Ž .Sensors and Actuators 77 1999 34–38www.elsevier.nlrlocatersna

Applications of TiNi thin film shape memory alloys inmicro-opto-electro-mechanical systems

M. Tabib-Azar ), B. Sutapun, M. HuffDepartment of Electrical Engineering and Computer Science, Case Western ReserÕe UniÕersity, Room 517, Glennan Building, CleÕeland, OH 44106, USA

Received 27 February 1998; received in revised form 5 December 1998; accepted 6 January 1999

Abstract

Ž .The application of shape memory alloy SMA thin films in optical devices is introduced and explored for the first time. Physical andŽ .optical properties of titanium–nickel TiNi SMA thin films change as these films undergo phase transformation upon heating. An optical

beam can be modulated either mechanically using a TiNi actuator or by the changes that occur in TiNi’s optical properties upon heatingŽ .and phase transformation. Reflection coefficient of TiNi films were measured in their martensitic at room temperature and austenitic

Ž .elevated temperature phases. The reflection coefficient of the austenitic phase were higher than those of the martensitic phase by morethan 45% in the wavelength range between 550 and 850 nm. A microfabricated TiNi diaphragm with a 0.26-mm-diameter hole was usedas a prototype light-valve. The intensity of the transmitted light through the hole was reduced by 10–17% when the diaphragm washeated. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Shape memory devices; Microactuators; Optical modulators; MOEMS

1. Introduction

Ž .Shape memory alloys SMAs have interesting shape-recovery characteristics upon heating. These alloys aredeformed below a martensite finish temperature and canrecover their initial shape when heated above a tempera-ture corresponding to the austenite temperature. Martensiteand austenite solid phases differ in microstructures, Youngmodulus, thermal expansion coefficient, and electrical con-

Ž .ductivity. Among the SMAs, titanium–nickel TiNi sys-tem has received most attention due to its ability togenerate large forces with large power-to-weight ratio,high recoverable stress, low power consumption, and longlifetime. These characteristics clearly make the TiNi sys-tem a suitable candidate for applications in microactuatorsw x1 .

Thin films of TiNi have been successfully deposited onw xsilicon substrates by means of magnetron sputtering 2 , rf

w x w xsputtering 3 and pulsed laser deposition 4 , using a

) Corresponding author. Tel.: q1-216-368-6431; Fax: q1-216-368-6039; E-mail: mxt7@po.cwru.edu

multiple target. Shape memory characteristics of thin filmTiNi are somewhat similar to its bulk characteristics. SMAactuators utilizing thin-film structures are preferred in mi-crosystems due to their large surface-to-volume ratios andtheir small thermal mass resulting in higher operatingspeed. Using micromachining techniques, many microde-

w x w xvices, such as micropumps 5 , microvalves 6–8 , andw xmicrogrippers 9 , that use TiNi SMA actuators have been

reported.In the present work, the application of TiNi thin films

in optical devices is introduced and explored. The basis ofour research reported here was the observation that as theTiNi films undergo phase transformation, they becomemore reflecting and ‘shiny’ and loose their ‘cloudy’ ap-pearance. Additionally, it is also noticed that as these alloyundergo phase transformation, their lattice constant changesresulting in changes in their physical dimensions or modi-fication of their internal or residual stresses. These changesbecome more important and dominant in thin films. We setout to explore and measure any changes that may occur inthe scattering, the refractive index, physical dimensionsand residual stress of the TiNi films upon heating andphase transformation.

0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0924-4247 99 00053-9

( )M. Tabib-Azar et al.rSensors and Actuators 77 1999 34–38 35

2. Experimental results

As shown in the atomic force microscopy of a TiNi filmŽ .in Fig. 1, the martensitic phase room temperature of TiNi

films are textured. The surface morphology in these filmsis due to the crystallographic twin planes. Hence, it can beexpected that some long range order are present in theirmartensitic phase. It has been observed that, at roomtemperature, the film surface appears to be opaque and‘cloudy’. When heated, the TiNi surface becomes shinywith a metallic luster. In order to study the reflection

coefficient of TiNi in its two phases, we performed reflec-tion measurement. TiNi thin films were deposited on sili-con wafers using an rf sputtering system with a multipletarget. As-deposited films were amorphous and the filmswere subsequently annealed at 5508C while still undervacuum to achieve crystallinity. The film thicknesses werearound 2–3 mm and the film compositions were approxi-mately 49.2% Ti and 50.8% Ni.

Martensite start and austenite finish temperatures wereobtained using resistivity vs. temperature measurementsand they were 40 and 658C, respectively. To perform

Fig. 1. The atomic force microscopy of the martensitic phase of 49% Ti, 51% Ni films. The regular surface structure is due to the twin planes.

( )M. Tabib-Azar et al.rSensors and Actuators 77 1999 34–3836

Fig. 2. Experimental setup used to measure the reflection coefficient ofthe TiNi film.

optical reflection measurements, the wafers were cut into2=2-cm pieces. Electrical contacts were made by attach-ing conductor wires near the edges of samples usingconducting silver epoxy. These samples were then mountedonto the sample holder. For these samples, 6–10 W wasneeded to cause the phase transformation.

The reflection coefficients were measured for bothmartensite and austenite phases using the setup schemati-cally shown in Fig. 2. The measuring optical fiber was anarm of a 3-dB coupler that was located perpendicular tothe sample. The 3-dB coupler arrangement was used todirect the reflected light to a detector as shown schemati-cally in Fig. 2. The fiber had a 100-mm core diameter with20-mm cladding. The monochromator was used to performspectroscopy to examine the TiNi reflectance at differentwavelengths. A chopper along with a lock-in amplifierwere used to perform synchronous measurement of theoptical signal which was detected using a photomultiplier.A microscope objective, 20= , was used to focus light into

Ž .the input fiber input arm of the 3-dB coupler and theentire experiment and data acquisition and analysis wereperformed using a computer. Reflection from the 3-dBcoupler and the reflecting end of the measuring fiber,

Fig. 3. Reflection outputs as a function of wavelength at 22.0 and 40.68C.The solid line shows the output from a 3-dB coupler alone at 22.08C.

Fig. 4. Normalized reflection output spectra for martensitic and austeniticTiNi.

shown in Fig. 3, were subtracted from the measure spectraof TiNi discussed in Section 3.

Fig. 3 shows a typical reflection spectrum of TiNi thinfilms. At room temperature, the film surface showed acloudy texture corresponding to the martensitic phase. Asthe film underwent phase transformation by Joule heating,the cloudiness disappeared and the film surface becamehighly reflective. From 550 to 850 nm wavelength, thereflection of austenitic phase was higher than martensiticphase by 15–45% depending on the wavelength. Thedifference of the reflection coefficients between the twophases reached its maximum at 500–600 nm as shown inFig. 4.

3. Discussion

When the TiNi film undergoes a phase transformation,both its surface roughness, which results to light scattering,and its refractive index change. Thus, it is important todifferentiate these two contributions to the overall opticalreflection of the TiNi film before and after its phasetransformation. Surface scattering measurements and theo-retical modeling of how surface scattering contributes tothe surface reflection are subject of intense research by

w xmany researchers, as discussed in Ref. 10 . These studies,however, are beyond the scope of the present investigation.Nevertheless, we note that a series of simple reflectionmeasurements can be performed to differentiate the abovetwo contributions. These measurements consist of using afiber optic system to perform reflection measurements as a

Ž .function of wavelength and distance d between the fiberŽ .and the TiNi surface see Fig. 5 inset . In optical fibers,

light emitted from the fibers emerges in a cone of radiationŽ .with an angle given by the numerical aperture NA . For a

Ž .small distance d- the fiber core diameter between thefiber and the sample, a large fraction of the scattered lightwill be within the NA of the fiber and, thus, will be guidedback to the photodetector. Thus, at these smaller distances,the scattering contribution is very small. However, atlarger fiber optic to TiNi distances, scattering may take

( )M. Tabib-Azar et al.rSensors and Actuators 77 1999 34–38 37

Fig. 5. Reflection of an Al mirror and a martensitic TiNi as a function ofŽ .distance d between the fiber optic probe and the reflecting surface. d is

shown in the inset.

Žaway optical power from the back-reflected light actually,there is an anomaly called the ‘opposition’ effect thatresults in the enhancement of the back-reflected light at

w x .normal incidences 10 as discussed next . Thus, at largerdistances, the combination of both the surface scatteringand the refractive index contribute to TiNi’s reflectioncoefficient.

Fig. 5 shows the reflected light from a martensitic TiNifilm and an aluminum mirror as a function of the distanceof the fiber optic probe. Fig. 5 inset shows the fiberrsam-ple arrangement and defines the probe-to-sample distance,d. For small d, the reflection from the A1 mirror is higherdue to its higher refractive index. However, as d becomescomparable to the fiber optic’s core diameter, the outputfrom the TiNi becomes higher. This increase can be due tothe ‘opposition’ effect that comes about due to the multi-path scatterings from the surface of the ‘cloudy’ TiNi film.Such an enhanced reflection is well known in roughsurfaces and it is explained in terms of coherent interfer-ence between the forward and backward waves at thefilm’s surface. Another contribution to the back-reflectedlight is to note that scattering at the surface may ‘scatter’light in the direction of the fiber’s collection cone. Such anenhanced collection is absent from the smooth surface ofthe A1 mirror.

Ž .Fig. 6. Reflection coefficient at ds0.05 solid line and at ds1.5 mmŽ .dashed line detected as the TiNi film underwent the phase transforma-tion. The reflection increased by 34 and 7% for ds0.05 and 1.50 mm,

Žrespectively. d is the distance between the fiber optic probe and the TiNi.surface as shown in Fig. 5 inset .

Fig. 7. Optical reflection of a TiNi film as a function of temperatureduring a continuous cooling and heating cycle. The resistance vs. temper-ature behavior is shown in inset.

On the other hand, the difference in the reflectioncoefficient of the two phases of TiNi decreased as dincreased. At ds0.05 mm, the normalized reflected lightincreased by 34% as compared to only 7% increase at

Ž .ds1.50 mm see Fig. 6 . Fig. 6 also shows the reflectioncoefficient at ds0.05 and 1.5 mm detected as the TiNifilm underwent the phase transformation. The reflectionincreased by 34 and 7% for ds0.05 and 1.50 mm,respectively. The experimental result shown in Fig. 6 alsoindicates that the TiNi’s phase transformation changes itseffective refractive index by a large factor and that thescattering contribution to its reflection coefficient is not adominant factor.

The change in optical and electrical properties of TiNifilms during heating and cooling cycles are somewhatcorrelated. Fig. 7 shows the reflections during the phasetransformations. Hysteresis and nonlinearity of the reflec-tion observed in this figure are similar to those in thewell-known resistivity vs. temperature characteristics of

Ž .TiNi also shown in Fig. 7 . Thus, the optical reflectionmeasurements can be used to identify the transformationtemperatures of SMA thin films, which are usually charac-terized using DSC or resistivity measurements. UnlikeDSC and resistivity measurements, reflection measure-ments are nondestructive and noncontact.

Fig. 8. The outputs from the photodetector when the diaphragm is heatedŽ . Ž .close and unheated open .

( )M. Tabib-Azar et al.rSensors and Actuators 77 1999 34–3838

We also explored the possibility of fabricating lightvalves using TiNi films. The first simple prototype wasmade from a 3-mm-thick 0.8=0.8 cm TiNi diaphragm. Ahole with 0.26-mm diameter was manually punched on thediaphragm using a tungsten probe pin. This diaphragm wasthen placed between the light source and the photodetector.The data were collected at room temperature and after thediaphragm was heated. We found that the intensity oftransmitted light through the hole was reduced by 17%

Ž .when the diaphragm was heated Fig. 8 . Time responsebetween the on and off states for this structure was lessthan 1 s. The data indicates that the hole is not completelyclosed when heated. When the hole was created, someportion of the membrane may have been broken or stressedbeyond the recoverable stress limit of this material. Infuture, we will fabricate these optical valves using pho-tolithography, which will eliminate stressing the film.

The difference in the reflection coefficient between thetwo phases of TiNi thin films may be useful for manyinteresting optical applications and devices such as inoptical switches. TiNi film itself can be used as a devicewith nonreflecting surface, or ‘off state’, at room tempera-ture and reflecting, or ‘on state’, when heated. Fast re-sponse time, a crucial factor for optical switches, can beachieved by using near-free standing structures such asTiNi cantilever beams or membranes. Compared to mi-

w xcromirrors optical switches 11,12 , TiNi optical switchescan be designed to have no moving mechanical partsresulting in less power consumptions and simple designand fabrication process and device assembly.

Last, but not least, we note that the combination ofoptical, thermal, and mechanical properties of TiNi SMAscan be used in a variety of micro-opto-electro-mechanical

Ž . w xdevices and systems MOEMS 13 . For example, anŽoptical probe can be used to detect the state of a TiNi or

.other SMA microfluidic switch. Or more importantly, anoptical signal can be used to drive such a switch inflammable environments where electrical wires and signalsare undesirable.

In these applications, the speed of the device may notbe very critical. Although, as discussed above, the speed ofTiNi devices can also be made quite fast by appropriatethermal designs.

4. Conclusion

In conclusion, we have shown for the first time how thereflection coefficient of TiNi SMA films change as afunction of phase transformation and wavelength. Ourinvestigation showed that the reflection coefficient of TiNi

Ž .changes by a large factor 45% as TiNi undergoes a phasetransformation. We also discussed some possible applica-tions of TiNi in optical devices and spatial light modula-tors.

Acknowledgements

The diaphragms used in this work were fabricated byW.L. Benard.

References

w x1 M. Tabib-Azar, Microactuators, Kluwer Academic Publishers,Boston, 1997.

w x2 J.D. Bush, A.D. Johnson, C.H. Lee, D.A. Stevenson, J. Appl. Phys.Ž .68 1990 6224.

w x Ž .3 R. Wolf, A.H. Heuer, J. Microelectromech. Syst. 4 1995 206.w x4 K. Ikuta, H. Fujishiro, M. Hayashi, T. Matsuura, in: A.R. Pelton, D.

Ž .Hodgson, T. Duerig Eds. , Proceedings of the First IntrnationalConference on Shape Memory and Superelastic Technologies, SMSTInternational Committee, Pacific Grove, CA, 1995, pp. 13–18.

w x5 W.L. Benard, H. Kahn, A.H. Heuer, M.A. Huff, Proceedings ofIEEE International Conference on Solid-State Sensors and Actua-tors, IEEE Proceedings, Chicago, 1997, pp. 361–365.

w x6 J.D. Johnson, E.J. Shahoian, Proceedings IEEE Micro Electro Me-chanical Systems, IEEE Proceedings, Amsterdam, 1995, pp. 216–220.

w x7 H. Kahn, W.L. Benard, M.A. Huff, A.H. Heuer, MRS Symp. Proc.Ž .444 1997 227.

w x8 K.D. Skrobanek, M. Kohl, S. Miyazaki, Proceedings IEEE MicroElectro Mechanical Systems, Nagoya, Japan, 1996, pp. 256–261.

w x9 A.B. Lee, D.R. Ciarlo, P.A. Krulevitch, S.C. Lehew, J.T. Trevino,Ž .M.A. Northrup, Sensors and Actuators A 54 1996 755.

w x10 J.C. Dainty, The Opposition Effect in Volume and Surface Scatter-Ž .ing, in: J.W. Goodman Ed. , International Trends in Optics, Aca-

demic Press, San Diego, 1991, pp. 207–219.w x Ž .11 L.J. Hornbeck, IEEE Trans. Electron Devices ED 30 1983 539.w x12 O. Solgaard, F.S.A. Sandejas, D.M. Bloom, Optical Letters 17

Ž .1992 688.w x13 M. Tabib-Azar, B. Sutapun, MOEM pressure and other physical

sensors using photon tunneling and optical evanescent fields withexponential sensitivities and excellent stabilities, SPIE InternationalSymposium on Microelectronic Structures and MEMS for OpticalProcessing IV, 1998, p. 210.

Massood Tabib-Azar received the MS and PhD degrees in ElectricalEngineering from the Rensselaer Polytechnic Institute in 1984 and 1986,respectively. He joined the faculty of Case Western Reserve University in1987. He is author and co-author of more than 50 journal publications,two book chapters, a book on Integrated Optics and MicrostructureSensors and a book on Microactuators. Prof. Tabib-Azar has been theChairman of the International Conference on Integrated Optics andMicrostructures since 1991. His main areas of interest include novelsensors, electronic devices and quantum computing. Dr. Tabib-Azar is arecipient of the 1991 Lilly Foundation Fellowship and he is a seniormember of IEEE Electron Devices, and a member of APS, AAPT, andSigma Xi Research Societies.