1 Fracture toughness measurement of thin nanoporous films on stiff substrates D.J. Morris , † R.F. Cook National Institute of Standards and Technology, Gaithersburg, Maryland USA Nanoporous low-dielectric-constant films constitute a class of materials that are plagued by fracture concerns and are not amenable to traditional fracture toughness measurement techniques. An indentation fracture toughness measurement technique has been developed for these materials. The experiment utilizes nanoindentation in combination with cube-corner indenters which create flaws are on the scale of the film thickness, about a micrometer. Interpretation of experimental results are a far-reaching generalization of the traditional Vickers based indentation test used for ceramics at the mesoscale. Cube-corner indentation fracture is dominated by crack-wedging effects that are not important for Vickers indentation. Film-substrate elastic coupling is very important, and is manifested in three distinct ways. After film-substrate coupling phenomena are identified, they are combined with acute indentation fracture models to form a complete thin-film indentation fracture mechanics model. The fracture toughness of two materials have been measured to be 0.09 MPa m 1/2 and 0.05 MPa m 1/2 . Variables a contact radius  indenter axis-to-face angle c surface crack length  film stress c´ crack depth  SIF amplitude E plane-strain elastic modulus  Poisson’s ratio G mechanical energy release rate (MERR)  shear modulus H hardness K stress-intensity factor (SIF) Super- and sub-scripts  Crack aspect ratio A apparent P indentation load CH channel crack R fracture resistance C contact s Zak-Williams stress-singularity exponent F film t film thickness I indentation T fracture toughness PC partially-cracked W wedginess S substrate TH threshold  Dundurs elastic mismatch parameter (1 of 2) W wedging  Dundurs elastic mismatch parameter (2 of 2)  channel crack geometry factor † Corresponding author. dylan.morris@nist.gov