Mechanical Stress, Grain-boundary Relaxation, and Oxidation of Sputtered CuNi(Mn) Films
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Mechanical stress, grain-boundary relaxation, and oxidation of sputtered CuNi(Mn) films W. Br¨uckner, W. Pitschke, S. Baunack, and J. Thomas Institute of Solid State and Materials Research Dresden, D-01171 Dresden, Germany (Received 22 September 1997; accepted 27 October 1998)
This paper focuses on understanding stress development in CuNi42Mn1 thin films during annealing in Ar. In addition to stress-temperature measurements, resistance-temperature investigations and chemical and microstructural characterization by Auger electron spectroscopy, scanning and transmission electron microscopy, x-ray diffraction, and atomic force microscopy were also carried out. The films are polycrystalline with a grain size of 20 nm up to 450 ±C. To explain the stress evolution above 120 ±C, atomic rearrangement (excess-vacancy annihilation, grain-boundary relaxation, and shrinkage of grain-boundary voids) and oxidation were considered. Grain-boundary relaxation was found to be the dominating process up to 250–300 ±C. A sharp transition from compressive to tensile stress between 300 and 380 ±C is explained by the formation of a NiO surface layer due to reaction with the remaining oxygen in the Ar atmosphere. This oxidation is masking the inherent structural relaxation above 300 ±C.
I. INTRODUCTION
Thin films of CuNi(Mn) with about 40 wt% Ni and sometimes 1 wt% Mn are used for resistive and thermoelectrical applications.1–4 Like the commercial° alloy ¢ Konstantan (CuNi44Mn1), they have a resistivity r of about 50 3 1028 V m, a small temperature coefficient of resistance (TCR) of ,50 3 1026 K21 , and a high thermopower against copper of about 40 mVyK, at room temperature (RT).5 Concerning the stability and reliability of the CuNi(Mn) films, high biaxial stresses are a problem.6 After sputtering the films are already under tensile stresses of a few hundred MPa. The stress development during heat treatment results in RT stresses of more than 1000 MPa. Irreversible stress changes during annealing were observed above 120 ±C, being especially large between 300 and 400 ±C.7 Different atmospheres result in different stress-temperature curves. Particularly sharp stress changes were found for measurement in Ar. Such stress changes include transitions from compressive to tensile stress7 or, for thicker films, also an increase in tensile stress.8 The reasons for these stress changes, however, are not yet clear. This paper focuses on understanding stress development during annealing by investigating related processes. After deposition at RT, the films are in nonequilibrium state. Subsequent heat treatment leads to temperature driven and, possibly, stress-supported atomic rearrangement (annihilation of excess vacancies, grainboundary relaxation, shrinkage of grain-boundary voids, grain growth), occasionally to phase transitions and interfacial processes (oxidation, film-substrate reaction), 1286
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 4, Apr 1999
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