X-ray porosimetry: a new method for the characterization of porous low-dielectric-constant thin films adaptable for the semiconductor industry. (General Developments).

NIST scientists have developed a new measurement method--x-ray porosimetry (XRP)--for characterization of porous low-dielectric-constant (low-k) films. The method has application to the semiconductor industry in their pursuit of new interlayer materials with low dielectric constants to reduce cross-talk and to increase processor speed as device features reach nanometer sizes. While candidate materials differ in their base chemistries, a common theme emerges in the push to develop low-k dielectric materials--nano-scale porosity must be introduced in a controlled manner to further reduce the dielectric constant. Measurement techniques are needed to accurately and noninvasively characterize the porosity in these films while attached on a silicon substrate. Over the past several years, NIST scientists in the Polymers Division and at the Center for Neutron Research have successfully addressed these needs through the development of a methodology based upon x-ray reflectivity (XR), small angle neutron scattering, an d ion scattering. However, these measurements require specialized facilities not readily accessible to industrial laboratories. In contrast, the new method requires x-ray equipment commercially available to industrial laboratories.

In the new method, a controlled solvent environment is created around the thin film so that an equilibrium amount of adsorption occurs. Under such conditions, XR instrumentation gives accurate values of the total density that is a combination of wall density and solvent-filled pores. The mass uptake as a function of partial pressure is calculated from these results.

An additional potential advantage of XRP is the ability to quantify not only the average film density, but also the density profile normal to the film surface. Ongoing work investigates the potential of XRP to extract pore size distributions as a function of depth into film. To prevent dielectric breakdown, the semiconductor industry prefers to have low porosity or very small pores near the surfaces with the majority of the porosity in the localized in the center of the thin film. XRP has the potential to become a useful method for characterizing these types of structures using commercially available XR within industrial laboratories.

CONTACT: Christopher Soles, [phone]; [e-mail] or Barry Bauer, [phone]; [e-mail] or Wen-li Wu, [phone]; [e-mail].