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

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364 T. Mineta and Y. Haga

1mm

(a)

(b)

1mm

Fig. 6.6 Tubular actuator fabricated from a ﬂash-evaporated TiNi ﬁlm (10 μm) on cylindrical sur-

face of a core Cu pipe with an outer diameter of 3 mm [26]. (Reprinted with permission. Copyright

2004 The Institute of Electrical Engineers of Japan) (a) Patterned by electrochemical etching with

a non-planar photoresist mask, (b)coreCupiperemoval

Properties of tubular SMA ﬁlm, in particular crystalline orientation, are not

always similar to those of a ﬁlm obtained by one-side deposition on a ﬂat surface

[27, 28].

6.2.3 Micromachining

TiNi alloy is a difﬁcult material to machine, thus etching is generally used for

MEMS applications. Grinding and micromilling are possible, but the optimal

machining condition is limited and considerable tool wear can be expected [29].

Carbide tooling is suitable for machining.

Micromachining with a water jet requires high water pressure, which can cause

undesired distortion of the SMA. A study of erosion resistance of TiNi alloy using

a water jet has indicated that good erosion resistance of the TiNi alloy is due to its

elastic behavior which relives local strain [30].

Laser cutting and electro-discharge machining (EDM) exhibit relatively good

results. Several designs of SMA microactuators can be realized with laser machin-

ing. A Nd:YAG laser is widely used for micromachining of SMA, for example,

intravascular stents or micro actuators, as shown in Fig. 6.7 [31]. A heat-affected

zone (HAS) formed on the outer surface of SMA and debris and TiO

particles may

slightly deteriorate the SME [32]. Femtosecond laser ablation allows micromachin-

ing of memorized SMA with no deterioration of its SME since the laser pulse width

is very short (nanosecond order), such that heat generation is sufﬁciently reduced

during laser machining [8, 33].

Electro-discharge machining (EDM) is an electro-thermal process by which the

material is removed by electro-discharges occurring between the workpiece and tool

electrode immersed in a liquid dielectric medium. In the case of micro-EDM, low-

speed machining is required to obtain a smooth surface. After EDM, re-cast hard

materials, the SME of which is slightly deteriorated, are observed on the EDM sur-

faces of TiNi SMA [34]. A disadvantage of EDM is that a starting hole is needed

for the wire unless the wire starts from the side of the workpiece [35].

6 Materials and Processes in Shape Memory Alloy 365

Elsevier)

6.2.4 Etching and Lift-Off

6.2.4.1 Case and Example

Chemical Etching and Dry Etching

It is important to establish micro-patterning techniques for SMA materials to realize

micro actuators with ﬁne patterns. TiNi alloy is well known as a corrosion-resistant

material in other words, it is a difﬁcult-to-etch material. There are few chemical

etching techniques; mixed solution of ﬂuoric acid and nitric acid is known as a

chemical etchant of TiNi alloy [3, 36]. There have been many reports on chemical

etching of TiNi thin ﬁlms in ﬂuoric and nitric acid.

Reactive ion etching (RIE) with SiCl

as a process gas has also been attempted

[36]. The etch rate of the RIE was about 10 nm/min. It is difﬁcult to obtain a high

etch rate due to the low vapor pressure of Ni compound.

Lift-Off

The lift-off process has also been attempted for patterning of deposited SMA ﬁlm

as shown in Fig. 6.8 [14]. On a thin evaporated ﬁlm of Cr (150 nm) on an SiO

substrate, an 11 μm thick polyimide layer is spin-coated and cured. A second Cr

ﬁlm is evaporated and patterned on the polyimide layer. The polyimide layer is

patterned using O

RIE with a Cr mask. The polyimide is slightly under-cut during

the RIE to facilitate the subsequent lift-off process. A TiNi ﬁlm with a thickness of

7–10 μm is deposited using DC sputtering. After the TiNi deposition, the polyimide

layer is removed in hot KOH solution, resulting in the lift-off patterning. The TiNi

with a narrow-pattern is released by wet etching of the sacriﬁcial Cr layer due to the

side etching of Cr.

Electrochemical Etching

Electrochemical etching is one of the most suitable methods to etch TiNi SMA

with a high etch rate [37–41]. Compared to conventional chemical etching,

366 T. Mineta and Y. Haga

Fig. 6.8 Process chart of

TiNi ﬁlm sputtering and

lift-off patterning. (1)

Deposition of Cr/polyimide

(11 μm)/Cr layers, (2) Upper

Cr patterning and O

RIE, (3)

TiNi sputtering (7–10 μm),

(4) Lift-off by polyimide

removal KOH solution, (5)

and (6) Cr wet etching to

release movable parts [14]

(Reprinted with permission.

of Electrical Engineers of

Japan)

electrochemical etching has features including high etching rate, low side etching

and a smooth etched surface [38–40]. An etch factor (etched depth/under-cut) higher

than 1.5 can be obtained. Another advantage of electrochemical etching is that nor-

mal photoresist can be easily used as an etching mask. Large-scale equipment is not

necessary for electrochemical etching. Electrochemical etching can be carried out

using a simple setup, as shown in Fig. 6.9 [40, 41]. Shape memory alloy coated with

a photoresist is used as an anode. The anode and a counter cathode, for example a

stainless-steel plate, were facing each other with a gap of several tens of millimeters

in the electrolyte solution. A solution of 5 vol% (about 1 mol/L) H

-methanol

has been used as an electrolyte for electrochemical etching of SMA [37, 38]. When a

direct-current (DC) voltage, typically 8–10 V, is applied between the electrodes, the

SMA anode is etched electrochemically. If the electrolyte solution contains water,

anodic oxidation of the SMA surface occurs simultaneously, resulting in inhibition

of uniform etching.

In addition to continuous voltage, pulse voltage is also useful for the electro-

chemical etching. The application of pulse voltage slightly suppresses under-cutting

in narrow groove etching because reaction products are replaced with fresh solution

when the voltage is turned off [40]. However, the etched surface tends to be rough

in the case of pulse etching.

Through-layer etching is very important for the precise fabrication of micro actu-

ators from an SMA sheet. During over-etching, the remaining SMA patterns tend

to be etched non-uniformly due to imbalance of the electrolytic current distribution.

The etching proceeds at the places where the current is concentrated, while it stops

6 Materials and Processes in Shape Memory Alloy 367

–

Electrolyte

Solution

Generator

(dc or pulse)

Electric

connection

Insulator

Anode

(SMA with resist pattern)

Stirrer

Cathode

(Stainless steel plate)

Fig. 6.9 Setup for

electrochemical etching of

SMA [40, 41] (Reprinted

with permission. Copyright

2000 Elsevier, 2004 IOP)

at other places. In order to overcome the problem of non-uniform sheet-through

etching, a conductive dummy layer of Ni or Cu, which is formed on the back side

of the SMA sheet previously, is very effective in maintaining a uniform current dis-

tribution during the over-etching. The dummy layers of Ni and Cu can be easily

removed in concentrated nitric acid without damage to the SMA surface [40, 41].

Figure 6.10 shows the process of fabrication of an SMA actuator from an SMA

sheet by using the double-side electrochemical etching with Ni dummy layers.

Double-side etching is effective to depress the side-etching. Figure 6.11 shows

meandering-shaped SMA actuators fabricated from an SMA sheet by using the

etching process shown in Fig. 6.10.

(a) Electrochemical etching

SMA

(d) Nickel removal

(b) Nickel evaporation and electroplating

Fig. 6.10 Double-side

electrochemical etching of

TiNi sheet using a nickel

dummy layer [40] (Reprinted

with permission. Copyright

2000 Elsevier)

368 T. Mineta and Y. Haga

Fig. 6.11 Electrochemical

etching of SMA sheet (40 μm

thick, double-side etching)

[40] (Reprinted with

permission. Copyright 200

Elsevier)

Electrochemical etching also can be used for fabrication of non-planar SMA

structures. Figure 6.12 shows a tubular SMA actuator unit for an active catheter

bending mechanism, in which meandering-shaped SMA actuators are formed. An

SMA tube, covered with a photoresist pattern formed by non-planar photolithogra-

phy, is electrochemically etched in a cylindrical vessel with a cylindrical cathode,

resulting in the fabrication of a tubular SMA actuator-unit [42].

Fig. 6.12 Tubular SMA

actuator-unit

electrochemically etched

from a SMA tube (outer/inner

dia.: 600/500 μm) [42]

(Reprinted with permission.

A non-corrosive, non-toxic and stable electrolyte solution is required for indus-

trial applications. Inorganic salt of LiCl, which has high solubility in alcohol [41],

is useful for the electrolyte solution. In 1 mol/L LiCl-ethanol solutions, good etch-

ing with a high etch factor of 1.5 and a smooth etched surface can be carried out

when LiCl is used as the electrolyte. The etch rate can be depressed less than 4

μm/min. In addition to good etching properties, the solution has other advantages,

for example, it is non-corrosive, non-toxic and stable for a long term. The SMA

can be etched with a smooth surface in LiCl-ethanol solution because there are no

inﬂuences of other electrochemical reactions, such as anodic oxidation that tends to

inhibit surface smoothing.

6 Materials and Processes in Shape Memory Alloy 369

6.2.5 Assembly

For purposes of mechanical ﬁxation of SMA or parts with SMA, several assembly

methods, for example, mechanical ﬁxation, adhesion, welding and soldering, are

used. Electrical connections are also formed for actuation of SMA by supply of an

electrical current.

6.2.5.1 Mechanical Fixation

Crimping and fastening are used for mechanical ﬁxation of SMA. Crimping is

joining two parts by deforming one or both of them to hold the other, s olderless ter-

minals made of deformable metal being used, especially in the case of SMA wires

and coils. Fastening by screw bolts or other devices such as, hooks, pins, or wires,

is also used to ﬁx SMA. The advantage of this method is ease of reattachment.

These mechanical ﬁxation methods are advantageous in that they are simple and

the formation of conductive connections is relatively easy. Disadvantages are the

requirement of large connection areas, impreciseness of length adjustment, low pro-

ductivity because of the handmade process and fracture susceptibility in the ﬁxation

area because of local deformation and local stress of SMA.

6.2.5.2 Adhesion

In the assembly of SMA parts using polymer material for intermediate adhesion,

not only a nonconductive adhesion part but also a conductive adhesion part using

conductive resin are employed. As the nonconductive adhesive part, epoxy resin

[40] and UV curable resin [43] have been utilized. As the conductive adhesive part,

epoxy resin containing silver powder [40] or UV curable conductive epoxy resin

have been utilized [44].

Silane coupling agents can be used for improving adhesion between the SMA

and polymer material [45].

Laser assisted deposition has been carried out for ﬁxing an SMA coil to silicon

parts for active bending catheters [44], vaporized polymer source material being

selectively polymerized and deposited on the UV light-irradiated area.

A disadvantage of adhesion is that considerable time is necessary to make all

connections when these connections have to be formed individually. Cyanoacrylates

can be cured in a shorter time than epoxy resin but are relatively brittle. Adhesive

tape (single sided or double sided) can be used for temporary ﬁxation.

6.2.5.3 Welding

Welding of TiNi SMA to itself can be successfully performed if the heat-affected

zone is minimized and welded in an inert atmosphere. Lasers such as CO

lasers

[46], resistance welding and tungsten inert gas (TIG) welding can be used for this

purpose. Several self-expanding TiNi stents have been fabricated by welding at a

speciﬁc location [47]. A laser-cut patterned TiNi sheet was rolled up and welded

370 T. Mineta and Y. Haga

at a speciﬁc location in an early example. Almost all laser-cut stents are fabricated

from TiNi tubes and do not require welding. Wire-based stents have been fabricated

by welding TiNi wires at speciﬁc locations to form hexagonal cells, as shown in

Fig. 6.13 [47].

Welding TiNi SMA to dissimilar material can be performed, for example, attach-

ment of a radiopaque marker such as a tantalum marker to a stent (Fig. 6.14)[2, 3].

Attention should be paid to welding strength because of the brittle interface layer

between TiNi and dissimilar metals.

Springer)

6 Materials and Processes in Shape Memory Alloy 371

6.2.5.4 Soldering

Solder composed of fusible alloys has been successfully used for joining TiNi SMA

with other materials. As the surface oxide layer of TiNi alloy prevents solder wet-

ting, an aggressive ﬂux (such as strong acid, aluminum paste, etc.) or ultrasonic

soldering is used to remove the oxide layer. After removal of the oxide layer by the

ﬂux, standard solder such as Sn-Ag can be used. Ultrasonic soldering utilizes cavi-

tation by ultrasonic vibration energy to remove the oxide layer, and some specially

designed solder of Sn-Al is suitable for joining TiNi alloy with other materials.

Soldering is useful not only for joining SMA to SMA but also for joining SMA to

dissimilar material, such as stainless steel.

Plating of a solderable intermediate metal layer on the SMA surface before sol-

dering is effective. Ni, Au or Cu are suitable for this purpose. However, adhesion

between the oxide layer and the plated metal is problematic. For tight ﬁxation of an

SMA wire on a metal pattern, temporary-soldering and additional crimping of the

solder by a sawtooth punch can be employed [48].

6.2.6 Materials and Processes Selection Guidance

6.2.6.1 Materials (Bulk/Thin Film)

Common Mistakes

It is important to note that mechanical and thermal properties of SMA are not inde-

pendent but are related closely. For example, the limit of reversible cycles strongly

depends on maximum strain.

For use in repeated actuation, an SMA actuator should be designed with consid-

eration of fatigue. The limit of reversible cycles of actuation of TiNi SMA strongly

depends on maximum strain or maximum stress which is applied to the SMA.

A shape recoverable strain as high as 8% can be obtained under optimum condi-

tions without load [49]. However, usable strain for cyclic actuation is much smaller.

Table 6.2 shows typical reversible cycles of the martensitic transformation of poly-

crystalline TiNi SMA. The reversible actuation cycles decrease signiﬁcantly with

increasing maximum strain or maximum stress. The data in Table 6.2 should be

considered only as a rough guideline. It is important to understand in detail the

Table 6.2 Dependence of an achievable number of repeated actuation cycles as a function of

maximum strain and stress [49]

Cycles Maximum strain (%) Maximum stress (MPa)

100 4 275

10,000 2 140

>100,000 1 70

372 T. Mineta and Y. Haga

Stress

Temperature

SMA actuator

Thermal

actuator

Fig. 6.15 Comparison of

characteristics of SMA

actuators with thermal

actuators (stress vs.

temperature under constant

strain)

fatigue behavior of the SMA to be used. When designing SMA actuators, maximum

strain or maximum stress should be determined according to the required actuation

cycles.

It should be also noted that an SMA actuator is drastically deformed in a narrow

temperature range. Figure 6.15 is a schematic comparison of characteristics between

SMA and thermal actuators. A thermal actuator can generate actuation force linearly

due to thermal expansion in a wide temperature range, in spite of small force gen-

eration. In contrast, an SMA actuator generates force drastically in the range from

the start temperature to the ﬁnish temperature of phase transformation. Accurate

temperature control is necessary to control the force (displacement) of the SMA

actuator continuously.

Speciﬁc Features of Materials: TiNi Binary Alloys

Regarding TiNi binary alloy, it is well known that phase transformation tempera-

tures are extremely sensitive to alloy composition [2, 21, 50]. Figure 6.16 shows

an example of transformation temperature dependence on the alloy composition

in ﬂash-evaporated TiNi ﬁlm [21]. The composition change of only 1 at.% causes

drastic transformation temperature change greater than several tens of degrees. It is

necessary to accurately control the composition of TiNi alloy to obtain transforma-

tion temperatures with high reproducibility. It should be noted that in the case of

TiNi bulk alloys or sputtered alloy ﬁlms, the composition dependence on transfor-

mation temperature is slightly different from the data shown in Fig. 6.16. In addition

to the alloy composition, the phase transformation temperature of TiNi SMA also

strongly depends on annealing temperature and annealing time. It is important to

evaluate the phase transformation temperatures of the TiNi alloys to be actually

used.

6 Materials and Processes in Shape Memory Alloy 373

Fig. 6.16 Effect of

composition of TiNi ﬁlm on

phase transformation

temperatures [21] (Reprinted

with permission. Copyright

1998 Elsevier)

(Flash-evaporated TiNi ﬁlm

on silicon substrate, 6 μm

thick, annealed at 500

◦

Cfor

60 min)

TiNi Alloys with R-Phase Transformation (Low Thermal Hysterisis)

Fully annealed near-equiatomic TiNi alloys transform from the B2 parent phase

directly to the monoclinic B19’ phase martensitically during cooling. However,

thermo-mechanically treated near-equiatomic TiNi alloys and Ni-rich alloys show

two-step transformation with a rhombohedral (R) phase [51]. Typical strain and

DSC curves of TiNi SMA with the R-phase transformation are shown in Fig. 6.17.

When TiNi alloy is cooled from a temperature above A

, R-phase transformation

starts at R

, which corresponds to the start temperature of actuator deformation.

The ﬁrst peak in the DSC curve is due to the transformation from the B2 phase to

the R-phase. The second peak ranging from M

to M

in the DSC curve is due to

transformation from the R-phase to the B19’ phase.

When the SMA is re-heated from a temperature above M

, at which the alloy is

in the R-phase, only reverse-R-phase transformation (from R-phase to B2) occurs.

Actuation using the R-phase and reverse-R-phase transformations of TiNi alloy is

effective to obtain small thermal hysteresis.

It should be noted that the R-phase of TiNi alloys disappears at high temperature

and lengthy annealing. If the R-phase is lost completely, actuation of TiNi alloy is

caused by only the M-phase and reverse-M-phase transformations, resulting in large

thermal hysteresis between the heating and cooling processes.

TiNi-Base Ternary Alloys

There have been many studies on TiNi-based ternary alloys in which a third element

is contained to control the actuation properties such as deformation temperature and

thermal hysteresis. Material properties of TiNiCu alloys (Cu is substituted for Ni)