Ti-Containing Cu3N Nanostructure Thin Films - IEEE Xplore

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 43, NO. 6, JUNE 2015

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Ti-Containing Cu3N Nanostructure Thin Films: Experiment and Simulation on Reactive Magnetron Sputter-Assisted Nitridation Ali Rahmati Abstract— Ti-containing Cu3 N (Ti:Cu3 N) thin films were deposited on Si(111), quartz, and stainless steel substrates using reactive dc magnetron sputtering at N2 ambient. The significance of nitrogen pressure and that of Ti accommodation on structure and microstructure, composition, deposition rate, and mechanical hardness of the as-deposited Ti–Cu–N thin films were experimentally and theoretically discussed. Crystallinity was determined using X-ray diffractometry and varied from Cu to Cu+ Ti:Cu3 N composite and finally textured Ti:Cu3 N structure with 100 preferred orientation depending on N2 pressure. The mean crystallite size of Ti:Cu3 N is around 21 nm. Elemental concentration was recognized using energy-dispersive X-ray spectroscopy. The elemental Ti:Cu ratio in as-deposited films is around half of the original target. The reflected N neutrals from the cathode and their initial energy were calculated by means of the transport of ions in matter Monte Carlo simulation and simple binary collision model, respectively. The mean energy of the sputtered particles was estimated by introducing an appropriate distribution in the vicinity of the target surface. Energy dissipation during mass transport through the gas phase was considered to estimate the final energy of the sputtered particles toward the substrate surface. To predict the composition of Ti–Cu–N films, energy and angular contribution of sputtering yield was introduced. The calculated values for the elemental Ti:Cu ratio are in agreement with experimental ones. The pressure-dependent behavior of deposition rate was described using a proposed formula as well. Film hardness was measured by Vickers microhardness test and its minimal value was 1.75 GPa for Ti:Cu3 N films. Index Terms— Chemical composition, hardness, reactive magnetron sputtering, throw distance (TD).

I. I NTRODUCTION

S

PUTTERING-BASED methods have been vastly used to form compound thin films [1]–[3]. The deposition of a compound thin film usually depends on the formation of a compound layer on the surface of the metal target (i.e., the target mode variation). Therefore, sputtering parameters such as the partial pressure of the reactive gas and the sputtering power are carefully adjusted to maintain the target surface in an appropriate state [4]–[7]. When applying, for example, pulsed laser deposition or magnetron sputtering to grow multicomponent thin films, a multielemental material source is used to avoid loss of compositional control. In the latter case, it is clear that this

Manuscript received July 17, 2014; revised July 28, 2014 and December 12, 2014; accepted April 5, 2015. Date of current version June 8, 2015. This work was supported by the Iranian Nanotechnology Initiative. The author is with the Department of Physics, Faculty of Science, Vali-e-Asr University of Rafsanjan, Rafsanjan 7718897111, Iran, and also with the Nano-Structure Laboratory, Faculty of Science, Vali-e-Asr University of Rafsanjan, Rafsanjan 7718897111, Iran (e-mail: [email protected]). Digital Object Identifier 10.1109/TPS.2015.2422310

approach does not guarantee compositional control due to the transport of the sputter particles through the sputter gas and the preferential sputtering by energetic particles during thin film deposition. Nevertheless, this technique is vastly used in the industrial era. The simulation strategy may give an interesting perspective to control and predict the composition of complex thin films. In the present study, Ti containing Cu3 N (Ti:Cu3 N) thin films were grown using reactive dc magnetron sputtering at N2 ambient. The effect of N2 pressure dependence and that of Ti insertion on structure, microstructure, chemical, deposition rate, and hardness of Ti–Cu–N thin films were investigated. The energy and particle flux toward the substrate were estimated using simulation and models given in the literature. II. M ETHODS A. Experiments Ti-containing Cu3 N (Ti:Cu3 N) thin films were deposited by dc magnetron sputtering from a Ti13 Cu87 alloyed target at a nitrogen pressure of 0.07–1.0 Pa. The sputtered particles were condensed on ultrasonically precleaned Si(111), quartz, and stainless steel substrates to study structural, optical, and hardness properties, respectively. The working chamber was evacuated via a rotary pump and a turbo-molecular pump to allow a background pressure of 7 × 10−4 Pa to be maintained. The sputtering power, substrate temperature, and the target–substrate distance (L) were 80 W, 150 °C, and 19 cm, respectively. Crystalline phases were characterized using ex situ X-ray diffractometry (XRD, Siemens D5000) with a CuKα radiation in 2θ scan mode. The elemental composition of the films was identified using an energy-dispersive X-ray (EDX) spectrometer coupled with a scanning electron microscope (Philips XL30). The film thickness was calculated using optical interferometry. The film hardness was extracted from Vickers test. B. Simulations and Models 1) Particle Reflection and Energy Flux: Particle reflection coefficient (R N ) is defined as a fraction of the impinging ion flux reflected from target surface. TRIM.SP Monte Carlo simulation [8] was used to estimate RN . The energetic N’s are produced from N+ 2 ion flux after these ions are accelerated through the cathode sheath and are dissociatively reflected from cathode [9]. Energy reflection

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j

TABLE I M ATERIAL -D EPENDENT PARAMETERS FOR Ti AND Cu TARGET E LEMENTS B OMBARDED BY N + I ONS [11]

coefficient is defined as ratio between the energy of reflected neutrals N’s to that of impinging ions. The energy of the impinging ions is close to 0.75 of the discharge voltage, eVd [10]. Using a simple binary collision model (collision between N and each element of target), the reflected energy (E ref ) and the energy of the impinging ions (0.75 eVd ) ratio is predicted as E ref 1 M j − MN ≈ cj 0.75eVd 2 M j + MN 2

RE =

(1)

j =1

where c j is the mole fraction of each component and j = Ti, Cu. M j and MN are each target mass component and N atom mass, respectively. 2) Sputtering Yield: It is supposed that sputtering yield having separable form of energy and angle as Y (E, θ ) = Y (E) · S(θ )

(2)

where E is the energy of the incident ions and θ is the incident angle of the bombarding ions with respect to the surface normal. Behrisch and Eckstein [11] proposed a semiempirical formula for the energy-dependent part such as μ E − 1 E th (3) Y (E) = qsnKrC (ε) μ λ + EEth − 1 where q, E th , μ, and λ are material-dependent parameters. Table I indicates these parameters for pairs of incident ion and target atom. E th and snKrC (ε) are the threshold energy for sputtering and nuclear stopping power, respectively. Yamamura et al. [12] proposed a relation for angular distribution of sputtered atoms as S(θ ) = cos θ (1 + β cos2 θ )

(4)

where β depends on the mass and surface binding energy of the target material and the mass and energy of the incident ions. It may be expressed as β = BLnQ − Bc and Q =

Mt E Mg E sb

(5)

where Mt is the mass of the sputtered atom and E sb is the surface binding energy of the sputtered material (Table I). The values of B and Bc are approximated as 0.488 and 2.44, respectively. 3) Estimation of the Film Composition: The elemental Ti:Cu ratio is roughly estimated as N+ rN cbTi YTi YTi 2 Ti : Cu = · 1 − RN rN (6) + 1 − cbTi Y N2 YCu Cu

where cbTi is the Ti concentration in the target surface. Yi is the sputtering yield of the i th atom due to the j th bombardment, i = Ti, Cu and j = N2+ and reflected N neutrals. Here, it is supposed that N+ 2 bombardment acts as same as two separate N+ ions with half energy. The second term is resputtering contributions in changing the elemental composition due to the reflected N neutrals. 4) Effect of Gas Phase on Particle and Energy Transport: As sputtered Ti, Cu, and reflected N’s travel from the target to the substrate, collisions with background N2 gas cause the energy of these super-thermal species to reduce. Hence, the deposited energy of sputtered Ti, Cu atoms, and the reflected neutrals decays exponentially with pressure-distance products as Pd E f = E i exp μm η (7) kB T where η is the collision cross section for momentum transfer (exchange) between the sputtered particles or reflected neutrals and background gas [3]. μm is a function of the gas-to-particle mass ratio (M = Mg /Mt ) [3]

|1 − M|2 (1 + M) μm = 1 − Ln . (8) 2M |1 − M| The collision cross section η depends on energy, mass, and radius of the particles involved in collision. The collision cross sections are approximated by an empirical power low [3] −0.29 E η(E) = η(E 0 ) (9) E0 where E 0 = 1 eV and η(E 0 ) = π(r + rsymp )2 (1 + (Ms /Mg ))1/2 , and s = reflected N neutrals, Ti, and Cu. rsymp is the radius of the pseudohard spherical particle that occupies the same lateral area as the N2 molecule. In this study, it is assumed rsypm = (4/π)1/2rcov , where rcov is the covalent radius of the N atom (0.075 nm). The energy distribution of sputtered particle f (E) is assumed as follows [3]: E γ E in Ln (10) f (E in , E)d E ∝ d E E + E sb (E + E sb )2.5 in the vicinity of the target surface, where E 0 and γ are initial incident ion energy and energy transfer factor from collision theory, respectively. The mean energy (E ki ) of sputtered is determined as Emax E f (E)d E (11) E ki = 0 E max f (E)d E 0 where E max = γ E 0 − E sb and γ = 4Mg Mt /(Mg + Mt )2 . E ki is reduced to (E kf ) toward the substrate during mass transport due to collision with the background gas, according to (7). Throw-distance (TD) or characteristic pressure-distance product (PL)0 is expressed as [13] 1 kB 2 Ts + Tc (12) TD = (PL)0 = η(E) 3 3

RAHMATI: Ti-CONTAINING Cu3 N NANOSTRUCTURE THIN FILMS

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where Ts and Tc are absolute substrate and cathode temperature, respectively. Tc was kept at room temperature. Transport factor (TF) is introduced as TF ∝ | E¯ ki − E¯ kf | · TD.

(13)

where | E¯ ki − E¯ kf | is the energy dissipated during mass transport between target- substrate. For including the effect of mass transport mechanism on composition, elemental Ti:Cu ratio (7) must be multiplied by ratio of titanium (TF) to that of copper (i.e., Ti:Cu TF). III. R ESULTS AND D ISCUSSION A. Structural Properties Fig. 1 displays the role of nitrogen pressure and Ti addition together on the structure of Ti:Cu3 N thin films deposited on an Si(111) substrate at a sputtering power of 80 W. The films deposited at lower nitrogen pressure (PN2 < 0.2 Pa) were intended to grow as single Cu-rich phase. Whereas the films deposited at higher nitrogen pressure (0.2 Pa ≤ PN2 ≤ 0.6 Pa) showed quasi-Cu3N [100]-oriented growth. For PN2 > 0.6 Pa, the copper peaks are diminished. The top profile in Fig. 1 shows the XRD pattern of the Cu3 N thin film deposited at a sputtering power and a nitrogen pressure of 80 W and 1.0 Pa, respectively. The change in XRD pattern of Ti:Cu3 N thin film in comparison with that of Cu3 N reveals the effect of N2 pressure and Ti addition. No change appears in the Cu3 N structure due to Ti addition. This clarifies Ti soluted in the Cu3 N structure (Ti:Cu3 N), [14]. The deposition rate is increased as nitrogen pressure decreases, therefore, the Cu flux arriving at the substrate is increased and nitrification is incomplete. The appearance of the Cu (111) peak confirms this topic. On the other hand, Cu concentration is increased due to high sputtering yield at low nitrogen pressure. Hence, the films are composed of Cu3 N and phase mixture (composite). The neutral N’s reflected [9] from the surface of the single TiCu target and impinging on the growing film with enough energy (due to large TD) can produce atomic scale heating [13], [14] which decomposes the metastable Cu3 N to copper and nitrogen. This results in another mechanism for copper phase appearance in deposited films at low nitrogen pressure. The N neutral reflection coefficient (RN ) has been estimated to be RN ≈ 22% for 106 incident fluence of N+ ions on the Ti13 Cu87 target using TRIM.SP. At a target voltage of 360–405 V, RE and E ref were calculated to be above 31% and 85 eV using (1) and (7), respectively. B. Microstructural Charactristics The mean crystallite size (D) and microstrain (ε) were determined using the broadening of the peaks by Williamson–Hall plot [15] β hkl Cosθhkl K 4εSinθhkl = + (14) λ D λ where βhkl is the instrumental corrected peak broadening of Bragg reflection (hkl) originating from the small crystallite size and the strain. K is the shape factor and is equal to 0.94

Fig. 1. XRD pattern of Ti:Cu3 N films deposited on Si(111) at different nitrogen pressures. The change in XRD pattern of Ti:Cu3 N in comparison with that of Cu3 N reveals the effect of Ti addition.

for spherical crystallites. λ is the X-ray wavelength. Here, the microstrain ε is assumed to be uniform in all crystallographic directions. The dislocation density (δ) was calculated from [16] 15ε . (15) δ= a0 D The mean crystallite size (D), microstrain (ε), and dislocation density (δ) of the quasi Cu3 N structure were listed in Table II. For full nitridation at 1.0 Pa, microstructural characteristics have minimal values among all. The extra nitrogen precipitates around the Ti:Cu3 N crystallites and suppresses their growth. The film prepared at a N2 pressure of 1.0 Pa has minimal imperfections in comparison with others. It is due to nearly full occupation of the vacant site of the Cu3 N structure by Cu and Ti atoms. C. Chemical Composition Experimental elemental Ti:Cu ratio of the as-deposited films on Si(111) substrate is given in Table II, within the uncertainty range of EDX measurement. Simulation results on atomic Ti:Cu ratio were done using (6) in two cases: without and with TD consideration under nearly normal ejection. The simulated values were listed in Table II. The experimental value lies between two later values.

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TABLE II P HASE A NALYSIS , D ISCHARGE V OLTAGE (Vd ), M ICROSTRUCTURAL P ROPERTIES ( D, ε, δ), AND E XPERIMENTAL AND S IMULATED ATOMIC Ti:Cu R ATIOS IN Ti–Cu–N T HIN F ILM ON Si(111) S UBSTRATE W ITH AND W ITHOUT TD C ONSIDERATION AT VARIOUS N ITROGEN P RESSURES

Fig. 3. Variation of Ti-containing Cu3 N thin film hardness on stainless steel substrate versus nitrogen pressure (PN2 ).

in the plasma, P, and the substrate–cathode spacing, L, as Fig. 2. Variation of deposition rate of Ti-containing Cu3 N thin films on quartz substrate versus the product of nitrogen pressure (PN2 ) and target–substrate spacing (L). Experimental data (solid square); K–S formula (dashed line); and model function used in this paper (solid line).

D. Film Thickness and Deposition Rate For Ti:Cu3 N thin films grown on quartz substrate under different PN2 values and a deposition time of 20 min, the thickness was evaluated by optical interferometry method used in [14] and [17]. The thickness of Ti:Cu3 N thin films, deposited at different PN2 values and times (t), can be calculated as d = (t/20 min) d20 min and the deposition rate was obtained as t R20 min . R= (16) 20 min Fig. 2 shows the variation of the deposition rate (R) of Ti:Cu3 N thin films versus the product of N2 pressure (P) and the target–substrate spacing (L). The Keller–Simmons (K–S) formula is the well-known equation to describe plasma-assisted sputtering [13] This formula defines the deposition rate of the growing film, R, versus: the number of sputtered particles per unit time and per unit area, φ0 , predominantly neutral, the pressure of the particles

1 R(x) = φ0 [1 − exp(−x)] x

(17)

where x = P L/(P L)0 and the quantity (PL)0 is known as the characteristic pressure-distance product (throw distance), [13]. φ0 and (PL)0 were calculated as 0.95 and 8.20 Pa · cm, respectively. The K–S equation does not fit the experimental data as well. In this paper, the following relation was found: 1

R(x) = φ0 [x 2 exp(−x 2 ) + R0 x 4 ]

(18)

where φ0 , (PL)0 , and R0 are called model parameters. Here, it is supposed that ionization yield in the background gas varies in proportion to the square of the pressure (∼x 2 ). By increasing the background gas pressure, the sputtered atoms collide with gas molecule, thereby slowing them for a longer residence time in the plasma. The flux of sputtered atoms from the surface is reduced by ∼exp(−x 2 ). The second term in brackets can be related to the convection flux of the super-thermal sputtered particle (diffusion drift) toward the substrate. Equation (18) shows agreement with experimental data as well. φ0 , (PL)0 , and R were calculated as 2.78 and 2.89 Pa · cm, and 0.089,

RAHMATI: Ti-CONTAINING Cu3 N NANOSTRUCTURE THIN FILMS

respectively. The TD or characteristic pressure-distance product (PL)0 was calculated as 3.6–3.7 Pa · cm from (12). E. Hardness The total value of hardness for films deposited on steel substrate for 4 h were measured under loads of 10, 25, and 50 g using Vickers test. Fig. 3 shows the variation of H versus PN2 . It is seen that hardness follows the same trend as deposition rate versus N2 pressure. The hardness values of Cu and Cu3 N are 1.7 and 3.5 GPa, respectively [18]. The hardness of Ti–Cu–N thin films is less than that of Cu3 N, because Ti addition induces change in grain geometry from pyramid-like to sphere-like [19]. It is seen that the hardness for composite Cu+Ti:Cu3 N and that for pure Ti:Cu3 N structures have maximal and minimal values, respectively. IV. C ONCLUSION Ternary Ti:Cu3 N thin films were deposited on Si(111) quartz and stainless steel substrates using dc magnetron sputtering at a nitrogen atmosphere of 0.07–1.0 Pa. The crystalline phase varied from Cu, Cu+ Ti:Cu3 N composite, and finally textured Ti:Cu3 N structure with 100-preferred orientation depending on N2 pressure. The elemental Ti:Cu ratio in as-deposited films is around half of the original target. Vickers microhardness for composite Cu + Ti:Cu3 N and pure Ti:Cu3 N structures have maximal and minimal values, respectively. A simple model was constructed based on sputtering theory and gas phase transport to estimate the final composition of compound films. The calculated values for the elemental Ti:Cu ratio are in agreement with experimental ones with low uncertainty.

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[4] N. Malkomes and M. Vergöhl, “Dynamic simulation of process control of the reactive sputter process and experimental results,” J. Appl. Phys., vol. 89, no. 1, pp. 732–739, 2001. [5] S. Ohno et al., “Plasma emission control of reactive sputtering process in mid-frequency mode with dual cathodes to deposit photocatalytic TiO2 films,” Thin Solid Films, vol. 445, no. 2, pp. 207–212, 2003. [6] E. Kusano and A. Kinbara, “Investigation of the effects of pumping speed and Ar/O2 ratio on the transient time at mode transition in Ti-O2 reactive sputtering,” Thin Solid Films, vols. 281–282, pp. 423–426, Aug. 1996. [7] T. Kubart, O. Kappertz, T. Nyberg, and S. Berg, “Dynamic behaviour of the reactive sputtering process,” Thin Solid Films, vol. 515, no. 2, pp. 421–424, 2006. [8] (2006). The Stopping and Range of Ions in Matter. [Online]. Available: http://www.srim.org/ [9] Z. Wang, S. A. Cohen, D. N. Ruzic, and M. J. Goeckner, “Nitrogen atom energy distributions in a hollow-cathode planar sputtering magnetron,” Phy. Rev. E, vol. 61, no. 2, p. 1904, 2000. [10] S. Mahieu and D. Depla, “Reactive sputter deposition of TiN layers: Modelling the growth by characterization of particle fluxes towards the substrate,” J. Phys. D, Appl. Phys., vol. 42, no. 5, p. 053002, 2009. [11] R. Behrisch and W. Eckstein, Eds., Sputtering by Particle Bombardment: Experiments and Computer Calculations from Threshold to MeV Energies. Berlin, Germany: Springer-Verlag, 2007. [12] Y. Yamamura, T. Takiguchi, and M. Ishida, “Radiation damage in zircon by high energy electrons beams,” Radiat. Effects Defects Solids, vol. 118, no. 3, p. 237–261, 1991. [13] A. Palmero, H. Rudolph, and F. H. P. M. Habraken, “Generalized Keller–Simmons formula for nonisothermal plasma-assisted sputtering depositions,” Appl. Phys. Lett., vol. 89, no. 21, p. 211501, 2006. [14] A. Rahmati, “Reactive DC magnetron sputter deposited Ti–Cu–N nano-composite thin films at nitrogen ambient,” Vacuum, vol. 85, no. 9, pp. 853–860, 2011. [15] H. P. Klug and L. E. Alexander, X-Ray Diffraction Procedures: For Polycrystalline and Amorphous Materials. New York, NY, USA: Wiley, 1974. [16] B. E. Warren, X-Ray Diffraction. London, U.K.: Addison-Wesley, 1969. [17] A. Rahmati, H. Bidadi, K. Ahmadi, and F. Hadian, “Ti substituted nano-crystalline Cu3 N thin films,” J. Coat. Technol. Res., vol. 8, no. 2, pp. 289–297, 2011. [18] J. F. Pierson, “Structure and properties of copper nitride films formed by reactive magnetron sputtering,” Vacuum, vol. 66, no. 1, pp. 59–64, 2002. [19] X. Y. Fan et al., “Ti-doped copper nitride films deposited by cylindrical magnetron sputtering,” J. Alloys Compounds, vol. 440, nos. 1–2, pp. 254–258, 2007.

ACKNOWLEDGMENT The authors would like to acknowledge financial support of Iranian nanotechnology initiative. R EFERENCES [1] W. D. Westwood, Sputter Deposition. New York, NY, USA: AVS, 2003. [2] K. Wasa, M. Kitabatake, and H. Adachi, Thin Film Materials Technology. Norwich, U.K.: Williams Andrew, 2004. [3] D. Depla and S. Mahieu, Eds., Reactive Sputter Deposition. Berlin, Germany: Springer-Verlag, 2008.

Ali Rahmati was born in Palkanlou Sofla, Shirvan, Northern Khorasan, Iran, in 1980. He received the B.S. degree in applied physics from the Shahroud University of Technology, Shahroud, Iran, and the M.S. degree in solid-state physics from the University of Tabriz, Western Azarbaijan, Iran, in 2005, where he was worked toward the Ph.D. degree in same field in 2010. His current research interests include ion-solid interaction and modeling, sputtering method to thin-film growth, synthesis of compound semiconducting nanostructure, and simulation of Chalcopyrite-based solar cells.