Inline control of an ultra low-k ILD layer using Broadband Spectroscopic Ellipsometry

As chip dimensions are scaling down new challenges develop in the back-end-of-line. In order to keep the capacitance small while decreasing the volume of the inter-layer dielectric (ILD), new materials and processes have been introduced over the past years to lower the dielectric constant of the ILD layers. For design rules of 45 nm and below porous Ultra low-k materials are widely used in today's semiconductor process flows. Beside process challenges this introduces stringent requirements for metrology not only to monitor film thickness but other properties of the material as well. This paper discusses the development and implementation of a Broadband Spectroscopic Ellipsometer for inline process control of a SiCOH based porous ultra low-k film. After deposition the material is cured with UV light to introduce the porosity. The challenge for the metrology is to measure both the thickness and an adequate metric for the chemical properties which do no longer correlate with optical properties. In addition the chemical properties vary as a gradient from top to bottom of the film. We discuss the methodology to develop a metrology recipe resulting in the thickness of a metric layer and the percentaged thickness shrink being the best parameters to sense and track the process adequately. Results demonstrate the sensitivity of the technique to process variations. Short term precision, long term stability and tool-to-tool matching results prove that the technique enables routine process monitoring in a high volume automated semiconductor fab.