**1. Introduction**

One of the earliest reported thin film version of Nickel-Titanium shape memory alloy was done in 1990 [1]. The first several accounts of SMA film characterization on Si wafers showed measureable shape memory effects, but all transformations happened below ambient conditions, in part due to the fact that the films tended to be Ni-rich in composition if starting from equiatomic NiTi sputter target, due to the different sputter yields of Ni and Ti. Ni has a higher sputter rate than Ti, and Ti has a tendency to react with any residual oxygen in the deposition chamber. In order to make high performance thermal actuators, it is thus necessary to undergo the necessary processing to ensure transformations are measured above ambient conditions [2]. This is no easy task, but can nonetheless be done by carefully controlling Ni/Ti ratio and thermal processing (i.e., annealing). One of the first SMA-based MEMS actuators was reported out of Case Western University (CWU) in 2001, based on a

sputtered NiTi film capable of recovering about 250 MPa according to their stresstemperature loop on 4" Si wafer [2].

Of the many SMAs available, NiTi has become one of the most widely used due to its exceptional physical and mechanical properties (SME and SE), including large recoverable strains [3]. To understand the reason behind the SME/SE in NiTi, it is necessary to first understand the crystallography. The basis for SME/SE is the switching between two different crystallographic phases, namely the high temperature phase known as austenite (or) the parent phase, and the low temperature phase known as Martensite. The crystal structure of the austenite phase is a CsCl type B2 cubic structure and the low temperature Martensite phase is a complex monoclinic crystal structure (B19'). The martensitic transformation is a diffusionless solid-state phase transformation. During the martensitic transformation, the metal atoms move cooperatively in the matrix under shear stresses. As a result a new phase is formed from the parent phase. To accommodate the internal stresses caused by the transformation to the B19' phase, the formation of a combination of up to 24 multiple martensitic variants is possible, resulting in a twinned Martensite crystal form, also known as self-accommodated Martensite.

TiNi thin films are in demand for applications in actuators for micro-electromechanical systems (MEMS) [4–12], because these films exhibit large displacement, accompanied by the shape memory effect (SME) through the B2 austenite to B19' monoclinic Martensite transformation. The majority of TiNi films are fabricated by RF or DC magnetron sputtering methods [13–20], and these films are amorphous, unless the substrates are heated during deposition [16, 20]. Post deposition annealing at a temperature above 700 K (equivalent to 427°C) for crystallization is necessary for the films initially deposited in amorphous condition to show the shape memory effect [21]. It is noted that Ti–Ni thin films sputter-deposited at ambient temperature are often amorphous, thus require post-sputtering crystallizing at elevated temperature to obtain the desired shape memory property. It is also possible to crystallize TiNi films during deposition by utilizing a heated substrate above an ambient temperature [22]. The TiNi films deposited in this manner exhibit interesting behaviors such as lowered crystallization temperature and oriented crystallographic structure [18, 19]. For example, Ikuda observed that the NiTi film deposited onto a glass substrate at 673 K (or 400°C) produced crystallinity in the NiTi film. Other, more recent studies also looked into the SMA properties of in-situ annealed NiTi films.

Regarding the in-situ crystallization of NiTi, Gisser also observed that the films deposited on (100) silicon (Si) substrates at 733 K (equivalent to 500°C) showed a (110)-oriented crystalline structure of the austenite phase [23]. By incorporation of a Ru seed layer, epitaxial growth of the NiTi alloy can be achieved at some of the lowest deposition and crystallization temperatures, and thinnest films reported to date [24]. Hou also observed that the films deposited onto quartz and polyimide substrates above 623 K (equivalent to 350°C) showed a strong (110)-oriented crystalline structure [25–28]. This range of crystallization temperatures of the TiNi films suggests that the crystallization process is affected by the surface condition of the substrates. However, it is not clear why the heated substrates lowers the crystallization temperature and enhances a particular orientation for the TiNi thin films. This is due to lack of understanding the process of film growth during deposition. The composition and structure of sputtered Ni-Ti shape memory alloy (SMA) films are significantly affected by the sputtering conditions: target power, gas pressure, target to substrate distance, deposition temperature, substrate bias voltage, etc.

Some fundamental limitations for shape memory MEMS are related to (1) how thin could one go and still be able to measure reversible shape memory effects,

**15**

*Fabrication and Characterization of Nanoscale Shape Memory Alloy MEMS Actuators*

(2) how fast could one conceivably actuate the SMA MEMS device (limitations previously existed) that would not allow for heat transfer to happen much faster than several 10s, or at most, 100 Hz. Several accounts claim that NiTi films should be at least 100–400 nm thick [15] to help ensure shape memory effects which can be

suppressed by film/substrate interfacial strains and small grain sizes [29–33].

Combined with SMA's natural advantages of large displacements, and high work densities [8], our efforts have demonstrated major breakthroughs in the bandwidth, or speed with which NiTi could be actuated, and thus enabled additional possibilities to use NiTi in microelectronics and MEMS. Shape memory MEMS can certainly now be used for higher frequency actuation applications such as mechanical logic, signal routing, and switching, and at relatively low power and energy consumption. In thin films the roll of texture is extremely important in improving shape memory

In the preceding paragraphs we overviewed the important developments in NiTi thin films processing and characterization. As such, we aim to use the next paragraph to highlight some of the more relevant MEMS device implementations of

In 2004, high frequency actuation based on SMA MEMS was only 20–40 Hz [35], and considerable improvements have been made since then. By shrinking the volume and heat capacity of the SMA MEMS actuators, we showed for the first time, reversible actuation beyond 1 kHz frequencies, verifying that the heat transfer (in other words, the heating and cooling of shape memory alloy), could happen more than 1000 times per second. For example, green laser actuation of shape memory MEMS bimorph actuators was characterized in [9], whereby actuation response happened in just a few milliseconds. Here, the authors showed that NiTi bimorph cantilevers with nanometer thickness NiTi, could be actuated in under 100 ms, with as little as 2 W/

. In another paper, the high cycle frequency of actuation and electrical characterization of SMA MEMS device was characterized to include (resistance, current, and power requirements) [5]. Here, it was demonstrated that NiTi bimorph resistor actuators could be actuated with as little as 0.5 V, requiring just 5–15 mA of power, and at rates faster than 1000 times per second (up to 3 kHz) due to the small volume and rapid heat transfer facilitated by large surface to volume ratios. Expanding upon this work even further, these same NiTi films were integrated with nanoscale 3D printing to enable some impressive actuation metrics [36, 37]. Specifically, by 3D printing polymeric materials mated with NiTi films, the following metrics were achieved: >5000 reversible actuation cycles with very limited degradation, low voltage actuation of 3.7 V (which is compatible with common Li-ion batteries), large strokes (85 μm for 415 μm length cantilever), and large force-displacement product

N-m, with an impressively small volume and weight (1.04 × 10<sup>−</sup><sup>5</sup>

Stress versus temperature measurements were performed using a Toho FLX-2320-S wafer bow tool with controlled heating and cooling from 25 to 100°C with a heating

was achieved using a pulsed laser to actuate a FIB cut NiTi SMA microactuator spring of 25 μm thickness. Most recently NiTi SMA has even recently been integrated with Si photonics to form a physically actuated optical coupler/de-coupler type device with excellent nano-positioning accuracy to within 4 nm and on/off ratio of 9 dB [7, 39].

g, respectively). By comparison in [38], 1.6 kHz actuation frequency

cm3

*DOI: http://dx.doi.org/10.5772/intechopen.92762*

properties like reversible strain [34].

NiTi SMA films.

cm2

of 1.2 × 10<sup>−</sup><sup>7</sup>

and 1.27 × 10<sup>−</sup><sup>5</sup>

**2. Methods**

**2.1 Stress vs. temperature measurements**

#### *Fabrication and Characterization of Nanoscale Shape Memory Alloy MEMS Actuators DOI: http://dx.doi.org/10.5772/intechopen.92762*

(2) how fast could one conceivably actuate the SMA MEMS device (limitations previously existed) that would not allow for heat transfer to happen much faster than several 10s, or at most, 100 Hz. Several accounts claim that NiTi films should be at least 100–400 nm thick [15] to help ensure shape memory effects which can be suppressed by film/substrate interfacial strains and small grain sizes [29–33].

Combined with SMA's natural advantages of large displacements, and high work densities [8], our efforts have demonstrated major breakthroughs in the bandwidth, or speed with which NiTi could be actuated, and thus enabled additional possibilities to use NiTi in microelectronics and MEMS. Shape memory MEMS can certainly now be used for higher frequency actuation applications such as mechanical logic, signal routing, and switching, and at relatively low power and energy consumption. In thin films the roll of texture is extremely important in improving shape memory properties like reversible strain [34].

In the preceding paragraphs we overviewed the important developments in NiTi thin films processing and characterization. As such, we aim to use the next paragraph to highlight some of the more relevant MEMS device implementations of NiTi SMA films.

In 2004, high frequency actuation based on SMA MEMS was only 20–40 Hz [35], and considerable improvements have been made since then. By shrinking the volume and heat capacity of the SMA MEMS actuators, we showed for the first time, reversible actuation beyond 1 kHz frequencies, verifying that the heat transfer (in other words, the heating and cooling of shape memory alloy), could happen more than 1000 times per second. For example, green laser actuation of shape memory MEMS bimorph actuators was characterized in [9], whereby actuation response happened in just a few milliseconds. Here, the authors showed that NiTi bimorph cantilevers with nanometer thickness NiTi, could be actuated in under 100 ms, with as little as 2 W/ cm2 . In another paper, the high cycle frequency of actuation and electrical characterization of SMA MEMS device was characterized to include (resistance, current, and power requirements) [5]. Here, it was demonstrated that NiTi bimorph resistor actuators could be actuated with as little as 0.5 V, requiring just 5–15 mA of power, and at rates faster than 1000 times per second (up to 3 kHz) due to the small volume and rapid heat transfer facilitated by large surface to volume ratios. Expanding upon this work even further, these same NiTi films were integrated with nanoscale 3D printing to enable some impressive actuation metrics [36, 37]. Specifically, by 3D printing polymeric materials mated with NiTi films, the following metrics were achieved: >5000 reversible actuation cycles with very limited degradation, low voltage actuation of 3.7 V (which is compatible with common Li-ion batteries), large strokes (85 μm for 415 μm length cantilever), and large force-displacement product of 1.2 × 10<sup>−</sup><sup>7</sup> N-m, with an impressively small volume and weight (1.04 × 10<sup>−</sup><sup>5</sup> cm3 and 1.27 × 10<sup>−</sup><sup>5</sup> g, respectively). By comparison in [38], 1.6 kHz actuation frequency was achieved using a pulsed laser to actuate a FIB cut NiTi SMA microactuator spring of 25 μm thickness. Most recently NiTi SMA has even recently been integrated with Si photonics to form a physically actuated optical coupler/de-coupler type device with excellent nano-positioning accuracy to within 4 nm and on/off ratio of 9 dB [7, 39].
