Effect of oxygen on the thermomechanical behavior of passivated Cu thin films
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Thomas E. Mates Materials Department, University of California at Santa Barbara, California 93106
Shefford P. Baker Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853-1501 (Received 15 February 2003; accepted 11 June 2003)
The thermomechanical behavior of Cu thin films, 600–1125 nm thick and encapsulated between SiNx barrier and SiNx or AlNx passivation layers on silicon substrates, was studied during thermal cycling between room temperature and 400 or 500 °C using the substrate curvature method. Films were prepared with varying oxygen contents, and the distribution of oxygen through the thickness of selected films was studied before and after thermal cycling using secondary ion mass spectrometry. Large variations in the thermomechanical behavior with oxygen content were found and correlated with segregation of oxygen to the film/barrier and film/passivation interfaces. These variations are thought to be due to recovery of stored misfit dislocation energy, which is, in turn, controlled by oxygen in the film. Effects of oxygen on film deformation through variations in interfacial adhesion and diffusion-induced dislocation glide are considered.
I. INTRODUCTION
Thin films are widely used in microelectronics, microelectromechanical systems, and other applications. It is well known that high stresses in such films can lead to stress-voiding, cracking, or delamination that, in turn, can lead to device failure.1 Thus, prediction and improvement of device reliability requires an intimate knowledge of film stresses and film behavior. However, the mechanical behavior of a thin film can be very different from that of the same material in bulk form.1 These differences are not yet well understood. Thus, there remains great interest in the mechanical behavior of thin films. The deformation mechanisms in thin films on substrates are often studied by determining the stresses that arise during thermal cycling using substrate curvature2 or x-ray3 techniques, and Cu films are often studied.4 –13 The effects of film thickness6 –8,10 and presence of a passivation layer6 –8,10,11 on thermomechanical behavior have been investigated, and typical stress–temperature (–T ) hysteresis shapes have been identified for passivated and unpassivated films.6 –8 Recent work, however, has shown that exposure to very small amounts of air8,12–14 or oxygen12 can dramatically affect the thermomechanical behavior of passivated Cu films. An example is shown in Fig. 1, where the 2122
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J. Mater. Res., Vol. 18, No. 9, Sep 2003 Downloaded: 17 Mar 2015
thermomechanical behavior of two 1-m-thick passivated Cu films during thermal cycles between room temperature and 400 °C is shown. The preparation of these samples (HV films described in detail in Sec. II. A.1) was identical, except that one of the films was exposed to air before passivation. Stress–temperature data from the second cycle for each film are shown. The film prepared in uninterrupted vacuum shows the “normal” open (–T ) hy
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