Electrodeposition of CoMoP thin film as diffusion barrier layer for ULSI applications

Surface & Coatings Technology 203 (2009) 3692–3700

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

Electrodeposition of CoMoP thin film as diffusion barrier layer for ULSI applications Z. Abdel Hamid a,⁎, A. Abdel Aal a, Ali Shaaban a, H.B. Hassan b a b

Central Metallurgical Research & Development Institute (CMRDI), Helwan, Egypt Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt

a r t i c l e

i n f o

Article history: Received 5 April 2009 Accepted in revised form 2 June 2009 Available online 9 June 2009 Keywords: Electrodepostion CoMoP Barrier layers Capping layers Thin films Ferromagnetic Corrosion

a b s t r a c t CoMoP thin films were fabricated by electrodeposition technique from citrate based bath onto Cu sheets for the application as diffusion barriers and metal capping layers in the copper interconnect technology. The study focused on the effect of (NH4)6Mo7O24·4H2O concentrations in the plating solution on the plating rate and chemical composition of the deposited layer. It was found that the Mo wt.% in the deposited layer increased from 13 to 22 wt.% with increasing (NH4)6Mo7O24·4H2O concentration. The influence of deposition current density, solution pH and deposition temperature at certain (NH4)6Mo7O24·4H2O concentration in the plating bath on the plating rate and chemical composition was studied. Polarization behavior of induced codeposition of CoMoP at various electrolyte pH values was studied using cyclic voltammetry and chronoamperometry to estimate the current efficiency (CE%) of the plating solutions and the optimum pH for the plating process. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques have been applied to characterize the morphology and chemical composition of the deposited layer. CoMoP alloys of high P wt.% as-deposited films showed irregular microcracks amorphous structure and of low P wt.% showed amorphous/nanocrystalline structure while, after annealing at 400 °C for 1 h, the films deposited with low and high P wt.% converted into polycrystalline structure. The results of oxidation property showed that, the Co–13.2 wt.% Mo–10.3 wt.% P alloy has highest stability against oxidation and lowest electrical resistance values (100–150 µΩ). The ferromagnetism nature of coated materials has been studied by hysteresis loop measurements. The electrochemical corrosion results were calculated from polarization studies for as-plated and annealed CoMoP coatings in 3.5% NaCl solution. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Recently, the focus of the interest is moving into integration technologies of copper and low dielectric constant materials. The problem of the usage of Cu is its oxidation and diffusion into SiO2 layer; which degrades the electrical performance of the microelectronic devices. To prevent this problem, barrier layers with dielectric or metallic materials have been introduced. Among them, nickel and cobalt are commonly used as barrier materials because of its better barrier capacity and performance at high temperature [1]. The co-deposition of P atoms can stuff the voids in the grain boundaries of Ni films, thereby improving their barrier function [2]. However, amorphous Ni–P film is of only moderate mechanical strength and it has a high enough magnetic susceptibility to be classified as ferromagnetic [3] after its structural is transformation into crystalline phase. The addition of MoO2− 4 expected to solve the above problem and eliminate the film stress generated in Ni serving as efficient inhibitors for electroless deposition. as a consequence of The incorporation of molybdenum oxide MoO2− 4 ⁎ Corresponding author. Surface Protection & Corrosion Control Lab., Central Metallurgical Research & Development Institute, CMRDI, P.O.87 Helwan, Egypt. Tel.: +20 12340792; fax: +20 25010639. E-mail address: [email protected] (Z.A. Hamid). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.06.004

cobalt–molybdenum induced co-deposition has been demonstrated by Gómez et al. [4]. Furthermore, the introduction of molybdenum to the cobalt coating producing a material has good soft-magnetic properties [5]. Additionally, Mo has a high melting point (2623 °C), high thermal stability, high thermal conductivity (1.246 W/cm/°C), good electrical conductivity and a very low diffusivity. Among these advantages, the most important factor is that Mo has varied workability for the pattern fabrication. Line patterns or structures can easily be obtained from Mo by using dry or wet etching. In addition, there are no intermetallic compounds between Cu and Mo while the temperature is below 1000 °C [6]. This implies that, the electrical resistance does not rise rapidly, as Cu and Mo come into contact. Although a lot of literatures have been reported about interfacial phenomena between Cu and Mo or Monitride [7–9], there is still some information lacking about the Cu/Mo/Si multi-layer structure. Thus, the applicability of Mo buffer layers to Cubased interconnects should be urgently investigated. Barrier layers can be deposited by several techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), and wet methods by electrochemical or electroless technique [10–12]. However, electroless deposition method can be considered more suitable than electrolytic one due to the possibility of achieving uniform surface coverage and plating micromagnetic patterns on variety of substrates [13]. On the other hand, the electrodeposition is one

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of the most cost-effective techniques for the fabrication of nanostructured materials. The advantage of electrodeposition includes: room temperature operation; thus reducing problems with thermal stress; low equipment cost, high deposition rates, and easy scalability. Furthermore, electrodeposition is the appropriate method for preparing both low and high coating thickness. Many studies have been done by our group on electroless cobaltbased ternary alloys for electronic applications [14] and for magnetic application [15]. In the present work, efforts have been done to deposit diffusion barrier layers for copper metallization. So, the central goal of this study is to fabricate an excellent antioxidant CoMoP layer using electroplating technique. The work mainly focused on the development of a bath and optimization of deposition conditions for electroplating CoMoP thin film with required barrier layer properties. The influence of various factors on the current efficiency, the deposition rate, the chemical composition, surface morphology, microstructure, corrosion resistance, electrical and magnetic properties have been examined. In addition, the work presents the effect of annealing in air on the thin films characteristics.

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thin-plate sample containing four very small ohmic contacts placed on the periphery, preferably in the corners, of the plated Cu sheet. The magnetic properties of electroplating CoMoP ternary alloy deposits were studied using a vibrating magnetometer device (9600VSM) at an applied field of 16 kOe. The magnetic parameters, viz. saturation magnetization MS, remanence Mr, coercivity Hc, squareness S, uniaxial anisotropy K and anisotropy angle θ derived from the hysteresis loop parallel and perpendicular with respect to the film surface with 16 kOe magnetic field. Potentiodynamic polarization tests were carried out on CoMoP working electrodes (as-deposited and annealed) to study the general corrosion resistance of the samples at room temperature using an Auto lab1 Pgstate 30 with corrware software. The electrochemical measurements were made in a conventional three-electrode cell using a saturated calomel electrode (SCE) as a reference electrode and a platinum wire was used as an auxiliary electrode. Cleaned coupons were dipped in 3.5% NaCl solution and the change in the potential with respect to SCE was monitored continuously at regular intervals. 3. Results and discussion

2. Experimental procedures 2.1. CoMoP electrodeposition Electrodeposition was performed on copper substrates of 0.6 mm thickness. Prior to the deposition, the substrates were degreased in alkaline solution to remove oil and greases then pickled in dilute nitric acid for 30 s to remove the native oxide layer from the copper sheet surface. For electrodeposition of Co alloy, the citrate bath was selected which has the following composition CoSO4·7H2O, 25 g l− 1; (NH4)6Mo7O24·4H2O, 2.5–12.5 g l− 1 and NaH2PO2·H2O, 25 g l− 1; as metals sources and Na3C6H5O7·2H2O, 80 g l− 1; as a complexing agent to control the rate of release of free metal ions for the reduction reaction. Tri-sodium citrate was chosen as a complexing agent because it is non-toxic and has brightening, leveling and buffering actions. The plating process took place under the following conditions: 25–50 °C, 6.1–9.3 pH, 1–10 A dm− 2 and at plating time 15 min. For pH-dependent experiments the pH was adjusted by the addition of either NH3 solution or H2SO4. After deposition the samples were washed with distilled water and dried. 2.2. CoMoP characterization and analysis Polarization behavior of induced co-deposition of CoMoP on copper electrode at various electrolyte pH values was studied using cyclic voltammetry and chronoamperometry to estimate the current efficiency (CE%) of the plating solutions and the optimum pH for the plating process. The electrochemical measurements were conducted using a computer-controlled potentiostat (Amel 5000) driven by an IBM PC for data processing. Electrochemical measurements were performed on the electrode surface area of 1 cm2. The electrode was immersed in CoMoP electrolyte in the electrolytic cell at the room temperature. A saturated Hg/Hg2SO4/1 M H2SO4 (MMS) which is (680 mV vs NHE) was used as reference electrode and Pt wire was used as counter electrode. The phase structure of the films was characterized by X-ray diffraction analysis using Bruker AXS X-ray diffractometer with Cu Kα1 target of wavelength 1.5046 Å. The scanning step, rate and range were 0.02°, 0.05° s− 1 and 10–100°, respectively. A scanning electron microscope (SEM) equipped with energy dispersive X-ray system (EDX) model JEOL, JSM-5410 was used to study surface morphologies and elemental analysis of the coating layers. The oxidation test was performed by annealing the coated samples at 400 °C. The electrical resistance of the films was measured before and after oxidation as a function of time up to 2 h in air. The electrical current and potential difference were measured using a four probe approximated Van der Pauw technique [16]. Van der Pauw, one uses an arbitrarily shape,

The electrochemical behavior at different pH values was investigated in the plating solution by employing cyclic voltammetry and chronoamperometry on copper electrode to estimate the optimum pH value for the plating process and the current efficiency (CE%) of the plating solutions. Fig. 1(a–c) show the cyclic voltammetric behaviors of electrolytic solutions containing cobalt, molybdenum, hypophosphite and citrate ions at various pH values and scan rate 50 mV s− 1 at copper electrode. The polarization was started from −1200 mV up to −100 mV (MMO) in the anodic direction, then the scan was reversed to −1200 mV (MMO) in the cathodic direction. At the selected potential range, all cyclic voltammograms are characterized by one peak in the anodic direction and another one in the cathodic direction, their shapes, current densities and potential values are depending on the pH value. It is clear that, the voltammograms proved that the codeposition of CoMoP occurs at around −550 to −650 mV (MMO). A gradual shift of onset potential for the reduction of ions toward more negative direction is also observed. The onset potentials and calculated current efficiencies at various pH values for the deposition of CoMoP film from citrate bath are given in Table 1. An increasing in cathodic current is found at −615 mV during potential scan toward more negative direction at pH 9.3. The shifting of the onset potential for reduction at various pH values is influenced by proton in case of low pH (6.1) and by OH− ions in the case of higher pH values (8.5, 9.3). Fig. 1 also shows that, on the reverse scan, the stripping potential shifts toward cathodic direction with increasing the pH of the electrolyte. In the case of pH 6.1 electrolyte, the stripping current (anodic current) is observed at −250 mV. On the other hand, the dissolution of the deposited layer starts at much higher negative potential −450 mV in the pH 9.3 electrolyte. In addition, the cyclic voltammetry has been applied to estimate the current efficiency (CE%) for the deposition of CoMoP film using the following equation: CE =

QO × 100 QR

ð1Þ

where QO and QR are oxidation and reduction charges, respectively. The quantity of charge was calculated from the integrated area within cathodic and anodic regions. From Table 1, we can find that; the current efficiency reaches the ultimate value at pH 6.1. Further increasing tends to CE% decreasing; these results are in agreement with Dulal et al. [17]. At pHN 6 the evolution of large amount of hydrogen from the bulk of the electrolyte decreases the efficiency of the solution while, the decrease in current efficiency with increasing pH (b6) is due to the formation of cobalt hydroxide (Co(OH)2) in the solution [18]. The highest current efficiency (69%) has been found when CoMoP is plated from an

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Fig. 2. Current transient of CoMoP deposition at different pH electrolytes on copper sheets at applied potential of − 1000 mV (MMO) for 5 min. 2+ ] metal ratio at constant cobalt sulfate The effect of [MoO2− 4 ]:[Co −1 on the deposition rate and chemical composition of CoMoP 25 g l films is shown in Fig. 4. It is clear that the deposition rate increases gradually with increasing the metal ratio up to 0.25. These results are disagreed with Y. WU et al. who have reported that the coating thickness of NiMoP deposited by electroless technique was decreased 2+ ] ratios [20]. In addition, Fig. 4 with increasing of [MoO2− 4 ]:[Ni illustrates the correlation between metal ratio and the contents of Mo and P in CoMoP thin film. As can be seen in this figure, the weight percentage of Mo increases with increasing in metal ratio up to 0.25, on the other hand; the concentration of P decreases with increasing the metal ratio. The results were in agreement with Koiwa et al. [21]. At higher concentration of metal ratio than 0.25, EDX analysis shows the presence of oxygen which means that the oxidation of the deposited layer and there is no enough citrate to form the mixed metal complex of Co, Mo which leads to decomposition of the electrolyte. According to Ivanova et al. [22] Mo in the anionic form [MoO2− 4 ] has the following equilibrium:

Fig. 1. Voltammograms of the CoMoP electrolyte at different pH values; a) pH 6.1, b) pH 8.5 and c) pH 9.3. −

electrolyte of pH 6 ± 0.1 due to the amount of CoCit species in the solution is also the highest at this pH during the deposition. This also observed by Dulal et al. in the deposition of CoWP ternary alloy [19]. In the potentiostatic deposition technique the transient currents at various pH values are recorded and represented in Fig. 2. It is obvious from the current transient of different pH electrolytes on copper sheets at applied potential of − 1000 mV (MMO) for 5 min that the lowest current required for deposition was at the nearly neutral pH (6.1). This steady state current was at more negative values (−5 to −6 mA). The deposited CoMoP films were then stripped in the same solution by holding the electrode at a constant potential of −100 mV until the anodic current reached to minimum value or nearly zero. The stripping current of the deposited film at different pH were recorded and represented in Fig. 3.

6−

2−

þ

½Mo7 O24  þ 4H2 O↔½MoO4  þ 8H

ðIÞ

This equilibrium in acidic media is virtually quantitatively shifted to the formation of pentamolybdate ions. In the simplest case, the

Table 1 Onset potentials for oxidation and reduction and calculated current efficiencies at various pH for the deposition of CoMoP film on Cu by cyclic voltammetry. Electrolyte pH

Onset potential for reduction (mV) MMO

Onset potential for oxidation (mV) MMO

CE%

6.1 8.3 9.3

−379 − 544 −616

−147 −261 − 340

69.7 52.7 19.35

Fig. 3. Stripping Currents of CoMoP deposition in different pH electrolytes from copper sheets at applied potential of − 100 mV (MMO) for 5 min after 5 min of deposition at − 1000 mV (MMO).

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following reactions take place through an electrochemical reduction of molybdate ions to molybdate oxide and then a chemical reduction of molybdate oxide to metallic molybdate [23]. 2−

þ

−

2½MoO4  þ 4H þ 4e → 2MoO3 þ 2H2 O

ðIIÞ

þ

−

ðIIIÞ

þ

−

ðIVÞ

MoO3 þ 2H þ 2e → MoO2 þ H2 O MoO2 þ 4H þ 4e → Mo þ 2H2 O

At the same time, the sodium hypophosphite supplies the H2PO− 2 ion as phosphorus in the coating and cobalt ions reduced to cobalt metal. −

þ

−

H2 PO2 þ 2H þ e → P↓ þ 2H2 O 2þ

Co 2+ Fig. 4. The effect of [MoO2− ] metal ratio at constant cobalt sulfate 25 g l− 1 on the 4 ]:[Co deposition rate and the contents of Mo and P in CoMoP films.

−

þ 2e → Co

ðVÞ ðVIÞ

The morphological analysis of CoMoP thin films deposited at 25 °C, 6 A dm− 2, pH 6.1 and at different metal ratios was investigated. Fig. 5 illustrates the SEM micrographs of the CoMoP thin films as deposited and after annealing at 400 °C for 1 h. It can be seen that, the deposition with low metal ratio 0.04 produces coherent and smooth films as in Fig. 5a. Thus, the lowest metal ratio is suitable to prepare finer

Fig. 5. SEM of CoMoP thin films deposited from electrolytes containing different metallic ratios at pH 6.1, 15 min, 6 A dm− 2, 25 °C before and after annealing at 400 °C for 1 h. 0.04 b) 0.16 and c) 0.24; after annealing d), e) and f).

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Fig. 6. The effect of deposition current densities at 0.04 metal ratio on the deposition rate of CoMoP films and the contents of Mo and P at pH 6.1, 15 min, 25 °C.

morphological CoMoP alloy films. While, an increasing of metal ratio than 0.04, irregular microcracks are observed at the surface of asdeposited CoMoP film. These cracks increase by increasing in the metal ratio Fig. 5(b, c). These microcracks may be attributed to the internal stresses in as-deposited films. In addition, oxygen was detected in the films and its content increased with increasing metal ratios. However, these deposited showed cracks. After annealing at 400 °C for 1 h the images show significant morphological change as in Fig. 5(d, e, f). More compact and uniform homogenous surface morphology was observed for CoMoP thin films deposited with low metal ratios as in Fig. 5d. The layer deposited at 25 °C, 6 A dm− 2, pH 6.1 and low metal ratio 0.04 was the best one with smaller grain size and homogeneous distribution of particles which aligned perpendicularly to the surface of the film. Fig. 6 shows the effect of the current density on the chemical composition and the deposition rate of the coating layers. The influence of current density was studied at pH 6.1, 15 min and 25 °C. As can be seen in Fig. 6 the deposition rate increases by the increasing of current density from 1 to 10 A dm− 2 and then burned layer is observed with further increasing in the current density. In addition, Fig. 6 demonstrates the dependence of CoMoP deposit composition on the applied current density. It was observed from EDX data that, both of Mo wt.% and P wt.% significantly increase; with increasing of current density; from 2.2 wt.%

Fig. 7. SEM of CoMoP thin films deposited at pH 6.1, 15 min, 0.04 metal ratio, 25 °C and at different current densities before and after annealing at 400 °C for 1 h. a) 1, b) 3 and c) 6 A dm− 2; after annealing d), e) and f).

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Fig. 8. The effect of deposition temperature on the deposition rate and elemental composition of CoMoP films deposited at pH 6.1, 15 min, 6 A dm− 2, and 0.04 metal ratio.

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to 13.3 wt.% and from 3 wt.% to 14 wt.%, respectively. The results suggest that, the deposition of Co and P is facilitate at low current density and a higher current density is more favorable for the deposition of Mo when plated from a complex solution containing cobalt, molybdenum, hypophosphite and citrate ions. The SEM images of the CoMoP thin film deposited at different current density are shown in Fig. 7. The images show a compact and homogeneous surface morphology of the deposited layer (Co–13.2 wt.% Mo– 10.3 wt.% P) after annealing at 400 °C for 1 h. The images reveal that, at low current density (1 A dm− 2) the surface morphology takes the spherical or nodular shape of about 1.5 µm in diameter particles while, at (3 A dm− 2) the structure takes the network shape of particles smaller than that of corresponding one deposited at lower current density. At current density 6 A dm− 2 the micro-crystals of 1 µm in diameter grown and self aligned perpendicularly to the surface. Fig. 8 illustrates the effect of bath temperature on the deposition rate and the chemical composition of the CoMoP films deposited from electrolyte contains 2.5 g l− 1 (NH4)6Mo7O24·4H2O at pH 6.1 and 6 A dm− 2. It is clear that, the deposition rate increase very sharply when the bath temperature is raised from 25 to 50 °C. Additionally, cobalt content in the film increases with the temperature increasing. Both Mo and P contents in the films decrease with bath temperature. As discussed earlier, the co-

Fig. 9. SEM of CoMoP thin films deposited at pH 6.1,15 min, 6 A dm− 2 and 0.04 metal ratio at different temperatures as deposited before a) 25, b) 40 and c) 50 °C and after annealing at 400 °C for 1 h; d), e) and f).

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deposition of Mo and P occurs under diffusion control. The composition analysis suggests that the influence of temperature on the diffusivity of Co species in the solution is dominant than that of molybdate and hypophosphite ions. The SEM images of the CoMoP thin films deposited at different temperatures before and after annealing at 400 °C for 1 h are shown in Fig. 9. The images reveal that, the deposited layer at 25 °C (Co– 13.2 wt.% Mo–10.3 wt.% P) after annealing converted into compact and homogeneous surface morphology of fine grained while, further increasing in the deposition temperature the annealing process tends to particles agglomeration. Fig. 10 shows X-ray diffraction patterns of electrodeposited CoMoP films with low and high Mo wt.% before and after annealing at 400 °C for 1 h. XRD pattern of as-deposited Co–13.2 wt.% Mo–10.3 wt.% P (Fig. 10a) shows peaks of Cu substrate doesn't show any cobalt or related peaks as an indication of amorphous coating presence. This is because of higher phosphorus (10.33 wt.%) content in this film. Fig. 10b shows XRD pattern of as-deposited Co–15 wt.% Mo–3.5 wt.% P. The pattern reveals that, the deposit consists of amorphous/nanocrystalline phase. From the equilibrium-phase diagram of Co–Mo [24], at room temperature, the ε-Co phase was able to dissolve substantial amounts of molybdenum forming Co3Mo solid solution at 2θ ≈90.1°. After annealing both CoMoP thin films, besides the CoO phase existence due to thermal oxidation, weak peaks of Mo and its phosphide (MoP), fcc-Co and solid solution hcp-Co3Mo phases are detected. The appearance of MoP is attributed to the higher phosphorous segregation in grain boundary regions of Co3Mo solid solution. Relative sheet resistance ((Ra − Rd)/ Ra) of the CoMoP alloy films was measured as a function of annealing time at 400 °C, where Ra and Rd are the resistances of annealed and as-deposited alloy, respectively. It was found that the sheet resistance of CoMoP films containing different Mo wt.% in the range (200–250 µΩ). The Relative sheet resistance data demonstrated in Fig. 11 shows relatively small deviation in its values (±0.4) after annealing at 400 °C for different time periods up to 120 min. The oxidation resistance decreased with increasing of the Mo wt.% in the deposited layer. Improvement of oxidation resistance and reducing the oxidation rate are explained by slower diffusion of reaction components (oxygen and cobalt with molybdenum) in alloy films. Increasing of the Mo content in the deposited layer over 13 wt.% leads to deterioration in oxidation stability due to oxidation behavior of molybdenum and formation of fast diffusion pathways along grain boundaries. This observation is in agreement with studies done by Sverdlov et al. [25] on another alloying element W in the electrolessed CoWP alloy. Furthermore Mo and Co may be oxidized by increasing the Mo wt.% in the deposited layer. The oxidation of Co is due to the applied mechanical

Fig. 10. X-rays thin film for a) (high P and low Mo wt.%) Co–13.2 wt.% Mo–10.3 wt.% P alloy before annealing b) (low P and high Mo wt.%) Co–15 wt.% Mo–3.5 wt.% P alloy before annealing c) both CoMoP alloys after annealing at 400 °C for 1 h.

Fig. 11. Relative sheet resistance of deposited CoMoP films containing different Mo wt.% deposited at 6 A dm− 2, pH 6.1, 15 min after annealing for 2 h at 400 °C.

stress on the alloy grain boundaries which allow the oxygen penetration to oxidized Co [26]. In conclusion, a highest stability against oxidation and lowest resistance values (100–150 µΩ) were characterized for the Co–13.2 wt.% Mo–10.3 wt.% P films of thickness≈0.2 µm. This may be attributed to that, the films of low Mo wt.% are comprised of amorphous phase and otherwise nanocrystalline or microcrystalline structure was detected for high Mo wt.% films. Consequently, the relative resistance values of low Mo wt.% films fluctuated especially after 45 min of annealing at 400 °C because of the crystallization process occurrence but its overall behavior was decreasing. The relative resistance of the high Mo wt.% films (Co– 22.5 wt.% Mo–7 wt.% P–3 wt.% O) also fluctuated after the same time of annealing may be due to the starting of the oxidation process of Mo but its overall behavior was increasing. The amorphous component is expected to exhibit low electrical conductance in comparison to crystalline Co [27]. The magnetic hysteresis of as-deposited and annealed CoMoP films is measured as a function of the applied field in the parallel and perpendicular directions and is shown in Fig. 12. It is obvious that Co– 13.2 wt.% Mo–10.3 wt.% P film showed a very strong magnetic moment (BS) before and after annealing compared with Co–15 wt.% Mo–3.5 wt.% P and Co–22.5 wt.% Mo–7 wt.% P–3 wt.% O films. While Co–15 wt.% Mo– 3.5 wt.% P, Co–22.5 wt.% Mo–7 wt.% P–3 wt.% O films after annealing enhance BS due to the nanocrystalline phase formation. This observation is in agreement with the above statement in the XRD part. The magnetic properties, viz. saturation magnetization MS, remanence Mr, coercivity Hc, squareness S, uniaxial anisotropy K and anisotropy angle θ values of different alloys are illustrated in Table 2. As shown in Table 2 Hc values of perpendicular direction reveals the hard ferromagnetic nature of the deposited alloys. The Hc values of the films were found to decrease with increasing in P wt.%. These results indicated that the alloy Co–15 wt.% Mo–3.5 wt.% P shows the highest value of Hc in the perpendicular direction due to low P wt.% and coarse crystallites, as shown in SEM images Fig. 5b. According to Tarozaite et al. [28] for Ni– Co–P system the formation of certain crystallites shape, which determines high Hc, takes place only at definite phosphorus content. If phosphorus content exceeds 6 wt.%, films consisting of very fine grains are deposited Fig. 5(a and c) which leads to a decreasing in Hc. It may be concluded that the relation between the P wt.% and the Hc is an inverse proportionality relationship. Furthermore, Hc is directly proportional with the film surface crystallite size. The previous studies on ternary magnetic alloys of Ni–Co–P system by Sankara Narayanan et al. [29] and Matsubara et al. [30] reveal that the MS and Mr are found to increase with cobalt content of the deposit. García-Arribas et al. show that the reduction in the phosphorous content in Co–P alloys produces an increase in the Ms and, therefore, in the total

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magnetic moment of the film [31]. While the predominant factor in our study was the ratio of non magnetic inclusions to each other (P:Mo) in the deposited layer. The MS and Mr are directly proportional to the P:Mo ratio. When the P:Mo ratio increased three-fold the MS also increased five times. So we can say that, the alloy Co–13.2 wt.% Mo–10.3 wt.% P has the highest magnetic moments orientation. Furthermore, these ratios reflect the increasing in the probability of formation of non magnetic phases such as (MoP) and increasing in the probability of existence of cobalt as a magnetic phase.

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Table 2 Magnetic parameters of CoMoP thin film of different compositions measured at perpendicular direction with respect to the film surface in 16 kOe magnetic field. Alloy

Annealing HcPer

Co–13.2 wt.% Mo–10.3 wt.% P Before After Co–15 wt.% Mo–3.5 wt.% P Before After Co–22.5wt.% Mo–7wt.% Before P–3wt.% O After

BrPer BsPer

742.3 34.9 213.5 356.2 18 155.3 1366 11.6 38.8 983 31.2 132 808.6 11.1 52.75 1490 17.3 57.2

SPer K, × E6 θ° 16 12 30 23 21 30

2.68 2.01 0.53 1.70 0.72 0.77

41.9 42.8 44.4 42.7 44.4 44.1

The value of the uniaxial anisotropy constant (K) and the angle θ of the resulting anisotropy with respect to the film normal may be found using the expressions [32]: Hc == =

2K cos2 θ MS

Hc == + Hc8 = 4πMS +

ð2aÞ 2K MS

ð2bÞ

where MS is the saturation magnetization, Hc//, Hc are the parallel and perpendicular direction coercivities, respectively. Uniaxial anisotropy values of CoMoP are from 0.53 up to 2.68 × 106 erg cm− 1 and in angle around 44° were found. A relatively weak uniaxial anisotropy was observed for the CoMoP films, if it compared with results obtained by Vallés et al. [33] uniaxial anisotropy of 6.4 × 106 erg cm− 1 and θ = 55° were found for hcp CoMo system. This decreasing in both uniaxial anisotropy and anisotropy direction values may be due to the existence of the phosphorous element. The polarization resistance Rp was measured by scanning the potential in a range between 100 to − 1000 mV around the corrosion potential Ecorr at 0.1 mV s− 1. From the current C–V plot the reciprocal of the slope of the curve dE/dI was determined at the corrosion potential in resistance units (Ω cm− 2). The corrosion current densities in (μA cm− 2) were calculated from the polarization resistance values using the Stern–Geary equation [34]: Icorr =

Ba × Bc 2:303ðBa + Bc Þ

ð3Þ

where the Tafel constants Ba and Bc, are the slopes of tangents drawn on the respective anodic and cathodic polarization plots in mV dec− 1. Subsequently, the corrosion rate (CR) was determined in mpy using the relationship: CR =

0:13 × Icorr × eq:wt: ρ

ð4Þ

where ρ the density of the alloy (g cm− 3) and Icorr the current density (μA cm− 2).

Fig. 12. Hysteresis loops of a) Co–13.2 wt.% Mo–10.3 wt.% P, b) Co–15 wt.% Mo–3.5 wt.% P and c) Co–22.5 wt.% Mo–7 wt.% P–3 wt.% O films before; b; and after; a; annealing obtained at room temperature for parallel and perpendicular direction of 16 kOe applied field.

Fig. 13. Electrochemical polarization curves of CoMoP films containing different Mo wt.% in 3.5% NaCl solution a) 13.3, b) 15 and c) 22.5 wt.% before annealing and d), e) and f) after annealing for 1 h at 400 °C.

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Table 3 Electrochemical corrosion data related to polarization curves of coated Cu sheet with thin films of different composition in 3.5% NaCl solution before and after heat treatment at 400 °C for 1 h. Alloy

βa

Co–13.2 wt.% Mo–10.3 wt.% P 0.196 0.253 Co–15 wt.% Mo–3.5 wt.% P 0.179 0.147 Co–22.5 wt.% Mo–7 wt.% 0.141 P–3 wt.% O 0.182

βc

Rp, Ω cm− 2 Icorr, μA cm−2 Ecorr, V

0.107 964.6 0.122 886.0 0.074 801.0 0.091 505.0 0.077 1421 0.165 989.0

30.79 37.38 28.05 47.76 15.04 37.55

− 0.567 − 0.430 − 0.547 − 0.407 − 0.557 − 0.414

CR, mpy 14.22 17.26 12.96 22.07 7.226 18.04

Fig. 13 shows the electrochemical results obtained from polarization studies of CoMoP coatings in 3.5% sodium chloride solution before and after heat treatment. The electrochemical corrosion parameters obtained by Tafel polarization curves are listed in Table 3. The Tafel slopes Bc and Ba were determined by fitting of a theoretical polarization curve to the experimental polarization curve. The corrosion current Icorr is representative of the degree of degradation of the alloy. An alloy with a tendency toward passivation will have a value of Ba greater than Bc, whereas an alloy that corrodes will have a Ba lower than Bc [35]. As shown in Table 3 and Fig. 13, the increasing in Mo wt.% slightly moved the potentiodynamic polarization curve in the noble direction, i.e. the corrosion potential slightly changed positively from − 0.567 to −0.557 V Fig. 13(a, b and c) and downed positively to around −0.414 V after annealing Fig. 13(d, e and f). Furthermore the lower current density in the anodic polarization curve of Co–22.5 wt.% Mo–7 wt.% P–3 wt.% O coating suggests a lower corrosion rate for the coatings containing high Mo wt.%. In the present study, the improvement of the corrosion resistance arises from two effects; increasing Mo wt.% in the as-deposited and decreasing crystallite size by heat treatment. Increasing the Mo content in the alloy initially increased the corrosion resistance owing to preferentially migrated of Mo toward the surface and formed oxides. Mo modifies the polarity of the passive film by formation of molybdates. The presence of negative MoOn− ions in the outer part of the film changes its intrinsically anionic selectivity into a cationic one, inducing the formation of a bipolar layer which, promotes the migration of O− 2 [36–41], as well as prevents Cl− ingress making the passive film more stable against breakdown [42–45]. While, after heat treatment the amorphous coated layer transformed into crystalline. Consequently the corrosion resistance decreased by decreasing in the crystallite size owing to the increasing in the volume of the intercrystalline region of the triple junction [46]. The high intercrystalline surface fraction or defect density provides the increase in the overall corrosion current from as-deposited alloys (15.04–30.79 µA cm− 2) to annealed alloys (37.38–47.76 µA cm− 2). Even though the corrosion current is high, owing to uniformly distributed grains the corrosion is not concentrated at one point, i.e. uniform corrosion is taking place in case of nanocrystalline materials rather than the localized corrosion. 4. Conclusion 1. Ternary CoMoP films of thickness ≈ 200 nm were deposited onto Cu sheets from cobalt-citrate bath of CE ≈ 69% at pH 6.1, 6 A dm− 2 and 25 °C. 2. The film composition, microstructure and surface morphology are 2+ ] ratio in the electrolyte. strongly depending on the [MoO2− 4 ]:[Co Before annealing the low metal ratio bath showed the amorphous structure of Co–13.2 wt.% Mo–10.3 wt.% P alloy which crystallized after 1 h of annealing at 400 °C to give smaller grain size and homogeneous distribution of particles.

3. The film surface of Co–13.2 wt.% Mo–10.3 wt.% P alloy showed the highest stability against oxidation and lowest electrical resistance values (100–150 µΩ). 4. The variety of the film's magnetic nature between soft and hard ferromagnetic films would be expected to enlarge its applications in both R/W and storage devices. 5. Mo wt.% incorporation strongly improved the corrosion resistance of the amorphous structure alloys in 3.5% NaCl solution. Furthermore, the corrosion rate slightly increased; after annealing; due to the transformation of the corrosion state from localized corrosion to uniform corrosion by the action of the grain refinement. 6. It is necessary to adjust the amount of alloying element (Mo) in the deposited alloy in order to balance between existence of good antidiffusion layer and good anti-corrosion layer which is accomplished in the Co–13.2 wt.% Mo–10.3 wt.% P alloy. References [1] M. Paunovic, M. Schlesinger, Fundamentals of Electrochemical Deposition, John Wiley & Sons, Inc., New York, 1998, p. 157. [2] M.A. Nicolet, Thin Solid Films 52 (1978) 415. [3] A.F. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 5th ed., John Wiley & Sons, Inc., New York, 1988, p. 686. [4] E. Gómez, E. Pellicer, E. Vallés, J. Appl. Electrochem. 33 (2003) 245. [5] W.P. Taylor, M. Schneider, H. Baltes, M.G. Allen, Proceedings of the International Conference on Solid-State Sensors and Actuators, Transducers '97, Chicago, 1997. [6] P.R. Subramanian, D.E. Laughlin, Bull. Alloy Phase Diagr. 11 (1990) 169. [7] Y. He, J.Y. Fen, J. Cryst. Growth 263 (2004) 203. [8] Y.L. Kuo, H.H. Lee, C. Lee, J.C. Lin, S.L. Shue, M.S. Liang, B.J. Dianiels, Electrochem. Solid-State Lett. 7 (2004) C35. [9] Y.L. Kuo, C. Lee, J.C. Lin, Y.W. Yen, W.H. Lee, Thin Solid Films 484 (2005) 265. [10] L. Vanasupa, Y.C. Joo, P.R. Besser, S. Pramanick, J. Appl. Phys. 85 (1999) 2583. [11] G. Oksam, P.M. Vereecken, P.C. Searson, J. Electrochem. Soc. 146 (1999) 1436. [12] V.M. Dubin, Y. Shacham-Diamand, B. Zhao, P.K. Vasudev, C.H. Ting, J. Electrochem. Soc. 144 (1997) 898. [13] K. Yoshino, M. Paunovic, I. Ohno, Y. Migoshi (Eds.), PV 93-26, The Electrochemical Society Proceedings Series, Pennington, NJ, 1993, p. 370. [14] A. Abdel Aal, H. Barakat, Z. Abdel Hamid, Surf. Coat. Technol. 202 (2008) 4591. [15] A. Abdel Aal, A. Shaaban, Z. Abdel Hamid, Appl. Surf. Sci. 254 (2008) 1966. [16] Yicai Sun, Semicond. Sci. Technol. 11 (1996) 805. [17] S.M.S.I. Dulal, E.A. Charles, J. Alloys Compd. 455 (2008) 274. [18] N. Petrov, Y. Sverdlov, Y. Shacham-Diamand, J. Electrochem. Soc. 149 (2002) 187. [19] S.M.S.I. Dulal, E.A. Charles, S. Roy, J. Appl. Electrochem. 34 (2004) 151. [20] Y. Wu, C.C. Wan, Y.Y. Wang, J. Electron. Mater. 34 (2005) 541. [21] I. Koiwa, M. Usudo, K. Yamada, T. Osaka, J. Electrochem. Soc. 135 (1988) 718. [22] N. Ivanova, S. Ivanov, E. Boldyrev, O. Stadnik, ISSN 0033 1732, Prot. Met. 42 (2006) 354. [23] R. Saifullin, R. Fomina, G. Mingazova, R. Khaidarov, Prot. Met. 38 (2002) 471. [24] M. Hansen, Constitution of Binary Alloys, Mc Graw Hill, New York, 1958. [25] Y. Sverdlov, V. Bogush, Y. Shacham-Diamand, Microelectron. Eng. 83 (2006) 2243. [26] Y. Shacham-Diamand, Y. Sverdlov, V. Bogush, R. Ofek-Almog, J. Solid State Electrochem. 11 (2007) 929. [27] K.I. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1969. [28] R. Tarozaite, G. Stalnionis, A. Sudavicius, M. Kurtinaitiene, Surf. Coat. Technol. 138 (2001) 61. [29] T.S.N. Sankara Narayanan, S. Selvakumar, A. Stephen, Surf. Coat. Technol. 172 (2003) 298. [30] H. Matsubara, A. Yamada, J. Electrochem. Soc. 141 (1994) 2386. [31] A. García-Arribas, J.M. Barandiarán, y J. Herreros, J. Magn. Magn. Mater. 131 (1994) 129. [32] J.Q. Xiao, C.L. Chien, A. Gavrin, J. Appl. Phys. 79 (1996) 5309. [33] E. Gómez, E. Pellicer, X. Alcobe, E. Vallés, J. Solid State Electrochem. 8 (2004) 497. [34] M. Stern, A.L. Geary, J. Electrochem. Soc. 104 (1957) 56. [35] F. Mansfeld, J. Electrochem. Soc. 120 (1973) 515. [36] C.R. Clayton, Y.C. Lu, Corros. Sci. 29 (1989) 881. [37] M.A. Ameer, A.M. Fekry, F. El-Taib Heakal, Electrochim. Acta 50 (2004) 43. [38] N. Boucherit, A. Hugot-le Goff, S. Joiret, Corrosion 48 (1992) 569. [39] W.J. Tobler, S. Virtanen, Corros. Sci. 48 (2006) 1585. [40] J.M. Bastidas, C.L. Torres, E. Cano, J.L. Polo, Corros. Sci. 44 (2002) 625. [41] M. Sakashita, N. Sato, in: Kruger Frenkental (Ed.), Passivity of Metals, The Electrochem. Soc., Pennington, 1978, p. 479. [42] C.R. Clayton, Y.C. Lu, J. Electrochem. Soc. 133 (1986) 2465. [43] W.D. Robertson, J. Electrochem. Soc. 98 (1951) 94. [44] Y.C. Lu, C.R. Clayton, R. Brooks, Corros. Sci. 29 (1989) 863. [45] K. Ogura, T. Ohama, Corrosion 40 (1984) 47. [46] K.R. Sriraman, S. Ganesh Sundara Raman, S.K. Seshadri, Mater. Sci. Eng. A 460–461 (2007) 39.