Latching Shape Memory Alloy Microactuator

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

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

3. 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

4. Si island over valve Joule heating NiTi SMA arm 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, 2001. • Uses SMA arms to raise and lower a Si island to seal the valve. • Uses continuous Joule heating to keep valve open. TOPVIEW: SIDEVIEW:

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

6. Heat SMA2 valve opens SMA2 valve stayscools open SMA1 magnet keepscools valve closed Heat SMA1 valve closes INITIALDESIGN

7. Heat SMA1 valve closes SMA1 magnet keepscools valve closed SMA2 valve stayscools open Heat SMA2 valve opens FINALDESIGN

8. Cantilever Positions and Forces • Based on beam theory • Non-uniform shape change between SMA and substrate causes cantilever bending • Thermal expansion causes bulk strain (a2-a1)DT • Martensite-austenite transformation creates lattice strain e=1-(aaust/amart) • Ω = [(a2-a1)DT] or [e]

9. Material Properties http://www.keele.ac.uk/depts/ch/resources/xtal/classes.html, http://cst-www.nrl.navy.mil/ lattice/struk/b2.html

10. 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

11. Simulation

12. 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)

13. 0.3L 0.03L L Tip Deflection Scaling Tip deflection (m) Length (um) SMA thickness (um)

14. 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.

15. Small green circles indicate needle placement with respect to cantilever wafer Side view of needle apparatus 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

16. 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

17. 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.

18. Power Considerations • Heat cantilevers to induce shape memory effect • P = (m•c•DT)/t = I2R • m - mass of cantilever, c - specific heat of cantilever, ΔT - difference between Af 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 = 0.097 W

19. outside world device Applications and Requirements • Electrical Contacts • Sensor • Circuit breaker • Optical Switching • Telescope mirrors • Gas/liquid Valves • Drug release system TI thermal circuit breaker, http://www.ti.com/mc/docs/precprod/docs/tcb.htm Sandia pop-up mirror and drive system, http://mems.sandia.gov/scripts/images.asp

20. 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

21. Backup

22. Shape Memory Effect Free-energy versus temperature curves for the parent (Gp) and martensite (Gm) structures in a shape memory alloy. From Otsuka (1998), p.23, fig. 1.17. Martensite-austenite phase transformation in shape memory alloys. From http://www.tiniaerospace.com/sma.html.

23. Property Value Transformation temperature -200 to 110 C Latent heat of transformation 5.78 cal/g Melting point 1300 C Specific heat 0.20 cal/g Young’s modulus 83 GPa austenite; 28 to 41 GPa martensite Yield strength 195 to 690 MPa austenite; 70 to 140 MPa martensite Ultimate tensile strength 895 MPa annealed; 1900 MPa work-hardened % Elongation at failure 25 to 50% annealed; 5 to 10% work-hardened Material Choice: NiTi SMA • Near-equiatomic NiTi most widely used SMA today From http://www.sma-inc.com/NiTiProperties.html

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