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Fabrication of 316-L stainless steel micro parts by softlithography and powder metallurgy
Materials Letters 62 (2008) 4213–4216
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Fabrication of 316-L stainless steel micro parts by softlithography and powder metallurgy Mohamed Imbaby a,1, Kyle Jiang a,⁎, Isaac Chang b a b
School of Mechanical Engineering, University of Birmingham, Edgbaston, Birmingham, UK School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, UK
A R T I C L E
I N F O
Article history: Received 8 May 2008 Accepted 15 June 2008 Available online 28 June 2008 Keywords: SU-8 mould PDMS 316-L stainless steel Metallurgy Sintering
A B S T R A C T This paper presents an approach to fabricate 316-L stainless steel micro parts with complex shapes using soft lithographical and powder metallurgical techniques. The process includes production of high quality deep micro SU-8 master moulds and their negative replicas in polydimethylsiloxane (PDMS). Then the PDMS soft moulds are filled with slurry containing superfine (b 4 um) stainless steel particles and binder. When the slurry is dry, green patterns are removed from the PDMS moulds before the patterns are de-bound and sintered in the tube furnace at 1200 in forming gas atmosphere. The resultant micro parts show high shape retention. The linear shrinkage of the sintered part is measured and found to be 17%. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Micro fabrication technology was evolved from silicon based semiconductor fabrication technology [1]. The demands for multiple material microcomponents encourage researchers to develop new fabrication techniques in this area. In the past few years, fabrication of metallic microcomponents has rapidly increased due to the demands from different applications, such as sensors, medical devices and micro machines. 316-L grade stainless steel components possess excellent mechanical and corrosion resistive properties, as well as biocompatibility [2]. These properties make it an excellent candidate for micro-medical and micro-implant applications. There are various ways for microcomponent fabrication at present, such as LIGA process based on synchrotron X-ray radiation lithography and galvanoforming [3], focus ion beam (FIB) [4], laser micro machining [5], and micro-electro-discharge machining (MicroEDM) [6]. These methods are either very expensive because of the resources it relies on or slow in production because of the nature of individual fabrication. Micro injection moulding (MIM) is another fabrication technique which has been used for polymers [7], metals [8], and ceramics [9]. Nevertheless, micro injection moulding relies on precise metal moulds fabricated either by microEDM or by electroforming process [10]. As a consequence, MIM bears the similar weakness in slow process and high overall costs. ⁎ Corresponding author. Tel.: +44 121 414 6800; fax: +44 121 414 3958. E-mail addresses: [email protected] (M. Imbaby), [email protected] (K. Jiang). 1 Assistant Lecturer on leave, Department of Mechanical Design Engineering, Helwan University, Egypt. 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.06.049
This paper presents a study on fabrication of defect-free 316-L stainless steel micro parts with high shape retention using softlithography in combination with powder metallurgy. Softlithography relies on soft mould insert. It has the features of low cost in fabrication and suitability for volume production. The research work is based on previous successful experience in fabrication of microcomponents [11–13] and the techniques have been developed further. The softlithography process adopted in this research includes fabrication of SU-8 master moulds and replication of PDMS moulds from the masters. Stainless steel powder slurry was then filled in the soft moulds to form green patterns before they are sintered in a furnace. The fabrication process steps were investigated and the linear shrinkage of the sintered micro part was investigated. 2. Experimental 2.1. Fabrication of SU-8 master moulds and PDMS negative replicas SU-8 is a negative tone epoxy type photoresist sensitive to wavelengths ranging from 360–420 nm. The usual fabrication techniques of US-8 patterns include X-ray [14] and UV lithography [15]. SU-8 has a very low light absorbance property, which allows UV radiation to go through the entire resist thickness without losing much energy. This ensures that from top to bottom, the SU-8 layer can get homogenous exposure and result in excellent vertical sidewalls. SU-8 microstructures of 1 mm in thickness were reported before [11,12,15]. In addition, SU-8 is highly resistant to solvents and acids when cured. All the properties together make it an excellent material for fabricating ultrathick micro moulds.
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The fabrication process of SU-8 components used in this research was based on those introduced in references [11,12,15], and a little modification was made due to the change of the SU-8 type. In this work, SU-8 2075 [MicroChem, USA] was used for fabricating micro spur gear moulds with 2.5 mm pitch diameter and 1 mm in thickness. The SU-8 master mould fabrication process consists of the following steps: (i) cast SU-8 resist onto a 4 inch silicon wafer and leave the wafer on a level surface until the SU-8 spreads evenly; (ii) bake the coated wafer on a hot plate at 65 °C for 2 h followed by 95 °C for 34 h; (iii) expose the wafer in Canon PLA-501 FA UV-mask aligner; (iv) bake the wafer again at 65 °C for 15 min followed by 95 °C for 25 min; (v) develop the exposed wafer in EC solvent in ultrasonic bath for 40 min; (vi) Finally, hard bake the wafer at 105 °C for 5 min. PDMS slurry was prepared by well mixing Sylgard Silicone Elastomer 184 with Sylgard Curing Agent 184 (Dow Corning Corp.) at a 10:1 ratio in weight in a beaker. Then the beaker was put in vacuum to remove all the bubbles formed during the mixing. Afterward, the PDMS slurry was poured on SU-8 moulds and degassed again in a vacuum chamber until all trapped bubbles were removed. Finally, PDMS soft moulds were cured in an oven at 90 °C for 2 h and peeled off from the SU-8 moulds. The SU-8 and the PDMS moulds were examined under SEM (Philips XL-30) and their images are shown in Fig. 1. 2.2. Preparation of powder–binder mixture
Fig. 1. (a) SU-8 master mould (b) PDMS mould.
316-L stainless steel powder (Sandvik Osprey Ltd, UK) was used in this research. The chemical compositions are: 18.5% Cr, 11.6% Ni, 2.3% Mo, 0.048% C, 0.027% P, 0.65% Si, 0.008% S, and Fe balance, and the particles size distribution of the powder are: D10 = 1.1 µm, D50 = 1.8 µm, and D90 = 3.6 µm as delivered by supplier.
Fig. 2. (a) PDMS moulds filled with metallic slurry without applying vacuum; (b) filled PDMS moulds and treated with vacuum; (c) a green part taken out from the PDMS mould (d) internal structure of a fractured green part. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
M. Imbaby et al. / Materials Letters 62 (2008) 4213–4216
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Duramax D-3005 (Chesham Speciality Ingredients Limited, UK) was selected as the binder. Duramax D-3005 is ammonium salt of an acrylic homo-polymer and has been used as a dispersant in ceramics [12,13] and metallic Ni powder [16]. The metallic slurry was prepared by mixing stainless steel powder, binder, and distilled water in 10:1:1.5 wt. respectively in which the samples were weighed to ±0.01 mg. The mixture was well mixed by using mechanical stirrer for 2.5 h and put under vacuum at a pressure of 0.2 bar for 2 min to remove the bubbles formed during the stirring. 2.3. Filling the PDMS micro moulds and obtaining the green parts PDMS micro moulds were put in a Petri dish. Then the slurry was poured onto the PDMS cavities under gravity with help of vacuum to fill the voids in the moulds. After the moulds were completely filled, the excess slurry on the top of the moulds was removed with a razor blade to keep the patterns flat with sharp edges. Afterwards, the samples were left to dry in the air for 2 h. The green samples are demoulded by bending the PDMS moulds slightly and taking the samples out gently with tweezers. 2.4. De-binding and sintering processes The green samples were de-bound and sintered in a tube furnace including forming gas atmosphere of 90% nitrogen and 10% hydrogen. The maximum sintering temperature reached 1200 °C. A detailed de-binding and sintering process is provided and discussed in the next section. 3. Results and discussion 3.1. Vacuum assisted mould filling The filling methods investigated in the research include pouring the slurry onto the PDMS cavities under gravity without using vacuum and with using vacuum and their resultant differences were observed. In the first case where no vacuum was used, air was trapped in the gear tooth areas during the filling and the green patterns had incomplete shapes, as shown in Fig. 2a. Then vacuum was applied after the moulds were filled with slurry in order to remove trapped air bubbles. This resulted in complete patterns with gear tooth tips well filled. Fig. 2b shows a group of microgear soft moulds filled with the metallic slurry without voids. After drying, the green parts were demoulded and found free of cracks. The green parts and its internal structure were examined under SEM and their images were shown in Fig. 2c and d respectively. It was found that excellent shape retention was obtained and the internal structure of the dried green samples had homogeneous particle distribution. 3.2. Binder degradation and de-binding–sintering process The degradation behaviour of Duramax D-3005 dry sample was investigated using NETZSCH thermal gravimetric analysis (TGA) in the temperature range between 25 and 800 °C with argon flow and 10 °C/min heating rate. Fig. 3 shows the wt.% in relation with the temperature. It can be observed that the maximum weight loss occurred between 150 and 450 °C. Also, the binder was degraded when the temperature reached 650 °C and the residual was burnt out to less than 3% at 800 °C. Therefore, de-binding and sintering steps were carried out in one heating cycle. In the de-binding stage, the temperature was slowly ramped up to 700 °C at 1.2 °C/min to remove the binder gradually and to avoid
Fig. 4. SEM micrographs of (a) a sintered gear, and (b) its teeth (c) internal structure of fractured sintered-gear. deformation of the samples before sintering occurred. Afterwards, it came to the sintering stage and the temperature was ramped to 1200 °C at a rate of 5 °C/min before being remained at this temperature for 90 min. Finally, the samples were cooled down to room temperature in the furnace without interference. 3.3. Sintered gear and shrinkage The sintered gears were examined under SEM and one of the images was shown in Fig. 4a. It can be seen that it has uniformly straight teeth. Even the tiny lines on the SU-8 moulds have been reproduced on the top of the teeth, Fig. 4b. Fig. 4c shows the fracture surface of a sintered microgear. The sintered metallic structure looks dense and uniform. The linear shrinkage of the sintered gear is measured in reference to the SU-8 master mould and found to be 17%. The shrinkage is found to be homogenous and no deformation is found on the gears. The complete micrometallic component fabrication process has been used in producing microgears, microlinkages and some other micropatterns. The experiments demonstrated about 75% success rate in producing these acceptable microcomponents.
4. Conclusions
Fig. 3. TGA of dried D-3005.
316-L stainless steel microcomponents with complex shapes were successfully fabricated using softlithography and powder metallurgy
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M. Imbaby et al. / Materials Letters 62 (2008) 4213–4216
processes. The results show excellent shape retention with uniformly straight side wall. Duramax D-3005 is successfully used as the binder. The linear shrinkage of the sintered part is found to be 17% and it could be reduced when high solid loading applied. This research demonstrates a valid approach to fabricate microcomponents from metallic powder. Therefore, the process is regarded suitable for mass production of micrometallic components. Acknowledgement The authors would like to thank Sandvik Osprey Ltd. for the supply of superfine stainless steel 316L powders. The research was partly supported by the Chinese Ministry of Science and Technology on International Project 2006DFA73620. References [1] Beeby S, Ensell G, Kraft M, White N. MEMS Mechanical Sensors. London: Artech House; 2004. [2] Ranter DB, Hoffman AS, Schoen FJ, Lemons JE. Biomaterial Science. London: Academic Press; 1996.
[3] Becker EW, Ehrfeld W, Hagmann P, Maner A, Munchmeyer D. Microelectron Eng 1986;4:35–56. [4] Walker JF, Moore DF, Whitney JT. Microelectron Eng 1996;30:517–22. [5] Molpeceres C, Lauzuricaa S, García-Ballesterosa JJ, Moralesa M, Ocañaa JL. Microelectron Eng 2007;84:1337–40. [6] Liu HS, Yan BH, Huang FY, Qiu KHJ. Mater Process Technol 2005;169:418–26. [7] Liou AC, Chen RH. Int J Adv Manuf Technol 2006;28:1097–103. [8] Fu G, Loh NH, Tor SB. Mater Des 2004;24:29–33. [9] Gietzelt T, Jacobi O, Piotter V, Ruprecht R, Hausselt JJ. Mater Sci 2004;39:2113–9. [10] Moon S, Lee N, Kang S. J Micromechanics Microengineering 2003;13:98–103. [11] Kim J, Jiang K, Chang I. J Micromechanics Microengineering 2006;16:48–52. [12] Zhu Z, Wei X, Jiang K. J Micromechanics Microengineering 2007;17:193–8. [13] Zhang D, Su B, Button T. Adv Eng Mater 2003;5:924–7. [14] Cremers C, Bouamrane F, Singleton L, Schenk R. Microsyst Technol 2001;7:11–6. [15] Jin P, Jiang K, Sun N. J Microlith Microfab Microsys 2004;3:569–73. [16] Sanchez-Herencia AJ, Millan AJ, Nieto MI, Moreno R. Acta Mater 2001;49:645–51.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Fabrication of 316-L stainless steel micro parts by softlithography and powder metallurgy Mohamed Imbaby a,1, Kyle Jiang a,⁎, Isaac Chang b a b
School of Mechanical Engineering, University of Birmingham, Edgbaston, Birmingham, UK School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, UK
A R T I C L E
I N F O
Article history: Received 8 May 2008 Accepted 15 June 2008 Available online 28 June 2008 Keywords: SU-8 mould PDMS 316-L stainless steel Metallurgy Sintering
A B S T R A C T This paper presents an approach to fabricate 316-L stainless steel micro parts with complex shapes using soft lithographical and powder metallurgical techniques. The process includes production of high quality deep micro SU-8 master moulds and their negative replicas in polydimethylsiloxane (PDMS). Then the PDMS soft moulds are filled with slurry containing superfine (b 4 um) stainless steel particles and binder. When the slurry is dry, green patterns are removed from the PDMS moulds before the patterns are de-bound and sintered in the tube furnace at 1200 in forming gas atmosphere. The resultant micro parts show high shape retention. The linear shrinkage of the sintered part is measured and found to be 17%. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Micro fabrication technology was evolved from silicon based semiconductor fabrication technology [1]. The demands for multiple material microcomponents encourage researchers to develop new fabrication techniques in this area. In the past few years, fabrication of metallic microcomponents has rapidly increased due to the demands from different applications, such as sensors, medical devices and micro machines. 316-L grade stainless steel components possess excellent mechanical and corrosion resistive properties, as well as biocompatibility [2]. These properties make it an excellent candidate for micro-medical and micro-implant applications. There are various ways for microcomponent fabrication at present, such as LIGA process based on synchrotron X-ray radiation lithography and galvanoforming [3], focus ion beam (FIB) [4], laser micro machining [5], and micro-electro-discharge machining (MicroEDM) [6]. These methods are either very expensive because of the resources it relies on or slow in production because of the nature of individual fabrication. Micro injection moulding (MIM) is another fabrication technique which has been used for polymers [7], metals [8], and ceramics [9]. Nevertheless, micro injection moulding relies on precise metal moulds fabricated either by microEDM or by electroforming process [10]. As a consequence, MIM bears the similar weakness in slow process and high overall costs. ⁎ Corresponding author. Tel.: +44 121 414 6800; fax: +44 121 414 3958. E-mail addresses: [email protected] (M. Imbaby), [email protected] (K. Jiang). 1 Assistant Lecturer on leave, Department of Mechanical Design Engineering, Helwan University, Egypt. 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.06.049
This paper presents a study on fabrication of defect-free 316-L stainless steel micro parts with high shape retention using softlithography in combination with powder metallurgy. Softlithography relies on soft mould insert. It has the features of low cost in fabrication and suitability for volume production. The research work is based on previous successful experience in fabrication of microcomponents [11–13] and the techniques have been developed further. The softlithography process adopted in this research includes fabrication of SU-8 master moulds and replication of PDMS moulds from the masters. Stainless steel powder slurry was then filled in the soft moulds to form green patterns before they are sintered in a furnace. The fabrication process steps were investigated and the linear shrinkage of the sintered micro part was investigated. 2. Experimental 2.1. Fabrication of SU-8 master moulds and PDMS negative replicas SU-8 is a negative tone epoxy type photoresist sensitive to wavelengths ranging from 360–420 nm. The usual fabrication techniques of US-8 patterns include X-ray [14] and UV lithography [15]. SU-8 has a very low light absorbance property, which allows UV radiation to go through the entire resist thickness without losing much energy. This ensures that from top to bottom, the SU-8 layer can get homogenous exposure and result in excellent vertical sidewalls. SU-8 microstructures of 1 mm in thickness were reported before [11,12,15]. In addition, SU-8 is highly resistant to solvents and acids when cured. All the properties together make it an excellent material for fabricating ultrathick micro moulds.
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The fabrication process of SU-8 components used in this research was based on those introduced in references [11,12,15], and a little modification was made due to the change of the SU-8 type. In this work, SU-8 2075 [MicroChem, USA] was used for fabricating micro spur gear moulds with 2.5 mm pitch diameter and 1 mm in thickness. The SU-8 master mould fabrication process consists of the following steps: (i) cast SU-8 resist onto a 4 inch silicon wafer and leave the wafer on a level surface until the SU-8 spreads evenly; (ii) bake the coated wafer on a hot plate at 65 °C for 2 h followed by 95 °C for 34 h; (iii) expose the wafer in Canon PLA-501 FA UV-mask aligner; (iv) bake the wafer again at 65 °C for 15 min followed by 95 °C for 25 min; (v) develop the exposed wafer in EC solvent in ultrasonic bath for 40 min; (vi) Finally, hard bake the wafer at 105 °C for 5 min. PDMS slurry was prepared by well mixing Sylgard Silicone Elastomer 184 with Sylgard Curing Agent 184 (Dow Corning Corp.) at a 10:1 ratio in weight in a beaker. Then the beaker was put in vacuum to remove all the bubbles formed during the mixing. Afterward, the PDMS slurry was poured on SU-8 moulds and degassed again in a vacuum chamber until all trapped bubbles were removed. Finally, PDMS soft moulds were cured in an oven at 90 °C for 2 h and peeled off from the SU-8 moulds. The SU-8 and the PDMS moulds were examined under SEM (Philips XL-30) and their images are shown in Fig. 1. 2.2. Preparation of powder–binder mixture
Fig. 1. (a) SU-8 master mould (b) PDMS mould.
316-L stainless steel powder (Sandvik Osprey Ltd, UK) was used in this research. The chemical compositions are: 18.5% Cr, 11.6% Ni, 2.3% Mo, 0.048% C, 0.027% P, 0.65% Si, 0.008% S, and Fe balance, and the particles size distribution of the powder are: D10 = 1.1 µm, D50 = 1.8 µm, and D90 = 3.6 µm as delivered by supplier.
Fig. 2. (a) PDMS moulds filled with metallic slurry without applying vacuum; (b) filled PDMS moulds and treated with vacuum; (c) a green part taken out from the PDMS mould (d) internal structure of a fractured green part. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
M. Imbaby et al. / Materials Letters 62 (2008) 4213–4216
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Duramax D-3005 (Chesham Speciality Ingredients Limited, UK) was selected as the binder. Duramax D-3005 is ammonium salt of an acrylic homo-polymer and has been used as a dispersant in ceramics [12,13] and metallic Ni powder [16]. The metallic slurry was prepared by mixing stainless steel powder, binder, and distilled water in 10:1:1.5 wt. respectively in which the samples were weighed to ±0.01 mg. The mixture was well mixed by using mechanical stirrer for 2.5 h and put under vacuum at a pressure of 0.2 bar for 2 min to remove the bubbles formed during the stirring. 2.3. Filling the PDMS micro moulds and obtaining the green parts PDMS micro moulds were put in a Petri dish. Then the slurry was poured onto the PDMS cavities under gravity with help of vacuum to fill the voids in the moulds. After the moulds were completely filled, the excess slurry on the top of the moulds was removed with a razor blade to keep the patterns flat with sharp edges. Afterwards, the samples were left to dry in the air for 2 h. The green samples are demoulded by bending the PDMS moulds slightly and taking the samples out gently with tweezers. 2.4. De-binding and sintering processes The green samples were de-bound and sintered in a tube furnace including forming gas atmosphere of 90% nitrogen and 10% hydrogen. The maximum sintering temperature reached 1200 °C. A detailed de-binding and sintering process is provided and discussed in the next section. 3. Results and discussion 3.1. Vacuum assisted mould filling The filling methods investigated in the research include pouring the slurry onto the PDMS cavities under gravity without using vacuum and with using vacuum and their resultant differences were observed. In the first case where no vacuum was used, air was trapped in the gear tooth areas during the filling and the green patterns had incomplete shapes, as shown in Fig. 2a. Then vacuum was applied after the moulds were filled with slurry in order to remove trapped air bubbles. This resulted in complete patterns with gear tooth tips well filled. Fig. 2b shows a group of microgear soft moulds filled with the metallic slurry without voids. After drying, the green parts were demoulded and found free of cracks. The green parts and its internal structure were examined under SEM and their images were shown in Fig. 2c and d respectively. It was found that excellent shape retention was obtained and the internal structure of the dried green samples had homogeneous particle distribution. 3.2. Binder degradation and de-binding–sintering process The degradation behaviour of Duramax D-3005 dry sample was investigated using NETZSCH thermal gravimetric analysis (TGA) in the temperature range between 25 and 800 °C with argon flow and 10 °C/min heating rate. Fig. 3 shows the wt.% in relation with the temperature. It can be observed that the maximum weight loss occurred between 150 and 450 °C. Also, the binder was degraded when the temperature reached 650 °C and the residual was burnt out to less than 3% at 800 °C. Therefore, de-binding and sintering steps were carried out in one heating cycle. In the de-binding stage, the temperature was slowly ramped up to 700 °C at 1.2 °C/min to remove the binder gradually and to avoid
Fig. 4. SEM micrographs of (a) a sintered gear, and (b) its teeth (c) internal structure of fractured sintered-gear. deformation of the samples before sintering occurred. Afterwards, it came to the sintering stage and the temperature was ramped to 1200 °C at a rate of 5 °C/min before being remained at this temperature for 90 min. Finally, the samples were cooled down to room temperature in the furnace without interference. 3.3. Sintered gear and shrinkage The sintered gears were examined under SEM and one of the images was shown in Fig. 4a. It can be seen that it has uniformly straight teeth. Even the tiny lines on the SU-8 moulds have been reproduced on the top of the teeth, Fig. 4b. Fig. 4c shows the fracture surface of a sintered microgear. The sintered metallic structure looks dense and uniform. The linear shrinkage of the sintered gear is measured in reference to the SU-8 master mould and found to be 17%. The shrinkage is found to be homogenous and no deformation is found on the gears. The complete micrometallic component fabrication process has been used in producing microgears, microlinkages and some other micropatterns. The experiments demonstrated about 75% success rate in producing these acceptable microcomponents.
4. Conclusions
Fig. 3. TGA of dried D-3005.
316-L stainless steel microcomponents with complex shapes were successfully fabricated using softlithography and powder metallurgy
4216
M. Imbaby et al. / Materials Letters 62 (2008) 4213–4216
processes. The results show excellent shape retention with uniformly straight side wall. Duramax D-3005 is successfully used as the binder. The linear shrinkage of the sintered part is found to be 17% and it could be reduced when high solid loading applied. This research demonstrates a valid approach to fabricate microcomponents from metallic powder. Therefore, the process is regarded suitable for mass production of micrometallic components. Acknowledgement The authors would like to thank Sandvik Osprey Ltd. for the supply of superfine stainless steel 316L powders. The research was partly supported by the Chinese Ministry of Science and Technology on International Project 2006DFA73620. References [1] Beeby S, Ensell G, Kraft M, White N. MEMS Mechanical Sensors. London: Artech House; 2004. [2] Ranter DB, Hoffman AS, Schoen FJ, Lemons JE. Biomaterial Science. London: Academic Press; 1996.
[3] Becker EW, Ehrfeld W, Hagmann P, Maner A, Munchmeyer D. Microelectron Eng 1986;4:35–56. [4] Walker JF, Moore DF, Whitney JT. Microelectron Eng 1996;30:517–22. [5] Molpeceres C, Lauzuricaa S, García-Ballesterosa JJ, Moralesa M, Ocañaa JL. Microelectron Eng 2007;84:1337–40. [6] Liu HS, Yan BH, Huang FY, Qiu KHJ. Mater Process Technol 2005;169:418–26. [7] Liou AC, Chen RH. Int J Adv Manuf Technol 2006;28:1097–103. [8] Fu G, Loh NH, Tor SB. Mater Des 2004;24:29–33. [9] Gietzelt T, Jacobi O, Piotter V, Ruprecht R, Hausselt JJ. Mater Sci 2004;39:2113–9. [10] Moon S, Lee N, Kang S. J Micromechanics Microengineering 2003;13:98–103. [11] Kim J, Jiang K, Chang I. J Micromechanics Microengineering 2006;16:48–52. [12] Zhu Z, Wei X, Jiang K. J Micromechanics Microengineering 2007;17:193–8. [13] Zhang D, Su B, Button T. Adv Eng Mater 2003;5:924–7. [14] Cremers C, Bouamrane F, Singleton L, Schenk R. Microsyst Technol 2001;7:11–6. [15] Jin P, Jiang K, Sun N. J Microlith Microfab Microsys 2004;3:569–73. [16] Sanchez-Herencia AJ, Millan AJ, Nieto MI, Moreno R. Acta Mater 2001;49:645–51.