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 micro-actuation 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

MicrogripperTheory of Operation• 2 Main designs of microgrippers

• Location of TiNi 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 TiNi with another element such as Cu

Recommended Improvements of Microgripper

• Reduce residual stress in the thin film– Need to reduce

thermal mismatch between NiTi 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 bioMEMS 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