Latching Shape Memory Alloy Microactuator ENMA490, Fall 00 S. Cabrera, N. Harrison, D. Lunking, R. Tang, C. Ziegler, T. Valentine
Outline Background Problem Project Development Design Evaluation Applications Device and Process Flow Summary/Future Research Materials Applications
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
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.-EE8.3.6, 00. Uses SMA arms to raise and lower a Si island to seal the valve. Uses continuous Joule heating to keep valve open. TOP VIEW: Si island over valve SIDE VIEW: Joule heating NiTi SMA arm
Shape Memory Alloys Martensite-Austenite Transformation Cooling Applied Stress Applied Stress Re-heating Austenite Polydomain Martensite Single-domain Martensite Austenite Twinned domains (symmetric, inter-grown crystals)
Heat SMA valve opens SMA cools magnet keeps valve closed INITIAL DESIGN SMA cools valve stays open Heat SMA valve closes
Heat SMA valve closes SMA cools valve stays open FINAL DESIGN SMA cools magnet keeps valve closed Heat SMA valve opens
Cantilever Positions and Forces Based on beam theory Non-uniform shape change between SMA and substrate causes cantilever bending Thermal expansion causes bulk strain (α-α) T Martensite-austenite transformation creates lattice strain ε=-(a aust /a mart ) Ω =[(α-α) T] or [ε] ) 3 ( ) ( ) ( ) ( 6 t t t t t t E E b b t E b E t b t t t t E E b b k + + + + Ω + = kl d = 3 3 L EId F =
Material Properties Young s Modulus (GPa) Thermal Expansion Coefficient (*0-6 /K) Lattice Parameter (nm) Si 90.33 N/A GaAs 85.5 5.73 N/A NiTi (martensite) 8-4 0.889 (smallest axis) NiTi (austenite) 83 6.6 0.305 http://www.keele.ac.uk/depts/ch/resources/xtal/classes.html, http://cst-www.nrl.navy.mil/ lattice/struk/b.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 00µm long, 30µm wide,.5µm thick substrate, 0.5µm thick SMA Tip deflection 39µm, Deflection <, Tip force 0.3mN Heat/cool cantilever : F() > F(magnet) > F() Heat/cool cantilever : F() > F(magnet) > F()
Tip Deflection Scaling 0. 0.5 5 0 5 0 5 30 35 40 45 50 0 50 00 500 000 5000.E-0 Tip deflection (m).e-03.e-04.e-05.e-06 L 0.3L SMA thickness (um) Length (um) 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 Small green circles indicate needle placement with respect to cantilever wafer Thin film will remember its trained shape when it transforms to austenite Degree of actuation determined by deflection of cantilever during training process Side view of needle apparatus
Non-Latching Power Cycle Cumulative Energy Consumed (arb. units) 00 80 60 40 0 0 Non-latching Duty Cycle Max energy usage 0 0 0 30 40 50 60 70 80 90 00 Time Close d (%) Normally open Normally Closed 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.
Latching Power Cycle Cumulative Energy consumed (arb. units) 30 5 0 5 0 5 0 Latching Duty Cycles 0 3 4 5 6 7 8 9 0 3 4 5 Switches (cycles * ) Low Power, Low Freq Low Power, High Freq 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. High Power, Low Freq High Power, High Freq
Power Considerations Heat cantilevers to induce shape memory effect P = (m c T)/t = I R m - mass of cantilever, c - specific heat of cantilever, T - 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 = 0.7 ma Required power = 0.097 W
Applications and Requirements Electrical Contacts Sensor Circuit breaker Optical Switching Telescope mirrors Gas/liquid Valves Drug release system outside world device 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
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 (998), p.3, fig..7. Martensite-austenite phase transformation in shape memory alloys. From http://www.tiniaerospace.com/sma.html.
Material Choice: NiTi SMA Near-equiatomic NiTi most widely used SMA today Property Transformation temperature Latent heat of transformation Melting point Specific heat Young s modulus Yield strength Value -00 to 0 C 5.78 cal/g 300 C 0.0 cal/g 83 GPa austenite; 8 to 4 GPa martensite 95 to 690 MPa austenite; 70 to 40 MPa martensite Ultimate tensile strength 895 MPa annealed; 900 MPa work-hardened % Elongation at failure 5 to 50% annealed; 5 to 0% work-hardened From http://www.sma-inc.com/nitiproperties.html
Nickel-Titanium Parent β (austenite) phase with B structure Martensite phase with monoclinic B9 structure B (cesium chloride) crystal structure. From http://cst-www.nrl.navy.mil/ lattice/struk/b.html B9 crystal structure. From Tang et al., p.3460, fig.5.