Indentation fracture of low-dielectric constant films: Part I. Experiments and observations
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Advanced microelectronic interconnection structures will need dielectrics of low permittivity to reduce capacitive delays and crosstalk, but this reduction in permittivity typically necessitates an increase in the porosity of the material, which is frequently accompanied by reduced mechanical reliability. Failure by brittle fracture remains a typical manufacturing and reliability hurdle for this class of materials. Part I of this two-part work explores the instrumented indentation and indentation fracture responses of a variety of organosilicate low-dielectric constant (low-) films. Three different chemical varieties of low- material were tested. The influence of film thickness on the fracture response is also explored systematically. Correlations are made between instrumented indentation responses and differing modes of fracture. It is demonstrated that the elastic response of the composite film + substrate systems can be simply tied to the fraction of the total indentation strain energy in the film. These results are then used in the companion paper, Part II [D.J. Morris and R.F. Cook, J. Mater. Res. 23, 2443 (2008)], to derive and use a fracture mechanics model to measure fracture properties of low- films.
I. INTRODUCTION
Incorporation of low-dielectric constant (low-) materials into silicon-based integrated circuits is a major technological challenge facing the microelectronics industry. As the required permittivity of the dielectric material in the back-end-of-line interconnection structure approaches that of a vacuum, it is inevitable that the density of the dielectric material falls, no matter what its chemical makeup. However, the dielectric material functions as a structural material in integrated circuits, and the reduction of density has important consequences for the mechanical properties of these materials. Improvement of the fracture toughness T of low- dielectrics remains a difficult challenge, according to the current International Technology Roadmap for Semiconductors.1 Certainly, then, a method to quantitatively measure the fracture toughness of low- dielectrics is needed. In this work, the extension of a well-established method for fracture toughness estimation, indentation fracture,2,3 is described. Part I of this two-part work summarizes indentation experiments that were performed on various organosili-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0294
cate low- material films with the goal of measuring film fracture toughness. There are several hurdles to studying the fracture properties of low- films. Principally, it must be verified that sharp indentation can be used to create controlled flaws in low- materials. The low-dielectric constant—and accompanying inclusion of porosity (or increased openness of the molecular structure)—will almost certainly mean that the conventional residual-stress elastic-plastic mechanism of radial indentation fracture will not work for these materials.4 The conventional indentation fracture model
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