NIST creates toughness test for low–k films


Researchers at the National Institute of Standards and Technology (NIST) have updated a well–known technique for measuring material properties to gauge the mechanical strength of low–k films, to help devicemakers better identify interconnect dielectric film candidates.

low–k dielectric layers are increasingly utilized as insulating films on top of a substrate (e.g. silicon) and between layers of conductors and components in semiconductor devices. Increasing their porosity has helped prevent cross–talk (electrical interference) but also makes them more brittle, affecting yields. NIST notes that no accurate method has been devised to measure fracture resistance of such films, which limits design and development of new and improved dielectrics.

Typical low–k film test for material toughness using the new NIST technique. The indentation instrument that punches the triangular hole registers the forces involved. That, plus the length of the resulting cracks, determines the toughness of the film, which is about 2.4μm thick. Color added for clarity. (Credit: NIST)
Click here to enlarge image

So, the researchers have adapted a known test technique called nanoindentation, which involves pressing a sharp, hard object (e.g., a diamond tip) against the surface of a material to determine the pressure required to deform the material. Typically, this test has been used to measure elasticity and plasticity, the forces required to bend a material temporarily or permanently.

The new technique, described in the Sept. 2008 issue of the Journal of Materials Research, modifies the probe with a sharper, more acute point, pressing on a dielectric film to generate cracks as small as 300nm, which are measured by electron microscopy. How the cracks form depends on indentation force, film thickness, film stress, and the elasticity of both the film and substrate underneath. Plugging the variables into a fracture mechanics model allows the researchers to predict the fracture toughness as well as critical film thickness for “spontaneous fracture.” This methodology is expected to help devicemakers eliminate candidate interconnect dielectric films without more expensive device testing. —J.M.