Tantalum Nitride Seed Layers for Bcc Tantalum Coatings Deposited on Steel by Magnetron Sputtering
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Tantalum Nitride Seed Layers for Bcc Tantalum Coatings Deposited on Steel by Magnetron Sputtering
Anamika Patel, Leszek Gladczuk, Charanjeet Singh Paur, Marek Sosnowski, Dept. of Electrical Engineering, New Jersey Institute of Technology, Newark, NJ. ABSTRACT Tantalum, a tough refractory metal with excellent corrosion resistance, is an attractive protective coating material for steel subjected to mechanical wear in harsh chemical environments. The coatings deposited by sputtering usually contain a hard and brittle tetragonal phase mixed with the desirable tough and ductile bcc phase of Ta. One of the methods of promoting the growth of the bcc phase at a moderate substrate temperature is deposition of an interfacial or seed layer of tantalum nitride. Ta coatings were deposited by dc magnetron sputtering in argon, and the interfacial nitride layers by reactive sputtering performed in the same chamber with nitrogen gas added to argon. The critical thickness, stoichiometry, and structure of the seed layer required for bcc Ta growth were studied by scanning electron microscopy and Xray diffraction. The results of adhesion testing of the Ta coatings deposited on the interfacial layers are also reported. INTRODUCTION The body centered cubic (bcc) structure of Ta has physical properties, which makes it an attractive material for protective coating against corrosive environments and mechanical wear. Besides the desirable bcc phase, the tetragonal structured beta phase is usually found in sputtered Ta. The presence of hard and brittle beta phase makes Ta coatings susceptible to failure. Knoop hardness for beta-Ta was found to be greater than 900, while for bcc Ta it was 300-400 [1]. Beta phase of Ta is a meta stable and transforms to the bcc phase at temperatures above 750 °C [2]. Bcc, beta or mixed phases have been observed in sputtered Ta depending on the deposition conditions. Structure of the substrate or the seed layer deposited on the substrate prior to deposition, strongly affects the crystallographic structure of sputtered Ta. Niobium and titanium seed layer were reported to promote the growth of Ta bcc phase [3,4]. Colgan and Fryer [5] described the method of making bcc Ta thin film by introducing an intermediate layers as thin as 20 Å of Ta(N) on silicon. IBM’s first generation CMOS chips with copper interconnect had a bilayer of Ta/TaN used as the diffusion barrier [6]. To our knowledge no work has been done to investigate the phase characteristics of sputtered Ta (> 1 µm) on steel with tantalum nitride seed layer. The goal of this work was to investigate the structure and critical thickness of tantalum nitride seed layer required to deposit bcc Ta on steel. Tantalum nitride seed layer was formed, by reactive sputtering of Ta in argon and nitrogen gas mixture. This method has an advantage, over the method of deposition of a different metal seed layer promoting the bcc phase in sputtered Ta that the reactive sputtering is done with the same source (target) as used for Ta deposition. In this work, propertie
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