological and thermal and structural characteristics.Maximum microhardness of the composition corresponds to a film formed at bias voltage of 200 V, temperature range of 645-725 K, film heating rate of V filmheat = 14.2 K/min and distinguished with minimum width of X-ray diffraction cones (111)TiN and (101)TiN 0.3 , deviation of interplanar space from tabular values, various internal stresses in the film.Exceeding the optimal value of the bias voltage leads to the formation on the surface of the film numerous and maximum size of extended structural formations and surface discontinuities.In the case of reduction of the bias voltage with respect to the optimum, globular, cellular, or disordered columnar structure will be formed.To form polycrystalline TiN film by electric-arc evaporation, a combination of optimum and stable process parameters; temperature range and film heating rate while depositing it necessary for sequential occurrence of all formation stages is required.

FIG. 1 :
FIG. 1: Results of X-ray diffraction phase analysis of areas of TiN films produced by electric-arc evaporation at various substrate bias voltage values.(a) Comparative spectrum of diffraction pattern fragments within the double Bragg angle range of 33-44 • , and (b) comparative diagram of diffraction peak intensities of cubic TiN phase reduced to 100% maximum peak, within 25-80 • .

3 .
FIG. 3: Formation of 3D structures with granular substructure: influence of bias voltage on the shape, the orientation with respect to the substrate and the size of the structure.(a) 150 V-ϕ1.4 µm, H ≈ 2.0 µm, and (b) 200 V-ϕ720 nm to 2.5 µm, H ≈ 1.0 µm.

Fig. 9 .FIG. 9 :
Fig. 9. Formation stages of TiN film depending on substrate voltage value, film temperature and Forming of 3D objects with grain substructure, max contact area, normal to substrate Integration of 3D objects with grain substructure

TABLE I :
Process Parameters of Ion Cleaning and Ti Underlayer Deposition

TABLE III :
Structural characteristics of TiN-based films produced by magnetron spraying.U b is the substrate bias voltage; V is the volume percentage of phases; dTiN/dTiN tab is the interplanar space; I101TiN0.3,I111TiNarethe reflection intensities of the corresponding phases; ITiN and ITiN0.3 are the total reflection intensities of the cubic and hexagonal phases; maxI111TiN/IΣ and maxI101TiN0.3/IΣ is the ratio of (111) and (101) reflection intensities to total reflection intensity of all TiN phases, width β0 of the X-ray diffraction line.

TABLE IV :
Location of Diffraction Peaks