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On work function and characteristics of anomalous electrodeposited nickel–cobalt films
Thin Solid Films 519 (2011) 2075–2078
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Thin Solid Films 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 / t s f
On work function and characteristics of anomalous electrodeposited nickel–cobalt films C.K. Chung ⁎, W.T. Chang, M.W. Liao Department of Mechanical Engineering, and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 17 December 2009 Received in revised form 8 October 2010 Accepted 20 October 2010 Available online 4 November 2010 Keywords: Ni–Co Electrodeposition Work function Kelvin probe Ultraviolet photoelectron spectroscopy
a b s t r a c t The anomalous cobalt content in the electrodeposited nickel–cobalt (Ni–Co) alloy films significantly influenced by the current density was related to the variation of morphology and electron work function (EWF) of the films. The characteristics and EWF of Ni–Co films were investigated by scanning electron microscope with an attached energy dispersive X-ray spectrometer, X-ray diffraction, ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe technique, respectively. As the current density increased from 1 to 20 ampere per square decimeter (A/dm2), the Co atomic concentration (at.%) in Ni–Co greatly decreased from 22.5 at.% to 13.2 at.% correspondingly. The surface morphology of film obtained at low current density became smoother than that at high current density. Both UPS and Kelvin probe results showed the same trend of EWF variation which increased with increasing current density from 1 to 10 A/dm2 and kept nearly unchanged at 10–20 A/dm2. The smooth Ni–Co film with low EWF could be achieved at low current density. In comparison, Kelvin probe operated at atmosphere ambient could be a good candidate for EWF measurement because of the lower cost and easier operation than UPS at ultra high vacuum. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The nickel–cobalt (Ni–Co) alloy films have been widely used for the traditional protective and decorative coatings as well as for the modern permanent magnetic memories. The electrodeposition of Ni–Co films has been recognized as an anomalous codeposition: the composition of alloy films is different from that in the electrolyte composition [1,2]. The anomalous composition related to the current density, temperature, pH value, applied potential and other variables in electrolyte would affect the property of alloy. Iron group alloys, i.e., Fe, Co and Ni usually exhibit the anomalous phenomena during codeposition. Recently, an increasing interest in the electrochemical deposition of alloys has emerged, mostly in microfabrication for the microelectronics industry [3]. Therefore, the electronic properties of Ni–Co alloy films e.g. electron work function (EWF) will be an important issue for investigation. The EWF is defined as the minimum energy needed to remove an electron from a solid to a point outside the solid surface or energy needed to move an electron from the Fermi energy level into vacuum. The EWF is uniform across the interior of the homogeneous solid and may alter at the surface due to the induced states of the surface structure by stress [4,5] or contamination [6]. Moreover, it can also be used to judge the electron emissive performance [7]. EWF is an important quantity for the physical and chemical property of a solid surface. Two methods are commonly used for determining the EWF: one is Kelvin probe [8–11] and the other is ultraviolet photoemission spectroscopy (UPS) [11,12].
The former measures the EWF difference between two dissimilar solids which is so-called contact potential difference (CPD) while the latter counts on the yield spectrum near threshold of photoelectron emission for the work function of a solid. Recently, Kelvin probe has been widely applied to the process monitoring [14], solar cell [15], corrosion science [16–18], semiconductor device [19–21], protein/DNA analysis [22] and even human hair study [23]. The home-made Kelvin probe was ever applied to the evaluation of Ni–Co alloys [10] but the resolution was poor due to the raw probe fabrication and the difficulty of parallel control between the sensing probe and the specimen. The better resolution of Kelvin probe is worthy of studying for more accurate measurement in materials application. In addition, Chung and Chang reported that the anomalous cobalt content in the Ni–Co films was greatly influenced by the current density [24]. It is of thus interest to further examine the effect of cobalt content in Ni–Co on EWF of the films for possible application in future. In this paper, the anomalous codeposited Ni–Co alloy films under varied current densities were prepared and investigated on the surface morphology and EWF using the scanning electron microscopy, selfmade Kelvin probe and UPS. The parallel control and resolution of Kelvin probe were much improved by inserting a universal connector to the sensing probe and judging the zero-Kelvin signal. The anomalous codeposition of Ni–Co films was discussed on electrochemical principles and correlated to the characteristics and EWF property of films. 2. Experimental procedures
⁎ Corresponding author. Tel.: +886 6 2757575x62111; fax: +886 6 2352973. E-mail address: [email protected] (C.K. Chung). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.10.044
The p-type Si(100) wafers were diced into 5 mm × 5 mm chips as the substrates of electrodeposition. The Cr/Au seed layers of about 40/
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C.K. Chung et al. / Thin Solid Films 519 (2011) 2075–2078
80 nm thick were deposited on the Si substrates by an electron beam evaporator (ULVAC, VT1-10CE, Japan) and then cleaned by conventional steps of acetone, isopropanol and sulfuric acid solutions prior to electrodeposition in the electrolyte. The composition of the electrolyte bath solution consisted of nickel sulphamate of 500 ± 20 g/l, cobalt sulfamate of 20 ± 1 g/l, nickel chloride of 4 ± 1 g/l, wetting agent (dihexyl sulfosuccinate) of 0.50 ± 0.05 ml/l and boric acid of 42 ± 2 g/l. The temperature was kept at 50 ± 1 °C. The initial pH of the electrolyte was controlled at around 4.2. The potentiostat (Jiehan 5000, Taiwan) was used for electrochemical codeposition. The directcurrent (DC) current density was varied from 1 to 20 ampere per square decimeter (A/dm2). The microstructure, phases, and crystallinity of Ni–Co films were identified by grazing incidence x-ray diffractometry (GIXRD, PANalytical X'Pert PRO, Philips, Netherlands) using Cu Kα radiation (0.15418 nm). The surface morphology and composition were determined by Field-Emission Scanning Electron Microscope (FE-SEM) with an attached Energy Dispersive X-ray Spectrometer (JEOL JSM-7000, Japan). The thickness of the coatings was examined by the SEM cross section of films and the average thicknesses of four films were around 800–900 nm. The EWF was determined by two different methods: one is UPS and the other is Kelvin probe. The light source of UPS (Sigma probe, Thermo VG-Scientific, England) is HeI whose photon energy equals to 21.22 eV. UPS is based on the photoelectron effect. The monochromatic light of energy hν is absorbed by a solid to cause ejection of electron. The kinetic energies of electrons are evaluated via an electron energy analyzer and calculated for work functions of films. Fig. 1 shows the schematic self-made Kelvin probe including the vibrating capacitor, pre-amplifier and data acquisition system. The vibrating Kelvin capacitor was driven by function generator. The parallel control and resolution of Kelvin probe were much improved by inserting a universal connector to the sensing probe and judging the zero-Kelvin signal. Therefore, the S/N ratio is significantly enhanced for better judgment of Kelvin signal due to the improved parallelism between the sensing probe and the specimen. The DC power supply was used to drive the pre-amplifier and to compensate the CPD induced capacitance. Then, the signal processing system acquired the data and processed through the commercial software LabVIEW for the Kelvin signal (V), CPD and EWF values of films.
reasons are related to the anomalous behavior. One is high exchange current in Co than that in Ni [24–26] to result in high Co content in the Ni–Co alloy. The other possible reason is the generation of hydrogen [25,26]. Increasing current density would rapidly consume metal ions to reduce the concentration gradient of metal ions with the deposition time and result in the decrease of anomalous behavior with the reduced diffusion. The crystallinity and phases of Ni-Co films codeposited at current density of 1–20 A/dm2 had been examined by GIXRD patterns [24]. All films exhibit the polycrystalline characteristics with multiple diffraction peaks of Ni phase but no Co peak. It indicated that the codeposited Ni–Co film is a solid solution with Co solutes dissolved in a Ni-matrix solvent. The anomalous Co content at different current density may affect the morphology of film. Fig. 2(a) and (b) shows the top-view and cross-sectional SEM micrographs of the Ni–Co films codeposited at current densities of 1, 5, 10 and 20 A/dm2, respectively. The average thicknesses of four films are around 800– 900 nm. It reveals that the thicknesses of all the specimens are wellcontrolled although the current densities are in a wide range. The close thickness of deposits is helpful for the discussion of morphology evolution in terms of current density. The smoothest morphology is achieved at the lowest current density of 1 A/dm2 and becomes rougher with increasing current density. It is attributed to the common
3. Results and discussion The effect of current density on the anomalous Co content in the codeposited Ni–Co films has been done in the previous work [24]. The Co content distinctly decreases with increasing current density from 22.5% (1 A/dm2), to 20.8% (5 A/dm2), to 15.3% (10 A/dm2) to 13.2% (20 A/dm2). Compared to Co content of about 4% in electrolyte, the anomalous Co atomic composition (at.%) in the Ni–Co films is 13.2 22.5% due to the anomalous codeposition. The mechanism of anomalous codeposition is not completely understood. Two possible
Fig. 1. Schematic diagram of the self-made Kelvin probe including the vibrating capacitor, pre-amplifier and data acquisition system.
Fig. 2. (a) Top-view and (b) cross-sectional SEM micrographs of the Ni–Co films codeposited at current densities of 1, 5, 10 and 20 A/dm2.
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phenomenon of low deposition rate at low current density with dense and small grain for low roughness. Fig. 3 shows the energy diagram for UPS measurement. The kinetic energy (KE) of photoelectrons is activated and emitted by means of ultraviolet photons overcoming the binding energy (BE) of electrons of the film. For a given photon energy (hν = 21.22 eV, HeI) the EWF (Φ) of the film surface can be written as [13]: Φ = hν−BE–KE
ð1Þ
The summation of BE and KE is equal to the energy width △E of UPS spectra; Φ = hν−△E
ð2Þ
Fig. 4 shows the UPS spectra of Ni–Co alloys codeposited at current densities of 1, 5, 10 and 20 (A/dm2). The inset is the magnification spectra around the Fermi level (EF). The energy width △E of the Ni–Co films at 1, 5, 10 and 20 A/dm2 are 17.33, 17.27, 17.15 and 17.15 eV, respectively. Therefore, the EWF of Ni–Co films are 3.89 eV (1 A/dm2), 3.95 eV (5 A/dm2) and 4.07 eV (both 10 and 20 A/dm2), respectively. The EWF of Ni–Co films increases with increasing current density because the EWF of Ni is higher than Co [27]. The Ni–Co films with low Co content at high current density exhibits high EWF. The nearly same EWF values (4.07 eV) are obtained at 10 A/dm2 and 20 A/dm2 due to the difference of low Co content in both films is beyond the resolution of sensitivity in UPS. In comparison with UPS, the Kelvin probe is used to measure the difference of EWF between the probe and the Ni–Co film which is socalled contact potential difference (CPD). The probe is the electroformed Ni whose EWF is 4.20 eV by means of UPS. Therefore, the EWF of Ni–Co film can be obtained by the following equation: Φ = j4:20 ðeVÞ−CPDj
ð3Þ
Fig. 5 shows the Kelvin signal, i.e., the current-converted voltages from actuating Kelvin capacitor as a function of external voltage
Fig. 4. The UPS spectra of the Ni–Co films: the width ΔE of films at 1, 5, 10 and 20 A/dm2 are 17.33, 17.27, 17.15 and 17.15 eV, respectively.
(±Vb). When Kelvin probe is sinusoidally vibrating, the varied capacitance results in the altered charge flow for producing the current signal. This current signal is converted to voltage signal via the pre-amplifier and monitored by oscilloscope. The magnitude of the current signal (I) can be derived from the varied capacitive charges. It is a function of the external voltage (Vb), voltage (V) across the capacitor and time-dependent capacitance (C(t)) as expressed in Eq. (4). I = ðV + Vb Þ
dC ðt Þ dt
ð4Þ
If the current from the capacitor is zero, the external voltages Vb will be equal to CPD signal V in negative sign as Eq. (5). I = 0; VCPD = −Vb
ð5Þ
The time-dependent capacitance C(t) is related to the surface area A, time-dependent displacement d(t) and dielectric constant (ε) as Eq. (6); C ðt Þ =
εA dðt Þ
ð6Þ
where the time-dependent spacing d(t) can be expressed in Eq. (7) dðt Þ = do + d1 cos ωt
Fig. 3. The energy band diagram for UPS measurement: the kinetic energy (KE) of ultraviolet activated photoelectrons is obtained by overcoming the binding energy (BE) and work function Φ of electrons in materials.
ð7Þ
Fig. 5. The results of Kelvin probe measurement of the Ni–Co films: the CPDs of films at 1, 5, 10 and 20 A/dm2 are −0.110, −0.052, −0.001 and −0.001 eV, respectively.
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by substituting Eqs. (7) into (6) yields (8); C ðt Þ =
εA εA = dðt Þ do + d1 cos ωt
ð8Þ
probe at an atmosphere ambient could be a good candidate for EWF measurement because of the lower cost and easier operation than UPS at ultra high vacuum. 4. Conclusions
by substituting Eqs. (8) into (4) yields (9); iðt Þ =
dQ ðt Þ εωd1 A sin ωt = ðVCPD + Vb Þ dt ðdo + d1 cos ωt Þ2
ð9Þ
Kelvin signal can be governed by Eq. (9) and the magnitude of the current signal depends on the gap distance (d0, d1), surface area of the metal (A) and the applied voltage (Vb). The current signal is converted into the voltage signal, i.e., Kelvin signal, through the pre-amplifier. And no charges can be ideally detected while −Vb = VCPD. Both the probe and the specimen form as a parallel capacitor during Kelvin probe measurement. The bias voltage is applied on the probe and varied from −0.5 V to 0.5 V in order to compensate the CPD induced capacitance (Kelvin signal). The capacitance will become zero at a specific bias voltage which is equal to the CPD but in opposite polarization (Vbias = −VCPD). Then the CPD can be obtained by judging the zero-capacitance (zero-Kelvin signal) point. That is, the CPDs of Ni–Co films at 1, 5, 10 and 20 A/dm2 are −0.110, −0.052, −0.001 and −0.001 eV, respectively. The EWF can be estimated by substituting CPDs into Eq. (4), and those at 1, 5, 10 and 20 A/dm2 are 4.090, 4.148, 4.199 and 4.199 eV, respectively. The trend of Kelvin probe measured EWF variation is similar to that by UPS. It increases with increasing current density and keeps the same EWF at 10 and 20 A/dm2. It implies that the sufficiently low Co content could lead to nearly no fluctuation in EWF. Fig. 6 shows the comparison of measured EWFs between both Kelvin probe and UPS methods. The trend of both measured values variation is the same but there is a relative shift between them. The shift is attributed to the difference in the basic theory and the measurement environment. The UPS measurement must be operated in ultra high vacuum environment while Kelvin probe measurement is only in air. In reference [12], the relationship of EWF between Kelvin probe and UPS under various surface treatments on indium tin oxide (ITO) is very difficult to identify because EWF is extremely sensitive to surface conditions such as contaminations or adsorptions. Moreover, the evolution of EWF form UPS and Kelvin probe is not identical and the variation between UPS and Kelvin probe can be from 0.1 to 1 eV because EWF is sensitive to surface condition of materials. Therefore, the data in the published papers often showed a wide range of EWF for the same material [27–29]. Fortunately, the values of EWFs are more strongly dependent on the element contents. The similar relationship between the EWF and the Co content of films form Kelvin probe and UPS has been obtained in our study. Kelvin
Fig. 6. The comparison of EWF values measured by means of both Kelvin probe and UPS methods.
The anomalous composition, surface morphology and EWF of the codeposited Ni–Co alloy films have been investigated at varied current densities of 1–20 A/dm2. Increasing current density leads to the reduced Co atomic concentration from 22.5 at.% (1 A/dm2) to 13.2 at.% (20 A/dm2). The surface morphology of Ni–Co films is smoother at low current density due to growth of dense and small grains at low deposition rate. The EWF is sensitive to surface condition of materials and often reported a wide range values for the same material in the publications. Moreover, it is significantly influenced by the element content in the film. The relationship between the EWF and the Co content of films form Kelvin probe and UPS has been successfully examined in this study. The UPS measured EWFs of Ni–Co films increases with increasing current density and ranges from 3.89 to 4.09 eV. The Kelvin probe measured EWFs have the same trend of variation but exhibit a relative shift from 4.090 to 4.199 eV due to the difference in the basic theory and the measurement environment. Kelvin probe at an atmosphere ambient could be a good candidate for EWF measurement doe to the low cost and easy operation compared to UPS at ultra high vacuum. Acknowledgements This work is partially sponsored by National Science Council under grant no. 99-2221-E-006-032-MY3. We pay our great thanks to Center for Micro/Nano Science and Technology (CMNST) in National Cheng Kung University and National Nano Device Laboratories (NDL), Tainan, Taiwan for the access of analysis, processing equipments and technical support. References [1] G. Qiao, T. Jing, N. Wang, Y. Gao, X. Zhao, J. Zhou, W. Wang, J. Electrochem. Soc. 153 (2006) C305. [2] Y. Zhuang, E.J. Podlaha, J. Electrochem. Soc. 150 (2003) C219. [3] R. Orinakova, A. Turonova, D. Kladekova, M. Galova, R.M. Smith, J. Appl. Electrochem. 36 (2006) 957. [4] D.Y. Li, L. Wang, W. Li, Mater. Sci. Eng. A 384 (2004) 335. [5] F. Rossi, G.I. Opat, A. Cimmino, Rev. Sci. Instrum. 63 (1992) 3736. [6] G. Kiss, E.B. Varhegyi, J. Mizasei, O.H. Krafcsik, K. Kovacs, G. Negyesi, B. Ostrick, H. Meixner, F. Reti, Sens. Actuators B 68 (2000) 240. [7] S. Yamamoto, Appl. Surf. Sci. 251 (2005) 4. [8] L. Kelvin, Phiols. Mag. J. Sci. 46 (1989) 82. [9] W.A. Zisman, Rev. Sci. Instrum. 3 (1932) 367. [10] C.K. Chung, W.T. Chang, R.X. Zhou, Microsyst. Technol. 14 (2008) 1389. [11] I.D. Baikie, P.J. Estrup, Rev. Sci. Instrum. 69 (1998) 3902. [12] J.S. Kim, B. Lagel, E. Moons, N. Johansson, I.D. Baikie, W.R. Salaneck, R.H. Friend, F. Cacialli, Synth. Met. 111–112 (2000) 311. [13] C. Bromberger, H.J. Jansch, D. Fick, Surf. Sci. 506 (2002) 129. [14] Y. Yang, T. Kurfess, S. Liang, S. Danyluk, Wear 225 (1999) 1215. [15] T. Glatzel, H. Steigert, S. Sadewasser, R. Klenk, M.C. Lux-Steiner, Thin Solid Films 480 (2005) 177. [16] M. Stratmann, H. Streckel, Corr. Sci. 30 (1990) 681. [17] M. Stratmann, H. Streckel, Corr. Sci. 30 (1990) 697. [18] M. Stratmann, H. Streckel, K.T. Kim, S. Crockett, Corr. Sci. 30 (1990) 715. [19] M. Tanimoto, O. Vatel, J. Vac. Sci. Technol. B 14 (1996) 1547. [20] S. Kamiya, M. Iwami, T. Tsuchiya, M. Kurouchi, J. Kikawa, T. Yamada, Appl. Phys. Lett. 90 (2007) 213511. [21] A. Kikukawa, S. Hosaka, R. Imura, Appl. Phys. Lett. 66 (1995) 3510. [22] A.K. Sinensky, A.M. Belcher, Nat. Nanotechnol. 2 (2007) 653. [23] R.A. Lodge, B. Bhushan, J. Vac. Sci. Technol. 25 (2007) 893. [24] C.K. Chung, W.T. Chang, Thin Solid Films 517 (2009) 4800. [25] A. Bai, C.C. Hu, T.C. Wen, Electrochem. Acta 48 (2003) 2425. [26] A. Bai, C.C. Hu, Electrochem. Acta 47 (2002) 3447. [27] MichaelsonH.B. , 73rd ed., CRC Handbook of Chemistry and Physisc, 12, 1992, p. 108. [28] T. Durakiewicz, S. Halas, A. Arko, J.J. Joyce, D.P. Moore, Phys. Rev. B 64 (2001) 045101. [29] S. Halas, T. Durakiewicz, J. Phys. Condens. Matter 10 (1998) 10815.
Contents lists available at ScienceDirect
Thin Solid Films 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 / t s f
On work function and characteristics of anomalous electrodeposited nickel–cobalt films C.K. Chung ⁎, W.T. Chang, M.W. Liao Department of Mechanical Engineering, and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 17 December 2009 Received in revised form 8 October 2010 Accepted 20 October 2010 Available online 4 November 2010 Keywords: Ni–Co Electrodeposition Work function Kelvin probe Ultraviolet photoelectron spectroscopy
a b s t r a c t The anomalous cobalt content in the electrodeposited nickel–cobalt (Ni–Co) alloy films significantly influenced by the current density was related to the variation of morphology and electron work function (EWF) of the films. The characteristics and EWF of Ni–Co films were investigated by scanning electron microscope with an attached energy dispersive X-ray spectrometer, X-ray diffraction, ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe technique, respectively. As the current density increased from 1 to 20 ampere per square decimeter (A/dm2), the Co atomic concentration (at.%) in Ni–Co greatly decreased from 22.5 at.% to 13.2 at.% correspondingly. The surface morphology of film obtained at low current density became smoother than that at high current density. Both UPS and Kelvin probe results showed the same trend of EWF variation which increased with increasing current density from 1 to 10 A/dm2 and kept nearly unchanged at 10–20 A/dm2. The smooth Ni–Co film with low EWF could be achieved at low current density. In comparison, Kelvin probe operated at atmosphere ambient could be a good candidate for EWF measurement because of the lower cost and easier operation than UPS at ultra high vacuum. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The nickel–cobalt (Ni–Co) alloy films have been widely used for the traditional protective and decorative coatings as well as for the modern permanent magnetic memories. The electrodeposition of Ni–Co films has been recognized as an anomalous codeposition: the composition of alloy films is different from that in the electrolyte composition [1,2]. The anomalous composition related to the current density, temperature, pH value, applied potential and other variables in electrolyte would affect the property of alloy. Iron group alloys, i.e., Fe, Co and Ni usually exhibit the anomalous phenomena during codeposition. Recently, an increasing interest in the electrochemical deposition of alloys has emerged, mostly in microfabrication for the microelectronics industry [3]. Therefore, the electronic properties of Ni–Co alloy films e.g. electron work function (EWF) will be an important issue for investigation. The EWF is defined as the minimum energy needed to remove an electron from a solid to a point outside the solid surface or energy needed to move an electron from the Fermi energy level into vacuum. The EWF is uniform across the interior of the homogeneous solid and may alter at the surface due to the induced states of the surface structure by stress [4,5] or contamination [6]. Moreover, it can also be used to judge the electron emissive performance [7]. EWF is an important quantity for the physical and chemical property of a solid surface. Two methods are commonly used for determining the EWF: one is Kelvin probe [8–11] and the other is ultraviolet photoemission spectroscopy (UPS) [11,12].
The former measures the EWF difference between two dissimilar solids which is so-called contact potential difference (CPD) while the latter counts on the yield spectrum near threshold of photoelectron emission for the work function of a solid. Recently, Kelvin probe has been widely applied to the process monitoring [14], solar cell [15], corrosion science [16–18], semiconductor device [19–21], protein/DNA analysis [22] and even human hair study [23]. The home-made Kelvin probe was ever applied to the evaluation of Ni–Co alloys [10] but the resolution was poor due to the raw probe fabrication and the difficulty of parallel control between the sensing probe and the specimen. The better resolution of Kelvin probe is worthy of studying for more accurate measurement in materials application. In addition, Chung and Chang reported that the anomalous cobalt content in the Ni–Co films was greatly influenced by the current density [24]. It is of thus interest to further examine the effect of cobalt content in Ni–Co on EWF of the films for possible application in future. In this paper, the anomalous codeposited Ni–Co alloy films under varied current densities were prepared and investigated on the surface morphology and EWF using the scanning electron microscopy, selfmade Kelvin probe and UPS. The parallel control and resolution of Kelvin probe were much improved by inserting a universal connector to the sensing probe and judging the zero-Kelvin signal. The anomalous codeposition of Ni–Co films was discussed on electrochemical principles and correlated to the characteristics and EWF property of films. 2. Experimental procedures
⁎ Corresponding author. Tel.: +886 6 2757575x62111; fax: +886 6 2352973. E-mail address: [email protected] (C.K. Chung). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.10.044
The p-type Si(100) wafers were diced into 5 mm × 5 mm chips as the substrates of electrodeposition. The Cr/Au seed layers of about 40/
2076
C.K. Chung et al. / Thin Solid Films 519 (2011) 2075–2078
80 nm thick were deposited on the Si substrates by an electron beam evaporator (ULVAC, VT1-10CE, Japan) and then cleaned by conventional steps of acetone, isopropanol and sulfuric acid solutions prior to electrodeposition in the electrolyte. The composition of the electrolyte bath solution consisted of nickel sulphamate of 500 ± 20 g/l, cobalt sulfamate of 20 ± 1 g/l, nickel chloride of 4 ± 1 g/l, wetting agent (dihexyl sulfosuccinate) of 0.50 ± 0.05 ml/l and boric acid of 42 ± 2 g/l. The temperature was kept at 50 ± 1 °C. The initial pH of the electrolyte was controlled at around 4.2. The potentiostat (Jiehan 5000, Taiwan) was used for electrochemical codeposition. The directcurrent (DC) current density was varied from 1 to 20 ampere per square decimeter (A/dm2). The microstructure, phases, and crystallinity of Ni–Co films were identified by grazing incidence x-ray diffractometry (GIXRD, PANalytical X'Pert PRO, Philips, Netherlands) using Cu Kα radiation (0.15418 nm). The surface morphology and composition were determined by Field-Emission Scanning Electron Microscope (FE-SEM) with an attached Energy Dispersive X-ray Spectrometer (JEOL JSM-7000, Japan). The thickness of the coatings was examined by the SEM cross section of films and the average thicknesses of four films were around 800–900 nm. The EWF was determined by two different methods: one is UPS and the other is Kelvin probe. The light source of UPS (Sigma probe, Thermo VG-Scientific, England) is HeI whose photon energy equals to 21.22 eV. UPS is based on the photoelectron effect. The monochromatic light of energy hν is absorbed by a solid to cause ejection of electron. The kinetic energies of electrons are evaluated via an electron energy analyzer and calculated for work functions of films. Fig. 1 shows the schematic self-made Kelvin probe including the vibrating capacitor, pre-amplifier and data acquisition system. The vibrating Kelvin capacitor was driven by function generator. The parallel control and resolution of Kelvin probe were much improved by inserting a universal connector to the sensing probe and judging the zero-Kelvin signal. Therefore, the S/N ratio is significantly enhanced for better judgment of Kelvin signal due to the improved parallelism between the sensing probe and the specimen. The DC power supply was used to drive the pre-amplifier and to compensate the CPD induced capacitance. Then, the signal processing system acquired the data and processed through the commercial software LabVIEW for the Kelvin signal (V), CPD and EWF values of films.
reasons are related to the anomalous behavior. One is high exchange current in Co than that in Ni [24–26] to result in high Co content in the Ni–Co alloy. The other possible reason is the generation of hydrogen [25,26]. Increasing current density would rapidly consume metal ions to reduce the concentration gradient of metal ions with the deposition time and result in the decrease of anomalous behavior with the reduced diffusion. The crystallinity and phases of Ni-Co films codeposited at current density of 1–20 A/dm2 had been examined by GIXRD patterns [24]. All films exhibit the polycrystalline characteristics with multiple diffraction peaks of Ni phase but no Co peak. It indicated that the codeposited Ni–Co film is a solid solution with Co solutes dissolved in a Ni-matrix solvent. The anomalous Co content at different current density may affect the morphology of film. Fig. 2(a) and (b) shows the top-view and cross-sectional SEM micrographs of the Ni–Co films codeposited at current densities of 1, 5, 10 and 20 A/dm2, respectively. The average thicknesses of four films are around 800– 900 nm. It reveals that the thicknesses of all the specimens are wellcontrolled although the current densities are in a wide range. The close thickness of deposits is helpful for the discussion of morphology evolution in terms of current density. The smoothest morphology is achieved at the lowest current density of 1 A/dm2 and becomes rougher with increasing current density. It is attributed to the common
3. Results and discussion The effect of current density on the anomalous Co content in the codeposited Ni–Co films has been done in the previous work [24]. The Co content distinctly decreases with increasing current density from 22.5% (1 A/dm2), to 20.8% (5 A/dm2), to 15.3% (10 A/dm2) to 13.2% (20 A/dm2). Compared to Co content of about 4% in electrolyte, the anomalous Co atomic composition (at.%) in the Ni–Co films is 13.2 22.5% due to the anomalous codeposition. The mechanism of anomalous codeposition is not completely understood. Two possible
Fig. 1. Schematic diagram of the self-made Kelvin probe including the vibrating capacitor, pre-amplifier and data acquisition system.
Fig. 2. (a) Top-view and (b) cross-sectional SEM micrographs of the Ni–Co films codeposited at current densities of 1, 5, 10 and 20 A/dm2.
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phenomenon of low deposition rate at low current density with dense and small grain for low roughness. Fig. 3 shows the energy diagram for UPS measurement. The kinetic energy (KE) of photoelectrons is activated and emitted by means of ultraviolet photons overcoming the binding energy (BE) of electrons of the film. For a given photon energy (hν = 21.22 eV, HeI) the EWF (Φ) of the film surface can be written as [13]: Φ = hν−BE–KE
ð1Þ
The summation of BE and KE is equal to the energy width △E of UPS spectra; Φ = hν−△E
ð2Þ
Fig. 4 shows the UPS spectra of Ni–Co alloys codeposited at current densities of 1, 5, 10 and 20 (A/dm2). The inset is the magnification spectra around the Fermi level (EF). The energy width △E of the Ni–Co films at 1, 5, 10 and 20 A/dm2 are 17.33, 17.27, 17.15 and 17.15 eV, respectively. Therefore, the EWF of Ni–Co films are 3.89 eV (1 A/dm2), 3.95 eV (5 A/dm2) and 4.07 eV (both 10 and 20 A/dm2), respectively. The EWF of Ni–Co films increases with increasing current density because the EWF of Ni is higher than Co [27]. The Ni–Co films with low Co content at high current density exhibits high EWF. The nearly same EWF values (4.07 eV) are obtained at 10 A/dm2 and 20 A/dm2 due to the difference of low Co content in both films is beyond the resolution of sensitivity in UPS. In comparison with UPS, the Kelvin probe is used to measure the difference of EWF between the probe and the Ni–Co film which is socalled contact potential difference (CPD). The probe is the electroformed Ni whose EWF is 4.20 eV by means of UPS. Therefore, the EWF of Ni–Co film can be obtained by the following equation: Φ = j4:20 ðeVÞ−CPDj
ð3Þ
Fig. 5 shows the Kelvin signal, i.e., the current-converted voltages from actuating Kelvin capacitor as a function of external voltage
Fig. 4. The UPS spectra of the Ni–Co films: the width ΔE of films at 1, 5, 10 and 20 A/dm2 are 17.33, 17.27, 17.15 and 17.15 eV, respectively.
(±Vb). When Kelvin probe is sinusoidally vibrating, the varied capacitance results in the altered charge flow for producing the current signal. This current signal is converted to voltage signal via the pre-amplifier and monitored by oscilloscope. The magnitude of the current signal (I) can be derived from the varied capacitive charges. It is a function of the external voltage (Vb), voltage (V) across the capacitor and time-dependent capacitance (C(t)) as expressed in Eq. (4). I = ðV + Vb Þ
dC ðt Þ dt
ð4Þ
If the current from the capacitor is zero, the external voltages Vb will be equal to CPD signal V in negative sign as Eq. (5). I = 0; VCPD = −Vb
ð5Þ
The time-dependent capacitance C(t) is related to the surface area A, time-dependent displacement d(t) and dielectric constant (ε) as Eq. (6); C ðt Þ =
εA dðt Þ
ð6Þ
where the time-dependent spacing d(t) can be expressed in Eq. (7) dðt Þ = do + d1 cos ωt
Fig. 3. The energy band diagram for UPS measurement: the kinetic energy (KE) of ultraviolet activated photoelectrons is obtained by overcoming the binding energy (BE) and work function Φ of electrons in materials.
ð7Þ
Fig. 5. The results of Kelvin probe measurement of the Ni–Co films: the CPDs of films at 1, 5, 10 and 20 A/dm2 are −0.110, −0.052, −0.001 and −0.001 eV, respectively.
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by substituting Eqs. (7) into (6) yields (8); C ðt Þ =
εA εA = dðt Þ do + d1 cos ωt
ð8Þ
probe at an atmosphere ambient could be a good candidate for EWF measurement because of the lower cost and easier operation than UPS at ultra high vacuum. 4. Conclusions
by substituting Eqs. (8) into (4) yields (9); iðt Þ =
dQ ðt Þ εωd1 A sin ωt = ðVCPD + Vb Þ dt ðdo + d1 cos ωt Þ2
ð9Þ
Kelvin signal can be governed by Eq. (9) and the magnitude of the current signal depends on the gap distance (d0, d1), surface area of the metal (A) and the applied voltage (Vb). The current signal is converted into the voltage signal, i.e., Kelvin signal, through the pre-amplifier. And no charges can be ideally detected while −Vb = VCPD. Both the probe and the specimen form as a parallel capacitor during Kelvin probe measurement. The bias voltage is applied on the probe and varied from −0.5 V to 0.5 V in order to compensate the CPD induced capacitance (Kelvin signal). The capacitance will become zero at a specific bias voltage which is equal to the CPD but in opposite polarization (Vbias = −VCPD). Then the CPD can be obtained by judging the zero-capacitance (zero-Kelvin signal) point. That is, the CPDs of Ni–Co films at 1, 5, 10 and 20 A/dm2 are −0.110, −0.052, −0.001 and −0.001 eV, respectively. The EWF can be estimated by substituting CPDs into Eq. (4), and those at 1, 5, 10 and 20 A/dm2 are 4.090, 4.148, 4.199 and 4.199 eV, respectively. The trend of Kelvin probe measured EWF variation is similar to that by UPS. It increases with increasing current density and keeps the same EWF at 10 and 20 A/dm2. It implies that the sufficiently low Co content could lead to nearly no fluctuation in EWF. Fig. 6 shows the comparison of measured EWFs between both Kelvin probe and UPS methods. The trend of both measured values variation is the same but there is a relative shift between them. The shift is attributed to the difference in the basic theory and the measurement environment. The UPS measurement must be operated in ultra high vacuum environment while Kelvin probe measurement is only in air. In reference [12], the relationship of EWF between Kelvin probe and UPS under various surface treatments on indium tin oxide (ITO) is very difficult to identify because EWF is extremely sensitive to surface conditions such as contaminations or adsorptions. Moreover, the evolution of EWF form UPS and Kelvin probe is not identical and the variation between UPS and Kelvin probe can be from 0.1 to 1 eV because EWF is sensitive to surface condition of materials. Therefore, the data in the published papers often showed a wide range of EWF for the same material [27–29]. Fortunately, the values of EWFs are more strongly dependent on the element contents. The similar relationship between the EWF and the Co content of films form Kelvin probe and UPS has been obtained in our study. Kelvin
Fig. 6. The comparison of EWF values measured by means of both Kelvin probe and UPS methods.
The anomalous composition, surface morphology and EWF of the codeposited Ni–Co alloy films have been investigated at varied current densities of 1–20 A/dm2. Increasing current density leads to the reduced Co atomic concentration from 22.5 at.% (1 A/dm2) to 13.2 at.% (20 A/dm2). The surface morphology of Ni–Co films is smoother at low current density due to growth of dense and small grains at low deposition rate. The EWF is sensitive to surface condition of materials and often reported a wide range values for the same material in the publications. Moreover, it is significantly influenced by the element content in the film. The relationship between the EWF and the Co content of films form Kelvin probe and UPS has been successfully examined in this study. The UPS measured EWFs of Ni–Co films increases with increasing current density and ranges from 3.89 to 4.09 eV. The Kelvin probe measured EWFs have the same trend of variation but exhibit a relative shift from 4.090 to 4.199 eV due to the difference in the basic theory and the measurement environment. Kelvin probe at an atmosphere ambient could be a good candidate for EWF measurement doe to the low cost and easy operation compared to UPS at ultra high vacuum. Acknowledgements This work is partially sponsored by National Science Council under grant no. 99-2221-E-006-032-MY3. We pay our great thanks to Center for Micro/Nano Science and Technology (CMNST) in National Cheng Kung University and National Nano Device Laboratories (NDL), Tainan, Taiwan for the access of analysis, processing equipments and technical support. References [1] G. Qiao, T. Jing, N. Wang, Y. Gao, X. Zhao, J. Zhou, W. Wang, J. Electrochem. Soc. 153 (2006) C305. [2] Y. Zhuang, E.J. Podlaha, J. Electrochem. Soc. 150 (2003) C219. [3] R. Orinakova, A. Turonova, D. Kladekova, M. Galova, R.M. Smith, J. Appl. Electrochem. 36 (2006) 957. [4] D.Y. Li, L. Wang, W. Li, Mater. Sci. Eng. A 384 (2004) 335. [5] F. Rossi, G.I. Opat, A. Cimmino, Rev. Sci. Instrum. 63 (1992) 3736. [6] G. Kiss, E.B. Varhegyi, J. Mizasei, O.H. Krafcsik, K. Kovacs, G. Negyesi, B. Ostrick, H. Meixner, F. Reti, Sens. Actuators B 68 (2000) 240. [7] S. Yamamoto, Appl. Surf. Sci. 251 (2005) 4. [8] L. Kelvin, Phiols. Mag. J. Sci. 46 (1989) 82. [9] W.A. Zisman, Rev. Sci. Instrum. 3 (1932) 367. [10] C.K. Chung, W.T. Chang, R.X. Zhou, Microsyst. Technol. 14 (2008) 1389. [11] I.D. Baikie, P.J. Estrup, Rev. Sci. Instrum. 69 (1998) 3902. [12] J.S. Kim, B. Lagel, E. Moons, N. Johansson, I.D. Baikie, W.R. Salaneck, R.H. Friend, F. Cacialli, Synth. Met. 111–112 (2000) 311. [13] C. Bromberger, H.J. Jansch, D. Fick, Surf. Sci. 506 (2002) 129. [14] Y. Yang, T. Kurfess, S. Liang, S. Danyluk, Wear 225 (1999) 1215. [15] T. Glatzel, H. Steigert, S. Sadewasser, R. Klenk, M.C. Lux-Steiner, Thin Solid Films 480 (2005) 177. [16] M. Stratmann, H. Streckel, Corr. Sci. 30 (1990) 681. [17] M. Stratmann, H. Streckel, Corr. Sci. 30 (1990) 697. [18] M. Stratmann, H. Streckel, K.T. Kim, S. Crockett, Corr. Sci. 30 (1990) 715. [19] M. Tanimoto, O. Vatel, J. Vac. Sci. Technol. B 14 (1996) 1547. [20] S. Kamiya, M. Iwami, T. Tsuchiya, M. Kurouchi, J. Kikawa, T. Yamada, Appl. Phys. Lett. 90 (2007) 213511. [21] A. Kikukawa, S. Hosaka, R. Imura, Appl. Phys. Lett. 66 (1995) 3510. [22] A.K. Sinensky, A.M. Belcher, Nat. Nanotechnol. 2 (2007) 653. [23] R.A. Lodge, B. Bhushan, J. Vac. Sci. Technol. 25 (2007) 893. [24] C.K. Chung, W.T. Chang, Thin Solid Films 517 (2009) 4800. [25] A. Bai, C.C. Hu, T.C. Wen, Electrochem. Acta 48 (2003) 2425. [26] A. Bai, C.C. Hu, Electrochem. Acta 47 (2002) 3447. [27] MichaelsonH.B. , 73rd ed., CRC Handbook of Chemistry and Physisc, 12, 1992, p. 108. [28] T. Durakiewicz, S. Halas, A. Arko, J.J. Joyce, D.P. Moore, Phys. Rev. B 64 (2001) 045101. [29] S. Halas, T. Durakiewicz, J. Phys. Condens. Matter 10 (1998) 10815.