Latching Shape Memory Alloy Microactuator ENMA490, Fall 2002 S. Cabrera, N. Harrison, D. Lunking, R. Tang, C. Ziegler, T. Valentine.
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Latching Shape Memory Alloy Microactuator
ENMA490, Fall 2002S. Cabrera, N. Harrison, D. Lunking,
R. Tang, C. Ziegler, T. Valentine
Outline
• Background
• Problem
• Project Development
• Design
• Evaluation
• Applications
• Summary/Future Research
ApplicationsDevice 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, 2001.
– Uses SMA arms to raise and lower a Si island to seal the valve.
– Uses continuous Joule heating to keep valve open.
Joule heatingTOPVIEW:
SIDEVIEW:
• 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
SMA2 valve stayscools open
SMA1 magnet keepscools valve closed
INITIALDESIGN
Heat SMA1
valve close
s
Heat SMA2
valve open
s
SMA1 magnet keepscools valve closedSMA2 valve
stayscools open
FINALDESIGN
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-(aaust/amart)
– Ω = [(2-1)T] or []
)232(2)()(
)(62221
21212121
22222
22111
21212121
ttttttEEbbtEbtEb
ttttEEbbk
2
2kLd 3
3
L
EIdF
Material PropertiesYoung’s Modulus (GPa)
Thermal Expansion Coefficient (*10-6/K)
Lattice Parameter (nm)
Si 190 2.33 N/A
GaAs 85.5 5.73 N/A
NiTi (martensite) 28-41 11 0.2889 (smallest axis)
NiTi (austenite) 83 6.6 0.3015
http://www.keele.ac.uk/depts/ch/resources/xtal/classes.html, http://cst-www.nrl.navy.mil/ lattice/struk/b2.html
Cantilever Positions and Forces
• Major assumptions:– Can calculate martensiteaustenite 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)
0.1 0.5 1 5 10 15 20 25 30 35 40 45 50
1050
100500
100050001.E-06
1.E-05
1.E-04
1.E-03
1.E-02
Tip Deflection Scaling
SMA thickness (um)Length
(um)
Tip
def
lect
ion
(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.
Non-latching Duty Cycle
0
20
40
60
80
100
0 10 20 30 40 50 60 70 80 90 100
Time Closed (%)
Cu
mu
lati
ve
En
erg
y C
on
su
me
d
(arb
. u
nit
s)
Normally open Normally Closed
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.
Latching Duty Cycles
0
5
10
15
20
25
30
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Switches (cycles * 2)
Cu
mu
lati
ve
En
erg
y c
on
su
me
d
(arb
. un
its
)
Low Power, Low Freq Low Power, High Freq
High Power, Low Freq High Power, High Freq
Power Considerations
• Heat cantilevers to induce shape memory effect– P = (m•c•T)/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
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, http://www.ti.com/mc/docs/precprod/docs/tcb.htmSandia 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 (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.
Material Choice: NiTi SMA
• Near-equiatomic NiTi most widely used SMA today
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
From http://www.sma-inc.com/NiTiProperties.html
Nickel-Titanium
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.
Parent β (austenite) phase with B2 structure
Martensite phase with monoclinic B19’ structure
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