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A 2.3 kJ pulsed plasma focus device was used to deposit thin films of titanium nitride at room temperature onto the aluminum substrates. Films were deposited with 20 and 40 numbers of focus shots, at a distance of 9 cm from the top of the anode axis. Deposited films were characterized for their structure by X-ray diffractometer (XRD), surface morphology by scanning electron microscope (SEM) and hardness using a nano-indenter. A study of structure, surface morphology and hardness of films for 20 and 40 focus shots is reported. XRD patterns show the growth of polycrystalline TiN thin film with nano crystallites. Behavior of lattice constant, grain size, micro-strain, dislocation density and texture coefficient developed in the film for different phases of deposited film is discussed. SEM micrographs show smooth, dense and uniformly distributed film with fine-grained morphology. Hardness is found to follow inverse Hall-Petch relation.
Titanium nitride is a member of the refractory transition metal nitrides family exhibiting characteristics of both covalent and metallic compounds [1,2].Titanium nitrides have excellent mechanical, thermal, and electronic properties, such as good thermal stability, high corrosion resistance, and low electrical resistivity, and therefore have many applications ranging from coatings on cutting tools to diffusion barriers in microelectronic applications. This broad range of applications has resulted in the development of a wide variety of deposition methods. However, most of these techniques require moderate to high substrate temperatures to form crystalline films. The high deposition temperature inhibits the use of TiN films in some applications where the substrate cannot withstand elevated temperature. It is therefore of interest to study the possibility of TiN deposition at lower substrate temperatures.
The major obstacle to low temperature growth is the difficulty of obtaining the high surface mobility required for the nucleation and growth of crystals at low substrate temperatures. This limitation can be overcome by delivering highly energetic charged or highly excited species of the material to the substrate. The latter objective may be accomplished by the use of Dense Plasma Focus (DPF).
The dense plasma focus device [3,4] is a simple pulsed plasma device in which the electrical energy of a capacitor bank, upon discharge, is initially stored as the magnetic energy behind the moving current sheath as the sheath is accelerated along the coaxial electrode assembly. A portion of this magnetic energy is then rapidly converted into plasma energy during the collapse of the current sheath towards the axis beyond the end of the central electrode resulting in the formation of short lived, but hot (≈1-2 keV) and dense (≈1025-26 m-3) plasma consisting of molecules, ions, and electrons. With little loss of energy, the plasma is transported to and deposited onto substrate.
In this paper, we report the deposition of TiN thin films at room temperature on the aluminum substrates using plasma focus device. The films were deposited with different numbers of focus shots at 9 cm axial position with respect to anode axis. A systematic study of the structure, surface morphology and hardness, for films deposited using different numbers of focus shot, is presented.
2. Experimental setup and methodology
Deposition of the TiN thin films was done on polished 10-10-5 mm3 aluminum substrate by dense plasma focus. The substrates were cleaned by rinsing in ultrasonic bath of water. The deposition process was performed in a Mather-type DPF device powered by single 32 µF, 15 KV capacitor. Details of the plasma focus device are given in earlier work [3-5]. The schematic diagram of the system is shown in Fig. 1. The conventional Mather-type plasma focus electrode assembly has a hollow copper anode being surrounded by six cylindrical copper cathode rods in a squirrel type cage. For deposition of titanium nitride films, copper anode was replaced with titanium fitted anode. The focus chamber and cathode rods were kept at ground potential. Nitrogen was used as a working gas. The chamber was evacuated up to 1 - 10-2 mbar by a rotary vane pump and filled with high purity nitrogen gas before plasma focus operation. The focusing action was monitored using a simple resistive voltage probe and Rogowski coil. The intense voltage spike in the high voltage probe signal and a steep current dip in Rogowski coil are an indication of the strong focusing action, which ensures efficient energy transfer, and heating of plasma column. The substrates are mounted, downstream of the anode axis, at a fixed distance of 9 cm from the top of the anode using a substrate holder behind a moveable metallic shutter, shown in the Fig. 1. It always takes several focus shots to get strong focusing after each fresh loading of gas for film deposition. A metallic shutter in between the anode and the substrate was used to prevent the exposure of substrate to these initial weak focusing shots as shown in Fig. 1. The shutter is removed generally after two or three focus shots, after obtaining good focusing. The focus shots are fired at a frequency of one shot per minute; a time long enough to ensure thermal relaxation of specimen after being heated by the preceding ion beam. The films are deposited, at room temperature substrates, using 20 and 40 focus shots.
The qualitative understanding of thin film deposition process, in dense plasma focus device, is as follows: DPF transfers the electric energy stored in the capacitor to the chamber by a spark gap switch. The dielectric breakdown of gas occurs along the insulator surface in between anode and cathode and an axisymmetric current sheath forms around the insulator. This current sheath moves towards the open end of electrode assembly under J -B force. When this current sheath reaches the top of electrode assembly, it collapses radially inward during the final focus phase. This is the instant where micro instabilities, mainly m = 0 instabilities start to grow and in turn enhances the induced electric field locally. This enhanced electric field, coupled with magnetic field, breaks the focused plasma column by accelerating ions towards the top of chamber and electrons towards the positively charged anode. After this, disruption of the plasma column starts and it breaks up completely, to form hot (≈1-2 keV) and dense (≈1025-26 m-3) plasma.
The deposited TiN films are characterized for their structure, surface morphology and hardness by a variety of techniques. The crystalline structure of the films is characterized by X-ray diffraction (XRD) using X'Pert PRO MPD X-ray Diffractometer (XRD). HITACHI S-3400N Scanning Electron Microscope (SEM) is used to study surface morphology of the films. The hardness measurement is done using Wilson Wolpert 401MVA Vickers Micro-hardness tester.
3. Result and discussion
3.1 Phase identification
The XRD patterns of plasma focus deposited thin films at 20 and 40 shots, are shown in Fig. 2. The patterns were recorded over a range of 2θ angles from 35o to 80o and crystalline phases of TiN matches with JCPDS card no. 38-1420. Only a single phase of TiN with FCC structure and the peaks corresponding to (111), (200), (220) and (311) planes are observed. There is good correspondence between the PDF data peaks and measured peaks. The patterns also show peaks of AlN (111), (200) and (220) planes in addition to those peaks identified for TiN planes.
The observed intensity of the TiN planes for film grown at 20 focus shots were higher than films grown at 40 focus shots, which is possible because a greater compaction in the film could have been produced, taking into account that the intensity of diffraction peaks are related to the quantity of planes that generate diffraction . The plausible explanation of decreasing intensity with increasing focus shots is as follows: The highly energetic filling gas species from the following focus shots, besides helping formation of titanium nitride due to its nitrogen ion content, can also cause substantial radiation damage of the film that has already been deposited with previous shots. This energetic ion induced radiation damage can be in the form of etching, change of phase, change of stoichiometry of deposited film. The radiation damage of deposited film is dependent on the energy and flux of ions that are impinging the film surface. As more focus shots were fired the focus becomes more stable with stronger and stronger focusing action resulting in generation of high energy, high flux ions causing greater radiation damage . In the plasma focus thin films deposition process, many growth mechanisms occur, such as adsorption, nucleation, coalescence and re-sputtering. Up to 20 focus shots, the film continues to grow, but after that the re-sputtering process start to be dominant, because the ad-atoms gain enough energy, returning to the plasma, less material is deposited . This produces diffraction peaks with less intensity. In all physical processes, there is an equilibrium point, where the system could have its greater efficiency, which is 20 focus shots in our case.
3.2 Lattice constant
In order to further investigate the micro-structural changes of plasma focus deposited TiN thin films, the lattice constants are calculated.
Lattice constants of as deposited samples shown in Fig. 3 are found in good agreement with the reported value (~0.424 nm, JCPDS No. 38-1420). When number of focus shots increases the lattice parameter decreases in accordance with the decrease of crystallographic volume . The value of lattice constant for the TiN thin film is lower than the reported value for the bulk  means that the lattice has many nitrogen vacancies . These vacancies increase with the focus shots because re-sputtering process affects the lighter N atoms more than heavier Ti atoms. The pronounced ion induced radiation damage accumulated in the TiN thin film may cause a decrease in the lattice constant. The change in lattice constant may be due to the film grains that are strained and that may be present owing to change of nature and concentration of the native imperfection .
3.3 Grain Size
Using the broadening of the peaks, it is possible to determine the crystallite size, strain and dislocation density from Scherrer formula . The grain size (D) of the thin films was estimated from equation:
where β is the FWHM of the diffraction peak, λ is the wavelength of the incident Cu Kα X-ray (1.514 Çº), and θ is the diffraction angle. While increasing the focus shots, grain size increase whereas strain and dislocation density were found to decrease.
The mean crystallite size was found to be ~25 nm and ~ 28 nm for samples deposited for 20 and 40 focus shots respectively. The grain size curve (Fig. 4) is influenced by number of focus shots. When shots are low, the adatoms have low mobility and the grouping for nucleation and formation of island is difficult, thereby producing smaller grain size. When numbers of focus shot is increased to 40, the surface mobility of adatoms increases, favoring the grouping and the nucleation, producing larger grain size . This provides large area of contact between adjacent crystallite, facilitating coalescence process to form still larger crystallites . Additionally, the observed structural rearrangement of the film is based on principle of minimization of the surface energy of the film resulting from a competition between surface and strain energy. The formation of smaller crystallites reflects a relatively high destabilizing effect of defects to the total free energy in crystals . This could also be combined with short range diffusion to grain boundaries, the mechanism responsible for the formation of the small crystalline grains .
3.3 Micro-Strain and Dislocation Density
The micro-strain () is calculated from relation :
The dislocation density (δ)  is defined as the length of the dislocation per unit volume of the crystal, was calculated using the relation given below:
where a is lattice constant. Fig. 5 and Fig. 6 present the graphics of micro-strain () and dislocation density (δ) as a function of focus shots respectively.It is observed that both micro-strain () and dislocation density (δ) decrease with increasing focus shots. The decreasing trend is due to the movement of interstitial atoms from inside the crystallites to its grain boundary, which dissipates leading to reduction in the concentration of lattice imperfection . The lattice distortions are responsible for reduction in dislocations . This becomes possible due to the fact that at higher number of focus shots, the dislocations get more thermal energy and higher mobility . From above analysis the values of lattice constant, mean grain size, microstrain and dislocation density obtained for film deposited with 20 and 40 focus shots are summarized in table 1.
3.3 Texture Coefficient
The texture coefficients of the TiN films as a function of focus deposition shots are calculated from their respective XRD peaks using the following formula; the results are shown in Fig 7.
Texture Coefficient (Tc) = I(hkl)/[I(111)+I(200)+I(220)+I(311)] (4)
where hkl represents the (111), (200), (220) or (311) orientations.
The Tc value for a particular set of (hkl) planes is proportional to the number of grains that are oriented with this plane parallel to the surface of sample . The observed changes in textures are influenced by number of shots. The texture coefficients of the (111) and (200) orientation are high compared to other orientations in the plasma focus deposited TiN thin films. The competition between surface energy and strain energy during film growth might contribute to these changes.
3.4 Surface Morphology
The surface morphological studies of the plasma focus deposited TiN thin film observed by scanning electron microscope (SEM) is shown in Fig. 8.
The sample exposed to 20 focus shots shows relatively dense, smooth and fine-grained morphology with few voids and boundaries present throughout the deposited thin films. It is noticeable that there is no trace of columnar growth which is indeed striking. Columnar growth results from self shadowing during deposition process and is characteristic feature for evaporation and sputter deposition [20-22]. For sufficiently high energies, the impacting cluster compresses and anneals the area hit directly, so that columnar growth is not possible. The energetic cluster leads also to a self-smoothing of surface [23, 24]. A few micro-cracks are also present. These micro cracks may be attributed due to hardness and brittleness of the film by quenching after transient temperature rise with the short pulse of ion beam of plasma focus. The strong thermal shock, taking place during ion beam incidence consisting of the fast heating and strong temperature gradients may also be the reason .
3.5 Micro Hardness
The Vickers micro-hardness (HV) as a function of imposed load for plasma focus deposited TiN thin films is shown in Fig 9. Four hardness measurements at 25, 50,100 and 200 gf loads, applied for dwell time of 5 seconds, for each sample are used for micro hardness profile. The hardness of untreated aluminum sample is also presented for comparison. The hardness observed to be increased with increase in plasma focus shots. A steep fall of the micro hardness values in the near surface region of the samples exposed to focus shots suggests a concentration gradient towards the bulk. The morphology of the material i.e. microstructure related effects severally affects the hardness of the material . The increase in the micro hardness values may be attributed to increase in ion flux and incorporation of nitrogen ions into the deposited TiN thin film .
There are numerous factors that may affect the measured hardness of deposited thin films, including the crystallite size, residual stress and densification of coatings. A hardening due to a reduction in average crystallite size d according to the Hall-Petch relationship H α 1/d1/2 might be excluded in present case: the validity of this relation is limited to a critical crystallite size dcrit., above which dislocation slip is main plastic deformation mechanism . For d < dcrit., Inter-particle sliding is assumed to be dominant plastic deformation mechanism. In this case a softening of material with decreasing grain size is often the consequence of inverse Hall-Petch relation . In our case, the deposited TiN thin films seem to have such small grain sizes that the case d < dcrit seems to apply. This can be deduced from the observation that hardness increases as grain size increases from about 25 nm to 28 nm (refer to Fig. 4) for deposited films at 20 and 40 focus shots respectively.
Successful deposition of TiN thin films onto aluminum substrates at room temperature have been achieved using plasma focus device. Films have been deposited on aluminum substrates using 20 and 40 focus shots. A study of structure, surface morphology and hardness of deposited films is reported. Behavior of lattice constant, grain size, micro strain, dislocation density and texture coefficient developed in the film for different phases of deposited film is discussed. SEM micrographs show smooth, dense and uniformly distributed film with fine-grained morphology. Hardness is found to follow inverse Hall-Petch relation. The observed variation in XRD, SEM and hardness results have been explained on the basis of the ion emission characteristics of the focus device. The results showed that 20 focus shots are adequate for deposition of polycrystalline, smoother and harder thin films of TiN onto aluminum substrates at room temperature.