Technique of TiNi-based shape memory alloy thin film coating on optical fibers

Abstract

<sec>Intelligent, integrated and cost-effective micro-electro-mechanical system (MEMS) and micro sensors can be developed with TiNi-based memory alloy thin film and optical fibers. Such devices can work in harsh environment, like in deep sea, in space with flammable or explosive objects, or with strong electromagnetic interference; and examples of their possible applications include gas concentration detection in underground mines, dynamic detection of production parameters in oil or gas mining, etc. As TiNi-based memory alloy thin film possesses good biocompatibility, such devices can also be used in intracranial/endocardial pressure test, surgical resection, early cancer assessment, etc. The successful development of the above MEMS and micro sensors involve optical fibers coated with memory alloy films. However, unlike the common planar substrates, optical fiber is of a cylinder with a small diameter, and how to grow good-quality memory alloy film on its surface remains to be explored.</sec><sec>In this work, the silica fibers are coated with TiNi memory alloy films by magnetron sputtering. How to choose the proper operating parameters in the sputtering process, and also the effects of subsequent annealing treatment on the films, are discussed in detail. Uniform thin films are grown on the 125-μm-diameter cylindrical surfaces of optical fibers with our built coating mask device specially designed for fibers. The experiments show that when target-substrate distance, background vacuum degree, Ar gas flow and sputtering time are fixed in the sputtering process, the sputtering power can be optimized, while a higher sputtering pressure results in lower film deposition rate but better surface roughness. The thin film is well crystallized under annealing, and the major martensite B19′ phase and minor austenite B2 phase coexist in the Ti<sub>49.09</sub>Ni<sub>50.91</sub> film. In the experiments, with the optimal operating parameters (sputtering power of 150 W and sputtering pressure of 0.23 Pa), TiNi memory alloy film about 852.2 nm in thickness is grown on the fiber at a deposition rate of 0.118 nm/s, and surface root mean square roughness of the unannealed film is 15.1 nm. Annealing at temperatures of 500, 550 and 600 ℃ are respectively tried, and such a thermal treatment evidently refines the crystalline grains inside the film. Surface root mean square roughness of the film annealed at 600 ℃ is reduced to 6.32 nm.</sec><sec>This work indicates that a glass fiber can be coated with high-quality TiNi-based memory alloy film, and it thus forms a part of the bases of further development of relevant MEMS and micro sensors.</sec>