NiTi thin films prepared by biased target ion beam deposition co-sputtering from elemental Ni and Ti targets Huilong Hou, Reginald F. Hamilton ⁎, Mark W. Horn, Yao Jin Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802, United States abstract article info Article history: Received 21 March 2014 Received in revised form 29 August 2014 Accepted 3 September 2014 Available online 16 September 2014 Keywords: Biased target ion beam deposition NiTi ultrathin films Film quality Crystallization B2 and B19′ crystal structures NiTi thin films are fabricated using biased target ion beam deposition technique. By design, the technique operates over a broad range of processing pressures; enables control of adatom energies; facilitates low energy bombardment; and promotes uniformity and repeatability. Thus, the technique is advantageous for preparing smooth and dense ultrathin films. Typically NiTi shape memory alloy thin films are deposited using the magnetron-sputtering technique and alloy targets. In this work films are co-sputtered from pure Ti and pure Ni targets and the technique is contrast with magnetron co-sputtering. Approximately 100 nm thick NiTi thin films are prepared with Ni-rich (N 50.5 at.% Ni), near equiatomic, and Ti-rich (b 49.5 at.% Ni) compositions. Atomic force microscopy reveals that films are consistently ultra-smooth over the broad range of compositions. The cur- rent findings confirm that biased target ion beam deposition can facilitate the preparation of high quality ultrathin NiTi films. After heat-treatment, the films deposited exhibit B2 and B19′ crystal structures and thus pos- sess potential for martensitic phase transformation, which is the prerequisite for functional shape memory behavior. © 2014 Elsevier B.V. All rights reserved. 1. Introduction NiTi based thin films have been primarily fabricated by vacuum magnetron sputtering as the common form of physical vapor deposition [1]. The films that exhibit shape memory behavior have found applica- tions as micro-grippers, micropumps, stents, and endoscopes, among other micro-scale applications [2–4]. Reports on conventional NiTi thin films—used in practical application—recommend fabrication using vacuum magnetron sputtering utilizing a NiTi alloy targets. There are in- herent drawbacks: Ni sputters at a rate nearly three times than Ti; Ti can be lost to residual oxygen; the target composition may not be absolutely uniform [1]. Essential to fabrication of NiTi thin films with the shape memory effect are controlling chemical composition, having uniformity of composition over a large area and run to run, and ensuring quality [1, 2]. To date, NiTi films exhibit shape memory behavior when thickness ranges from 0.5 to 2 μm [1]. Technological advancement for smaller scale applications requires the exploration of thinner films. In this work, NiTi alloy film with thicknesses on the order of 100 nm are co-sputtered from separate Ni and Ti targets utilizing biased target ion beam deposition (BTIBD) technique. BTIBD allows differential bias- ing of each target BTIBD and combines ion beam deposition and sputtering deposition, and offers low-pressure operation [5–8]. A sche- matic of the BTIBD system is shown in Fig. 1. A low energy end-Hall (or closed-drift Hall) type ion source, typically lower than 25 eV, is introduced to the negative biased targets. A stable plasma must be maintained in the hollow cathode. The hollow cathode (HC) generates electrons and ejects electrons to the end-Hall ion source, rendering the plasma at the end-Hall constantly stable. A “wave” of ions carries the sputtered material to the substrate. Each target bias can be modulat- ed differently by controlling the pulsed width and period. The capability of individual modulation can facilitate precisely controlling composition uniformity and stably maintaining deposition repeatability. Further- more, bias target deposition allows control of medium energy Ar ions from the ion gun to impinge on the growing film and enables low ion energy bombardment with less impurities being sputtered. By design, on the other hand, the BTIBD technique enables controlling adatom en- ergy (for high and low adatom energy process capabilities); decoupling adatom energy and deposition rate as well as target and substrate plas- ma; and directing the flux while providing low energy ion bombard- ment at low processing pressures 1 × 10 -4 to 5 × 10 -3 Torr [8]. Thus, the technique is advantageous for the fabrication of ultra thin and dense ultra smooth films. Co-sputtering enabled the fabrication of NiTi thin films with Ni-rich (N 50.5 at.% Ni); near equiatomic; and Ti-rich (b 49.5 at.% Ni) composi- tions prepared using BTIBD. For preparing NiTi alloy thin films, the im- portance of controlling the alloy composition is reflected in Fig. 2. The ordinate axis is the temperature at with the thermal-induced martens- itic transformation (TIMT) starts (referred to as M s ); the TIMT being the foundation of shape memory behavior for practical application. Near- equiatomic ranges from 49.5 to 50.5 at.% Ni based on Ref. [1]. The Ni- rich and Ti-rich regions are assigned outside that range. Note that, Thin Solid Films 570 (2014) 1–6 ⁎ Corresponding author. http://dx.doi.org/10.1016/j.tsf.2014.09.004 0040-6090/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf