Thin Shape Memory Films for Micro-Actuators
Brian Dance, Contributing Editor -- Semiconductor International, 10/1/2000
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The French group has investigated the composition and structure of NiTiCu films deposited by rf sputtering onto unheated 2-in. single-silicon substrates. This sputtering was carried out in argon at a pressure of 10-2 to 10-3 mbar using a target rich in titanium. The target was produced by arc melting and cold rolling followed by homogenization at 880°C. Its atomic composition was adjusted to 49.1% titanium, 41.4% nickel and 9.5% copper. It had a martensitic structure with a transformation temperature measured as 60°C.
The film composition was determined by energy-dispersive X-ray analysis in a JEOL microscope. Film thickness was measured with a mechanical profilometer by scanning over steps obtained with a lithographic process and etching in a mixture of hydrofluoric and nitric acids.
After the rf sputter deposition, the NiTiCu thin films were annealed without removing the substrate in order to crystallize them. The annealing was carried out in an ultrahigh vacuum (about 10-9 mbar) using an electrically heated filament located at the rear side of the sample. A compact thermoelectric cooler and heater were used to check the shape memory behavior by X-ray diffraction over a range of temperatures as their crystalline structure evolved.
The deposited films were amorphous, but their composition was uniform on the wafers, and all showed excellent adhesion to the silicon substrates. The surface roughness was so low it could not be measured by atomic force microscopy. The deposition rates increased slightly with increasing argon pressure and more noticeably with increasing rf sputtering power. This suggests these rates are controlled by the energy of the Ar+ ions. The films generally contained a smaller percentage of titanium than the target, except when the argon pressure and the power were both high.
The electrical resistivity of bulk Ni40Ti50Cu10 metal in a martensite structure is 0.8 µV-m. All of the electric resistivities of the deposited films were some 3x larger than that of the bulk metal, perhaps due to their amorphous structure. This resistivity decreased with increasing argon pressure during sputtering. There was a slight increase in density with increasing argon pressure and rf power.
The films were annealed at 600°C for 15 minutes to 1 hour. Nearly equi-atomic films, annealed for 15 minutes, had 100% austenitic structures. The martensitic structure began to appear after annealing for 1 hour, while the grain size of the austenite matrix decreased to 26 nm.
Annealed Ni48Ti48Cu4 thin films were found to exhibit a structural transformation with temperature change characteristic of shape memory alloys. The start of the martensite transformation was estimated as near 0°C, while the transformation temperature was around -30°C, the low value of the latter being possibly due to the copper present and the annealing step.
The workers conclude that a target with a larger titanium content is required for achieving transitions above room temperature, as required for microsystem applications. They suggest the production of such a target should be possible by melting and rolling with an atomic titanium concentration of up to 52%. They plan further work with such a target and in situ annealing during the deposition.
The work was carried out by N. Frantz-Rodriguez et al (Institut d'Electronique Fondamental, Université Paris Sud, Orsay), G. Nouet (LERMAY, Caen Cedex) and J.L. Lebrun (LM3, ENSAM Paris), and was reported in the Journal of Micromechanics & Microengineering, 10(2), 147-151 (2000).
