Background: Reliable photomask metrology is required to reduce the risk of yield loss in the semiconductor manufacturing process as well as for the research on absorber materials. Actinic pattern inspection (API) of EUV reticles is a challenging problem to tackle with a conventional approach. For this reason, we developed RESCAN, an API platform based on coherent diffraction imaging. Aim: We want to verify the sensitivity of our platform to absorber and phase defects. Approach: We designed and manufactured two EUV mask samples with absorber and phase defects, and we inspected them with RESCAN in die-to-database mode. Results: We reconstructed an image of an array of programmed absorber defects, and we created a defect map of our sample. We inspected two programmed phase defect samples with buried structures of 3.5 and 7.8 nm height. Conclusions: We verified that RESCAN, in its current configuration, can detect absorber defects in random patterns and buried (phase) defects down to 50 × 50 nm 2 .

Reliable photomask metrology is required to reduce the risk of yield loss in the semiconductor manufacturing process. Actinic pattern inspection (API) of EUV reticles is a challenging problem to tackle with a conventional approach. For this reason we developed an API platform based on coherent diffraction imaging. Aim:We want to verify the sensitivity of our platform to absorber and phase defects. Approach:We designed and manufactured two EUV mask samples with absorber and phase defects and we inspected them with RESCAN in die-to-database mode. Results:We reconstructed an image of an array of programmed absorber defects and we created a defect map of our sample. We inspected two programmed phase defect samples with buried structures of 3.5 nm and 7.8 nm height. Conclusions:We verified that RESCAN in its current configuration can detect absorber defects in random patterns and buried (phase) defects down to 50 × 50 nm 2 .

Coherence control and flexible pupil fill play a key role in the imaging of EUV reticles. This is also true for lensless metrology applications based on coherent diffraction imaging. We describe the concept and the key components of a Fourier synthesis illuminator designed to provide the RESCAN microscope with flexible illumination capabilities and to improve its resolution limit. In particular, we discuss the characteristics of the three mirrors of the new illuminator and the requirements for their multilayer coating.

RESCAN is an actinic patterned EUV mask metrology tool based on coherent diffraction imaging. An image of the reticle is reconstructed from recorded diffraction patterns using a phase retrieval algorithm. As semiconductor manufacturing has moved to EUV lithography to meet the next technology node, accurate photomask metrology with resolution in the nanometer range is crucial for high production yield. To find the optimal reconstruction strategy to achieve the highest resolution, sensitivity and reconstruction speed in RESCAN, we compared three algorithms. We demonstrate that, for the current setup, the best approach is the difference map algorithm.

The production of modern semiconductor devices is based on photolithography, a process through which a pattern engraved on a mask is projected on a silicon wafer coated with a photosensitive material. In the past few decades, continuous technological progress in this field allowed the industry to follow Moore's law by reducing the size of the printed features. This was achieved by progressively increasing the numerical aperture of the projection system and reducing the wavelength. The latest lithography platforms for semiconductor manufacturing employ Extreme Ultra Violet (EUV) light at a wavelength of 13.5 nm. The metrology for the optics and the components of such platforms is not fully mature yet. Specifically, the inspection of the EUV photomask is still an open issue as no commercial solutions are currently available. Here we describe a lensless approach to this problem, based on coherent diffraction imaging at EUV that overcomes the main technological issues linked to the conventional mask inspection approach.