Immersion scatterometry for improved feature resolution and high speed acquisition of resist profiles

Specular-mode spectroscopic scatterometry is currently being used as an in-line metrology tool for wafer-to-wafer process monitoring and control in lithography and etch processes. Experimental real-time, in situ demonstrations of critical dimension monitoring and control have been made for reactive ion etching. There have been no similar demonstrations of real-time control in the critical step of resist development. In this paper, we will show the results of a simulation study on the use of scatterometry in an immersion mode both to improve resolution and to act as a real-time monitor for photoresist topography evolution during development. We have performed realistic simulations of the experimental performance by using Prolith™ to generate developing resist profiles vs. time and a rigorous couple wave algorithm (RCWA) simulator (modified to include the immersion ambient) to generate simulated scatterometry data. We have examined several modes of operation of the proposed measurement including specular and 1 st order scattered modes using spectroscopic ellipsometry and reflectometry. For our simulations, we have used pure water to approximate the developer refractive index. We have created realistic simulation data by adding appropriate amounts of random noise to perfect simulations, and then used regression analysis to extract profiles from these data. Water immersion increases feature shape resolution for small period gratings by increasing the scattering into real diffracted modes.