- Email: [email protected]
Ta multilayered films and NdFeB magnetic powder
Journal of Magnetism and Magnetic Materials 323 (2011) 2696–2700
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Fabrication of NdFeB microstructures using a silicon molding technique for NdFeB/Ta multilayered films and NdFeB magnetic powder Yonggang Jiang a,b,n, Takayuki Fujita b,c, Minoru Uehara d, Yuki Iga b, Taichi Hashimoto c, Xiuchun Hao b, Kohei Higuchi b, Kazusuke Maenaka b,c a
School of Mechanical Engineering and Automation, Beihang University, Xueyuan Road No. 37, Haidian District, Beijing 100191, China Maenaka Human-Sensing Fusion project, Japan Science and Technology Agency, 2167 Shosha, Himeji, Hyogo 671-2280, Japan c Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan d NEOMAX Co. Ltd., 2-15-17, Egawa, Shimamoto-Cho, Mishima-gun, Osaka 618-0013, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 February 2011 Received in revised form 12 May 2011 Available online 14 June 2011
The silicon molding technique is described for patterning of NdFeB/Ta multilayered magnetic films and NdFeB magnetic powder at the micron scale. Silicon trenches are seamlessly filled by 12-mm-thick NdFeB/Ta multilayered magnetic films with a magnetic retentivity of 1.3 T. The topography image and magnetic field distribution image are measured using an atomic force microscope and a magnetic force microscope, respectively. Using a silicon molding technique complemented by a lift-off process, NdFeB magnetic powder is utilized to fabricate magnetic microstructures. Silicon trenches as narrow as 20 mm are filled by a mixture of magnetic powder and wax powder. The B–H hysteresis loop of the patterned magnetic powder is characterized using a vibrating sample magnetometer, which shows a magnetic retentivity of approximately 0.37 T. & 2011 Elsevier B.V. All rights reserved.
Keywords: NdFeB Silicon molding Magnetic MEMS Magnetic powder
1. Introduction High-performance magnetic materials and microstructures have many potential applications in magnetic micro-electromechanical systems (MEMS), such as electromagnetic sensors, micro-actuators, and energy harvesting devices [1–3]. Micromachining techniques are required to integrate magnetic materials with MEMS devices. Three kinds of preparation techniques for magnetic films have been developed for magnetic MEMS applications, which include electroplating, sputtering, and packed magnetic powder. Magnetic films such as Co–Pt and NiFe have been fabricated by electroplating techniques for MEMS devices. Co80Pt20 cylinder magnets with a thickness of 2 mm fabricated by electroplating have yielded a perpendicular retentivity of 0.62 T and an energy product of 52 kJ/m3 [4,5]. NdFeB alloyed prepared by high-rate triode sputtering have achieved an energy product as high as 400 kJ/m3 [6]. NdFeB magnetic powder was used for energy harvesting application by dry packed method with compression to achieve an integrated mini-magnet with dimensions of several hundred micrometers [7]. One of our authors reported NdFeB/Ta multilayered films deposited by DC
magnetron sputtering with an excellent magnetic performance that is comparable with that of commercial sintered NdFeB magnets [8]. For micro-structuring of magnetic films, both wet chemical etching and CO/NH3 gas based reactive ion etching (RIE) methods have been reported [9–11]. However, it is difficult to achieve fine patterned microstructures by wet etching method as the isotropic etching method leads to a large sidewall etching. Carbon monoxide and ammonia based RIE is not applicable to multilayered NdFeB/Ta films due to its high selectivity between tantalum and NdFeB. In order to fabricate NdFeB magnetic structures as thick as tens of micrometers, we reported a high power plasma etching method based on Ar/Cl2 composite gases [12]. However, it is a technological challenge to precisely stop the etching at the interface of the NdFeB film and the silicon substrate. In this paper, we present a silicon molding technique for micro-structuring of NdFeB/Ta multilayered magnetic films and NdFeB magnetic powder with a feature size of several micrometers. The magnetic properties of the magnetic structures are characterized based on a magnetic force microscope (MFM) and a vibrating sample magnetometer (VSM).
2. Fabrication process n
Corresponding author at: School of Mechanical Engineering and Automation, Beihang Univeristy, 705#, Xueyuan Road No. 37, Haidian District, Beijing 100191, China. Tel./fax: þ 86 10 8231 6603. E-mail address: [email protected] (Y. Jiang). 0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.05.061
The silicon molding processes for micro-structuring of NdFeB/ Ta multilayered magnetic films and NdFeB magnetic powder are shown in Fig. 1. The starting material is a silicon substrate with a
Y. Jiang et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2696–2700
2697
Fig. 1. Silicon molding process for sputtered NdFeB/Ta multilayered magnetic films and NdFeB magnetic powder.
thickness of 525 mm (Fig.1a). The silicon substrate is coated with photoresist and patterned by photolithography (Fig. 1b). For micro-structuring of NdFeB/Ta multilayered magnetic films, deep RIE is utilized to fabricate the silicon trenches and the photoresist is removed after RIE (Fig. 1c1). NdFeB/Ta multilayered films are formed on the silicon mold by a DC magnetron sputtering system (Fig. 1d1). NdFeB microstructures are finally fabricated by a mechanical polishing process (Fig. 1e1). For NdFeB magnetic powder, the photoresist remains after the deep RIE process (Fig. 1c2). The magnetic powder (MQFP-B; Magnequench, Inc.) with an average particle diameter of 5 mm is homogeneously mixed with wax powder (0CON-196; Logitech Ltd.) loading fraction of 8 wt%. The mixture is packed into the silicon molds with compression by wiping across the wafer surface using a flat edge (Fig. 1d2). The silicon wafer is heated on a hotplate at 150 1C for 2 min to have the wax powder melted and bonded with the NdFeB powder and then cooled down rapidly. Finally, the wafer is immersed in N-methylpyrrolidone (NMP) solutions at 100 1C for 30 min to remove the photoresist and magnetic particles from the wafer surface (Fig. 1e2).
3. Results and discussion 3.1. Silicon molding process for sputtered NdFeB/Ta multilayered films After the NdFeB/Ta multilayered film with a thickness of 12 mm is formed by sputtering, the silicon substrate is diced to examine the step-coverage of the magnetic films on silicon trenches using scanning electron microscope (SEM). As shown in Fig. 2(a), silicon trenches with a width greater than 50 mm are seamlessly filled by sputtered NdFeB/Ta magnetic films. However, when the width of the silicon trenches is less than 10 mm (aspect ratio larger than 1), cracks occur in the magnetic filling process as shown in Fig. 2b. The magnetic property of the NdFeB/Ta multilayered magnetic film is plotted in Fig. 3. The retentivity and coercivity of the magnetic film are approximately 1.3 T and 1220 kA/m, respectively. Mechanical polishing process is utilized to flatten the silicon surface and form the silicon microstructures. The magnetic pattern with a width of 50 mm fabricated by silicon
Fig. 2. SEM images of Ta/NdFeB multilayered magnetic films sputtered on silicon trenches with a thickness of 12 mm: (a) silicon trench with a width of 50 mm, (b) silicon trenches with a width less than 10 mm.
mold technique without any defects as shown in Fig. 4a, whilst there are some cracks for the magnetic pattern with a width of 7 mm as shown in Fig. 4b.
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Fig. 5. Topography and MFM images of the magnetic microstructures: (a) AFM image of NdFeB microstructures (b) MFM image of magnetic field distribution at 1 mm above the surface of NdFeB microstructures. Fig. 3. B–H hysteresis loops of sputtered NdFeB/Ta multilayered magnetic films on silicon substrate.
field line of the NdFeB microstructure is narrower than its corresponding topography line, which probably originates from the oxidation of the edges of the magnetic microstructure. It is important to protect the magnetic microstructures from oxidation by sputtering another Ta or other metal layers in the future.
3.2. Silicon molding process for NdFeB magnetic powder The magnetic patterns with various geometries and dimensions are successfully fabricated using the silicon molding technique for NdFeB magnetic powder as illustrated in Fig. 6. Fig. 6(a) shows NdFeB magnetic strips with a width of 50 mm, and Fig. 6(b) shows rectangular magnetic dots with a feature size of 70 mm. The cross-sectional view of the NdFeB magnetic microstructure is investigated using SEM as shown in Fig. 7. Both Magnetic strips with a width of 100 mm (Fig. 7a) and magnetic strips with a width of 20 mm (Fig. 7b) are seamless filled by NdFeB magnetic powder with a thickness of 20 mm. To the best of our knowledge, it is the finest magnetic pattern achieved using packed magnetic powder. The magnetic property of the micro-patterned NdFeB magnetic powder is measured by VSM using a sample integrated with magnetic strips with a width of 100 mm and a thickness of 20 mm. As shown in Fig. 8, the retentivity in both the vertical direction and in-plane direction is greater than 0.37 T.
3.3. Discussion
Fig. 4. Cross-sectional SEM micrograph of the polished NdFeB magnetic layer: (a) silicon trenches with a width of 50 mm, (b) silicon trenches with a width of 7 mm.
The NdFeB/Ta multilayered microstructures are magnetized in a magnetic field with a peak magnetic flux density of 3 T by a pulse magnetizer. In order to characterize the magnetic field distribution generated by the magnetic strips, magnetic force microscopy (MFM) is carried out using an atomic force microscope (AFM) and a MFM microprobe (MFMR-10; Toyo Corporation, Japan). Fig. 5 (left) illustrates the atomic force microscope image of the surface of the magnetic strips with a width of 30 mm. Fig. 5(right) shows the MFM image of the magnetic field distribution at 1 mm above the NdFeB magnetic surface. The magnetic
Magnetic microstructures were successfully fabricated by silicon molding processes for sputtered NdFeB/Ta multilayered magnetic films and NdFeB magnetic powder. Compared to magnetic films obtained from sputtering method, magnetic microstructures fabricated from NdFeB magnetic power shows a smaller retentivity, coercivity, and energy product. It is because the magnetic poles of the NdFeB powder are randomly distributed that the B–H hysteresis loops in vertical direction and in-plane direction are almost the same, whilst the magnetic poles of the sputtered NdFeB/Ta magnetic films are aligned in a direction perpendicular to the surface of the silicon substrate. However, magnetic microstructures using NdFeB magnetic powder can be obtained at a process temperature as low as 150 1C. On the contrary, the process temperature for NdFeB sputtering is as high as 500 1C. In addition, it is very difficult to deposit NdFeB/Ta multilayered magnetic films with a thickness greater than 20 mm using magnetron sputtering. Much thicker magnetic microstructures can be fabricated using packed NdFeB powder with the silicon molding technique.
Y. Jiang et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2696–2700
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Fig. 7. Cross-sectional SEM images of magnetic powder filled in silicon trenches with a width of 100 mm (a) and 20 mm (b).
Fig. 6. Micrographs of the magnetic mictrostructures fabricated by a silicon molding process for NdFeB magnetic powder: (a) magnetic strips with a width of 50 mm, (b) rectangular magnetic pattern with a width of 70 mm.
4. Conclusions Micro-fabrication of high-performance NdFeB micro-magnets using silicon molding processes for sputtered NdFeB/Ta multilayered film and NdFeB powder was demonstrated. It has been found that the sputtering deposition of NdFeB/Ta multilayered magnetic film onto pre-patterned silicon trenches with a thickness of 12 mm works well, though small cracks occur when the width of the silicon trench is less than 10 mm. The NdFeB/Ta multilayered magnetic film obtained by sputtering shows a magnetic retentivity as high as 1.3 T. The silicon substrate with trenches filled by
Fig. 8. B–H hysteresis loops of the patterned NdFeB magnetic powder using the silicon molding technique.
NdFeB/Ta multilayered magnetic film was flattened by a mechanical polishing process. A silicon molding process complemented by a lift-off process was employed to fabricate magnetic microstructures using the mixture of NdFeB magnetic powder and wax powder. Silicon trenches with a thickness of 20 mm and widths varying from 20 to 100 mm were seamless filled by the NdFeB magnetic powder. The VSM measurement indicates that the magnetic microstructures fabricated from NdFeB magnetic powder can achieve a magnetic retentivity of 0.37 T in both vertical and
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in-plane directions. In summary, the silicon molding technique for NdFeB/Ta multilayered films and NdFeB magnetic powder was established for magnetic MEMS applications.
Acknowledgments The author would like to thank Dr. Kanda and Mr. Kasai for kind discussions and advices on device fabrication and evaluation. The author also appreciates the assistance from Mr. Olinver for proof reading the paper. References [1] Je´rˆome Orphe´e Cugat, Delamare, Gilbert Reyne, Magnetic micro-actuators and systems (MAGMAS), IEEE Trans. Magn. 39 (2003) 3607–3612. [2] K. Kobayashi, K. Ikuta, 3D magnetic microactuator made of newly developed magnetically modified photocurable polymer and application to swimming micromachine and microscrewpump, in: Proceedings of the IEEE MEMS Conference, Italy, 2009, pp. 11–14. [3] M. Tang, et al., Micromachined passive magnetostatic relays for portable applications, in: Proceedings of the IEEE MEMS Conference, Hongkong, China, 2010, pp. 779–782.
[4] I. Zana, G. Zangari, J.-W. Park, M.G. Allen, Electrodeposition of Co–Pt micronsize magnets with strong perpendicular magnetic anisotropy for MEMS applications, J. Magn. Magn. Mater. 272–276 (2004) e1775–e1776. [5] I. Zana, G. Zangari, Electrodeposited Co–Pt permanent micromagnet arrays on Cu(1 1 1)/Si(1 1 0) substrate, magnetics, IEEE Trans. Magn. 38 (5) (2002) 2544–2546. [6] B.A. Kapitanov, N.V. Kornilov, Ya.L. Linetsky, V.Yu. Tsvetkov, Sputtered permanent Nd–Fe–B magnets, J. Magn. Magn. Mater. 127 (1993) 289–297. [7] N. Wang, D.P. Arnold, Fully batch-fabricated MEMS magnetic vibrational energy harvesters, in: Proceedings of the PowerMEMS, Washington DC, USA, 2009, pp. 348–351. [8] M. Uehara, N. Gennai, M. Fujiwara, T. Tanaka, Improved perpendicular anisotropy andpermanent magnetic properties in Co-doped Nd–Fe–B films multilayered with Ta, IEEE Trans. Magn. 41 (2005) 3838–3843. [9] A. Walther, C. Marcoux, B. Desloges, R. Grechishkin, D. Givord, N.M. Dempsey, Micro-patterning of NdFeB and SmCo magnet films for integration into micro-electro-mechanical-systems, J. Magn. Magn. Mater. 321 (2009) 590–594. [10] H. Cho, K.P. Lee, Y.B. Hahn, E.S. Lambers, S.J. Pearton, Plasma etching of magnetic multilayers—effect of concurrent UV illumination, Mater. Sci. Eng. B67 (1999) 145–151. [11] S. Kishioka, R. Aogaki, J. Nakagawa, Comparison of Nd–Fe–B surface treated by chemical etching in the absence and presence of a magnetic field, Chem. Lett. B67 (1999) 589–590. [12] Y.G. Jiang, S. Masaoka, M. Uehara, T. Fujita, K. Higuchi, K. Maenaka, Microstructuring of thick NdFeB film using high-power plasma etching for magnetic MEMS application, J. Micromech. Microeng. 21 (2011) 045011.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Fabrication of NdFeB microstructures using a silicon molding technique for NdFeB/Ta multilayered films and NdFeB magnetic powder Yonggang Jiang a,b,n, Takayuki Fujita b,c, Minoru Uehara d, Yuki Iga b, Taichi Hashimoto c, Xiuchun Hao b, Kohei Higuchi b, Kazusuke Maenaka b,c a
School of Mechanical Engineering and Automation, Beihang University, Xueyuan Road No. 37, Haidian District, Beijing 100191, China Maenaka Human-Sensing Fusion project, Japan Science and Technology Agency, 2167 Shosha, Himeji, Hyogo 671-2280, Japan c Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan d NEOMAX Co. Ltd., 2-15-17, Egawa, Shimamoto-Cho, Mishima-gun, Osaka 618-0013, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 February 2011 Received in revised form 12 May 2011 Available online 14 June 2011
The silicon molding technique is described for patterning of NdFeB/Ta multilayered magnetic films and NdFeB magnetic powder at the micron scale. Silicon trenches are seamlessly filled by 12-mm-thick NdFeB/Ta multilayered magnetic films with a magnetic retentivity of 1.3 T. The topography image and magnetic field distribution image are measured using an atomic force microscope and a magnetic force microscope, respectively. Using a silicon molding technique complemented by a lift-off process, NdFeB magnetic powder is utilized to fabricate magnetic microstructures. Silicon trenches as narrow as 20 mm are filled by a mixture of magnetic powder and wax powder. The B–H hysteresis loop of the patterned magnetic powder is characterized using a vibrating sample magnetometer, which shows a magnetic retentivity of approximately 0.37 T. & 2011 Elsevier B.V. All rights reserved.
Keywords: NdFeB Silicon molding Magnetic MEMS Magnetic powder
1. Introduction High-performance magnetic materials and microstructures have many potential applications in magnetic micro-electromechanical systems (MEMS), such as electromagnetic sensors, micro-actuators, and energy harvesting devices [1–3]. Micromachining techniques are required to integrate magnetic materials with MEMS devices. Three kinds of preparation techniques for magnetic films have been developed for magnetic MEMS applications, which include electroplating, sputtering, and packed magnetic powder. Magnetic films such as Co–Pt and NiFe have been fabricated by electroplating techniques for MEMS devices. Co80Pt20 cylinder magnets with a thickness of 2 mm fabricated by electroplating have yielded a perpendicular retentivity of 0.62 T and an energy product of 52 kJ/m3 [4,5]. NdFeB alloyed prepared by high-rate triode sputtering have achieved an energy product as high as 400 kJ/m3 [6]. NdFeB magnetic powder was used for energy harvesting application by dry packed method with compression to achieve an integrated mini-magnet with dimensions of several hundred micrometers [7]. One of our authors reported NdFeB/Ta multilayered films deposited by DC
magnetron sputtering with an excellent magnetic performance that is comparable with that of commercial sintered NdFeB magnets [8]. For micro-structuring of magnetic films, both wet chemical etching and CO/NH3 gas based reactive ion etching (RIE) methods have been reported [9–11]. However, it is difficult to achieve fine patterned microstructures by wet etching method as the isotropic etching method leads to a large sidewall etching. Carbon monoxide and ammonia based RIE is not applicable to multilayered NdFeB/Ta films due to its high selectivity between tantalum and NdFeB. In order to fabricate NdFeB magnetic structures as thick as tens of micrometers, we reported a high power plasma etching method based on Ar/Cl2 composite gases [12]. However, it is a technological challenge to precisely stop the etching at the interface of the NdFeB film and the silicon substrate. In this paper, we present a silicon molding technique for micro-structuring of NdFeB/Ta multilayered magnetic films and NdFeB magnetic powder with a feature size of several micrometers. The magnetic properties of the magnetic structures are characterized based on a magnetic force microscope (MFM) and a vibrating sample magnetometer (VSM).
2. Fabrication process n
Corresponding author at: School of Mechanical Engineering and Automation, Beihang Univeristy, 705#, Xueyuan Road No. 37, Haidian District, Beijing 100191, China. Tel./fax: þ 86 10 8231 6603. E-mail address: [email protected] (Y. Jiang). 0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.05.061
The silicon molding processes for micro-structuring of NdFeB/ Ta multilayered magnetic films and NdFeB magnetic powder are shown in Fig. 1. The starting material is a silicon substrate with a
Y. Jiang et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2696–2700
2697
Fig. 1. Silicon molding process for sputtered NdFeB/Ta multilayered magnetic films and NdFeB magnetic powder.
thickness of 525 mm (Fig.1a). The silicon substrate is coated with photoresist and patterned by photolithography (Fig. 1b). For micro-structuring of NdFeB/Ta multilayered magnetic films, deep RIE is utilized to fabricate the silicon trenches and the photoresist is removed after RIE (Fig. 1c1). NdFeB/Ta multilayered films are formed on the silicon mold by a DC magnetron sputtering system (Fig. 1d1). NdFeB microstructures are finally fabricated by a mechanical polishing process (Fig. 1e1). For NdFeB magnetic powder, the photoresist remains after the deep RIE process (Fig. 1c2). The magnetic powder (MQFP-B; Magnequench, Inc.) with an average particle diameter of 5 mm is homogeneously mixed with wax powder (0CON-196; Logitech Ltd.) loading fraction of 8 wt%. The mixture is packed into the silicon molds with compression by wiping across the wafer surface using a flat edge (Fig. 1d2). The silicon wafer is heated on a hotplate at 150 1C for 2 min to have the wax powder melted and bonded with the NdFeB powder and then cooled down rapidly. Finally, the wafer is immersed in N-methylpyrrolidone (NMP) solutions at 100 1C for 30 min to remove the photoresist and magnetic particles from the wafer surface (Fig. 1e2).
3. Results and discussion 3.1. Silicon molding process for sputtered NdFeB/Ta multilayered films After the NdFeB/Ta multilayered film with a thickness of 12 mm is formed by sputtering, the silicon substrate is diced to examine the step-coverage of the magnetic films on silicon trenches using scanning electron microscope (SEM). As shown in Fig. 2(a), silicon trenches with a width greater than 50 mm are seamlessly filled by sputtered NdFeB/Ta magnetic films. However, when the width of the silicon trenches is less than 10 mm (aspect ratio larger than 1), cracks occur in the magnetic filling process as shown in Fig. 2b. The magnetic property of the NdFeB/Ta multilayered magnetic film is plotted in Fig. 3. The retentivity and coercivity of the magnetic film are approximately 1.3 T and 1220 kA/m, respectively. Mechanical polishing process is utilized to flatten the silicon surface and form the silicon microstructures. The magnetic pattern with a width of 50 mm fabricated by silicon
Fig. 2. SEM images of Ta/NdFeB multilayered magnetic films sputtered on silicon trenches with a thickness of 12 mm: (a) silicon trench with a width of 50 mm, (b) silicon trenches with a width less than 10 mm.
mold technique without any defects as shown in Fig. 4a, whilst there are some cracks for the magnetic pattern with a width of 7 mm as shown in Fig. 4b.
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Fig. 5. Topography and MFM images of the magnetic microstructures: (a) AFM image of NdFeB microstructures (b) MFM image of magnetic field distribution at 1 mm above the surface of NdFeB microstructures. Fig. 3. B–H hysteresis loops of sputtered NdFeB/Ta multilayered magnetic films on silicon substrate.
field line of the NdFeB microstructure is narrower than its corresponding topography line, which probably originates from the oxidation of the edges of the magnetic microstructure. It is important to protect the magnetic microstructures from oxidation by sputtering another Ta or other metal layers in the future.
3.2. Silicon molding process for NdFeB magnetic powder The magnetic patterns with various geometries and dimensions are successfully fabricated using the silicon molding technique for NdFeB magnetic powder as illustrated in Fig. 6. Fig. 6(a) shows NdFeB magnetic strips with a width of 50 mm, and Fig. 6(b) shows rectangular magnetic dots with a feature size of 70 mm. The cross-sectional view of the NdFeB magnetic microstructure is investigated using SEM as shown in Fig. 7. Both Magnetic strips with a width of 100 mm (Fig. 7a) and magnetic strips with a width of 20 mm (Fig. 7b) are seamless filled by NdFeB magnetic powder with a thickness of 20 mm. To the best of our knowledge, it is the finest magnetic pattern achieved using packed magnetic powder. The magnetic property of the micro-patterned NdFeB magnetic powder is measured by VSM using a sample integrated with magnetic strips with a width of 100 mm and a thickness of 20 mm. As shown in Fig. 8, the retentivity in both the vertical direction and in-plane direction is greater than 0.37 T.
3.3. Discussion
Fig. 4. Cross-sectional SEM micrograph of the polished NdFeB magnetic layer: (a) silicon trenches with a width of 50 mm, (b) silicon trenches with a width of 7 mm.
The NdFeB/Ta multilayered microstructures are magnetized in a magnetic field with a peak magnetic flux density of 3 T by a pulse magnetizer. In order to characterize the magnetic field distribution generated by the magnetic strips, magnetic force microscopy (MFM) is carried out using an atomic force microscope (AFM) and a MFM microprobe (MFMR-10; Toyo Corporation, Japan). Fig. 5 (left) illustrates the atomic force microscope image of the surface of the magnetic strips with a width of 30 mm. Fig. 5(right) shows the MFM image of the magnetic field distribution at 1 mm above the NdFeB magnetic surface. The magnetic
Magnetic microstructures were successfully fabricated by silicon molding processes for sputtered NdFeB/Ta multilayered magnetic films and NdFeB magnetic powder. Compared to magnetic films obtained from sputtering method, magnetic microstructures fabricated from NdFeB magnetic power shows a smaller retentivity, coercivity, and energy product. It is because the magnetic poles of the NdFeB powder are randomly distributed that the B–H hysteresis loops in vertical direction and in-plane direction are almost the same, whilst the magnetic poles of the sputtered NdFeB/Ta magnetic films are aligned in a direction perpendicular to the surface of the silicon substrate. However, magnetic microstructures using NdFeB magnetic powder can be obtained at a process temperature as low as 150 1C. On the contrary, the process temperature for NdFeB sputtering is as high as 500 1C. In addition, it is very difficult to deposit NdFeB/Ta multilayered magnetic films with a thickness greater than 20 mm using magnetron sputtering. Much thicker magnetic microstructures can be fabricated using packed NdFeB powder with the silicon molding technique.
Y. Jiang et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2696–2700
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Fig. 7. Cross-sectional SEM images of magnetic powder filled in silicon trenches with a width of 100 mm (a) and 20 mm (b).
Fig. 6. Micrographs of the magnetic mictrostructures fabricated by a silicon molding process for NdFeB magnetic powder: (a) magnetic strips with a width of 50 mm, (b) rectangular magnetic pattern with a width of 70 mm.
4. Conclusions Micro-fabrication of high-performance NdFeB micro-magnets using silicon molding processes for sputtered NdFeB/Ta multilayered film and NdFeB powder was demonstrated. It has been found that the sputtering deposition of NdFeB/Ta multilayered magnetic film onto pre-patterned silicon trenches with a thickness of 12 mm works well, though small cracks occur when the width of the silicon trench is less than 10 mm. The NdFeB/Ta multilayered magnetic film obtained by sputtering shows a magnetic retentivity as high as 1.3 T. The silicon substrate with trenches filled by
Fig. 8. B–H hysteresis loops of the patterned NdFeB magnetic powder using the silicon molding technique.
NdFeB/Ta multilayered magnetic film was flattened by a mechanical polishing process. A silicon molding process complemented by a lift-off process was employed to fabricate magnetic microstructures using the mixture of NdFeB magnetic powder and wax powder. Silicon trenches with a thickness of 20 mm and widths varying from 20 to 100 mm were seamless filled by the NdFeB magnetic powder. The VSM measurement indicates that the magnetic microstructures fabricated from NdFeB magnetic powder can achieve a magnetic retentivity of 0.37 T in both vertical and
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in-plane directions. In summary, the silicon molding technique for NdFeB/Ta multilayered films and NdFeB magnetic powder was established for magnetic MEMS applications.
Acknowledgments The author would like to thank Dr. Kanda and Mr. Kasai for kind discussions and advices on device fabrication and evaluation. The author also appreciates the assistance from Mr. Olinver for proof reading the paper. References [1] Je´rˆome Orphe´e Cugat, Delamare, Gilbert Reyne, Magnetic micro-actuators and systems (MAGMAS), IEEE Trans. Magn. 39 (2003) 3607–3612. [2] K. Kobayashi, K. Ikuta, 3D magnetic microactuator made of newly developed magnetically modified photocurable polymer and application to swimming micromachine and microscrewpump, in: Proceedings of the IEEE MEMS Conference, Italy, 2009, pp. 11–14. [3] M. Tang, et al., Micromachined passive magnetostatic relays for portable applications, in: Proceedings of the IEEE MEMS Conference, Hongkong, China, 2010, pp. 779–782.
[4] I. Zana, G. Zangari, J.-W. Park, M.G. Allen, Electrodeposition of Co–Pt micronsize magnets with strong perpendicular magnetic anisotropy for MEMS applications, J. Magn. Magn. Mater. 272–276 (2004) e1775–e1776. [5] I. Zana, G. Zangari, Electrodeposited Co–Pt permanent micromagnet arrays on Cu(1 1 1)/Si(1 1 0) substrate, magnetics, IEEE Trans. Magn. 38 (5) (2002) 2544–2546. [6] B.A. Kapitanov, N.V. Kornilov, Ya.L. Linetsky, V.Yu. Tsvetkov, Sputtered permanent Nd–Fe–B magnets, J. Magn. Magn. Mater. 127 (1993) 289–297. [7] N. Wang, D.P. Arnold, Fully batch-fabricated MEMS magnetic vibrational energy harvesters, in: Proceedings of the PowerMEMS, Washington DC, USA, 2009, pp. 348–351. [8] M. Uehara, N. Gennai, M. Fujiwara, T. Tanaka, Improved perpendicular anisotropy andpermanent magnetic properties in Co-doped Nd–Fe–B films multilayered with Ta, IEEE Trans. Magn. 41 (2005) 3838–3843. [9] A. Walther, C. Marcoux, B. Desloges, R. Grechishkin, D. Givord, N.M. Dempsey, Micro-patterning of NdFeB and SmCo magnet films for integration into micro-electro-mechanical-systems, J. Magn. Magn. Mater. 321 (2009) 590–594. [10] H. Cho, K.P. Lee, Y.B. Hahn, E.S. Lambers, S.J. Pearton, Plasma etching of magnetic multilayers—effect of concurrent UV illumination, Mater. Sci. Eng. B67 (1999) 145–151. [11] S. Kishioka, R. Aogaki, J. Nakagawa, Comparison of Nd–Fe–B surface treated by chemical etching in the absence and presence of a magnetic field, Chem. Lett. B67 (1999) 589–590. [12] Y.G. Jiang, S. Masaoka, M. Uehara, T. Fujita, K. Higuchi, K. Maenaka, Microstructuring of thick NdFeB film using high-power plasma etching for magnetic MEMS application, J. Micromech. Microeng. 21 (2011) 045011.