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Electrodepositing amorphous Ni-W alloys for MEMS
Microelectronic Engineering 87 (2010) 1901–1906
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Electrodepositing amorphous Ni-W alloys for MEMS Hong Wang *, Rui Liu, FengJi Cheng, Ying Cao, GuiFu Ding, XiaoLin Zhao Key Laboratory for Thin Film and Microfabrication Technology of Ministry of Education, National Key Laboratory of Micro/Nano Fabrication Technology, Research Institute of Micro /Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 1 August 2008 Received in revised form 12 May 2009 Accepted 5 November 2009 Available online 10 November 2009 Keywords: Ni-W amorphous corrosion resistance wearing resistance MEMS
a b s t r a c t The wearing resistance and corrosion resistance of Ni-W alloy films is more excellent than that of Ni film. As a contrast, wearing resistance and corrosion resistance of Ni and NiW films are investigated in this paper. The results show that electrodeposited amorphous Ni-W alloy films have good wearing resistance and excellent corrosion resistance compared with electrodeposited Ni films. Moreover, the Ni-W alloy films still had good properties after heat treatment. The good wearing resistance and corrosion resistance of amorphous Ni-W alloy films are associated with crystal structure. The micro-spinneret is fabricated successfully with amorphous Ni-W alloys by UV-LIGA technology. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction With the rapid development of micro electro mechanical systems (MEMS) technology, a lot of micro-devices require more materials with excellent physical and mechanical properties. Traditional materials such as Ni, Cu and Ag in MEMS devices have their own advantages, but the range of properties they can offer is rather limited in some special devices such as micro-mold, micro-engines and micro-motor, etc. In recent years, several researchers pay more attention to Ni-W alloy films because they have excellent wear resistance, good corrosion resistance and high hardness. Atanassov [1] studied properties of nickel tungsten alloys electrodeposited from sulfa-mate electrolyte. Schuh [2] researched abrasion resistance of nanocrystalline Ni-W alloys and they observed that nanocrystalline Ni-W alloys have better abrasion resistance than nanocrystalline Ni. Friction and wear behavior of electrodeposited Ni-W alloys were deposited at different current densities by Siraman [3]. They found that the wear rate of Ni-W alloys is associated with the grain size. The sliding friction and wear characteristics of Ni-W alloys with different tungsten contents were investigated by Haseeb [4]. They observed that Ni-W alloys have lower friction coefficient against steel counter body compared with that of the nicked-steel pair. Yamasaki [5] reported that electrodeposited Ni-W alloys with amorphous structure exhibit excellent properties, such as high hardness, good thermal stability and excellent corrosion resistance. Though Ni-W alloys have these good properties, a major problem of electrodeposited Ni-W alloys * Corresponding author. Tel.: +86 21 34206687; fax: 21 34206686. E-mail address: [email protected] (H. Wang). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.11.018
is that they suffer from high internal stress. The internal stress has an inverse effect on the quality of Ni-W alloy films, and it would result in cracks and cause damage to MEMS devices. In our previous work, it is concluded that electrodeposited amorphous Ni-W alloy films have low internal stress [6]. So the amorphous Ni-W alloy films may be used in MEMS devices. Atomic arrangement of amorphous state is long-rang disorder, and there are no grain boundaries and dislocations in the amorphous structure. Moreover, amorphous structure is isotropic compared with crystalline structure. So the chemical stability of amorphous Ni-W alloy films is better than crystalline Ni-W alloy films. In this paper, wearing resistance and corrosion resistance of amorphous Ni-W alloy films are investigated carefully, and micro-spinneret is fabricated successfully with amorphous Ni-W alloys by UV-LIGA technology. 2. Experimental procedures The electrolytes consist of 0.4 mol/L (NH4)2C6H5O7, 0.25mol/L C2H8O7P2, 0-0.25mol/L NiCl26H2O and 0-0.25mol/L K2WO42H2O. The bath is kept at 50°C during deposition. Ni-W alloy films are electrodeposited on copper substrate. The copper substrate with 99.9% purity is provided by ShangHai Nonferrous Metal Pressing Factory (China). The copper substrate is polished with HNO3 and H3PO4 mixture and coated with insulating cement on one side. The nitric acid (HNO3) and phosphoric acid (H3PO4) in 1:3 compositions (V/V) are used in this experiment. Then Ni-W alloy films are electrodeposited on the other side with pulse current. The current density and pulse ratio is 70 mA/cm2 and 2 (ton=1 ms, toff=0.5 ms), respectively.
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The heat treatment is conducted in quartz furnace, and the atmospheric pressure is 1 KPa. The speed rate of heating-up is 10°C/min during heat treatment. The surface topography of films is examined with scanning electron microscopy (S-520, Hitachi Ltd, Japan), and the structure of alloy films is measured by D/max2200VPC X-ray diffraction instrument (Rigaku International Corp, Japan). Wearing tests are conducted in MMU-5G device at room temperature. Non-corrosive steel loop are used as the counter body with the 100N load. The rotary speed of counter body is 100 rpm/min for wearing with 30 minutes. The surface topography of fricative alloy films is investigated by Veeco Dektak 6M profile testing instrument and Multimode Nanoscope IIIa AFM (Digital Instrument Ltd, USA).
3. Results and discussion 3.1. structure of Ni-W alloy films
Fig. 2. X-ray diffraction spectra of Ni-W alloy films with different heat treatment temperature.
Fig. 1 presents X-ray diffraction spectra of Ni-W alloy films with different W contents. It shows that diffraction peak become low and wide gradually with the W content increasing, and there is only one diffraction peak when the W content is about 59wt%. Yamasaki [5] reported that the structure of Ni-W alloy films is crystalline when the W content is less than 44wt%, and it becomes amorphous when the W content exceeds 44wt%. Zhu [5] studied that up to 44wt% of W content is deposited in the Ni-W alloys in crystalline form. Moreover, an amorphous structure is formed when W content in the range of 44-67wt%, and above that an orthorhombic crystal structure is observed. The change of diffraction peak of Ni-W alloy films in Fig. 1 indicates that the amorphous structure convert into crystalline state. In order to obtain amorphous Ni-W alloy films, the W content is in the range of 4560wt% in this work. X-ray diffraction spectra of Ni-W alloy films with different heat treatment temperature are shown in Fig. 2. The diffraction peak of electrodeposited Ni-W alloy films with 59wt% W content is low and wide, and they become sharp gradually with temperature increasing. The results show that the amorphous structure of electrodeposited Ni-W alloy films become crystalline state after heat treatment. The amorphous state is meta-stable, and it has no long-range order crystal lattice. Moreover, there is obvious enthal-
py difference compared with stable crystalline state [7]. Amorphous Ni-W alloy films are easy to convert into crystalline state during heat treatment. In order to avoid grains growing exceedingly, the heat treatment temperature is 550°C in the experiment. 3.2. Wearing resistance of Ni-W alloy films Fig. 3 shows surface profile curve of Ni and Ni-W films after wearing at room temperature. It can be seen that there is slight difference of surface profile after wearing between electrodeposited and heat treated Ni-W alloy films. Moreover, wearing depth of electrodeposited Ni-W alloy films is about 1.8 lm, and it is about 1.4 lm of heat treated Ni-W alloy films when heat treatment temperature is 550°C. Electrodeposited and heat treated Ni film are also observed in Fig. 3. Wearing depth of electrodeposited Ni film is about 3.5 lm, and it increases to 8.4 lm when heat treated temperature is 550°C. Electrodeposited Ni-W alloy films have better wearing resistance than that of electrodeposited Ni film because W atoms dissolve into Ni crystal lattice. Moreover, heat treatment affect wearing resistance of Ni-W alloy films slightly while wearing resistance of Ni film changes obviously after heat treatment. In or-
Fig. 1. X-ray diffraction spectra of Ni-W alloy films with different W content.
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Fig. 3. Profile curve of worn surface of Ni and Ni-W films (a) electrodeposited Ni (b) electrodeposited Ni-W (c) heat treated Ni (d) heat treated Ni-W.
Fig. 4. AFM images of electrodeposited and heat treated Ni and Ni-W films after wearing (a) electrodeposited Ni (b) heat treated Ni (c) electrodeposited Ni-W (d) heat treated Ni-W.
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der to research the effect of heat treatment on wearing resistance of Ni and Ni-W films, microstructure of Ni and Ni-W films are investigated with AFM (atom force microscope) and SEM (scanning electron microscope). AFM images of Ni and Ni-W films after wearing are presented in Fig. 4. Electrodeposited Ni films have obvious deformation after wearing, and some spalling chip are observed on the surface (Fig. 4a). Moreover, deformation of heat treated Ni film is more serious than that of electrodeposited Ni film (Fig. 4b). Electrodeposited Ni-W alloy films have little deformation, and some wearing dots are observed on the surface (Fig. 4c). Heat treated Ni-W alloy films also have slight deformation, and there are some wear trace along the direction of wearing (Fig. 4d). This results indicates that
both electrodeposited and heat treated Ni-W alloy films have good wearing resistance while heat treatment has an obvious effect on the wearing resistance of Ni film. The structure of electrodeposited Ni-W alloy films is amorphous, and its surface is smooth and flat. However, there are many grains in Ni-W alloy films after heat treatment. The structure of NiW alloy films convert into crystalline after heat treatment and this result is accord with the X-ray diffraction spectra. There are no regular grain boundaries in amorphous structure. Moreover, atomic bonding force of amorphous state is stronger than that of crystalline state. Therefore, amorphous Ni-W alloy films have good wearing resistance. There are many grains of NiW alloy films after heat treatment, and these grains have strong
Fig. 5. Morphology of Ni films (a) electrodeposited Ni (b) heat treated Ni (c) electrodeposited Ni after wearing (d) heat treated Ni after wearing.
Fig. 6. Anodic oxidation curves of Ni-W alloy films.
H. Wang et al. / Microelectronic Engineering 87 (2010) 1901–1906
Fig. 7. Anodic oxidation curves of different films.
capability of resistance to deformation. So the amorphous Ni-W alloy films still have good wearing resistance after heat treatment. SEM photograph of electrodeposited and heat treated Ni film are shown in Fig. 5. It can be seen that electrodeposited Ni films contain a lot of microcrystallines (Fig. 5a), and size of these microcrystallines increase dramatically after heat treatment (Fig. 5b). The wear surface of electrodeposited and heat treated Ni film are also presented in Fig. 5. Deformation of heat treated Ni film (Fig. 5d) is more serous than that of electrodeposited Ni films (Fig. 5c). The grain sizes increase obviously while the amount of grain boundaries decreases rapidly after heat treatment. The Ni film with large grains is easy to deform when it is worn by external force. Therefore, the wearing resistance of Ni films is decreased obviously by heat treatment [9]. 3.3. Erosion resistance of Ni-W alloy films Fig. 6 presents the anodic oxidation curve of electrodeposited and heat treated Ni-W alloy films in 1M HCL solution. It can be seen that trend of electrodeposited and heat treated Ni-W alloy films are similar in the same solution. Oxide films in the electrodeposits begin to dissolve with the electrode potential increasing. Then, the Ni-W alloy films begin to dissolve when the oxide films are depleted completely, and dissolution rate of alloy films increases regularly when the potential becomes positive gradually. However, the dissolution rate of Ni-W alloy films decreases dramatically when potential increase to passivation value. Moreover, the dissolution rate becomes stable gradually and changes slightly with the potential increasing. From the two curves, it also can be seen that corrosion current of electrodeposited Ni-W alloy films is slight greater than that of heat treated Ni-W alloy films when potential is less than 0 V(VS Saturated Calomel Electrode: SCE). However, the corrosion current of electrodeposited Ni-W alloy films is lower than that of heat treated Ni-W alloy films when the potential is in the range of 0-0.8V (VS SCE), and the change tendency of corrosion current is similar when the potential is less than 0 V (VS SCE) and above 0.8 V (VS SCE). Corrosion current of electrodeposited Ni-W alloy films is about 3 times than that of heat treated Ni-W alloy films when potential is 1.0V (VS SCE). In general, the erosion resistance of electrodeposited Ni-W alloy films is better than that of heat treated Ni-W alloy films in 1M HCL solution. This result indicates that the erosion resistance of Ni-W alloy films is associated with crystal structure. In previous research, we know that structure of electrodeposited Ni-W alloy films is amorphous, and it becomes crystalline after heat treatment [8].
Fig. 8. Major fabrication steps of the micro-spinneret.
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Fig. 9. Optical image of the micro-spinneret (a) optical image of micro-spinneret (b) cross-section of micro-spinneret.
Fig. 7 shows the anodic oxidation curve of electrodeposited and heat treated Ni films, stainless steel, electrodeposited and heat treated Ni-W alloy films. It can be seen that erosion resistance of Ni-W alloy films is better that of stainless steel, and the erosion resistance of heat treated Ni films is worst in the five types of sample. Moreover, the corrosion rate has obvious difference between them. For example, the corrosion rate of electrodeposited Ni film is about 100 times than that of electrodeposited Ni-W alloy films, and the corrosion rate of heat treated Ni films is about 40 times than that of heat treated Ni-W alloy films. Therefore, the corrosion resistance of Ni-W alloy films is more excellent than that of Ni film. 4. Applications Spinneret of textile is the typical micro-mold structure. In order to extend the operating life of spinneret, the surface of jet expansion should have high wearing resistance and excellent erosion resistance. In our research, it is proved that amorphous Ni-W alloys have good wearing resistance and excellent erosion resistance. Therefore, the micro spinneret is fabricated with amorphous NiW alloys by UV-LIGA technology and lamella electrodepositing method in this work. The major steps required to fabricate microspinneret utilizing the electrodeposited amorphous Ni-W alloys are shown in Fig. 8. They are: (a) sputtering Ti seed layer on the substrate and oxidizing it, (b) patterning of the photo-resist ultraviolet lithographic technology, (c) sputtering Ti/Cu seed layer, (d) electrodepositing the buffer layer Cu which could release the stress of Ni-W alloy films (e) electrodepositing Ni/W layer, (f) electrodepositing supporting layer Ni, (g) releasing of the micro-spinneret. Fig. 9 (a) is the optical image of micro-spinneret fabricated with amorphous Ni-W alloys, and the cross-section of micro-spinneret is presented in Fig. 9 (b). It can be seen that the surface of microspinneret is smooth and the interface of Ni-W and Ni films is stable. 5. Conclusions Wearing resistance and corrosion resistance of amorphous NiW alloy films have been carefully investigated. The results show
that the amorphous Ni-W alloy films have better wearing resistance and corrosion resistance than that of Ni films. Moreover, they still maintain these excellent properties after heat treatment, and these properties are related to crystal structure. Based on these results, the micro spinneret is fabricated successfully with amorphous Ni-W alloys by UV-LIGA technology and lamella electrodepositing method. Acknowledgments Financial support for this research from the National High Technology Research and Development Program of China (No. 2006AA4Z326) and the Foundation of the National Key Laboratory of Nano/Micro Fabrication Technology of China (No. 9140C7903020907) are gratefully acknowledged. References [1] M. Bratoeva, N. Atanassov, Nickel-tungsten alloy electrodeposition from a sulfamate electrolyte, Metal Finish 96 (6) (1998) 98–99. [2] C.-A. Schuh, T.-G. Nieh, H. Iwasaki, The effect of solid solution W additions on the mechanical properties of nanocrystalline Ni, Acta Materialia 51 (2003) 431– 443. [3] K.-R. Sriraman, S.-G. Raman, S.-K. Seshadri, Synthesis and evaluation of hardness and sliding wear resistance of electrodeposited nanocrystalline NiW alloys, Mater Sci Eng 418 (2006) 303–311. [4] A.-S. Haseeb, U. Albers, K. Bade, Friction and wear characteristics of electrodeposited nanocryatalline nickel-tungsten alloy films, Wear 246 (2) (2007) 106–112. [5] T. Yamasaki, P. Schloßmacher, K. Ehrlich, Y. Ogino, Formation of amorphous electrodeposited Ni-W alloys and their nanocrystallization, Nanostruct Mater 10 (1998) 375–388. [6] R. Liu, H. Wang, J.-Y. YAO, X.-P. Li, G.-F. Ding, Preparing Ni-W alloy films with low internal stress and high hardness by heat treating, Surf Rev Lett 14 (6) (2007) 1107–1112. [7] L. Zhu, O. Younes, N. Ashkenasy, D.-Y. Shacham, E. Gileadi, STM/AFM studies of the evolution of morphology of electroplated Ni/W alloys, App Surf Sci 200 (1) (2002) 1–14. [8] S.H. Chen, Glassy metals, Rep Prog Phys 43 (1980) 353–432. [9] L.L. Wang, C.S. Guan, C.Z. Sun, Effect of heat treatment on bonding strength and microhardness of Ni-P coating, Plating and Finishing 30 (2) (2008) 4–6 (in Chinese).
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Electrodepositing amorphous Ni-W alloys for MEMS Hong Wang *, Rui Liu, FengJi Cheng, Ying Cao, GuiFu Ding, XiaoLin Zhao Key Laboratory for Thin Film and Microfabrication Technology of Ministry of Education, National Key Laboratory of Micro/Nano Fabrication Technology, Research Institute of Micro /Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 1 August 2008 Received in revised form 12 May 2009 Accepted 5 November 2009 Available online 10 November 2009 Keywords: Ni-W amorphous corrosion resistance wearing resistance MEMS
a b s t r a c t The wearing resistance and corrosion resistance of Ni-W alloy films is more excellent than that of Ni film. As a contrast, wearing resistance and corrosion resistance of Ni and NiW films are investigated in this paper. The results show that electrodeposited amorphous Ni-W alloy films have good wearing resistance and excellent corrosion resistance compared with electrodeposited Ni films. Moreover, the Ni-W alloy films still had good properties after heat treatment. The good wearing resistance and corrosion resistance of amorphous Ni-W alloy films are associated with crystal structure. The micro-spinneret is fabricated successfully with amorphous Ni-W alloys by UV-LIGA technology. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction With the rapid development of micro electro mechanical systems (MEMS) technology, a lot of micro-devices require more materials with excellent physical and mechanical properties. Traditional materials such as Ni, Cu and Ag in MEMS devices have their own advantages, but the range of properties they can offer is rather limited in some special devices such as micro-mold, micro-engines and micro-motor, etc. In recent years, several researchers pay more attention to Ni-W alloy films because they have excellent wear resistance, good corrosion resistance and high hardness. Atanassov [1] studied properties of nickel tungsten alloys electrodeposited from sulfa-mate electrolyte. Schuh [2] researched abrasion resistance of nanocrystalline Ni-W alloys and they observed that nanocrystalline Ni-W alloys have better abrasion resistance than nanocrystalline Ni. Friction and wear behavior of electrodeposited Ni-W alloys were deposited at different current densities by Siraman [3]. They found that the wear rate of Ni-W alloys is associated with the grain size. The sliding friction and wear characteristics of Ni-W alloys with different tungsten contents were investigated by Haseeb [4]. They observed that Ni-W alloys have lower friction coefficient against steel counter body compared with that of the nicked-steel pair. Yamasaki [5] reported that electrodeposited Ni-W alloys with amorphous structure exhibit excellent properties, such as high hardness, good thermal stability and excellent corrosion resistance. Though Ni-W alloys have these good properties, a major problem of electrodeposited Ni-W alloys * Corresponding author. Tel.: +86 21 34206687; fax: 21 34206686. E-mail address: [email protected] (H. Wang). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.11.018
is that they suffer from high internal stress. The internal stress has an inverse effect on the quality of Ni-W alloy films, and it would result in cracks and cause damage to MEMS devices. In our previous work, it is concluded that electrodeposited amorphous Ni-W alloy films have low internal stress [6]. So the amorphous Ni-W alloy films may be used in MEMS devices. Atomic arrangement of amorphous state is long-rang disorder, and there are no grain boundaries and dislocations in the amorphous structure. Moreover, amorphous structure is isotropic compared with crystalline structure. So the chemical stability of amorphous Ni-W alloy films is better than crystalline Ni-W alloy films. In this paper, wearing resistance and corrosion resistance of amorphous Ni-W alloy films are investigated carefully, and micro-spinneret is fabricated successfully with amorphous Ni-W alloys by UV-LIGA technology. 2. Experimental procedures The electrolytes consist of 0.4 mol/L (NH4)2C6H5O7, 0.25mol/L C2H8O7P2, 0-0.25mol/L NiCl26H2O and 0-0.25mol/L K2WO42H2O. The bath is kept at 50°C during deposition. Ni-W alloy films are electrodeposited on copper substrate. The copper substrate with 99.9% purity is provided by ShangHai Nonferrous Metal Pressing Factory (China). The copper substrate is polished with HNO3 and H3PO4 mixture and coated with insulating cement on one side. The nitric acid (HNO3) and phosphoric acid (H3PO4) in 1:3 compositions (V/V) are used in this experiment. Then Ni-W alloy films are electrodeposited on the other side with pulse current. The current density and pulse ratio is 70 mA/cm2 and 2 (ton=1 ms, toff=0.5 ms), respectively.
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The heat treatment is conducted in quartz furnace, and the atmospheric pressure is 1 KPa. The speed rate of heating-up is 10°C/min during heat treatment. The surface topography of films is examined with scanning electron microscopy (S-520, Hitachi Ltd, Japan), and the structure of alloy films is measured by D/max2200VPC X-ray diffraction instrument (Rigaku International Corp, Japan). Wearing tests are conducted in MMU-5G device at room temperature. Non-corrosive steel loop are used as the counter body with the 100N load. The rotary speed of counter body is 100 rpm/min for wearing with 30 minutes. The surface topography of fricative alloy films is investigated by Veeco Dektak 6M profile testing instrument and Multimode Nanoscope IIIa AFM (Digital Instrument Ltd, USA).
3. Results and discussion 3.1. structure of Ni-W alloy films
Fig. 2. X-ray diffraction spectra of Ni-W alloy films with different heat treatment temperature.
Fig. 1 presents X-ray diffraction spectra of Ni-W alloy films with different W contents. It shows that diffraction peak become low and wide gradually with the W content increasing, and there is only one diffraction peak when the W content is about 59wt%. Yamasaki [5] reported that the structure of Ni-W alloy films is crystalline when the W content is less than 44wt%, and it becomes amorphous when the W content exceeds 44wt%. Zhu [5] studied that up to 44wt% of W content is deposited in the Ni-W alloys in crystalline form. Moreover, an amorphous structure is formed when W content in the range of 44-67wt%, and above that an orthorhombic crystal structure is observed. The change of diffraction peak of Ni-W alloy films in Fig. 1 indicates that the amorphous structure convert into crystalline state. In order to obtain amorphous Ni-W alloy films, the W content is in the range of 4560wt% in this work. X-ray diffraction spectra of Ni-W alloy films with different heat treatment temperature are shown in Fig. 2. The diffraction peak of electrodeposited Ni-W alloy films with 59wt% W content is low and wide, and they become sharp gradually with temperature increasing. The results show that the amorphous structure of electrodeposited Ni-W alloy films become crystalline state after heat treatment. The amorphous state is meta-stable, and it has no long-range order crystal lattice. Moreover, there is obvious enthal-
py difference compared with stable crystalline state [7]. Amorphous Ni-W alloy films are easy to convert into crystalline state during heat treatment. In order to avoid grains growing exceedingly, the heat treatment temperature is 550°C in the experiment. 3.2. Wearing resistance of Ni-W alloy films Fig. 3 shows surface profile curve of Ni and Ni-W films after wearing at room temperature. It can be seen that there is slight difference of surface profile after wearing between electrodeposited and heat treated Ni-W alloy films. Moreover, wearing depth of electrodeposited Ni-W alloy films is about 1.8 lm, and it is about 1.4 lm of heat treated Ni-W alloy films when heat treatment temperature is 550°C. Electrodeposited and heat treated Ni film are also observed in Fig. 3. Wearing depth of electrodeposited Ni film is about 3.5 lm, and it increases to 8.4 lm when heat treated temperature is 550°C. Electrodeposited Ni-W alloy films have better wearing resistance than that of electrodeposited Ni film because W atoms dissolve into Ni crystal lattice. Moreover, heat treatment affect wearing resistance of Ni-W alloy films slightly while wearing resistance of Ni film changes obviously after heat treatment. In or-
Fig. 1. X-ray diffraction spectra of Ni-W alloy films with different W content.
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Fig. 3. Profile curve of worn surface of Ni and Ni-W films (a) electrodeposited Ni (b) electrodeposited Ni-W (c) heat treated Ni (d) heat treated Ni-W.
Fig. 4. AFM images of electrodeposited and heat treated Ni and Ni-W films after wearing (a) electrodeposited Ni (b) heat treated Ni (c) electrodeposited Ni-W (d) heat treated Ni-W.
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der to research the effect of heat treatment on wearing resistance of Ni and Ni-W films, microstructure of Ni and Ni-W films are investigated with AFM (atom force microscope) and SEM (scanning electron microscope). AFM images of Ni and Ni-W films after wearing are presented in Fig. 4. Electrodeposited Ni films have obvious deformation after wearing, and some spalling chip are observed on the surface (Fig. 4a). Moreover, deformation of heat treated Ni film is more serious than that of electrodeposited Ni film (Fig. 4b). Electrodeposited Ni-W alloy films have little deformation, and some wearing dots are observed on the surface (Fig. 4c). Heat treated Ni-W alloy films also have slight deformation, and there are some wear trace along the direction of wearing (Fig. 4d). This results indicates that
both electrodeposited and heat treated Ni-W alloy films have good wearing resistance while heat treatment has an obvious effect on the wearing resistance of Ni film. The structure of electrodeposited Ni-W alloy films is amorphous, and its surface is smooth and flat. However, there are many grains in Ni-W alloy films after heat treatment. The structure of NiW alloy films convert into crystalline after heat treatment and this result is accord with the X-ray diffraction spectra. There are no regular grain boundaries in amorphous structure. Moreover, atomic bonding force of amorphous state is stronger than that of crystalline state. Therefore, amorphous Ni-W alloy films have good wearing resistance. There are many grains of NiW alloy films after heat treatment, and these grains have strong
Fig. 5. Morphology of Ni films (a) electrodeposited Ni (b) heat treated Ni (c) electrodeposited Ni after wearing (d) heat treated Ni after wearing.
Fig. 6. Anodic oxidation curves of Ni-W alloy films.
H. Wang et al. / Microelectronic Engineering 87 (2010) 1901–1906
Fig. 7. Anodic oxidation curves of different films.
capability of resistance to deformation. So the amorphous Ni-W alloy films still have good wearing resistance after heat treatment. SEM photograph of electrodeposited and heat treated Ni film are shown in Fig. 5. It can be seen that electrodeposited Ni films contain a lot of microcrystallines (Fig. 5a), and size of these microcrystallines increase dramatically after heat treatment (Fig. 5b). The wear surface of electrodeposited and heat treated Ni film are also presented in Fig. 5. Deformation of heat treated Ni film (Fig. 5d) is more serous than that of electrodeposited Ni films (Fig. 5c). The grain sizes increase obviously while the amount of grain boundaries decreases rapidly after heat treatment. The Ni film with large grains is easy to deform when it is worn by external force. Therefore, the wearing resistance of Ni films is decreased obviously by heat treatment [9]. 3.3. Erosion resistance of Ni-W alloy films Fig. 6 presents the anodic oxidation curve of electrodeposited and heat treated Ni-W alloy films in 1M HCL solution. It can be seen that trend of electrodeposited and heat treated Ni-W alloy films are similar in the same solution. Oxide films in the electrodeposits begin to dissolve with the electrode potential increasing. Then, the Ni-W alloy films begin to dissolve when the oxide films are depleted completely, and dissolution rate of alloy films increases regularly when the potential becomes positive gradually. However, the dissolution rate of Ni-W alloy films decreases dramatically when potential increase to passivation value. Moreover, the dissolution rate becomes stable gradually and changes slightly with the potential increasing. From the two curves, it also can be seen that corrosion current of electrodeposited Ni-W alloy films is slight greater than that of heat treated Ni-W alloy films when potential is less than 0 V(VS Saturated Calomel Electrode: SCE). However, the corrosion current of electrodeposited Ni-W alloy films is lower than that of heat treated Ni-W alloy films when the potential is in the range of 0-0.8V (VS SCE), and the change tendency of corrosion current is similar when the potential is less than 0 V (VS SCE) and above 0.8 V (VS SCE). Corrosion current of electrodeposited Ni-W alloy films is about 3 times than that of heat treated Ni-W alloy films when potential is 1.0V (VS SCE). In general, the erosion resistance of electrodeposited Ni-W alloy films is better than that of heat treated Ni-W alloy films in 1M HCL solution. This result indicates that the erosion resistance of Ni-W alloy films is associated with crystal structure. In previous research, we know that structure of electrodeposited Ni-W alloy films is amorphous, and it becomes crystalline after heat treatment [8].
Fig. 8. Major fabrication steps of the micro-spinneret.
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Fig. 9. Optical image of the micro-spinneret (a) optical image of micro-spinneret (b) cross-section of micro-spinneret.
Fig. 7 shows the anodic oxidation curve of electrodeposited and heat treated Ni films, stainless steel, electrodeposited and heat treated Ni-W alloy films. It can be seen that erosion resistance of Ni-W alloy films is better that of stainless steel, and the erosion resistance of heat treated Ni films is worst in the five types of sample. Moreover, the corrosion rate has obvious difference between them. For example, the corrosion rate of electrodeposited Ni film is about 100 times than that of electrodeposited Ni-W alloy films, and the corrosion rate of heat treated Ni films is about 40 times than that of heat treated Ni-W alloy films. Therefore, the corrosion resistance of Ni-W alloy films is more excellent than that of Ni film. 4. Applications Spinneret of textile is the typical micro-mold structure. In order to extend the operating life of spinneret, the surface of jet expansion should have high wearing resistance and excellent erosion resistance. In our research, it is proved that amorphous Ni-W alloys have good wearing resistance and excellent erosion resistance. Therefore, the micro spinneret is fabricated with amorphous NiW alloys by UV-LIGA technology and lamella electrodepositing method in this work. The major steps required to fabricate microspinneret utilizing the electrodeposited amorphous Ni-W alloys are shown in Fig. 8. They are: (a) sputtering Ti seed layer on the substrate and oxidizing it, (b) patterning of the photo-resist ultraviolet lithographic technology, (c) sputtering Ti/Cu seed layer, (d) electrodepositing the buffer layer Cu which could release the stress of Ni-W alloy films (e) electrodepositing Ni/W layer, (f) electrodepositing supporting layer Ni, (g) releasing of the micro-spinneret. Fig. 9 (a) is the optical image of micro-spinneret fabricated with amorphous Ni-W alloys, and the cross-section of micro-spinneret is presented in Fig. 9 (b). It can be seen that the surface of microspinneret is smooth and the interface of Ni-W and Ni films is stable. 5. Conclusions Wearing resistance and corrosion resistance of amorphous NiW alloy films have been carefully investigated. The results show
that the amorphous Ni-W alloy films have better wearing resistance and corrosion resistance than that of Ni films. Moreover, they still maintain these excellent properties after heat treatment, and these properties are related to crystal structure. Based on these results, the micro spinneret is fabricated successfully with amorphous Ni-W alloys by UV-LIGA technology and lamella electrodepositing method. Acknowledgments Financial support for this research from the National High Technology Research and Development Program of China (No. 2006AA4Z326) and the Foundation of the National Key Laboratory of Nano/Micro Fabrication Technology of China (No. 9140C7903020907) are gratefully acknowledged. References [1] M. Bratoeva, N. Atanassov, Nickel-tungsten alloy electrodeposition from a sulfamate electrolyte, Metal Finish 96 (6) (1998) 98–99. [2] C.-A. Schuh, T.-G. Nieh, H. Iwasaki, The effect of solid solution W additions on the mechanical properties of nanocrystalline Ni, Acta Materialia 51 (2003) 431– 443. [3] K.-R. Sriraman, S.-G. Raman, S.-K. Seshadri, Synthesis and evaluation of hardness and sliding wear resistance of electrodeposited nanocrystalline NiW alloys, Mater Sci Eng 418 (2006) 303–311. [4] A.-S. Haseeb, U. Albers, K. Bade, Friction and wear characteristics of electrodeposited nanocryatalline nickel-tungsten alloy films, Wear 246 (2) (2007) 106–112. [5] T. Yamasaki, P. Schloßmacher, K. Ehrlich, Y. Ogino, Formation of amorphous electrodeposited Ni-W alloys and their nanocrystallization, Nanostruct Mater 10 (1998) 375–388. [6] R. Liu, H. Wang, J.-Y. YAO, X.-P. Li, G.-F. Ding, Preparing Ni-W alloy films with low internal stress and high hardness by heat treating, Surf Rev Lett 14 (6) (2007) 1107–1112. [7] L. Zhu, O. Younes, N. Ashkenasy, D.-Y. Shacham, E. Gileadi, STM/AFM studies of the evolution of morphology of electroplated Ni/W alloys, App Surf Sci 200 (1) (2002) 1–14. [8] S.H. Chen, Glassy metals, Rep Prog Phys 43 (1980) 353–432. [9] L.L. Wang, C.S. Guan, C.Z. Sun, Effect of heat treatment on bonding strength and microhardness of Ni-P coating, Plating and Finishing 30 (2) (2008) 4–6 (in Chinese).