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Structural and electrical resistivity characteristics of vacuum arc ion deposited zirconium nitride thin films
Materials Science in Semiconductor Processing 30 (2015) 486–493
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Structural and electrical resistivity characteristics of vacuum arc ion deposited zirconium nitride thin films Shakil Khan a,n, Mazhar Mehmood a, Ishaq Ahmad b, Farhat Ali a, A. Shah c a Department of Metallurgy and Materials Engineering (DMME), Pakistan Institute of Engineering and Applied Sciences (PIEAS), Islamabad, Pakistan b National Centre for Physics, Quiad-e-Azam University, Islamabad, Pakistan c National Institute of Laser and Optronics (NILOP), Islamabad, Pakistan
a r t i c l e in f o
Keywords: ZrN Cathodic arc ion XRD RBS
abstract Zirconium nitride (ZrN) thin films were grown on glass and aluminum substrates using a dual cathodic arc ion deposition technique. The effects of various negative bias voltages and flow ratios of N2/Ar on the stoichiometric ratio of nitrogen to zirconium (N/Zr), deposition rate, structure, surface morphology and electrical resistivity of the ZrN layer were investigated. Rutherford backscattering spectroscopy measurements indicated a drop in the deposition rate and a slight increase in stoichiometric ratio (N/Zr) with the increase of bias voltage up to 400 V, although the latter still remained slightly less than unity ( 0.92). Deposition rate of the film showed an increase with the argon addition. X-ray diffraction patterns depicted mostly polycrystalline nature of the films, with preferential orientation of (2 0 0) planes in the 100 V to 300 V bias voltage range. For 70–50% nitrogen and at a bias voltage of 400 V, the (1 1 1) orientation of ZrN film predominated. The films were smoother at a lower bias of 100 V, while the roughness increased slightly at a higher bias voltage possibly due to (increased) preferential re-sputtering of zirconium-rich clusters/islands. Changes in the resistivity of the films were correlated with stoichiometry, crystallographic orientation and crystalline quality. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Transition metal nitrides are popular as wear and corrosion-resistant coatings. Among these, zirconium nitride (ZrN) offers relatively higher thermal stability, higher hardness, better tribological properties, lower electrical resistivity and higher corrosion resistance [1–4]. In general, an interlayer of metallic zirconium and preferential orientation of (1 1 1) are considered desirable for corrosion protection [5,6] and reduced diffusivity [7], respectively.
n
Corresponding author. E-mail address: [email protected] (S. Khan).
http://dx.doi.org/10.1016/j.mssp.2014.10.029 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
The energy of the incident particles/species on the growing film surface plays an important role in physical vapor deposition (PVD) processes. ZrN coatings are mostly prepared by thermal evaporation or DC magnetron sputtering. The latter is preferred for relatively higher energy of the adatoms reaching the substrate. There are numerous reports on ZrN films, prepared by DC magnetron sputtering deposition at various nitrogen conditions and bias voltages. Cathodic arc ion deposition is in many ways similar to DC magnetron sputtering, but it offers comparatively higher throughput and larger fraction of ions (in addition to the neutral species) of the target material. Particularly due to bias applied on the substrate, ions of argon and nitrogen extracted from plasma with an optimal kinetic energy, impart their energy to the surface atoms and thus enhance
S. Khan et al. / Materials Science in Semiconductor Processing 30 (2015) 486–493
their mobility which facilitates the formation of dense and adherent coatings [8–10]. Nitrogen content in the plasma also affects the quality of ZrN film [11,12]. In the present study, thin films of ZrN have been deposited on glass/aluminum substrate with a pre-deposited Zr layer using a dual cathodic arc ion technique. The inter-metallic layer was grown for the improvement of ZrN film adhesion. The purpose of this work was to establish an empirical correlation between deposition parameters (substrate bias voltage and flow ratio of N2/Ar) and the stoichiometric ratio, film texture (crystallographic orientation), surface morphology and electrical resistivity of ZrN film. Aluminum and glass substrates were selected to study the dependence of the films orientation on the choice of substrates.
2. Experimental A commercial dual cathodic arc ion deposition system was employed for Zr/ZrN thin films growth. The system was equipped with two cathodes and two Kaufman ion sources as shown in a schematic diagram of Fig. 1. To get higher throughput of the depositing species/particles, both targets were utilized. The base vacuum obtained in the growth chamber was 10 4 Pa. The targets were two zirconium discs (Purity: 99.99%).
Fig. 1. Schematic diagram of the deposition chamber.
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Electropolished aluminum strips and glass discs were employed as substrate. These were rinsed in de-mineralized water and acetone, followed by ultrasonic cleaning in trichloroethylene for about 15 min. Deposition temperature was measured with the help of a thermocouple attached to the base of substrate holder. To avoid/minimize contamination of growing films by oxygen and other impurities, a three-stage procedure was adopted. In first step, the chamber was evacuated and substrates were degassed at 150 1C temperature to remove adsorbed species on the substrates. Secondly, the substrates were sputter-etched for 15 min using two Kauffman argon ion sources (1 keV each). Lastly, to clean the targets surfaces, pre-sputtering of the targets in pure argon atmosphere was performed for 10 min. During pre-sputtering, the substrates were shadowed by shutter. Two series of experiments were conducted coded as series A and B as described in Table 1. In series A, pure Ar gas was injected in the synthesis chamber and its flow was adjusted at 28 sccm. The arcs were created to settle the target current and bias voltage at 80 A and 100 V, respectively. Once the parameters established and targets were cleaned, shutter from targets front was removed and metallic species were allowed to deposit on the substrate surface. Metallic thin film was grown for a period of 5 min. Temperature during film growth was maintained in a range of 80–100 1C, as it rapidly rises during film growth owing to the high throughput of a cathodic arc ion process. After the completion of metallic coating, only nitrogen (N2) gas (flow 90 sccm) was injected and nitride layer was prepared for a period of 15 min keeping the current, bias voltage and temperature at 80 A, 100 V and 80–100 1C, respectively. Next samples of series-A (S2 and S3) were grown at 300 V and 400 V substrate bias, respectively while keeping the other parameters constant. In series-B samples, nitride film was deposited under different nitrogen conditions, ranging from 100% to 15% in the deposition chamber. The bias voltage was kept at 400 V for this series. The atomic percent compositions and thicknesses of the films were measured by means of Rutherford backscattering (RBS) spectroscopy. A collimated 2.0 MeV 4He þ 2 beam produced by 5UDH-2 Pelletron was used having charge of 15 mC and current 35 nA. To control the orientation of samples relative to the 4He þ 2 beam, these were mounted on a high precision (0.011) five-axis goniometer in a vacuum chamber. Au–Si barrier detector collected the backscattered particles. The detection angle
Table 1 Deposition parameters constant parameter (time of deposition for ZrN layer: 15 min). Sample # Series A S1 S2 S3 Series B S3 S4 S5 S6 S7 S8
Flow ratio N2/(Arþ N2)%
Biasing (V)
Deposition pressure (Pa)
100 100 100
100 300 400
2 10 1 2 10 1 2 10 1
100 70 50 22 19 15
400 400 400 400 400 400
2 10 1 2 10 1 2 10 1 2 10 1 2 10 1 2 10 1
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was 1701 and energy resolution of the detector was about 25 keV. The results were analyzed by Rump software. For investigation of the film crystalline phases and growth directions, an X-ray diffractometer D8 Discoverer HRXRD BRUKER axes (Germany) equipped with Cu Kα radiation was employed. Measurement was executed at grazing incidence diffraction angle with parallel beam geometry. To maximize the counts from films surfaces, the incident X-ray beam was set at 1.5–11 with respect to the films surfaces. To restrict the radial divergence to 0.121, a long soller slit was used. Analysis of vibrational modes of molecules was performed by means of a Nicolet 6700 Fourier transform infra-red (FTIR) spectrophotometer. For the investigation of surface features/morphology of the deposited films, an Atomic Force microscope (Model: QScope™ 350) was employed. All the samples were scanned over an area of 2 2 mm2 in a taping mode. To measure film resistivity, a four-point probe method was adopted. 3. Results and discussion 3.1. Rutherford backscattering spectroscopy (RBS) Thickness and stoichiometric measurements were executed by means of RBS spectroscopy. The thickness and
Energy(MeV) 80
0.5
1.0
1.5
2.0
Zr Experimental Simulation
NormalizedYield
60
Zr(N) ZrN
40
O 20
N Si
Ca
0 500
1000
1500
2000
Channel Fig. 2. Simulated and experimental graph of RBS analysis for samples prepared at 100% nitrogen condition.
concentration profile of the films were acquired by simulating the experimental particle energy spectrum of each sample employing the RUMP code. Fig. 2 offers the measured and simulated RBS spectra for samples prepared at 100% nitrogen in the environment. The whole energy spectrum was simulated with the elements shown in Fig. 2. The zirconium and nitrogen counts/band arise from the ZrN and the underlying Zr layers. The counts of sodium (Na), calcium (Ca), magnesium (Mg) and silicon (Si) originated from scattering in the glass substrate. The positions of the band edges (maximum energy corresponding to a particular band), width of the band and the counts/intensity in an RBS spectrum, depend on composition and thickness of the coating and the underlying layer, which can be determined by comparing the measured spectrum with a calculated/simulated one. The spectrum exhibits distinct edges corresponding to the ZrN film, the underlying Zr layer and the glass substrate. Thickness of both layers and the composition of ZrN layer used in simulation are given in Table 2. An excellent match between the measured spectrum and the simulated one can be clearly noticed in Fig. 2. Fig. 3(a) depicts the RBS spectra for the series “A” samples (listed in Table 1) prepared at different substrate bias voltages with 100% nitrogen in the environment (i.e., without argon). Variation of the Zr (representing the nitride layer) width and relative heights with the deposition parameters are clearly evident. The decreased width of zirconium band with bias voltage is related with lower thickness as determined by comparing with the simulated data (Table 2). The film prepared at a substrate bias voltage of 100V, exhibited the thickness of 282 nm. It drops to 259 nm for a film grown at 400 V bias conditions. The drop is attributable to enhanced re-sputtering of the deposited species at the film surface with higher bias voltage [13]. It may, however, be noted that stoichiometric ratio (N/Zr) of the deposited films improves with the increase of bias voltage (Table 2). This is in agreement with observation made in TiN [14] and AlN films [15] prepared by cathodic arc ion deposition. It seems that the macroparticles of (metallic) Zr exhibit a higher re-sputtering rate than the surrounding (compound) ZrN film due to their lower stability in comparison with the latter, as a result of which an overall improved stoichiometry is observed [15]. The other possibility is the larger penetrating/sticking ability of accelerating nitrogen ions at higher bias voltage leaving behind the metallic phase
Table 2 RBS results. Sample # Series A S1 S2 S3 Series B S3 S4 S5 S6 S7 S8
Zr thickness (nm)
ZrN layer thickness (nm)
Zr concentration in ZrN layer
N concentration in ZrN layer
Stoichiometric (N/Zr) ratio
121 106 107
282 273 259
0.540 0.530 0.520
0.460 0.470 0.480
0.851 0.886 0.923
107 116 110 112 107 109
259 288 325 337 345 351
0.520 0.529 0.557 0.720 0.770 0.822
0.480 0.470 0.442 0.280 0.230 0.177
0.923 0.888 0.794 0.388 0.298 0.215
S. Khan et al. / Materials Science in Semiconductor Processing 30 (2015) 486–493
Energy(MeV)
Energy(MeV) 80
0.5
1.0
1.5
2.0
120
0.5
20
ZrN Zr(N)
NormalizedYield
NormalizedYield
40
O N
1.5
2.0
Zr
70%
Zr
-400V B E -300V G -100V
1.0
100%
100 60
489
b d 22% h i 19% j 15%
60
40
ZrN
50% c
80
O N
Si
Si
20
Ca
Ca
0
0 500
1000
1500
2000
500
1000
1500
Channel
Channel
Fig. 3. Combined RBS analysis graphs for ZrN films deposited at (a) different bias voltage (series A), and (b) various nitrogen fractions (series B).
3.2. XRD study Fig. 4 show XRD patterns of series “A” samples. The patterns depict the formation of ZrN phase with a face centered cubic lattice and space group Fm-3m (2 2 5). An additional peak at 36.21 can be assigned to underlying hcp Zr layer. For the substrate bias voltages of 100 V and
(111) (200) (220) Zr (101)
-400V
Intensity (arb units)
having higher re-sputtered rate and thus resulting in improved stoichiometry. Fig. 3(b) depicts RBS spectra for the series “B” samples prepared at a bias voltage of 400 V and at various nitrogen concentrations in the environment. Variation of the nitride layer thickness and stoichiometric ratio (N/Zr), as determined by comparing the simulated data with the measured spectra, are listed in Table 2. A pronounced decrease in N/Zr and a rise in growth rate have been observed with the introduction of argon in the environment as depicted in Fig. 3(b). The rise of the growth rate could be related to increased rate of yield at the target, greater sticking ability of the depositing atoms due to increased bonding force (example enthalpy of formation), and a lower rate of re-sputtering of the deposited species. As far as the third possibility is concerned, argon is a heavier element; hence increase in the concentration of its ions in the environment should not cause a decrease in re-sputtering. As will be observed later, nitride formation is hindered with the rise in argon content; the energy of formation of the product formed might therefore be decreasing with the argon content in the atmosphere. Hence, the second possibility also seems less probable. It is natural to expect that nitrogen in the environment may cause nitride formation at the target's surface as well; as a result of which, vaporization of the species at target surface might be lower at higher content of nitrogen in the environment. Therefore, we expect that the smaller growth rate at higher nitrogen conditions is due to a lower evaporation rate at the target with nitride formation at its surface.
(311)
-300V
-100V 30
40
50 2 theta (degree)
60
70
Fig. 4. The effect of bias voltage on the XRD pattern of ZrN film.
300 V, the intensity of (2 0 0) orientation is significantly higher than that of the other reflections. A significant rise in the intensity of (1 1 1) reflection is observed for the bias voltage of 400 V. This suggests that a bias voltage of 400 V is responsible for a shift in preferred orientation from (2 0 0) to (1 1 1) plane. Such a behavior was also reported by Zhang et al. [16] for the titanium nitride film deposited at 100% nitrogen condition using cathodic arc ion technique. The other noticeable aspect is the enhancement of (2 2 0) peak intensity with the increase of bias voltage. ZrN has Cubic-F type lattice with atomic arrangements of NaCl prototype structure, as do TiN. Different planes
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Zr Zr (002) (101) (111) (200)
(220)
Al substrate peaks
(311) (111)
Zr (200) (101)
Intensity (arb units)
Intensity (arb units)
19% N 2
22% N 2
50% N 2
50% N2
(220)
(311)
70% N2
70% N 2
100% N2
100% N 2 30
40
50
60
2 theta (degree)
70
30
40
50
60
70
2 theta (degree)
Fig. 5. XRD pattern of ZrN film prepared under various nitrogen conditions on (a) glass and, (b) aluminum substrate.
have varying surface and strain energies, depending on charge neutrality and atomic arrangements. For instance, its (2 0 0) surface is assumed to have the lowest surface energy while, its (1 1 1) surface corresponds to minimum strain energy [17,18]. It appears that at the bias voltage of 100 to 300 V, ZrN film has some tendency to acquire (2 0 0) plane parallel to the surface for a considerably larger number of grains (crystals); while, at a bias voltage of 400 V, the relative intensity of (1 1 1) peak rises considerably. It could be related to the competition of surface and strain energies of the growing film. Higher bias voltage corresponds to the increased energy of incident ions, which consequently imparts the energy to the depositing species. When the bias voltage exceeds the threshold value such as in our case it is Z 300 V, the coalescent energy of the adatoms for the growth of crystallites with (1 1 1) orientation becomes available. The higher adatoms energy causes the film to grow in (1 1 1) preferred orientation to lower the increased strain energy [18]. Fig. 5(a) and (b) exhibits the XRD patterns of the films grown at varying nitrogen fraction on glass and aluminum substrates, respectively. It may be noticed that lowering the nitrogen fraction, favors the (1 1 1) preferred orientation as its relative intensity rises considerably. This is independent of the choice of substrate (amorphous glass or crystalline aluminum). The most prominent (1 1 1) orientation is observed for the sample prepared at bias voltage of 400 V and the nitrogen content of 70%. As described earlier that the strain energy is responsible for the growth of (1 1 1) orientation, lowering the nitrogen
content means an increase fraction of argon fraction in the plasma gas. The impinging higher mass argon ions (relative to nitrogen) impart more energy to the growing species to lower the strain energy. However, reducing the nitrogen ratio below 50% in the environment abruptly decreases the nitrogen content in the film (shown in Table 2). At this stage, metallic zirconium predominates along with amorphous zirconium nitride or the one with extremely poor crystallinity because of significant deviation from stoichiometry. There are numerous reports on ZrN film deposited by magnetron sputtering with varying nitrogen fractions in the environment. Mostly in these cases, nitrogen rich zirconium nitride is formed approaching to Zr3N4 or nitrogen-rich amorphous phase for the nitrogen fraction approaching to 100% [19–21]. In our case, cathodic arc tends to evaporate the target mostly at local spots where clusters of metal/target atoms and even droplets also leave the target and deposit at the substrate. This may partly be the reason for the formation of crystalline zirconium nitrides with non-stoichiometry (lower nitrogen content) in comparison with the films prepared by magnetron sputtering at similar nitrogen fractions in the environment. 3.3. FTIR and Raman analysis FTIR spectroscopy was employed to yield information about the chemical bonds of ZrN compound. FTIR transmittance spectrum was taken in the region of interest from 500 to 2000 cm 1 in reflection mode. The resolution was kept at 4 cm 1 and number of scans were 16. Spectrum of the sample
S. Khan et al. / Materials Science in Semiconductor Processing 30 (2015) 486–493
491
182 cm -1
327 cm-1
470 cm-1
621 cm-1
19%
Intensity (arb units)
Transmitance %
S2
-1
1075 cm
22% 50% 70%
-1
680 cm
100% 600
800
1000
1200
1400
1600
1800
2000
Wavenumber (cm-1)
200
300
400
500
600
700
800
900
Wavenumer (cm-1)
Fig. 6. Exhibiting the (a) FTIR spectrum of ZrN film deposited at 300 V and 100% N2 and (b) Raman spectra of films grown at different nitrogen fractions.
grown at 300 V and 100% N2 condition is depicted in Fig. 6(a). It indicates a broad band centered at 680 cm 1 corresponding to the stretching vibrations of Zr–N simple bonds [22]. Spectrum also depicts a band for ZrO2 at 1075 cm 1, possibly due to thin oxide layer formed on ZrN surface. The formation of oxide layer at the film surface is attributable to its difference in heat of formation (261.5 kcal mol 1) compared to ZrN (87.3 kcal mol 1) [23]. Raman spectroscopy is an important spectroscopic analysis to probe the vibrational states of crystals or semiconductors. The Raman analysis was performed using a high resolution (4 cm 1) Raman spectroscopy system (Model: MST-4000A). Fig. 6(b) offers the Raman spectra of ZrN films grown at different nitrogen conditions exhibiting peaks at 182 cm 1, 327 cm 1, 470 cm 1 and 620 cm 1. The first order Raman effect is prohibited in a perfect single crystal of fcc phase [24]. However, defects produced in ZrN during film growth may be responsible for the first order Raman phonons [25]. The nitrogen vacancies in the film may produce any disorder or defect and cause the translational invariance loss, which gives Raman peaks depicted in Fig. 6 (b). The peak at 182 cm 1 and 327 cm 1 correspond to the disorder induced single acoustic phonons and second order acoustic phonons processes respectively [26]. The peaks at 470 cm 1 and 620 cm 1 arise due to the oxygen related defects that replaces the nitrogen interstitial sites. The appearance of sharp and well defined peaks at 70% nitrogen condition demonstrated the local structural order of the grown films and is in agreement with the XRD results. 3.4. AFM results Atomic force microscopy (AFM) was utilized primarily for surface analysis. The AFM images of ZrN films prepared at different bias voltages and nitrogen conditions are shown in Fig. 7. It can be seen that the films are fairly smooth, as the variation in Z-height remains only a few nanometers. Still, the surface morphology depends on the substrate bias. For
instance, the film formed at a bias voltage of 300 V (an rms value 6.679 nm) is smoother than that prepared at 100 V (rms value 10.14 nm) depicted in Fig. 7(a) and (b), respectively. The reduction in rms roughness may possibly be related to better compactness and quality of the film by imparting higher energy to the surface atoms/species during film growth. It may, however, be interesting to note that a further increase in bias voltage to 400 V or reducing the nitrogen fraction (Fig. 7(c) and (d), respectively), imparts some greater roughness in comparison with Fig. 7(b). It is known that in the present deposition technique, clusters and droplets numerously emerge from the target and deposit at the substrate. On the other hand, re-sputtering rate depends on the composition of the film at a particular location. It appears that increased roughness at 400 V or 70% nitrogen fraction is related to more pronounced variation in re-sputtering, that is, an enhanced re-sputtering of zirconium-rich (metallic) islands, clusters, particles, etc., in the zirconium nitride film occurs during growth. 3.5. Electrical resistivity Electrical resistivity of zirconium nitride films prepared at different nitrogen conditions is shown in Fig. 8. The resistivity lies in the range of 1.1–13 mΩ-m depending on the growth conditions. The values are in agreement with the reported data [27,28]. It is a well known fact that crystallinity and stoichiometric ratio strongly affect film resistivity [29]. The total resistivity of the films is a contribution of several independent electron scattering processes due to phonons, impurity atoms and defects [30]. As shown in Fig. 8, the film grown in 100% nitrogen in the environment exhibits resistivity of 6.315 mΩ-m, which increases to 12.51 mΩ-m for the film prepared at 70% nitrogen. This change in resistivity could be attributed to the (1 1 1) orientation growth (depicted by XRD patterns) of the deposited film. The (1 1 1) preferred orientation/texture is a close packed structure where the scattering of electron with phonons will be much enhanced. The resistivity drops rapidly
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S. Khan et al. / Materials Science in Semiconductor Processing 30 (2015) 486–493
Fig. 7. AFM images of ZrN films grown at bias of (a) 100 V (b) 300 V (c) 400 V and (d) 70% N2 and 400 V.
by an amorphous halo (broad peak) with superimposed peaks of metallic zirconium.
15
4. Conclusions Resistivity (μΩ .m)
10
5
0
0
20
40
60
80
100
Nitrogen fraction (%) Fig. 8. Influence of various nitrogen conditions on the ZrN film resistivity.
to 0.551 mΩ-m for the film deposited at 50%. This decrease may correspond to the lower atomic ratio (N/Zr) of the deposited film, which drops sharply as observed in RBS results. As nitrogen flow rate decreases below 22% nitrogen, resistivity increases again. This is related to the fact that the crystallinity of zirconium nitride (Fig. 5) rapidly diminishes at this level of lower nitrogen content in the environment, as it can be noticed that XRD peaks of crystalline ZrN are almost replaced
Thin film of ZrN was deposited using the dual cathodic arc ion deposition technique. An improvement of film stoichiometry (N/Zr) at 400 V bias is revealed by RBS analysis. However, the atomic ratio (N/Zr) was found less than one regardless of nitrogen content in the deposition chamber. Nitrogen fraction (N2/ArþN2) strongly affected the film stoichiometry and thickness. XRD patterns showed polycrystalline films. The film prepared at 100 V substrate bias voltage depicted a (2 0 0) preferred orientation. A transformation to (1 1 1) preferred orientation was observed while increasing the bias voltage to 400 V. A further growth of (1 1 1) preferred orientation was noticed while reducing the N2 fraction from 100% to 70% in the synthesis chamber. Below 22% nitrogen conditions, ZrN film exhibited amorphous phase. Crystallographic orientations of ZrN films were found to be independent of substrate choice. Grooves/pits were appeared on the film surface deposited at higher substrate biasing. The change in film resistivity was correlated with the film texture and stoichiometry. A near stoichiometric film, with preferred (1 1 1) orientation having lower surface roughness and higher resistivity was prepared at 70% nitrogen condition while keeping the bias voltage at 400 V.
S. Khan et al. / Materials Science in Semiconductor Processing 30 (2015) 486–493
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Structural and electrical resistivity characteristics of vacuum arc ion deposited zirconium nitride thin films Shakil Khan a,n, Mazhar Mehmood a, Ishaq Ahmad b, Farhat Ali a, A. Shah c a Department of Metallurgy and Materials Engineering (DMME), Pakistan Institute of Engineering and Applied Sciences (PIEAS), Islamabad, Pakistan b National Centre for Physics, Quiad-e-Azam University, Islamabad, Pakistan c National Institute of Laser and Optronics (NILOP), Islamabad, Pakistan
a r t i c l e in f o
Keywords: ZrN Cathodic arc ion XRD RBS
abstract Zirconium nitride (ZrN) thin films were grown on glass and aluminum substrates using a dual cathodic arc ion deposition technique. The effects of various negative bias voltages and flow ratios of N2/Ar on the stoichiometric ratio of nitrogen to zirconium (N/Zr), deposition rate, structure, surface morphology and electrical resistivity of the ZrN layer were investigated. Rutherford backscattering spectroscopy measurements indicated a drop in the deposition rate and a slight increase in stoichiometric ratio (N/Zr) with the increase of bias voltage up to 400 V, although the latter still remained slightly less than unity ( 0.92). Deposition rate of the film showed an increase with the argon addition. X-ray diffraction patterns depicted mostly polycrystalline nature of the films, with preferential orientation of (2 0 0) planes in the 100 V to 300 V bias voltage range. For 70–50% nitrogen and at a bias voltage of 400 V, the (1 1 1) orientation of ZrN film predominated. The films were smoother at a lower bias of 100 V, while the roughness increased slightly at a higher bias voltage possibly due to (increased) preferential re-sputtering of zirconium-rich clusters/islands. Changes in the resistivity of the films were correlated with stoichiometry, crystallographic orientation and crystalline quality. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Transition metal nitrides are popular as wear and corrosion-resistant coatings. Among these, zirconium nitride (ZrN) offers relatively higher thermal stability, higher hardness, better tribological properties, lower electrical resistivity and higher corrosion resistance [1–4]. In general, an interlayer of metallic zirconium and preferential orientation of (1 1 1) are considered desirable for corrosion protection [5,6] and reduced diffusivity [7], respectively.
n
Corresponding author. E-mail address: [email protected] (S. Khan).
http://dx.doi.org/10.1016/j.mssp.2014.10.029 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
The energy of the incident particles/species on the growing film surface plays an important role in physical vapor deposition (PVD) processes. ZrN coatings are mostly prepared by thermal evaporation or DC magnetron sputtering. The latter is preferred for relatively higher energy of the adatoms reaching the substrate. There are numerous reports on ZrN films, prepared by DC magnetron sputtering deposition at various nitrogen conditions and bias voltages. Cathodic arc ion deposition is in many ways similar to DC magnetron sputtering, but it offers comparatively higher throughput and larger fraction of ions (in addition to the neutral species) of the target material. Particularly due to bias applied on the substrate, ions of argon and nitrogen extracted from plasma with an optimal kinetic energy, impart their energy to the surface atoms and thus enhance
S. Khan et al. / Materials Science in Semiconductor Processing 30 (2015) 486–493
their mobility which facilitates the formation of dense and adherent coatings [8–10]. Nitrogen content in the plasma also affects the quality of ZrN film [11,12]. In the present study, thin films of ZrN have been deposited on glass/aluminum substrate with a pre-deposited Zr layer using a dual cathodic arc ion technique. The inter-metallic layer was grown for the improvement of ZrN film adhesion. The purpose of this work was to establish an empirical correlation between deposition parameters (substrate bias voltage and flow ratio of N2/Ar) and the stoichiometric ratio, film texture (crystallographic orientation), surface morphology and electrical resistivity of ZrN film. Aluminum and glass substrates were selected to study the dependence of the films orientation on the choice of substrates.
2. Experimental A commercial dual cathodic arc ion deposition system was employed for Zr/ZrN thin films growth. The system was equipped with two cathodes and two Kaufman ion sources as shown in a schematic diagram of Fig. 1. To get higher throughput of the depositing species/particles, both targets were utilized. The base vacuum obtained in the growth chamber was 10 4 Pa. The targets were two zirconium discs (Purity: 99.99%).
Fig. 1. Schematic diagram of the deposition chamber.
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Electropolished aluminum strips and glass discs were employed as substrate. These were rinsed in de-mineralized water and acetone, followed by ultrasonic cleaning in trichloroethylene for about 15 min. Deposition temperature was measured with the help of a thermocouple attached to the base of substrate holder. To avoid/minimize contamination of growing films by oxygen and other impurities, a three-stage procedure was adopted. In first step, the chamber was evacuated and substrates were degassed at 150 1C temperature to remove adsorbed species on the substrates. Secondly, the substrates were sputter-etched for 15 min using two Kauffman argon ion sources (1 keV each). Lastly, to clean the targets surfaces, pre-sputtering of the targets in pure argon atmosphere was performed for 10 min. During pre-sputtering, the substrates were shadowed by shutter. Two series of experiments were conducted coded as series A and B as described in Table 1. In series A, pure Ar gas was injected in the synthesis chamber and its flow was adjusted at 28 sccm. The arcs were created to settle the target current and bias voltage at 80 A and 100 V, respectively. Once the parameters established and targets were cleaned, shutter from targets front was removed and metallic species were allowed to deposit on the substrate surface. Metallic thin film was grown for a period of 5 min. Temperature during film growth was maintained in a range of 80–100 1C, as it rapidly rises during film growth owing to the high throughput of a cathodic arc ion process. After the completion of metallic coating, only nitrogen (N2) gas (flow 90 sccm) was injected and nitride layer was prepared for a period of 15 min keeping the current, bias voltage and temperature at 80 A, 100 V and 80–100 1C, respectively. Next samples of series-A (S2 and S3) were grown at 300 V and 400 V substrate bias, respectively while keeping the other parameters constant. In series-B samples, nitride film was deposited under different nitrogen conditions, ranging from 100% to 15% in the deposition chamber. The bias voltage was kept at 400 V for this series. The atomic percent compositions and thicknesses of the films were measured by means of Rutherford backscattering (RBS) spectroscopy. A collimated 2.0 MeV 4He þ 2 beam produced by 5UDH-2 Pelletron was used having charge of 15 mC and current 35 nA. To control the orientation of samples relative to the 4He þ 2 beam, these were mounted on a high precision (0.011) five-axis goniometer in a vacuum chamber. Au–Si barrier detector collected the backscattered particles. The detection angle
Table 1 Deposition parameters constant parameter (time of deposition for ZrN layer: 15 min). Sample # Series A S1 S2 S3 Series B S3 S4 S5 S6 S7 S8
Flow ratio N2/(Arþ N2)%
Biasing (V)
Deposition pressure (Pa)
100 100 100
100 300 400
2 10 1 2 10 1 2 10 1
100 70 50 22 19 15
400 400 400 400 400 400
2 10 1 2 10 1 2 10 1 2 10 1 2 10 1 2 10 1
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was 1701 and energy resolution of the detector was about 25 keV. The results were analyzed by Rump software. For investigation of the film crystalline phases and growth directions, an X-ray diffractometer D8 Discoverer HRXRD BRUKER axes (Germany) equipped with Cu Kα radiation was employed. Measurement was executed at grazing incidence diffraction angle with parallel beam geometry. To maximize the counts from films surfaces, the incident X-ray beam was set at 1.5–11 with respect to the films surfaces. To restrict the radial divergence to 0.121, a long soller slit was used. Analysis of vibrational modes of molecules was performed by means of a Nicolet 6700 Fourier transform infra-red (FTIR) spectrophotometer. For the investigation of surface features/morphology of the deposited films, an Atomic Force microscope (Model: QScope™ 350) was employed. All the samples were scanned over an area of 2 2 mm2 in a taping mode. To measure film resistivity, a four-point probe method was adopted. 3. Results and discussion 3.1. Rutherford backscattering spectroscopy (RBS) Thickness and stoichiometric measurements were executed by means of RBS spectroscopy. The thickness and
Energy(MeV) 80
0.5
1.0
1.5
2.0
Zr Experimental Simulation
NormalizedYield
60
Zr(N) ZrN
40
O 20
N Si
Ca
0 500
1000
1500
2000
Channel Fig. 2. Simulated and experimental graph of RBS analysis for samples prepared at 100% nitrogen condition.
concentration profile of the films were acquired by simulating the experimental particle energy spectrum of each sample employing the RUMP code. Fig. 2 offers the measured and simulated RBS spectra for samples prepared at 100% nitrogen in the environment. The whole energy spectrum was simulated with the elements shown in Fig. 2. The zirconium and nitrogen counts/band arise from the ZrN and the underlying Zr layers. The counts of sodium (Na), calcium (Ca), magnesium (Mg) and silicon (Si) originated from scattering in the glass substrate. The positions of the band edges (maximum energy corresponding to a particular band), width of the band and the counts/intensity in an RBS spectrum, depend on composition and thickness of the coating and the underlying layer, which can be determined by comparing the measured spectrum with a calculated/simulated one. The spectrum exhibits distinct edges corresponding to the ZrN film, the underlying Zr layer and the glass substrate. Thickness of both layers and the composition of ZrN layer used in simulation are given in Table 2. An excellent match between the measured spectrum and the simulated one can be clearly noticed in Fig. 2. Fig. 3(a) depicts the RBS spectra for the series “A” samples (listed in Table 1) prepared at different substrate bias voltages with 100% nitrogen in the environment (i.e., without argon). Variation of the Zr (representing the nitride layer) width and relative heights with the deposition parameters are clearly evident. The decreased width of zirconium band with bias voltage is related with lower thickness as determined by comparing with the simulated data (Table 2). The film prepared at a substrate bias voltage of 100V, exhibited the thickness of 282 nm. It drops to 259 nm for a film grown at 400 V bias conditions. The drop is attributable to enhanced re-sputtering of the deposited species at the film surface with higher bias voltage [13]. It may, however, be noted that stoichiometric ratio (N/Zr) of the deposited films improves with the increase of bias voltage (Table 2). This is in agreement with observation made in TiN [14] and AlN films [15] prepared by cathodic arc ion deposition. It seems that the macroparticles of (metallic) Zr exhibit a higher re-sputtering rate than the surrounding (compound) ZrN film due to their lower stability in comparison with the latter, as a result of which an overall improved stoichiometry is observed [15]. The other possibility is the larger penetrating/sticking ability of accelerating nitrogen ions at higher bias voltage leaving behind the metallic phase
Table 2 RBS results. Sample # Series A S1 S2 S3 Series B S3 S4 S5 S6 S7 S8
Zr thickness (nm)
ZrN layer thickness (nm)
Zr concentration in ZrN layer
N concentration in ZrN layer
Stoichiometric (N/Zr) ratio
121 106 107
282 273 259
0.540 0.530 0.520
0.460 0.470 0.480
0.851 0.886 0.923
107 116 110 112 107 109
259 288 325 337 345 351
0.520 0.529 0.557 0.720 0.770 0.822
0.480 0.470 0.442 0.280 0.230 0.177
0.923 0.888 0.794 0.388 0.298 0.215
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Energy(MeV)
Energy(MeV) 80
0.5
1.0
1.5
2.0
120
0.5
20
ZrN Zr(N)
NormalizedYield
NormalizedYield
40
O N
1.5
2.0
Zr
70%
Zr
-400V B E -300V G -100V
1.0
100%
100 60
489
b d 22% h i 19% j 15%
60
40
ZrN
50% c
80
O N
Si
Si
20
Ca
Ca
0
0 500
1000
1500
2000
500
1000
1500
Channel
Channel
Fig. 3. Combined RBS analysis graphs for ZrN films deposited at (a) different bias voltage (series A), and (b) various nitrogen fractions (series B).
3.2. XRD study Fig. 4 show XRD patterns of series “A” samples. The patterns depict the formation of ZrN phase with a face centered cubic lattice and space group Fm-3m (2 2 5). An additional peak at 36.21 can be assigned to underlying hcp Zr layer. For the substrate bias voltages of 100 V and
(111) (200) (220) Zr (101)
-400V
Intensity (arb units)
having higher re-sputtered rate and thus resulting in improved stoichiometry. Fig. 3(b) depicts RBS spectra for the series “B” samples prepared at a bias voltage of 400 V and at various nitrogen concentrations in the environment. Variation of the nitride layer thickness and stoichiometric ratio (N/Zr), as determined by comparing the simulated data with the measured spectra, are listed in Table 2. A pronounced decrease in N/Zr and a rise in growth rate have been observed with the introduction of argon in the environment as depicted in Fig. 3(b). The rise of the growth rate could be related to increased rate of yield at the target, greater sticking ability of the depositing atoms due to increased bonding force (example enthalpy of formation), and a lower rate of re-sputtering of the deposited species. As far as the third possibility is concerned, argon is a heavier element; hence increase in the concentration of its ions in the environment should not cause a decrease in re-sputtering. As will be observed later, nitride formation is hindered with the rise in argon content; the energy of formation of the product formed might therefore be decreasing with the argon content in the atmosphere. Hence, the second possibility also seems less probable. It is natural to expect that nitrogen in the environment may cause nitride formation at the target's surface as well; as a result of which, vaporization of the species at target surface might be lower at higher content of nitrogen in the environment. Therefore, we expect that the smaller growth rate at higher nitrogen conditions is due to a lower evaporation rate at the target with nitride formation at its surface.
(311)
-300V
-100V 30
40
50 2 theta (degree)
60
70
Fig. 4. The effect of bias voltage on the XRD pattern of ZrN film.
300 V, the intensity of (2 0 0) orientation is significantly higher than that of the other reflections. A significant rise in the intensity of (1 1 1) reflection is observed for the bias voltage of 400 V. This suggests that a bias voltage of 400 V is responsible for a shift in preferred orientation from (2 0 0) to (1 1 1) plane. Such a behavior was also reported by Zhang et al. [16] for the titanium nitride film deposited at 100% nitrogen condition using cathodic arc ion technique. The other noticeable aspect is the enhancement of (2 2 0) peak intensity with the increase of bias voltage. ZrN has Cubic-F type lattice with atomic arrangements of NaCl prototype structure, as do TiN. Different planes
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Zr Zr (002) (101) (111) (200)
(220)
Al substrate peaks
(311) (111)
Zr (200) (101)
Intensity (arb units)
Intensity (arb units)
19% N 2
22% N 2
50% N 2
50% N2
(220)
(311)
70% N2
70% N 2
100% N2
100% N 2 30
40
50
60
2 theta (degree)
70
30
40
50
60
70
2 theta (degree)
Fig. 5. XRD pattern of ZrN film prepared under various nitrogen conditions on (a) glass and, (b) aluminum substrate.
have varying surface and strain energies, depending on charge neutrality and atomic arrangements. For instance, its (2 0 0) surface is assumed to have the lowest surface energy while, its (1 1 1) surface corresponds to minimum strain energy [17,18]. It appears that at the bias voltage of 100 to 300 V, ZrN film has some tendency to acquire (2 0 0) plane parallel to the surface for a considerably larger number of grains (crystals); while, at a bias voltage of 400 V, the relative intensity of (1 1 1) peak rises considerably. It could be related to the competition of surface and strain energies of the growing film. Higher bias voltage corresponds to the increased energy of incident ions, which consequently imparts the energy to the depositing species. When the bias voltage exceeds the threshold value such as in our case it is Z 300 V, the coalescent energy of the adatoms for the growth of crystallites with (1 1 1) orientation becomes available. The higher adatoms energy causes the film to grow in (1 1 1) preferred orientation to lower the increased strain energy [18]. Fig. 5(a) and (b) exhibits the XRD patterns of the films grown at varying nitrogen fraction on glass and aluminum substrates, respectively. It may be noticed that lowering the nitrogen fraction, favors the (1 1 1) preferred orientation as its relative intensity rises considerably. This is independent of the choice of substrate (amorphous glass or crystalline aluminum). The most prominent (1 1 1) orientation is observed for the sample prepared at bias voltage of 400 V and the nitrogen content of 70%. As described earlier that the strain energy is responsible for the growth of (1 1 1) orientation, lowering the nitrogen
content means an increase fraction of argon fraction in the plasma gas. The impinging higher mass argon ions (relative to nitrogen) impart more energy to the growing species to lower the strain energy. However, reducing the nitrogen ratio below 50% in the environment abruptly decreases the nitrogen content in the film (shown in Table 2). At this stage, metallic zirconium predominates along with amorphous zirconium nitride or the one with extremely poor crystallinity because of significant deviation from stoichiometry. There are numerous reports on ZrN film deposited by magnetron sputtering with varying nitrogen fractions in the environment. Mostly in these cases, nitrogen rich zirconium nitride is formed approaching to Zr3N4 or nitrogen-rich amorphous phase for the nitrogen fraction approaching to 100% [19–21]. In our case, cathodic arc tends to evaporate the target mostly at local spots where clusters of metal/target atoms and even droplets also leave the target and deposit at the substrate. This may partly be the reason for the formation of crystalline zirconium nitrides with non-stoichiometry (lower nitrogen content) in comparison with the films prepared by magnetron sputtering at similar nitrogen fractions in the environment. 3.3. FTIR and Raman analysis FTIR spectroscopy was employed to yield information about the chemical bonds of ZrN compound. FTIR transmittance spectrum was taken in the region of interest from 500 to 2000 cm 1 in reflection mode. The resolution was kept at 4 cm 1 and number of scans were 16. Spectrum of the sample
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491
182 cm -1
327 cm-1
470 cm-1
621 cm-1
19%
Intensity (arb units)
Transmitance %
S2
-1
1075 cm
22% 50% 70%
-1
680 cm
100% 600
800
1000
1200
1400
1600
1800
2000
Wavenumber (cm-1)
200
300
400
500
600
700
800
900
Wavenumer (cm-1)
Fig. 6. Exhibiting the (a) FTIR spectrum of ZrN film deposited at 300 V and 100% N2 and (b) Raman spectra of films grown at different nitrogen fractions.
grown at 300 V and 100% N2 condition is depicted in Fig. 6(a). It indicates a broad band centered at 680 cm 1 corresponding to the stretching vibrations of Zr–N simple bonds [22]. Spectrum also depicts a band for ZrO2 at 1075 cm 1, possibly due to thin oxide layer formed on ZrN surface. The formation of oxide layer at the film surface is attributable to its difference in heat of formation (261.5 kcal mol 1) compared to ZrN (87.3 kcal mol 1) [23]. Raman spectroscopy is an important spectroscopic analysis to probe the vibrational states of crystals or semiconductors. The Raman analysis was performed using a high resolution (4 cm 1) Raman spectroscopy system (Model: MST-4000A). Fig. 6(b) offers the Raman spectra of ZrN films grown at different nitrogen conditions exhibiting peaks at 182 cm 1, 327 cm 1, 470 cm 1 and 620 cm 1. The first order Raman effect is prohibited in a perfect single crystal of fcc phase [24]. However, defects produced in ZrN during film growth may be responsible for the first order Raman phonons [25]. The nitrogen vacancies in the film may produce any disorder or defect and cause the translational invariance loss, which gives Raman peaks depicted in Fig. 6 (b). The peak at 182 cm 1 and 327 cm 1 correspond to the disorder induced single acoustic phonons and second order acoustic phonons processes respectively [26]. The peaks at 470 cm 1 and 620 cm 1 arise due to the oxygen related defects that replaces the nitrogen interstitial sites. The appearance of sharp and well defined peaks at 70% nitrogen condition demonstrated the local structural order of the grown films and is in agreement with the XRD results. 3.4. AFM results Atomic force microscopy (AFM) was utilized primarily for surface analysis. The AFM images of ZrN films prepared at different bias voltages and nitrogen conditions are shown in Fig. 7. It can be seen that the films are fairly smooth, as the variation in Z-height remains only a few nanometers. Still, the surface morphology depends on the substrate bias. For
instance, the film formed at a bias voltage of 300 V (an rms value 6.679 nm) is smoother than that prepared at 100 V (rms value 10.14 nm) depicted in Fig. 7(a) and (b), respectively. The reduction in rms roughness may possibly be related to better compactness and quality of the film by imparting higher energy to the surface atoms/species during film growth. It may, however, be interesting to note that a further increase in bias voltage to 400 V or reducing the nitrogen fraction (Fig. 7(c) and (d), respectively), imparts some greater roughness in comparison with Fig. 7(b). It is known that in the present deposition technique, clusters and droplets numerously emerge from the target and deposit at the substrate. On the other hand, re-sputtering rate depends on the composition of the film at a particular location. It appears that increased roughness at 400 V or 70% nitrogen fraction is related to more pronounced variation in re-sputtering, that is, an enhanced re-sputtering of zirconium-rich (metallic) islands, clusters, particles, etc., in the zirconium nitride film occurs during growth. 3.5. Electrical resistivity Electrical resistivity of zirconium nitride films prepared at different nitrogen conditions is shown in Fig. 8. The resistivity lies in the range of 1.1–13 mΩ-m depending on the growth conditions. The values are in agreement with the reported data [27,28]. It is a well known fact that crystallinity and stoichiometric ratio strongly affect film resistivity [29]. The total resistivity of the films is a contribution of several independent electron scattering processes due to phonons, impurity atoms and defects [30]. As shown in Fig. 8, the film grown in 100% nitrogen in the environment exhibits resistivity of 6.315 mΩ-m, which increases to 12.51 mΩ-m for the film prepared at 70% nitrogen. This change in resistivity could be attributed to the (1 1 1) orientation growth (depicted by XRD patterns) of the deposited film. The (1 1 1) preferred orientation/texture is a close packed structure where the scattering of electron with phonons will be much enhanced. The resistivity drops rapidly
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Fig. 7. AFM images of ZrN films grown at bias of (a) 100 V (b) 300 V (c) 400 V and (d) 70% N2 and 400 V.
by an amorphous halo (broad peak) with superimposed peaks of metallic zirconium.
15
4. Conclusions Resistivity (μΩ .m)
10
5
0
0
20
40
60
80
100
Nitrogen fraction (%) Fig. 8. Influence of various nitrogen conditions on the ZrN film resistivity.
to 0.551 mΩ-m for the film deposited at 50%. This decrease may correspond to the lower atomic ratio (N/Zr) of the deposited film, which drops sharply as observed in RBS results. As nitrogen flow rate decreases below 22% nitrogen, resistivity increases again. This is related to the fact that the crystallinity of zirconium nitride (Fig. 5) rapidly diminishes at this level of lower nitrogen content in the environment, as it can be noticed that XRD peaks of crystalline ZrN are almost replaced
Thin film of ZrN was deposited using the dual cathodic arc ion deposition technique. An improvement of film stoichiometry (N/Zr) at 400 V bias is revealed by RBS analysis. However, the atomic ratio (N/Zr) was found less than one regardless of nitrogen content in the deposition chamber. Nitrogen fraction (N2/ArþN2) strongly affected the film stoichiometry and thickness. XRD patterns showed polycrystalline films. The film prepared at 100 V substrate bias voltage depicted a (2 0 0) preferred orientation. A transformation to (1 1 1) preferred orientation was observed while increasing the bias voltage to 400 V. A further growth of (1 1 1) preferred orientation was noticed while reducing the N2 fraction from 100% to 70% in the synthesis chamber. Below 22% nitrogen conditions, ZrN film exhibited amorphous phase. Crystallographic orientations of ZrN films were found to be independent of substrate choice. Grooves/pits were appeared on the film surface deposited at higher substrate biasing. The change in film resistivity was correlated with the film texture and stoichiometry. A near stoichiometric film, with preferred (1 1 1) orientation having lower surface roughness and higher resistivity was prepared at 70% nitrogen condition while keeping the bias voltage at 400 V.
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