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J. Micromech. Microeng. 8 (1998) 213221. Printed in the UK PII: S0960-1317(98)87262-8

The TiNi shape-memory alloy and its

applications for MEMS

H Kahn, M A Huff and A H Heuer

Department of Materials Science and Engineering, Case Western ReserveUniversity, 10900 Euclid Avenue, Cleveland, OH 44106, USA Department of Electrical Engineering and Applied Physics, Case WesternReserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA

Received 1 September 1997, accepted for publication 26 February 1998

Abstract. The shape-memory effect is a solid state phenomenon which exploits areversible phase transformation to repeatedly achieve an initial shape, even aftersome deformation of the material. Numerous metal alloys exhibit this effect. One ofthe most widely used shape-memory alloys is TiNi, due to its large range ofrecoverable deformations and its relative ease of processing. In bulk and wireform, TiNi has been applied to a number of applications, and as a thin film, TiNi isan excellent material for use as a microactuator in microelectromechanical systems(MEMS), due to its large recovery forces and high recoverable strains. SeveralTiNi-actuated MEMS devices have already been reported.

1. Titaniumnickel as MEMS actuators

In the past several years, considerable commercial and mil-

itary interest in MEMS has developed. In the most general

form, MEMS is the integration of mechanical elements,

sensors, actuators and electronics on a common silicon

substrate through the utilization of silicon microfabrication

technology. MEMS promises to revolutionize the perfor-

mance of a wide range of products by merging silicon-based

microelectronics with micromachining technologies, en-abling complete systems-on-a-chip to be realized. While

the electronics are fabricated using integrated circuit (IC)

processes (e.g., standard CMOS fabrication), the microme-

chanical components are fabricated using compatible mi-

cromachining processes that selectively etch away parts of

the silicon wafer or add new structural layers to form me-

chanical and electromechanical devices. MEMS enables the

development of smart products by augmenting the com-

putational ability of microelectronics with the perception

and control capabilities of microsensors and microactuators.

The microsensors gather information from the environment

through measuring mechanical, thermal, biological, chem-

ical, optical and magnetic phenomena; the microelectron-

ics process the information derived from the sensors and,through some decision-making capability, direct the mi-

croactuators to respond by moving, positioning, regulating,

pumping or filtering, thereby controlling the environment

for some desired outcome or purpose.

A crucial topic for the continued maturation of MEMS

technology is the development of suitable microactuators

for practical applications. Various types of microactuator

device have been reported in the literature, including

microvalves [13], micropumps [46], optical switches

[7], imaging displays [8,9] and microrelays [10]. Each

of these devices used one of a variety of integrated

actuator mechanisms based on electrostatic, magnetic,

piezoelectric, bimetallic or thermopnuematic phenomena.

Although it may at first glance appear that there are

a considerable number of microactuation technologies

to choose from, each of the ones listed above has

significant disadvantages which render them unsuitable

for many applications. In particular, many applications

require an integrated microactuation mechanism that is

compatible with microfabrication, and able to provide alarge displacement and a large actuation energy density.

None of these microactuator technologies is capable of

simultaneously satisfying these requirements.

Thin-film shape-memory alloys (SMAs) have been

recognized as a promising material from which to make

MEMS microactuators for nearly a decade. However, due

to a lack of understanding of the basic material properties

and lack of control of the deposition parameters, they have

not received as much attention from the MEMS community

as other microactuator technologies. Walker et al [11]

first described SMA actuators in 1990; they used a simple

doubly supported beam of sputter-deposited TiNi, which

had been surface micromachined and was made to undergothe martensite to austenite phase transformation by Joule

heating. (The terms martensite and austenite are defined

in the next section.) The residual stress in the released

beam caused the beam to be deformed in the martensite

state, but upon heating and transformation to the austenite

state, the beam remembered its parent shape and became

straight and taut. By turning off the Joule heating current,

the beam was allowed to cool back to room temperature

and transform to the martensite phase, at which point the

beam deformed. This cycle could be repeated many times.

0960-1317/98/030213+09$19.50 c 1998 IOP Publishing Ltd 213

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H Kahn et al

Also in 1990, Busch and Johnson reported on a

normally closed gas microvalve using thin-film TiNi

microactuation [12]. Their device used a pneumatic

pressure bias to deform a TiNi diaphragm positioned over

an outlet orifice and thereby close the valve. The martensite

to austenite phase transformation was induced by passing a

current through the thin film to cause heating, whereupon

the diaphragm became flat and the valve opened to allow

the fluid to flow. This device employed one important

advancement, a mechanical bias to close the device in themartensite state, which enabled the valve to have repeatable

characteristics under cyclical use. In 1992, Ray et al

[13] reported on an improved microvalve design which

employed a mechanical spring to bias the valve closed in the

off state. A mechanical spring is preferable to a pneumatic

bias because of its smaller temperature sensitivity and easier

fabrication.

Kim et al [14] reported on a thermo-mechanical switch

employing a TiNi thin-film microactuator. This device was

primarily used to study the interaction of the substrate, in

this case silicon, and a sputter-deposited TiNi thin film.

The substrate stiffness had a profound influence on the

output energy of the TiNi layer. Further, the switchingcharacteristics were greatly improved by depositing a

second thin film of amorphous TiNi on the other side of

the device. This additional TiNi thin film, which was not

re-crystallized and therefore did not undergo any thermally

induced phase transformation, compensated for the thermal

stress in the transforming TiNi layer and allowed well

behaved switching behavior to be realized.

Kuribayashi and Taniguchi reported on a microactuator

suitable for robotic manipulators using a reversible TiNi

thin film [15]. This microactuator employed a two-layer

film of TiNi, in which the top layer was composed of

nearly stoichiometric TiNi and was able to undergo the

shape-memory effect. The bottom TiNi layer contained

disc-shaped Ti3Ni4 precipitates oriented parallel to the film

surface, and acted as a biasing spring for the composite thin

film. Consequently, this device overcomes the need for an

additional spring as in previously reported designs, greatly

simplifying fabrication. Using such a microactuator design,

this group successfully realized a micro-flexible robotic arm

able to undergo large movements out of the plane of the

substrate.

More recently, our group reported on a normally closed

microvalve suitable for modulating the flow of liquids

using TiNi thin-film microactuators [16]. All previously

reported microvalves were designed for control of gases.

The control of liquids using a microvalve is one application

where a large displacement is imperative; the maximumdisplacement of the actuator sets the stroke of the valve,

and a low stroke causes the open flow resistance to be

too high to allow liquids to flow through the structure at

normal pressures. High strokes are possible with TiNi

SMAs due to the high recoverable strains. For example,

previously reported microvalves (using other integrated

actuation schemes) were capable of producing strokes of

only a few tens of microns, a deflection such that the

maximum strain in silicon is below the maximum fracture

strain of 0.2%. In comparison, the maximum recoverable

Figure 1. Schematic cross-sectional drawings of a

TiNi-actuated microvalve in the (a) closed and (b) openpositions. See text for more details.

strain of TiNi is at least 3%, resulting in over an order of

magnitude increase in the stroke of the microvalve. As

the flow resistance of the open microvalve is inversely

proportional to the stroke cubed, a TiNi microvalve displays

an open flow resistance three orders of magnitude lower

than microvalves using other actuation methods. Further,

TiNi microactuators are capable of providing sufficient

force to modulate fluids at pressures seen in practical

applications. Our first microvalve design employed an

external biasing spring [16], while subsequent designs

employed integrated polyimide diaphragm springs [17] to

bias the structure closed in the off state, and was actuated

to the open state using Joule heating. Testing with filtered

DI water demonstrated that this liquid-handling microvalve

had an open flow rate of over 5 ml min1 at a differential

pressure of 0.2 psid, the highest open flow rate reported to

date in a microvalve, thereby demonstrating the feasibility

of MEMS devices for controlling liquid flows. The latest

version of the microvalve employs a microfabricated single-

crystal silicon spring [17]. In the off/unpowered position

(figure 1(a)) the microspring deflects the martensitic TiNi

film downward, pressing the boss against the orifice

opening. When heated (figure 1(b)), the austenitic TiNi

becomes nearly flat, deflecting the microspring upward,

lifting the boss away from the orifice and allowing fluidto flow. This microvalve exhibits an on:off flow rate ratio

of 1000:1 for water flows of 0.9 ml min1.

Our group also recently reported on the first

MEMS micropump device using a thin-film TiNi

microactuator [18]. MEMS-based micropumps are

generating considerable attention for many microfluidic

applications, such as micro on-chip chemical analysers

and implantable drug delivery systems, due to their small

reagent usage and small dead volumes. Similar to the

microvalve application, TiNi is an attractive material for use

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The TiNi shape-memory alloy and its applications for MEMS

Figure 2. Schematic cross-sectional drawings of aTiNi-actuated micropump. See text for more details.

as an actuator for micropumps because its high recoverable

strain levels and actuation forces enable large pumping

rates and pressure heads to be realized. The novel feature

of our pump design was the recognition that in order to

maximize the stroke of the device, the biasing spring should

have a nonlinear loaddeflection characteristic. That is,

the stiffness of the spring should decrease as the spring

compression is increased. To realize a nonlinear spring,

a second TiNi thin-film layer is used and operated in

tandem with the first TiNi layer. These two TiNi layers are

separated from each other by a silicon spacer layer. The

micropump is operated by alternating the Joule heating to

the two layers to achieve cyclic motion of the actuator.

Upon heating of the top TiNi layer and the lack of Joule

heating of the bottom TiNi layer, the actuator is positioned

in its most downward position (figure 2(a)). On the next

half-cycle, the Joule heating to the top layer is turned off

and the bottom layer is heated. This causes the actuator to

move to its most upward position (figure 2(b)). This cycle

is repeated to achieve a cyclical pumping motion. Check

valves made from polyimide are positioned on either side

of the micropump actuator to achieve unidirectional fluid

flow in the desired direction. Testing with filtered DI waterdemonstrated that the micropump had a maximum pumping

rate of over 49 ml min1 at an excitation frequency of

0.9 Hz. The excitation current and voltage were 0.9 A and

0.6 V, respectively, resulting in a required pumping power

of 0.5 W.

2. The shape-memory effect (SME)

Having discussed SMA-based MEMS devices, we now

discuss the material science of SMAs. The shape-memory

effect is displayed in materials which exhibit two distinct

solid phases, where the lower-temperature phase (known

as martensite) is relatively compliant due to its particularly

glissile boundaries, and the higher-temperature phase

(known as austenite) is less compliant. The material

assumes its parent shape while in the austenite phase.

When cooled through the phase transformation, it becomes

compliant and can be deformed. Then, when re-heated

to the austenite phase field, the material recovers its

remembered parent shape. This shape recovery generatesa displacement with a significant accompanying force. The

transformation temperature can be engineered to be either

above or below the ambient temperature, depending on the

specific material and the desired application. Since the

transformation temperature varies with the applied stress

(higher stress yields a lower transformation temperature), if

the austenitic material is subjected to a high enough stress,

it will isothermally transform into martensite, which can

then be deformed. When the applied stress is relieved,

the material returns to the austenite phase and assumes its

parent shape. This phenomenon is called pseudoelasticity

(PE).

As just noted, the basis of the SME is a reversiblesolid state phase transformation, of the type known to

materials scientists as martensitic transformations. These

are diffusionless transitions, meaning that the atomic

displacements involved in the transformation are small with

respect to interatomic distances, allowing the reaction front

to advance at near the speed of sound. Furthermore, no

chemical differences exist between the parent and product

phases. The transformation to martensite involves a small

shape change of the sample (much smaller than the shape

change of the unit cell), which is accommodated in material

exhibiting the SME by the establishment of alternating

twin or martensite variants, as is shown schematically in

figure 3. When a force is applied to the martensite, the

twin boundaries migrate in order to increase the fractionof those twin variants oriented parallel or nearly parallel

to the applied stress. This results in strain created solely

by twin boundary motion, which can occur at relatively

low stresses. Of course, there is a limit to the strains that

can be achieved in this manneri.e., when only a single

twin variant remains. At this point, a dramatic increase

in force is required to achieve higher strains, which will

be produced by standard plastic deformation, accompanied

by dislocation formation, migration and multiplication. If

the martensitic material is deformed exclusively by twin

boundary migration, the parent shape can be regained by

re-heating into the austenite phase field, as seen in figure 3.

The two solid phases differ not only in microstructure,but also in a variety of properties, such as Youngs modulus,

thermal and electrical conductivity, coefficient of thermal

expansion and damping capacity. Consequently, the shape-

memory effect offers many opportunities for dynamic

sensing and actuation.

The martensitic phase transformation is hysteretic, in

that it takes place over a range of temperatures, as

seen in the generic graph in figure 4. On cooling, the

Glissile in materials science jargon implies the ability to migrate under

an applied shear stress.

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Figure 3. Schematic drawing of the shape-memory effect:(a) high-temperature austenite phase, (b) low-temperature

martensite phase, (c) twin boundary migration toaccomodate stress, d) re-heating to austenite.

Figure 4. Generic graph for a shape-memory material,showing transformation temperatures on heating andcooling.

transformation begins at the martensite start temperature

(Ms) (i.e., the onset of the forward transformation) and

concludes at the martensite finish temperature (Mf). On

re-heating, the martensite persists to a higher temperature,

and begins transforming back to austenite at the austenite

start temperature (As) and ending at the austenite finishtemperature (Af) (i.e., the start and finish of the reverse

transformation). The hysteretic behavior is one hallmark

of martensitic transformations. An analogous hysteresis is

found in the stress versus strain curves associated with the

PE effect.

The shape-memory behavior described above, and

shown schematically in figure 3, is known as the one-way

SME, because spontaneous shape change occurs only on

heating. It is possible to achieve two-way shape memory,

whereby the material will alternate between two distinct

Figure 5. Apparent stress (as determined by substratecurvature) as a function of temperature for a 1.5 m TiNifilm on a silicon wafer, showing the hysteretic behavior forboth full and partial transformation.

shapes as it is thermally cycled through the martensitic

transformation. The material must be trained to establish

two-way behavior. This is accomplished by appropriate

stress and thermal cycling, which limits the number of

variants of martensite formed [19]. Stressing the material

while cooling favors the initial formation of particular

variants; repeating the cycle eventually trains the structure

to achieve both shapes in the absence of an external stress.

The microstructural basis for this training is not well

understood, but it is believed that individual dislocations

and dislocation arrays play a role [2022], and possibly also

precipitates [15]. Inasmuch as the martensite is inherently

compliant in SME materials, relatively small applied forceswill deform the material, even if it has been trained.

Therefore, the two-way SME is limited to those applications

where the material is not subjected to any substantial forces

when in the martensite phase.

The hysteresis at the heart of the SME suggests

analogies to ferromagnetic and ferroelastic behavior, and

indeed, alloys capable of the SME can be said to be

ferroelastic. These three classes of ferroic materials all

depend on domain wall migration to switch the material

from one state to the other. (A material is ferroic when

it has two or more orientation states in the absence of a

mechanical stress, an electric field and a magnetic field and

can shift from one to another of these states by means ofa mechanical stress, an electric field, a magnetic field or a

combination of these [23].)

A striking example of ferroic behavior involving thin-

film SMAs is shown in figure 5. Here, the transformation

behavior of a 1.5 m TiNi thin film sputter deposited onto

a 4 inch silicon wafer is monitored by the curvature of

the wafer; not only is the hysteretic behavior evident, but

a minor loop due to partial transformation can be seen.

Thin-film TiNi SMAs are discussed further in a later section

of this paper.

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3. The discovery of shape-memory alloys (SMAs)

Shape-memory-related materials properties were first

reported in 1932 for a goldcadmium alloy [24] and

in 1938 for brass [25]. (The martensitic transformation

in steels has been known since the 19th century [26].)

However, it was not until 1951 that the phenomenon was

explained crystallographically [27]. Later that decade,

shape-memory behavior was described for CuZn [28] and

CuAlNi [29] alloys, two viable engineering materials. In1961, researchers at the US Naval Ordnance Laboratory

discovered the SME in TiNi; they were first alerted to the

unique properties of the alloy when they noticed that a rod

would give a dull thud when dropped cold, but produced

a sharp ring when dropped warm [30]. The martensitic

transformation of TiNi was described fully in 1965, when

the term memory was first used to describe the shape

recovery behavior [31].

Since that time, the SME has been discovered in

numerous metallic alloys, including AgCd, NiAl, InAl,

NbTi, NbU, UMo, Nb3Sn and V3Si [32]. Currently,

the three most commonly used SMAs in engineering

applications are TiNi and the copper-based alloys, CuZnAl

and CuAlNi. Of these, TiNi is better suited for use as a

microactuator, because it is less brittle and is capable of

maximum recoverable strains about twice that of the Cu-

based alloys [33]. This large strain allows for increased

travel of moving parts for such applications as liquid-

flow microvalves. Several iron-based SMAs, such as FePt,

FePd and FeMnSi, are also being developed for industrial

applications [34].

Ternary additions to TiNi are also being explored.

Substituting the ternary element Cu for Ni in TiNi (in

amounts between 10 and 20 percent) can reduce the

hysteresis; however, the Af temperatures are slightly lower

than for the equiatomic TiNi [35]. In addition, for TiNiCu

alloys with Cu contents greater than 15 percent, the low-temperature martensitic phase has orthorhombic, rather than

monoclinic symmetry, resulting in a slightly decreased

recoverable strain [36]. Other ternary additions to TiNi

(in particular Zr, Hf, Pd, Pt and Au) have been studied as

a means of increasing the transformation temperatures for

high-temperature applications [37].

4. The titaniumnickel shape-memory alloy

The TiNi SMA displays a martensitic transformation

between a cubic high-temperature austenite phase and

a low-temperature monoclinic martensite phase. The

transformation temperature is highest (centered at 150

C)for the equiatomic TiNi alloy, dropping sharply as the Ni

content is increased (to 50 C at 53% Ni) and dropping

gradually as the Ti content is increased (to 75 C for 53%

Ti) [31]. The crystal structure of the austenite is the

cubic B2 or CsCl structure, with a lattice parameter of

0.3015 nm [38]. The martensite phase has a monoclinic

crystal structure, known as B19, with lattice parameters

of a = 0.2889, b = 0.4120 and c = 0.4622 nm, with a

angle of 96.8 [38] or a = 0.2898, b = 0.4108 and

c = 0.4646 nm, with a angle of 97.78 [39].

As discussed above, the crystallographic shape change

brought about by the transformation from austenite to

martensite is accommodated by the formation of a set

of differently oriented martensite twin variants. The

change from cubic to monoclinic symmetry provides many

possible shape changes by combining various monoclinic

variants, and allows cooperative deformation between

grains [33]. This leads to significant recoverable strains.

For TiNi single crystals, the recoverable strains range from

approximately 3% near the [001] direction to approximately10% for directions close to [011] and [111] [40, 41]. For

polycrystalline specimens, observed recoverable strains are

typically 68% for both the thermally driven SME [41, 42]

and the PE effect [43].

In some instances, the austenitic TiNi will undergo an

intermediate transformation to a rhombohedral (R-) phase

before achieving the monoclinic martensite phase. The R-

phase crystal structure can be thought of as a rhombohedral

distortion along the [111] direction of the B2 parent lattice;

the R-phase is often described by a hexagonal unit cell with

lattice parameters of a = 0.738 and c = 0.532 nm [44].

It is not completely understood why the R-phase appears

in certain TiNi specimens; however, various processingconditions such as cold working, thermal annealing, thermal

cycling, variation of Ni content and introduction of ternary

elements can bring about its occurrence [4447]. Since the

SME associated with the R- phase transformation is limited

to less than 1% recoverable strain [48], the austenite to

rhombohedral phase transformation is not as effective for

actuator applications as the austenite to monoclinic phase

transformation.

Along with the enhanced compliance as TiNi

transforms from austenite to martensite, there is also an

increase in the damping capacity of the material, so that

TiNi alloys have attracted attention for use in vibration

suppression applications [49,50]. The origin of the

damping is related to the internal friction associated with

the movement and interaction of internal interfaces such as

twin or intervariant boundaries [51]. In fact, the damping

capacity of TiNi peaks during the martensitic and R-phase

transformations, presumably due to the motion of interphase

interfaces.

Another important property of TiNi that changes

with the solidsolid phase transformation is the electrical

resistivity [49, 52]. The variation is seen for both thermally

induced and strain-induced transformations, with austenite

having a lower resistivity than martensite. The resistivity

versus temperature or resistivity versus strain curves display

the hysteresis characteristic of martensitic transformations,

as do the unit cell or macroscopic sample dimensions.For any engineering applications, it is essential that the

SME be repeatable and predictable, even after many cycles

through the phase transformation. Unfortunately, cycled

TiNi actuators sometimes show a decreased recovery force

and decreased recoverable strain, an increased permanent

strain and undergo shifts in the transformation temperatures.

This fatigue should properly be called transformation

fatigue; i.e., for SMAs, the term fatigue has a different

meaning than for traditional structural alloys, where it

pertains to mechanical cycle-induced fracture.

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On the other hand, a small number of cycles has

actually been demonstrated to have a beneficial effect on

SMA behavior. Cycling through the transformation 50

times while under 100 MPa stress improved the recoverable

strain of wires of TiNi containing 6% Cu from 4.8 to

5.3%; at 50 MPa, 250 cycles were required to increase the

recoverable strain from 2 to 3% [53]. This enhancement

in the SME is the result of training of the material. As

noted above, however, larger numbers of cycles can have

a detrimental effect. Under zero load, the Af and Mftemperatures of TiNi wires decreased by 15 and 20 C,

respectively, after 600 cycles; under 35 MPa stress, Mf

was unchanged, but Af was lowered by 15 C after 800

cycles [54]. TiNi wires experienced a 0.5% permanent

strain after 1800 cycles at 100 MPa stress; similar strain was

found after 50 000 cycles when the maximum temperature

in the wires (created by current-induced Joule heating)

was closely controlled to be just above the Af temperature

[55]. This result emphasizes the importance of preventing

overheating the material.

TiNi springs have also demonstrated fatigue, showing

decreases in recovery force and recoverable strain by 30%

after 1000 cycles, and by 60% after 10 000 cycles [56].

Deflection of TiNi springs against a bias force also degraded

by at least 20% after aging at 95 C for 2000 hours [57].

The stress-induced transformations of the PE effect

display similar fatigue phenomena as the thermal

transformations. Permanent strain in TiNi wires reached

1% after only 10 cycles and 2.5% after 800 cycles, for

a maximum stress of 600 MPa [54]. This unrecoverable

strain increases upon cycling at higher strain rates [58].

Also, a TiNi alloy with 50.9% Ni performed better than

a 50.0% Ni alloy, because of the higher critical stress

for slip [59]. The origins of fatigue in TiNi are not

completely understood, but likely issues include dislocation

formation due to overstraining, and precipitate formation

due to overheating. It is not surprising that processingconditions affect the observed resistance to fatigue [57, 60].

TiNi in bulk or wire form has been applied to a

number of engineering applications. One of the earliest

uses was as mechanical couplings for pipes [19]. The

coupling is expanded in the martensite phase, fitted over

the ends of two pipes, and then heated to induce austenite

formation, thereby forming a tight seal. TiNi springs

are utilized in the automotive industry in a number of

ways, including compensating for changing temperatures

in automatic transmissions by adjusting pressure [42],

improving the performance of shock absorbers [42] and

opening fog-lamp louvers [19]. TiNi springs are also used

to control air conditioner systems [61] and the positioning

of the space shuttle antennae [62]. TiNi wires and springsprovide motion in many robotics operations.

Because of its biocompatibility, TiNi is found in

many medical applications. The continuous low-magnitude

applied forces are ideal for orthodontic tasks [63].

Bronchial prostheses [64] and bone clamps [65] have

also been developed, as have eyeglass frames [66]. The

potential for controlled snakelike motion is being explored

for small, active catheters [67], and expandable TiNi filters

can be fed through the catheters to break up blood clots

and prevent embolisms [66].

5. Titaniumnickel thin films

As discussed above, the shape-memory properties of TiNi

are also being exploited in thin-film form for use as

microactuators in microeletromechanical systems (MEMS).

The TiNi films are typically sputtered onto silicon wafers

using a TiNi alloy target [6870]. Films deposited

onto unheated substrates are amorphous as deposited and

are usually crystallized by annealing while still under

vacuum. While sputtering is a well understood depositionprocess, the fabrication of TiNi films suitable for MEMS

applications is still complex. Titanium is a very reactive

species. Thus, a very low base pressure is necessary in

the sputtering chamber before the admittance of the argon

sputtering gas, in order to reduce contaminants (principally

Ti oxide precipitates) in the film which could adversely

affect the SME. For a target-to-substrate distance of 6 cm,

a base pressure of at least 107 Torr is required [71].

It was discovered quite early that the composition of the

sputtered film does not match the composition of the target;

specifically, the films are Ti poor with respect to the target

(by 24 percent) [72]. This results from a combination

of effects: differential sputtering rates of the two elementsfrom an alloy target; differential re-sputtering rates from the

substrate and differential capture by reactive contaminants;

in addition, as the target begins to wear, the typical

magnetron sputtering target develops a circular groove in

use, and the sputtering profile and ejection angle will

change, which can also lead to composition changes [72].

The simplest and most common solution to this problem is

to place small pieces of pure Ti onto the target, to achieve

the correct film stoichiometry [68, 73,74]. Increasing or

decreasing the amount of Ti (or Ni, or any ternary element)

can result in a film of any desired composition. It has

also been demonstrated that adjusting the target-to-substrate

distance and the sputtering gas pressure can change the

TiNi stoichiometry from 47 to 52% Ti, while using a

stoichiometric (50% Ti) target [75].

Another strategy to control stoichiometry is the use

of multiple sputtering guns. This approach was tried and

reported for TiNiCu films; however, the sputtering system

was incapable of substrate rotation during deposition, and

so the relative stoichiometry across the substrate varied

significantly [76]. For multiple sputtering guns, inasmuch

as the atomic species no longer originate at the same

location, it is critical to rotate the substrate in order to

achieve uniform deposition. In addition, with the sputtering

rate of each element under separate control, an in situ

monitor of the atomic ratio in the plasma (which is

possible using optical emission spectroscopy [75] or atomicabsorption spectroscopy [77]) and the appropriate electronic

feedback would allow the adjustment of the deposition rates

during film deposition to obtain precise film stoichiometry.

As mentioned above, the as-deposited amorphous films

must be annealed to achieve crystallinity. This also

promotes adhesion to the substrate, almost certainly through

the formation of thin (40 nm) reaction layers [68, 78].

An alternative would be to heat the substrates during

deposition, resulting in an as-deposited crystalline state, but

this leads to very fine grain sizes (100200 nm) [72],

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Figure 6. The titaniumnickel binary equilibrium phasediagram [90]. Equiatomic TiNi could exist in equilibriumbelow 630 C in a very narrow composition region [90].

which inhibits the SME, and thick (200 nm) reaction

layers [68], which could alter the film stoichiometry.

The crystallization temperature of TiNi films has been

variously determined by differential scanning calorimetry to

be 464 C [79], by in situ transmission electron microscopy

(TEM) to be 477 C [73] and by ex situ TEM to be 400 C[80]. It was determined by cross-sectional TEM of TiNi

films on oxidized silicon substrates that a 100 nm thick

region of TiNi adjacent to the substrate will not transform

to martensite, presumably due to the mechanical constraints

imposed by the substrate [81]. TEM experiments also

determined that freestanding Ti50.2% Ni films thinner than

100 nm will not transform to martensite [82], though TEM

sample preparation and experimental vagaries could also be

important. TiNi films exhibit a strong (110) texture in the

austenite phase when deposited on oxidized [81] or bare

[83] silicon substrates.

Besides crystallization, the annealing procedure will

strongly affect the film microstructure and the SME.Slightly Ni-rich films underwent the transformation to the

R-phase, as well as the normal martensitic transformation,

when annealed at 500 C, but only the martensitic

transformation when annealed at 700 C [84]. TiNi films

which had first been crystallized by a 700 C anneal

exhibited a single transformation; subsequent aging at

500 C induced the intermediate R-phase transformation

[73].

As would be expected from the TiNi binary

phase diagram (figure 6), nonequiatomic TiNi films can

experience precipitate formation when annealed, due to

conventional diffusive processes. After annealing at

500 C, Ti50.0% Ni films exhibited no precipitates,

Ti48.4% Ni and Ti46.8% Ni films contained Ti2Niprecipitates and Ti51.4% Ni films contained Ti3Ni4precipitates [74]. Experiments on Ti48.5% Ni films

showed very fine plate-shaped Ti-rich precipitates forming

at 500 C and spherical Ti2Ni precipitates forming above

600 C [80]; the plate-shaped precipitates increased the

stress necessary for permanent deformation, leading to an

improved SME [79]. Ti3Ni4 precipitates found in Ni-

rich films have been associated with training of the TiNi,

presumably due to the establishment of internal stress fields

caused by the preferential orientation of the precipitates

along the (111) planes [15, 72]; however, this issue is still

under investigation.

In general, TiNi thin films demonstrate the same

transformation temperatures as bulk specimens, although

transformation in thin films can also be affected by

heat treatments. For equiatomic TiNi, no change in

transformation temperatures was found for annealing

between 500 and 700 C; however, Ti-rich films displayed

increased transformation temperatures as the annealing

temperature increased, whereas Ni-rich films displayeddecreased transformation temperatures [85]. It is likely that

the higher annealing temperatures led to greater precipitate

formation, which affected the overall stoichiometry of the

film, and thus changed the transformation temperatures.

TiNi thin films also display the PE effect, similar to bulk

specimens [85].

The maximum recoverable strains reported for TiNi

films range from 2.6 to 6% for recovery forces of 117 to

600 MPa [86, 74, 79]. TiNi square diaphragm experiments

have demonstrated recoverable deflections of 400 m at

0.02 MPa for 8.4 mm wide diaphragms [16] and 125 m

at 0.2 MPa for 2 mm wide diaphragms [78], corresponding

to recoverable strains of 1.3% and 1.7%, respectively. Thework density performed by TiNi films varies between 5

and 25 MJ m3 [68, 12, 87], significantly higher than for

any other MEMS actuation material.

TiNi thin films are expected to experience the same

fatigue problems as bulk specimens, though there has been

little published work in this area. For TiNi(7% Cu) films

attached to silicon substrates, the recovered stress for the

martensitic transformation was reduced from 500 MPa for

the first cycle to 350 MPa after 2000 cycles (the film strain

is 0.2%) [87]. From this result, one prediction would

be that stressing the films at 350 MPa would result in

no fatigue effects for up to 2000 cycles. Freestanding

7 m thick TiNi films were cycled through the martensitic

transformation up to 100 times, with no observed change inthe recoverable strain, which ranged from 0.7 to 1.8 to 2.2%

for 100, 250 and 400 MPa of applied force, respectively;

however, permanent strain developed in the films subjected

to 250 MPa (0.3%) and 400 MPa (3.2%) [88]. It was

also reported that a TiNi thin-film actuated microvalve,

operating at 1% strain, survived more than two million

cycles (possible degradation of the valve performance was

not mentioned) [89].

6. Conclusions

The shape-memory effect is a solid state phenomenon

whereby materials which have been deformed can recovertheir initial shape merely by heating through a phase

transformation. In the process, high forces can be

generated. One particular shape-memory alloy, TiNi, can

recover at least 3% strain in either bulk, wire or thin-film

form. As a thin film, TiNi is an excellent material for use as

a microactuator in microelectromechanical systems, due to

its high recoverable strains and its large recovery forces,

and several TiNi-actuated MEMS devices have already

been reported. One potential drawback to this technology

is the stability of the shape-memory effect in thin-film

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H Kahn et al

TiNi over the millions of cycles required of these devices;

however, development efforts under way by our group

and others are expected to ameliorate this phenomenon,

resulting in the desired device stability.

Acknowledgments

Research at CWRU on TiNi SMA-actuated microdevices

is supported by DARPA under contract No DABT63-95-

C0070.

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