Optical metrology beyond Abbe and Rayleigh

Abstract

For many years, it was believed that optical microscopy and metrology was limited in resolution related to the light wavelength as suggested by Ernst Abbe and Lord Rayleigh. In recent past, several approaches have been developed to overcome these limitations such as Nobel price honored STED or optical CD as widely used in semiconductor metrology. Unfortunately, both techniques need special samples. While STED relies on fluorescence, OCD requires grating samples. In our contribution, we present two model based (mb) approaches to overcome some of these restrictions. One is mb Laser Focus Scanning (mLFS). Here, we show how to improve the accuracy of edge detection from several hundred nm to about 10 - 20 nm by exploiting rigorous modeling. The second one is Scanning Coherent Fourier Scatterometry (SCFS) where the diffracted Fourier spectrum is detected and the attempt is undertaken to retireve the sample profile. It is shown that this technique is very sensitive, particularly when the phase is recorded by means of a wave-front sensor. Measurements and simulations for periodic as well as for aperiodic sub-resolution features are show already good agreement. Moreover, we strongly believe that the observed high sensitivity of the Fourier spectra opens the path to quantitatively measurements below the resolution limits of light.