Scatterfield microscopy using back focal plane imaging with an engineered illumination field

Abstract

We have implemented back focal plane (conoscopic) imaging in an optical microscope that has also been modified to allow selection of the illumination angles and polarization at the sample, and collected back focal plane images of silicon on silicon grating scatterometry targets with varying line widths. Using a slit illumination mask, the zero-order diffraction versus angle for -60° to +60° incident angles at a given polarization was obtained from a single image. By using reference images taken on a flat silicon background, we correct the raw target images for illumination source inhomogeneities and polarization-dependent transmission of the optics, and convert them to reflectance versus angle data for s-and p-polarizations, similar to that obtained from angle-resolved grating scatterometry. As with conventional scatterometry, the target lines need not be resolved for the reflectance signature to show sensitivity to small changes in the grating parameters. For a series of 300 nm pitch targets with line widths from 150 nm to 157 nm, we demonstrate nanometer-level sensitivity to line width with good repeatability, using 546 nm illumination. Additionally, we demonstrate a technique for separating the zero order from higher order diffraction on targets with multiple diffraction orders, allowing collection of both zero and higher order diffraction versus angle from the back focal plane image. As conventional images can be easily collected on the same microscope, the method provides a powerful tool for combining imaging metrology with scatterometry for optical critical dimension measurements in semiconductors.

193 nm angle-resolved scatterfield microscope for semiconductor metrology

“…Many alternative methods have been developed to overcome this limitation, such as scatterometry, confocal microscopy, etc. [1][2][3][4][5] Recently, scatterfield microscopy has demonstrated a potential for improving optical measurement sensitivity and performance. [3][4][5] Scatterfield microscopy combines scatterometry and bright-field optical microscopy, and features Köhler illumination so that an off-axis point in the back focal plane (BFP) of an objective lens makes a plane wave illuminating the sample plane at a specific angle.…”

“…[1][2][3][4][5] Recently, scatterfield microscopy has demonstrated a potential for improving optical measurement sensitivity and performance. [3][4][5] Scatterfield microscopy combines scatterometry and bright-field optical microscopy, and features Köhler illumination so that an off-axis point in the back focal plane (BFP) of an objective lens makes a plane wave illuminating the sample plane at a specific angle. The intensity distribution of the back focal plane can be modified through a conjugate to the back focal plane (CBFP), allowing control of the illumination angle to generate angleresolved images of the sample that yield image and scattered field information simultaneously.…”

Analysis of Köhler illumination for 193 nm scatterfield microscope

“…Following initial research to quantitatively characterize grating topography [1], scatterfield microscopy has been demonstrated as a technique with potential for improving optical measurement sensitivity and performance [2][3][4]. The scatterfield microscopy method combines scatterometry and bright field imaging using the underlying principle in the Köhler illumination configuration that each point in the back focal plane (BFP) of an objective lens creates a plane wave illuminating the sample plane at an angle base on the position of the point in a conjugate of the BFP (CBFP).…”

Angle-resolved scatterfield microscope for linewidth measurement

“…Scatterometer 1-3 is a non-contact and non-destroyed metrology for linewidth and overlay measurement and it has higher optical resolution than the image-based optical microscope. The angle-resolved scatterfield microscope (ARSM) [4][5][6] is developed for several years by R. M. Silver, et al It combines the optical microscope with the angle-resolved scatterometer, thus it is simple to be used in the metrology.…”