Selective measurement of small metrology targets using CD-GISAXS

Abstract

To address the increasing requirements of the semiconductor industry for accurate measurement of small features, Critical Dimensions (CD) Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) is being considered. GISAXS offers fast, surface sensitive measurements from which relevant structural parameters can be reconstructed. However, an essential limitation for the practical application is the large measurement spot size resulting from the shallow incidence angle. Here we show that despite a large spot size, GISAXS measurements of small targets are possible if the targets are designed such that the scattering from the targets is separable from the scattering of other structures in the measurement spot. For grating targets, in particular rotation of the targets in the sample plane such that the grating lines are not parallel to the predominant direction of the surroundings allows reliable measurement of the target scattering irrespective of the X-ray beam size. We discuss the effect of X-ray source, rotation angle, target size and grating pitch on the minimum measurement times and number of measurable grating diffraction orders and conclude that a single 50 μm × 50 μm area grating target can be measured with high-end laboratory sources in tens of seconds if it is rotated by at least 0.2°. For smaller targets, significantly longer measurement times and larger rotations are necessary.