A novel method for pushing the limits of line edge roughness detection by scatterometry

Abstract

Fluctuations in the line edge of lithographic features, termed line edge roughness (LER) always exist. At 32 nm line width (and below), LER can be a significant fraction of the feature dimensions. LER can be simply detected by AFM or SEM techniques, however, fast and nondestructive optical techniques should be developed in order to enable effective process control. Optical scatterometry is preferable over other existing measurement techniques, due to the relatively simple implementation in production and lower photoresist damage. In this article we show simulations of LER by 3-dimensional Rigorous Coupled Wave Theory (RCWT) calculations. The prediction of tool capabilities was done using simulations. The outcome of these simulations results where analyzed and used for the basic design of photoresist structures. The conclusions from sensitivity and correlation analysis of the simulation data were verified against measured scatterometry data. Well-defined features with controlled LER, in the range of 2.5 to 15nm, were fabricated by e-beam direct write technique (IMEC, Belgium). The photoresist features we created were a large matrix of different scatterometry targets with varying parameters of CD, Period, LER level, and LER frequency. These features were characterized by electron microscopy and AFM in order to verify the LER values and a NovaScan 3090 system and NovaMARS modeling software were used for the Scatterometry characterization. To achieve better sensitivity to the lower roughness dimensions, we used an option of Effective Medium Approximation (EMA) modeling for spectra analysis. Based on this reference data and the scatterometry measurements we have developed a novel scatterometry method that is sensitive to very low level of LER. This method is based on design of a special test structure which can show better sensitivities than the basic noise levels of the tool. The basic idea in this design is the calibration of the scatterometry measurement on a series of sites with LER steps. It will be shown that LER changes of about 1 nm can be detected based on these designed test structures. This is well below the normal capabilities of current optical tools.