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Manufacturing issues of thin film NiTi microwrapper
Sensors and Actuators A 93 (2001) 148±156
Manufacturing issues of thin ®lm NiTi microwrapper John J. Gilla,1, David T. Changb, Leslie A. Momodab, Greg P. Carmana,* a
Mechanical and Aerospace Engineering Department, School of Engineering, University of California Los Angeles, 420 Westwood Plaza Engineering IV, Los Angeles, CA 90095, USA b HRL Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265-4799, USA Received 27 November 2000; received in revised form 28 March 2001; accepted 29 March 2001
Abstract Manufacturing issues related to a thin ®lm NiTi shape memory alloy (SMA) microactuator (i.e. microwrapper) have been investigated using both wet and dry etching techniques. Results show that wet etching the amorphous ®lm produces a cleaner pattern than the crystallized ®lm. Transformation temperatures are not affected by the pre-exposure of the NiTi ®lm to air before crystallization. However, this process produces breakage in the NiTi ®lm at sacri®cial layer steps. This is believed to be due to residual stresses developed between the ®lm and substrate during sputtering. The ®lm breakage is overcome by dry etching the ®lm with an ion-milling technique. Curvature in the microwrapper arms is induced using either a bi-layer material (i.e. polyimide and NiTi) or a functionally gradated NiTi ®lm. Results show that when heated the microwrapper arms ¯atten due to shape memory effect and curl up to form a cage-like structure when cooled. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Shape memory alloy; NiTi; Transformation temperatures; Ion-milling
1. Introduction The term shape memory alloy (SMA) refers to a material which returns to a predetermined shape when heated. This phenomenon is caused by atomic shuf¯ing within the material during phase transformation. At low temperatures, the phase of the SMA material is martensite, which is ductile and can be deformed easily. However, by simply heating the deformed material to an elevated temperature, the material's phase changes to austenite and the deformation induced at low temperature is fully recovered. Therefore, by repeating the deformation/heating cycle, the SMA material can be used as an actuator in a mechanical device. SMAs were ®rst discovered in 1930s by Olander [1]. After that initial discovery, many alloys have been found to exhibit the SMA effect. Among those, NiTi alloys are used in industrial applications due to low manufacturing cost, good corrosion resistance and tailorable material properties [2]. NiTi has been predominantly applied at the macroscale (i.e. * Corresponding author. Tel.: 1-310-825-6030. E-mail addresses: [email protected] (J.J. Gill), [email protected] (G.P. Carman). 1 Present address: Jet Propulsion Laboratory, M/S 302-306, 4800 Oak Grove Dr., Pasadena, CA 91109, USA. Tel.: 1-818-393-0456; fax: 1-818-393-4540.
wires and thick plates) over the last several decades. However, its application to microelectromechanical systems (MEMS) started in the early 1990s when Walker and Gabriel fabricated a shape memory alloy coil spring on a silicon wafer to demonstrate the compatibility of thin ®lm NiTi with MEMS fabrication processes [3]. Since that initial work, many research studies have been conducted on integrating sputter-deposited thin ®lm SMA into micro devices. Applications include micro¯uid control, optics, sensors, relays and actuators [4±18]. One major advantage associated with NiTi is its biocompatibility. Commercial biomedical applications include a bone anchor, vena-cava ®lter, and stents [19,20]. While a number of macroscopic biomedical applications exist, many possibilities equally exist for microscale applications such as minimal invasive surgery. However, to begin investigating these new applications, we must ®rst develop the fabrication process to produce thin ®lm NiTi SMA actuator. One possible bio-medical use for thin ®lm NiTi SMA is a micrograbbing device (i.e. microwrapper). The purpose of the microwrapper is to grab microsize objects for in situ analysis in a living organism or possibly grab pieces of cancerous tumor for removal from the body via a small catheter. Micrograbbing devices using different actuation mechanisms have been previously reported (i.e. electrostatic and pneumatic actuation) [21,22]. However, these devices
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 6 4 6 - X
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Fig. 1. Illustration of microwrapper: (a) plan-view of microwrapper; (b) schematic diagram of actuation.
are not necessarily bio-compatible. Since the device works in a living organism, the bio-compatibility requirement is a critical issue. Therefore, NiTi appears to be an excellent actuator candidate for microwrapper or other small-scale bio-medical devices. 2. Actuation scheme The actuation scheme of the proposed shape memory microwrapper is illustrated in Fig. 1. Fig. 1(a) represents a plan-view of the microwrapper. The gray outline in the ®gure identi®es the NiTi area. The bonding pad, current path, and central portion of the ``arms'' are attached to the substrate while the remaining parts of the microwrapper arms are detached from the substrate. The arms of the microwrapper have two stable con®gurations: (1) curledup shape; (2) ¯at shape (Fig. 1(b)). When released from the substrate, the arms curl up to form a cage-like structure (i.e. residual stress). When heated, the arms ¯atten out due to the shape memory effect. Heat required for actuation is provided by passing a current from one bonding pad to the other which causes a current ¯ow in the entire NiTi structure (i.e. Joule heating). When the current is removed and heat dissipates to the environment, the individual wrapper arms return to the cage-shaped con®guration. The returning process requires residual stress using a bias spring mechanism or a shape memory material exhibiting the two-way effect [23].
Ê thick) (LPCVD) polysilicon sacri®cial layer (2000 A (Fig. 2(a)). The polysilicon layer is patterned with xenon± di¯uoride (XeF2) photo-lithographically (Fig. 2(b)). Following the sacri®cial layer patterning, the NiTi ®lm is sputterdeposited from a near equiatomic target of NiTi onto the substrate and crystallized. The sputter-deposition parameters for the NiTi ®lm are presented in Table 1 [24]. Following deposition, the NiTi ®lm is etched (Fig. 2(c)). Typically a mixture of hydro¯uoric acid (HF), nitric acid (HNO3), and deionized (DI) water with the respective ratio of 1:1:20 is used to wet etch the NiTi ®lm. A thick photoresist (AZ-4620, Clariant, 5 mm thick) is used as an etch mask. It was reported previously that the NiTi ®lm could be etched uniformly with this approach [23]. Polyimide (PI) layer is deposited on top of the NiTi ®lm to induce curvature (i.e. bias spring) when the NiTi ®lm is released (Fig. 2(d)). Finally, the polysilicon sacri®cial layer is removed with XeF2 and the arms of the microwrapper are released. Due to residual stress, the arms curl up and form the cage-like structure once released (Fig. 2(e)). During this fabrication process, several problems occur in patterning the NiTi ®lm. The ®rst problem is a nonuniform undercut of the NiTi ®lm (Fig. 3(a)). In some regions, the undercut is so severe the ®lm is completely etched away. We believe this problem could be alleviated by etching the amorphous ®lm rather than the crystallized ®lm. The unannealed NiTi ®lm is amorphous: therefore its material Table 1 Sputtering parameters of NiTi film deposition
3. Fabrication process The process ¯ow to manufacture the microwrapper is described in Fig. 2. First, a silicon wafer is wet-oxidized and coated with low-pressure chemical-vapor deposition
Base pressure Argon pressure Substrate to target distance Power Crystallization temperature
<5 10 8 Torr 2 mTorr with 99.999% purity 3 cm 300 W dc 5008C for 6 min
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Fig. 2. Schematic diagram of microwrapper fabrication process: (a) wet oxidation and LPCVD polysilicon sacrificial layer deposition; (b) patterning polysilicon layer by XeF2; (c) deposition and patterning NiTi film; (d) deposition and patterning polyimide layer; (e) removal of polysilicon layer by XeF2.
Fig. 3. Wet etching of NiTi film: (a) crystallized film; (b) amorphous film.
strength is thought to be weaker and consequently, easier to be etched than an annealed one. Therefore, the fabrication procedure is modi®ed such that following deposition, NiTi ®lm is etched prior to crystallization. However, altering the fabrication process in this manner raises a question regarding the in¯uence on the shape memory effect [8]. To investigate this issue, a sputter-deposited amorphous NiTi ®lm is exposed to air, dipped into DI water, and followed by crystallization. Following this, the transformation temperatures (As: austenite-start temperature; Af: austenite-®nish temperature; Ms: martensite-start temperature; Mf: martensite-®nish temperature) of the ``pre-exposed'' NiTi ®lm are
measured by the wafer curvature technique and compared to in situ crystallized ®lm. As one can see in Fig. 4 and Table 2, the pre-exposed NiTi ®lm has similar phase transformation temperatures when compared to in situ crystallized NiTi Table 2 Transformation temperature measurement of differently processed NiTi film NiTi film
As (8C)
Af (8C)
Ms (8C)
Mf (8C)
Pre-exposed Conventional
88 90
104 110
74 70
60 58
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Fig. 4. Transformation temperature measurement result of in situ crystallized and pre-exposed NiTi film.
®lm. Therefore, it appears that crystallization can be done either prior to (in situ) or following the patterning of the ®lm without impacting the shape memory response. To evaluate the etching characteristics of the amorphous ®lm, NiTi ®lm is sputter-deposited on a test wafer and etched using the same etchant and mask as described previously. In Fig. 3(b), results show that delamination of the photoresist does not occur and a clean etch pattern of the ®lm is produced. It is noted that the amorphous ®lm etches approximately three times faster than the crystallized ®lm (i.e. etch rate of amorphous ®lm is 0.6 mm/min, and that of crystallized ®lm is 0.2 mm/min). Therefore, etching amorphous ®lm produces a more distinctly de®ned line pattern than etching crystallized ®lm (i.e. compare Fig. 3(a) and (b)). In addition to the undercut problem, it is observed the patterned NiTi ®lm breaks at the sacri®cial layer step (Fig. 5). Scanning electron microscopy (SEM) shows that a hole is produced during etching at the interface of the NiTi ®lm and sacri®cial layer step (Fig. 6(a) and (b)). As etching proceeds, the hole enlarges and eventually breaks the ®lm (Fig. 6(c)). The breakage of the NiTi ®lm appears to be related to the residual stresses in the ®lm. When a ®lm is deposited, residual stresses develop which could be either tensile or compressive depending on the nature of the process or the materials [25,26]. The NiTi ®lm has a tensile stress of 200 MPa in the martensite phase and 310 MPa in the
austenite phase as determined by wafer curvature technique (see Fig. 4). It is believed that this large tensile stress produces a tiny gap at the interface of the sacri®cial layer step and the ®lm (Fig. 7(a)) [27]. Once the etchant reaches the sacri®cial layer, the gap provides a pathway for the etchant to ¯ow in and back-etching occurs to break the NiTi ®lm (Figs. 7(b)±(d) and 8). We attempted unsuccessfully to verify the existence of this tiny gap prior to etching with SEM. The inability to ®nd the gap may be due to its small physical size. To reduce the residual stress and thus eliminate the gap, NiTi ®lm is deposited onto a heated substrate at 3508C. When the NiTi atoms arrive on a heated substrate, they have more thermal energy to align themselves and relieve the stress. As described previously, the in situ heating temperature should not exceed 5008C in order to prevent crystallization of the ®lm and minimize the number of precipitates (i.e. partial crystallization starts at 4008C) [28]. Once deposited, the ®lm is etched using the same procedure described previously. The presence of the etch hole is investigated at the interface of the NiTi layer and sacri®cial layer with SEM (Fig. 9). SEM pictures reveal that the etch hole is still present but is substantially smaller in size when compared to the ®lm deposited onto an unheated substrate. This result indicates substrate heating during deposition works to reduce the residual stress in the ®lm. While the size of the etch hole
Fig. 5. Center part of microwrapper illustrating the location of NiTi film breakage at the sacrificial layer step.
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Fig. 6. Pictures showing the NiTi film breakage at the polysilicon sacrificial layer step: (a) etch hole grows between the NiTi film and the substrate; (b) almost broken NiTi film; (c) completely broken NiTi film.
Fig. 8. SEM picture of the broken NiTi film at the sacrificial layer step which shows the result of Fig. 7(d).
Fig. 7. Schematic diagram of the NiTi film breakage: (a) a hole exists due to the residual stress; ((b) and (c)) the hole enlarges as etching proceeds; (d) the NiTi film breaks.
Fig. 9. SEM picture of in situ heated NiTi film.
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Fig. 10. NiTi film patterned by ion-milling (no film breakage is shown).
is reduced, its presence is still unacceptable from a fatigue perspective. The ®lm breakage problem is solved by ion-milling the NiTi ®lm (i.e. dry etching technique). Unlike chemical wet etchants, which etch isotropically, ion-milling is anisotropic etching technique producing a vertical side-wall. Therefore, back-etching does not occur in the gap and the ®lm breakage can be prevented. Using a thick photoresist (AZ-4620) as an etch mask, the NiTi ®lm is ion-milled in the following conditions: (a) working gas argon; (b) acceleration of argon beam 150 V; (c) beam current 8 mA; (d) etching time 90 min and overetching time 30 min. Figure 10 shows the result of ion-milling of the NiTi ®lm at the sacri®cial layer step. As one can see from the picture, without an etch hole, a clean etch pro®le is produced over the sacri®cial layer step. Also ion-milling produces a clean etch pro®le for crystallized ®lm. This is bene®cial because manufacturing process becomes simpler than wet etching the amorphous ®lm due to in situ crystallization. Therefore, by patterning the NiTi ®lm with ion-milling, the undercutting and breakage of the NiTi ®lm can be prevented. 4. Curling-up microwrapper arm One approach to induce curvature in the microwrapper is with a bi-layer. PI is deposited and patterned on the top of the microwrapper arm to induce the curvature. When the polysilicon sacri®cial layer is removed and the microwrapper arms are released, the arms curl up due to the residual stresses developed between the NiTi ®lm and PI. To investigate the degree of curvature produced, a beam equation presented by Timoshenko is used [29]. The equation is
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30 GPa (martensite phase), PI 2:7 GPa), a the thickness of each layer (NiTi is varying, PI 2:0 mm), I the moment of inertia of each layer (a3/12) and h is the total thickness of the NiTi and PI. In using Timoshenko's equation, the following assumptions are made: (1) temperature is uniform throughout the microwrapper arm; (2) bending is constant throughout the arm of the microwrapper; (3) curvature of the arm is achieved by residual stress only, which is generated from the thermal expansion difference between the NiTi ®lm and PI during cooling from 2508C (i.e. curing temperature of PI) to room temperature (i.e. 258C); (4) residual stresses from other sources are negligible. Based on the model, a 1.8 mm thick NiTi ®lm produces a semi-circle curvature (i.e. r 320 mm for 1000 mm long beam) when combined with 2 mm thick PI ®lm on top of the arm. This is assumed to be the ideal curvature for the microwrapper to form. Based on the model, several different bi-layer structures are manufactured. In this study, the PI layer thickness is held constant at 2 mm while the thickness of the NiTi layer is varied from 2.5 to less than 1 mm. Results show that a 0.8 mm thick NiTi layer rather than the predicted 1.8 mm thick NiTi layer produces the desired curvature (Fig. 11). The difference between the modeling and the test may be attributed to a variety of reasons including material property variations which are not included in the model. Curvature of the microwrapper can also be induced using functionally gradated NiTi ®lm through the thickness [30]. In Fig. 12, a photograph of a microwrapper fabricated using the gradated ®lm is presented. To illustrate the functional gradation through the thickness of the ®lm, Fig. 13(a) presents the results of Rutherford backscattering (RBS) Ê NiTi ®lm measurements performed on the ®rst 6000 A deposited onto the wafer. As shown in the ®gure, the deposited NiTi ®lm has a Ni-rich gradated layer (i.e. Ê thick) adjacent to the substrate. It is well known 6000 A that Ni-rich ®lm exhibits a phenomenon known as pseudoelasticity [19]. As the ®lm is deposited, the Ni content decreases. RBS measurement is also performed on the Ê thick) deposited. top portion of NiTi ®lm (i.e. 8000 A
1 a1 a2 DT r h=2 2 E1 I1 E2 I2 =h 1=E1 a1 1=E2 a2 where r is the radius of the curvature, a the thermal expansion coefficient (NiTi 6 ppm/K (martensite phase), PI 57 ppm/K), DT the temperature difference (difference of PI curing temperature and room temperature, i.e. 225 K), E the Young's modulus of each layer (NiTi
Fig. 11. Fully closed thin film NiTi microwrapper (0.8 mm thick NiTi film combined with 2.0 mm thick PI film).
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property changes in the functionally gradated microwrapper and thus the larger strain to failure of the pseudo-elastic layer when compared to the double layers of PI and NiTi in PI microwrapper. 5. Results
Fig. 12. Fabricated microwrapper with NiTi only.
This portion of the ®lm (while deposited simultaneously) sits on top of the Ni-rich layer. Results show that the ®lm consists of near-equiatomic Ni and Ti (Fig. 13(b)). Equiatomic NiTi behaves as a shape memory material. This compositional difference between the top and bottom NiTi layers serves as a functionally gradated bimorph and induce a curvature in the microwrapper arm when released from the substrate. The functionally gradated microwrapper represents a signi®cant improvement over the PI microwrapper. We base this conclusion on the absence of discrete
Ê thick NiTi film deposited on Fig. 13. RBS measurement: (a) initial 6000 A silicon substrate; (b) NiTi film deposited on Ni-rich layer (Y-axis: atomic composition of Ni and Ti, X-axis: thickness of NiTi film).
The fabricated microwrapper is initially placed on a hot plate to evaluate the response. It is observed that when the temperature approaches austenite-®nish temperature (Af), the arms of the microwrapper ¯atten to open up. When the microwrapper is removed from the hot plate, the arms curl up to form the cage-like structures. This same SMA-effect actuation is also observed when the device is actuated by Joule heating (Fig. 14(a) and (b)). To actuate the microwrapper using Joule heating, approximately 400 mW is required. This fairly large value is due to the heat transfer to the silicon substrate from the microwrapper. The supplied power heats both the microwrapper and the silicon substrate [23]. The transferred heat to the substrate is so signi®cant that another microwrapper on the same substrate also actuates due to heat conduction. Currently, silicon dioxide is used as both a thermal and electrical insulator
Fig. 14. Actuation of microwrapper by Joule heating: (a) opened microwrapper; (b) closed microwrapper.
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between the NiTi ®lm and the substrate. Thus, to minimize power required to actuate the microwrapper, a better thermal isolation mechanism is needed. 6. Conclusion A thin ®lm NiTi shape memory alloy microwrapper is fabricated. Etching the amorphous ®lm appears to be more successful in terms of line de®nition and speed when compared to crystallized ®lm. Transformation temperatures of the NiTi ®lm is independent of being in situ crystallized or pre-exposed to air before annealing. However, wet etching causes the NiTi ®lm to break at the sacri®cial layer step. Using ion-milling, problems associated with wet etching can be prevented. Results show that when heated, the microwrapper arms ¯atten and return to the curled-up state when cooled. Further re®nement of the interface between the NiTi ®lm and the substrate is needed to reduce the required power levels. Acknowledgements This work was sponsored by the Air Force Of®ce of Scienti®c Research Grant/Contract no. F49620-98-1-0058 and partially by a DARPA CHAP program managed by Kyle Henderson. References [1] A. Olander, An electrochemical investigation of solid cadmium±gold alloys, Journal of America Chemical Society, Oct. 1932, pp. 3819± 3833. [2] H. Funakubo, Shape Memory Alloys, Gordon and Breach, London, 1987. [3] J.Walker, K. Gabriel, Thin film processing of TiNi shape memory alloy, Sens. Actuators A 21±23 1990, pp. 243±246. [4] J. Busch, A. Johnson, Prototype microvalve actuator, in: Proceedings of the IEEE Annual Workshop of Microelectromechanical Systems, CA, USA, February 1990, pp. 40±41. [5] C. Ray, C. Sloan, A. Johnson, J. Busch, B. Petty, A silicon-based shape memory alloy microvalve, in: Proceedings of the Material Research Society, Vol. 276, 1992, pp. 161±166. [6] A. Jardine, P. Mercado, Dynamics of thin film NiTi cantilevers on Si, in: Proceedings of the Material Research Society Symposium, Vol. 311, 1993, pp. 161±166. [7] A. Johnson, Thin film shape-memory technology: A tool for MEMS, Micromachine Devices 4 (12) (1999) 1±3. [8] P. Kruelevitch, A. Lee, P. Ramsey, M. Northrup, Thin film shape memory alloy microactuators, J. Microelectromech. Syst. 5 4 1996, pp. 270±282. [9] Y. Nakamura, S. Nakamura, L. Buchaillot, M. Ataka, H. Fujita, Two thin film shape memory alloy microactuators, Transactions of the Institute of Electrical Engineers of Japan, Part E, vol. 117-E, no. 11, 1997, pp. 554±559. [10] M. Kohl, K. Skrobanek, Linear microactuators based on the shape memory effect, in: Proceedings of the Transducer'97 International Conference on Solid-State Sensors and Actuators, June 1997, pp. 785±788.
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[11] D. Allen, T. Chen, The electrochemical micromachining of microactuator devices from sputtered NiTi thin films, in: Proceedings of the 3rd International Symposium on Microstructures and Microfabricated Systems III, The Electrochemical Society Inc., Pennington, NJ, 1997, pp. 72±78. [12] W. Bernard, M. Huff, Thin film shape memory alloy actuated mciropumps, J. Microelectromech. Syst. 7, 2 1998, pp. 245±251. [13] B. Sutapun, M. Tabib-Azar, M. Huff, Applications of shape memory alloys in optics Appl. Opt. 37, 28 1998, pp. 6811±6815. [14] K. Kuribayashi, T. Fuji, A new microthin film actuator prestrained by polyimide, in: Proceedings of the 1998 International Symposium on Micromechanics and Human Science, 1998, pp. 165±170. [15] S. Takeuchi, I. Shimoyama, Three-dimensional SMA microelectrodes with clipping structure for insect neural recording, in: Proceedings of the IEEE Annual Workshop of Microelectromechanical Systems, January 1999, pp. 464±469. [16] M.Bendahan, H. Carchano, NiTi thin film as a gate of MOS capacity sensors, Sens. Actuators A 74 1999, pp. 242±245. [17] A. Johnson, Thin film shape memory technology: a tool for MEMS, Micromachine Dev. 4 (12) (1999). [18] M. Tabib-Azar, B. Sutapun, M. Huff, Applications of TiNi thin film shape memory alloys in micro-opto-electromechanical systems, Sens. Actuators A 77 1999, pp. 34±38. [19] T. Duerig, K. Melton, D. Stockel, C. Wayman, Engineering Aspect of Shape Memory Alloys, Butterworth±Heinemann, London, 1990. [20] Web site of Johnson and Johnson, http://www.johnsonandjohnson.com/news_finance/214.htm. [21] G. Lin, C.-J. Kim, S. Konishi, H, Fujita, Design, fabrication, and testing of a C-shaped actuator, in: Proceedings of the 8th International Conference on Solid-State Sensors and Actuators, Sweden, June 1995, pp. 416±419. [22] J. Ok, M. Chu, C.-J. Kim, Pneumatically driven microcage for microobjects in biological liquid, in: Proceedings of the IEEE Annual Workshop of Microelectromechanical Systems, FL, USA, 1999, pp. 459±463. [23] J. Gill, K. Ho, G. Carman, Three-dimensional thin film shape memory microactuator with two-way effect, J. Microelectromech. Syst., 2000, submitted for publication. [24] K. Ho, A. Jardine, G. Carman, Sputter deposition of NiTi thin film exhibiting the SME at room temperature, in: Proceedings of the SPIE, CA, 1999. [25] M. Ohring, The Materials Science of Thin Films, Academic Press, New York, 1992. [26] S. Ghandhi, VLSI Fabrication Principles, 2nd Edition, Wiley, New York, 1994. [27] D. Smith, Thin Film Deposition, McGraw-Hill, New York, 1995. [28] K. Mohanchandra, K. Ho, G. Carman, Electrical resistivity measurement is an important probe in the studies of different phases in sputtered NiTi films, Thin Solid Films, 2000, submitted for publication. [29] S. Timoshenko, Analysis of bi-metallic thermostats, J. Opt. Soc. Am. 11 1925, pp. 233. [30] K. Ho, G. Carman, Sputter deposition of NiTi thin film shape memory alloy using a heated target, Thin Solid Films 370 2000, pp. 18±29.
Biographies John J. Gill received his BS and MS degree in mechanical engineering from University of California Los Angeles, Los Angeles, in 1997. From 1997 to 1998, he worked at Jet Propulsion Laboratory in Pasadena, CA, where he was involved in developing quantum-well-infrared-photodetector. He received his PhD degree in mechanical engineering with MEMS from University of California Los Angeles, Los Angeles, in 2001. Currently, he is with Jet Propulsion Laboratory working on developing microwave devices and microsensors. His research interest includes
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design, fabrication and characterization of microactuators and microsensors using silicon, smart materials and III±V materials for MEMS and microelectronics applications. David T. Chang received the BS degree in 1990, the MS degree in 1992 both in physics, and the PhD degree in 1998 in experimental solid-state physics, all from the University of California Los Angeles, Los Angeles. His dissertation research focused on the phase transition of ferroelectric thin films fabricated on MEMS structures. Since 1998, he has been involved in the design, fabrication and characterization of several surfaceand bulk-micromachined tunneling sensors at HRL Laboratories (formerly Hughes Research Laboratories) in Malibu, CA. His research interests include microfabrication technologies, inertial and infrared sensors, heterogeneous integration of dissimilar semiconductors, and the applications of ferroelectric materials to both MEMS and microelectronics. Leslie A. Momoda got her BS degree in chemical engineering from University of California Los Angeles, Los Angeles, in 1985 and MS and PhD degree in materials science and engineering from University of California Los Angeles, Los Angeles, in 1987 and 1990, respectively. She has 15 years of experience in the field of materials synthesis, processing and characterization for electronic and structural applications. Currently she is a research department manager at HRL Laboratories, LLC in charge
of several major projects in the area of smart materials, materials for thermal management, gas sensing and the modeling and prediction of materials reliability. She has authored or co-authored 14 published papers and one issued patent. Greg P. Carman is an associate professor and supervises the Active Materials Lab in the Mechanical and Aerospace Engineering Department at UCLA. He received his BS (1985) in engineering sciences and mechanics from Virginia Tech, his MS (1988) in metallurgical and materials engineering from the University of Alabama, Tuscaloosa, and his PhD (1991) in engineering mechanics from Virginia Tech. He has held research faculty positions at the Jet Propulsion Lab and Wright Patterson Air Force Base. He is presently chairman for the adaptive structures and material systems of the ASME, and has organized a number of symposia on this topical area. He currently holds a position as associate editor for the Journal of Intelligent Material Systems and Structures and Journal of Composite Materials. He was awarded the Northrop Grumman Young Faculty in 1995 for his research work at UCLA on active materials and the best paper award from the American Society of Mechanical Engineering Adaptive Structures and Material Systems Committee in 1996. He is primarily interested in the basic mechanics and materials issues related to a wide variety of coupled electromagnetothermomechanical materials.
Manufacturing issues of thin ®lm NiTi microwrapper John J. Gilla,1, David T. Changb, Leslie A. Momodab, Greg P. Carmana,* a
Mechanical and Aerospace Engineering Department, School of Engineering, University of California Los Angeles, 420 Westwood Plaza Engineering IV, Los Angeles, CA 90095, USA b HRL Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265-4799, USA Received 27 November 2000; received in revised form 28 March 2001; accepted 29 March 2001
Abstract Manufacturing issues related to a thin ®lm NiTi shape memory alloy (SMA) microactuator (i.e. microwrapper) have been investigated using both wet and dry etching techniques. Results show that wet etching the amorphous ®lm produces a cleaner pattern than the crystallized ®lm. Transformation temperatures are not affected by the pre-exposure of the NiTi ®lm to air before crystallization. However, this process produces breakage in the NiTi ®lm at sacri®cial layer steps. This is believed to be due to residual stresses developed between the ®lm and substrate during sputtering. The ®lm breakage is overcome by dry etching the ®lm with an ion-milling technique. Curvature in the microwrapper arms is induced using either a bi-layer material (i.e. polyimide and NiTi) or a functionally gradated NiTi ®lm. Results show that when heated the microwrapper arms ¯atten due to shape memory effect and curl up to form a cage-like structure when cooled. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Shape memory alloy; NiTi; Transformation temperatures; Ion-milling
1. Introduction The term shape memory alloy (SMA) refers to a material which returns to a predetermined shape when heated. This phenomenon is caused by atomic shuf¯ing within the material during phase transformation. At low temperatures, the phase of the SMA material is martensite, which is ductile and can be deformed easily. However, by simply heating the deformed material to an elevated temperature, the material's phase changes to austenite and the deformation induced at low temperature is fully recovered. Therefore, by repeating the deformation/heating cycle, the SMA material can be used as an actuator in a mechanical device. SMAs were ®rst discovered in 1930s by Olander [1]. After that initial discovery, many alloys have been found to exhibit the SMA effect. Among those, NiTi alloys are used in industrial applications due to low manufacturing cost, good corrosion resistance and tailorable material properties [2]. NiTi has been predominantly applied at the macroscale (i.e. * Corresponding author. Tel.: 1-310-825-6030. E-mail addresses: [email protected] (J.J. Gill), [email protected] (G.P. Carman). 1 Present address: Jet Propulsion Laboratory, M/S 302-306, 4800 Oak Grove Dr., Pasadena, CA 91109, USA. Tel.: 1-818-393-0456; fax: 1-818-393-4540.
wires and thick plates) over the last several decades. However, its application to microelectromechanical systems (MEMS) started in the early 1990s when Walker and Gabriel fabricated a shape memory alloy coil spring on a silicon wafer to demonstrate the compatibility of thin ®lm NiTi with MEMS fabrication processes [3]. Since that initial work, many research studies have been conducted on integrating sputter-deposited thin ®lm SMA into micro devices. Applications include micro¯uid control, optics, sensors, relays and actuators [4±18]. One major advantage associated with NiTi is its biocompatibility. Commercial biomedical applications include a bone anchor, vena-cava ®lter, and stents [19,20]. While a number of macroscopic biomedical applications exist, many possibilities equally exist for microscale applications such as minimal invasive surgery. However, to begin investigating these new applications, we must ®rst develop the fabrication process to produce thin ®lm NiTi SMA actuator. One possible bio-medical use for thin ®lm NiTi SMA is a micrograbbing device (i.e. microwrapper). The purpose of the microwrapper is to grab microsize objects for in situ analysis in a living organism or possibly grab pieces of cancerous tumor for removal from the body via a small catheter. Micrograbbing devices using different actuation mechanisms have been previously reported (i.e. electrostatic and pneumatic actuation) [21,22]. However, these devices
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 6 4 6 - X
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Fig. 1. Illustration of microwrapper: (a) plan-view of microwrapper; (b) schematic diagram of actuation.
are not necessarily bio-compatible. Since the device works in a living organism, the bio-compatibility requirement is a critical issue. Therefore, NiTi appears to be an excellent actuator candidate for microwrapper or other small-scale bio-medical devices. 2. Actuation scheme The actuation scheme of the proposed shape memory microwrapper is illustrated in Fig. 1. Fig. 1(a) represents a plan-view of the microwrapper. The gray outline in the ®gure identi®es the NiTi area. The bonding pad, current path, and central portion of the ``arms'' are attached to the substrate while the remaining parts of the microwrapper arms are detached from the substrate. The arms of the microwrapper have two stable con®gurations: (1) curledup shape; (2) ¯at shape (Fig. 1(b)). When released from the substrate, the arms curl up to form a cage-like structure (i.e. residual stress). When heated, the arms ¯atten out due to the shape memory effect. Heat required for actuation is provided by passing a current from one bonding pad to the other which causes a current ¯ow in the entire NiTi structure (i.e. Joule heating). When the current is removed and heat dissipates to the environment, the individual wrapper arms return to the cage-shaped con®guration. The returning process requires residual stress using a bias spring mechanism or a shape memory material exhibiting the two-way effect [23].
Ê thick) (LPCVD) polysilicon sacri®cial layer (2000 A (Fig. 2(a)). The polysilicon layer is patterned with xenon± di¯uoride (XeF2) photo-lithographically (Fig. 2(b)). Following the sacri®cial layer patterning, the NiTi ®lm is sputterdeposited from a near equiatomic target of NiTi onto the substrate and crystallized. The sputter-deposition parameters for the NiTi ®lm are presented in Table 1 [24]. Following deposition, the NiTi ®lm is etched (Fig. 2(c)). Typically a mixture of hydro¯uoric acid (HF), nitric acid (HNO3), and deionized (DI) water with the respective ratio of 1:1:20 is used to wet etch the NiTi ®lm. A thick photoresist (AZ-4620, Clariant, 5 mm thick) is used as an etch mask. It was reported previously that the NiTi ®lm could be etched uniformly with this approach [23]. Polyimide (PI) layer is deposited on top of the NiTi ®lm to induce curvature (i.e. bias spring) when the NiTi ®lm is released (Fig. 2(d)). Finally, the polysilicon sacri®cial layer is removed with XeF2 and the arms of the microwrapper are released. Due to residual stress, the arms curl up and form the cage-like structure once released (Fig. 2(e)). During this fabrication process, several problems occur in patterning the NiTi ®lm. The ®rst problem is a nonuniform undercut of the NiTi ®lm (Fig. 3(a)). In some regions, the undercut is so severe the ®lm is completely etched away. We believe this problem could be alleviated by etching the amorphous ®lm rather than the crystallized ®lm. The unannealed NiTi ®lm is amorphous: therefore its material Table 1 Sputtering parameters of NiTi film deposition
3. Fabrication process The process ¯ow to manufacture the microwrapper is described in Fig. 2. First, a silicon wafer is wet-oxidized and coated with low-pressure chemical-vapor deposition
Base pressure Argon pressure Substrate to target distance Power Crystallization temperature
<5 10 8 Torr 2 mTorr with 99.999% purity 3 cm 300 W dc 5008C for 6 min
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Fig. 2. Schematic diagram of microwrapper fabrication process: (a) wet oxidation and LPCVD polysilicon sacrificial layer deposition; (b) patterning polysilicon layer by XeF2; (c) deposition and patterning NiTi film; (d) deposition and patterning polyimide layer; (e) removal of polysilicon layer by XeF2.
Fig. 3. Wet etching of NiTi film: (a) crystallized film; (b) amorphous film.
strength is thought to be weaker and consequently, easier to be etched than an annealed one. Therefore, the fabrication procedure is modi®ed such that following deposition, NiTi ®lm is etched prior to crystallization. However, altering the fabrication process in this manner raises a question regarding the in¯uence on the shape memory effect [8]. To investigate this issue, a sputter-deposited amorphous NiTi ®lm is exposed to air, dipped into DI water, and followed by crystallization. Following this, the transformation temperatures (As: austenite-start temperature; Af: austenite-®nish temperature; Ms: martensite-start temperature; Mf: martensite-®nish temperature) of the ``pre-exposed'' NiTi ®lm are
measured by the wafer curvature technique and compared to in situ crystallized ®lm. As one can see in Fig. 4 and Table 2, the pre-exposed NiTi ®lm has similar phase transformation temperatures when compared to in situ crystallized NiTi Table 2 Transformation temperature measurement of differently processed NiTi film NiTi film
As (8C)
Af (8C)
Ms (8C)
Mf (8C)
Pre-exposed Conventional
88 90
104 110
74 70
60 58
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Fig. 4. Transformation temperature measurement result of in situ crystallized and pre-exposed NiTi film.
®lm. Therefore, it appears that crystallization can be done either prior to (in situ) or following the patterning of the ®lm without impacting the shape memory response. To evaluate the etching characteristics of the amorphous ®lm, NiTi ®lm is sputter-deposited on a test wafer and etched using the same etchant and mask as described previously. In Fig. 3(b), results show that delamination of the photoresist does not occur and a clean etch pattern of the ®lm is produced. It is noted that the amorphous ®lm etches approximately three times faster than the crystallized ®lm (i.e. etch rate of amorphous ®lm is 0.6 mm/min, and that of crystallized ®lm is 0.2 mm/min). Therefore, etching amorphous ®lm produces a more distinctly de®ned line pattern than etching crystallized ®lm (i.e. compare Fig. 3(a) and (b)). In addition to the undercut problem, it is observed the patterned NiTi ®lm breaks at the sacri®cial layer step (Fig. 5). Scanning electron microscopy (SEM) shows that a hole is produced during etching at the interface of the NiTi ®lm and sacri®cial layer step (Fig. 6(a) and (b)). As etching proceeds, the hole enlarges and eventually breaks the ®lm (Fig. 6(c)). The breakage of the NiTi ®lm appears to be related to the residual stresses in the ®lm. When a ®lm is deposited, residual stresses develop which could be either tensile or compressive depending on the nature of the process or the materials [25,26]. The NiTi ®lm has a tensile stress of 200 MPa in the martensite phase and 310 MPa in the
austenite phase as determined by wafer curvature technique (see Fig. 4). It is believed that this large tensile stress produces a tiny gap at the interface of the sacri®cial layer step and the ®lm (Fig. 7(a)) [27]. Once the etchant reaches the sacri®cial layer, the gap provides a pathway for the etchant to ¯ow in and back-etching occurs to break the NiTi ®lm (Figs. 7(b)±(d) and 8). We attempted unsuccessfully to verify the existence of this tiny gap prior to etching with SEM. The inability to ®nd the gap may be due to its small physical size. To reduce the residual stress and thus eliminate the gap, NiTi ®lm is deposited onto a heated substrate at 3508C. When the NiTi atoms arrive on a heated substrate, they have more thermal energy to align themselves and relieve the stress. As described previously, the in situ heating temperature should not exceed 5008C in order to prevent crystallization of the ®lm and minimize the number of precipitates (i.e. partial crystallization starts at 4008C) [28]. Once deposited, the ®lm is etched using the same procedure described previously. The presence of the etch hole is investigated at the interface of the NiTi layer and sacri®cial layer with SEM (Fig. 9). SEM pictures reveal that the etch hole is still present but is substantially smaller in size when compared to the ®lm deposited onto an unheated substrate. This result indicates substrate heating during deposition works to reduce the residual stress in the ®lm. While the size of the etch hole
Fig. 5. Center part of microwrapper illustrating the location of NiTi film breakage at the sacrificial layer step.
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Fig. 6. Pictures showing the NiTi film breakage at the polysilicon sacrificial layer step: (a) etch hole grows between the NiTi film and the substrate; (b) almost broken NiTi film; (c) completely broken NiTi film.
Fig. 8. SEM picture of the broken NiTi film at the sacrificial layer step which shows the result of Fig. 7(d).
Fig. 7. Schematic diagram of the NiTi film breakage: (a) a hole exists due to the residual stress; ((b) and (c)) the hole enlarges as etching proceeds; (d) the NiTi film breaks.
Fig. 9. SEM picture of in situ heated NiTi film.
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Fig. 10. NiTi film patterned by ion-milling (no film breakage is shown).
is reduced, its presence is still unacceptable from a fatigue perspective. The ®lm breakage problem is solved by ion-milling the NiTi ®lm (i.e. dry etching technique). Unlike chemical wet etchants, which etch isotropically, ion-milling is anisotropic etching technique producing a vertical side-wall. Therefore, back-etching does not occur in the gap and the ®lm breakage can be prevented. Using a thick photoresist (AZ-4620) as an etch mask, the NiTi ®lm is ion-milled in the following conditions: (a) working gas argon; (b) acceleration of argon beam 150 V; (c) beam current 8 mA; (d) etching time 90 min and overetching time 30 min. Figure 10 shows the result of ion-milling of the NiTi ®lm at the sacri®cial layer step. As one can see from the picture, without an etch hole, a clean etch pro®le is produced over the sacri®cial layer step. Also ion-milling produces a clean etch pro®le for crystallized ®lm. This is bene®cial because manufacturing process becomes simpler than wet etching the amorphous ®lm due to in situ crystallization. Therefore, by patterning the NiTi ®lm with ion-milling, the undercutting and breakage of the NiTi ®lm can be prevented. 4. Curling-up microwrapper arm One approach to induce curvature in the microwrapper is with a bi-layer. PI is deposited and patterned on the top of the microwrapper arm to induce the curvature. When the polysilicon sacri®cial layer is removed and the microwrapper arms are released, the arms curl up due to the residual stresses developed between the NiTi ®lm and PI. To investigate the degree of curvature produced, a beam equation presented by Timoshenko is used [29]. The equation is
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30 GPa (martensite phase), PI 2:7 GPa), a the thickness of each layer (NiTi is varying, PI 2:0 mm), I the moment of inertia of each layer (a3/12) and h is the total thickness of the NiTi and PI. In using Timoshenko's equation, the following assumptions are made: (1) temperature is uniform throughout the microwrapper arm; (2) bending is constant throughout the arm of the microwrapper; (3) curvature of the arm is achieved by residual stress only, which is generated from the thermal expansion difference between the NiTi ®lm and PI during cooling from 2508C (i.e. curing temperature of PI) to room temperature (i.e. 258C); (4) residual stresses from other sources are negligible. Based on the model, a 1.8 mm thick NiTi ®lm produces a semi-circle curvature (i.e. r 320 mm for 1000 mm long beam) when combined with 2 mm thick PI ®lm on top of the arm. This is assumed to be the ideal curvature for the microwrapper to form. Based on the model, several different bi-layer structures are manufactured. In this study, the PI layer thickness is held constant at 2 mm while the thickness of the NiTi layer is varied from 2.5 to less than 1 mm. Results show that a 0.8 mm thick NiTi layer rather than the predicted 1.8 mm thick NiTi layer produces the desired curvature (Fig. 11). The difference between the modeling and the test may be attributed to a variety of reasons including material property variations which are not included in the model. Curvature of the microwrapper can also be induced using functionally gradated NiTi ®lm through the thickness [30]. In Fig. 12, a photograph of a microwrapper fabricated using the gradated ®lm is presented. To illustrate the functional gradation through the thickness of the ®lm, Fig. 13(a) presents the results of Rutherford backscattering (RBS) Ê NiTi ®lm measurements performed on the ®rst 6000 A deposited onto the wafer. As shown in the ®gure, the deposited NiTi ®lm has a Ni-rich gradated layer (i.e. Ê thick) adjacent to the substrate. It is well known 6000 A that Ni-rich ®lm exhibits a phenomenon known as pseudoelasticity [19]. As the ®lm is deposited, the Ni content decreases. RBS measurement is also performed on the Ê thick) deposited. top portion of NiTi ®lm (i.e. 8000 A
1 a1 a2 DT r h=2 2 E1 I1 E2 I2 =h 1=E1 a1 1=E2 a2 where r is the radius of the curvature, a the thermal expansion coefficient (NiTi 6 ppm/K (martensite phase), PI 57 ppm/K), DT the temperature difference (difference of PI curing temperature and room temperature, i.e. 225 K), E the Young's modulus of each layer (NiTi
Fig. 11. Fully closed thin film NiTi microwrapper (0.8 mm thick NiTi film combined with 2.0 mm thick PI film).
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property changes in the functionally gradated microwrapper and thus the larger strain to failure of the pseudo-elastic layer when compared to the double layers of PI and NiTi in PI microwrapper. 5. Results
Fig. 12. Fabricated microwrapper with NiTi only.
This portion of the ®lm (while deposited simultaneously) sits on top of the Ni-rich layer. Results show that the ®lm consists of near-equiatomic Ni and Ti (Fig. 13(b)). Equiatomic NiTi behaves as a shape memory material. This compositional difference between the top and bottom NiTi layers serves as a functionally gradated bimorph and induce a curvature in the microwrapper arm when released from the substrate. The functionally gradated microwrapper represents a signi®cant improvement over the PI microwrapper. We base this conclusion on the absence of discrete
Ê thick NiTi film deposited on Fig. 13. RBS measurement: (a) initial 6000 A silicon substrate; (b) NiTi film deposited on Ni-rich layer (Y-axis: atomic composition of Ni and Ti, X-axis: thickness of NiTi film).
The fabricated microwrapper is initially placed on a hot plate to evaluate the response. It is observed that when the temperature approaches austenite-®nish temperature (Af), the arms of the microwrapper ¯atten to open up. When the microwrapper is removed from the hot plate, the arms curl up to form the cage-like structures. This same SMA-effect actuation is also observed when the device is actuated by Joule heating (Fig. 14(a) and (b)). To actuate the microwrapper using Joule heating, approximately 400 mW is required. This fairly large value is due to the heat transfer to the silicon substrate from the microwrapper. The supplied power heats both the microwrapper and the silicon substrate [23]. The transferred heat to the substrate is so signi®cant that another microwrapper on the same substrate also actuates due to heat conduction. Currently, silicon dioxide is used as both a thermal and electrical insulator
Fig. 14. Actuation of microwrapper by Joule heating: (a) opened microwrapper; (b) closed microwrapper.
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between the NiTi ®lm and the substrate. Thus, to minimize power required to actuate the microwrapper, a better thermal isolation mechanism is needed. 6. Conclusion A thin ®lm NiTi shape memory alloy microwrapper is fabricated. Etching the amorphous ®lm appears to be more successful in terms of line de®nition and speed when compared to crystallized ®lm. Transformation temperatures of the NiTi ®lm is independent of being in situ crystallized or pre-exposed to air before annealing. However, wet etching causes the NiTi ®lm to break at the sacri®cial layer step. Using ion-milling, problems associated with wet etching can be prevented. Results show that when heated, the microwrapper arms ¯atten and return to the curled-up state when cooled. Further re®nement of the interface between the NiTi ®lm and the substrate is needed to reduce the required power levels. Acknowledgements This work was sponsored by the Air Force Of®ce of Scienti®c Research Grant/Contract no. F49620-98-1-0058 and partially by a DARPA CHAP program managed by Kyle Henderson. References [1] A. Olander, An electrochemical investigation of solid cadmium±gold alloys, Journal of America Chemical Society, Oct. 1932, pp. 3819± 3833. [2] H. Funakubo, Shape Memory Alloys, Gordon and Breach, London, 1987. [3] J.Walker, K. Gabriel, Thin film processing of TiNi shape memory alloy, Sens. Actuators A 21±23 1990, pp. 243±246. [4] J. Busch, A. Johnson, Prototype microvalve actuator, in: Proceedings of the IEEE Annual Workshop of Microelectromechanical Systems, CA, USA, February 1990, pp. 40±41. [5] C. Ray, C. Sloan, A. Johnson, J. Busch, B. Petty, A silicon-based shape memory alloy microvalve, in: Proceedings of the Material Research Society, Vol. 276, 1992, pp. 161±166. [6] A. Jardine, P. Mercado, Dynamics of thin film NiTi cantilevers on Si, in: Proceedings of the Material Research Society Symposium, Vol. 311, 1993, pp. 161±166. [7] A. Johnson, Thin film shape-memory technology: A tool for MEMS, Micromachine Devices 4 (12) (1999) 1±3. [8] P. Kruelevitch, A. Lee, P. Ramsey, M. Northrup, Thin film shape memory alloy microactuators, J. Microelectromech. Syst. 5 4 1996, pp. 270±282. [9] Y. Nakamura, S. Nakamura, L. Buchaillot, M. Ataka, H. Fujita, Two thin film shape memory alloy microactuators, Transactions of the Institute of Electrical Engineers of Japan, Part E, vol. 117-E, no. 11, 1997, pp. 554±559. [10] M. Kohl, K. Skrobanek, Linear microactuators based on the shape memory effect, in: Proceedings of the Transducer'97 International Conference on Solid-State Sensors and Actuators, June 1997, pp. 785±788.
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[11] D. Allen, T. Chen, The electrochemical micromachining of microactuator devices from sputtered NiTi thin films, in: Proceedings of the 3rd International Symposium on Microstructures and Microfabricated Systems III, The Electrochemical Society Inc., Pennington, NJ, 1997, pp. 72±78. [12] W. Bernard, M. Huff, Thin film shape memory alloy actuated mciropumps, J. Microelectromech. Syst. 7, 2 1998, pp. 245±251. [13] B. Sutapun, M. Tabib-Azar, M. Huff, Applications of shape memory alloys in optics Appl. Opt. 37, 28 1998, pp. 6811±6815. [14] K. Kuribayashi, T. Fuji, A new microthin film actuator prestrained by polyimide, in: Proceedings of the 1998 International Symposium on Micromechanics and Human Science, 1998, pp. 165±170. [15] S. Takeuchi, I. Shimoyama, Three-dimensional SMA microelectrodes with clipping structure for insect neural recording, in: Proceedings of the IEEE Annual Workshop of Microelectromechanical Systems, January 1999, pp. 464±469. [16] M.Bendahan, H. Carchano, NiTi thin film as a gate of MOS capacity sensors, Sens. Actuators A 74 1999, pp. 242±245. [17] A. Johnson, Thin film shape memory technology: a tool for MEMS, Micromachine Dev. 4 (12) (1999). [18] M. Tabib-Azar, B. Sutapun, M. Huff, Applications of TiNi thin film shape memory alloys in micro-opto-electromechanical systems, Sens. Actuators A 77 1999, pp. 34±38. [19] T. Duerig, K. Melton, D. Stockel, C. Wayman, Engineering Aspect of Shape Memory Alloys, Butterworth±Heinemann, London, 1990. [20] Web site of Johnson and Johnson, http://www.johnsonandjohnson.com/news_finance/214.htm. [21] G. Lin, C.-J. Kim, S. Konishi, H, Fujita, Design, fabrication, and testing of a C-shaped actuator, in: Proceedings of the 8th International Conference on Solid-State Sensors and Actuators, Sweden, June 1995, pp. 416±419. [22] J. Ok, M. Chu, C.-J. Kim, Pneumatically driven microcage for microobjects in biological liquid, in: Proceedings of the IEEE Annual Workshop of Microelectromechanical Systems, FL, USA, 1999, pp. 459±463. [23] J. Gill, K. Ho, G. Carman, Three-dimensional thin film shape memory microactuator with two-way effect, J. Microelectromech. Syst., 2000, submitted for publication. [24] K. Ho, A. Jardine, G. Carman, Sputter deposition of NiTi thin film exhibiting the SME at room temperature, in: Proceedings of the SPIE, CA, 1999. [25] M. Ohring, The Materials Science of Thin Films, Academic Press, New York, 1992. [26] S. Ghandhi, VLSI Fabrication Principles, 2nd Edition, Wiley, New York, 1994. [27] D. Smith, Thin Film Deposition, McGraw-Hill, New York, 1995. [28] K. Mohanchandra, K. Ho, G. Carman, Electrical resistivity measurement is an important probe in the studies of different phases in sputtered NiTi films, Thin Solid Films, 2000, submitted for publication. [29] S. Timoshenko, Analysis of bi-metallic thermostats, J. Opt. Soc. Am. 11 1925, pp. 233. [30] K. Ho, G. Carman, Sputter deposition of NiTi thin film shape memory alloy using a heated target, Thin Solid Films 370 2000, pp. 18±29.
Biographies John J. Gill received his BS and MS degree in mechanical engineering from University of California Los Angeles, Los Angeles, in 1997. From 1997 to 1998, he worked at Jet Propulsion Laboratory in Pasadena, CA, where he was involved in developing quantum-well-infrared-photodetector. He received his PhD degree in mechanical engineering with MEMS from University of California Los Angeles, Los Angeles, in 2001. Currently, he is with Jet Propulsion Laboratory working on developing microwave devices and microsensors. His research interest includes
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design, fabrication and characterization of microactuators and microsensors using silicon, smart materials and III±V materials for MEMS and microelectronics applications. David T. Chang received the BS degree in 1990, the MS degree in 1992 both in physics, and the PhD degree in 1998 in experimental solid-state physics, all from the University of California Los Angeles, Los Angeles. His dissertation research focused on the phase transition of ferroelectric thin films fabricated on MEMS structures. Since 1998, he has been involved in the design, fabrication and characterization of several surfaceand bulk-micromachined tunneling sensors at HRL Laboratories (formerly Hughes Research Laboratories) in Malibu, CA. His research interests include microfabrication technologies, inertial and infrared sensors, heterogeneous integration of dissimilar semiconductors, and the applications of ferroelectric materials to both MEMS and microelectronics. Leslie A. Momoda got her BS degree in chemical engineering from University of California Los Angeles, Los Angeles, in 1985 and MS and PhD degree in materials science and engineering from University of California Los Angeles, Los Angeles, in 1987 and 1990, respectively. She has 15 years of experience in the field of materials synthesis, processing and characterization for electronic and structural applications. Currently she is a research department manager at HRL Laboratories, LLC in charge
of several major projects in the area of smart materials, materials for thermal management, gas sensing and the modeling and prediction of materials reliability. She has authored or co-authored 14 published papers and one issued patent. Greg P. Carman is an associate professor and supervises the Active Materials Lab in the Mechanical and Aerospace Engineering Department at UCLA. He received his BS (1985) in engineering sciences and mechanics from Virginia Tech, his MS (1988) in metallurgical and materials engineering from the University of Alabama, Tuscaloosa, and his PhD (1991) in engineering mechanics from Virginia Tech. He has held research faculty positions at the Jet Propulsion Lab and Wright Patterson Air Force Base. He is presently chairman for the adaptive structures and material systems of the ASME, and has organized a number of symposia on this topical area. He currently holds a position as associate editor for the Journal of Intelligent Material Systems and Structures and Journal of Composite Materials. He was awarded the Northrop Grumman Young Faculty in 1995 for his research work at UCLA on active materials and the best paper award from the American Society of Mechanical Engineering Adaptive Structures and Material Systems Committee in 1996. He is primarily interested in the basic mechanics and materials issues related to a wide variety of coupled electromagnetothermomechanical materials.