Tantalum Nitride for Copper Diffusion Blocking on Thin Film (BiSb) 2 Te 3
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Tantalum Nitride for Copper Diffusion Blocking on Thin Film (BiSb)2Te3 H. H. Hsu1, C. H. Cheng*,2, C. K. Lin1, K. Y. Chen1 and Y. L. Lin1 1 Green Energy and Environment Research Lab., Industrial Technology Research Institute, Hsinchu, Taiwan, R.O.C. 2 Department of Mechatronic Technology, National Taiwan Normal University, Taipei, Taiwan, R.O.C ABSTRACT This study demonstrates the feasibility of introducing a TaN thin film as a copper diffusion barrier for p-type (BiSb)2Te3 thermoelectric material. Compared to conventional Ni diffusion barrier, remarkably little void generation in Cu bulk or near Cu/TaN interface originated from Cu penetration is observed for TaN barrier after suffering the thermal budget of close to soldering. Diffusion behaviors of the barriers were analyzed by transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS) to make a deep understanding in clarifying interface diffusion effects among the Cu electrode, the barrier layer, and the (BiSb)2Te3 thermoelectric layer. INTRODUCTION The solid-state thermoelectric devices are exploited for the direct conversion between heat and electric energy due to their advantages of no moving parts, no refrigerant needed, and high reliability [1]. Among thermoelectric materials, bismuth telluride compounds have been attracting considerable attention on micro-cooler applications because of superior thermoelectric characteristics [2,3]. However, copper (Cu) diffusion initiated by soldering process would degrade the thermoelectric properties due to the formation of copper telluride [4] or changing carrier concentration of thermoelectric materials [5,6]. Although quite a few studies have been reported to solve the diffusion issues for bulk thermoelectric materials [7-9], little research including observation of diffusion behavior was carried out for micro-thermoelectric devices. Many anti-diffusion materials such as Au, Ag, Ni, Ta, TiN and TiW were reported in a previous study [10], but these turned out not working for p-type Bi0.5Sb1.5Te3 thin films. It was demonstrated that no thermoelectric cooling functionality might be due to diffusion of titanium into the thermoelectric elements [11]. To improve these issues, we proposed a robust metal nitride, namely tantalum nitride, to replace conventional Ni and investigated their stabilities against Cu diffusion. EXPERIMENT The experimental procedure is concisely described as follows. The 170-nm-thick p-type (BiSb)2Te3 films were deposited by a multi-chamber sputter system with a processing temperature of 150°C on 500-nm-thick SiO2/Si substrate. After film deposition, in-situ annealing at 250°C was performed to enhance film quality. Subsequently, a lift-off process was used to define transfer length method (TLM) pattern and then followed by metal depositions of 110-nmthick Ni or TaN as Cu diffusion barrier layers. After that, 300-nm-thick Cu was deposited by sputtering as top electrodes. Finally, the post metal annealing at 200°C close to soldering process
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