rapid thermal annealing of ti-rich tini thin films: a new...

Materials and Design xxx (2010) xxx–xxx

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Materials and Design

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Rapid thermal annealing of Ti-rich TiNi thin films: A new approachto fabricate patterned shape memory thin films

Y. Motemani a, M.J. Tan a,⇑, T.J. White b,c, W.M. Huang a

a School of Mechanical & Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singaporeb School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singaporec Center for Advanced Microscopy, Australian National University, Canberra, ACT 2601, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 May 2010Accepted 31 July 2010Available online xxxx

Keywords:IntermetallicsHeat treatmentsCrystalline stateX-ray analysis

0261-3069/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.07.039

⇑ Corresponding author. Tel.: +65 67905582; fax: +E-mail address: [email protected] (M.J. Tan).

Please cite this article in press as: Motemani Y eory thin films. J Mater Design (2010), doi:10.10

This paper reports the rapid thermal annealing (RTA) of Ti-rich TiNi thin films, synthesized by theco-sputtering of TiNi and Ti targets. Long-range order of aperiodic alloy could be achieved in a fewseconds with the optimum temperature of 773 K. Longer annealing (773 K/240 s), transformed the filmto a poorly ordered vitreous phase, suggesting a novel method for solid state amorphization. Reitveldrefinement analyses showed significant differences in structural parameters of the films crystallized byrapid and conventional thermal annealing. Dependence of the elastic modulus on the valence electrondensity (VED) of the crystallized films was studied. It is suggested that RTA provides a new approachto fabricate patterned shape memory thin films.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

TiNi-based shape memory alloy thin films are attractive as actu-ation materials in micro-electro-mechanical systems (MEMS), suchas: micro-valves, micro-fluid pumps and micro-grippers [1–5].When prepared at low temperatures, the as-deposited state ofthe sputtered thin films is aperiodic and annealing is necessaryto realize a transition to the crystalline state [6]. Consequently,understanding the nature of this process is critical for microstruc-tural control and optimization for MEMS actuators.

Based on the employed heating source, there are two generaltechniques to crystallize the films after deposition: the first is con-ventional thermal annealing (CTA) using an electric tube furnace athigh vacuum with ramping rate less than 100 K/min, and the sec-ond is rapid thermal processing with an electromagnetic irradia-tion source with ramping <100 K/s in controlled atmosphere.

There has been extensive research to optimize crystallizationprocedures of TiNi-based thin films by CTA, where temperaturesfrom 673 to 773 K and ramp rates <100 K/min are typically em-ployed [7–12]. However, few studies have been done to investigatethe crystallization of these thin films with different heating mech-anism like irradiation (rapid thermal processing). There are a fewresearches on the ability of a laser annealing to selectively crystal-lize an amorphous TiNi film [6,13–15]. Tong et al. [16] have inves-tigated the crystallization behavior of TiNiCu ribbons modified by

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65 67911859.

t al. Rapid thermal annealing of16/j.matdes.2010.07.039

rapid thermal annealing (RTA), where the samples is annealed byirradiation of the photons from a halogen lamp and showed abilityof RTA to crystallize the ribbons in a shorter dwell times allowingmore precise control of microstructure and functionality. There re-main many aspects of rapid thermal annealing of thin films to bestudied.

In this paper, a systematic study was carried out of the crystal-lization behavior of Ti-rich NiTi thin films, during RTA. Ti-rich TiNifilms prepared by magnetron co-sputtering of TiNi and Ti targets atroom temperature were annealed by either rapid or conventionalthermal annealing and examined by X-ray diffraction (XRD). Struc-tural parameters of the films crystallized by RTA and CTA werestudied by quantitative X-ray diffraction. On this basis, we exam-ined the electronic structures of the crystallized samples and cor-related them to the mechanical properties. Finally, we proposeda new technique to fabricate patterned TiNi-based shape memorythin films using RTA treatment.

2. Experimental details

TiNi films were prepared by co-sputtering of Ti50Ni50 and Titargets at a base pressure <1.5 � 10�6 Torr onto rotating 4-in.Si(1 0 0) wafer substrates at room temperature. In this manner,uniform coatings were obtained at a substrate-to-target distance100 mm and an argon partial pressure of 1.5 mTorr. Film composi-tion was estimated by energy dispersive X-ray spectroscopy con-ducted in a scanning electron microscope operated at 20 keV(Table 1).

Ti-rich TiNi thin films: A new approach to fabricate patterned shape mem-

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Table 1Sputtering conditions and composition of TiNi film.

Target specifications/power Time (h) Thickness (nm) Composition

Ti50Ni50, 99.99% purity, Kurt J. Lesker Co., r.f. (W) Ti, 99.99% purity, super conductor materials, inc., DC (W) Ti (at.%) Ni (at.%)

400 60 2.5 1440 57.3 42.7

Table 2Crystallographic data of the austenite, martensite and Ti2Ni models [18,19].

Phase (space group) Atom Site x y z

Austenite (Pm–3m) Ti 1 a 0 0 0Ni 1 b 1/2 1/2 1/2

Ti2Ni (Fd–3mS) Ni 32 e 0.8292 0.8292 0.8292Ti1 16 d 5/8 5/8 5/8Ti2 48 f 0.196 0 0

Martensite (P1m1) Ti1 1 a 0 0 0Ti2 1 b 0 1/2 0.625Ni1 1 a 1/2 0 1/2Ni2 1 b 1/2 1/2 1/8

Fig. 2. XRD patterns of Ti-rich films in as sputtered state and after RTA for 180 s at

2 Y. Motemani et al. / Materials and Design xxx (2010) xxx–xxx

As-sputtered samples were annealed in a Jipelec Jetfirst 100 ra-pid thermal processor in an argon atmosphere while resting on adummy silicon wafer. Fig. 1 shows the schematic of the RTA treat-ment, where the temperature was measured using a pyrometercalibrated against a thermocouple. Successive Ar purging and evac-uation were employed for 35 min to minimize the oxygen contentof the RTA chamber before annealing (stage 1). The samples heatedto the annealing temperature (773 K) in two steps (stages 2 and 3),held isothermally (2–240 s) (stage 4), then cooled in two ramps(stages 5 and 6). To prevent the crack formation, a few samplesespecially those used for mechanical and surface analysis sub-jected to the lower heating rates during stages 2(ramp �5 K/s)and 3 (ramp �10 K/s) of treatment. Alternatively, the films werecrystallized by CTA in a vacuum tube furnace with 5 K/min ramp-ing and 10�5 Torr vacuum at 753 K for 1 h.

The crystallinity of the films was determined from X-ray pat-terns collected with a Rigaku diffractometer using Cu Ka radiationover a 2h range from 10 to 100� with a step size of 0.02�. Quantita-tive phase analyses were conducted using the fundamental param-eter Rietveld procedure as implemented in TOPAS (version 3) [17]by fitting to the full experimental pattern inclusive of calculateddiffracted intensity and background modelled with a Chebyshevpolynomial. The model for simulating the thin film patterns in-cluded crystallographic and instrumental parameters that are var-ied in a recurrent process to minimize the weighted squareddifference between the experimental data (Yi) and the theoreticallycalculated value (Yical) at each angular position, according to thefollowing equation [17]:

R ¼X

i

wiðYi � YicalÞ2 ð1Þ

where i varies from 1 to the number of observations and wi = 1/Yi isthe reciprocal of the variance associated to the ith observation [17].The crystalline phases included monoclinic (P1m1) B190 NiTi mar-tensite, cubic (Pm-3 m) B2 Austenite and (Fd-3mS) Ti2Ni as startingmodels (Table 2, [18,19]). Each refinement included the backgroundparameter, scale factor, cell parameter, zero point correction, sam-ple displacement, Lorenzien crystal size, strain factor, atom posi-

Fig. 1. Typical heating and cooling profile showing the setting temperature and thesamples temperature during RTA treatment.

Please cite this article in press as: Motemani Y et al. Rapid thermal annealing ofory thin films. J Mater Design (2010), doi:10.1016/j.matdes.2010.07.039

tions, site occupancy factors, isotropic thermal parameters, andpreferred orientation. The atomic arrangements of the crystallizedphases were prepared using ATOMS.

A Digital Instruments S-3000 atomic force microscope (AFM)operating in tapping mode, with Si3N4 tip cantilevers of 7 nm nom-

different crystallization temperatures.

Fig. 3. XRD patterns of Ti-rich films in as sputtered state and of the RTA at 773 K fordifferent duration times.



Fig. 4. Rietveld refinement profiles of the X-ray diffraction data for (a) crystallized film by RTA at 773 K for 180 s and (b) crystallized film by CTA at 753 K for 1 h. � Indicatesreflections associated with the possibly oxide phases formed during RTA. Experimental data are shown as blue line overlaying a simulated pattern (red line). The lower traceis the difference between the calculated and experimental intensities. The Bragg markers from top to bottom are austenite, martensite and Ti2Ni intermetallic compoundrespectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Y. Motemani et al. / Materials and Design xxx (2010) xxx–xxx 3

inal curvatures and a resonance frequency of 325 kHz was used todetermine morphology and the roughness of the films. The rough-ness was represented as a root mean square (rms) value (definedby the standard deviation of the height distribution) determinedover an area of 5 � 5 lm2. Surface features of the annealed materi-als were examined by scanning electron microscopy (SEM) in sec-ondary electron image (SEI) mode.

The elastic modulus of the films was determined using a CSMNanoindentation system with a diamond Berkovich tip (radius ofcurvature approximately 20 lm). Indentation was performed for5, 10 and 15 mN loads at room temperature.

Table 3Refined atomic coordinates for Ti-rich TiNi crystallized thin films by rapid and convention

Sample Phase (space group) Atom x

Crystallized by RTA (773 K for 180 s) Austenite (Pm–3m) Ti 0Ni 1/2

Ti2Ni (Fd–3mS) Ni 0.8Ti1 5/8Ti2 0.2

Martensite (P1m1) Ti1 0Ti2 0Ni1 1/2Ni2 1/2

Crystallized by CTA (753 K for 1 h) Austenite (Pm–3m) Ti 0Ni 1/2

Ti2Ni (Fd–3mS) Ni1 0.8Ti1 5/8Ti2 0.1

Martensite (P1m1) Ti1 0Ti2 0Ni1 1/2Ni2 1/2


3. Results and discussion

3.1. Crystallization of Ti-rich TiNi thin films

The optimum temperature for crystallization was establishedby RTA treatment of the films at 673, 723 and 773 K for 180 s.XRD showed a broad peak centered at �40� in the sputtered sam-ple that persisted at 673 K with Bragg diffraction becoming evidentafter annealing at 723 K. At 773 K, sharp peaks appeared and this istaken as crystallization temperature of these Ti-rich thin films(Fig. 2). No diffraction peaks were observed in the as-sputtered

al thermal annealing.

y z Occ. B Rb (%)

0 0 1 1 0.81/2 1/2 1 1

422 (3) 0.8422 (3) 0.8422 (3) 0.77 (2) 1 2.05/8 5/8 0.99 (4) 1

013 (5) 0 0 0.92 (3) 10 0 0.99 (2) 1.7 (6) 1.71/2 0.704 (5) 1 10 1/2 0.43 (2) 11/2 1/8 0.8 (2) 1

0 0 1 1 1.01/2 1/2 1 1

398 (5) 0.8398 (5) 0.8398 (5) 0.94 (2) 1 1.35/8 5/8 1 1

882 (7) 0 0 0.97 (2) 1.1 (2)0 0 0.75 (6) 1.4 (1) 1.91/2 0.724 (6) 0.85 (5) 10 1/2 0.24 (2) 11/2 1/8 1 1



Table 4


sample (Fig. 2), demonstrating the typical behavior of amorphousor disordered materials [20].

The crystallization behavior at 773 K was further examined forvariable holding times (2–240 s), and showed that the sputteredsamples partially crystallize in a few seconds (2–30 s), with sharpand intense diffraction peaks appearing after 120 s (Fig. 3). Thetrend of the XRD patterns changes dramatically after 120 s, withdiffracted intensity at �40�, diminishing at 180 s and the range or-der removed after 240 s. This novel phenomenon of solid stateamorphization in Ti-rich TiNi thin films during RTA using photons

Fig. 5. Schematic of the phases for Ti rich film crystallized by RTA at 773 K for180 s; (a) austenite, (b) martensite and (c) Ti2Ni. The red and black balls represent Tiand Ni atoms respectively. The unit cells are drawn by ATOMS software afterextracting the crystallographic information by Rietveld refinement. (For interpre-tation of the references to colour in this figure legend, the reader is referred to theweb version of this article.)


is quite distinct from established solid state amorphization meth-ods including particle irradiation (electrons, neutrons and ions),diffusion annealing, hydrogen-induced amorphization, cold rollingand mechanical alloying [21–27]. It is likely that at longer holdingtimes (240 s), the increasing amounts of absorbed photons due tothe photo effect (i.e. interaction of the photons with the surface),will transform the ordered structure to the aperiodic state. Further

Obtained structural parameters of the crystallized films by rapid and conventionalthermal annealing after Rietveld refinement.

Sample Phase Volumefraction(%)

Latticeparameters(Å)

Cellvolume(Å3)

Crystallitesize(nm)

Latticestrain(%)

RTA Austenite 16 a = 2.9971(3)

26.922(9)

40 0.30 (5)

Martensite 21 a = 2.904 (3) 56.20 (4) 20 0.5 (1)b = 4.158 (1)c = 4.688 (3)b = 96.8�

Ti2Ni 63 a = 11.2502(3)

1423.9(1)

17 2.30 (8)

CTA Austenite 30 a = 2.9883(1)

26.69 (3) 40 3.27 (8)

Martensite 24 a = 2.7315(8)

53.83 (6) 15 0.1

b = 4.2645(5)c = 4.6543(5)b = 96.8�

Ti2Ni 46 a = 11.341(2)

1458.6(9)

20 0.63 (4)

Fig. 6. Secondary electron images of the Ti-rich films after thermal annealing; (a)crystallized by RTA and (b) crystallized by CTA.




studies including microstructural analysis, local atomic structureinvestigation, and thermal analysis are required to understandthe physical bases of this phenomenon.

The kinetics of crystallization by RTA is distinct from conven-tional techniques since the samples crystallize in a few seconds[16,28–29]. In this type of annealing, the photons generated fromlight source (Tungsten Halogen Lamp) are shone on infinite regionsof the thin films, and consequently the temperature increases rap-idly and uniformly throughout the material leading to enhancedthermal motion that promotes long-range order [20]. In rapid ther-mal annealing, radiation of the source yielded photonic absorptionin the visible, ultraviolet (UV) and vacuum ultraviolet (VUV) maypromote certain reactions with higher activation energies [30].Furthermore, the interaction of the irradiated photons with thesurface atoms (photo-effect) may enhance atomic diffusion [31].Thus, the crystallization time is reduced to the order of seconds.Consequently, grain growth will be limited, the microstructurecan be easily controlled, and thermal budget significantly reduced.

3.2. Crystal structure refinement of the crystallized Ti-rich TiNi thinfilms

By comparing the crystal structure and phase development offilms annealed by RTA and CTA insights may be gained into the dif-ferent mechanisms involved. In particular, comparisons are drawnbetween the Ti-rich TiNi films annealed by RTA at 773 K for 180 s(RTA sample) and those crystallized by CTA at 753 K for 1 h (CTA

Fig. 7. 2D and 3D AFM images of the Ti-rich films; (a) as-sput


sample). The Rietveld analyzed XRD patterns of the RTA and CTAsamples are shown in Fig. 4. The refined pattern included austenite(cubic), martensite (monoclinic) and a Ti2Ni intermetallic (cubic).The refinements resulted in good fits to the experimental dataand Bragg reliability factor (Rb) values from 0.8% to 2% were ob-tained (Table 3). The Rb (R-Bragg) value defined by the followingequation [17]:

Rb ¼PðF0 � FcÞP

F0ð2Þ

where F0 and Fc are the observed and calculated structure factorsrespectively. In the RTA sample, two peaks are not well simulatedand may be related to surface oxidation during annealing in Aratmosphere, while in CTA, where annealing was in high vacuumcondition, these features were not observed. The atomic positions,site occupancies, thermal parameters and Bragg reliability factor(Rb) of the crystallized films are listed in Table 3. The variation ofthe atomic positions and site occupancies of Ti and Ni in crystallizedfilm by RTA and CTA are explained with respect to the atomicparameters from initial crystallographic models (Table 2 [18,19]).There is no change in atomic positions of the Ni and Ti in austenitefor both CTA and RTA films. The Z-parameter of Ti1 in martensite isincreased and it is slightly higher in annealed film by CTA. Thechange in atomic displacement of Ni and Ti in Ti2Ni intermetallicis more pronounced than that of martensite and austenite whereasboth Ni1 and Ti2 are varied. The x, y and z parameters of Ni1 areincreased and the displacements are higher in RTA crystallized film.

tered (b) crystallized by RTA and (c) crystallized by CTA.



Fig. 8. Indenter load versus indentation depth curves at 5, 10 and 15 mN maximumloads for TiNi films; (a) crystallized film by RTA and (b) crystallized film by CTA.

Fig. 9. Variation of the elastic modulus versus the indentation load for crystallizedfilms by rapid and conventional thermal annealing. Error bars denote standarddeviation above and below the corresponding mean values, calculated fromindentation data obtained from three different regions on the surface of the films.

Table 5Valence electron density (VED) of the austenite, martensite and Ti2Ni phases. N is thenumber of the atoms per unit cell.

Sample Phase N Valenceelectrons

Cell volume(Å3)

Valence electrondensity (VED)(e/Å3)

RTA Austenite 1 Ni 10 26.9 0.521 Ti 4

Martensite 2 Ni 20 56.2 0.502 Ti 8

Ti2Ni 32 Ni 320 1423.9 0.4064 Ti 256

CTA Austenite 1 Ni 10 26.7 0.521 Ti 4

Martensite 2 Ni 20 53.8 0.522 Ti 8

Ti2Ni 32 Ni 320 1458.6 0.3964 Ti 256


The x-parameter of Ti2 in RTA is increased, while the reduction hastaken place in CTA sample. The results show the profound differ-ence in atomic displacements and site occupancies of the Ti andNi atoms in martensite and Ti2Ni phases during crystallization forRTA and CTA films. Based on the crystallographic refined parame-ters (Table 3), the schematic of the crystallized phases of the RTAsample is drawn with aid of ATOMS software (Fig. 5). The volumefraction, lattice parameter, cell volume, crystallite size and latticestrain of the crystallized phases are presented in Table 4. The cellvolume of the Ti2Ni intermetallic phase in CTA sample is signifi-cantly higher than that observed in RTA, while there is no clearchange in austenite phase. However, the unit cell volume of mar-tensite in RTA film is larger compared to the CTA film. The origin

Fig. 10. Schematic of different configurations of TiNi thin films placing for RTA treatmenfilm; (c) two TiNi thin films were placed face-to-face.


of the changes in volume is clear when the relative changes in lat-tice parameters are considered (Table 3). The (a) cell edge of austen-ite in RTA is slightly higher than that observed in CTA film. The (a)and (c) cell parameters of RTA is larger than CTA film, while the (b)cell edge is smaller and the monoclinic angle (b) remains constant(96.8�). The (a) lattice parameter of RTA crystallized film is smallerthan that observed in CTA. Total lattice strain (LSTotal) of the filmscan be defined by the following equation:

t. (a) TiNi thin film deposited on Si wafer; (b) a Si wafer placed on top of TiNi thin



Fig. 11. (a) Schematic of the RTA in face-to-face configuration and XRD pattern ofthe film after annealing; (b) schematic of annealing when the other patternedsample has been placed above the film and the expected patterned shape memorythin film after annealing.


LSTotal ¼ faustenite � LSaustenite þ fmartensite � LSmartensite þ fTi2Ni � LSTi2Ni

ð3Þ

where faustenite, fmartensite and fTi2Ni are volume fractions for austenite,martensite and Ti2Ni and LSaustenite, LSmartensite and LSTi2Ni are thecorresponding lattice strains of austenite, martensite and Ti2Nirespectively (Table 4). The total lattice strain of the RTA film(�1.61%) is clearly higher than that observed in CTA sample(�1.3%) which could be due to the higher ramping rate of the RTAsamples compared to the CTA film.

3.3. Surface morphology of the crystallized Ti-rich TiNi thin films

Fig. 6 presents SEM micrographs of the Ti-rich films after RTAand CTA treatments at 753 K. No cracks were observed in the sur-faces of the films in both annealing techniques. Fig. 7a shows theatomic force microscopy (AFM) 2D and 3D images of the as-sput-tered film. The roughness (rms value) of the as-sputtered filmwas 1.3 nm, demonstrating the good quality of the deposited films.The surface morphologies of the RTA and CTA crystallized films aredemonstrated in Fig 7b and c. The roughness of the RTA crystal-lized film (’1.9 nm) is larger than that observed in CTA crystallizedsample (’1.1 nm). This means that the CTA produces a smoothersurface compared to the films prepared by the RTA treatment.

3.4. Dependence of elastic modulus on valence electron density

Elastic modulus of the crystallized films by RTA and CTA areinvestigated by nanoindentation method. The load-depth re-sponses of the RTA and CTA samples at 5, 10 and 15 mN indenta-tion loads are shown in Fig. 8, where each curve is an average offour measurements. The elastic modulus of the films was extractedfrom the load-depth curves using the Oliver–Pharr method [32].Fig. 9 presents the variation of the elastic modulus of the films atdifferent indentation loads. The elastic modulus of the CTA filmis larger than that observed in RTA sample. Gilman [33] reportedan established relationship between numbers of valence electronsper unit volume or valence electron density (VED) and bulk elasticmodulus of metallic material. It is known that with increasing theVED, the elastic modulus is increased [33,34]. From the Rietveld re-fined parameters of the crystallized films, the VED of each phasecan be calculated as:

VED ¼Number of valence electrons per unit cellcell volume

ð4Þ

Table 5 shows the calculated VED of the phases. As the crystal-lized films are composed of austenite, martensite and Ti2Ni phases,the VEDTotal is defined as:

VEDTotal ¼ faustenite � VEDaustenite þ fmartensite � VEDmartensite

þ fTi2Ni � VEDTi2Ni ð5Þ

where faustenite, fmartensite and fTi2Ni are the phase volume fractions.The calculated results confirmed that the VEDTotal of the CTA sample(�0.46) is larger than that observed in the RTA film (�0.44), indic-ative of the higher elastic modulus of the CTA crystallized film.

3.5. A new approach to fabricate patterned TiNi-based thin films

TiNi-based thin films can crystallize in a few seconds by RTAand suggests a new method to fabricate shape memory alloy films.Fig. 10 illustrates different configurations of TiNi thin films ar-ranged for RTA treatment. In the first configuration, a Si wafer isplaced on the top of the film and RTA (773 K/180 s) employed to


crystallize the sample (Figs. 1 and 10b). This produces a strongbond between the wafer and film, due to diffusion bonding. Inthe second configuration, two thin films were placed face-to-faceand the same treatment employed (Fig. 10c). However, no bondingwas created between the thin films.

Fig. 11a shows a schematic of the process and the XRD patternof the thin film after RTA. It is evident that the film is poorly or-dered and the sample has vitrified. Based of the preliminary re-sults, RTA has ability to crystallize the sample in a few secondsby directly illuminating the surface and thereby fabricating a pat-terned (see Fig. 11b) thin film. The crystallized part retains theshape memory effect, whilst the glassy part does not posses thisproperty. This approach can provide promising applications in mi-cro-electro-mechanical systems (MEMS).




4. Conclusion

In this paper, a systematic study was carried out of the crystal-lization behavior of Ti-rich NiTi thin films, during rapid thermalannealing. Crystallization took place in a few seconds, with theoptimum crystallization temperature of 773 K with ordering initi-ated after 2 s. Longer annealing (773 K/240 s), transformed the filmto a poorly ordered vitreous phase, and is a novel method for solidstate amorphization. Reitveld refinement showed significant dif-ferences in structural parameters of the films crystallized by RTAand CTA. The valence electron density of CTA sample is larger thanfor RTA film consistent with a higher elastic modulus of the former.Finally, a new approach based on the crystallization behavior ofRTA is proposed to fabricate patterned shape memory thin films.

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