熱動式撓性微型機構的設計與製作(II)

  

Design and Fabrication of Thermally Driven Compliant

Micromechanisms

ÒIIÓ

 : NSC87-2218-E-009-015 . : 86.08.01 - 87.07.31
  :   
....  
   !"#$%& '()*
%&'(+,-.% /#0(12.%/#0,
345$6789:;<= >!?@).%/#0 ABCD.EF G!?@H*IJKLM >NOPQNORST/"QUV WG!%&'(XYZP[\>] ^"_`Xabc<de@fg"h ijk
750µm x 3.2µm x 2µm̗ QNO%&'(,lmnoRpq) r

Here we present laterally driven electro-thermal microgrippers which can deflect in two-way(opposite) directions by only varying driving voltage modes. The main unit of these microgrippers is composed of a pair of adjacent cantilever beams with different lengths to exhibit the elastic deflection under uneven thermal expansion. To demonstrate the design principle, the microgrippers made of two different materials including single crystal silicon (SCS) and polysilicon are fabricated by IC compatible and simple processes with only one mask. The analytical model of the microgrippers is derived and verified by finite element model and experiment. Some key design parameters are also

discussed. A 750µm long, 3.2µm wide,

and 2µm thick polysilicon-made

microgripper, for example, can have

gripping force about 0.41 µN and

gripping range(including inward and

outward displacements) around 10 µm

with a current of 1.4mA at 15 V. The maximum error between simulation and calibrated is 7 %.

(Introduction)

Microgrippers utilizing a grip method by two or more fingers have the potential applications in microrobotics, microoptics, and biomedicine to accomplish micro-macro size interfacing for real world. Various couple field effects such as piezoelectric, shape memory, electrostatic, or thermal-mechanic effects have been applied to the actuation mechanism of micro-grippers. Although different types of microgrippers have been presented, they perform deflection only in one direction(inward or outward). Thermally actuating microgrippers are capable of large deflection and high forces in a current/voltage regime that is compatible with standard IC fabrication process. In this project, we accomplish electro-thermally and laterally driven microgrippers which can deflect in opposite (two-way) directions by only

varying voltages modes. They are based on the effect of Pan and Hsu’s operating principle.

 (Current Approach) Concept Design and Operating Principle

Figure 1 shows the schematic diagram of the basic electro-thermally driven microactuator. Due to the unequal thermal expansions of the two connected beams as they are heated by applying voltage(current), the microactuator produces deflection toward the short or cold beam.

By combining two basic microactuators with extended jaws symmetrically, then a two-way deflection microgripper can be formed. Figure 2 displays two operating conditions of the microgripper with its equivalent circuit. In Figure 2(a), the central two beams are heated along full beams, and in Figure 2(b), only partial length of the central two beams are heated.

Analytical Modeling

From analytical derivation, the values of axial force P, transverse force F, bending moment M, and the displacement of the tip can be calculated. It is found that Young’s modulus of the structure and the thickness of the beams don’t affect the displacement of the tip at all.

Since two cantilever beams are much longer than the length of the tip beam, one dimensional model is used to find the electro-thermal relationship between applied voltage(V) and temperature distribution along two cantilever beams. From heat flow equation under

steady-state condition, the following

second-order differential equation is obtained,

K d T(x

dx J T(x

⋅ 2 )= − ⋅2

( ))

ρ

(1)

where K is the thermal conductivity, T(x) is the temperature distribution along the beams, J is the current density, and

ρ(T(x)) is the temperature dependent

electrical resistance. Then the average temperature changes along the long and short beams can be found analytically, so is the displacement of the tip.

Exerting force

The gripping force of the proposed microgripper, in fact, is distributed along the actuating beams. However, a

concentrate force F_c exerting at tip point

O is considered. The detailed derivation is performed.

Finite Element Modeling

The commercial finite element code ANSYS 5.0A has been used to perform coupled field analysis of the electro-thermo-mechanical behavior to verify the analytical results of outward deflection. It also perform the inward deflection behavior. Material properties such as the thermal expansion coefficient, thermal conductivity and the Young’s modulus are treated as constant values. Only resistance is regarded as a linear function of temperature.

      (Results and Discussions)

Figure 3 displays the relation between the calculated displacement and the gap distance (S) of a polysilicon-made microgripper. It indicates that increasing gap distance will reduce the displacement.

Figure 4 displays the calculated gripping forces at the extended jaw tip of a polysilicon microgripper as function of various applied voltage for two-way deflection.

The surface micromachining processes are used to fabricate polysilicon-made microgrippers. As for SCS-made microgrippers, a wet etching bulk micromachining technique is used. The comparison between simulation results and experimental results is shown in Figure 5. The dimensions of the testing microgrippers are not optimal design here. They just demonstrate the two-way deflection effect and verify the simulation results.

The average error of outward deflection is about 5% between experimental results and analytical results for polysilicon-made microgripper, and is around 3% between experimental results and FEM results. The average error of inward deflection is about 7 % between experimental results and FEM results, which seems larger than that of outward deflection. This may be resulted from the complex influence of the heat transformation through the “r” beam structure and the corresponding thermal expansion.

Figure 6 displays the SEM micrographs of the two-way-deflection microgrippers made of SCS and polysilicon, respectively. It is found that only low applied voltage(0 ~ 20V) and power consumption(0 ~ 48mW) are required.

(References)

[1] J. G. Smits, “Design Consideration of

a Piezoelectric-on-silicon

Micro-robot”, Sensors and Actuators a, 35, pp.129-135, 1992.

[2] Yoshitaka Tatsue and Tokio Kitahara ,”Micro-grip system”, J. Robot. Mechatron., Vol.3, No.1, pp.57-59, 1991.

[3] S. Ballandras, W. Daniau, S Basrour, L. Robert, M. Rouillay, P. Blind, P. Bernede, D. Robert, S. Rocher, D. Hauden. S. Megtert, A. Labeque, L. Zewen, H. Dexpert, R. Comes, F. Rousseaux, M. F. Ravet and H. Launois, “Deep etch x-ray lithography using silicon-gold masks fabricated by deep etch UV lithography and electroforming”, J. of Micromech. Microeng., Vol. 5, pp.203-208, 1995.

[4] Steven Ashley, “Getting a Microgrip in the Operating Room”, Mechanical Engineering, Sept., pp.91-93, 1996.

[5] C. J. Kim and A. P. Pisano,

“Polysilicon Microgrippers”, Sensors and Actuators A, 33, pp.221- 227, 1992.

[6] Patrick B. Chu, Kristofer S. J.

Pister, “Analysis of closed-loop control of parallel-plate electrostatic microgrippers”, IEEE International Conference on Robotics and Automation, Vol.1, pp.820-825,1994.

[7] W. Riethmuller and W. Benecke, Thermally Excited Silicon Microactuators, IEEE Trans. Electron Devices, Vol.35, No. 6, June 1988, pp.758-763.

[8] W. Benecke and W. Riethmuller,

Applications of Silicon-Microactuators Based on Bimorph structures, IEEE Proc. Micro Electro Mechanical System, 1989, pp. 116-120.

[9] R. A. Buser, N. F. DeRooij, H. Tischhauser, A. Dommann and G. Staufert, Biaxial Scanning Mirror Activated by Bimorph Structures for Medical Applications, Sensors and Actuators A, 31, 1992, pp. 29-34. [10] Yoshihiko Suzuki, “Fabrication and

evaluation of flexible microgripper”, Jpn. J. Appl. Phys. Vol.33, 4A, pp.2107-2112,1994.

[11] G. Keller and R. T. Howe, “Nickel-filled Hexsil Thermally Actuated Tweezers”, Conference on Solid-State Sensors and Actuators, Sweden, pp.376-379.

[12] Ph Lerch, C Kara Slimane, B

Romanowicz, and Ph Renaud, Modelization and Characterization of Asymmetrical Thermal Micro-actuator, J. Micromech. and Microeng., Vol. 6, 1996, pp.134-137

[13] H. Guckel, J. Klein, T. Christenson, K. Skrobis, M. Landon, and E. G. Lovell, “Thermo-Magnetic Metal Flexure Actuators”, Technical Digest, IEEE Solid State Sensor and Actuator Workshop, pp.73-75, 1992

[14] J. H. Comtois and V. M. Bright, “Applications for Surface-micromachined Polysilicon Thermal Actuators and Arrays”, Sensors and Actuatos A 58, pp. 19-25, 1997

[15] C. S. Pan and Wensyang Hsu, “An

Electro-thermally Driven Polysilicon Microgripper”, J.

Micromech. and Microeng., Vol. 7, pp.7-13, 1997.

[16] K. E. Petersen, Silicon as

Mechanical Material, Proc. of the IEEE, Vol. 70, No. 5, pp. 420-429, 1982.

[17] S. P. Timoshenko and J. M. Gere, Mechanics of Materials, PWS publishing company, 3rd ed, pp.632-647, 1990.

[18] Gary K. Fedder and Roger T. Howe, “Thermal assembly of polysilicon microstructures”, Proc.IEEE Micro Electro Mechanical System Wokshop, pp.63-68, 1991

[19] Yasumasa Okada and Yozo

Tokumaru, “Precise determination of lattice parameter and thermal expansion coefficient of silicon between 300K and 1500K”, J. Appl. Phys., Vol. 56, No. 2, pp.314-320, 1984.

[20] Wia-shing Choi and Jan G. Smits, “A Method to Etch Undoped Silicon Cantilever Beams”, J. Micro Electro Mechanical System, Vol. 2, No. 2, pp.82-86,1993.



Figure 1. The schematic diagram of the basic electro-thermally driven microactuator proposed by Pan and Hsu[15].

(a) outward deflection

(b) inward deflection

Figure 2(a)-(b) Two operating conditions of a two-way-deflection microgripper by applying the voltage in different ways.

4.0 5.0 6.0 7.0 8.0 G ap d i st an c e, S (u m ) 1 .0 2 .0 3 .0 4 .0 5 .0 Gri ppi ng Di sp lacem en t ( um) Ou t wa rd In w ar d T o tal A p p l ie d Vo l ta g e = 1 5 V

Figure 3 The calculated displacement versus the variation of the gap distance of a polysilicon-made microgripper at different applied voltages for two-way deflections.

0 10 20 30 A pplie d V olta ge (V ) 0.00 0.20 0.40 0.60 0.80 1.00 Gri ppi ng For ce ( u N) O utw a rd, FE M In w ard , F EM

Figure 4 The gripping forces at the tip point O of a polysilicon-made microgripper under various applied voltages for two-way deflections.

0 10 20 30 A pplie d Voltage (V) 0.0 2.0 4.0 6.0 8.0 Gri pp in g Di sp la cemen t ( um ) Outw ard, FE M Inw ard, FE M Outw ard, A nalytical

Outw ard, Experim ent Inw ard, Experiment

Figure 5 The simulating and experimental displacements of the extended jaw tip under different applied voltages for two-way deflection.

Extended jaw Long/Short beams Contact Pads “r” beam

(a) SCS-made microgripper

(b) Polysilicon-made microgripper

Initial State

Ou tward

State In ward

Sta te

Two-way deflection of SCS-made microgripper by image capture pictures

Figure 6 SEM micrographs of two-way deflection microgrippers made of SCS and polysilicon. The operating status of two-way deflection by image capture pictures are also presented.