The Fundamental Investigation On Magnetron Sputtering Ti-Ni Alloy Thin Film In Micro Electro Mechanical Systems

Microelectromechanical systems (MEMS) have paved a new way for technological industry by mechanical miniaturization, informational integration and functional diversification. In 21st century the development of MEMS is promising and has penetrated into different departments of national economy. Microactuators are the important components to exchange the matter and energy between MEMS and environment. Their properties are close related to the function and progress of the whole MEMS. Ti-Ni thin films made by Magnetron sputtering are one kind of attractive materials for fabricating microactuators in virtue of their simple structure, strong working capability, fast response speed and tendency to integrated fabrication. Based on the application in MEMS field, deep catch on the relation between the process, microstructure and the properties of Ti-Ni thin films is the key to predict, improve and use the behavior of the films, optimize the design of microactuators and increase the life and reliability of MEMS furniture. In this study, the alloy target was used to fabricate Ti-Ni thin films with different compositions by magnetron sputtering. After post-annealed, Ti-Ni thin films exhibit different phases. The dependences of microstructure, phase transformation on annealing temperature were investigated and the isothermal and non-isothermal crystallization behaviors of amorphous Ti-Ni thin films were revealed as well. The surface morphology, surface roughness and adhesive strength between thin film and substrate were analyzed. Meanwhile the nanoscale pseudoelasticity and mechanical property of Ti-Ni thin films were evaluated. Besides the high temperature oxidation behaviors of (110) textured Ti-Ni thin films were studied. These investigations can not only provide guidance for the application of Ti-Ni thin films in MEMS field, but also broaden the basic research on Ti-Ni thin films, enrich the theory of shape memory alloy and develop the related technology of functional thin films.Ti-Ni thin films deposited on the substrate at ambient temperature are amorphous and will be crystallized after annealed at 450oC～600oC. The matrix phases of annealed Ti47.8Ni52.2,Ti48.7Ni51.3 and Ti50.9Ni49.1 thin films are B2 phase, R-phase and martensite, respectively. When annealed at high temperatures (above and including 500oC), the non-equiatomic Ti-Ni thin films will precipitate the second phase. The precipitates of Ti47.8Ni52.2,Ti48.7Ni51.3 thin films are Ni4Ti3, and The precipitates of Ti50.9Ni49.1 are Ti2Ni. With the increase of annealing temperature, the (reverse) martensitic transformation temperatures (As, Af, Ms, Mf) of Ti47.8Ni52.2 thin film increased, while those of Ti48.7Ni51.3 thin film first decreased and then increased. R-phase transformation temperature (Rs, Rf) of Ti48.7Ni51.3 thin film was independent of the annealing temperatures. The martensitic transformation temperatures (Ms, Mf) of Ti50.9Ni49.1 thin film were not sensitive to annealing temperatures, while the reverse martensitic transformation temperatures (As, Af) increased with annealing temperatures. As for sputter-deposited binary Ti-Ni alloy thin films, the high dislocation density is the cause of R-phase transformation, while the composition and precipitation are not the necessary conditions inducing R-phase transformation. With the increase of sputtering power, the dislocation density of Ti-Ni thin films will decrease and suppress R-phase transformation.Based on the analysis on DSC curves at different heating rates, the non-isothermal crystallization process of amorphous Ti-Ni thin films presented one-step crystallization process and the crystallization temperatures were between 450~550oC. As the heating rate increased, the crystallization temperatures would rise. The Kissinger calculation for the crystallization kinetics of Ti-Ni thin films with different compositions showed that with the increase of the deviation from non-equiatomic composition, the crystallization activation energy increased gradually and the stability improved.The near equiatomic Ti-Ni thin films(49.88at.%Ti:50.02at.%Ni)cannot isothermally crystallize at 420oC. With the rise of annealing temperatures, Ti-Ni thin films will exhibie B2 phase structure. XRD results showed that when crystallized between 425~450oC, Ti-Ni thin films exhibited strong preferred orientation and presented (110) fiber texture. By fitting XRD data to the JMA model, the kinetic parameters for crystallizing amorphous Ti-Ni thin films were determined, giving a value of Avrami exponent between 2.0~2.4 and of activation energy 358.1 kJ/mol. Based on these, a site-saturation kinetics was proposed with the feature of interface-controlled one-dimensional growth of grains and columnar microstructure.When annealed at the temperature between 450oC～600oC, surface roughness of Ti-Ni thin films increased with the increase of annealing temperatures. AFM morphology analysis indicated that such phenomena could be ascribed to the growth of columnar grains. The root mean square roughness (Rq) of B2 phase in Ti-Ni thin films is between 1.2~3.0; that of R-phase in Ti-Ni thin films is between 5.5~9.0;that of martensite in Ti-Ni thin films is between 8~16. Moreover according to the critical normal loads by the nanoscratch test on different phases of Ti-Ni thin films on the Si(100) of single crystal, the adhesion strength of B2 phase with the substrate was the largest, while that of martensite with the substrate was the least and that of R-phase was between the formers.The nanoscale pseudoelasticity and mechanical property of Ti-Ni thin films were evaluated by nanoindentation technology. The results showed that the microhardness and elastic modulus of annealed Ti47.8Ni52.2,Ti48.7Ni51.3 and Ti50.9Ni49.1 increased gradually, which was related to the crystal structures of their matrix phases and slipping systems. Meanwhile the microhardness and elastic modulus of annealed Ti-Ni thin films increased with the increase of annealing temperatures normally. However it is worth noticing that annealed Ti48.7Ni51.3 at 500oC exhibited the largest microhardness and elastic modulus, which could be attributed to the precipitation of Ni4Ti3 phase.As-deposited Ti-Ni thin films and annealed Ti48.7Ni51.3,Ti50.9Ni49.1 thin films did not exhibit pseudoelasticity. Annealed Ti47.8Ni52.2 thin films would exhibit complete pseudoelasticity process during nanoindentation because their B2 matrix phase and reverse martensitic temperature above room temperature. When annealed at the temperature between 450oC～600oC, pseudoelasticity energy recovery ratioηfirst increased to the maximum value and then decreased. Its values were between 44.9%～72.7%. Besides the values ofηshowed dependence on indented loads. At the maximum load of 10mN,ηvalue of annealed Ti47.8Ni52.2 at 550oC reached 72.7%. It is obvious that pseudoelasticity energy recovery ratio of Ti-Ni thin films is larger that that of Ti-Ni bulk alloy (below 45%).As-deposited Ti-Ni thin films on heating substrate presented (110) fiber texture and the oxidation behavior of Ti-Ni thin films at 550oC, 600oC and 650oC was investigated. The results showed that the oxidation kinetics of Ti-Ni thin films obeys a near-parabolic law and the oxidation rate increased with the increase of oxidation temperatures. Rutile TiO2, TiNi3 and B2 phases are the compositions of oxidized Ti-Ni thin films. AES depth profiles indicate that a double-layered scale including the outermost layer and the Ni-rich layer is formed outside the B2 matrix of oxidized Ti-Ni thin films. Particles of TiO2 phases distribute in both layers and decrease with the increasing depth. The thickness of the outmost layer and the Ni-rich layer increases with the increasing oxidation temperature. Thermal oxidation induces a surface smoothening of Ti-Ni thin films and surface roughness of oxidized Ti-Ni film decreases with the increasing oxidation temperature.