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394 T. Mineta and Y. Haga

Fig. 6.40 Schematic of the actuation mechanism of cantilever of a scanning nano-lithography

It should be noted that the limit of reversible cycles of actuation strongly depends

on the maximum strain or maximum stress [49].

Latch Mechanism

In the case of bistable r epeated motion, a latch mechanism is effective for low power

consumption and long service life by avoiding unnecessary SMA actuation at steady

state. Magnetic latch mechanisms utilize the magnetic attraction force between mag-

netic material and a permanent magnet. Using SMA coils and a magnetic latch

mechanism, a tactile pin display [8] and an optical switch [90] have been fabri-

cated. A bistable actuator using SMA beams and the magnetic latch mechanism has

been developed for large contact force and large stroke with low power consumption

[91].

6.3.6.2 Heating and Cooling

Large scale bulk SMA actuators tend to have a slow response due to the large heat

capacity of the actuator itself, which results in lengthy heating and cooling cycles.

As mentioned in Section 6.1.1 (Basic principle), an SMA actuator can be rapidly

deformed when it is miniaturized so as to reduce its heat capacity. An SMA micro

actuator can be heated above its phase transformation temperature rapidly by small

thermal energy and can be rapidly cooled down by ambient air without the addition

of a cooling mechanism.

6 Materials and Processes in Shape Memory Alloy 395

Direct Heating

TiNi SMA can be actuated by Joule heat created by direct ﬂow of an electrical cur-

rent into the SMA when electrical resistance of the SMA is relatively high. The

advantage of the actuation method is its simple structure without any heater or

cooler. Disadvantages of direct heating are the requirement of high electrical resis-

tance and electrical connections with low electrical resistance between the SMA and

dissimilar conductive material.

Relatively high electrical resistance of SMA is required to avoid a high electrical

current for actuating the SMA. Though the small or thin structure of SMA has high

electrical resistance, it cannot generate high power. An array of small or thin SMA

structures connected as a series circuit can solve this problem, but the structure is

relatively complex.

Electrical connections with low electrical resistance between SMA and dissimi-

lar conductive material, for example, copper as electrical lead wires, is required to

avoid undesirable consumption of electrical current for heat generation at the con-

nection. In the case of bulk SMA, assembly methods suitable for making electrical

connection and for joining the SMA to dissimilar material are required, for exam-

ple, mechanical ﬁxation, conductive adhesive, soldering and welding. In the case

of thin ﬁlm or plate SMA, it is relatively easy to form a low resistance connection

by coating a conductive metal layer on the SMA or coating an SMA layer on the

conductive metal layer.

Indirect Heating

Indirect heating of an SMA actuator by additional heater lines with high resistance

is advantageous for reducing the electric power needed for actuation. To realize

indirect heating, it is necessary to select an appropriate mechanical design and fabri-

cation processes of the heater and insulator so that they can deform without fracture

during the SMA actuation. Figure 6.41 is a close-up view of the SMA clip actuator

mentioned above [72, 73]. Micro heaters of Pt thin ﬁlm (100 nm thick) are formed

on an SMA thick ﬁlm actuator with a polyimide insulator ﬁlm (2 μm thick).

Figure 6.42 shows an indirect heating SMA coil actuator [92]. A thin Ni ﬁlm (0.6

μm) on a Parylene-coated SMA coil is used as an indirect heater. The Parylene layer

(1 μm thick) is an insulator between the Ni layer and the SMA surface. Parylene can

be deposited uniformly in a vacuum on the entire surface of the SMA coil. A thin Ni

heater layer is formed by electroless plating on the Parylene. No damage is observed

in the Ni or Parylene ﬁlms after 3000 cycles of prolonged experiments with applied

power of 80 mW at 2% strain of SMA coil.

Control

Improvement of Displacement Accuracy

One of the disadvantages of SMA is low accuracy of displacement because of

unstable heat radiation to the circumference, for example, air or solid material.

396 T. Mineta and Y. Haga

Fig. 6.41 Micro SMA clip

with thin ﬁlm heater lines for

indirect heating [72, 73]

(Reprinted with permission.

Fig. 6.42 Indirect heating

SMA coil actuator [92]

(Reprinted with permission.

University Press)

Additional feedback control s ystems are effective for improvement of the dis-

placement accuracy. The temperature of SMA [56] or the strain of heated SMA

[48] are estimated by measuring its electrical resistance. Figure 6.21 shows a micro

manipulator which has SMA ribbons with a thin ﬁlm strain sensor and a heater

for multi-directional bending motion [60]. A Ti/Pt resister pattern on the SMA

plate can measure temperature and displacement of SMA separately because the

two materials have different gauge factors and temperature coefﬁcients, as shown

in Fig. 6.43 [60].

6 Materials and Processes in Shape Memory Alloy 397

Fig. 6.43 Relation of strain and change of resistance [60] (Reprinted with permission. Copyright

1996 SAGA)

6.4 Summary

SMA materials, particularly TiNi-based alloys, have excellent mechanical prop-

erties for actuators, such as large force generation and large deformation. As

introduced in this chapter, many techniques for the preparations of bulk and thin ﬁlm

SMA materials, micromachining and etching processes, and assembly processes

have been developed by many researchers since the 1990s.

In particular, techniques for the deposition of thin ﬁlm SMA have been remark-

ably developed. Shape memory alloy thin ﬁlms with mechanical properties similar

to those of bulk SMAs have been realized. Several assembly techniques, for exam-

ple, mechanical ﬁxation, adhesion, welding, and soldering have been developed and

used as practical applications. Technical progress has resulted in the development of

many SMA micro devices. SMA micro actuators have some disadvantages such as

low displacement controllability. However, integration with other MEMS technolo-

gies, for example, micro thermal and position sensors, micro heaters, bias spring

mechanism, and latch mechanism, can overcome the disadvantages of SMA actua-

tors. Shape memory alloy actuator technologies will continue to evolve in integrated

MEMS systems.

Acknowledgements The authors wish to thank Professor Masayoshi Esashi of Tohoku uni-

versity for support and advices about MEMS process and development of microactuators. The

authors also thank to Professor Kiyoshi Yamauchi of Tohoku university and Professor Eiji

Makino of Hirosaki university for helpful advices on SMA material properties and fabrication

technologies.

398 T. Mineta and Y. Haga

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Chapter 7

Dry Etching for Micromachining Applications

Srinivas Tadigadapa and Franz Lärmer

Abstract Dry etching processes provide the tools to machine precision high-

aspect-ratio structures that form the basic building blocks of microelectromechani-

cal systems. Dry etching processes consist of (1) purely chemical (spontaneous gas

phase etching), (2) purely physical (ion beam etching or ion milling), and (3) a com-

bination of both methods (reactive ion or plasma etching) for the controlled removal

of desired substrate materials. Although some of the pioneering work in the ﬁeld

was performed as early as the 1970s and 1980s, the area of plasma etching has con-

tinued to evolve due to the continuing technological developments in high-density

plasma sources, high-throughput vacuum pumps, and process control and instru-

mentation. Excellent reviews are currently available that provide details in various

aspects of the technology and etching process development. This chapter is aimed

at providing the reader with a broad understanding of the parameters that inﬂuence

the results in dry etching techniques and to provide information that is useful to

explore dry etching processes for the fabrication of next-generation MEMS devices.

Practical recipes suitable for most commonly used plasma reactor conﬁgurations are

provided and motivated in t erms of the inﬂuence of the various control parameters

and chemicals (gases used). Because process parameters can rarely be transferred

directly across equipment or fabrication facilities, it is important to be able to tune

the process parameter to speciﬁc process ﬂow requirements and constraints. The

material presented is not necessarily exhaustive, but focuses instead on practical

issues to tackle f or the development of dry etching processes suitable for MEMS

applications across a broad range of materials.

S. Tadigadapa (B)

Department of Electrical Engineering, The Pennsylvania State University, University Park, PA,

USA; Materials Research Institute, The Pennsylvania State University, University Park, PA, USA

e-mail: srinivas@engr.psu.edu

403

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

MEMS Reference Shelf, DOI 10.1007/978-0-387-47318-5_7,

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Springer Science+Business Media, LLC 2011