Stock material or miscellaneous articles – All metal or with adjacent metals – Having composition – density – or hardness gradient
Reexamination Certificate
2002-10-28
2004-02-10
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Having composition, density, or hardness gradient
C148S402000, C204S192150, C204S298090, C204S298130
Reexamination Certificate
active
06689486
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a shape memory device that exhibits cyclical shape change, and the process for producing the same. In one embodiment, a multi-alloy NiTi thin film is deposited by DC sputtering.
BACKGROUND OF THE INVENTION
NiTi is a shape memory alloy (SMA) that is capable of recovering strains on the order of 10%. This effect, referred to as the shape memory effect (SME), occurs when the material undergoes a phase transformation from the low temperature martensitic phase to the high temperature austenitic phase. In the martensitic phase the material is deformed by preferential alignment of twins. Unlike permanent deformations associated with dislocations, deformation due to twinning is fully recoverable when heated to the austenite phase.
A difficulty in using thin film SMA is that the deposited films exhibit the one way shape memory effect (SME) only. An SME material recovers its original shape after heating to the austenite phase but does not revert back to its deformed state when cooled. In order to achieve cyclic actuation, a biasing force such as a spring is necessary to deform the material when in the martensite phase. Implementing a bias force on thin film structures present significant manufacturing obstacles, an additional challenge for using thin film SME in MEMS actuators.
The first work to incorporate thin film NiTi in devices used a micro-machining process developed by Walker et al. in 1990 [J. A. Walker, K. J. Gabriel, and M. Mehregany, Sens. Actuators, Vols. A21-A23, p. 243, 1990]. Walker et al. used a wet chemical etchant (HF+HN03+H20) to pattern a free standing serpentine NiTi spring. The structures were curled when released and uncurled when heated, they attributed this to the shape memory effect. However, the films were amorphous as deposited. In 1990 Bush and Johnson at the TiNi Alloy Company showed the first definitive evidence of SME in NiTi films [J. D. Busch, A. D. Johnson, et. al., “Shape-memory properties in Ni—Ti sputter deposited film”, J. Appl. Phy., Vol.68, p.6224, 1990]. Using a single target (50/50 atm % NiTi), with a DC magnetron sputtering system they pre-sputtered for 3 hours. Sputtering of the film was performed with a P
Ar
=0.75 mTorr, V=450V, I=0.5A, and a target substrate distance of 2.25 inches was used. The as-deposited, film was shown by XRD to be amorphous and, after vacuum annealing at 550° C. for 30 minutes, exhibited the SME although transformation temperatures were 100° C. lower than the target material.
To achieve a cyclical, two-way effect, abiasing force is required to reshape the NiTi when cooled. Kuribayshi introduced a biasing force by tailoring precipitates in his films such that there were compressive and tensile stresses on opposite sides of his film [K. Kuribayashi, T. Taniguchi, M. Yositake, and S. Ogawa, “Micron sized arm using reversible TiNi alloy tin film actuators”. Mat. Res. Soc. Symp. Pro., vol.276, p.167, 1992]. The film curled when in the martensitic phase and when heated to the austenite phase flattened because the higher modulus overcomes the residual stresses. The fabrication process required complicated heat treatments. The stability of these precipitates can degrade over numerous thermal cycles.
Thin film TiNi actuators are well suited for MEMS devices because of their large work energy densities. However, the difficulties associated with depositing this material has limited its access by the MEMS community. To address this issue, researchers focused on deposition, heat treatments, and thermomechanical characterization of the film [J. D. Busch, M. H. Berkson, and A. D. Johnson, Phase transformations in sputtered NiTi film: effects of heat treatment and precipitates. Mat. Res. Soc. Symp. Proc., vol.230, p. 91, 1992; D. S. Grummon and T. J. Pence, “Thermotractive titanium-nickel thin films for microelectromechanical systems and active composites”, Mat. Res. Soc. Symp. Pro., Vol. 459, p. 331, 1997; Q. Su, S. Z. Hua and M. Wuttig, “Martensitic transformation in NiTi films”, J. of Alloys and Compound, vol. 211, p.460, 1994; S. Miyazaki, et.al., “Shape memory characteristics of sputter-deposited Ti—Ni base thin films”, SPIE, vol. 2441, p. 156, 1995; and A. Ishida, A. Takei, M. Sato and S. Miyazaki, “Shape memory behavior of Ti—Ni thin films annealed at various temperatures”, Mat. Res. Soc. Symp. Proc., vol.360, p. 381, 1995. 11-15]. Few researchers developed actual micro-devices.
The TiNi Alloy Co. has a working microvalve it markets, which closes using a bias mass and opens when the thin film NiTi ligaments are heated [C. A. Ray, C. L. Sloan, A. D. Johnson, J. D. Busch, B. R. Petty: Mat. Res. Soc. Symp. Proc. 276, 161 (1992)]. Krulevitch et al. fabricated a 900 m long, 380 m wide, and 200 m tall microgripper from 5 m thick NiTi—Cu film, as well as a functioning microvalve [P. Krulevitch, et al, supra]. Benard et al. fabricated a micro-pump from NiTi film using two designs: polyimide as the biased actuator in one and a complementary NiTi actuator in the other [W. L. Benard, H. Kahn, A. H. Heuer and M. A. Huff, “Thin film shape memory alloy actuated micropumps”, J. of Microelectromechanical Systems, vol.7, no. 2, 1998]. Kuribayashi et al. used TiNi films to actuate a microrobotic manipulator [K. Kuribayashi, S. Shimizu, T. Nishinohara and T. Taniguchi, “Trial fabrication of micron sized arm using reversible TiNi alloy thin film actuators”, Proceedings International Conf. On Intel. Robots and Sys., Yokohama, Japan, p. 1697, 1993]. While the potential applications for SMA MEMS are large, the difficulties with fabricating quality material and achieving the two-way effect is preventing wide spread use of this actuator material.
NiTi films with transformation temperatures above room temperature are difficult to manufacture. Sputtering directly from a 50/50 atm % NiTi target results in films with dramatically lowered transformation temperatures, prohibiting its use as an actuator [J. D. Busch, et. al., supra]. This is caused by the fact that NiTi alloys are strongly dependent on composition, annealing temperatures, aging time, and sputtering parameters [S. Miyazaki, et.al., “Effect of heat treatment on deformation behavior associated with R-phase and martensitic transformations in Ti—Ni thin films”, Trans. Mat. Res. Soc. Jpn., Vol. 18B, pp1041, 1994; A. Ishida, M. Sato, A. Takei and S. Miyazaki, “Effect of heat treatment on shape memory behavior of Ti-rich Ti—Ni thin films”, Materials Transactions, JIM, vol. 36, p. 1349, 1995; and A. Peter Jardine, “Deposition parameters for sputter-deposited thin film TiNi”, Mat Res. Soc. Symp. Proc., Vol 360, p. 293, 1995]. Of these factors, alloy composition is the most critical.
NiTi alloys and other shape memory alloys are strongly dependent on composition, annealing temperatures, aging time, and sputtering parameters. Composition is the most critical sputtering parameter. Typically, small changes in composition occur during sputtering because titanium readily reacts with other materials.
FIG. 1
shows the dependence of transformation temperature on Ni—Ti stoichiometry, a shift in composition of as little as 1 atm % can alter transformation temperatures by 100° C. [T. W. Duerig, K. N. Melton, D. Stockel and C. M. Wayman, Engineering Aspects of Shape Memory Alloys, 1990]. Titanium is typically used to getter materials, and is often used in vacuum systems to pull down a vacuum by reacting with the gases and condensing. Miyazaki, et al., compensated for the titanium loss by placing titanium plates on top of the alloy target, thereby effectively altering the composition of the target [S. Miyazaki and K. Nomura, “Development of perfect shape memory effect in sputter-deposited Ti—Ni thin films”, Proceedings IEEE Microelectro Mechanical Sys., p. 176, 1994]. Wolf et al. similarly compensated with titanium foils [R. H. Wolf and A. H. Heuer, “TiNi (Shape Memory) Films on Silicon for MEMS Applicat
Carman Gregory P.
Ho Ken K.
Jardine Peter A.
Ostrolenk Faber Gerb & Soffen, LLP
Zimmerman John J.
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