TiNi shape memory alloys for MEMS : thin film deposition , thermophysical properties and laser micromachining characteristics

The growth of TiNi thin films by ion beam sputter deposition using a Kaufmann type ion source is described. Argon ions are used to sputter separate Ti and Ni targets to deposit nearequiatomic TiNi thin films. Typically, ion energies and current densities of 1500 eV and 1 mA cm respectively are used, with an argon overpressure of around 0.05 mtorr, to achieve deposition rates of order 1 μm hr. The thermophysical properties of the deposited films were investigated by thermal imaging. Patterning of TiNi films and foils with micrometre resolution using KrF excimer laser ablation at 248 nm wavelength, with beam fluence up to 2.5 J cm, 15 ns pulse duration and pulse rates up to 100 Hz has also been investigated. Introduction A shape memory alloy (SMA) possesses the property that if plastically deformed below a temperature at which it has wholly transformed to the martensitic state (the martensitic finish temperature MF) and then heated above a temperature at which it has wholly transformed to the austenitic state (the austenitic finish temperature AF), it will recover the original, undeformed, martensitic shape. The effect has long been known in TiNi alloys and is a form of diffusionless phase transformation in which the material will exert a considerable force if constrained. Consequently, TiNi SMAs in thin film form are now being extensively investigated as microactuators for MEMS. A range of potential applications have already been demonstrated, including microvalves, micropumps, micromirrors, microelectrodes and microgrippers and have been reviewed by Fu et al[1]. Desirable properties of TiNi microactuators include high specific work output (~ 1 J g), large displacements and composition-dependent phase transition temperatures (from –100 to +100 °C). Other considerations for specific applications include corrosion resistance and biocompatibility. Disadvantages, compared with other microactuation technologies, include low efficiency (around 5%), large hysteresis and relatively long thermal cycling times. Deposition of TiNi Thin Films In most instances reported in the literature, films have been prepared by DC or RF magnetron deposition. Typically, an argon overpressure of around 10 mtorr, ion energy of a few hundred eV and input power of ~ 100 W is used, giving deposition rates of around 1 μm hr. Considerable variability in film properties and quality is apparent for films deposited by these means. These include variations in film composition, transformation temperatures, yield strength and fatigue life. An alternative approach adopted here uses ion beam sputter deposition (IBSD). Advantages include precise control of ion beam energy, current density, and angle of incidence. In addition, the average energies of sputtered atoms depositing on the substrate can be readily influenced. Ion Beam Sputter Deposition. A 3-cm Kaufmann-type arrangement was used as a source of argon ions with energies in the range 1000 – 1500 eV and current densities around 1 mA cm in order to prepare near-equiatomic TiNi thin films by IBSD. The measured beam divergence was ~ 5° (FWHM) and was collimated such that the source subtended 0.1 radian at the centre of the target. A Materials Science Forum Vols. 539-543 (2007) pp 3151-3156 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland Online available since 2007/Mar/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.33-17/04/08,13:04:01) source-to-target separation of ~ 10 cm and target-to-substrate separation of 2 cm was used. Argon overpressure during deposition was 0.05 mtorr. Film growth rates were observed to be ~ 1 μm hr. Sputter Yields. Precise compositional control during film growth is essential as transformation temperatures are extremely sensitive to changes in composition. Changes of 100°C in transformation temperature occur per at.% change in composition. However, lack of sufficiently precise experimental data on sputter yields as a function of ion energy and angle of ion incidence is a problem for controlling composition using IBSD. Empirical determination of yields for the desired deposition geometry (usually with the aim of achieving both compositional control and maximising deposition rates) is necessary and can be expedited by theoretical considerations. Hence a simplified collisional cascade model[2] has been used to estimate sputter yields at normal incidence for Ti and Ni as a function of energy in the range 50 – 1500 eV. The yield Y (atoms/ion) is given by this model as :