Latching Shape Memory Alloy Microactuator ENMA490, Fall 2002 S. Cabrera, N. Harrison, D. Lunking, R. Tang, C. Ziegler, T. Valentine

Outline Background Problem Project Development Design Evaluation Applications Summary/Future Research Applications Device and Process Flow Materials

Problem Statement Assignment: Develop a design for a microdevice, including materials choice and process sequence, that capitalizes on the properties of new materials. Survey: functional materials and MEMS Specific Device Goals: – Actuates – Uses Shape Memory Alloys – Uses power only to switch states Concept: – Latching shape-memory-alloy microactuator

NiTi SMA arm Si island over valve Project Stimulus State of the Art: SMA microactuator –Lai et al. “The Characterization of TiNi Shape-Memory Actuated Microvalves.” Mat. Res. Soc. Symp. Proc. 657, EE8.3.1-EE8.3.6, –Uses SMA arms to raise and lower a Si island to seal the valve. –Uses continuous Joule heating to keep valve open. Joule heating TOP VIEW: SIDE VIEW:

Martensite-Austenite Transformation Twinned domains (symmetric, inter-grown crystals) Shape Memory Alloys Austenite Cooling Polydomain Martensite Applied Stress Single-domain Martensite Re-heating Austenite Applied Stress

Heat SMA2 valve opens Heat SMA1 valve closes SMA2valve stays coolsopen SMA1magnet keeps coolsvalve closed INITIAL DESIGN

Heat SMA1 valve closes Heat SMA2 valve opens SMA1 magnet keeps cools valve closed SMA2 valve stays cools open FINAL DESIGN

Cantilever Positions and Forces Based on beam theory Non-uniform shape change between SMA and substrate causes cantilever bending –Thermal expansion causes bulk strain (  2-  1)  T –Martensite-austenite transformation creates lattice strain  =1-(a aust /a mart ) –Ω = [(  2-  1)  T] or [  ]

Material Properties Young’s Modulus (GPa) Thermal Expansion Coefficient (*10 -6 /K) Lattice Parameter (nm) Si N/A GaAs N/A NiTi (martensite) (smallest axis) NiTi (austenite) lattice/struk/b2.html

Cantilever Positions and Forces Major assumptions: –Can calculate martensite  austenite strain from differing lattice constants –Properties change linearly with austenite-martensite fraction during transformation Deflection –Large effect from SMA, negligible effect (orders of magnitude less) from thermal expansion

Simulation

Simulation – Deflection Results 100μm long, 30μm wide, 2.5μm thick substrate, 0.5μm thick SMA Tip deflection ≈ 39μm, Deflection < ≈ 21°, Tip force ≈ 0.23mN Heat/cool cantilever 1: F(1) > F(magnet) > F(2) Heat/cool cantilever 2: F(2) > F(magnet) > F(1)

Tip Deflection Scaling SMA thickness (um) Length (um) Tip deflection (m) L 0.3L 0.03L

Process Flow (Single Cantilever) -Silicon wafer (green) with silicon dioxide (purple) grown or deposited on front and back surfaces. -Application of photoresist (orange), followed by exposure and development in UV (exposed areas indicated by green). -Buffered oxide etch removes exposed oxide layer. Oxide underneath unexposed photoresist remains. -Removal of photoresist in acetone/methanol is followed by KOH etch to remove exposed silicon until desired cantilever thickness is reached. -Deposition of NiTi (yellow) via sputtering, followed by 500C anneal under stress to train SMA film. -Deposition of magnetic material (blue) using a mask via sputtering on bottom of cantilever.

Process Flow (SMA Training) Small needles hold down cantilevers during post-deposition anneal Training process usually carried out at 500°C for 5 or more minutes Thin film will “remember” its trained shape when it transforms to austenite Degree of actuation determined by deflection of cantilever during training process Small green circles indicate needle placement with respect to cantilever wafer Side view of needle apparatus

Non-Latching Power Cycle Energy use based on time spent in secondary state. –Energy = Power * Time Max energy used when 50% of time spent in secondary state. Above 50%, other type of actuator more efficient. Max energy usage

Latching Power Cycle Energy use based solely on number of switches. –Energy = Energy per cycle * frequency of switching * time used –Least energy used at low power to switch, low frequency of switching Low energy to switch, low frequency, latching is more energy efficient.

Power Considerations Heat cantilevers to induce shape memory effect –P = (mc  T)/t = I 2 R m - mass of cantilever, c - specific heat of cantilever, ΔT - difference between A f and room temperature, t - desired response time –Power differs slightly for martensite and austenite for constant I because of differing resistivity. From simulation: –Required current = 0.27 mA –Required power = W

Applications and Requirements Electrical Contacts –Sensor –Circuit breaker Optical Switching –Telescope mirrors Gas/liquid Valves –Drug release system device outside world TI thermal circuit breaker, Sandia pop-up mirror and drive system,

Summary Final design: dual cantilever system with SMA and magnetic materials to provide latching action Power consumption lower than that of a non-latching design when switching occurs infrequently and uses little energy Future work: –Research magnetic material, packaging –Specify application –Continue analysis and optimization –Build device

Backup

Shape Memory Effect Free-energy versus temperature curves for the parent (G p ) and martensite (G m ) structures in a shape memory alloy. From Otsuka (1998), p.23, fig Martensite-austenite phase transformation in shape memory alloys. From

Material Choice: NiTi SMA Near-equiatomic NiTi most widely used SMA today PropertyValue Transformation temperature -200 to 110  C Latent heat of transformation5.78 cal/g Melting point 1300  C Specific heat0.20 cal/g Young’s modulus83 GPa austenite; 28 to 41 GPa martensite Yield strength195 to 690 MPa austenite; 70 to 140 MPa martensite Ultimate tensile strength895 MPa annealed; 1900 MPa work-hardened % Elongation at failure25 to 50% annealed; 5 to 10% work-hardened From

Nickel-Titanium B2 (cesium chloride) crystal structure. From lattice/struk/b2.html B19’ crystal structure. From Tang et al., p.3460, fig.5. Parent β (austenite) phase with B2 structure Martensite phase with monoclinic B19’ structure