Transcript Latching SMA Microactuator

Comments from GWR in yellow boxes.
An excellent project and presentation overall.
Presentation grade = A.
- GWR
Latching Shape Memory Alloy
Microactuator
ENMA490, Fall 2002
S. Cabrera, N. Harrison, D. Lunking,
R. Tang, C. Ziegler, T. Valentine
Outline
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Background
Problem
Project Development
Design
Evaluation
Device and
Process Flow
Applications
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, EE8.3.1-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.
TOP
VIEW:
SIDE
VIEW:
Si island over
valve
NiTi SMA arm
Joule
heating
Shape Memory Alloys
• Martensite-Austenite Transformation
Cooling
Austenite
Applied
Stress
Applied
Stress
Re-heating
Polydomain
Martensite
Austenite
Single-domain
Martensite
• Twinned domains (symmetric, inter-grown crystals)
Heat
SMA2
valve
opens
SMA1
cools
magnet keeps
valve closed
INITIAL
DESIGN
Heat
SMA1
valve
closes
SMA2
cools
valve stays
open
This slide would benefit from labeling the
SMA1 and SMA2 for describing
the actuation
Heat
SMA1
sequence
valve
closes
SMA2
cools
valve stays
open
FINAL
DESIGN
Heat
SMA2
valve
opens
SMA1
cools
magnet keeps
valve closed
Cantilever Positions and Forces
Wuttig
•Dr.Based
oncommented
beam theorythat we should be more
general in describing the lattice strain portion,
•notNon-uniform
shape change
making the specific
strainsbetween
directly SMA
and substrate
causes
cantilever
bending
coupled
to specific
lattice
constants.
– Thermal expansion causes bulk strain
(a2-a1)DT
– Martensite-austenite transformation
creates lattice strain e=1-(aaust/amart)
– Ω = [(a2-a1)DT] or [e]
d 
kL
2
F 
L
2
k 
3 EId
3
6 b1 b 2 E 1 E 2 t 1 t 2 ( t 1  t 2 ) 
( b1 E 1 t 1 )  ( b 2 E 2 t 2 )  2 b1 b 2 E 1 E 2 t 1 t 2 ( 2 t 1  3 t 1 t 2  2 t 2 )
2
2
2
2
2
2
Material Properties
Young’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)
6.6
0.3015
83
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 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
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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
1.E-03
1.E-04
1.E-05
Tip deflection (m)
1.E-02
0.1
25
30
35
40
45
50
20
15
10
5
1
5000
1000
500
100
50
10
0.5
1.E-06
0.3L
0.03L
L
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).
This schematic is more complete than that in
the report. It would
be helped if the wafer
-Buffered oxide etch removes exposed oxide layer.
bonding/glueing
underneath
step unexposed
could bephotoresist
indicated
remains.
somehow.
Oxide
-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
Non-latching Duty Cycle
• Energy use based on time
spent in secondary state.
Cumulative Energy Consumed
(arb. units)
100
– Energy = Power * Time
Max energy usage
80
60
• Max energy used when
50% of time spent in
secondary state.
40
20
0
0
10
20
30
40
50
60
70
80
Time Closed (%)
Normally open
Normally Closed
90
100
• Above 50%, other type of
actuator more efficient.
Cumulative Energy consumed
(arb. units)
Latching Power Cycle
The x-axis here is confusing, maybe
misleading. If the x-axis is # switching
Latching Duty Cycles
• Energy use based solely
actions, then the steps in energy expended
on number of switches.
should occur only at the switches. However,
– Energy = Energy per cycle
if the axis is time, then the faster switch
* frequency of switching *
frequency comes out right. More about
time this
used in
the final report.
– Least energy used at low
30
25
20
15
10
power to switch, low
frequency of switching
5
0
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Switches (cycles * 2)
Low Power, Low Freq
High Power, Low Freq
Low Power, High Freq
High Power, High Freq
• Low energy to switch, low
frequency, latching is
more energy efficient.
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
Applications and Requirements
• Electrical Contacts
– Sensor
– Circuit breaker
• Optical Switching
– Telescope mirrors
• Gas/liquid Valves
outside world
– Drug release system
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 (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
Parent β (austenite)
phase with B2 structure
B2 (cesium chloride) crystal structure. From
http://cst-www.nrl.navy.mil/ lattice/struk/b2.html
Martensite phase with
monoclinic B19’ structure
B19’ crystal structure. From Tang et
al., p.3460, fig.5.