**2. Methods**

#### **2.1 Stress vs. temperature measurements**

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

*Advanced Functional Materials*

temperature loop on 4" Si wafer [2].

form, also known as self-accommodated Martensite.

sputtered NiTi film capable of recovering about 250 MPa according to their stress-

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

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

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,

**14**

annealed NiTi films.

and cooling rate of 1°C/min. For these experiments, we prepared films of NiTi by sputtering onto 4-inch silicon (Si) wafers and vacuum annealing at 600, 500, and 450°C to crystallize the material. Additionally, we measured several NiTi on Si wafers where the NiTi was sputtered under 600°C substrate conditions. In later efforts, we characterized these films with NiTi in-situ anneals of 325, 350, 375, 400, 425, and 500°C. Wafer bow was measured experimentally from 25 to 100°C at a 1°C/min heating and cooling rates, which allowed us to calculate and plot the temperature-dependent residual stress in the NiTi film for each wafer sample based on Stoney's equation (1). <sup>σ</sup> = \_*<sup>E</sup>*

$$
\sigma = \frac{E}{6(1-\nu)} \frac{h\_s^{-2}}{h} \left(\frac{1}{R} - \frac{1}{R\_o}\right) \tag{1}
$$

Here, σ is the stress in the thin film, and E, ν, and hs are Young's modulus of Si, Poisson ratio of the Si substrate and the thickness of the Si substrate, respectively. h represents the NiTi thin film thickness and R and Ro represent the radii of curvature of the NiTi film- Si substrate composite and the curvature of the bare Si substrate. We used an extended version of Stoney's equation (2) in order to calculate the stress in the NiTi layer when deposited on a thin Pt film on Si wafer. Here, σNiTi is the stress in the NiTi layer, and σPt represents the stress in the annealed Pt layer. The variable hNiTi represents the NiTi thin film thickness, and R and Ro represent the radii of curvature of the NiTi film and the annealed Pt/Si substrate, respectively. <sup>σ</sup>*NiTi* = *<sup>E</sup> hs* \_ \_

$$\text{The NiTi film and the annealed Pt/Si substrate, respectively.}$$

$$\sigma\_{\text{NTI}} = \frac{E \, h\_i^2}{6(1 - \nu) \, h\_{\text{NTI}}} \left( \frac{1}{R} - \frac{1}{R\_o} \right) - \left( \frac{6(1 - \nu)}{E} \right) \left( \frac{\sigma\_{\text{Pt}} \{h\_{\text{NTI};\text{Pt}} - h\_{\text{NTI}} \}}{h\_i^2} \right) \tag{2}$$

#### **2.2 Laser actuation of SMA MEMS**

We also used a 400 mW, 532 nm green laser exiting a 400 μm diameter optical fiber to irradiate and heat released cantilevers with a known optical intensity level. We used optical density filters (ThorLabs) to control the laser irradiance levels. The distance from the optical fiber exit and therefore laser spot size were fixed at 1 mm, which allowed calculation of the optical intensity. We used a Photron Fastcam camera connected to a microscope to record video at 2000 frames per second (fps) or more to measure temporal data on activation time.

#### **2.3 Electrical actuation (joule heating) SMA MEMS**

We build stressed bimorph actuators out of SU-8 and NiTi, whereby a pulsed current through the freestanding NiTi 'resistor' caused rapid heating and cooling through Joule Heating. Deflection was monitored using laser Doppler Vibrometry (LDV) experimental setup. A Keithly power meter was used to pulse current (square wave) at various frequencies (2–3000 Hz) through the NiTi resistively heated MEMS actuator.
