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-of-the-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 three-dimensional 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 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.

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.

Section 1 Figures

Figures 1a and 1b show power usage and cumulative energy comparisons between latching and non-latching actuators for several switching cycles.

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.

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.

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.

Figure 2b shows different versions for an actuator to deal with production difficulties.

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 questioned in terms of whether or not the key component, the SMA thin film, would laminate upon cycling.