Indentation plastic displacement field: Part I. The case of soft films on hard substrates
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Indentation plastic displacement field: Part I. The case of soft films on hard substrates T. Y. Tsuia) Advanced Micro Devices, One AMD Place, Sunnyvale, California 94088
Joost Vlassak and William D. Nix Department of Materials Science and Engineering, Stanford University, Stanford, California 94305 (Received 31 August 1998; accepted 10 March 1999)
The plastic deformation behavior of Knoop indentations made in a soft, porous titanium/aluminum multilayered thin film on a hard silicon substrate is studied through use of the focused-ion-beam milling and imaging technique. Pileup is observed for indentations with depths larger than 30% of the total film thickness. Analysis of the indentation cross sections shows that plastic deformation around the indentation is partly accommodated by the closing of the pores within the multilayers. This densification process reduces the amount of pileup formed below that predicted by finite element simulations. Experimental results show that the pileup is formed by an increase of the titanium layer thickness near the edges of the indentation. The thickness increase is largest near the film/substrate interface and decreases toward the surface of the multilayered film. The amount of normal compression near the center of the indenter is characterized, and it is demonstrated that the deformation becomes more nonuniform with increasing indentation depth.
I. INTRODUCTION
Industry has long relied on indentation techniques for measuring the hardness of a wide range of materials. Recently, depth-sensing indentation techniques have been used to study the mechanical properties of thin films on substrates.1–4 Indentation techniques have been of particular interest to thin films used in the semiconductor and magnetic storage industry. As film thicknesses used in these industries continue to decrease, however, it becomes more and more difficult to obtain useful information from these techniques. For example, the thicknesses of gate oxides and barrier films currently used in semiconductor devices are often less than 50 nm. The film stack thickness used for magnetic data storage is less than 100 nm. On the other hand, indentations with contact depths of less than 10–20% of the film thickness are needed in order to obtain intrinsic film properties and to avoid the so-called substrate effect.5,6 Due to equipment limitations such as machine resolution, signal-to-noise ratio, and tip rounding effects, it is very difficult to obtain meaningful experimental results for indentation depths less than 10 nm.2,7 In order to obtain intrinsic film properties from larger indentations, one needs to understand how the mechanical properties of the substrate affect measurements of film stiffness and hardness. Many models and analytia)
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J. Mater. Res., Vol. 14, No. 6, Jun 1999
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