Applications of Shape Memory Alloys to MEMS MAE

Applications of Shape Memory Alloys to MEMS MAE 268 Greg Jarmer and Garrett Uyema

Outline of Presentation • Shape Memory Alloys (SMA) • Advantages of SMA’s for actuation of MEMS Devices • Example of an Application: Microgripper • Recommended Improvements of Microgripper • Future Applications of SMA in MEMS

Shape Memory Alloys • Principles of Shape Memory Alloys – Shape Memory Alloys (SMA) are alloys that exhibit the shape memory effect. – The shape memory effect is the process of restoring a deformed material back to an initial shape through a thermally induced crystalline transformation – The crystalline transformation occurs between a low temperature ductile martensitic phase and a high temperature high strength austenitic phase.

Shape Memory Alloys

Advantages of SMA’s in MEMS • The main advantages of SMA’s for microactuation are: – SMA’s are capable of producing a large actuation force – SMA’s are capable of producing large displacements – SMA’s are activated through thermal heating

Disadvantages of SMA’s in MEMS • The main disadvantages of SMA’s are: – Sensitivity of material properties in fabrication – Residual Stress’s developed in thin films – Nonlinearity of actuation force – Lower maximum frequency compared to other microactuator devices

Verification of Activation Force with FEA

Microgripper Theory of Operation • 2 Main designs of microgrippers • Location of Ti. Ni thin film • SMA actuation • Transformation from martensite to austenite heating SMA thin film on the inside

Microgripper • Fabrication of the Microgripper – 2 identical cantilevers and a silicon spacer – Processes used: • • Deposit and etch Thin film sputtering Annealing Eutectic bonding

Recommended Improvements of Microgripper • Be able to control hysteresis temperature range – Change composition of SMA • Can shift hysteresis curve left or right – Alloy Ti. Ni with another element such as Cu

Recommended Improvements of Microgripper • Reduce residual stress in the thin film – Need to reduce thermal mismatch between Ni. Ti and Si substrate – Add a layer of tungsten (W) • Difference in the coefficients of thermal expansion is reduced

Recommended Improvements of Microgripper • Increase the maximum frequency – Decrease the time it takes for phase transformation to occur – Increase surface area to volume ratio • Heat can be dissipated faster

Future Applications of Microgrippers • Grab tiny foreign objects for removal from the body – Facilitates access to intricate regions of the body • Microassembly for MEMS devices • Intravascular Therapy

Future Applications of SMA in MEMS • MEMS and bio. MEMS applications – Eliminate vibrations of read/write heads in hard disk drives – Microstents • Promote flow in tubular passages • Reinforce weak blood vessels – Microsurgery • Cardiovascular applications • Orthopedic applications

Future Applications of SMA in MEMS Simon filter Microstents SMA basket Microwrapper