Title: Fabrication and Characterization of Nanoscale Shape Memory Alloy Mems Actuators
Author: Cory Ray Knick
Date & Time: July 1, 2020 – 2:30-3:30pm
Committee: Dr. Hugh Bruck, Associate Dean for Faculty Affairs, Chair
Dr. Christopher Morris, Branch Chief, US Army Research Laboratory
Dr. Patrick McCluskey, Professor, Dept. of Mechanical Engineering
Dr. Don DeVoe, Professor and Associate Chair, Dept. of Mechanical Engineering
Dr. Miao Yu, Professor, Dept. of Mechanical Engineering
Dr. Ichiro Takeuchi, Deans Representative, Professor of Materials Science and Engineering
Abstract: The miniaturization of engineering devices has created interest in new actuation methods capable of large displacements and high frequency responses. Shape memory alloy (SMA) thin films have exhibited one of the highest power densities of any material used in these actuation schemes with thermally recovery strains of up to 10%. With the use of a biasing force, such as from a passive layer in a bilayer structure, homogenous SMA films can experience reversible shape memory effects provided they are thick enough that the crystal structure is capable of transforming. However, thick films exhibit lower actuation displacements and speeds because of the larger inertial resistance. Therefore, there is a need to find a way to process thinner SMA films with grain structures that are capable of transformation in order to realize larger actuation displacements at higher speeds.
In this work, a near-equiatomic NiTi magnetron co-sputtering process was developed to create nanoscale thick SMA films. By using a metallic seed layer, it was possible to induce the crystallization of epitaxial, columnar grains exhibiting the shape memory effects in nanoscale films. It was also possible to crystalize these SMA films at lower processing temperatures compared to directly sputtering thicker films onto Si wafers. The transformation behavior associated with the SME in these films were characterized using x-ray diffraction (XRD), differential scanning calorimetry (DSC), and stress-temperature measurements at wafer level. After quantifying the shape memory effects at wafer-level, the SMA films were used to fabricate various microscale MEMS actuators. The SMA films were mated in several “bimorph” configurations to induce out of plane curvature in the low-temperature martensite phase. The curvature radius vs. temperature was characterized on MEMS cantilever structures to elucidate a relationship between residual stress, recovery stress, radius of curvature, and degree of unfolding. SMA MEMS actuators were fabricated and tested using joule heating to demonstrate rapid electrical actuation of NiTi MEMS devices at some of the lowest powers (5 – 15 mW) and operating frequencies (1 – 3 kHz) ever reported for SMA actuators. By developing a process to create nanoscale thickness NiTi SMA films, we enabled the fabrication of MEMS devices with full, reversible, actuation as low as 0.5 V. This indicated the potential of these devices to be used for high frequency, low power, and large displacement applications in power constrained environments (i.e. on chip).