## Abstract

This study aims to develop a new fabrication process to create high-precision patterned shape memory alloy (SMA) bimorph micro-actuators by the e-beam evaporation technique. To examine the effect of the annealing process on nitinol (NiTi) thin film characteristics, the as-deposited and annealed NiTi thin films are, respectively, investigated. X-ray diffraction (XRD) results demonstrate the crystallization of NiTi thin films after annealing at 600 °C. The transformation behaviors of NiTi thin films during heating and cooling are studied using the differential scanning calorimeter (DSC). Furthermore, scanning electron microscopy (SEM) images indicate that SMA bimorph micro-actuators with high-precision features can be fabricated by the lift-off process, without any wet or dry etching procedures, and their thermomechanical behaviors are experimentally verified by comparing them with that of finite element analysis simulation results.

## 1 Introduction

Over the years, micro-actuators have been widely applied in a variety of disciplines such as aerospace [1,2], biomedical field [3,4], and microrobotics [57] and require delicate handling of micro-objects and high-precision coordination in the micro-environment. For the past three decades, the rapid advancements of micro-electromechanical system (MEMS) techniques have enabled the research of an extensive range of miniaturized micro-actuators using MEMS fabrication methods. Based on the actuation principles, micro-actuators can be classified into electrostatic [8], piezo-electric [9], electromagnetic [10], thermal and shape memory alloy (SMA) [11]. SMA has attractive advantages compared to other actuation mechanisms such as high power density, large actuation force, large displacement range, low cost, resistance to corrosion, and biocompatibility [12]. These advantages make SMA the preferred material to fabricate micro-actuators in various specific applications [1315], including endovascular surgery, neural interfaces, drug delivery, and intestinal obstruction. Among various SMA materials, the NiTi SMA composition is commonly used.

Shape memory alloy actuators, however, have often been described as having a slow response speed, which is seen as a major hindrance for the broad implementation [16]. That long response time is caused by the slow heating and cooling speed of the SMA [17]. To overcome the drawback of SMA, NiTi thin films based on MEMS have been studied by Walker [18] in 1990. The actuators formed by NiTi thin films can not only provide larger forces over longer displacements but also possess the fast response speed due to the larger surface area to volume ratio of NiTi thin films [19]. In addition, the advancements of NiTi thin film micro-actuators have been further achieved with the number of applications, which have been developed in MEMS fields. For example, using NiTi thin films, microgrippers [20,21], micropumps [22,23], and micro-imaging sensors [24] have been presented and fabricated. In general, there are several deposition techniques to fabricate NiTi thin films, namely, radio frequency or direct current (DC) magnetron sputtering, pulse laser deposition, plasma enhanced chemical vapor deposition, and flash evaporation [25]. Researchers commonly characterize NiTi thin films formed by sputtering deposition methods [26,27]. However, it seems to be difficult to micromachine NiTi thin films with high-precision features. In many applications, very uniform micropatterns with precise small feature sizes are particularly required [28]. Based on the sputtering deposition methods, NiTi films are usually lithographically fabricated into patterns using wet etching procedures, which produce nonuniform micropatterns with the severe undercut [28]. To achieve micropatterns with smooth edges, an anisotropic dry etching method has been proposed. But the drawback of this method is the slow etching rate, which is unacceptable in many cases [29]. According to the research of metal thin film, the e-beam evaporation technique might be a potential approach to fabricate the very uniform micropatterns of NiTi thin films with high-precision features, combining with lift-off resist method [30]. To study the characteristic of NiTi thin films formed by the e-beam evaporation technique, Noh [31] deposited NiTi thin films using e-beam evaporation method instead of sputtering. However, NiTi thin film microstructures with high-precision features were not micromachined in that paper. Therefore, it is necessary to develop a new approach to fabricate high-precision patterned SMA micro-actuators formed by the e-beam evaporation to meet specific requirements in various fields.

In this paper, a novel fabrication process is proposed for producing NiTi-aluminum SMA bimorph micro-actuators. In order to achieve the high-precision features, the fabrication process is combined with the e-beam evaporation and lift-off resist methods. To be more specific, this paper demonstrates the feasibility to fabricate the high-precision patterned NiTi thin film microstructures using the e-beam deposition technique and examines the effect of annealing process on the crystallization of NiTi thin films. Furthermore, thermomechanical behaviors of SMA bimorph micro-actuators are experimentally verified and compared with that of finite element analysis simulation results. Such studies may, therefore, contribute to a cost-effective fabrication process for the microrobotics applications.

This paper is organized as follows. The fabrication and annealing processes of SMA bimorph micro-actuators are described first, followed by the characterization analysis of NiTi thin films formed by the e-beam evaporation. Then, thermomechanical behaviors of the SMA bimorph micro-actuators are studied, and finite element analysis (FEA) and experiments are carried out to validate the feasibility. Finally, some conclusions are drawn.

## 2 Sample Preparation

In this paper, a novel fabrication process for producing NiTi-aluminum SMA bimorph micro-actuators with high-precision features is presented. For every fabrication step, the approach used to construct the features is described.

Fig. 1
Fig. 1
Close modal
Fig. 2
Fig. 2
Close modal

## 3 Results and Discussion

### 3.1 Fabricated Patterns.

Scanning electron microscopy (SEM) images (Hitachi S-4800, Hitachi High-Tech Company, Schaumburg, IL) were utilized to evaluate the high-precision patterned NiTi micro-actuators. Figures 3(a)3(c) show SEM images of patterned NiTi thin film microstructures obtained by the fabrication process proposed in this paper. Both the low magnification and high magnification SEM images were taken after releasing NiTi microstructures from the silicon substrates using XeF2 dry etcher. In addition, Fig. 3(c) illustrates that even in the features with sharp corners, this fabrication process produces quite smooth and relatively robust micropatterns. Furthermore, thanks to the perfect unique reactivity of silicon material using XeF2, these NiTi micropatterns do not induce the fracture or tearing problems reported after releasing from the silicon wafer [28]. Since this fabrication procedure is combined with lift-off resist and the e-beam evaporation methods, the limit of fabrication precision is attributed to the photolithography technique, which depends on the UV wavelength utilized during the exposure process.

Fig. 3
Fig. 3
Close modal

### 3.2 Material Characterization.

In this study, to examine the effect of annealing process on NiTi thin film characteristics, the as-deposited and annealed NiTi thin films were, respectively, investigated. A Rigaku Ultima III X-ray diffractometer (Applied Rigaku Technologies Inc., Austin, TX) was employed at room temperature to determine the crystalline structures of NiTi thin films. Figure 4(a) illustrates the X-ray diffraction (XRD) test results of as-deposited NiTi thin films without annealing process. It is obvious that the results exhibit only one strong peak (2θ = 45 deg), which can be identified as NiTi monoclinic structure (111) (martensite B19'), indicating that the as-deposited films are amorphous in nature. Subsequently, as shown in Fig. 4(b), it can be found that the annealing process shows a great influence on the XRD results. For annealed NiTi thin films, three strong peaks in total are observed, one peak (2θ = 42.5 deg) corresponds NiTi cubic structure (110) (austenite B2), and the other two peaks (2θ = 43.7 deg and 45 deg) are identified as (111) and (001) NiTi monoclinic structure (martensite B19'), which reveals that austenite and martensite co-existed in the same annealed NiTi thin films at room temperature. The XRD results demonstrate that crystallization can be clearly detected when NiTi thin films annealed at 600 °C.

Fig. 4
Fig. 4
Close modal

The transformation behaviors of NiTi thin films during heating and cooling were studied using the differential scanning calorimeter (DSC). Figure 5 shows the DSC results of the annealed NiTi thin films. The symbols As, Af, Ms, and Mf indicate the starting and finishing temperatures of austenitic and martensitic phase-transformation. During the heating process, one endothermic peak corresponding to a martensite to austenite phase-transformation is observed. The starting and finishing temperatures (As and Af) of austenitic phase-transformation are determined to be 32.04 °C and 46.89 °C. The transformation temperatures of As and Af, above room temperature, which can be used as the thermal actuation command of the SMA actuators, will be beneficial to apply NiTi films in the micro-actuators.

Fig. 5
Fig. 5
Close modal

### 3.3 Shape Memory Alloy Bimorph Micro-Actuator.

Following successful e-beam deposition of patterned NiTi thin films, aluminum was used as the stress layer for the fabrication of NiTi-aluminum bimorph micro-actuators. A schematic NiTi-Al SMA bimorph micro-actuator structure is shown in Fig. 6. The micro-actuator has two legs, 10 × 200 μm2, connected with a 50 × 50 μm2 square-pad at a free end and two fixed contact pads at the other. The multilayer metals were deposited and bonding together naturally during the e-beam deposition processes. The aluminum was used as the bottom layer with the thickness of 0.5 μm and NiTi as the top layer of the bimorph structure, as well as with the thickness of 0.5 μm. In addition, there were two microrulers next to the legs of the micro-actuator. These microrulers helped to measure the planar displacement of the micro-actuators when thermally actuated. The metals vapored from the crucibles were deposited on the wafer at a quite low temperature (almost 75 °C), thanks to the cooling system employed in the vacuum chamber during the deposition process. After the deposition of NiTi and aluminum thin films, the temperature change of 55 °C between room temperature (20 °C) and physical vapor deposition (PVD) machine induced the residual thermal stress throughout the whole structures of SMA bimorph micro-actuators. Therefore, with the combination of NiTi and aluminum, the bending of the bimorph micro-actuator occurred after releasing it from silicon wafer following XeF2 etching process, due to the mismatch in coefficient of thermal extension (CTE) of these two different materials. The radius of curvature R of the bent bimorph micro-actuator, could be expressed as
$R=(w1E1t12)2+(w2E2t22)2+2w1w2E1E2t1t2(2t12+3t1t2+2t22)6w1w2E1E2t1t2(t1+t2)(a1−a2)ΔT$
(1)
where α is CTE of the materials, E is Young's modulus of the materials, t is the thickness of the layer, w is the width of the beam, ΔT is the temperature change, and subscripts 1 and 2 represent the NiTi and the aluminum layer, respectively [32]. When the parameters of the bimorph actuator are determined, the R is the function of temperature change ΔT
$R=f(ΔT)$
(2)
Fig. 6
Fig. 6
Close modal
In addition, the radius of curvature R, the bending angle θ (in radian), and the length of the beam L have the following relationship:
$θ=LR$
(3)
Meanwhile, as shown in Fig. 6, the bending of the bimorph micro-actuator formed the planar displacement Δx and the vertical displacement Δz, along the x-direction and z-direction, respectively. According to the geometry theory, the geometry relationship between displacement Δx, Δz, and the bending angle θ can be expressed as
$Δx=ΔLx+Δlx=L−L cos θ2+l−l cos θ$
(4)
$Δz=ΔLz+Δlz=L sin θ2+l sin θ$
(5)
Combining equations (1), (2), (3), and (4), the displacement Δx of the bimorph micro-actuator along the x-direction due to the thermal effect from a temperature change becomes
$Δx=L−L cos(L2R)+l−l cos(LR)=L−L cos(L2f(ΔT))+l−l cos(Lf(ΔT))=g(ΔT)$
(6)
Combining equations (1), (2), (3), and (5), the displacement Δz of the bimorph micro-actuator along the z-direction due to the thermal effect from a temperature change becomes
$Δz=L sin(L2R)+l sin(LR)=L sin(L2f(ΔT))+l sin(Lf(ΔT))=h(ΔT)$
(7)

Then, the thermomechanical behaviors of SMA bimorph micro-actuators were investigated using the FEA method via Coventor Ware. As shown in Fig. 7(a), the SMA bimorph micro-actuator finite element models are built, with one end fixed and one end free. The bottom parts are the main body of micro-actuators using the aluminum material, and the other parts employ as the top layer using NiTi thin films to induce residual stress throughout the whole structure to make this bimorph micro-actuators bend. To determine the displacement of bimorph SMA micro-actuator along the x-direction and z-direction due to the thermal effect from a temperature change (ΔT = 55 K), FEA simulations were performed using the thermomechanical properties as shown in Table 1. As a result, Fig. 7(b) shows the maximum displacement Δx (1.98 μm) and Δz (29.91 μm) of the micro-actuator when the temperature decreased from 75 °C to 20 °C.

Fig. 7
Fig. 7
Close modal
Table 1

Thermomechanical properties of NiTi and Al

PropertiesNiTiAl
CTE (10−6 1/K)6.6 (Martensite)23.1
11 (Austenite)
Density (g/cm3)6.452.7
Young's modulus (GPa)8070
Poisson's ratio0.30.35
PropertiesNiTiAl
CTE (10−6 1/K)6.6 (Martensite)23.1
11 (Austenite)
Density (g/cm3)6.452.7
Young's modulus (GPa)8070
Poisson's ratio0.30.35

Figures 8(a) and 8(b), respectively, show the bimorph SMA micro-actuators before and after a dry etching process observed by an optical microscope. Before releasing from the silicon wafer, as shown in Fig. 8(a), the whole microstructure is fixed. Following the XeF2 etching process, the SMA bimorph micro-actuator was released from silicon, as shown in Fig. 8(b), and the significant bending result can be observed. The maximum displacement Δx (2 μm) of the micro-actuator can be measured with the help of microrulers deposited next to the microstructures. According to Eq. (6), we can calculate the temperature change ΔT = 55.4 K. Finally, the displacement Δz (30.13 μm) of the bimorph micro-actuator along the z-direction can be calculated using Eq. (7), without any additional measurement of Δz, which is experimentally verified compared with FEA simulation results.

Fig. 8
Fig. 8
Close modal

Furthermore, Figs. 9(a) and 9(b) show the optical microscopy picture and SEM picture of the same bimorph SMA micro-actuator in the static response experiment, which demonstrate the feasibility to fabricate NiTi SMA micro-actuator using the novel deposition process proposed in this paper.

Fig. 9
Fig. 9
Close modal

## 4 Conclusion

NiTi films formed by the sputtering techniques are usually lithographically fabricated into patterns using wet or dry etching procedures, which produce nonuniform micropatterns with severe undercut, as well as slow etching rate problems. In this paper, aiming to solve the above problems, a novel fabrication process combined with the e-beam evaporation and lift-off resist techniques has been evaluated, which can fabricate SMA micro-actuators with high-precision features. Meanwhile, the effect of the annealing process on the crystallization of NiTi thin films has been examined. The following conclusions can be drawn. The as-deposited films formed by e-beam evaporation are amorphous in nature, and crystallization of NiTi thin films can be clearly detected after annealing at 600 °C. The phase-transformation temperatures, which can provide the thermal actuation command of the proposed SMA actuators, have been determined above room temperature based on the DSC results. Furthermore, the SMA bimorph micro-actuators have been successfully fabricated, and their thermomechanical behaviors have been experimentally verified and compared with FEA simulation results. Our work may contribute to a cost-effective fabrication process for the microrobotics applications.

## Acknowledgment

Hao Sun acknowledges support from National Key Laboratory of Aerospace Flight Dynamics, Northwestern Polytechnical University, China, and also from Department of Mechanical Engineering, Stevens Institute of Technology, U.S. The authors most gratefully appreciate funding provided for this work by the Stevens NPU Ph.D. Program from China Scholarship Council. Also, this research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

## Funding Data

• China Scholarship Council (Funder ID: 10.13039/501100004543).

• U.S. Department of Energy, Office of Science, Basic Energy Sciences (Contract No. DE-SC0012704; Funder ID: 10.13039/100000015).

## Nomenclature

• E1 =

the Young's modulus of NiTi, Pa

•
• E2 =

the Young's modulus of Al, Pa

•
• l =

the length of the square-pad, m

•
• L =

the length of the beam, m

•
• R =

the curvature radius of the bent bimorph microactuator, m

•
• t1 =

the total thickness of the NiTi layer in microactuator, m

•
• t2 =

the total thickness of the Al layer in microactuator, m

•
• w1 =

the width of the beam formed by NiTi, m

•
• w2 =

the width of the beam formed by Al, m

•
• α1 =

the coefficient of thermal extension of NiTi in microactuator, 10−6 1/K

•
• α2 =

the coefficient of thermal extension of Al in microactuator, 10−6 1/K

•
• ΔT =

the temperature change, K

•
• Δx =

the planar displacement of the bimorph microactuator along the x-direction, m

•
• Δz =

the vertical displacement of the bimorph microactuator along the z-direction, m

•
• θ =

the bending angle of the bent bimorph microactuator, rad

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