Measurement of Ultrathin Film Mechanical Properties by Integrated Nano-scratch/indentation Approach
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Measurement of Ultrathin Film Mechanical Properties by Integrated Nanoscratch/indentation Approach Ashraf Bastawros1,2, Wei Che3, and Abhijit Chandra1,2 1 Aerospace Engineering, Iowa State University, Ames, IA, 50011 2 Mechanical Engineering, Iowa State University, Ames, IA, 50011 3 Saint Gobain, Inc., Boston, MA, 01606 ABSTRACT The thickness and property measurements of thin films on substrates are crucial for wide range of applications. Classical techniques have relied on various physical properties to identify film thickness, independent of its mechanical properties. Here, a new experimental technique is devised to evaluate the film thickness, its flow stress and its stiffness. The technique utilizes a combination of nano-scratch and dynamic stiffness measurements carried out by a nano-indenter. The technique relies on measuring the depth variation of normal and tangential forces, and indentation modulus. These combined measurements are calibrated through a simple statically admissible stress model to yield the unknown quantities. The measurements are ascertained by XPS film thickness measurements, and reasonably agree with the finite element predictions. The technique is applied to study the formed oxide nano-layer during copper chemical mechanical planarization. INTRODUCTION Ultrathin films are widely used in many applications such as dielectric films, copper liners and capping materials for microelectronics; native oxides for high temperature alloys, or evolving chemical product layer in chemical mechanical planarization (CMP) [1]. The physical size limitation of thin/ultra-thin films prohibits direct measurements of the film mechanical properties, except in some special circumstances (e.g. blanket wafer curvature to measure film stresses [2], or blister test to probe monolithic film properties [3]). Nano-indentation, on the other hand, provides a convoluted data, which depends on several sets of the film mechanical properties as well as its thickness [4, 5]. We are proposing a novel approach to measure the mechanical properties of ultra-thin (relatively soft) on a substrate (relatively hard), through several sets of measurements, combining nano-scratch and continuous stiffness measurements through the thickness of the film and up to an indentation depth of 8-10 and scratch depth of 2-3 times the film thickness. These measurements are analyzed by the simple limit analysis model and compared against a single phenomenological correlation, calibrated by finite element analysis. PROPOSED METHODOLOGY The proposed methodology utilizes the imposed deformation field by a nano-scratch to determine the film thickness and its mechanical properties. A sketch of a nano-scratch is depicted in Fig. 1, which ploughs the entire thickness of the film, t, and continue into the substrate. In the current simplified analysis, full contact is assumed between the indenter and
film/substrate combination across the entire depth of indentation. Also, pile-up or sink-in is ignored for the soft film on hard substrate syste
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