Papers by Author: S.J. Bull

Abstract: In most coating applications damage resistance is controlled by the mechanical properties of the coating, interface and substrate. For electronic and optical applications the design of coating-substrate systems has been predominantly controlled by their functional properties but more recently the mechanical response of the system has been used to enhance functional properties, as in the case of strained silicon/SiGe microelectronic devices where tensile strain has been used to enhance mobility and increase device speed. As coatings become more complex, with multilayer and graded architectures now in widespread use, it is very important to obtain the mechanical properties (such as hardness, elastic modulus, fracture toughness, etc.) of individual coating layers for use in design calculations and have failure-related design criteria which are valid for such multilayer systems. Nanoindentation testing is often the only viable approach to assess the damage mechanisms and properties of very thin coatings (<m) since it can operate at the required scale and provides fingerprint of the indentation response of the coating/substrate system. If coating properties are to be assessed, the key point is to ensure any measured value is free from the influence of the deforma-tion of the substrate or lower coating layers. Finite element analysis of indentation load displace-ment curves can be used to extract materials properties for design; as coating thicknesses decrease it is observed that the yield strength required to fit the curves increases and scale-dependent materials properties are essential for design. Since plasticity is less likely, non-linear elasticity is increasingly important as the size of a nanostructure is reduced. Similarly the assessment of fracture response of very thin coatings requires modeling of the indentation stress field and how it is modified by plas-ticity during the indentation cycle. An FE approach using a cohesive zone model has been used to assess the locus of failure and demonstrates the complexity of adhesive failure around indentations for multilayer coatings. Finally the mechanical design of a metallization stress sensor based on na-noindentation-derived materials properties, non-linear elastic and plastic behavior and the treatment of geometrical non-linearities (stress stiffening) is discussed.

Abstract: The development of nanostructured materials and coatings has driven the development of indentation-based assessment techniques which aim to generate useful mechanical property information. This paper introduces an approach to determine the limits for which direct measurement of these properties are possible and highlights the importance of modelling if reliable data is to be obtained from very thin coatings (<200nm) and fine grained materials.

Abstract: A number of methods have been proposed to assess the adhesion of ceramic coatings including pull-off, shockwave, bending, scratch, and indentation tests. With the development of new coatings, the need for reliable adhesion tests is paramount, but it is not always easy to select an appropriate adhesion test, particularly for sub-micron coatings. Indentation tests are suitable in such cases provided that the interfacial failure mode is understood and can be modelled. This paper reviews the existing models and our newly developed models to extract quantitative adhesion data in such cases.

Abstract: The evaluation of stress in sub-micron tracks is critical for the microelectronics industry and there is a need for new methods of measurement. This paper advocates the use of a rotating beam sensor structure which can be fabricated on the wafer along side electronic devices and used to monitor stress generation and relaxation as a function of processing. The rotation can be observed with a reflected light microscope and correlated to the actual stress level. Several samples, assputtered and sintered, were prepared with the aim of having different residual stress states. X-ray diffraction with a low incident angle geometry, was used to evaluate the residual stresses on the aluminum layer. Computer simulations using ANSYS were also performed in order to correlate the sensor rotation with the experimental stress values. It was observed that the extrinsic stress from the mismatch in expansion coefficients between the aluminum layer and the silicon substrate dominates over the compressive stress from the sputter growth. Sintering the layers at temperatures above 150°C reduces this compressive stress due to the action of creep. The calibration of the rotation of the device with the direct measurements of the X-ray diffraction shows that the sensor has a resolution better than 2.8 MPa.