shape memory alloy latching microactuator

SHAPE MEMORY
ALLOY LATCHING
MICROACTUATOR
By Stacy Cabrera, Nicole Harrison, David Lunking,
Rebecca Tang, Theresa Valentine,
and Christopher Ziegler
ENMA 490 Fall 2002 Professor Gary Rubloff
The goal of this project was to design a MEMS device that uses cutting edge
materials. After researching different MEMS devices and smart materials, the group
decided on a shape memory alloy microactuator. Shape memory alloys make use of a
twinning property that allows them to change shape upon heating. The material
changes from a mechanically formed shape in its cold martensite state to a
“remembered” shape in its heated austenite state. When it cools, the material once
again can be mechanically deformed. A literature search only turned up one result that
detailed a valve using SMA arms to lift a Si island. The group decided to improve upon
this design by changing the mechanical structure and turning the actuator into a
latching device that reduces power usage. The design process evolved from a
mechanical latch and two-way SMAs to one-way SMAs deposited on cantilevers with a
magnetic latch. Calculations were done to figure out the possible cantilever deflection,
which ranged from 1 micron to 0.1 millimeter depending on the size of the cantilever
and the thickness of the SMA. The process flow developed for the actuator involved the
mask design and the training and annealing of the SMAs. The SMA used is NiTi and
the substrate is either Si or GaAs. The device constraints are evaluated and possible
applications are mentioned within the report. The primary application of the actuator
is for electrical contacts and secondary ones are optical mirrors, valves, and drug
delivery systems. Future research can be done regarding optimization of the device
properties and a model of the device can be built.
1. INTRODUCTION
Problem Statement
The goal of the project was to develop a latching shape-memory alloy (SMA)
microactuator. An existing design of a SMA microvalve that served as a microactuator
and had new technology that could be improved upon was found in an article, The
Characterization of TiNi Shape-Memory Actuated Microvalves (Lai, 2001). A newer
design was needed that incorporated SMAs and latching technology. The article State-ofthe-art Shape Memory Actuators (Johnson, Kramer 1998) describes the current state of
SMA actuator development and mentions a movement to a latching design and
manufacturing on the micron scale as the next step in processing. The existing design
consumed power the entire time the valve was kept in its secondary state (open or closed
depending on design) and was on the millimeter scale. A newer design was needed that
used microelectromechanical systems (MEMS), used lower power consumption, and was
smaller. The primary goal of reducing power consumption can be fixed by converting the
actuator to a latching design and the system can be downsized to the micrometer scale
(See Figure 1a).
Background
MEMS devices are microelectromechanical systems that can be used in many
applications such as sensors and actuators. In general, MEMS involve both electric and
non-electric parts and perform functions that include sensing, actuating, signal
processing, displaying, and controlling (Senturia, 2001). They can be used in chemical,
mechanical, and medical applications. In the case of this project, the group is working on
actuators--devices that trigger an action response.
MEMS are made from a variety of processes, mostly involving some form of
lithography-based microfabrication combined with micromachining. For threedimensional devices, thin-film deposition, etching, and wafer-bonding techniques are
used to make the creation of moving parts possible. Silicon is commonly used in these
processes because the machines are already calibrated to match the properties of silicon
wafers. Inorganic materials such as silicon, silicon dioxide, silicon nitride, aluminum, and
tungsten as well as some polymers are often used in MEMS (Senturia, 2001).
This project will investigate wafer-bonding, optical lithography, masking, and
etching as the processes used for manufacturing the actuator.
Initial Concept
The initial response to the problem was to develop a “latching” design for the
already established SMA microvalve to save power. The group wanted some way to hold
the secondary state of the valve (open or closed depending on design) without applying
continuous power. Several design concepts were offered including: a technical latching
model using ferromagnetics (see materials section) controlled by magnetic fields, a
mechanical latching model using a pushing object and Velcro (Figure 1c), and a smart
materials model using a two-way SMA. The group decided that a latching design was
possible. The next step was to decide how and why a latching model worked better than a
conventional actuator that required continuous power. The decision to pursue a latching
1
design was chosen after comparing the duty cycles of latching and non-latching designs
as seen in Figures 1a and 1b.
Previous Research
An article on SMA microvalve actuators was published in 2001 that discussed a
valve design that was normally closed (Lai, 2001). Power had to be applied continuously
to heat the TiNi SMA wires to keep the valve open. The design included 8 SMA wires
connected to a Si island in a square shaped valve on the millimeter scale. Figure 1d
shows the microactuator developed at Case Western University.
Dr. Manfred Wuttig in the Department of Materials Engineering at the University
of Maryland has also done extensive research on SMAs, thin film SMAs. Shape memory
alloys, especially nickel-titanium alloys, have been of interest lately because of their use
as microactuators in microelectromechanical systems (MEMS). Manfred Wuttig and his
colleagues at the University of Maryland, College Park have studied TiNi thin films in
order to better understand the properties of shape memory alloys as thin films. Research
shows that bimorphs of one-dimensional thin film strips patterned on to two-dimensional
silicon substrates under uniaxial stresses have increased transformation-induced
deflection (Mori et al., 2002). The bimorph cantilever systems also have decreased
thermo-elastically induced deflection compared to the bimorphs of unpatterned, planar
TiNi thin films on silicon substrates (Mori et al., 2002). When double layering the
patterned strips onto silicon substrates at different deposition temperatures, it has been
observed that depositing strips at decreasing temperatures produces high damping while
depositing TiNi strips upon heating produces larger actuation and stress relief (Wuttig,
2002). The results of the studies help to determine the type of patterning that provides
better actuation as well as whether microactuators should contain multiple layers of SMA
thin films in order to increase the properties of the device.
A literature search was performed in late October and early November to
determine if there has been any previous research or designing of a latching SMA
microactuator. Articles were found that described SMA microactuators but none with a
latching component. Power had to be continuously supplied in all cases. One abstract was
found that described thin film SMA latching microactuators. “In this paper, the
development of TiNi thin film actuated, high current carrying, latching microrelays are
described” (Galhotra et al., 2000). However, the full text of this paper could not be found.
Both US patents and academic articles were searched. The group proceeded with the
project under the assumption that its design of a latching SMA microactuator is
unprecedented.
2
Definitions
Shape Memory Alloys (SMAs): metal alloys that can recover apparent permanent strain
when they are heated to their phase transition temperature. These materials switch from
martensite phase to austenite phase upon heating and then back to martensite upon
cooling. When heated, the SMA returns to the shape it was formed in, resulting in what is
termed the shape memory effect.
Latching: in reference to a SMA actuator the device catches between modes so that
power does not have to be continuously applied. The device will heat to the transition
temperature, change to austenite and then remain in that state after the power is removed.
Then the device will “unlatch” when it needs to return to its original state.
Duty Cycle: the ratio of the on-time to the period of the power transfer switch when it is
taking energy from the source. It is (on-time)/(on-time + off-time), a dimensionless
parameter falling within the values 0 and 1.
Cantilever: a beam (much longer than it is wider) that is bendable and is attached on one
end and free on the other.
3
Section 1 Figures
Power Comparison
Cummulative Energy Comparison
Cummulative Energy
Power
1.2
0.8
0.4
0
0
Time
5
10
Latching
15
20
Non-latching
20
15
10
5
0
0
Time
5
10
Non-latching
15
20
Latching
Figures 1a and 1b show power usage and cumulative energy comparisons between latching and non-latching actuators for several
switching cycles.
Velcro hooks
Velcro loops
Velcro hooks
Velcro loops
Another TWSMA
under “loops”
Velcro hooks
Actuates with more force than
Velcro has, releases cantilever
Returns to cold shape, ready
for another cycle
Figure 1c shows one of the early designs using mechanical latching--Velcro that holds a two-way SMA.
Figure 1d is the valve from the Case Western University group that uses SMA arms to lift a Si island to open the valve.
4
2. PROJECT EVOLUTION
The first step in the design process was finding something to design. Our initial
constraints were that the device had to be MEMS, and that it should use a new material in
a novel way. After brainstorming individually, we each came up with design proposals
ranging from a telescope mirror arrays to implanted medical devices. For each proposal,
we tried to look at what materials were to be used, what would the device do, and what its
applications could be. This concept of a design triangle became the backbone of our
project.
After rethinking our ideas in terms of this triangle, we came to the conclusion that
all of the ideas were too complex and we would not be able to decide on just one of them.
We had to come up with something simple that all of our proposed devices could use.
Going back to our new materials constraint, we thought about new materials and came
upon shape memory alloys. This led to the idea for an actuating device. Some sort of
actuator was a major component in all of our designs. But our device would have to be a
new idea, and SMA actuators already exist at almost a MEMS scale. One thing that none
of us had seen was a latching SMA actuator. Since MEMS latching SMA actuators could
be used in all of our ideas, we already had a number of applications for it. Finally we had
completed the basic design triangle.
There were two different types of SMA considered, thermally actuated and
magnetically actuated. The thermally actuated TiNi had already been used and tested
giving us a basis to work on. Not enough is known about magnetic SMA properties, and
it would have been too hard to control the magnetic field in the simple kind of device we
wanted. Since we knew more about it, we agreed on using TiNi as our SMA material.
The next step was determining out how to latch the actuator. The initial idea was
a swinging door-like device that gets caught on “micro-Velcro”, a physical barrier, or a
magnet (Figure 1c). Soon after this, the design became two blocks each suspended from
NiTi wires over the contact area (Figure 2a). A magnet would be used to seal the contact.
There were several different versions of each design to compensate for processing
restrictions (Figure 2b).
After weeks of trying to find a way to actually build one of these designs, another
design that might be easier to make was introduced. It used two cantilevers coated with
NiTi (Figure 2c) and a magnet to hold the latch. This became our final design. Although
it is easier to fabricate, there are still barriers such as attaching the two cantilevers
together and figuring out the specifics of the magnet.
In terms of organization, we had to split up group members’ work and prioritize.
We chose what each person in the group would work on according to interest and
expertise. We had to make some aspects of the design secondary to others. Neither the
glue needed for attaching the cantilevers together nor the magnet were specified,
assuming instead they could be found later under less of a time constraint.
5
Section 2 Figures
Figure 2a is the initial "block and spring" design going from the closed position to the open position
and then back to closed.
a.)
b.)
Contact
Direction of Flow
c.)
Fig 2.2 First design’s different processing schemes
versus flow in device
Direction of Latch Movement
Substrate
Figure 2b shows different versions for an actuator to deal with production difficulties.
6
Figure 2c is the final design using SMAs deposited on cantilevers with a magnetic latching device.
3. DEVICE DESIGN
Process Flow for Shape Memory Cantilever Actuator
The fabrication of the latching shape memory alloy device was a point of major
concern in the feasibility of earlier versions of the device. These versions utilized
separated masses that were maneuvered by freestanding NiTi alloy thin films. A new
version of the device design was adopted on the premise that two free standing masses, a
spring, and numerous NiTi thin film wires were too difficult to fabricate using current
MEMS fabrication technology.
Numerous process schemes were discussed in an attempt to fabricate the
“concept” designs. The process schemes included bulk wafer processing, front-side
wafer processing, and wafer bonding. Each of these process flows was analyzed
individually, and each was dismissed for a number of reasons. A common problem for
the “concept” design was the fabrication and placement of the NiTi thin film wires with
respect to both of the freestanding masses and the substrate. The old designs required
that the NiTi thin film wires be deposited in such a way that they were attached to both
freestanding masses and oriented at an angle (0<θ <90) in their rest positions. In order to
fabricate this design, techniques including shadowing, in which the thin film would be
deposited with the substrate at an angle with respect to the sputtering gun, and a
technique that would use a similar concept to the TiNi micro-bubble (Lin-Eftekhar, 2002)
in which the mechanical masses would be located above and under the micro-bubble
were discussed. Beyond the difficulty of processing, which would require a number of
complex masks and specialized equipment, the functionality of the fabricated device was
7
questioned in terms of whether or not the key component, the SMA thin film, would
laminate upon cycling.
A problem for the front-side processing (micro-device fabrication using only one
surface of the substrate) and bulk processing methods was the formation of two separated
mechanical masses. In order to obtain the two separated masses using just one substrate
required numerous steps as well as separation techniques that could not take advantage of
photolithography due to each mass blocking the light’s path to the other. This problem
led to a seemingly feasible “horizontal” design in which the mechanical masses moved in
a plane parallel to the substrate material. Upon further consideration, this feasible
horizontal design suffered from the same film deposition and placement issues as the
vertical design, yet allowed for relatively easy freestanding mass formation.
Wafer bonding was also discussed to create the freestanding masses. This
method, which would have used two separately patterned substrates and a specialized
substrate bonding technique, was quickly dismissed due to the high temperature
requirement for wafer bonding. With thin film already deposited on each individual
wafer, high temperature bonding probably would have caused the TiNi thin film to
degrade and lose functionality.
Keeping the requirement of freestanding masses controlled by shape memory thin
film, the initial concept design was discarded in favor of a revolutionary cantilever
design. This design incorporated the use of two bulk-processed cantilevers with thin
films separately deposited on each. The design eliminates the need for additional process
steps to include a spring, because the cantilever substrate material has an inherent spring
constant (Young’s modulus). It allows for the non-complicated processing of numerous
devices on each wafer, and significantly decreases the energy required for processing,
when compared to the high-temperature deposition steps discussed in previous design
process flows.
To fabricate each individual cantilever, a silicon substrate with a thin layer of
oxide on both front- and backsides (Figure 3a) is processed using photolithography. The
photolithography step requires each mask (Figure 3e and 3f) to be aligned exactly with
one another (Figure 3b). After both back and front sides are patterned, exposed, and
developed, exposed oxide is removed using HF solution (Figure 3c) leaving exposed
silicon. The exposed silicon is then etched away using potassium hydroxide until the
desired cantilever thickness is obtained (Figure 3d). The processed cantilever wafer can
then have NiTi thin film deposited on its front-side by sputtering and can then be
annealed at a temperature around 500oC. This process flow was adopted from University
of Maryland materials professor Ichiro Takeuchi’s combinatorial experiments. Two
cantilevers make up one device and are oriented parallel to each other in the final
packaging process.
Packaging
Device packaging is vital to the success of the final product, but has very little to
do with the basic functionality of the SMA latching actuator. The packaging of the
device requires that each cantilever be separately “glued” to the packaging material and
spaced accordingly based on application parameters. A demand on the packaging not
driven by application parameters is the placement of a magnet to hold the bottom
cantilever down when actuation occurs. It is critical that the packaging comply with our
8
goal of creating a SMA latching actuator. This eliminates most mechanical means of
holding the cantilever down (gears, wheels, and moving parts) as well as high power
electromagnets, piezoelectrics, and other shape memory alloy cantilevers to name a few.
A permanent magnet, chosen based on necessary cantilever deflection and accompanying
forces for specific applications should provide enough attractive force to the magnetic
material deposited on the cantilever to hold the cantilever down without expending
continuous energy. Upon reheating of the cantilever and NiTi thin film, the magnetic
force between the two magnets should be considered negligible compared to the force
generated by the contraction of the thin film in the device’s progress to its relaxed state.
Process Flow and Device Functionality
The shape memory alloy thin film must be trained during the annealing process in
the high temperature parent phase (austenite) in order for the film to remember its shape
and allow for SMA driven actuation. A simple structure that utilizes stiff needles (Figure
3g) would be used to push down the cantilevers to the application-specific degree of
actuation during annealing. The stress induced by the stiff needles will trigger the
martensite phase and allow the domains to orient themselves parallel with the applied
stress. Maintaining this stress for a given amount of time will result in the thin film
“remembering” its martensite orientation.
9
Section 3 Figures
Cantilever Fabrication
Figure 3a: Silicon (blue) wafer with front and back sides coated with oxide (purple)
Figure 3b: Wafer patterned with photoresist (orange) is exposed using a mask (green)
Figure 3c: Weakened photoresist is removed via developing and exposed oxide is removed in a BOE
etch step
Figure 3d: Exposed silicon is removed until desired thickness is achieved using a KOH etch step
10
Cantilever Wafer Design
Figure 3e: Exposure mask for back side of wafer (clear area represents area of wafer surface
exposed for positive resist methods)
Figure 3f: Exposure mask for front side of wafer (clear area represents area of wafer surface
exposed for positive resist methods)
11
Cantilever Training
Small green circles indicate needle
placement with respect to cantilever
wafer
Side view of needle apparatus
Figure 3g is the training equipment for the cantilevers, the wafer and then the needles.
4. MATERIAL CHOICE
Material Constraints
There are separate criteria for the materials used in the microactuator. The
actuator material needs to be a novel smart material that is capable of vertical actuation
when exposed to a stimulus. The material should be energy efficient or require low
amounts of power. It also needs to be capable of large strain because the larger the strain,
the larger the deflection of the material. Another characteristic the material should have
is fatigue resistance, important if cyclic actuation is required. The material also has to be
produced as a thin film. Several materials were researched, but nickel-titanium shape
12
memory alloys were the best choice for this design. The substrate material should
provide support for the SMA and be deformable when a stress is applied. Possible
substrate materials are copper, silicon, and galium arsenide. Both materials have to be
able to be fabricated using MEMS processing techniques.
Shape Memory Alloys
Shape memory alloys (SMAs) have been of interest since their discovery in the
1930's by Olander, but more recently because of their potential applications in
microelectromechanical systems (Gill et al., 2001). Shape memory alloys have desirable
properties like high work output per unit volume, super-elasticity, and high damping
capacity that make them suitable for use in actuators. The fact that they can be actuated
by joule heating with an electric current or with heat transfer is also a plus. However, it
is their shape memory effect that makes them unique compared to other actuator
materials.
The shape memory effect is the thermally induced transformation between a
martensite phase at low temperatures and an austenite phase at high temperatures (Gu et
al., 1998). Figure 1 shows how SMAs start off in their parent phase, the ordered
austenite phase and when the material is cooled below the martensitic temperature, the
structure of the SMA becomes twinned. When in the martensite phase the material can be
easily deformed into other shapes with relatively little force. Applying a stress to the
shape memory alloy reorganizes the twin orientations along the direction of the stress
(Surbled et al, 2001). Upon heating above the austenite temperature, the crystal structure
converts back to its highly ordered parent phase and the original SMA structure is
recovered.
In order to use shape memory alloys for specific applications the materials have to
be set or trained to a specific shape. The SMAs are constrained to mandrels or fixtures
with the desired shape and are then heat treated for specific amounts of time. The time
and temperature needed to train the SMA depends on the type of SMA used. The heat
treatment is applied so that at that temperature the material remembers to go to that
particular shape. When heat treating, care must be taken to make sure the SMA reaches
the required temperature and is held at that temperature for enough time. If heat
treatments are to high then there can be an increase in the actuation temperature and the
material can have a sharper thermal response.
When comparing bulk SMAs to SMA thin films, SMA thin films are preferred
because of their large energy density, high frequency response, and long lifetime at
smaller dimensions. SMA thin films can be fabricated in batch, patterned with standard
lithographic techniques, and engineered into micron-size structures (Fu & Du, in press).
According to Ren, Wang, Xu, and Cai (2000), the properties of SMA thin films strongly
depend on the metallurgical factors as well as deposition conditions used to fabricate
them. For nickel titanium thin films, their properties have been assumed to be the same
as bulk nickel-titanium.
Nickel-Titanium
Nickel titanium is considered to be the best type of shape memory alloy compared
to other popular polycrystalline SMAs like CuZnAl and CuAlNi (Huang, 2002). TiNi
thin films have small thermal masses and large surface to volume ratios that enable them
13
to reduce the time needed for the SMA thin films to cool down, leading to faster heat
transfer. TiNi thin films are known to have superior fatigue, corrosion resistance,
work/weight ratio, biocompatibility, and ductility (Frantz et al., 2002). A very important
quality of the thin films is their large recoverable stress of up to 500MPa without
permanent deformation (Gabry et al., 2000). Microactuators containing the TiNi SMA
thin films have large force and actuation, but have slow response times when the
temperature hysteresis is large (Ren et al., 2000). A way to increase the response time is
by decreasing the thickness of the film. Another advantage of TiNi thin films is that they
require low voltages since their resistivity is high.
For the latching shape memory alloy microactuator, near-equiatomic nickeltitanium thin films will be used. Since TiNi is the most popular and most researched
shape memory alloy, more information is available on this material. Due to the martensite
and austenite phase transformations, nickel-titanium shape memory alloys have more
than one value for some of their thermal and mechanical properties [refer to Table 4.1].
The austenite phase typically has higher values than the martensite phase.
Substrates
The microactuator design contains a cantilever made up of the shape memory
alloy thin film and a substrate material. The substrate materials researched were silicon,
copper, and gallium arsenide. Copper is not a common substrate material, but it was
considered because it would have better crack resistance with cyclic actuation compared
to the silicon and gallium arsenide. Also we expected copper to bend in the opposite
direction as the other two materials.
The values that were considered were the Young's modulus and the coefficient of
thermal linear expansion [Table 4.2]. Early on in the project, it was decided that the
deflection due to the phase change should cooperate with the thermal expansion of the
substrate material. Therefore a simulation was made to determine the most deflection as
well as the direction of bending for each material. According to initial calculations,
gallium arsenide bent in the opposite direction as silicon. The values for the thermal
expansion coefficients were similar so they should have acted similarly. It was then
found that the data for the coefficient of thermal expansion for gallium arsenide was
incorrect. When the calculations were repeated the materials acted the same. Having the
SMA on the silicon or gallium arsenide substrate will cause the cantilever to bend
downward and the deflections are also very close.
It was later decided that the bending caused by the phase change is not important.
Shape memory alloys can be trained to bend in either direction, up or down. Since the
substrate material is connected to the SMA thin film, it will move in the same direction
assuming that the SMA thin film is stronger. Also it was decided that the microactuator
would not be used for heavy cyclic applications, so good crack resistance was not
considered critical. Therefore, for this design silicon or gallium arsinide could be used as
the substrate material for this design.
14
Section 4 Figures and Tables
Applied
Stress
Cooli
ng
Austenite
Applied
Stress
Polydomain
Martensit
e
Reheating
Austenite
Singledomain
Martensite
Figure 4a: This diagram shows the transformation of the SMA under heating and stress.
Property
Transformation temperature
Value
-200 to 110° C
Latent heat of transformation
5.78 cal/g
Melting point
1300° C
Specific heat
0.20 cal/g
Young’sTable
modulus
83 ofGPa
austenite;
28attofor41substrate
GPa martensite
4.2: This table shows properties
the materials
looked
choices.
Yield strength
195 to 690 MPa austenite; 70 to 140 MPa martensite
Ultimate tensile strength
% Elongation at failure
895 MPa annealed; 1900 MPa work-hardened
25 to 50% annealed; 5 to 10% work-hardened
Table 4.1 has the material properties for the three substrates that were investigated.
Substrate Material
Silicon (single crystal)
Gallium Arsenide (bulk)
Copper (thin film)
Young's Modulus
(Gpa)
190.8
85.5
127.4
Coefficient of Thermal
Linear Expansion (K-1)
2.33*10-6
5.73*10-6
16.12*10-6
Table 4.2: Properties of Equiatomic Nickel-Titanium Shape Memory Alloy from http://www.smainc.com/NiTiProperties.html
15
5. DESIGN EVALUATION
Power Calculations and Tradeoff
Duty Cycles
Non-Latching: Traditional non-latching actuator designs require the constant
input of energy to remain in their secondary state. As a result, the amount of energy
expended is a function of the time the actuator remains in the secondary state and the
power needed to maintain the switched state. This translates to a linear energy consumed
= power * time
(See Figure 5a).
There are two types of non-latching actuators: normally open and normally
closed. When power is shut off, each of these types will revert to its base state, (e.g., open
for a normally open actuator). If the actuator application requires it to spend only a small
percentage of its time in the secondary state, then the amount of energy expended will be
very small. If the application requires that the majority of the time be spent in the
secondary state, then using the other actuator type (such as normally closed instead of
normally open) will result in lower power consumption. When 50% of the time is spent in
the secondary state, then either type of actuator will consume the same amount of energy.
This results in one actuator choice being better for large secondary state times, while both
are equally suited for near half and half times.
Latching: The latching actuator however, requires energy only to shift states, e.g.
from open to closed. Therefore, the energy used is merely a multiple of the number of
times the actuator shifts state, i.e. opens or closes. Power is consumed at a rate that is
independent of the total amount of time the actuator is in its secondary state. Instead, the
cumulative energy consumed = energy per cycle * frequency of operation * amount of
time used.
As a result, there are several different possible energy configurations for a
latching actuator. These are the combinations of low power, high power, low frequency,
and high frequency. From these possibilities there are four different duty cycles that
become apparent (See Figure 5b). Power is dependent on the actual device itself and how
much energy is needed to actuate the device. Frequency is an application parameter that
depends on how the device is used.
As can be seen in Figure 5b, low power and low frequency offers the best options
for energy savings. However, high power is merely a scalar multiple of low power, and is
determined by the properties of the device.
The significant difference appears with high frequency applications. Rapidly
switching the state of the latching actuator requires significantly more energy per time
than low frequency shifting. This means that at some critical operational frequency, a
non-latching actuator will in fact use the same amount of energy as the latching actuator.
At any higher frequency, the non-latching actuator is in fact more energy efficient.
This cycle dependent energy efficiency yields interesting application possibilities.
First, for applications where the state needs to be changed at a low frequency and held for
a long time, the latching actuator is more energy efficient. Secondly, if the needs of the
application are uncertain or varying but are require the actuator to be in the secondary
16
state approximately 50% of the, then the latching actuator can potentially offer significant
energy savings for low frequency applications.
Power for Heating SMAs
To determine the power consumed by our latching microactuator, we created a
simple simulation with VisSim 4.5 (Visual Solutions, Inc.). By equating the power
equations for current flow (Equation 5.1) and thermal conductivity (Equation 5.2), the
current necessary to heat the actuating cantilever to the austenite-finish temperature (Af)
could be determined. Because of different thermal conductivity and resistivity of the
martensite and austenite phases of the SMA film, the current required to heat the
cantilever changes during the transformation. Taking the maximum current value (either
austenite or martensite) and assuming that a power source would be programmed to
deliver this current constantly, the power required throughout the SMA-heating process
could be calculated from Equation 5.1. Figure 5c shows this relationship for a response
time of 0.01 seconds and an Af of 140ºC; the power is turned off once the cantilever
reaches this temperature. A total of 0.97mJ of energy is necessary per heating cycle for
the chosen configuration; therefore, 0.97mJ is needed to change the state of the actuator
(assuming both cantilevers are the same).
1
(5.1)
P = I 2R = I 2
1
1
+
RSMA Rsubstrate
∆T
(5.2)
P = mC
t response
m - mass of cantilever, c - specific heat of cantilever, ∆T - difference between Af and room temperature, t desired response time, I – current applied, R – resistance of the cantilever
Cantilever Deflections and Forces
Modeling the positions of the cantilevers with respect to time throughout the
switching cycle of the actuator was extremely complicated. The basis for the entire
simulation was the idea that non-uniform shape changes in the SMA film and substrate
cause curvature in the cantilever (see Figure 5d for a graphical “rationale” of this idea).
The non-uniform shape change could be produced either by thermal expansion on heating
or by lattice-parameter change during the martensite-to-austenite transformation. For the
case of thermal expansion, equations for tip deflection of a bimetallic cantilever had
already been derived in the literature (Chu, 1993). Using beam theory, Equation 5.3
could be derived for the curvature of the beam*. Equations 5.4 and 5.5 relate the
curvature to tip deflection and tip deflection to tip force, respectively.
*
For this equation, Figure 5f shows the notation for the cantilever dimensions, E represents the Young’s
modulus of the material, and Ω represents a strain term that equals ∆T(α2 – α1) for thermal expansion and |1
- (aaust/amart)| for shape memory effect. The parameters aaust and amart are the lattice constants of the austenite
and martensite phases, selected in such a manner that the shape change is consistent with the shape change
from thermal expansion (i.e., if thermal expansion makes the SMA film expand more than the substrate, we
want the austenite SMA to get even bigger than the martensite to enhance the bending). These lattice
constants were found in the literature (Huang, 1999).
17
k=
6b1b2 E1 E 2 t1t 2 (t1 + t 2 )Ω
(b1 E t ) + (b2 E 2 t 22 ) 2 + 2b1b2 E1 E 2 t1t 2 (2t12 + 3t1t 2 + 2t 22 )
2 2
1 1
(5.3)
d=
kL2
2
(5.4)
F=
3EId
L3
(5.5)
To make the tip deflection and force of the cantilevers time-dependent, the
changing parameters (namely E and α, both of which have different values for martensite
and austenite SMA, and the addition of the shape-memory-effect deflection) had to be
related to the fraction of the SMA film that had transformed at a given time. First, ∆T
(the difference between room temperature and Af) was divided by the given response
time to determine the necessary temperature ramp per second. Then, using a case
statement to determine whether Af had been reached, a temperature function was
constructed that ramped linearly from room temperature to Af and back down. At each
point in time, the temperature of the SMA could be classified as martensite, austenite, or
transition; assuming a linear transformation with respect to time, the fractions of austenite
and martensite in the film could be calculated. Finally, these fractions could be used at
all times to determine effective values for E and α by combining martensite and austenite
values in their proper fractions and the proper fractional value of deflection due to the
shape memory effect.
Once the simulation of one cantilever’s motion was complete, the simulation was
expanded to show the interaction of both cantilevers when either is heated. To model this
interaction, the simulation must detect when the heated cantilever bends far enough to
reach the unheated cantilever, then change the position of the unheated cantilever as it is
pushed from its equilibrium position. Depending on the position in the actuation cycle,
when the power is turned off, either both cantilevers spring back as the heated cantilever
transforms to martensite, or only the heated cantilever springs back and the unheated
cantilever is held by the magnet. A series of Boolean tests and case statements were used
to develop the logic that controlled these complicated situations.
The final results of the simulation are shown in Figures 5f-5i. In Figures 5f and
5g, screen shots of the simulation show its structure and input parameters. Figure 5h
shows the positions of the cantilevers in both halves of the switching cycle – when
cantilever 1 is heated and when cantilever 2 is heated. When cantilever 1 is heated,
cantilever 2 is pushed down by cantilever 1 and held by the magnet when cantilever 1
cools and springs back. When cantilever 2 is heated, it does not move up until its force
overcomes the magnet’s force; it then bends upward, pushes cantilever 1 up, and both
spring back as cantilever 2 cools.
The forces caused by these motions are shown in Figure 5i. When cantilever 1 is
heated, it has a large force that can easily overcome the force that resists cantilever 2
being pushed down. The upward resistive force of cantilever 2 is also smaller than the
force of the magnet, so it stays down once it reaches the magnet’s capture range. When
cantilever 2 is heated, its upward force increases until it can finally break free of the
magnet. Its upward force decreases because it is no longer constrained, then increases
18
again as the transformation to austenite causes it to bend upward. As it moves up, it
comes in contact with cantilever 1 and pushes it upward, creating a smaller downward
force in cantilever 1 that causes it to spring back as cantilever 2 cools off and moves back
down. The relationships between these forces are shown schematically in Figure 5j.
It is clear from these graphs that with the given parameters, a deflection of about
39µm can be achieved using a silicon substrate. Also, the forces are of the correct
magnitude to produce the desired motion (if the magnet exerts a force of approximately
0.8mN). On the basis of the simulation, therefore, it seems that the design is functional.
Device Constraints
As simulated, the device is much smaller than previous devices in the literature.
The graphs in Figures 5h and 5i were constructed with the following parameters: 100µm
cantilever length, 30µm cantilever width, 2.5µm substrate thickness, and 0.5µm SMA
film thickness. These dimensions, as shown before, resulted in a tip deflection of 39µm
and a tip force of 0.23mN. Although these parameters must be optimized for each
application, the designed device is truly micron-scale.
Depending on the application, different deflections (actuator “stroke distances”)
may be necessary for the device. By scaling different parameters, the stroke distance and
tip force can be changed dramatically. While maintaining a 10:3 length-to-width ratio
and a 10:1 width-to-total-thickness ratio for the cantilever (constraints that produce a
long, thin cantilever), the cantilever width and SMA film thickness can be scaled. For
silicon substrates, this scaling produces the graphs in Figures 5i and 5j. The results are
not completely intuitive. Large gains in stroke distance are observed when SMA
thickness is increased with a constant total thickness, but not when the length of the
cantilever is increased for a given SMA thickness. For large stroke distances while
retaining a micron-scale device, therefore, it is desirable to increase the fraction of the
total cantilever thickness occupied by the SMA film. Tip force, however, increases both
with cantilever length and with SMA film thickness. To make cantilever 2 easier to bend
and be held by the magnet, it should either be made shorter or the SMA film should be
made thinner. In this way, material could be saved while increasing performance.
The lifespan of the device also depends on the application. If the device only
needs to switch a few times, then its fatigue properties are not as important. If it switches
very frequently and must last a long time, however, the cantilevers must be designed in
such a way as to reduce fatigue. These constraints are not known until an application is
chosen – for example, an electrical contact might need to switch at extremely high
frequencies and last a number of years, while a fluid valve in a drug delivery system
might only need to open for a fraction of a second every day for weeks or months.
19
Section 5 Figures
Cumulative Energy Consumed (arb.
units)
Non-latching Duty Cycle
100
Max energy
usage
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
100
Time Closed (%)
Normally open
Normally Closed
Figure 5a: This graph shows the energy that is used when a actuator is held in its “switched” state.
Cumulative Energy Consumed
(arb. units)
Latching Duty Cycles
30
25
20
15
10
5
0
0
5
10
15
Each step equals one actuation
Low Power, Low Freq
High Power, Low Freq
Low Power, High Freq
High Power, High Freq
Figure 5b: This graph shows the energy used when a latching model is used.
20
Figure 5c: Power required to heat cantilever to temperature Af (140ºC) in the desired response time
(0.01 seconds). After Af is reached, the power is automatically turned off.
Figure 5d: Rationale for cantilever bending caused by non-uniform shape change in cantilever.
21
Figure 5e: Schematic drawing of bi-layered cantilever showing dimension notation (Chu, 1993). In
the current project, material 2 is the SMA film and material 1 is the substrate.
22
Figure 5f: Screen shot from VisSim showing tree of calculation
blocks and structure of the cantilever force/
deflection simulation.
Figure 5g: Input menu from VisSim simulation showing
cantilever dimensions, transformation temperatures,
magnet data, and substrate material.
23
Figure 5h: Positions of cantilevers (SMA on Si substrate) for entire switching cycle.
Figure 5i: Forces in system (SMA on Si cantilevers) during device switching cycle.
Heating cantilever 1:
Heating cantilever 2:
F-cantilever 1 aust
F-cantilever 2 aust
F-cantilever 2 mart
CANTILEVER 2
F-magnet
F-cantilever 1 aust
>
F-magnet
>
F-cantilever 2 mart
F-cantilever 1 mart
CANTILEVER 2
F-magnet
F-cantilever 2 aust
>
F-magnet
>
F-cantilever 1 mart
Figure 5j: Necessary relationships of forces in cantilever system.
Figure 5k: Cantilever tip deflection as a function of cantilever length and SMA film thickness,
assuming a 10:3 length-to-width ratio and a 10:1 width-to-thickness ratio for the cantilever.
1
Figure 5l: Cantilever tip force as a function of cantilever length and SMA film thickness, assuming a
10:3 length-to-width ratio and a 10:1 width-to-thickness ratio for the cantilever.
6. APPLICATIONS
The applications of the latching device were not the focus of this project, but the
device design restricts the range of its applications. Both the placement and type of
contact the latch makes with the mounted device affect the applications. Other factors
are the reactivity of the material used with the latch’s environment, the operating
temperatures, size, and lifetime. Since it was nearly impossible to design the latch
without some constraints, a primary application of an electrical contact was kept in mind
throughout the design process as a reference. Our latch is designed in such a way that the
contact will be nearly a point. The NiTi transition temperature chosen is good for
electrical contacts, and the addition of a magnet will not affect the environment or be
affected by it in a negative way.
There were also secondary applications considered which would require
modifications to the design. However, due to complexity in the design that could not be
resolved in our timeframe, we did not focus on primary applications. The most prominent
of these was a valve for transporting liquids or gases in a biomedical device. NiTi is
biocompatible for short-term applications, but it is unknown how it would react after
long-term exposure to a body. However, SMA actuators for in vivo use are still being
resolved. The operating temperatures could be too high for implantation, so insulation
would need to be added. This application would also require a larger contact area and the
ability to create some sort of seal. Other secondary applications were optical switches
and telescope mirror actuators. Despite the lack of investigation into these applications,
they are still novel and worth researching in the future.
2
In addition to applications for the final design there were different applications for
each of the three versions of the first design. The two “vertical” designs and the
“horizontal” design actuated differently yielding different applications. The “vertical”
design actuated perpendicular to the plane of the substrate meaning the contact that it
makes would be best if placed between the bottom block and the substrate (Figure 2b(a)),
or on top of the top block (Figure 2b(b)) It could be placed on top of a larger object such
as a tube, stopping or allowing flow in a valve. The “horizontal” design actuated in plane
with the substrate, so the contact would be best next to or in between the blocks. (Figure
2b(c)) The object inserted between the blocks should be small enough to fit between
them, or even better, just an electrode. An electrical contact would be the best
application for this design. Although we did not follow through with the first design the
varying applications could be useful for future research.
7. CONCLUSION
This project began as a way to capitalize on the properties of smart materials in
MEMS applications. A small amount of general research in this area created interest in
designing a microactuator that took advantage of the shape memory effect of various
shape memory alloys. However, instead of merely duplicating the only SMA
microactuator we found in our literature search, we decided to add a new function,
latching, to the microactuator idea. Latching potentially offers not only significant power
savings, but allows one to determine how a valve will function if power is removed, as a
latching microactuator will remain in its chosen state until power is applied.
The initial design of our latching SMA microactuator underwent numerous
changes as the project evolved. For instance, the initial device design was very complex,
involving numerous freestanding structures that were at difficult to manufacture using
most standard MEMS techniques. The final cantilever design reflects the need to have a
device that can be easily fabricated using a fairly simple process flow. Also, the
technique used for the actual latching mechanism changed, from a MEMS version of
Velcro to utilize two-way SMAs to the final decision to use a magnetic interaction to
retain state. The SMA used in this device was also under consideration, with the widely
researched and applied TiNi system chosen over less effective thermal, ferroelectric,
ferromagnetic, or piezoelectric materials. Because of these wide changes from the initial
conceptual design, the process flow for building our device changed from nigh
impossible to something that can be easily fabricated.
For designing and analyzing the properties of our device, modeling software
proved invaluable in determining what would or would not be effective in the design. We
were capable of inputting material properties and defining the geometry of the device to
determine how much the actuator would deflect as a function of geometry and materials.
Modeling allowed us to change materials and dimensions to determine their
effectiveness, all without ever building a single device. All that remains now is to build
the device in order to compare it to the model.
Once needed improvements are made to the model, we can proceed to optimize
the design, as this design is by no means perfected. Certain processing steps, such as the
training of the SMA cantilevers, require a fair amount of direct force and heat, so the
thermal budget must be considered. Thus far we are also uncertain how much more
3
energy efficient this design is compared to the current state of the art, and whether or not
the actuator is capable of high operational frequency.
These necessary improvements lead to the future work this project requires to
bring this product to market. Once the device specifications have been set forward, the
design and process flow can be completed and the device built. Experimental testing of
the device will allow for several key aspects we were unable to complete during the
design process. First, SMA thin film properties are not adequately described in the
literature. Key materials aspects such as the Young’s modulus of thin films as well as
fatigue measurements are sorely lacking. We used the bulk moduli in the simulation and
experimental testing will demonstrate just how accurate these numbers are. Also, the key
selling point of our design, low power consumption due to the latching function, is still in
question. As described in the duty cycle section, our design offers the most power savings
for low frequency switching with a low activation cost. So far, we are also uncertain how
quickly the device will cool, which is the determining factor in the frequency of
operation. These factors determine which applications the device is suitable for. As an
example, a circuit breaker will be effective, but electrical switching may require higher
frequencies than this device can handle. Essentially, we cannot evaluate out device fully
until the device is actually built.
We have designed a latching shape memory alloy microactuator that offers power
savings and new functionality over the current state of the art MEMS microactuator.
Using modeling software we were able to compare different design dimensions and
materials choices without resorting to costly, time-consuming experimental testing. What
remains is to build and optimize the design and account for any variables and
considerations that escaped our modeling. We believe this design advances the state of
the art in MEMS actuators and capitalizes on the properties of smart materials.
4
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