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SEQUENTIAL BATCH ASSEMBLY OF 3-D MICROSTRUCTURES WITH ELASTIC HINGES BY A MAGNETIC FIELD Eiji Iwase, Shoji Takeuchi∗ , and Isao Shimoyama Dept. of Mechano-Informatics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ∗ CIRMM, IIS, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan Phone: +81-3-5841-6318, Fax: +81-3-3818-0835, E-mail: [email protected] ABSTRACT This paper describes three-dimensional (3-D) microstructure assembly using an external magnetic field. An external magnetic field perpendicular to a substrate lifts up a hinged structure due to the shape magnetic anisotropy. Micro flap structures (4.5 µm-thick electroplated Permalloy) having 0.2 µm-thick nickel elastic hinges with various lengths and widths are bent in out-of-plane direction in a magnetic field up to 50 kA/m. The volume of magnetic material and the stiffness of the hinges determine the sensitivity to a magnetic field. As a result, we realized sequential batch assembly using the difference of the sensitivity. By way of the example, we have erected plates 600 µm × 600 µm in size. Also, structures bent at 90 and 45 degrees out of the plane have been obtained at the same time.

Tfield Soft Magnetic Material Magnetization : I Elastic Hinge

Tmech

Substrate

Magnetic Field : Hext Figure. 1 The direction of torques acting on a flap structure in a magnetic field.

INTRODUCTION

on a permanent magnet. The similar work on the shape magnetic anisotropy was reported on by C. Liu et al. [5], where sequential assembly was achieved by two different pin joints. We have already reported on an out-of-plane structure with polyimide elastic joints [6]. In this current paper, we replace the polyimide with metal, which acts as a spring element with different stiffness. The volume of a magnetic material and the stiffness of a metal hinge determine the sensitivity to the magnetic field. As the volume is larger or the stiffness is smaller, even the smaller magnetic flux density folds the structure. Therefore, sequential batch assembly for a folding process without any actuators or any electrical circuitry can be achieved. In addition, the metal hinge can work as electrical interconnection between both connected plates so that moving elements or sensors can be allocated on an out-of-plane structure.

There are many tools to fabricate 3-D microstructures [1] such as active probes, actuators like SDA [2], surface tension of melting solder [3], and so on. These tools have not only advantages but also disadvantages as stated below. The probes assemble microstructures flexibly. There are, however, not good at batch and mass fabrication because of the many DOF. Although the actuators also have flexibility, they and their electrical circuitry require plenty of room on the substrate. The surface tension method is very difficult, especially of sequential assembly. In this paper, we propose an alternative way for microstructure assembly on a substrate. This assembly uses the shape magnetic anisotropy to lift up the structures out of a substrate. By utilizing the shape magnetic anisotropy, only one magnetic layer is necessary for assembly. The shape magnetic anisotropy is caused by the hardness of magnetization between short and long axes of a material in a magnetic field [4]. When a magnetic thin film, like Permalloy, is placed in a magnetic field, the direction of the longer axis is magnetized easier and the torque is generated between the magnetized axis and the direction of the magnetic field. We can observe this phenomenon, for example, iron sands stand

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Bent Angle : θ

PRINCIPLE Figure 1 and Figure 2 show the schematic diagrams of a flap structure made of soft magnetic material. The structural part is made of soft magnetic material where Vmag is its volume. Lh , Wh and Th are length, width and thickness of

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the hinge, respectively. The structural part can be regarded as rigid because of its thickness compared to the hinge. The torque, Tmech , generated by the elastic deformation of the hinge is determined by Tmech

Wh · Th3 = Eh θ 12Lh

(b) Photoresist

(1)

Tfield

A'

Electroplated Permalloy (d) (e) Side View

(2)

: Si, : SiO2, : Cr/Ni, : Permalloy, : Photoresist

In the strict sense, the direction of the magnetization is determined by minimizing the total energy. The total energy consists of a potential energy by an internal demagnetizing field and a magnetic energy by an external magnetic field. The magnetization can be also estimated as the direction of the long axis because the magnetization is harder in the axis of a thickness [7]. Tfield  Vmag IHext cos θ.

A

(c)

where Eh and θ are Young’s modulus and the bent angle of the structural part, respectively. The shape magnetic anisotropic torque, Tfield , acting on → − the magnetic material, which has magnetization, I , in the −−→ external magnetic field, Hext , is evaluated by − −−→ → = Vmag | I × Hext |.

Sputtered Cr/Ni SiO2 Top View Si

(a)

Figure. 2 Fabrication outline of a micro flap structure. Crosssectional view of A–A’ is illustrated.

100 mm (a)

(3)

(b)

(c)

The equilibrium point of the flap in the magnetic field is determined by Tfield = Tmech . 12I Vmag · Lh θ = · · Hext . cos θ Eh Wh · Th3
 

(4)

sensitivity factor : S

Figure. 3 Micro magnetic flaps in a magnetic field (5.0 kA/m). Flaps have the same dimensional hinges (length, width and thickness are the same). The volume, Vmag , and the sensitivity factor, S, are different: (a) S = 1.20 × 10 6 ; (b) S = 2.34 × 10 6 ; (c) S = 3.28 × 10 6 .

The sensitivity to the magnetic field depends on the material and the shape, where the shape dependency is called “sensitivity factor S” in this paper (Eq. 4). By use of the sensitivity factor, the equilibrium angle can be evaluated easily when the structure is placed in a magnetic field. For example, the necessary area of the structural part to lift up can be calculated based on the volume of magnetic material Vmag . Also the number of hinges can be determined by the width of the hinge Wh .

layer is adhesive between the oxide and nickel layer (Figure 2(a),(b)). Secondly, the hinges were covered with photoresist (Figure 2(c)). A 4.5 µm Permalloy is electroplated only on the patterned nickel layer because the area of the oxide layer was insulated (Figure 2(d)). After removing the photoresist and the oxide layer, the structure was released with CF4 and O2 plasma from the silicon substrate (Figure 2(e)). The sensitivity to magnetic field can be determined by the volume of a magnetic material as shown in Figure 3. If the metal hinges have the same dimensions, the larger volume of Permalloy on the structural part obtains more torque from the magnetic field. Figure 4 shows the relationship between the magnitude of the magnetic field and the bent angle. The magnetic field was produced by an industrialstrength electromagnet. Theoretical curves were obtained from Eq. 4 by substituting the saturated magnetization of electroplated Permalloy I =1.0 T [4] and the Young’s mod-

FABRICATION AND CHARACTER TO A MAGNETIC FIELD The fabrication process of a microstructure is shown in Figure 2. In order to deposit a magnetic material on a substrate, Permalloy (Ni 80% and Fe 20%) was electroplated on a thin nickel layer. The electroplated Permalloy plays an important part in both a structure and a lifting torque. The nickel layer was used as both elastic hinges and a seed layer for electroplating. First of all, a 0.28 µm thermal oxide layer was grown on a silicon substrate. Chromium and 0.2 µmthick nickel were sputtered and patterned. The chromium

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bent angle θ [degree]

80

S=8.49

60

106

(a)

S=4.32 106 S=2.34 106

40 20

Magnetic Field : Hext (b)

(e)

(c)

(f)

(g)

(h)

10000 20000 30000 40000 50000 external magnetic field Hext [A/m]

Figure. 4 The theoretical and experimental plots of the bent angle with different values of S in an external magnetic field.

magnetic field H80 [A/m]

(d)

Bent Angle : θ

theoretical prediction measured

0 0

500 µm

80000

theoretical prediction measured

60000 S=

40000

Vmag•Lh Wh•Th3

Erected Plate

20000 0 0

Elastic Hinges 6

2•10

4•106

6•106

8•106

sensitivity factor S [-]

Figure. 6 A sequential batch assembly of the microstructures with locking mechanisms. The erected plates (the larger plates) have S = 18.41 × 10 6 , H80 = 7.64 kA/m and the locking plates are designed as S = 7.78 × 10 6 , H80 = 18.10 kA/m. The steps of assembly: (a) 0 kA/m; (b) 0.5 kA/m (The erected plates were lifted up); (c) 2.0 kA/m; (d) 7.5 kA/m (The locking plates were lifted up); (e) 30 kA/m (The erected plates were locked); (f) 0 kA/m (Assembled). (g) The pattern before assembly (the top view of (a)). (h) The assembly drawing (the condition of (f)).

Figure. 5 Theoretical prediction and measurement of the external magnetic field that bends structures at 80 degrees to the substrate. As the value of S is larger, the smaller flux can fold the structure. If structures have large difference of the values of H80 , sequential assembly can be achieved.

ulus of nickel hinge, Eh = 210 GPa. Figure 5 shows the relationship between the sensitivity factor S and the magnitude of the magnetic field for bending the structure at 80 degrees to the substrate (H80 ). Since the value of H80 depends heavily on the volume of the Permalloy and the stiffness of the hinge, assembly can be sequentially achieved using the magnitude of the magnetic field by moving a structure from above to the permanent magnet. Since experimental results gave good agreement with theoretical calculation in Figure 4 and Figure 5, the sequential assembly is predictable in the design of structures. Also, the condition for bending the structure at the desired angles can be calculated in advance.

plate. At first, a plate with larger area is erected. The locking plate is lifted up to hold the erected plate. Note that the assembly fails if the structures are lifted in reverse order. In this case, the difference of H80 , 10.56 kA/m, was found to be enough for a sequential assembly. Figure 7 shows simple erected plane structures, which have different volume of Permalloy for sequential assembly. In Figure 7(d), the rectangle area at the center of erected plate, whose color is different from the rest, was not electroplated by Permalloy to reduce the volume of a magnetic material. The design in Figure 7 is a good example for selectivity to a magnetic field using the volume of a magnetic material on each plate. In Figure 8, the locking structures were controlled at the bent angle of 45 degrees. Although the volume of Permalloy of erected and locking plates was almost equal, the number of hinges with the same width was different. In this way, ac-

SEQUENTIAL ASSEMBLY Figure 6 shows a sequential batch assembly of the structure with locking mechanisms based on the theoretical prediction. The locking was attained by friction at the locking site. The friction was generated by Tmech of the locking

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Locking Plate

10•106

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Erected Plate

(d)

structures bent at 90 and 45 degrees out of the plane have been obtained at the same time. The control of bent angle as well as conductivity at the hinges implies the possibility of actively moving optical devices.

100 mm

(a)

ACKNOWLEDGMENT (c)

Locking (b) Plate

Photolithography masks were fabricated using EB lithography apparatus of VLSI Design and Education Center (VDEC), the University of Tokyo.

Figure. 7 The simple erected plane structures using a magnetic field: (a), (b), (c) steps of sequential assembly; (d) erected structures. The electing plate has S = 26.72 × 10 6 , H80 = 5.27 kA/m and the locking plate has S = 2.64 × 10 6 , H80 = 53.37 kA/m.

REFERENCES [1] K. S. J. Pister, M. W. Judy, S. R. Burgett, and R. S. Fearing, “Microfabricated hinges,” Sensors and Actuators A, Vol. 33, pp. 249-256, 1992. [2] T. Akiyama, Dominique Collard, and Hiroyuki Fujita, “Scratch Drive Actuator with Mechanical Links for Self-Assembly of Three-Dimensional MEMS,” Journal of Microelectromechanical Systems, Vol. 6, No. 1, pp. 10–17, 1997. [3] P. W. Green, R. R. A. Syms, and E. M. Yeatman, “Demonstration of Three-Dimensional Microstructure Self-Assembly,” Journal of Microelectromechanical Systems, Vol. 4, No. 4, pp. 170–176, 1995. [4] Juck W. Judy and Richard S. Muller, “Magnetically Actuated, Addressable Microstructures,” Journal of Microelectromechanical Systems, Vol. 6, No. 3, pp. 249–256, 1997. [5] Yong W. Yi and Chang Liu, “Magnetic Actuation of Hinged Microstructures,” Journal of Microelectromechanical Systems, Vol. 8, No. 1, pp. 10–17, 1999. [6] Kenji Suzuki, Isao Shimoyama, and Hirofumi Miura, “Insect-Model Based Microrobot with Elastic Hinges,” Journal of Microelectromechanical Systems, Vol. 3, No. 1, pp. 4–9, 1994. [7] Jun Zou, Jack Chen, Chang Liu, and J´ose E. SchuttAin´e, “Plastic Deformation Magnetic Assembly (PDMA) of Out-of-Plane Maicrostructures: Technology and Application,” Journal of Microelectromechanical Systems, Vol. 10, No. 2, pp. 302–309, 2001.

100 mm

Figure. 8 An SEM photograph of another out-of-plane microstructures bent at 90 and 45 degrees. The plates bent at 90 degrees have S = 42.28 × 10 6 , H80 = 3.33 kA/m and the plates bent at 45 degrees have S = 7.05 × 10 6 , H80 = 19.97 kA/m.

cording to the objectives we can search for executable ways to obtain various three-dimensional microstructures. CONCLUSIONS We designed three-dimensional microstructures using a 4.5 µm-thick electroplated Permalloy as a magnetic material and a 0.2 µm-thick sputtered nickel as an elastic hinge. The structure was deformed continuously by an external magnetic field due to elastic hinges. The deformation can be predicted theoretically. Especially we emphasize a factor depending on the shape: sensitivity factor S. To assign the suitable value to S, sequential assembly by controlling bent angles can be achieved. By way of the example, we have made erected plates 600 µm × 600 µm in size. The locking was attained by friction at the locking site. The friction was generated by the elastic deformation of the hinge. Also,

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