Alloying process of sputter-deposited Ti/Ni multilayer thin films

doi:10.1016/j.msea.2006.02.083

Materials Science and Engineering: A

Volumes 438–440, 25 November 2006, Pages 699-702

https://doi.org/10.1016/j.msea.2006.02.083 Get rights and content

Abstract

Alloying process of a Ti/Ni multilayer thin film was investigated in detail by differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The Ti/Ni multilayer thin film was prepared by depositing Ti and Ni layers alternately on a SiO₂/Si substrate. The number of each metal layer was 100, and the total thickness was 3 μm. The alloy composition was determined as Ti–51 at.%Ni by electron probe micro analysis (EPMA). The DSC curve exhibited three exothermic peaks at 621, 680 and 701 K during heating the as-sputtered multilayer thin film. In order to investigate the alloying process, XRD and TEM observation was carried out for the specimens heated up to various temperatures with the heating rate same as the DSC measurement. The XRD profile of the as-sputtered film revealed only diffraction peaks of Ti and Ni. But reaction layers of 3 nm in thickness were observed at the interfaces of Ti and Ni layers in cross-sectional TEM images. The reaction layer was confirmed as an amorphous phase by the nano beam diffraction analysis. The XRD profiles exhibited that the intensity of Ti diffraction peak decreased in the specimen heat-treated above 600 K. The peak from Ni became broad and shifted to lower diffraction angle. The amorphous layer thickened up to 6 nm in the specimen heated up to 640 K. The diffraction peak corresponding to Ti–Ni B2 phase appeared and the peak from Ni disappeared for the specimen heated up to 675 K. The Ti–Ni B2 crystallized from the amorphous reaction layer. After further heating above the third exothermic peak, the intensity of the peak from the Ti–Ni B2 phase increased, the peak from Ti disappeared and the peaks corresponding to Ti₂Ni appeared. The Ti₂Ni phase was formed by the reaction of the Ti–Ni B2 and Ti.

Introduction

Ti–Ni thin films fabricated by the sputter-deposition method are expected to be applicable in microdevices such as microvalves [1] and micropumps [2], since they exhibit excellent shape memory effect and good mechanical properties [3]. Controlling the composition of the deposited Ti–Ni thin film is very important in order to adjust martensitic transformation temperature, since they are martensitic transformation temperatures are strongly affected by Ni-content [3], [4], [5], [6], [7], [8], [9]. Alloyed Ti–Ni targets have been used in conventional sputter-deposition methods. The composition of the film is adjusted by placing pure Ti chips on the alloyed Ti–Ni target. However, the composition of the Ti–Ni thin film fabricated using the alloyed target is difficult to be adjusted precisely. On the other hand, the Ti–Ni thin films can be fabricated by a multi-source sputtering system, using pure Ti and pure Ni targets [10], [11]. By using this method, the controlling of the alloy composition of Ti–Ni thin film seems to be easier in comparison with the alloyed target method, since the composition of the Ti–Ni thin film can be adjusted by setting the sputtering powers for both metal targets. Furthermore, the Ti/Ni multilayer thin film composed of Ti and Ni layers can be fabricated by the alternating sputtering method [12], [13]. The average composition of the multilayer thin film is estimated by the thicknesses of Ti and Ni layers. The thicknesses of Ti and Ni layers can be adjusted by setting not only sputtering power but also sputtering time. When using the Ti/Ni multilayer thin film for making a shape memory thin film, it is necessary to make it alloyed. The microstructure and shape memory effect of annealed Ti/Ni multilayer thin film were already reported [12], [13]. But the alloying process of the Ti/Ni thin film has not been investigated in detail. In this study, the alloying process of the Ti/Ni multilayer film was investigated by X-ray diffractometry (XRD) and transmission electron microscopy (TEM).

Section snippets

Experimental procedure

A Ti/Ni multilayer thin film was prepared by a dual-source dc sputtering system. Ti and Ni layers were deposited alternately on a SiO₂/Si substrate which is placed on a turn-table. The sputtering gas was pure Ar at a pressure of 2.5 × 10⁻⁴ Pa. The dc powers for the Ti (purity 99.9 wt.%) and Ni (purity 99.99 wt.%) targets were fixed as 280 and 60 W, respectively. The sputtering was carried out at room temperature. The sputtering time was coordinated in order to obtain Ti–51 at.%Ni alloy composition.

Differential scanning calorimetry and X-ray diffraction

Fig. 1 shows differential scanning calorimetry (DSC) curve of an as-sputtered Ti/Ni multilayer thin film upon heating with a heating rate of 10 K/min. Three exothermic peaks were observed at 621, 680 and 701 K. A similar DSC curve was also reported by previous research [12], [13]. To investigate the microstructural change upon heating in detail in the present investigation, X-ray diffraction (XRD) profiles were obtained for specimens heated up to various temperatures with the same heating rate of

Conclusions

(i)
The diffraction peaks of the crystalline Ti and Ni were observed in XRD profiles for an as-sputtered Ti/Ni multilayer thin film. Amorphous reaction layers of 3 nm in thickness were observed at the interfaces of Ti and Ni layers in a cross-sectional TEM micrograph.
(ii)
Three exothermic peaks upon heating were observed at 621, 680 and 701 K in the DSC curve of the as-sputtered multilayer thin film. The XRD profiles exhibited that the intensity of (0 0 2)_Ti and {1 1 1}_Ni diffraction peaks decreased and

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Alloying process of sputter-deposited Ti/Ni multilayer thin films

Abstract

Introduction

Section snippets

Experimental procedure

Differential scanning calorimetry and X-ray diffraction

Conclusions

Sens. Actuators A

Mater. Sci. Eng. A

Thin Solid Films

Thin Solid Films

Sens. Actuators A

Mat. Sci. Eng. A

Thin Solid Films

Mat. Sci. Eng. A

Acta Mater.

Thin Solid Films

J. Magn. Magn. Mater.

Thin Solid Films