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

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Chapter 6

Materials and Processes in Shape Memory Alloy

Takashi Mineta and Yoichi Haga

Abstract Actuators are a key factor of micro electro mechanical systems (MEMS).

In particular, shape memory alloy (SMA) is an effective material for microactuators

to generate large force and large displacement. In this chapter, basic properties of

SMA materials, fabrication processes of bulk and thin ﬁlm SMA actuators, and

application devices (medical devices, ﬂuidic devices, switches, tactile displays and

cantilevers for atomic force microscopy (AFM)) are described. Microactuators of

SMA have some disadvantages such as difﬁculty to batch fabricate and assemble,

high electric power for thermal driving, and low controllability of displacement.

In this chapter, integration techniques with other MEMS technologies to overcome

the disadvantages of SMA actuators also are described with introductions of typical

application devices.

6.1 Introduction and Principle

6.1.1 Basic Principle

Shape memory alloys (SMAs) are excellent materials for microactuators which can

generate large force and large displacement. Shape memory alloy can recover its

original shape after it is deformed over its elastic limit by external force. As shown

in Table 6.1 compared with other types of actuators, the SMA actuators shows the

highest energy density (work/unit volume), on the order of 10

J/m

[1].

The response of SMA actuators tends to be slow due to the heat capacity of the

actuator, resulting in lengthy heating and cooling. Because deformation of SMA is

caused by a crystalline phase transformation which occurs without any diffusion of

T. Mineta (B)

Graduate School of Science and Engineering, Yamagata University, Yonezawa, Japan

e-mail: mineta-t@yz.yamagata-u.ac.jp

355

R. Ghodssi, P. Lin (eds.), MEMS Materials and Processes Handbook,

MEMS Reference Shelf, DOI 10.1007/978-0-387-47318-5_6,



Springer Science+Business Media, LLC 2011

356 T. Mineta and Y. Haga

Table 6.1 Work per unit volume for various microactuators

Actuator type W/v (J/m

) Equation Comments

TiNi SMA 2.5 × 10

6.0 × 10

σ · ε

Maximum one time output:

σ = 500 Mpa, ε = 5

Thousands of cycles:

σ = 300 Mpa, ε = 2

Solid-liquid phase

change

4.7 × 10

(

v

)

kk= bulk modulus = 2.2

GPa (H

8% volume change

(acetamide)

Thermo-pneumatic 1.2 × 10

F·δ

Measured values: F =

20 N, δ = 50 μm, v =

4mm×4mm×50 μm

Thermal expansion 4.6 × 10

)

(α · T)

Ideal, nickel on silicon,

s = substrate, f =

ﬁlm, T = 200 K

Electro-magnetic 4.06 × 10

F·δ

F =

−M

2μ

Ideal, variable reluctance:

v = total gap volume,

= 1Vsm

−2

2.8 × 10

F·δ

Measured values, variable

reluctance:

F = 0.28 mN,

δ = 250 μm, v =

100 × 100 × 250 μm

1.6 × 10

Measured values, external

ﬁeld: Torque =

0.185 nN m

−1

, v =

400 × 40 × 7 μm

Electrostatic 1.8 ×10

F·δ

A·gap

F =

εV

2δ

Ideal: V = 100 V,

δ = gap = 0.5 μm

3.4 × 10

F·δ

Measured values: comb

drive:

F = 0.2 mN (60 V),

v = 2 × 20 ×

3000 μm

(total gap),

δ = gap = 0.5 μm

7.0 × 10

F·δ

Measured values: integrated

force array:

v = device volume,

120 V

Piezoelectric 1.5 × 10

Calculated, PZT: E

60 GPa (bulk), d

500 (bulk),

E = 40 kV cm

−1

6 Materials and Processes in Shape Memory Alloy 357

substances [2], an SMA actuator can deform rapidly when it is miniaturized thus

reducing its heat capacity.

6.1.2 Introduction of TiNi and TiNi-Base Ternary Alloys

The shape memory effect (SME) has been discovered in many alloys, such as

TiNi-based alloys (typically Ti-50 at.%Ni), Cu-Al-Zn, Cu-Al-Ni, and Fe-Mn-Si [2].

However, only TiNi-based alloys, which were developed in 1963, have come into

wide practical use in industry. Since the 1970s, TiNi alloys have been used for ther-

mal switches, tube couplings and other applications. Thin ﬁlms of TiNi-based alloy

have been studied and applied for microactuators since the 1990s [3–5]. Compared

with other SMA materials, TiNi SMA has advantages such as high durability for

repeated actuation cycles, small thermal hysteresis, and chemical stability.

The principles of SMA materials and actuators have been published in many

review articles and books [1, 2, 6, 7], and reference to these publications is

very useful for understanding the principles of SMA actuators in detail. In this

section, an overview of the deformation mechanism of SMA is presented as

follows.

The shape memory effect based on the phase transformation of SMA can be

explained as resulting from deformation of the crystalline lattice, as shown in

Fig. 6.1 [2]. At high temperature, SMA has an austenitic phase, as shown in

Fig. 6.1a. When it is cooled below the start temperature of martensitic transfor-

mation (M

), the transformation of the austenitic phase into the martensitic phase

is initiated. At the ﬁnish temperature of martensitic transformation (M

), the trans-

formation is ﬁnished completely. This is a shear deformation without diffusion of

atoms, which is completed within a very short time (Fig. 6.1b). In the martensitic

phase, the shear strain is compensated due to the formation of twin crystals (variant)

which consist of opposing martensitic lattices. The shape of the SMA material does

not change in appearance despite the change of the crystalline phase from austenite

Cooling

External

Load

Heating (>A

)

(a) Austentic phase (b) Martensitic phase

(deformed)

(<M

)

Deformation

(shear strain)

(Variant)

Fig. 6.1 Principle of shape memory effect based on deformation of crystalline lattice [2]

358 T. Mineta and Y. Haga

to martensite. When an exterior load is applied, the SMA material is deformed due

to deformation of the twin phase as shown in Fig. 6.1c. When the SMA is heated

above the reverse-martensitic phase transformation temperature (start temperature;

, ﬁnish temperature: A

), the martensitic phase reverses to the austenitic phase,

resulting in recovery of the original shape, as shown in Fig. 6.1a. In the case of

plastic deformation of conventional metals, slip deformation caused by an exterior

load results in permanent deformation which cannot be recovered after the load is

released. In contrast, SMA can recover its original shape as mentioned above.

Shape memorization is achieved by thermal treatment. The SMA is clamped as

the shape to be memorized during the thermal treatment. Shape memory alloys with

one-way SME, which was described above, are widely used at present. Although

there is another SMA type with two-way deformation, as mentioned below (Section

6.1.4), it is not always suitable for actuations with a large applied load.

Shape memory alloy goes through a phase transformation loop of stress vs. tem-

perature under constant strain during heating and cooling, as shown in Fig. 6.2a.

Another phase transformation loop of strain vs. temperature under constant stress

is also useful to characterize SMA (Fig. 6.2b). In the process of cooling from high

temperature, martensitic transformation starts when the SMA is cooled below M

and is ﬁnished at M

. In the heating process, the reverse martensitic transformation

starts and is ﬁnished at A

and A

, respectively. In addition, a rhombohedral phase

(R-phase) often appears in TiNi alloy.

Stress

Temperature

Heating

Cooling

Thermal

hysteresis

recovery

Strain

Temperature

Cooling

Heating

Thermal

hysteresis

(a)

(b)

Fig. 6.2 Phase transformation loops during heating and cooling [1, 2]. (a) Stress vs. temperature

under constant strain, (b) Strain vs. temperature under constant stress

Differential scanning calorimetry (DSC), by which the exothermic and endother-

mic heat ﬂow can be measured during the phase transformations, is a common

method for characterizing SMA. It should be noted that phase transformation tem-

peratures are inﬂuenced by both external and intrinsic stress of the SMA. Electric

resistivity change in the heating and cooling cycle is also useful for measuring the

phase transformation temperatures of SMA.

6 Materials and Processes in Shape Memory Alloy 359

6.1.3 Super-Elasticity

At constant temperature, SMA shows a force-displacement relationship which cor-

responds to the stress-strain relationship, as shown in Fig. 6.3a. When an external

stress is applied to an SMA above its elastic limit, the SMA is pseudo-plastically

deformed in the low temperature phase. Although further strain causes permanent-

plastic strain, the SMA is strained reversibly (typically up to several percent) in the

pseudo-plastic range. After the external force (stress) is released from the pseudo-

plastic range, strain is retained. This retained strain can be recovered completely by

heating above the austenitic temperature (A

Stress

Strain

Stress

Strain

(Shape recovery

by heating)

(b)

(a)

Fig. 6.3 Stress-strain behavior of SMA at constant temperature. (a) Shape memory effect, (b)

super-elasticity

At temperatures higher than A

, SMA shows super-elasticity, as shown in

Fig. 6.3b. When external stress is applied to the SMA in the austenitic phase, it

is deformed over its elastic limit due to the stress-induced martensitic transforma-

tion. After the external stress is released, the martensitic phase re-transforms into

the high temperature phase, and then the SMA recovers its original shape by itself.

The super-elastic effect enables the SMA to recover its original shape, even though

it is deformed above its elastic limit, as shown in Fig. 6.3b.

Shape memory alloy materials which have a phase transformation temperature

(austenitic ﬁnish temperature) below room temperature are called “super-elastic

alloys”. TiNi-based super-elastic alloys have been used for commercial products

such as eyeglass frames, mobile phone antennas, and brassiere wire.

6.1.4 One-Way Type, Two-Way Type, All-Round-Way Type

The deformation mechanism mentioned above is usually called “one-way shape

recovery”. One-way SMA is deformed only once when it is heated. After the one-

way SMA recovers its original shape completely by heating above A

, it cannot be

deformed without external force. In most cases at present, one-way SMA with a bias

mechanism is widely used due to large force generation and small thermal hysteresis

[2, 7].

360 T. Mineta and Y. Haga

The two-way shape memory effect can be obtained in TiNi alloys with con-

trol of martensitic transformation by intrinsic stress [4, 7]. By the two-way effect,

TiNi alloy can be deformed to the memorized shape by heating, and then it can

be deformed in another direction by cooling, resulting in reciprocatory deforma-

tion between two shapes without any external bias force [4, 7]. A particular type of

the two-way effect is called “all-round type effect”, which occurs in Ni-rich TiNi

alloys after aging treatment under a constrained shape. Ni-rich precipitations (such

as Ti

) provide an intrinsic stress ﬁeld surrounding the TiNi parent phase [2].

Two-way SMA is suitable for simple structured actuators because reciprocatory

actuation can be performed by itself without an external bias mechanism. However,

there are disadvantages in two-way SMA. The shape recovery force generated by

the two-way effect is much less than that by the one-way type effect. Also, two-way

SMA tends to have large thermal hysteresis.

6.2 Materials Properties and Fabrication Process

of SMA Actuators

Figure 6.4a shows the force generation by an SMA as a function of displacement at

various temperatures. When the SMA is heated up to the A

, the force generation is

initiated. The force generation increases with increasing temperature until the SMA

is heated up to the A

. In order to design an SMA actuator accurately, it is important

to comprehend the actual force-displacement property (stress-strain property) of the

SMA to be used.

SMA (HT)

Stress

Strain

Reactive force

of bias spring

SMA(LT)

Low

temperature

Stress

Strain

(at high temperature)

(at low temperature)

High

temperature

(a)

(b)

Fig. 6.4 Force-displacement curves of SMA at various temperatures (a), and principle of one-way

SMA actuator with a bias spring (b)

6 Materials and Processes in Shape Memory Alloy 361

For repeated motion, one-way type SMA should be used with a bias spring to

recover the shape before the actuation, as shown in Fig. 6.4b [2]. Force-displacement

curves of the SMA at low temperature and at high temperature are generally illus-

trated as the curves of SMA (HT) and SMA (LT) in Fig. 6.4b, respectively. At low

temperature, the connection face between the SMA and the bias s pring is located at

the point B at which force of the SMA (LT) and reactive force of the bias spring are

balanced. When the SMA is heated above A

, the connection face moves to point A

at which the force of the SMA (HT) and reactive force of the bias spring are bal-

anced. Stroke of reciprocatory actuation (e

− e

) can be obtained by heating and

cooling.

6.2.1 Bulk Material

The bulk material for SMA is mainly TiNi alloy ingots which are processed into

required shapes.

Shape memory alloy wires produced by drawing are the most widely used prod-

ucts. Such wire comes in several diameters and can be actuated by Joule heat

generated by direct ﬂow of an electrical current into the SMA without any heater or

cooler when the wire diameter of the coil is small. The direction of movement of the

SMA wire is generally contractional, but bending or torsional direction is also used.

Shape memory alloy coil actuators, which are fabricated by winding SMA wire,

are also commercially produced and widely used. Shape memory alloy coil can be

obtained by helical cutting of an SMA tube or pipe [8]. The external diameter of the

SMA coils used as actuators is generally 100–500 μm, and the wire diameter is less

than 200 μm. The direction of movement is not only the contractional direction, but

also the extension and torsional directions are also available. Shape memory alloy

coil actuators have the advantages of ﬂexibility and large stroke per unit length.

SMA thin plates, sheets or ribbon are fabricated by rolling. There is also a tech-

nique for lamination and rolling of thin Ti and Ni sheets and thermal diffusion [9,

10]. The thickness of the bulk thin plates is generally around 10–100 μm and the

directions of movement are bending and torsional. A planar shape can be cut out for

several types of ﬁne patterning and micromachining, for example, photolithography

and etching.

Shape memory alloy pipes or tubes are also fabricated by drawing. The external

diameter is generally around 1–3 mm and wall thickness is around 10–100 μm.

Sintering of TiNi powder ﬁlled in a mold has also been studied for obtaining

arbitrary shapes of bulk SMA (Fig. 6.5)[11].

6.2.2 Thin Film

Many TiNi-based alloy materials have been prepared by melting metallurgy. On the

other hand, novel fabrication techniques for TiNi-base thin foils such as sputtering

362 T. Mineta and Y. Haga

Fig. 6.5 TiNi Microspring

structure [11] (Reprinted with

Elsevier)

[1, 3–6, 12–20], evaporation [21–23], laser ablation [24], and rapid solidiﬁcation

[25], have also been studied. In particular, the evolution of SMA ﬁlm deposition

techniques such as sputtering and evaporation are important for devices of micro

electro mechanical systems (MEMS). Shape memory alloy ﬁlms with high perfor-

mance have been realized by ﬁlm deposition techniques, for example, sputtered TiNi

ﬁlm can realize a shape recover force of 600 MPa and shape recovery strain of 6%,

which are sufﬁciently practical properties [16].

6.2.2.1 Sputtering

Sputtering has been used for fabrication of TiNi-based thin ﬁlms since the 1990s

[1, 3–6, 12–20]. In particular, magnetron sputtering has been used most frequently.

Because sputtered TiNi-base t hin ﬁlms tend to be amorphous, the ﬁlm should be

annealed above the crystallization temperature to obtain the shape memory effect.

There have been numerous reports on sputtering of TiNi-based alloy ﬁlms using

a single alloy target [3–5, 12–14]. In sputtering deposition using a single alloy tar-

get, the chemical composition of the target should be adjusted so that the sputtered

ﬁlm has a desired composition. Following are certain points which should be kept

in mind when sputtering with a s ingle alloy target. The surface of the alloy target

should be carefully cleaned by pre-sputtering to remove the oxide layer on the target

surface. However, lengthy pre-sputtering results in unbalance of the target composi-

tion, for example, titanium is lost during pre-sputtering due to its high sputter yield

[12]. In the case of magnetron sputtering, it should also be noted that sputtering

and re-sputtering of some materials onto the erosion grooves in the target cause

compositional variations in the ﬁlm deposited on the substrate [12].

To adjust the composition of SMA ﬁlm sputtered from a single alloy target, small

plates of Ti or Ni are used to partially cover an alloy target [6]. The chemical com-

position of the deposited ﬁlm can be adjusted by changing the ratio of the areas

covered by the Ti and Ni plates.

6 Materials and Processes in Shape Memory Alloy 363

There have also been many trials to sputter from separate metal targets of tita-

nium, nickel, and other additional elements. TiNiCu alloy ﬁlms [19] and TiNi alloy

ﬁlms [20] have been realized by sputtering deposition with separate metal targets.

When individual pure metal targets are sputtered, the substrate passes through each

deposition area of the metal target rapidly. The sputtering rate of each metal is con-

trolled by electric power individually applied to the target to control the composition

of the deposited alloy. Compared to sputtering with a single alloy target, sputtering

with separate multi targets is advantageous for easy adjusting the ﬁlm composition,

but special sputtering equipment with multi-targets is required. In sputtering with

separate targets, a substrate rotating mechanism is necessary to obtain a uniform

ﬁlm composition.

6.2.2.2 Evaporation

It is difﬁcult to obtain a uniform composition along ﬁlm thickness by using conven-

tional evaporation techniques. At the early stage of evaporation, Ni content tends to

be higher than that of Ti because the vapor pressure of Ni is higher than that of Ti.

There is a unique deposition method termed ﬂash-evaporation, by which small

alloy pellets as the evaporation source are supplied from a storage holder to a heated

tungsten boat via a guide pipe one by one [21–23]. To omit the deposition at an

early stage of the evaporation, the deposition starts after a certain closed-shutter

time has elapsed from the start of the pellet vaporization. The evaporation of the

pellets is subsequently repeated, resulting in the deposition of a thick ﬁlm with a

multi-layered structure.

6.2.2.3 Non-planar Thin Film Deposition

In addition to deposition on the ﬂat surface of substrates, there have been some

studies on SMA deposition on non-planar surfaces, particularly on the cylindri-

cal surface of a core rod or pipe. Figure 6.6 shows a tubular actuator-unit for the

multi-directional bending mechanism [26]. Three meandering-shaped SMA actu-

ators are formed in a TiNi cylindrical ﬁlm with a thickness of 10 μm, which are

ﬂash-evaporated on a core Cu pipe with an outer diameter of 3 mm. The TiNi ﬁlm

can be uniformly deposited on a cylindrical surface of the core pipe with rotation

during deposition. After the deposition, the TiNi ﬁlm on the core pipe surface is pat-

terned by electrochemical etching with a photoresist mask pattern which is formed

by non-planar photolithography. After the core Cu pipe is dissolved selectively in

concentrated HNO

, a self-standing tubular TiNi actuator-unit is obtained.

Sputtering has been also used for tubular TiNi ﬁlm deposition on core material.

TiNi ﬁlms with a thickness of 10 μm were obtained on Cu wires (50 μm in diameter)

by rotating deposition [27]. TiNi crystalline grains can grow more uniformly by

rotating deposition than two-step deposition on both sides of the core wire. Ion

beam sputtering has also been employed [28]. By using ion beam sputtering with Ni

and Ti targets, a TiNi ﬁlm with a thickness of 2 μm can be deposited on a Cu shaft

which is rotated during the sputtering.