Synthesis of nano-structure tungsten nitride thin films ...

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Synthesis of nano-structure tungsten nitride thin films on silicon using Mather-type plasma focus ab

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A. Hussnain , R.S. Rawat , R. Ahmad , Z.A. Umar , T. Hussain , P. a

Lee & Z. Chen

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Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, Singapore 637616, Singapore b

Department of Physics, GC University, Lahore 54000, Pakistan

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Centre for Advanced Studies in Physics (CASP), GC University, Lahore 54000, Pakistan d

School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore Published online: 16 Jul 2015.

To cite this article: A. Hussnain, R.S. Rawat, R. Ahmad, Z.A. Umar, T. Hussain, P. Lee & Z. Chen (2015): Synthesis of nano-structure tungsten nitride thin films on silicon using Mather-type plasma focus, Radiation Effects and Defects in Solids: Incorporating Plasma Science and Plasma Technology, DOI: 10.1080/10420150.2015.1052435 To link to this article: http://dx.doi.org/10.1080/10420150.2015.1052435

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Radiation Effects & Defects in Solids, 2015 http://dx.doi.org/10.1080/10420150.2015.1052435

Synthesis of nano-structure tungsten nitride thin films on silicon using Mather-type plasma focus Downloaded by [INASP - Pakistan (PERI)] at 22:26 28 August 2015

A. Hussnaina,b∗ , R.S. Rawata , R. Ahmadc , Z.A. Umarb , T. Hussainc , P. Leea and Z. Chend a Natural

Sciences and Science Education, National Institute of Education, Nanyang Technological University, Singapore 637616, Singapore; b Department of Physics, GC University, Lahore 54000, Pakistan; c Centre for Advanced Studies in Physics (CASP), GC University, Lahore 54000, Pakistan; d School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore (Received 23 December 2014; accepted 27 April 2015 ) Nano-structure thin film of tungsten nitride was deposited onto Si-substrate at room temperature using Mather-type plasma focus (3.3 kJ) machine. Substrate was exposed against 10, 20, 30, and 40 deposition shots and its corresponding effect on structure, morphology, conductivity and nano-hardness has been systematically studied. The X-ray diffractormeter spectra of the exposed samples show the presence of various phases of WN and WN2 that depends on number of deposition shots. Surface morphological study revealed the uniform distribution of nano-sized grains on deposited film surface. Hardness and conductivity of exposed substrate improved with higher deposition shots. X-ray photo-electron spectroscopy survey scan of 40 deposition shots confirmed the elemental presence of W and N on Si-substrate. Keywords: dense plasma focus; tungsten nitride; thin film; mechanical properties; XPS

1.

Introduction

Dense plasma focus is a source of energetic ions, relativistic electrons, X-rays and neutrons. It produces hot (1–2 keV), dense (1025 –1026 m−3 ), short-lived ( ∼ 10−7 s), pinched plasma column (1). Energetic ion beams emanating from pinched plasma column have been employed for deposition of nano-composite thin films at room temperature substrate (2, 3), surface modification (4), ion implantation (5–7), thermal surface treatment (8), phase transformations (9, 10), and so on. Plasma focus device acquires attractive features like energetic deposition, high deposition rate, and possible thin film deposition under reactive background gas pressure. A thin film deposition at room temperature is highly attractive as deposition at elevated temperature severely limits the selection of substrate material. Plasma focus device is a promising candidate in this respect offering thin film depositions at room temperature substrate; additionally, these deposited thin films have better adhesion to substrate surface. Thin films of tungsten nitride (WNx ) have significant properties like high melting point, high conductivity, better hardness, and chemically inert behavior (11). These properties of tungsten nitride films make it a promising and suitable material for hard-protective coatings on cutting tools (12), diffusion barriers in microelectronics devices (13). Various techniques have been employed for the synthesis of tungsten nitride films such as pulsed laser depositions (14), reactive magnetron sputtering (12, 15), atomic layer *Corresponding author. Email: [email protected] © 2015 Taylor & Francis

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deposition (16), physical vapor deposition (17, 18), and plasma enhanced chemical vapor deposition (18). We employed plasma focus device for deposition of tungsten nitride thin films on silicon substrate at room temperature by different deposition shots. In this work, we studied the crystal structure, phase growth, surface morphology, band gap, and hardness of deposited nanocrystalline tungsten nitride thin films on silicon substrates. The effect of varying focus deposition shots on microstructures, surface morphology, bonding structure, and mechanical properties was investigated.


2.

Experimental details

Nano-crystalline thin films of tungsten nitride are deposited on silicon material using United Nation University/International Centre for Theoretical Physics (UNU/ICTP) plasma focus. Figure 1 shows the schematic sketch of UNU/ICTP plasma focus. It is a low-energy (3.3 kJ) Mather-type plasma focus powered by single Maxwell (30 µF, 15 kV) fast discharging capacitor. Further details and operation of plasma focus can be found elsewhere (3, 19–21). A hot, dense, short-lived, pinched plasma column is developed in the plasma focus device. The elevated temperature of this dense, pinched plasma completely ionized the filling (nitrogen) gas species. High energetic ions move toward the top of the chamber and relativistic electrons (100 keV to above)

Figure 1.

Schematic sketch of Mather-type plasma focus device.


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down toward anode (22, 23). Sadowski et al. (24) found that in each good focus shot, ions and electrons are emitted from plasma column in more than one bunch during the period of few to 10s of nano-second. These relativistic electrons ablated the anode surface material (tungsten, in our case). Ablated plasma from tungsten surface contains tungsten ions required for tungsten nitride synthesis. Anode material and filling gas strongly influence the synthesis of ionic, atomic, and molecular species that are deposited on substrate surface placed above anode. Plasma chamber is filled by nitrogen gas at 2 m bar pressure and silicon wafers are placed at 14 cm from the anode tip. These samples are exposed for 10, 20, 30, and 40 numbers of deposition shots, with a time interval of 3 min between two consecutive shots. X-ray diffractormeter (XRD) (SIEMENS D5005) is used to investigate the structure of deposited thin films. XRD is operated at 40 kV and 40 mA. CuKα (λ = 1.54 Å) radiations are employed at grazing incident angle (3°) to record the diffraction patterns of exposed samples. Surface morphology of exposed samples is studied by field emission scanning electron microscope (FESEM, Jeol JSM-6700F) at an operating voltage of 5 kV. The band gap energy of deposited films was estimated using GENESYS 10S UV–VIS spectrophotometer. The characteristic bonding in the deposited thin films was analyzed using ® X-ray photo-electron spectroscopy (XPS) (Thermo Scientific Theta Probe). Nano Indenter XP (MTS system, TN, USA) was employed to check the hardness and elastic modulus of exposed samples.

3.

Structure and chemical commotional results

Figure 2 shows the XRD spectra of exposed sample for 10, 20, 30, and 40 shots along with unexposed. XRD results show the emergence of WN2 and WN phases with plan reflections (006), (107), (111), (200) and (220), respectively. Tungsten nitride deposited film exhibits the appearance of (006) and (107) plane reflection of the WN2 phase observed at 2θ values of 32.5° and 53.42° (ref # 01-075-0998) and the WN phase with plane reflections of (111), (200), and (220) observed at 2θ 37.6°, 43.93°, and 63.4° (ref # 01-075-1012). The peak intensities of various crystalline phases of tungsten nitride corresponding to different deposition shots are given in

Figure 2.

XRD patterns of films deposited for various (10, 20, 30, and 40) numbers of focus depositions shots.

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A. Hussnain et al. Table 1. Peak intensities of various crystalline phases with various deposition shots. Intensity (counts/s) of phases along with reflecting plane


WN2

WN

Deposition shots

(006)

(107)

(111)

(200)

(220)

10 20 30 40

63.7 102 205 158.4

52 82.8 90 90

79.9 119.9 226.4 426.3

43.5 76.5 55.6 74.7

68.6 83.9 104.6 102.6

Table 1. Intensities of diffraction peaks of WN2 and WN phases increase with the increase in the number of deposition shots, which suggests crystal growth. XRD results of un-exposed silicon show a mono-crystal phase that vanishes on increasing the number of focus deposition shots. It may be due to amorphization of silicon on exposing for energetic ions (9) or may be due to growth of thick deposited film on substrate surface. Formations and deposition of tungsten nitride films on Si-substrate using plasma focus deposition shots are as follows. Nitrogen ions emanate from pinched plasma column, accelerated earlier than the ablated tungsten ions from the anode surface. These energetic ions on reaching the Si-substrate transfer its energy to substrate which may cause etching, cleaning, restructuring, and phase changes (5, 6, 9). It has been reported earlier that the ions emanated in the form of micro-beams in more than one bunch in a single focus shot with the time duration of a few nano-seconds (24). Therefore, tungsten ions in ablated plasma may combine with next bunch of nitrogen ions forming tungsten nitride, accelerated and deposited on the Si-substrate. While it is also possible that all the ablated tungsten plasma may not combine with nitrogen ions and instead of it deposit on substrate as tungsten atoms. While the energetic ions coming from next focus shot may sputter/ablate previously deposited films (W and WN) and recombine with the next bunch of ions of the same focus shots, forming tungsten nitride (WN from W and WN2 from WN) and redepositing onto the substrate. These energetic nitrogen ions may also cause nitriding of previously deposited films, hence increasing the nitrogen content in the deposited films.

Figure 3.

Typical XPS survey scan of exposed sample for 40 focus deposition shots.



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XRD spectra of sample exposed for 10 shots indicate the formation of a WN2 phase with plane reflection of (006), (107) and a WN phase with plane reflections of (111), (200), and (220). The WN2 (006) phase increases at 20 and 30 focus shots, but slightly decreases at 40 shots which may be due to resputtering, while the WN2 (107) phase grows by an increase in the number of deposition shots. The WN (111) phase increases with the increase in deposition shots and highest peak intensity of this phase is observed at 40 shots, which is high as compared to other phases, while the peak intensities of WN phase having plane reflections of (200) and (220) increase by an increase in the deposition shots. Figure 3 shows the XPS survey scan of sample exposed for 40 deposition focus shots. The XPS result shows the presence of deposited species (W and N) on Si-substrate material, which confirms the deposition of tungsten nitride films on the Si-substrate. Some impurity (C, O) peaks are also present on survey scan spectrum, which may be due to un-mounting the sample from plasma focus chamber and shifting for XPS analysis.

4.

Surface morphology

Figure 4 shows FESEM graphs of exposed samples for 10, 20, 30, and 40 deposition shots. At 10 shots, the graph shows that the deposited thin film is very smooth and uniformly distributed over the substrate surface. The deposited film shows the presence of well-developed and sharp edged and irregular shaped grains. Graph A is at higher magnification and shows that the average grain size is around 50 ± 10 nm. At 20 shots, the graph shows the agglomeration of particles forming bigger grains of various sizes and shapes. The grains are well developed having sharp boundaries of various sizes ranging from smaller grains (40 ± 5 nm) to bigger grains (100–200 nm). Smaller graph C at lower magnification shows the uniform distribution of grains over substrate. At 30 shots, the grains of various shapes and size (40–100 nm) are clearly seen. Small graphs B and D show double-layer structure, indicating the deposition of thin films by focus shots, while graphs C and E at lower magnification show the uniform distribution of film on the substrate. The SEM graph for 40 shots shows that the agglomeration is much higher than pervious lower shots. These agglomerated grains have clear, distinct boundaries while the graph F at lower magnification shows the uniform distribution over the surface.

Figure 4.

Micrographs of samples exposed for 10, 20, 30, and 40 focus depositions shots.


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Figure 5. UV absorbance spectra plotted as a function of photon energy for the samples exposed for 10, 20, 30, and 40 focus deposition shots.

5.

UV analysis

Figure 5 shows the band gap values of deposited tungsten nitride films. UV-spectra of exposed samples show a continuous decrease in the band gap values (eV) on an increase in the number of focus deposition shots. Figure 6 shows the almost linearly decreasing behavior band gap values by an increase in the number of shots. It may be due to the increase in the deposited content (tungsten and nitrogen) in the films with the increase in the number of deposition shots, which is also supported by XRD spectra. The decrease in the band gap value indicates the increase in the conductivity of deposited tungsten nitride films, which increases with the increase in the number of deposition shots.

6.

Mechanical properties

The nano-hardness (GPa) of exposed samples as a function of indentation depth (nm) is given in Figure 7. It shows that the hardness of exposed sample for 10 deposition shots significantly increases as compared with reference sample. Further hardness of deposited film increases at 20, 30, and 40 deposition shots and maximum hardness is observed at 40 shots. It is obvious that the nitrogen content in deposited thin films increases by an increase in the number of shots. The enhancement of hardness may be attributed to the increase in nitrogen and tungsten content in



Figure 6.

Photon energy of exposed samples for various (10, 20, 30, and 40) deposition shots.

Figure 7.

Hardness results of exposed samples for 10, 20, 30, and 40 depositions shots.

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the deposited film, resulting in the formation of tungsten nitride coatings due to incorporation of nitrogen into interstitial sites of tungsten. Similar results with identical plasma focus device have been earlier reported that the hardness of deposited thin films improved with the increase in the content of nitrogen and transition metal (25, 26). Elastic modulus (GPa) of deposited films as a function of indentation depth (nm) is given in Figure 8. It shows the significant increase in the elastic modulus (272 GPa) of exposed sample for 10 deposition shots as compared with reference sample (185 GPA). The elastic modulus of exposed samples increases with the increase in the number of deposition shots, while maximum elastic modulus (334 GPa) is observed at 40 shots. Figure 9 shows the average nano-hardness (GPa) along with average elastic modulus (GPa). The average hardness at 10 shots is observed as 21.8 GPa, which shows much enhancement compared with the average hardness (12.8 GPa) of un-exposed. The hardness of exposed samples increases with the increase in the number of shots and maximum hardness 26.6 GPa is observed


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Figure 8.

Modulus results of exposed samples for 10, 20, 30, and 40 depositions shots.

Figure 9.

Average hardness and average modulus results of exposed samples for 10, 20, 30, and 40 depositions shots.

at 40 shots. Average elastic modulus of exposed samples increases with the increase in deposition shots. Average elastic modulus of exposed sample for 10 shots is 272 GPa while maximum elastic modulus is observed as 334 GPa for 40 shots.

7.

Conclusion

Tungsten nitride thin films are successfully deposited on Si-substrate at room temperature by utilizing plasma focus device. The XRD analysis shows the polycrystalline phases of tungsten nitride films and the intensities of these phases are highly dependent on the number of deposition


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shots. The FESEM graphs show nano-particles of tungsten nitride films and on increasing the number of focus shots, these particles agglomerated and formed grains of larger size. FESEM images show that the deposited thin films are smooth and uniformly distributed on the surface. The mechanical properties of the deposited film are enhanced on increasing the number of shots, while the conductivity increases and the band gap energy decreases with the increase in the number of shots. Acknowledgements


The authors would like to thank Dr Zheng Zhang and Dr Lu Shen (Institute of Material Research and Engineering, A*Star, Singapore) for providing technical support in XPS and nano-indentation analysis.

Disclosure statement No potential conflict of interest was reported by the authors.

Funding One of the authors (Ali Hussnain) would like to thank Higher Education Commission, Pakistan, for providing financial support to conduct this research at Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University (NTU), Singapore.

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