**3. Results and discussion**

#### **3.1 Stress vs. temperature measurements**

**Figure 1A** shows a reversible phase change with onset at 60°C upon heating for two different NiTi sputter deposition pressures of Ni50Ti50 on 200 nm Pt. Since NiTi was sputtered onto a thin film of Pt on Si for this set, the modified Stoney's equation (2) was used to figure the NiTi film stress. Assuming the thickness of the two films to be similar, the film sputtered at 5 mTorr exhibited a higher value of recovery stress,

**17**

**Figure 1.**

*thickness for SMA properties.*

**3.2 Laser actuation of SMA MEMS**

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

defined as the difference between initial stress and stress in the presumably austenitic phase at elevated temperature. Both films had a similar initial or residual stress of around 300 MPa. The maximum residual stress values peaked around 70°C for each wafer in this experiment, and the process was reversible when cooled back to RT. **Figure 1B** shows that the trend of higher recovery stress (approximately 900 MPa) at lower deposition pressure was the same for two 525 μm thick NiTi films, which in this case were deposited onto Si and stress values were determined with standard Stoney equation (1). Residual stress was lower (70 MPa) for NiTi sputtered onto Si at 5 mTorr compared to the NiTi sputtered at 10 mTorr (230 MPa). Lower residual stress would generally be desired to reduce unwanted deformation of MEMS structures fabricated based on NiTi. These results are also useful, providing

*(A) Stress vs. temperature plots for NiTi sputtered at 600°C under different pressures onto 200 nm Pt for NiTi (A) near 1 μm thickness, (B) near half micron thickness, and (C) approaching 200 nm minimum film* 

confirmation that the SME is similar when we deposited NiTi onto Pt or Si. We also performed wafer bow stress measurements on thinner films of 341 and 270 nm NiTi which were sputtered onto Si at 600°C substrate temperature. **Figure 1C** shows a reversible SME in 341 and 270 nm films sputtered at 10 mTorr. Therefore, significant micro actuation should be achievable in even thinner films.

We measured cantilever actuation under optical irradiation. Devices demonstrated rapid actuation ranging from 2 to 90 ms, depending on optical power

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

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

#### **Figure 1.**

*Advanced Functional Materials*

<sup>σ</sup> = \_*<sup>E</sup>*

<sup>σ</sup>*NiTi* = *<sup>E</sup> hs* \_

**2.2 Laser actuation of SMA MEMS**

heated MEMS actuator.

**3. Results and discussion**

**3.1 Stress vs. temperature measurements**

2 6(1 − ν) *hNiTi*

or more to measure temporal data on activation time.

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

 ( \_1 *<sup>R</sup>* <sup>−</sup> \_1 *Ro* ) − (

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).

> 6(1 − ν) *hs* 2 \_ *h* ( \_1 *<sup>R</sup>* <sup>−</sup> \_1 *Ro*

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.

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)

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

**Figure 1A** shows a reversible phase change with onset at 60°C upon heating for two different NiTi sputter deposition pressures of Ni50Ti50 on 200 nm Pt. Since NiTi was sputtered onto a thin film of Pt on Si for this set, the modified Stoney's equation (2) was used to figure the NiTi film stress. Assuming the thickness of the two films to be similar, the film sputtered at 5 mTorr exhibited a higher value of recovery stress,

\_ 6(1 − ν) *<sup>E</sup>* )(

) (1)

<sup>σ</sup>*Pt*(*hNiTi*<sup>+</sup>*Pt* <sup>−</sup>*hNiTi*) \_\_\_\_\_\_\_\_\_\_\_\_\_\_ *hs* 2

) (2)

**16**

*(A) Stress vs. temperature plots for NiTi sputtered at 600°C under different pressures onto 200 nm Pt for NiTi (A) near 1 μm thickness, (B) near half micron thickness, and (C) approaching 200 nm minimum film thickness for SMA properties.*

defined as the difference between initial stress and stress in the presumably austenitic phase at elevated temperature. Both films had a similar initial or residual stress of around 300 MPa. The maximum residual stress values peaked around 70°C for each wafer in this experiment, and the process was reversible when cooled back to RT.

**Figure 1B** shows that the trend of higher recovery stress (approximately 900 MPa) at lower deposition pressure was the same for two 525 μm thick NiTi films, which in this case were deposited onto Si and stress values were determined with standard Stoney equation (1). Residual stress was lower (70 MPa) for NiTi sputtered onto Si at 5 mTorr compared to the NiTi sputtered at 10 mTorr (230 MPa). Lower residual stress would generally be desired to reduce unwanted deformation of MEMS structures fabricated based on NiTi. These results are also useful, providing confirmation that the SME is similar when we deposited NiTi onto Pt or Si.

We also performed wafer bow stress measurements on thinner films of 341 and 270 nm NiTi which were sputtered onto Si at 600°C substrate temperature. **Figure 1C** shows a reversible SME in 341 and 270 nm films sputtered at 10 mTorr. Therefore, significant micro actuation should be achievable in even thinner films.

#### **3.2 Laser actuation of SMA MEMS**

We measured cantilever actuation under optical irradiation. Devices demonstrated rapid actuation ranging from 2 to 90 ms, depending on optical power

#### **Figure 2.**

*(A) Actuation time vs. laser irradiance for 600 nm NiTi on 20 nm Pt bimorphs, (B) calculated and measured curvature radius for thermally activated NiTi on Pt bimorphs (1.4 μm on 200 nm Pt), and (C) demonstration of passive laser irradiated device-specific actuation with a 532 nm "green" laser (~7.2 W/cm2 ) in 24 ms for the 1.4 μm thick NiTi device stack.*

density as shown in **Figure 2A**. As would be expected for a cantilever beam clamped on one end to a heat sink, the actuation time followed a 1/I2 (I, being intensity) power law. Overall, the devices could be fully actuated in under 20 ms with intensities as low as 2 W/cm<sup>2</sup> . The response time decreased to 3 ms with intensities over 14 W/cm<sup>2</sup> . As shown in **Figure 2C**, 1.4 μm thick NiTi on 200 nm Pt devices actuated into their downward state within 25 ms when irradiated at 7.2 W/cm2 . A slower actuation time of 230 ms was observed at 1.44 W/cm2 . The radius of curvature for the 600 nm NiTi/20 nm Pt stack was 5.4X tighter (200 μm), compared to the 1.2 mm curvature in the 1.4 μm thick NiTi stack. For the tightest curling (200 μm) 600 nm thick NiTi devices, we performed a dynamic optical actuation experiment where we measured actuation time at various laser intensities. These results are plotted in **Figure 2A**.

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

**Figure 3A** shows schematic of the joule heated SEM MEMS resistor actuator including cross section. The bond pads for electrical probe pads or wire bonding are comprised on 200 micron squares of NiTi alloy. The cross section is comprised of 1 micron thick SU-8 epoxy on top of the 270 nm thick NiTi, with reversible SMA properties. We fabricated joule heaters with various widths and lengths (10, 15, 20) and (100, 150, 200, 300, and 400) microns, respectively. The large CTE mismatch between SU-8 and NiTi drives the upward curvature of

**19**

**Figure 4.**

*(50% duty cycle) at 3 kHz.*

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

the MEMS actuator post-release. **Figure 3B** shows SEM of the released actuator. **Figure 3C** depicts the fabrication process flow used to build the actuator, whereby NiTi is patterned with ion milling, and the release etch is done in XeF2. **Figure 3D** shows the thermal actuation of the actuator which is characterized by large, non-linear changes in deflection upon subsequent heating and cooling

*(A) Schematic of the joule heated SEM MEMS resistor actuator including cross section, (B) SEM of released actuator, (C) fabrication process flow used to build the actuator, and (D) thermal actuation of the actuator.*

*Measured displacement of SMA MEMS resistively heated actuator vs. time for a 1 V pulsed square wave* 

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

cycles.

**Figure 3.**

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

the MEMS actuator post-release. **Figure 3B** shows SEM of the released actuator. **Figure 3C** depicts the fabrication process flow used to build the actuator, whereby NiTi is patterned with ion milling, and the release etch is done in XeF2. **Figure 3D** shows the thermal actuation of the actuator which is characterized by large, non-linear changes in deflection upon subsequent heating and cooling cycles.

**Figure 3.**

*Advanced Functional Materials*

density as shown in **Figure 2A**. As would be expected for a cantilever beam clamped on one end to a heat sink, the actuation time followed a 1/I2 (I, being intensity) power law. Overall, the devices could be fully actuated in under 20 ms with intensi-

*(A) Actuation time vs. laser irradiance for 600 nm NiTi on 20 nm Pt bimorphs, (B) calculated and measured curvature radius for thermally activated NiTi on Pt bimorphs (1.4 μm on 200 nm Pt), and (C) demonstration* 

for the 600 nm NiTi/20 nm Pt stack was 5.4X tighter (200 μm), compared to the 1.2 mm curvature in the 1.4 μm thick NiTi stack. For the tightest curling (200 μm) 600 nm thick NiTi devices, we performed a dynamic optical actuation experiment where we measured actuation time at various laser intensities. These results are

**Figure 3A** shows schematic of the joule heated SEM MEMS resistor actuator including cross section. The bond pads for electrical probe pads or wire bonding are comprised on 200 micron squares of NiTi alloy. The cross section is comprised of 1 micron thick SU-8 epoxy on top of the 270 nm thick NiTi, with reversible SMA properties. We fabricated joule heaters with various widths and lengths (10, 15, 20) and (100, 150, 200, 300, and 400) microns, respectively. The large CTE mismatch between SU-8 and NiTi drives the upward curvature of

into their downward state within 25 ms when irradiated at 7.2 W/cm2

*of passive laser irradiated device-specific actuation with a 532 nm "green" laser (~7.2 W/cm2*

actuation time of 230 ms was observed at 1.44 W/cm2

**3.3 Electrical actuation (joule heating) SMA MEMS**

. The response time decreased to 3 ms with intensities over

. A slower

*) in 24 ms for the* 

. The radius of curvature

. As shown in **Figure 2C**, 1.4 μm thick NiTi on 200 nm Pt devices actuated

**18**

ties as low as 2 W/cm<sup>2</sup>

*1.4 μm thick NiTi device stack.*

plotted in **Figure 2A**.

14 W/cm<sup>2</sup>

**Figure 2.**

*(A) Schematic of the joule heated SEM MEMS resistor actuator including cross section, (B) SEM of released actuator, (C) fabrication process flow used to build the actuator, and (D) thermal actuation of the actuator.*

#### **Figure 4.**

*Measured displacement of SMA MEMS resistively heated actuator vs. time for a 1 V pulsed square wave (50% duty cycle) at 3 kHz.*

**Figure 4** shows the measured deflection of the actuator using a 1 V pulsed current with 50% duty cycle at 3 kHz. For these devices the total power drawn was measured to be 5–15 mW.
