Ta multilayered films utilizing laser assisted heating

Sensors and Actuators A 220 (2014) 298–304

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Micromagnetization patterning of sputtered NdFeB/Ta multilayered films utilizing laser assisted heating Ryogen Fujiwara a,b , Tadahiko Shinshi c,∗ , Elito Kazawa d a

Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama, Japan Research Fellow of Japan Society for the Promotion of Science Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, Japan d Research and Development Department, Tokyo Metropolitan Industrial Technology Research Institute, Tokyo, Japan b c

a r t i c l e

i n f o

Article history: Received 5 June 2014 Received in revised form 12 September 2014 Accepted 11 October 2014 Available online 20 October 2014 Keywords: MEMS Permanent magnet film Micromagnetization patterning Laser heating

a b s t r a c t A process for magnetizing small areas of a multilayered several micrometer thick NdFeB/Ta permanent magnet (PM) deposited by magnetron sputtering was developed. In this process, areas of the PM film heated by laser beam scanning were magnetized under an external DC magnetic field. First, by measuring the demagnetization curve at room temperature and 300 ◦ C, the target temperature and external DC magnetic field were determined. These were found to be 300 ◦ C and 0.8–0.9 T, respectively. Then, in order to satisfy these requirements, the heat conduction during laser heating was simulated, resulting in glass being chosen as the substrate material and, in addition, determining the laser beam scanning conditions. We also fabricated a DC magnetic field generator consisting of an array of bulk magnets which was designed using a magnetic field simulator. By adjusting the laser beam trajectories, magnetization patterns consisting of stripes with a width of 100 ␮m were experimentally generated in the 4.5 ␮m thick PM film. Agreement between the measured and simulated magnetic flux densities over the PM film was obtained. Furthermore, a 70 ␮m wide striped pattern, a 100 ␮m diameter dot pattern and a pattern consisting of a set of letters were also generated in the PM film. The desired magnetic field pattern could be successfully observed. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Some current cell phone camera units require advanced functions, such as optical auto-focus (AF), optical image stabilization (OIS) and zooming. Usually, the AF and OIS functions are obtained using a multi-DOF actuator to move the camera lens backwards and forwards, and in the plane of the lens voice coil motor (VCM) based actuators using bulk permanent magnets (PM) are widely used for this [1–3]. Nevertheless, the use of bulk PMs and the time-consuming process required to assemble the many micro parts limits the ability to miniaturize the device and reduce the cost. The use of MEMS fabrication technologies is one of the promising ways of realizing micromechanical devices at low cost and with high productivity. Recently, because of these advantages, many MEMS-based lens drive actuators for cell phone camera units have been developed [4,5]. However, most of these actuators are driven

∗ Corresponding author. Tel.: +81 45 924 5095; fax: +81 45 924 5046. E-mail address: [email protected] (T. Shinshi). http://dx.doi.org/10.1016/j.sna.2014.10.011 0924-4247/© 2014 Elsevier B.V. All rights reserved.

by electrostatic or thermal forces, and they require high voltages (>20 V). The lithium-ion batteries in cell phones are limited to voltages below 3.7 V, so additional special power-consuming booster circuits are required. The goal of this research is to realize a MEMS-based electromagnetic multi-degree of freedom lens drive actuator consisting of a MEMS-based PM film and micro coils which can be operated using the available battery voltage. The proposed actuator is illustrated in Fig. 1. It has been shown recently that some types of PM film have almost the same magnetic properties as bulk sintered neodymium magnets [6,7]. These PM films are deposited by MEMS compatible magnetron sputtering processes. Nevertheless, in order to generate a large electromagnetic force, the demagnetization field generated by the PM film needs to be reduced. Micromagnetization pattering of the PM film is one possible solution. In order to minimize the demagnetization field in a PM and effectively use a PM film of several ␮m thickness, the aspect ratio (magnetized width-to-PM film thickness) should be close to 1.0. However, micromagnetization technology with a patterning resolution similar to the thickness of a high coercivity PM film has yet to be developed. Our group has investigated 500 ␮m

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299

Magnetized direction

PM film N S

N S

N S

Pulsed current Coil 1. PM film deposition and pulse magnetization External DC magnetic field N S

N N S S S S S Bulk PM Bulk PM N N N 2. Application of external DC magnetic field Fig. 1. Lens drive actuator for smartphone camera.

2. Micromagnetization patterning assisted by laser heating 2.1. Principle of micromagnetization Fig. 2 shows the principle of micromagnetization patterning. In the first step, a PM film is deposited onto a substrate and magnetized using conventional pulse magnetization in the vertical direction. Then, the PM film is partly and momentarily heated by laser beam scanning under an external DC magnetic field. The direction of the DC magnetic field is opposite to that of the initial magnetization. The coercive force in the heated parts of the PM film is weakened and these parts are magnetized by the external DC magnetic field. After cooling, the heated parts retain the reversed magnetization. 2.2. Target values of the heating temperature and the applied magnetic field In order to realize micromagnetization patterning as shown in Fig. 2, full magnetization of the heated parts of the substrate is required, but irreversible demagnetization of the non-heated parts

N S

S N

N S

4. Partial magnetization reversal N S N S N S N S N S N S N S 5. Micro magnetization Fig. 2. Principle of the micro magnetization assisted by laser heating.

needs to be avoided. Therefore, an examination of the temperature and magnetic field intensity needed for magnetization is necessary. Fig. 3 shows the demagnetization curves of a 4.5 ␮m thick PM film sputtered onto a 4 mm × 4 mm × 0.2 mm glass substrate at 22 ◦ C and 300 ◦ C measured using a vibrating sample magnetometer (VSM). Correction of the demagnetization effect was not carried out in Fig. 3. As shown in Fig. 3, in order to avoid irreversible demagnetization of the non-heated parts of the film, the reverse DC magnetic flux density should be less than 0.94 T. When the PM film is heated to 300 ◦ C, a reverse magnetic flux density of more than 0.63 T is sufficient for magnetic saturation even if there is a demagnetization field. Therefore, for magnetic field saturation, the target heating temperature and the target magnetic flux density are 300 ◦ C and from 0.63 to 0.94 T, respectively. Although the coercive force increases after laser heating, there is a possibility of irreversible demagnetization during cooling. In order to avoid this, the applied DC magnetic flux density should be large. Finally, we determined the target heating temperature and the target applied magnetic flux density to be 300 ◦ C and 0.8–0.9 T, respectively.

Magnetization [T]

wide micromagnetization patterning of a 6 ␮m thick multilayered NdFeB/Ta film using a pulsed magnetic field generated by a current of 4800 A through a meandering coil with a diameter of ␾0.26 mm [8]. However, due to the current limit and heat resistance in the coil, it is difficult to get magnetization patterns with a narrower width using a micro coil. Another group has examined micromagnetization patterning of a 4 ␮m thick sputtered NdFeB PM film using single pulsed laser irradiation through a photomask under an external DC magnetic field [9]. However, in this work, the temperature distribution in the PM film during heating was not considered, the thermal effect of the substrate material was not investigated, and the depth of the magnetized PM film was limited to 1.1–1.2 ␮m. The purpose of this study is to achieve micromagnetization patterning of a several micrometer thick PM film with widths of less than a hundred ␮m. We used a high performance multilayered NdFeB/Ta PM film with a high coercive force of 810 kA/m [6]. In this paper, we examine a micromagnetization patterning process using focused laser beam scanning under an external DC magnetic field. The basic principle of micromagnetization is the same as that with magneto-optical (MO) disks [10]. However, our PM film is about a hundred times thicker than a MO disk, and the Curie temperature is also higher, so that the micromagnetization method must take account of the thermal conduction. We used heat conduction analysis to investigate the effect of the substrate material on the temperature distribution during laser beam scanning, and determined the laser beam scanning conditions.

Laser beam N N S S S S S Bulk PM Bulk PM N N N 3. Laser heating

1.0

22℃

0.5 300℃

0.0 -0.5

-0.94T in air

-0.63T in air

-1 -0.75 -0.5 0 1 Applied magnetic field [MA/m] Fig. 3. Demagnetization curve of the PM film at 22 ◦ C and 300 ◦ C.

300

R. Fujiwara et al. / Sensors and Actuators A 220 (2014) 298–304

Reflecvity %

80 70 60 50 40 30 250

500

750

Fig. 5. Analytical model.

1000

Wavelength nm Fig. 4. Measured reflectivity of the PM film.

3. Micromagnetization method 3.1. Requirements of micromagnetization In order to realize the proposed method for micromagnetization pattering assisted by laser heating, a laser marking system combined with a DC magnetic field generator was prepared. Furthermore, parameters such as the laser power, scanning speed, etc. and the substrate material should be examined by thermal analysis. The requirements of the micromagnetization test are to achieve a temperature of more than 300 ◦ C, a DC magnetic flux density between 0.8 and 0.9 T and a processing range of more than 4 mm × 4 mm.

Fig. 6. Laser beam intensity distribution. Table 2 Physical properties related with heat conduction.

3.2. Laser heating method 3.2.1. Laser source In order to choose the laser wavelength, which is related to the absorptivity, the reflectivity of the PM film was measured using a spectrophotometer (Shimadzu Scientific Instruments, Solidspec 3700DUV). The relationship between reflectivity and wavelength is shown in Fig. 4, which shows that the reflectivity increases with wavelength. We chose a YVO4 laser marker (Keyence Corp., MD-S9910) with a wavelength of 532 nm and a pulse repetition frequency of 400 kHz. Its minimum laser spot diameter of 25 ␮m is suitable for heating a small area. The laser power (0–6 W, with a resolution of 0.001 W), the scanning trajectory (area of 120 mm × 120 mm × 42 mm) and scanning speed (maximum 12 m/s) can all be adjusted. 3.2.2. Analytical model for heat conduction Fig. 5 shows the analytical model for heat conduction. In this model, the laser beam heats the upper surface of the PM film deposited on the substrate. Table 1 shows the model conditions. Model 1 is to investigate the effect of the substrate material. Model

Thermal conductivity Specific heat Density

PM film

Silicon

Glass

9 W/(m K) 500 J/(kg K) 7600 kg/m3

124 W/(m K) 700 J/(kg K) 2330 kg/m3

1.35 W/(m K) 710 J/(kg K) 2200 kg/m3

2 is to determine the laser power and scanning speed. To simplify the model, the Ta layers between the NdFeB ones were ignored because the Ta layer thickness of 10 nm is much smaller than those of the 200 nm thick NdFeB layers. The laser beam was modeled as a constant heat flux. As shown in Fig. 6, the intensity distribution of the laser beam was measured by a beam profiler (Ophir Optronics Solutions Ltd., BGP-USB-SP620) and this was used in the simulation. The absorptivity of the laser beam was set to 50%, assuming that the laser beam does not penetrate the PM film. Table 2 shows the physical properties of candidate substrates and the PM film. 3.2.3. Analysis for determining the substrate material In Model 1, a fixed laser beam continuously heats the center part of the upper surface of the PM film. If the temperature of the PM film cannot be increased to more than 300 ◦ C within 0.5 ms, we

Table 1 Model conditions.

Heat source Laser beam

Substrate Initial temperature Boundary conditions Size

Power Absorptivity Spot diameter

Laser heating area Interface between PM film and substrate Outside surface excluding laser heating area w1 w2 hs hpm

Model 1

Model 2

Non-moving laser beam

Scanning laser beam (speed of 100 mm/s)

0.21 W 50% 25 ␮m (a) Silicon 20 ◦ C Constant heat flux No thermal contact resistance Adiabatic 2 mm 2 mm 0.2 mm 4.5 ␮m

(b) Glass

0.5 mm 0.5 mm 0.2 mm 4.5 ␮m

Glass

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301

Fig. 7. Element of Model 1. Fig. 9. Element of Model 2.

decide the substrate material is unsuitable for heating. Since (a) silicon and (b) glass are often used in the MEMS field we used these as substrate materials in the simulation. The laser power was set to be 0.21 W in this simulation. We used multiphysics simulation software (COMSOL AB, COMSOL Multiphysics Ver. 4.3b). Fig. 7 shows the triangular-prism elements generated. The element size in the laser heating area is approximately 2 ␮m in width and 0.9 ␮m in height. The number of elements and step time were 204,280 and 0.01 ms, respectively. Fig. 8 shows magnified cross-sectional views of the simulated temperature distributions at t = 0.1, 0.3, and 0.5 ms. As shown in Fig. 8(a), the temperature at the interface between the PM film and the silicon substrate reached only 40 ◦ C. Furthermore, the temperature almost saturated within 0.5 ms because the heat flux flows to the substrate which has a relatively large volume. The insufficient heating is due to the high thermal conductivity of the silicon substrate which acts as a heat sink. On the other hand, as shown in

Fig. 8. Simulated temperature distribution for determining the substrate material.

Fig. 8(b), the PM film on the glass substrate can be heated above the target temperature of 300 ◦ C. These results suggested that the low thermal conductivity of the glass substrate supports the heating of the PM. Therefore, we chose glass as the substrate material. 3.2.4. Analysis for determining the laser beam parameters In Model 2, the scanning laser beam heats the PM film deposited on the glass substrate. In order to realize the target temperature of 300 ◦ C, we can adjust the laser power and the scanning speed. In this simulation, the laser power and scanning speed were set to 0.21 W and 100 mm/s, respectively. The other conditions are the same as the simulation in Section 3.2.3. Fig. 9 shows the triangular-prism elements generated. The element size in the laser heating area is approximately 2 ␮m in width and 0.9 ␮m in height. The number of elements and step time were 299,730 and 0.01 ms, respectively. The simulated temperature distribution at t = 2.02 ms when the cross-sectional part reached its maximum temperature is illustrated in Fig. 10, which shows the temperature exceeds 300 ◦ C in a 25 ␮m wide region in the 4.5 ␮m thick PM film. Fig. 10 also shows the temperature distribution at t = 2.24 ms when the 70 ␮m wide region had reached its maximum temperature. This figure is used in the discussion in Section 4.2. The laser beam scanning conditions used in this simulation were used in the subsequent patterning experiments.

Fig. 10. Simulated temperature distribution.

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Fig. 11. DC magnetic field generator.

3.3. Method for applying the DC magnetic field In order to apply the target magnetic flux density to the PM film, a special DC magnetic field generator is necessary. The requirements are that: (1) it can generate a uniform magnetic flux density of 0.8–0.9 T in a 4 mm × 4 mm × 4.5 ␮m region, and (2) the PM film can be irradiated with a laser beam in the DC magnetic field. We decided to design the DC magnetic field generator using bulk PMs instead of a coil in order to avoid heat generation and heat conduction from the coil. We used three-dimensional magnetostatic analysis software (ANSYS, Inc., Maxwell 3D Ver. 15) to design the DC magnet field generator, which consists of seven 10 mm × 10 mm × 30 mm neodymium magnets as shown in Fig. 11(a). The PMs are arranged in a U-shaped Halbach array. The 4 mm × 4 mm × 4.5 ␮m PM film is placed in the PM array, and can be irradiated by the laser beam. Fig. 11(b) shows a cross-section of the simulated magnetic flux density distribution. Fig. 11(c) shows the magnetic flux densities in the X and Z directions in the PM film. A magnetic flux density of 0.853–0.863 T in the Z direction was obtained. The direction of the magnetic field is almost uniform because the magnetic flux density in the X direction is almost zero. Fig. 11(d) shows the assembled PM array fixed in a nonmagnetic stainless steel jig. The magnetic

flux density generated at the center of the PM film measured by a Gauss meter (F.W. BELL, Model 6010) and a Hall probe (F.W. BELL, HAD61-2508-05-T) was 0.84 T. 4. Micromagnetization experiments 4.1. Laser scanning trajectory As shown in Fig. 12, we aimed to produce 100 ␮m wide magnetization patterns in a 4.5 ␮m thick PM film on a 4 mm × 4 mm × 0.2 mm glass substrate (Matsunami Glass Ind., Ltd., silica glass). Because the heat-affected zone, which is the same as the magnetization width, is uncertain, an investigation to determine a suitable laser beam trajectory should be done. Fig. 13(a)–(c) shows the experimental trajectories of the center of the laser beam for the target magnetization pattern. According to the results of thermal analysis, a 25 ␮m wide stripe in the PM film can be heated to over 300 ◦ C by single line scanning of the laser beam. Therefore, the scanning pitch of the laser beam was set to be less than 25 ␮m. By scanning the PM film with the laser beam as shown in the schematics in Fig. 13(a)–(c), 95 ␮m, 65 ␮m, and 55 ␮m wide stripes, respectively, were heated to over 300 ◦ C. According to the results of thermal analysis, although the paths of the laser beam

(Not to scale) 4mm

100μm

100μm O

PM film

4mm

4.5μm

Residual magnetic flux density: 1.18 T Coercive force: 810 kA/m (Not to scale)

N S N S 100μm

…

Magnetized direction (a) Ge ne ral vie w

(b) T op vi ew Fig. 12. Target magnetization.

N S

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200μm 25μm 20μm 25μm

Laser beam (φ25μm) … 95μm

N 70μm

200μm 20μm 20μm N

Laser beam (φ25μm) …

Laser beam (φ25μm) …

200μm 15μm 15μm N 30μm

65μm

40μm

(a) Trajectory A

303

(b) Trajectory B

55μm

(c) Trajectory C

Fig. 13. Laser beam trajectories.

partly overlap, the temperature of the film in the overlapping areas is sufficiently reduced by the interval between each pass of the laser beam, and we were unable to detect any damage to the bonding at the interface. The laser setting for the experiment was the same as that in the analysis.

(0.84 T in air), the magnetization cannot be reversed at 130 ◦ C. In order to clarify the reason for this, the precise magnetization state at the boundary between the heated and non-heated areas needs to be investigated in the future. 4.3. Narrower and more complex micromagnetization patterns

4.2. Evaluation and discussion

Hall element Asahi Kasei Microdevices Corporation, HG-0711 Active area (50μm × 50μm) Hall probe

Moving direction

Piezo motor Base

Fig. 14. Magnetic flux density measurement setup.

Magnec flux density [mT]

Magnec flux density [mT]

By controlling the laser scanning trajectory, we can realize narrower and more complex magnetization patterns. According to the results of Section 4.2, we can produce a 70 ␮m wide striped magnetization pattern by setting the laser beam scanning pitch to 140 ␮m. We also realized magnetization patterns consisting of 100 ␮m diameter dots and a set of letters.

Trajectory A Trajectory B

6

Trajectory C Simulaon

4 2 0 -2 -4 -6

-2

4

-1

0 Posion [mm]

1

2

Simulation C B A

140 μm 110 μm

2 0

60μm

-2

90μm

-4 -0.5

-0.4

-0.3 -0.2 Posion [mm]

-0.1

0.0

Fig. 15. Magnetic flux density distribution.

Magnezaon [T]

Micro magnetized PM film

100μm

To clarify the magnetization of the PM film, the magnetic flux density distribution perpendicular to the PM film surface was measured by sliding a Hall element (Asahi Kasei Microdevices Corp., HG-0711) across the PM film. The Hall element has an active area of 50 ␮m × 50 ␮m and a sensitivity of 1.31 V/T, and the distance between the top surface of the PM film and the active area of the Hall element was 100 ␮m. Fig. 14 shows the measurement setup. The Hall element was fixed and the micromagnetized PM film was positioned using a piezo motor (Physik Instrumente GmbH & Co. KG, M227.25) with a speed of 10 mm/min. An analog low pass filter with a cut-off frequency of 10 Hz was used to reduce the signal noise. Fig. 15 compares the measured magnetic flux density distributions with one simulated by three-dimensional magnetostatic analysis software (ANSYS, Inc., Maxwell 3D Ver. 15). In the simulation model, the PM film is ideally magnetized as shown in Fig. 12. The residual flux density of 1.18 T and the coercive force of 810 kA/m used for the simulation were measured using a VSM. Fig. 15 shows three types of experimental magnetic field distribution generated by laser assisted magnetization and a simulated one. The experimental result using Trajectory C with a laser scanning width of 55 ␮m coincides with the design, which has a 100 ␮m wide striped magnetization pattern. On the other hand, the experimental result using Trajectory A with a laser scanning width of 95 ␮m is far from the designed values. These results suggest that the PM film outside the laser scanning area is magnetized even if the temperature there is less than 300 ◦ C. The simulated maximum temperature at the edge of magnetization reversal is 130 ◦ C as shown in Fig. 10. However, in practice, this temperature was too low to reverse the magnetization. As shown in Fig. 16, at a temperature of 150 ◦ C, a magnetic field of 1.6 mA/m (2 T in air) is needed to saturate the magnetization. Even if the demagnetization field is zero, a magnetic field of 0.88 mA/m (1.1 T in air) is still required. However, because the applied magnetic field is only 0.67 mA/m

1.0

150oC

0.5 0.0 -0.5 -0.9 -1.0 -2 -1.6 -1.26T in air

-1

0

1

Applied field [MA/m]

Fig. 16. Magnetization curve of the PM film at 150 ◦ C.

2

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References

Fig. 17. Photograph of the observed magnetic field.

In order to detect the magnetization quantitatively, we utilized the Faraday effect to observe the magnetic field generated by the micromagnetized PM film. To visualize the magnetic field, we put a magneto-optical sensor (Matesy GmbH, MO sensor Type A) directly on the film. Two-dimensional images of the magnetic field at the magneto-optical layer level were obtained by observing the polarization phenomenon through a polarizing microscope (Olympus Corp., BX51). Fig. 17(a)–(c) shows photographs of the observed magnetic field. The desired magnetic field patterns were successfully obtained. We can confirm that using our micromagnetization technology complicated magnetization patterns, which are difficult to achieve using conventional pulse magnetization technology, can be realized. 5. Conclusion The purpose of this study was to achieve few hundred ␮m wide micromagnetization patterns in a several micrometer thick sputtered PM film. We proposed and examined a micromagnetization patterning process that uses laser beam scanning under an external DC magnetic field. The substrate material for the PM film deposition and the laser beam scanning conditions were determined by simulating the temperature distribution in the film. The DC magnetic field was produced by an array of bulk PMs designed using a magnetic field simulator. A magnetization pattern comprising 100 ␮m wide stripes was generated in the 4.5 ␮m thick PM film. Agreement between the measured and simulated magnetic flux densities over the PM film was obtained. Furthermore, we also generated a 70 ␮m wide striped pattern, a 100 ␮m diameter dot pattern and a pattern comprising a set of letters in the PM film. The desired magnetic field patterns were successfully observed. In future work, we will investigate the precise magnetization state at the boundary between the heated and non-heated areas. We intend to use this technology to realize a multi-degree of freedom lens drive actuator for cell phone camera units, and we are planning to make magnetization patterns as small as the spot size of the laser beam by adjusting the laser heating conditions and utilizing heat insulating material. In addition we propose to investigate the damage to the PM film due to the laser heating and to apply this method to thicker permanent magnets without decreasing the width of the striped pattern. Acknowledgements We would like to express our gratitude to Mr. Uehara of Hitachi Metals, Ltd. for developing and providing us with the PM film. We are grateful to Mr. Ishii of Ophir Optronics Solutions Ltd. for measuring the laser beam intensity distributions, and Ms. Ebisawa of Tokyo Metropolitan Industrial Technology Research Institute for measuring the reflectance of the PM film. This work was partly supported by JSPS KAKENHI Grant Number 26630036, Grant-inAid for JSPS Fellows Grant Number 26·925 and Grant-in-Aid from Electro-Mechanic Technology Advancing Foundation.

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Biographies

Ryogen Fujiwara is a resident of Yokohama 226-8503, Japan. He got his graduation from Tokyo Institute of Technology during 2012-14, and continues to pursue his Ph.D from the same institution. His main works were on Positioning Characteristics of a MEMS Linear Motor and its fabrication and then on Multi-pole magnetization of thin film neodymium permanent magnet. He has Membership in a few Academic Societies such as JSPE, JSME and IEEJ.

Tadahiko Shinshi is a resident of Yokohama 226-8503, Japan. From the start of this millennium, he has been in teaching industry. His services set off as an Associate Professor, Precision and Intelligence Laboratory, Tokyo Institute of Technology; then he moved to Tokyo Medical and Dental University and is currently with Tokyo Institute of Technology. During 2004 to 2006, he worked concurrently as Senior Scientific Research Specialist for a Japan’s ministry. His main works were on Optically driven microcantilever type actuator, Miniaturization of one-axis-controlled magnetic bearing, and Dynamic characteristics of a magnetically levitated impeller. He has Membership in a few Academic Societies such as JSPE, JSME, ASPE, JSAO, JSAP and IEEJ. Elito Kazawa is a resident of Japan and works in Research and Development Department, Tokyo Metropolitan Industrial Technology Research Institute, Tokyo, Japan.