Ghodssi R., Lin P., MEMS Materials and Processes Handbook

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384 T. Mineta and Y. Haga

Fig. 6.25 Active bending

catheter using ﬂat meandering

SMAs [40] (Reprinted with

Elsevier)

SMA bending

actuator

Spacer

Bias coil

Heating

OFF

Lead wire

0.5mm

Fig. 6.26 Active guide wire using an SMA bending actuator [39] (Reprinted with permission.

6.3.1.4 Micro Clips and Grippers

Micro clips and grippers are typical applications of SMA micro actuators because

the large deformation and large force generation of SMA material is well suited to

these devices.

A micro gripper has been developed, as shown in Fig. 6.27 [1, 69].

The micro gripper consists of two micromachined (110) Si layers on which

5 μm thick Ti NiCu ﬁlm is sputtered. The two micromachined Si wafers are bonded

by using an Au-Si eutectic bonding technique. The TiNiCu ﬁlm has tensile thermal

stress because it is deposited at 500

◦

C. When the TiNiCu ﬁlm is cooled, the grip-

per closes due to the relaxation of the thermal stress. The gripper jaws can open by

shape recovery of the TiNiCu ﬁlm when heated to 70

◦

C. Opening and closing of the

jaws can be repeated because the Si layer acts as a bias spring.

6 Materials and Processes in Shape Memory Alloy 385

Fig. 6.27 Microgripper with sputtered TiNiCu ﬁlm actuators [1] (Reprinted with permission.

Figure 6.28 shows a micro gripper developed by Kohl et al. [70]. The gripper

mainly consists of two actuation units with a bonding pad, both of which are fab-

ricated from a cold-rolled TiNi sheet with a thickness of 100 μm by laser cutting.

Actuation unit 1 has a folded-beam structure for linear motion to close the grip-

ping jaws. Actuation unit 2 has two circular beams. The gripper is mounted on a

substrate in a pre-strained condition. By selective heating of actuation unit 1, the

folded-beams recover their initial shape so that the link is pulled. Consequently, the

circular beams are deformed and the gripping jaws are closed. By heating t he cir-

cular beams selectively, the condition is reset, resulting i n opening of the gripping

jaws.

Fig. 6.28 Structure and operation principle of the SMA micro gripper [70] (Reprinted with

Takeuchi and Shimoyama have developed an SMA micro electrode with a clip-

ping structure for neural recording [71]. An SMA ﬁlm (Ti-48 at.%Ni alloy ﬁlm,

6 μm thick) is deposited by RF magnetron sputtering and patterned by HF-HNO

386 T. Mineta and Y. Haga

wet etching (Fig. 6.29(1)). Two beams are bent and clamped by an Al wire bonded

to the s ubstrate during heat treatment so that the curved shape is memorized

(Fig. 6.29(2)). The upper and the lower beams are furthermore deformed by a

micro manipulator at room temperature to form a hook structure (Fig. 6.29(3)).

When the SMA arms are heated by an electric current directly for a very short

time less than a second, the SMA arms recover the memorized shape, resulting

in clipping of a nerve cord. The clipping force is enhanced by the 3-D structure.

Consequently, the SMA clip-shaped electrode can clip a nerve cord tightly. Peak

temperatures of martensitic and reverse martensitic transformation of the SMA ﬁlm

are 54 and 50

◦

C, respectively. The nerve is not damaged by the thermal actuation

of the SMA clip due to the short heating time. The electrical potential differ-

ence of the nerve of a living insect can be measured by using two SMA clips as

electrodes.

Figure 6.30 shows a micro-vascular clip fabricated from an SMA ﬁlm (11 μm

thick) [72, 73]. The SMA clip is used for pulsation measurement of an artery

sutured in micro-surgery to monitor obstruction of blood ﬂow by thrombus for-

mation. The holding mechanism consists of an outer arm and an inner cantilever

to hold a blood vessel with a diameter of 1 mm. The SMA clip is fabricated from

a ﬂash-evaporated TiNiCu ﬁlm with a memorized ﬂat planar shape. Micro-heater

circuits of platinum ﬁlm are formed on the root and the halfway point on the outer

arm with an insulator layer of polyimide ﬁlm. The outer arm is bent at the root

and the halfway point by external force before being attached to a blood vessel.

When the SMA clip is heated at the root point locally, the outer arm closes due

to deformation, resulting in the holding of a blood vessel between the outer arm

and the cantilever. Thin ﬁlm strain gauge circuits are also formed on the can-

tilever in the same platinum ﬁlm layer to detect the periodical variation of the

arterial pulsation. After use for pulsation monitoring, the SMA clip is heated at

the halfway point so that the outer arm opens and releases the blood vessel safely.

By local heating with the micro heaters, local actuation is realized, resulting in the

multi-step motion.

6 Materials and Processes in Shape Memory Alloy 387

Setting

Hold actuation

Release actuation

Pull out

Outer

arm

Strain gauge

Blood

vessel

Flat print circuit (FPC)

Heater-B

Heater-A

Strain

gauge

Reference

resister

Fig. 6.30 Micro clip actuator with integrate micro heater and strain sensor [72, 73] (Reprinted

6.3.2 Fluidic Devices

Shape memory alloy has been applied for micro valves and micro pumps as

a component for use in micro analysis systems. Johnson et al. have developed

micro valves consisting of TiNi thin ﬁlm ribbons and bulk-micromachined silicon

springs [74, 75].

Kohl and his research group have been active in the development of SMA micro

valves [76–79]. Shape memory alloy actuators micromachined from a TiNi thin

plate (100 μm thick) by laser cutting have been used for several types of micro

valves [76–78]. The operation principle, completed valve, and micromachined TiNi

thin plate actuator are shown in Fig. 6.31. The micro valve is conﬁgured by an

SMA actuator, a spacer and a polyimide membrane. The SMA actuator has multiple

beams which can be directly heated by an electric current. In the normal condition

of the unheated martensitic phase, the membrane is deﬂected by a pressure differ-

ence between the inlet and the outlet ports. By heating the SMA actuator above

the phase transformation temperature, the ﬂat shape is recovered and the valve is

closed. The micromachined TiNi thin plate beams have a stress-optimized shape

with homogeneous stress distribution. R-phase transformation of TiNi is used for

the valve operation.

In addition to the micromachined SMA-thin-plate beam actuator, Ti

and

ﬁlms (10 μm thick) deposited by DC-magnetron sputtering have also

been used for a similar type of micro valves [79]. Figure 6.32 shows the sputtered

TiNiPd thin ﬁlm actuator for the micro valve. The TiNiPd ﬁlm is patterned by

electrochemical etching. Sharply deﬁned edges and vertical sidewalls are obtained.

As shown in Fig. 6.33, a reciprocation micro pump with an TiNi actuator has

been developed by Benart et al. [80]. To realize cyclic motion, the micro pump con-

sists of two complementary TiNi actuators that are fabricated on silicon layers. The

TiNi ﬁlm is directly heated by an electrical current (0.9 A, 0.54 W). The two actua-

tors are heated complementarily by out-of-phase currents to achieve cyclic motion.

388 T. Mineta and Y. Haga

Fig. 6.31 Micro valve with stress optimized SMA actuator (micromachined SMA sheet type) [76]

Elsevier)

Polyimide check valves are fabricated in stacked silicon layers to ensure unidirec-

tional ﬂow through the pump chamber. When the upper TiNi actuator is heated,

the pump chamber is compressed, forcing liquid out through the right check valve

(State 1); the pump chamber is expanded when the lower TiNi actuator is heated,

drawing liquid in through the left check valve. The maximum pumping ﬂow r ate

of water is 50 μL/min, at an excitation frequency of 0.9 Hz. At higher drive fre-

quencies, the pumping ﬂow rate decreases presumably due to the incomplete phase

transformation of the TiNi material.

Figure 6.34 shows a micro pump which consists of two main parts, a ﬂash-

evaporated SMA ﬁlm actuator and a silicon check valve [81, 82]. TiNi thin ﬁlm

6 Materials and Processes in Shape Memory Alloy 389

Fig. 6.33 Schematic structure and fabrication sequence of a reciprocation micro pump with two

1997 IEEE)

Fig. 6.34 Structure and fabrication process of micro pump with SMA ﬁlm actuator [82] (Reprinted

with a thickness of about 6 μm was ﬂash-evaporated onto a Si substrate, and its ﬂat

shape was memorized by heat treatment in a vacuum. A noteworthy process is the

anodic bonding of the Pyrex glass cap to the TiNi ﬁlm surface. Pyrex glass can be

anodically bonded to TiNi ﬁlm with applying DC voltage of 600 V at 300–400

◦

in a vacuum successfully. The micro pump is driven by thermal cycles of resistive

heating and air-cooling under bias pressure applied by a nitrogen gas ﬂow. The TiNi

diaphragm of about 6 μm in thickness with a memorized ﬂat shape is deformed

when a bias pressure is applied to the chamber at room temperature. Then, when

the diaphragm is directly heated up by an electric current, it recovers its initial ﬂat

shape. The check valve consists of an inlet port and an outlet port which open and

close according to the movements of the TiNi diaphragm. It is easy to control the

390 T. Mineta and Y. Haga

pumping pressure by changing the bias pressure. The SMA micro pump gives a

pumping rate of about 0.4 mL/cycle under a bias pressure of 100 kPa and at a back

pressure of 0 kPa.

6.3.3 Optical Fiber Switch

Several optical ﬁber switches are widely used for telecommunication using an opti-

cal network. Mechanical 1×2 optical switches which adjust the optical axis of ﬁbers

by mechanical motion of a movable ﬁber have been developed using several micro

actuators, including SMA actuators. The switches have advantages of low insertion

loss and high cross-channel isolation because of the excellent optical property of

optical ﬁbers.

Using standard optical ﬁbers, a mechanical 1 × 2 optical switch has been fabri-

cated (Fig. 6.35)[31, 83]. A meandering SMA actuator for lateral displacement of

a movable ﬁber has been fabricated by laser cutting of an SMA sheet. V-grooves for

precise positioning of the movable ﬁber are fabricated by anisotropic etching of the

Si substrate.

Fig. 6.35 Optical ﬁber

switch using a ﬂat

meandering SMA actuator

[31] (Reprinted with

The Institute of Electrical

Engineers of Japan)

A mechanical 1 × 2 optical switch using plastic optical ﬁbers (POF) with a

relatively large core diameter have been developed (Fig. 6.36)[8]. A magnetic

latch maintains the position of the movable POF on the left or right side at steady

state. An SMA microcoil is ﬁxed to a magnetic material ring and electrodes by

soldering.

6.3.4 Tactile Pin Display

Tactile dynamic pin displays which depict graphic information or Braille points by

bistable up-and-down motion of an array of pins to visually impaired or sightless

people have been developed. The Braille points are varied arrangements of raised

6 Materials and Processes in Shape Memory Alloy 391

Stopper

SMA coil

Magnetic

material ring

Electrode

Electric

wire

Permanent

magnet

Acrylic substrate

Incident

light

Outgoing

light

Line 1

Line 2

POF/PCF

L30 mm x W10 mm x H5 mm

Cross section

Fig. 6.36 Plastic optical ﬁber switch using an SMA coil actuator [91] (Reprinted with permission.

dots representing characters identiﬁed by touch. Generally, six raised points repre-

sent one character. An SMA actuator has advantages of noiseless actuation and a

compact package with large displacement of SMA. As a disadvantage, the problem

of heat generation and its accumulation caused by repeated motion and a high den-

sity of the pin array remains to be solved. Appropriate actuation of SMA should be

considered for the required long service life.

Tactile pin displays using SMA wires and SMA coils have been fabricated. In

the case of SMA wires, contraction of SMA wire moves individual pins and return

force to restore the pin position is realized by a spring, as shown in Fig. 6.37 [84,

85] or by a rubber sheet on pin heads [86]. To improve heat dissipation, air cooling

by a fan [85] or water cooling by circulation of water [86] are utilized. In the case of

SMA coils, the return force to restore pin position is supplied by other SMA coils

at a counter position as shown in Fig. 6.38 [8, 87]. To avoid heat accumulation, air

coolingbyafan[87] or a magnetic latch mechanism (Fig. 6.38), which reduce heat

generation, are utilized [8].

Fig. 6.37 Side view of one

element of tactile display [84]

(Reprinted with permission.

Design of a tactile pin display using an SMA sheet or plate has been pro-

posed, and force and displacement of the actuator has been estimated. Return force

to restore pin position is assumed to be served by a TiNi/SiO

bi-material beam

392 T. Mineta and Y. Haga

Fig. 6.38 (a) Braille character (Japanese standard) and (b) structure of dynamic Braille display

structure [88]. Because of the large displacement of an SMA coil actuator, system

size is expected to be sufﬁciently small for mobile use.

6.3.5 AFM Cantilever

Ti-Ni shape memory alloy sputtered ﬁlm actuators have also been used for a sin-

gle crystal silicon multi-probe devices for scanning probe nano-lithography [89].

The device has TiNi SMA ﬁlm-actuated 8 × 8 writing probes with a cantilever

(2 μm thick) and an Au ﬁlm-coated tip (8 μm in height). The cantilevers and the

tips are fabricated from a silicon-on-insulator (SOI) wafer consisting of a 10 μm-

thick device layer and a 1 μm thick buried-oxide (BOX) layer. The TiNi SMA ﬁlm

is deposited by DC magnetron s puttering on the cantilever. To control the ﬁlm com-

position, several pure Ti blocks are placed on the erosion area of a Ti-50 at%Ni alloy

disk target. The sputtered TiNi ﬁlm (Ni-50.2 at.%Ti) is annealed at 450

◦

Cfor1h

in a high vacuum for crystallization. After patterning the Ti-Ni ﬁlm with a mixed

solution of HF and HNO

, Au ﬁlm lines are formed as lead wires for direct heating

of the Ti-Ni ﬁlm. A polyimide ﬁlm is deposited as an interlayer insulator in a cross

wiring section. A thick silicon substrate layer is etched by deep reactive ion etching

(DRIE) followed by removal of the remaining BOX layer from the backside. Each

cantilever is deﬂected to the lower side due to the compressive intrinsic stress of

the BOX layer, as shown in Fig. 6.39. When the SMA actuator is heated above the

phase transformation temperature, it recovers the memorized ﬂat shape as shown

in Fig. 6.40. The SMA actuation mechanism yields a contact or non-contact state

between the probe-tip and sample surface.

6 Materials and Processes in Shape Memory Alloy 393

Fig. 6.39 SEM images of scanning probe nano-lithography device using Ti-Ni SMA ﬁlm actuator

6.3.6 Case Studies and Lessons Learned

6.3.6.1 Designs

Shape Memory

As mentioned in Section 6.1 (Principle), “one-way shape recovery” SMA can be

deformed only once. When one-way SMA is used for repeated actuation, an elastic

bias spring should be added to deform the SMA [2, 7]. The two-way shape memory

effect can also be realized in TiNi alloys, by which the two-way TiNi alloy actuator

can be repeatedly deformed by heating and cooling without a bias mechanism [4].

Shape recovery force generated by two-way effect is much less than that of one-way

type SMA. A particular “all-round type effect”, which is a type of two-way effect,

can also be obtained in Ni-rich TiNi alloys after aging with the shape constrained.

However, it shows large thermal hysteresis at the deformation temperature.

Maximum Strain and Reversible Cycles

For SMA actuators to be used repeatedly, they should be designed with considering a

fatigue as mentioned in Section 6.2.6 (Materials and Processes Selection Guidance).