Resolution enhancement for lensless mask metrology with RESCAN

Abstract

The EUV photomask is a key component of the lithography process for semiconductor manufacturing. A critical defect on the mask could be replicated on several wafers, causing a significant production yield reduction. For this reason, actinic patterned mask inspection is an important metrology component for EUV lithography. The RESCAN microscope is a lensless imaging platform dedicated to EUV mask defect inspection and metrology. The resolution of the tool is about 35 nm, which is similar to that of state-of-the-art EUV microscopes. To improve the resolution of RESCAN, we designed an upgraded optical layout for the illumination system and we developed a coherent diffraction imaging-compatible method to synthesize a custom pupil structure. This new scheme will enable a lensless EUV microscope with a resolution down to 20 nm and thereby allow mask review capabilities for future technology nodes with EUV lithography.

Illumination control in lensless imaging for EUV mask inspection and review

“…The imaging numerical aperture along the axis parallel to the incidence plane (NA Y ) is limited by the extent of the condenser optics as shown in figure 1. 4 If α is the minimum incidence angle allowed by the condenser and β is the maximum reflection angle allowed by the multilayer on the EUV mask sample, the maximum non vignetted NA Y value for a chief ray angle θ 0 is given by:…”

“…This is a not a problem for the image reconstruction algorithm employed in RESCAN because each illumination probe and its corresponding sample image is reconstructed independently. 4…”

“…This limits the theoretical resolution of RESCAN to about 48 nm. 4 To overcome this problem, we decided to upgrade our system with a Fourier synthesis illuminator 5,6 to shift the portions of the diffraction spectrum that are normally unreachable, in the high-reflectivity angular range of the EUV mask. This approach will enable us to increase the resolution of RESCAN and will give it the flexibility to operate with arbitrary pupil shapes, paving the way towards EUV lensless mask review.…”